corrosion

Introduction 

Metals and alloys are important engineering materials with a wide range of applications. The surfaces of almost all metals begin to decay more or less rapidly, when exposed to  environmental conditions like gaseous atmosphere, water or another reactive liquid medium.  As a result of the decay, the metals are converted to their oxides, hydroxides, carbonates, sulphides etc. The process of decay of metal by environmental attack is called corrosion. For example formation of reddish brown layer of rust on the surface of iron and formation of green film of basic carbonate [CuCO3 + Cu(OH)2] on the surface of copper. The corrosion of metals is measured in the units of milli inches/year or mm/year.

causes of corrosion 

The basic reason for this attack is most of the metals (except Pt, Au) exist in nature in the form of their minerals or ores, in the stable combined forms as oxides, chlorides, silicates, carbonates, sulphides etc. During extraction of metals these ores are reduced to metallic state by supplying considerable amounts of energy. Hence the isolated pure metals are regarded as excited states than their corresponding ores. So metals have the natural tendency to go back to their combined state (ores/minerals).  As a result when the metal is exposed to environmental conditions like dry gases, moisture and liquids etc. the metal surface reacts and forms the more stable compounds of metals (thermodynamically stable state) like oxides, carbonates etc., more rapidly.  For example iron undergoes corrosion to produce brown coloured hydrated ferric oxide which is nothing but rust on the surface.  As a result of corrosion the loss of metal takes place and the process of corrosion is reverse of metal extraction, as shown below. In the pure form, metal posses the engineering properties like malleability ductility, and electrical conductivity etc. When corroded to compounds of metal which donot posses engineering properties, metals cannot be used as engineering materials.

effects or disadvantages of the corrosioN   process

1. The valuable metallic properties like conductivity, malleability, ductility etc.,  are lost due to corrosion.
2. The process of corrosion is very harmful and is responsible for the enormous wastage of metal in the form of its compounds.  It is estimated that the amount of iron wasted by corrosion each year is about  of its annual world production.
3. Life span of the metallic parts of the machineries is reduced.
4. The failure of the machinery takes place due to loses of useful properties of metals.
5. Losses occurring due to corrosion cannot be measured in terms of the cost of metals alone, but the high cost of fabrication into equipment, machine tool and structures should also be considered.  The approximate estimate of loss of  metal due to corrosion is 2 to 2.5 billion dollars  per annum all over the world.  Hence an engineer must understand the mechanism of corrosion so that the  effects can be minimised by avoiding the corrosion conditions and provide simultaneous protection against corrosion.

Types of corrosion 

Broadly the corrosion process proceeds in two types by direct chemical action of environment on the surface of metal in absence of moisture and in presence of a conducting liquid by the formation of electrochemical cells.  The former process is called dry corrosion or chemical corrosion and the later is called wet corrosior electrochemical corrosion. The different ways of corrosion of metals is summarised in the form of a chart as given below.
Corrosion of Metals


Dry Corrosion/Chemical Corrosions
         Wet Corrosion or Electrochemical Corrosion
(Direct chemical action of environment/ (Corrosion due to the conducting liquid- atmospheric gases/anhydrous inorganic in contact with cathodic and anodic areas).
liquid metal surface in the absence                                                                                                                           of moisture)


1. Oxidation 2. Corrosion by other 3. Liquid metal        Corrosion      gases    Corrosion
(Direct action of (Attack of gases like (Chemical action oxygen at high or SO2, CO2, Cl2, H2S, F etc.            of flowing liquid low temperatures on metal surface)  metal at high on metals)  temperatures)

1. Electrochemical Corrosion   2. Galvanic 3. Concentration cell 4. Underground or      by evolution of hydrogen      Corrosion    Corrosion    Soil Corrosion      and absorption of oxygen     (Corrosion    (Corrosion due to    (Corrosion  due to    (Corrosion due to flow of      between two    the formation of    corrosiveness of soil)    electron current between      dissimilar metals)      varying concentrations    cathodic and anodic areas)    of aeration)


(a) Pitting corrosion (b) Water line corrosion
(Corrosion due to (Corrosion due to difference formation of cavities in water level) around the metal)

5. Stress Corrosion 6.  Intergrannular corrosion
    (Corrosion due to static     (Corrosion along the     tensile strength)      grain boundaries)
3.5 theories of corrosion / MECHANISM OF CORROSION
Broadly the mechanism of corrosion takes place in three types as given below.
3.5.1  Dry or Chemical Corrosion
This type of corrosion occurs mainly through the direct chemical action of environment or atmospheric gases such as oxygen, halogen, hydrogen sulphide, sulphur dioxide, nitrogen or anhydrous inorganic liquid with metal surfaces in immediate proximity.  There are three main types of chemical corrosion.
a)     Oxidation corrosion
b) Corrosion by other gases
c) Liquid metal corrosion
(a) Oxidation Corrosion :   Direct attack of oxygen at high or low temperatures on metals in the absence of moisture is called oxidation corrosion.  Generally metals are oxidised to a smaller extent, however alkali metals (Li, Na, K, Rb etc.) and alkaline earth metals (Be, Ca, Sr etc.,) are very rapidly oxidised at lower temperatures.  Except Ag, Au and Pt all metals are oxidised at high temperatures.
2M 2M+ + 2e (Oxidation by loss of electron)
O2 + 2e 2O2– (Reduction by gain of electron)
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Total reaction 2M +O2  2M+ + 2O2–  2MO
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Mechanism of Oxidation Corrosion :  The oxidation of the metal occurs at the surface first, resulting in the formation of metal oxide scale, which restricts further oxidation (Fig. 3.1).
Further oxidation can continue if the metal diffuses out of the scale or the oxygen must diffuse in through the scale to the underlaying metal. Of the two types of diffusions, the diffusion of the metal is rapid because the size of the metal ion is smaller than oxygen ion, hence higher mobility to metal ion.

Nature of the metal oxide formed plays an important role in oxidation corrosion.
1. A stable metal oxide layer/film with fine-grained structure, tightly adheres to the metal surface is impermeable to the attacking oxygen.  Such film behaves as protective coating in nature, thereby shielding the metal from further corrosion (Fig. 3.2).  Eg : the oxide film on Al, Sn, Pb, Cu, Cr, W etc.
2. An unstable oxide formed on the metal surface decomposes back into the metal and oxygen. In such cases oxidation corrosion is not possible. (Fig. 3.3).
For example Ag, Au and Pt do not undergo oxidation corrosion.
3. A volatile oxide layer formed during corrosion evaporates as soon as it is formed, leaving the metal for further attack.  This causes rapid and continuous corrosion leading to excessive corrosion to occur (Fig. 3.4). For example molybdinum oxide (MoO3) is volatile.
4. A porous layer of metal oxide contains pores, cracks etc. provide access to oxygen to reach the underlying surface of the metal.  As a result the corrosion process continues to be unobstructed till the entire metal is completely converted to its metal oxide (Fig. 3.5). Alkalimetals and alkaline earthmetals (Li, K, Na, Mg) form such type of oxide film.
To express the extent of protection given by the corrosion layer to the underlaying metal Pilling Bedworth rule was postulated. According to Pilling Bedworth rule the smaller the specific volume ratio (volume of metal oxide/volume of metal), the greater is oxidation corrosion, which is explained as follows.
i) If the volume of the metal oxide layer is at least as great as the volume of the metal from which it is formed is non porous and becomes protective layer by tightly adhering to the base metal from which it is formed.  No corrosion occurs as the oxidation of metal reaches to zero.  For example, the specific volume ratios of W, Cr and Ni are 3.6, 2.0 and 1.6 respectively.  Consequently the rate of corrosion is least in Tungsten (W).
ii) If the volume of metal oxide is less than the volume of the metal, the oxide layer is porous, non continuous and non-protective and faces strains.  Hence cracks and pores are developed in the layer, creating access to atmospheric oxygen to reach the underlaying metal.  In this case corrosion is continuous and rapidly increase.  For example Li, Na and K.
(b) Corrosion by other gases :The gases like SO2, CO2, Cl2, H2S and F2 etc., also cause chemical corrosion and the extent of the corrosion mainly depends on the reactivity to the gas on the metal surface.  The corrosion product is metal compound. The nature of the metal compound determines the corrosion.  For example, if the volume of the corrosion film formed is more than the underlaying metal, it is strongly adherent, non-porous and does not allow the corrosive gas to reach the underlaying metal.  Hence it is protective film.  For example the attack of Cl2 gas on silver metal produces adherent, non porous AgCl film which is protective.  If the corrosion layer has less volume than the metal, then strain is developed on the layer to produce cracks and pits, creating access to the corrosive gases to reach the underlaying metal.  Thus the whole metal is gradually destroyed.  For example chlorine attacks the metal tin (Sn) forming volatile SnCl4 (which volatilises immediately), leaving fresh metal surface for further attack.  H2S gas attack steel in petroleum industry forming a porous FeS layer which further increases the corrosion.  
(c) Liquid metal corrosion : The chemical action of a flowing liquid metal at high temperature, on a solid metal or alloy produces liquid metal corrosion. The reason for corrosion is the dissolution of the solid metal by liquid metal or internal penetration of the liquid metal into the solid phase, weakening the solid metal.

Wet or Electrochemical Corrosion 

Wet corrosion or electrochemical corrosion takes place under wet or moist conditions through the formation of short circuited electrochemical cells.  Wet corrosion is more common than dry corrosion.  Electrochemical corrosion involves
1. Cathodic and anodic areas separated by the conducting medium.
2. The oxidation of the metal liberating electrons takes place at anodic areas.
M  M+ + e–
3. Depending on the nature of the corrosive environment the electrons liberated at anode are transported to cathodic area through the metal.
4. During the cathodic reaction either H+ or O2 and H2O consumes the electrons generating non metallic ions like OH--  or O2– either by evolution of hydrogen or absorption of oxygen.
5. The diffusion of the metallic (M+) and non metallic (OH--  or O2–) ions towards each other through conducting medium results the formation of corrosion product in between the anodic and cathodic areas. Without conducting medium no corrosion takes place.
Wet corrosion takes by the following ways.
a) Evolution of hydrogen : This type of corrosion occurs usually in acidic environments. The rusting of iron takes place in acidic environment in the following way.
At anode the dissolution (oxidation) of iron to ferrous ion with liberation of electrons takes place.
Fe Fe2+ + 2e– (oxidation)
The 2e– flow from anodic area to cathodic area through the metal where H+ ions (of acidic solution) are available and eliminated as hydrogen gas.
2H+ + 2e–  H2 (reduction)
Thus Fe2+ ions are liberated at anode and H2 gas is liberated at the cathode.  The over all reaction in acidic medium is given by
Fe + 2H+  Fe2+ + H2
In hydrogen evolution type of corrosion, the anodes are usually large and the cathodes are small areas (Fig. 3.6).
In neutral medium at the cathode formation of OH--  takes place along with the liberation of hydrogen
2H2O + 2e–  H2  + 2 OH--  
The Fe2+ and OH--  diffuse towards each other through the conducting medium forming the corrosion product rust in between cathodic and anodic areas.
Fe2+ + 2 OH--   Fe (OH)2
(ferrous hydroxide)
4 Fe (OH)2 + O2 + 2H2O  4 Fe (OH)3  2 Fe2 O3.3 H2O
                    (ferric hydroxide) (rust)
b) Absorption of Oxygen : In presence of neutral, aqueous solution of electrolytes like (NaCl solution), in the presence of atmospheric oxygen and some cracks developed in iron oxide film causes this type of electrochemical corrosion.  The surface of iron is always coated with a thin film of iron oxide.  If a crack is developed in this iron oxide film, the anodic areas are created on the surface, while the well coated metal parts act as cathode.  It follows that the anodic areas are small and the cathodic areas are large.
The following reactions take place taking rusting of iron as an example.
At the anodic areas, the metal oxidises as ferrous ions liberating two electrons which pass through metal to the cathodic area.
                  Fe  Fe 2+ + 2e–
At the cathode the electrons were accepted by water and oxygen to generate
OH--  ions by reduction.
                 O2 + H2O + e–  4 OH--
The combination of  Fe2+ and OH--  originated at anode and cathode respectively occurs at cathode, because the mobility of Fe2+ ions,  (which are smaller  than the OH--  ions)  is greater than OH-- ions. Corrosion occurs at anode and the rust occurs in between cathodic and anodic areas.
When the amount of O2 increases, it forces cathodic reaction to produce  more OH-- ions thus consuming more electrons by accelerating the corrosion at anode. In turn the rust formation is also accelerated.  The presence of oxygen accelerates the corrosion at anodic area and rust formation at cathodic area. (Fig. 3.7).
The Fe2+ ion at anode and OH-- ions at cathode diffuse towards cathodic and anodic areas respectively and they form rust when they meet.
Fe2+ + 2OH--   Fe (OH)2
If enough oxygen is present ferrous hydroxide is easily oxidised to ferric hydroxide and then to hydrated ferric oxide which is nothing but rust.
4Fe (OH)2 + O2 + 2H2O  4 Fe (OH)3  2[Fe2O3.3H2O]
(ferrous hydroxide) (ferric hydroxide) (rust)
Table - 3.1 : Comparison of chemical corrosion with electrochemical corrosion
 S.No.        Dry or Chemical Corrosion Wet or Electrochemical  Corrosion
1.  This corrosion occurs at dry conditions. This corrosion occurs in wet condition in presence of an electrolyte medium.
2.  Corrosion is uniform. Corrosion is not uniform if the anodic area is small, pitting is more frequent.
3.  It is a slow process. It is a rapid process.
4.  It involves direct chemical attack of the It involves the formation of large        metals by environment. number of electro-chemical cells.
5.  It is explained by absorption mechanism. It is explained by mechanism of electrochemical reactions.
6.   It occurs both on homogeneous and It takes place only on heterogeneous        heterogeneous solutions. processes.
7.  Corrosion products are produced at Corrosion occurs at the anode and        the place where corrosion takes place. corrosion product occurs in between cathodic and anodic areas.
Basic requirements of rusting of iron are listed below.
a) Both oxygen and water are essential for rusting.  Iron will not undergo rusting in dry air or water that is completely free from oxygen.
b) The rusting of iron is also accelerated by the following.
(i) rust itself  (ii) the presence of acids and electrolytes and (iii) contact with less corrosion active metals like Cu, Ag etc.
c) Production of anodic and cathodic areas on the metal surface.
Confirmation of the mechanism of corrosion
The mechanism of the electrochemical corrosion is confirmed by using ferroxyl                   indicator whose composition is
1)   Phenolphthalein (indicates the presence of OH--  ions) (turns pink)
2)   Potassium ferricyanide (indicates the presence Fe2+ ions) (turns blue)
3)   Sodium chloride (accelerates the corrosion for demonstration purpose).
On placing a drop of ferroxyl indicator on the surface of a polished metal, an electrochemical cell is formed quickly because the oxygen is used up at the centre of the drop which becomes anodic liberating ferrous ions, which gives blue colour with potassium ferricyanide of ferroxyl indicator at the centre of the drop.  Cathodic area is produced at the edge of the drop producing OH--  ions, whereby the ferroxyl indicator turns pink at the edge because of phenolphthalein.  This is a clear confirmation that Fe2+ ions are produced at the anonidic area and OH--  ions are produced at the cathodic area

Types of Wet/Electrochemical Corrosion

The different types of wet/electrochemical corrosion is given below.
(a)  Galvanic Corrosion
When two dissimilar metals are connected and jointly exposed to the corrosive atmosphere, the metal possessing higher oxidation potential or higher in the electrochemical         series becomes anodic and undergoes corrosion.  For example when zinc and copper are connected and exposed to corroding environment (containing moisture, which acts as an electrolyte), zinc becomes anodic because of its higher oxidation potential or higher position in the electrochemical series, zinc undergoes oxidation and corroded whereas copper undergoes reduction and protected as shown below (Fig. 3.9). The electrons released by zinc reaches copper through the metal.
Anodic metal  Zn    Zn2+ + 2e (oxidation)
Cathodic metal  Cu2+ + 2e  Cu  (reduction)
It is evident that corrosion occurs to metal which is anodic metal while the cathodic metal is protected.
When more than two metals are used in a structure, it is easy to find the more corrosive metal with the help of galvanic series.  In an equipment using zinc in contact with iron parts, zinc will be corroded because zinc is anodic to iron.  When iron is in contact with copper, iron will undergo corrosion because iron is anodic to copper.
From the electrode potentials a relative corrosion tendencies can be formed. For               example iron in contact with copper will corrode faster than iron in contact with lead.  In drawing conclusions the galvanic series and polarizing effects must be considered.
Galvanic Series : Although electrochemical series give very useful information                 regarding chemical reactivity of metals, it did not provide sufficient information in predicting the corrosion behaviour under a particular set of environmental conditions. Several side reactions taking place may influence the corrosion reactions. Hence, oxidation potentials of various metals and alloys commonly used are measured by using standard calomel electrode as the reference electrode and immersing the metals and alloys in sea water. When those oxidation potentials are arranged in the decreasing order of activity a series of metals arise. These series are known as galvanic series, which gives more practical information about the corrosive tendency of metals and alloys. The corrosive tendency may be influenced by other interfering factors. Thus it is clear that corrosion occurs at the anodic part and cathodic part is protected.
Examples of galvanic corrosion :
1. Steel screws is a brass marine hardware (steel becomes anodic).
2. Lead antimony solder around copper wire (lead, antimony solder becomes anodic).
3. A steel propeller shaft in bronze bearing (steel becomes anodic).
4. Steel pipe connected to bronze plumbing (steel becomes anodic).
Table - 3.2 :  Galvanic series
High anodic/active toward corrosion
Magnesium
Magnesium alloys
Zinc
Aluminium
Low carbon steel
Cast iron
Stainless steel (active)
Lead tin alloys
Lead
Tin
Brass
Copper
Bronze
Copper - Nickel alloys
Inconel
Silver
Stainless steel (passive)
Monel
Graphite
Titanium
Gold
Platinum
More cathodic or inactive towards corrosion
When metals are arranged in the order of their increasing reduction potentials, a series called e.m.f. series arise (discussed in unit-2, Electrochemistry) which also furnish information regarding the nature of metals to behave as anodic or cathodic under suitable conditions. The following are the differences between galvanic series and electrochemical series.
Table - 3.3
 S. No.       Galvanic series Electrochemical series
1. This series was developed by This series is developed by dipping studying corrosion of metals and pure metals in their salt solution alloys in unpolluted sea water, of 1M concentration without any without their oxide film. oxide film on them.
2. The position of a given metal may The position of a metal in electro- shift in galvanic series. chemical series is fixed.
3. The corrosion of alloys can be studied There is no information regarding from this series. the position of the alloys in this series.
4. The position of a metal is different The position of the metal is fixed. from that of the position of the alloy which contains this same metal in it.
5. This series predicts the relative This series predict the relative corrosion tendencies. displacement tendencies.
6. This series comprise of metals and alloys. This series comprise of metals and non-metals.
(b) Concentration Cell Corrosion
This type of corrosion occurs due to electrochemical attack on the metal surface exposed to an electrolyte of varying concentrations or varying aeration.
The concentration cell corrosion may arise due to
1. difference in the concentration of the metal ions or
2. difference in the exposure to air/oxygen (differential aeration corrosion) or
3. difference in the temperature or
4. inadequate agitation or
5. different diffusions of metal.
The most common type of concentration cell corrosion is differential aeration corrosion.  When a metal is exposed to different air concentrations or concentrations of salts etc. it has been found that the poorly oxygenated part of the metal becomes anodic and well oxygenated part of the metal becomes cathodic.  A difference of potential is created, which causes the flow of electrons (current) between the two differently aerated parts of the same metal.

Various types of concentration cell corrosion is summarised below.

 Various types of concentration corrosion

The general ways of production of oxygen deficient areas on metal surfaces can be summarised as follows.
i)  Differential aeration corrosion :
The different ways of creating differential aeration on the metal surface is listed below :
    a)  Partial immersion of metal in a solution :
The Zn rod immersed deep in NaCl = anodic solution
The Zn rod above NaCl solution   = cathodic
The flow of electrons take place from anodic to cathodic part through the metal.
The circuit is completed by the migration of ions through the electrolyte.  As a result zinc starts corroding in the anodic area immersed in sodium chloride solution (Fig. 3.10).  This is also called waterline corrosion.  Examples are ship hulls, storage tanks.
    b)  Faulty design of equipment :
An anodic area can be created due to difference in aeration of the metal parts.  When the metal surface is exposed to less oxygen supply due to faulty design of the equipment, the metal part less exposed to oxygen becomes anodic and starts corroding.  The metal part exposed to more oxygen supply becomes cathodic.  The flow of electrons takes place through the metal and atmospheric moisture acts as medium for the transfer of ions.  Some examples of this type of corrosion due to faulty design are listed below.
   c)  Accumulation of dirt, sand, scale and other contaminations :
When the dirt, sand, scale and other contaminations are deposited on the metal surface, the metal under the deposit is exposed to less oxygen supply and becomes anodic and remaining part of the metal becomes cathodic.  Transfer of electrons takes place through the metal and atmospheric moisture acts as medium for the transfer of ions.  Corrosion product is formed between cathodic and anodic areas
(ii)  Pitting corrosion :
A cavity, pinholes, pits and cracking of the protective film developed on the metal surface creates the formation of small anodic areas in the less oxygenated parts and large cathodic areas in well oxygenated parts.  The flow of electrons from anodic part to the cathodic part takes place and the ions move through medium (atmospheric moisture). (Fig. 3.13).  The corrosion product is formed between cathodic and anodic areas.  Cracking of protective film may be due to (1) surface roughness or non uniform stresses (2) scratches or cut edges (3) local strain of metal (4) alternating stresses (5) sliding under load (6) impingement attack (caused by the turbulent flow of a solution over metal (7) chemical attack.
    a)  Carry over in boiler causing corrosion to turbine plates :
The boiler water concentrated with dissolved salts, is carried along the steam or in the form of droplets of water and deposits on the turbine plates. The metal under the drop becomes anodic due to high concentration of the dissolved salts and starts corroding while the remaining large  areas of the turbine plates becomes cathodic. The flow of electrons takes place through the metal and the ions pass through the medium. Thus the electrochemical corrosion due to concentration cell occurs to the metal.
    b) Corrosion due to caustic embrittlement :
Boiler feed water contains certain amount of sodium carbonate which decomposes to sodium hydroxide under the high pressure of the boilers, which deposits in the hair cracks, pits etc. of the boiler plate creating a concentration cell. The metal deposited with NaOH becomes anodic while the metal surround the drop becomes cathodic.
Na2CO3 + H2O   2 NaOH + CO2
The concentrated alkali in the anodic area dissolves the metal as sodium ferrite (Na2FeO2), which decomposes a short distance away from the point of formation as magnetite and sodium hydroxide thereby enhancing corrosion.
6 Na2FeO2 + H2O + O2     12 NaOH + 2 Fe3O4
The concentration cell formed can be represented as
[Iron (–) anodic/Concentrated NaOH/Dilute NaOH/Iron (+) (cathodic)]
(iii)  Underground corrosion :
Underground corrosion, otherwise called as soil corrosion is due to the corrosiveness of soil depends upon
1. Acidity of the soils, as the acidity of the soil increases, the rate of corrosion increases.
2. Degree of aeration, as the degree of aeration increases the rate of corrosion increases.
3. Electrical conductivity of the soil increases, the corrosion intensity.
4. The moisture and salt content of the soil enhances the corrosion.
5. Soil texture is also responsible for the enhancement of corrosion. According to particle size, soils are classified as
a) Gravely or sandy soils, which are very porous, and strongly aerated, create corrosive conditions similar to those under wet condition.  The corrosion rate is governed by amount of moisture content in the soil.
b) Water logged soils, have very small amount of free oxygen available. In acidic soils the corrosion to the buried underground pipeline takes place hydrogen evolution type, whose rate of corrosion depends on the pH of the soil and the presence of salts and oxygen in the soils.
c) Intermediate character soils, containing fine gravel, sand and salt may produce localised intense corrosion to pipelines.  If air packets are present in the soil, differential aeration occurs.
(iv) Stress corrosion :
Stress corrosion or stress cracking is the combined effect of stratic tensile stresses and the corrosive environment on a metal.  It is highly localised attack occurring when overall corrosion is negligible.  The following are the causes for stress corrosion to occur.
Presence of tensile stresses is a main reason for stress corrosion, majorly seen in fabricated articles of alloys like high zinc brasses and Nickel brasses.  Heavy working like rolling, drawing or insufficient annealing produces stress in alloys.  Pure metals are immune to stress corrosion.
Localised electrochemical corrosion occurs along the grain boundaries (Fig. 2.14(a) and narrow paths, Fig. 3.14 (b) forming anodic areas and the remaining part of the metal becomes cathodic. The metal undergoes ionisation at the anodic area and the electrons diffuse through the metal to the cathodic area producing the hydroxyl ions.  The corrosion product is formed between the anodic and cathodic area.  The presence of stress produces high electrode potential on localised zone, finally resulting the formation of crack, which grows and propagates until failure occurs.
Some typical examples of stress corrosion are
a) Stress corrosion occurs to alloys mainly brasses
b) Mild steel
c) Stainless steel.
(v)  Intergranular corrosion :
This corrosion occurs along grain boundaries. The main reasons for the corrosion of grain boundaries is
a) the grain boundaries where the metal is sensitive to corrosive attack,
b) the corrosive attacking liquid/reagent possess the character of only attacking at the grain boundaries
c) the grain boundary contain a material which shows electrode potential more anodic at the grain boundary and
d) the precipitation of certain compounds at grain boundary producing a solid solution depleted in one constituent, making the grain boundary anodic
The metal at the grain boundary decays as it becomes anodic and the centre of the grain becomes cathodic which is protected.  The electron flow from anodic to cathodic areas through the metal.  The ions formed at cathodic and anodic areas moves to the centre of these two areas where the corrosion product is formed.
The typical example of this corrosion is that the welding of stainless is done by deposition of chromium carbide at the grain boundaries which makes the grain boundaries anodic and corrosion occurs.
(vi) Erosion corrosion :
Erosion corrosion is caused by the combined effect of the abrading action of turbulent flow of gases vapours and liquids and the mechanical rubbing action of solids over a metal surface.
The reasons for the erosion corrosion to occur are
a) Breakdown of protective film on the spot of abrasion and subsequent inability to repair under existing conditions.
b) Removal of the protective film at the localised spots on the metal surface resulting in the formation of differential cells, resulting in the anodic areas in the spots.
The erosion corrosion occurs in pipings, agitators, condenser tubes and the vessels in which streams of liquids or gases emerge from an opening and strike the side walls with high velocities.
3.6  Factors Influencing Corrosion
The rate of extent of corrosion depends on the following factors :
a)   Nature of metal
b)   Nature of corroding atmosphere
(a)  Nature of the metal :
1. Position in the galvanic series :  When two metals or alloys are in electrical contact, in presence of an electrolyte, the metal with higher oxidation potential suffers corrosion.  The metal higher in the electrochemical series becomes anodic.  The extent of corrosion is determined by the difference in the position of the metals.  The greater is difference, the faster and higher is the corrosion.
2. Over voltage :  When a metal say zinc is placed in 1N  H2SO4, it undergoes corrosion forming a film and evolving hydrogen gas, the initial rate of corrosion is slow because of the high over voltage i.e. 0.70 volts of the zinc metal which reduces the effective electrode potential to a small value.  If a few drops of CuSO4 are added, some copper gets deposited on the zinc metal, forming minute cathodes where hydrogen over voltage reduces to 0.33 volts.  The reduction in over voltage of corroding metal/alloy accelerates the corrosion rate.
3. Relative areas of the anodic and cathodic parts :  When two dissimilar metals or alloys are in contact, the corrosion of the anodic part is directly proportional to the areas of the cathodic and anodic parts.
For example, corrosion is more rapid, severe and highly localised if the anodic area is small.
For example, a small pipe made of steel fitted in a large copper tank.
a) The reasons rapid corrosion due to smaller anodic areas is the current density at a smaller anodic area is much greater and
b) the larger cathodic area demands more electrons, which should be met by smaller anodic areas only by undergoing corrosion rapidly.
4. Purity of metal :  Heterogenity is produced if impurities are present in a metal, which form tiny electrochemical cells at the exposed parts.  The anodic parts get corroded.  As the extent of exposure and impurities increase, the extent of corrosion increases with the increasing exposure and impurities. For example zinc metal containing impurity such as Pb or Fe undergoes corrosion due to the formation of local electrochemical cells.
5. Nature of surface film :  In aerated atmosphere, practically all metals produce a thin surface film of metal oxide.  The ratio of the volumes of the metal oxide formed to the metal is called “specific volume ratio”.  If the specific volume ratio is more, the rate of corrosion is less, because the surface of the metal is completely covered by the film, offering protection to the metal surface.  For example, the specific volume ratios of Ni, Cr, and W are 1.6, 2.0 and 3.6 respectively. The rate of corrosion for tungsten (W) is least even at elevated temperatures.
6. Physical state of metal :  The grain size, orientation of crystals, stress etc. of the metals influence the rate of corrosion.  The smaller the grain size of the metal or alloy greater will be the rate of corrosion, because of its high solubility.  The areas under stress become anodic and corrosion takes place in these areas.
7. Passive character of metal :  Metals like Ti, Al, Cr, Mg, Ni and Co are passive and they exhibit much higher corrosion resistance than expected from their position in the electrochemical series. This is because the metal form very thin, highly protective corrosion film, by reacting with atmospheric oxygen.  If the film is broken, it compensates the film by re exposure to oxidising conditions. Thus they produce “self healing film”. This property is called passive character of metal. For example the corrosion resistance of “stainless steel” is due to passivating character of chromium present in it.
8. Volatility of corrosion products :  If the corrosion produced volatilizes as soon as it is formed, the metal surface is exposed for further attack.  This creates rapid and excessive corrosion.  For example the corrosion product of molybdenum as molybdenum oxide (MoO3) is  volatile.
9. Solubility of corrosion product :  If the oxide film formed as corrosion product is soluble in corroding medium, the corrosion proceeds at a faster rate.  The corrosion product acts as a physical barrier between the metal and environment.
For example PbSO4 film formed by Pb on sulphuric acid medium.
(b)  Nature of corroding environment :
1. Temperature :  The rate of corrosion reaction and diffusion rate increases with increase in temperature, causing the increase in rate of corrosion.
2. Humidity of air :  The humidity of air is a deciding factor for rate of corrosion. The relative humidity above which, the rate of corrosion increases sharply is called “critical humidity”. The value of critical humidity depends on the physical characteristics of the metal and the nature of corroding atmosphere.  The reasons for increase of corrosion with humidity are
i) The moisture or vapours present in atmosphere furnish water to the electrolyte, essential for setting up an electrochemical cell.
ii) The oxide film formed has the tendency to absorb moisture, which creates another other electrochemical cell corrosion.
The rain water not only supplies necessary moisture for electrochemical attack but also wash away a good part of oxide film the metal surface, leading to enhanced atmospheric attack, unless the oxide film is exceptionally adherent.
3. Presence of impurities in atmosphere : Atmosphere is contaminated in the vicinity of industrial areas with
1. Gases like CO2, SO2, H2S etc., and fumes of H2SO4, HCl etc.
2. Suspended particles of chemically active/inactive by nature like NaCl, (NH4)2SO4 and charcoal.
The gases and acidity of the atmosphere increases the electrical conductivity, thereby increases corrosion.  In marine atmosphere the presence of sodium and other chlorides lead to increased conductivity thereby increased corrosion.
The suspended particles absorb moisture and act as strong electrolytes increasing the rate of corrosion. These suspended impurities also absorb both gases and moisture and slowly enhance the rate of corrosion.
4. Nature of ions present in the medium : The presence of ions like silicates in the medium leads to the formation of “insoluble reaction products” like silica gel, on the metal surface which “inhibit” further corrosion.  On the other hand chloride ions, if present in the medium destroys the protective film on the surface of the metal, thereby exposing the metal surface for fresh corrosion attack.  Presence of traces of copper in marine waters enhances the corrosion of the iron pipes or steel body of the ships.
5. Conductance of the corroding medium : For the corrosion of underground or submerged structures the conductance of the corroding  medium plays an important role. The conductance of dry sandy soils is lower than the conductance of clayey and mineralised soils.  Hence the rate of corrosion is more in clayey and mineralised soils, causing severe damage to metallic structures buried.
6. Amount of oxygen in atmosphere :  As the percentage of oxygen in atmosphere increases, the rate of corrosion also increases due to the formation of oxygen concentration cell.  The decay of metal occurs at the anodic part and the cathodic part of the metal is protected.
anodic reaction for iron : Fe     Fe2+ + 2e–
cathodic reaction : 2 H2O + O2 + 4e–   4OH--
corrosion product : 2 Fe2+ + 4 OH--  2Fe (OH)2
 2Fe (OH)2 + H2O + 1/2 O2  2Fe(OH)3  Fe2O3.3 H2O   rust
7.  Velocity of ions which flow in the medium : As the velocity of the diffusion of ions in the medium increases, the rate of corrosion increases.
8.  pH value of the medium :  pH value of the medium has greater effect on corrosion.  When pH value is lowered, the corrosion is increased.
Thus all the factors effecting the rate of corrosion can be summarized as follows.
                                Factors influencing rate of corrosion

 Nature of Metal                                      Nature of Corroding Environment
1. Position in galvanic series 1. Temperature
2. Over voltage 2. Humidity of air
3. Relative areas of cathodic and anodic parts 3. Presence of impurities in atmosphere
4. Purity of metal 4. Nature of ions present
5. Physical state of metal 5. Conductance of corroding medium
6. Nature of surface film 6. Amount of oxygen in atmosphere
7. Passive character of metal 7. Velocity of ions which flow in the medium.
8.   Volatility of corrosion products 8. pH value of the medium.
9. Solubility of corrosion products

corrosion control methods 

As we have discussed the disadvantages and different mechanisms of corrosion so far, it is essential to know the different corrosion control methods.  The following are the important control methods of corrosion.
1. Proper designing
2. Using pure metal
3. Using metal alloys
4. Modifying the environment
5. Use of inhibitors
6. Cathodic protection
7. Application of protection coatings.
 Proper Designing
The design of the metal under corroding atmosphere must be such that corrosion, if occurs is uniform and does not produce intense and localised corrosion.  Important principles of proper designing are :
i) To avoid the contact of two dissimilar metals in the presence of corroding solution.
ii) When two dissimilar metals are in contact, the anodic metal must possess large surface area, where as cathodic metal must posses smallest surface area so the corrosion takes place is minimum.
iii) If two dissimilar metals are in contact, their position in the electrochemical series must be very close, so that minimum corrosion occurs.
iv) A direct metal to metal contact between two dissimilar metals must be avoided by fixing an insulating fitting in between them, so that the corrosion velocity can be minimised.
v) The anodic metal should not be painted or coated.
When the anodic metal is in contact with the cathodic metal, the anodic metal should not be painted or coated, because any break in the paint would lead to rapid localisation corrosion.
vi) In homogenities in metal must be prevented by proper design of the equipment.
(a) A proper design should avoid the presence of crevices, between the adjacent parts of the structure, even in the case of the same metal.  For example, electrical box (Fig. 3.16) the design shown in Fig. 3.16 (a) is such that rain water collects at the top and also seeps between the bolt and two housing and remains there because of capillary action producing anodic parts.  The defects can be corrected by slight modification in the design
    (a) Poor Design                                      (b) Best Design
(b) The design of the equipment should allow free circulation of air proper drainage and easy washing as shown in the
(c) The design should eliminate sharp corners and stresses as shown in the Fig. 3.18 (b), alongwith a modification of the poor design

(a) Poor design
(b) Best design

Use of  Pure Metals and Alloys   

The impurities in metal produces heterogenity, which decreases corrosion resistance by producing electrochemical cells.  Thus corrosion resistance of a metal is increased by increasing the purity of metal.  The pure metals provide adherent and impermeable protective metal oxide film on the surface of metals when exposed to environment.  In presence of alkaline environment both the metal oxide film and metal are attacked.  Due to the following reasons it is not possible to produce high chemically pure metal.
a) Cost of production is high.
b) Very pure metal possess inadequate mechanical properties like softness and low  strength.
c) Only electrochemical corrosion can be avoided in pure metals not direct chemical attack.
Corrosion in alloys can be avoided by alloying the metals completely in a homogeneous state. Chromium is best suitable metal for iron or steel, because it forms a uniform, resistant film which is self healing.
Thus steel containing 13 to 25% chromium are called “ferrite stainless steels” are used in turbine brackets, heat resisting parts, cutlery, surgical instruments, springs etc.

Modifying the Environment

The corrosive nature of the environment can be reduced by
1. The removal of harmful constituents
2. The addition of specific substances, which neutralise the effect of corrosive constituents of the environment.
The following are some of the methods adopted for corrosion control.
a) Deaeration : In oxygen concentration type of corrosion removal of oxygen from aqueous environment reduces metal corrosion.  The dissolved oxygen is removed by adjustments of temperature along with mechanical agitation.  This method is adopted for steel pipelines carrying steam condensates from boilers.
b) Deactivation : The corrosion control adopted by the addition of chemicals, which combine with oxygen in aqueous solution is called deactivation for example sodium sulphite (Na2SO3).
2Na2SO3 + O2  2 Na2SO4
Hydrazine hydrate is another reagent used for the removal of oxygen present in the aqueous solutions and is more advantageous than sodium sulphite, because it generates N2 and H2O as products.
N2H4 + O2  N2  + 2 H2O
c) Dehumidification :  The process of reduction of moisture content of air to such an extent that the amount of water condensed on metal is too small to cause corrosion.  For example by keeping alumina and silica gel in closed atmosphere like air conditioning rooms absorb moisture preferably on their surfaces and reduces the corrosion.
d) Alkaline neutralisation : The corrosion process in presence of acidic atmosphere can be prevented by neutralising the acidic character of the corrosive environment.  The presence of gases like H2S, HCl, CO2, SO2 etc., enhance the acidic nature of the environment, thereby enhancing the rate of corrosion.  Some examples of alkaline neutralisers are NH3, NaOH, lime, naphthlenic soaps etc., which are injected either in vapour or liquid form to the corroding system or its parts.
This method is used in controlling the corrosion of refinery equipments.

Cathodic Protection 

The method of protection given to a metal by forcibly making it to behave like a cathode is called cathodic protection. There are two types of cathodic protection.
(a)  Sacrificial anodic protection
(b)  Impressed current cathodic protection
(a) Sacrificial anodic protection : In this method of protection, the metallic structure to be protected called “base metal” is connected to more anodic metal through a wire.  The anodic metal undergoes corrosion slowly, while the base metal is protected.  The corroded sacrificial anode block is replaced by a fresh one.  The commonly used anodic metals are magnesium, zinc and aluminium and their alloys.  The important applications of sacrificial anodic protection are given below Fig. 3.19 (a) and (b).
1. To protect marine structures and ship-hulls, which are made of steel are connected to a sacrificial anode, which undergoes corrosion leaving the base metal protected.          

       (a) Unprotected metal   (b) Sacrificial anodic                   (c) Protection of ship-hull    
              being corroded             protection                           below water line by                          sacrificial anode
Fig. 3.19 Protection of metal by sacrificial anodic method
2. Protection of buried pipelines, underground cables, water tanks, piers etc.  are also protected by sacrificial anode method.  By referring to electrochemical series, a small piece of the metal, anodic base metal is attached to the metal.  The anodic metal undergoes corrosion and it is replaced from time to time
(a) Buried pipeline protected by                (b) Industrial hot water tank protected           connecting to Mg block                       by suspending Zn or Mg rod
(b) Impressed current cathodic protection :  In this method an impressed current little more than corrosion current is applied in the opposite direction to nullify the corrosion current producing a reverse cell reaction.  Thus the anodic corroding metal becomes cathodic and protected from corrosion.  The impressed current is taken from a battery or rectified on a.c. line.  The anode is usually insoluble anode like graphite, high silica iron, scrap iron, stainless steel, or platinum.  Usually a sufficient D.C. current is passed on to the insoluble anode kept in a ‘back fill’ composed of coke or gypsum, so as to increase the electrical contact with the surrounding soil (Fig. 3.21).
This type of impressed current cathodic protection is given to (1) open water
box coolers (2) water tanks (3) buried water or oil pipeline (4) condensers (5) transmission line towers (6) marine pipes (7) this type of protection is more useful for large structures for long term operations.
by impressed current cathodic protection
Disadvantages of cathodic protection :
1. The cathodic protection may be efficient in protecting a pipeline but it may increase the corrosion of the adjacent pipelines or metal structure because of stray currents.
2. Capital investment and maintenance costs are more.
3. Special care must be taken that the metal is not over protected, i.e., the use of much higher potential than the open circuit voltage for the metal/metal ion couple in case of impressed current method and the higher anodic metal in the series must be avoided. Otherwise problems related to cathodic reactions like evolution of H2 or formation and accumulation of OH-- will take place.
Inspite of these disadvantages, cathodic protection has been widely used with success, when suitable precautions are taken.

Corrosion Inhibitors 

A substance which when added in small quantities to the aqueous corrosive environment, effectively decrease the corrosion of a metal is called corrosion inhibitor.  Inhibitors are two types (a) anodic inhibitors and (b) cathodic inhibitors.
(a) Anodic inhibitors :  These inhibitors avoid the corrosion reaction occurring at the anode by forming sparingly soluble compound with the newly produced metal ion and absorbed on the surface forming a protective film on the surface of the metal, reducing the corrosion. If certain areas are left unprotected severe corrosion occurs to the metal.  Some common anodic corrosion inhibitors are chromates, phosphates, tungstates and other ions of transition metals with high oxygen content.
(b) Cathodic inhibitors :  In acidic environment, the corrosion reaction takes place with evolution of hydrogen.
2H+ + 2e–   H2 (g)
Corrosion may be reduced by (1) slowing down the diffusion of hydrated H+ ions to the cathode or (2) by increasing the over voltage of hydrogen evolution.
The diffusion of H+ is decreased by the addition of organic inhibitors like amines, mercaptans, heterocyclic nitrogen compounds, substituted ureas and thioureas which are capable of being adsorbed on the metal surface.  Antimony and arsenic oxides are used as inhibitors because they produce adherent film of metallic arsenic or antimony at the cathodic area, considerably increasing the hydrogen over voltage.  The cathodic reaction in neutral medium is
H2O (l) +  O2 + 2e–   2 OH-- (aq)
Hence the corrosion can be controlled by
1. Eliminating oxygen from the corrosion medium by the addition of Na2SO3.
2. Retarding the diffusion to the cathodic area by the inhibitors like Mg, Zn or Ni salts to the environments, which react with hydroxy ions forming the corresponding insoluble hydroxides which form impermeable self barrier between the metal and the corrosion environment.
3.8 surface coatings - methods of application on metals
The oldest and common method of protection of a metal from its surroundings is applying a protective coating on the surface of the metal.  The coated surface isolates the metal from its corroding environment. The requirements of this type of protection are
a) The coating applied must be chemically inert to the environment under particular conditions of temperature and pressure.
b) Coatings must prevent the penetration of the environment to the material, which they protect.
Functions of the protective coatings :
1. To protect the metal from corrosion.
2. To impart esthetic sense (beautiful appearance or decoration) to the metal surface.
3. To impart specific mechanical and physical properties such as wear resistance, hardness, electrical properties etc.
4. To give oxidation resistance and thermal insulating properties.
The different types of protection coatings and their methods of application are discussed below, along with the preparation of metal surface for the application of protective coating.
Methods of cleansing the metal surface before applying the surface coatings :
Before applying any protective coating, the metal surface must be thoroughly cleaned to remove dirt, old protective coatings like paints, greases, corrosion products etc.  The following are some of the methods adopted for metal cleansing.
1. Solvent cleaning :  This method is used to remove oils, greases, buffing compounds, and fatty substances.  This method involves the cleaning of metal surface by the applications of organic solvents like naphtha, chlorinated hydrocarbons like carbon tetrachloride, toluene, xylene or acetone.  Then the metal surface is cleaned with steam and hot water containing wetting reagents and alkalis.
This treatment is specially adopted for electroplating where the metal surface requires wetting.
2. Alkali cleaning :  The metal surface is cleaned with alkali cleansing agents like trisodium phosphate, along with soaps and wetting agents like caustic soda.  This method removes old paint coating from metal surfaces. Alkali treatment is followed by thorough rinsing with water and then immersion in 0.1% chromic acid, to remove the last traces of alkalis.
A more effective cleaning with alkali is made by the application electric current, by making the metal to be coated as cathode.  This removes effectively the grease or oily substances attached to the metal surface.
3. Mechanical cleaning : This method removes loose rust, scales, dirt etc. from the metal surface.  Different methods are adopted for cleaning.
a)  Cleaning of the metal surface is done by using bristle brushes and metal      brushes to remove the loosely adhering impurities.
b)  Sand paper are also used to remove strongly adhering scales and rust.
c)  Detergents like soap are adopted for cleaning the metal surface.
This method is followed by steam or hot water treatment.
4. Flame cleaning :  This method involves heating the metal surface with a hot flame to remove loosely adhering scales, followed by wire brushing.
5. Sand blasting :  The oxide scales, particularly when a slightly roughen surface is desired are removed by this method.  For sand blasting the samples of sand of different grain size were blasted on the metal surface with a pressure of 25 to 100 atmospheres.  This blast impact
a) removes the scales on the metal surface
b) also hardens the metal surface and
c) the protective coating is more durable.
Usually this method is adopted for cleaning large surface areas like large steel structures etc.
The only disadvantage of this method is that the workers are affected by the disease ‘Silicosis’.
6. Pickling and etching :  This is a more convenient method for the removal of scales, not removed by mechanical cleaning or sand blasting. In this process the metals ‘except aluminium’ are immersed in acid pickling solution.  Aluminium is pickled in alkaline solution.  The following are some of the acid pickling solutions.
1. Acid pickling of steel is carried out in warm dil. H2SO4 solution or cold HCl solution to which some ‘inhibitor’ has been added.
2. For cleaning articles made of copper, brass and nickel the pickling bath consists of dil. HNO3 or a mixture of dil. HNO3 and dil. H2SO4.
This method provides clean, smooth surface for applying protective coating.
After pickling process the surface must be thoroughly cleaned with water and steam to remove the acid layer adhering to the metal surface, otherwise the metal surface undergoes dry corrosion.

Different methods adopted for cleaning the metal surface

Metallic coatings :  Metallic coatings are produced by coating one metal on the surface of another metal.  The metal which is protected is called base metal and the metal which is coated on the surface of the base metal is called coating metal.  Broadly we can classify the metallic coatings into two categories.
a) Anodic coatings
b) Cathodic coatings
a) Anodic coatings :  This type of coatings are produced by coating an anodic coating metal on the surface of base metal.  For example coating of Zn, Al and Cd on iron are anodic, because their reduction potentials are lower than that of base metal iron.
If any pores, breaks, or discontinuation in metallic coating, exposing the base metal, a galvanic cell is formed between coating metal and base metal, where the coating metal dissolves anodically and the base metal is protected.  Hence no corrosion occurs to the base metal until all the coating method corrodes and disappear from vicinity of the exposed base metal spot.
b) Cathodic coatings : Cathodic coatings are obtained by coating a more noble metal (having higher reduction potential) than the base metal.  The coating metal has higher reduction potential than the base metal and protect the base metal due to their higher corrosion resistance than the base metal.  For example coating of tin on the surface of iron.  Cathodic coatings provide effective protection to the base metal only when the coating is (a) completely continuous and (b) free from pores, breaks, and discontinuities.
When a break pore or discontinuity occurs in the cathodic coating more corrosion damage is done to the base metal.  When the base metal which is anodic to the coating metal is exposed to environment, a galvanic cell is set up and an intense localized attack at the small exposed part occurs resulting severe pitting and perforation in the base metal because of a small anode and large cathode area
        Anodic coatings           Cathodic coatings
1. These coatings involve coating of an These coatings involve coating of a anodic metal on the surface of noble metal on the surface of base metal. base metal.
2. Protects the underlying base Protects the base metal by their noble metals “sacrificially” character and higher corrosion resistance.
3. The reduction potential of the coating The reduction potential of the coating metal is lower than that of base metal. metal is higher than that of the base metal.
4. If pores, breaks or discontinuities are If pores, breaks, and discontinuities produced in the metallic coating, the are produced in the metallic coating, base metal is not corroded because it is the base metal becomes anodic and cathodic to coating metal.  The and faces severe corrosion due to coating metal undergoes severe smaller anodic area and the protecting corrosion and consumed. metal becomes cathodic and do not undergo corrosion.
5. The coating metal possess lower The coating metal possess higher reduction potential than base metal. reduction potential than base metal.
6. Example :  Galvanisation. Example : Tinning.
3.9 Methods of application of metal coatings
The following are the methods of application of the metallic coatings.
3.9.1  Hot Dipping
This is a method of coating a low melting metal such as Zn (m.p = 419 0C), Sn (m.p = 232 0C) Pb, Al, etc., on iron, steel and copper which have relatively higher melting points.The base metal is dipped in a molten bath of the coating metal, which is covered by a molten flux layer (usually ZnCl2 if Zn is used as a coating metal) which cleans the base metal surface and prevents the oxidation of the coating metal.  For good adhesion of the coating metal on the surface of base metal, the base metal surface must be very clean.
The most widely used hot dipping methods are (a) Galvanisation  (b) Tinning.
Galvanisation
It is a process of coating iron or steel sheets with a thin coat of zinc to prevent iron from rusting.
The base metal iron or steel sheet is cleaned by acid pickling method with
dil. H2SO4 for 15 - 20 minutes at 60 - 90 0C. The sheet is then washed well and dried.  It is dipped in a bath of molten zinc maintained at 425 - 430 0C. The surface of the bath is kept covered with ammonium chloride flux to prevent oxide formation. The sheet is taken out and excess Zn is removed by passing in between a pair of hot rollers. Then the sheet is subjected to annealing process at 650 0C and cooled slowly (Fig. 3.24).  An alloy of iron and zinc were formed at the junction of the base metal and coating metal.

Applications of galvanisation method 

It is mostly used to protect iron used for roofing sheets, wires, pipes, nails, bolts, screws, buckets and tubes.
Galvanised utensils cannot be used for preparing and storing food stuffs especially acidic in nature, because zinc dissolves to form highly toxic or poisonous compounds.
3.9.1.2 Tinning
Coating tin over iron or steel articles is called tinning.  In this process the surface of the base metal i.e., iron sheet is cleaned by acid pickling with dilute H2SO4 and passed through a bath of zinc chloride flux.  The flux help the molten metal to adhere to the metal surface.  Then the sheet is passed through molten tin bath and pressed between two rollers from a layer of palm oil.  Palm oil helps to protect the tin coated surface against oxidation.  The rollers remove excess of tin and produce a thin film of uniform concentration, (Fig. 3.25).  An alloy of the base metal and coating metal at their junction is produced.
The sheet passes through a layer of a flux  into the molten tin and then emerges between pairs   of rollers from a layer of palm oil
Application of tinning :
1. Tin metal possess good resistance against atmospheric corrosion.
Tin is non toxic and widely used for coating steel, copper, and brass sheet.
2. The containers coated with tin are used for storing food stuffs, ghee, oils etc. and packing food materials.
3. Tinned copper sheets are used for making cooking utensils and refrigeration equipment.
Table - 3.5 :  Comparison of galvanisation with tinning
                Galvanisation         Tinning
1. The process of coating Zn on the surface The process of coating tin (Sn) on the of iron or steel is called galvanisation. surface of iron or steel is called tinning.
2. Zinc is anodic to the base metal iron or Tin is cathodic to the base metal iron steel. or steel.
3. Zinc protects iron ‘sacrificially’. Tin protects iron due to its ‘noble  nature’.
4. Since zinc is more electropositive Since tin is less electropositive than than iron, it does not permit iron to iron, it protects the iron till the coating pass into the outside solution in is perfect. other words corrosion does not occur to iron.
        Galvanisation         Tinning
5. Any break in the protective zinc Any break in the protective layer of tin, layer “does not cause corrosion” cause “severe corrosion” to base metal to base metal iron, because iron is iron because iron is anodic to tin. cathodic to zinc.
6. Galvanised containers cannot be Tin coated containers are used for used for storing food stuffs, because storing food materials because tin zinc reacts with the acids that produces non toxic protective layer are present in the food materials on the surface of metal and avoids and produces toxic compounds. any food poisoning.
7. After galvanisation, the galvanised No annealing process. sheet is subjected to annealing
process.
8. Galvanised articles are good Tinned articles are used majorly in engineering materials. food processing industries.

 Metal Cladding   

In this process a dense, homogeneous layer of coating metal is bonded (cladded) firmly and permanently to the base metal on one or both sides.  The metal cladded is  called cladding metal.  
The choice of cladding metals depends on the corrosion resistance required for any particular environment.
The corrosion resistant metals like Nickel, Copper, Lead, Silver, Platinum etc., and alloys like stainless steel, nickel alloys, copper alloys, lead alloys are used as cladding materials.  The common base metals cladded are mild steel, aluminium, copper, nickel and their alloys. A 99.5% pure aluminium is subjected to cladding to protect duraluminium to produce ‘alclad’, which is widely used in air craft industry.
The process of metal cladding is a kind of “metal sandwiching” where the base metal is arranged with protecting metal on either side like sandwich and passed through rollers under the action of heat and pressure (Fig. 3.26).  In this method the cladding metal is bonded/attached firmly to the base metal.
Applications :  This method is widely adopted in aircraft industry and automobile industry.

 Electroplating or Electrodeposition  

The process of depositing the coating metal on the surface of base metal/non metal by electrolysis is called electroplating.
The base metals are inferior or anodic to the coating metal (or alloy) and electrolyte is the solution containing the metal ions to be deposited on the base metal.  The base metal is made cathode and the coating metal is made anode or an inert material of good electrical conductivity is also used as an anode.  A direct current (d.c.) is passed through the electrolytic  solution.  If anode is made of graphite, the electrolytic salt is added continuously to maintain the proper coating metal ion concentration in the electrolyte bath.
The article to be electroplated is subjected to solvent cleaning by trichlorethylene to remove oil, greases etc. Then it is subjected to acid pickling with dil.HCl or
dil. H2SO4 to remove any scales, oxides etc. The cleaned article is made cathode of the electrolytic cell and the anode can be made of the pure coating metal or graphite.  The electrodes were dipped in the salt solution of the coating metal, which acts as an electrolyte.  When direct current is passed, the coating metal ions migrate to the cathode and deposit on the base metal article in the form of a thin layer.  For brighter and smooth deposits, conditions like low temperatures, medium current density, low metal ion concentration are used
 For example for electroplating of copper on iron article, the following are maintained
1. Electrolytic bath solution : Copper sulphate
2. Temperature maintained : 40 - 70 0C (Low temperature for brighter and smooth surface)
3. Current density : 20 - 30 mA/cm2 (Medium current density for high efficiency)
4. Anode : Copper pellets (99% pure) taken in a titanium mesh basket or Graphite.
5. Cathode : Base metal article.
Applications :
Electroplating is a most important and frequently used technique in industries to produce metallic coatings.  Both metals and non metals can be electroplated.  In metals the electroplating increase resistance to corrosion, chemical attack, hardness, wear resistance and surface properties.  In non metals electroplating increases strength and decorates the surface of non metals like plastics, wood, glass etc.
3.9.4  Electroless  Plating
The method of deposition of a metal from its salt solution on a catalytically active surface by a suitable reducing agent without using electrical energy is called electroless plating. This process is also called chemical plating or autocatalytic plating.
The metallic ions (M+) are reduced to the metal with the help of reducing agents (R-1). When the metal (M) is formed, it gets plated over a catalytic surface.

The metal surface to be subjected to electroless plating is prepared by a) Etching (acid treatment) b) For non conducting surfaces like plastics or printed circuit boards, treatment with stannons chloride and palladium chloride alternatively is used. c) To get an active surface electroplating followed by heat treatment is adopted. d) For Ni surface treatment of organic solvents followed by acid treatment is used.
The following are the requirements of electroless plating :
a) Soluble electroactive metal in the form of metal chloride or sulphate.
b) The reducing agents like formaldehyde, hypophospites.
c) Complexing agents like citrate, tartrate and succinate.
d) Exaltants like succinates, glycinates and flourides to improve the rate of plating.
e) Stabilizers which prevent decomposition of bath. Eg : Pb++, Ca++, Th++ ions and Thio urea.
f) To control pH of the bath buffer solution was added.
Electroless plating of Nickel :
The base object, a plastic material like acrylonitrile-butadiene styrene (ABS) can be coated with Nickel.
The base metal surface is activated by stannous chloride followed by palladium chloride to get a layer of Pd and the surface is dried and dipped in a solution of nickel chloride, sodium hyposulphite (reducing agent), sodium acetate buffer, sodium succinate (complexing agent and exalt) pH of the bath maintained is 4.5 and temperature maintained is 93°C. The following reactions take place and Ni get plated electrolessly on the surface of the base object.

Properties :
Electroless plated Ni object has better corrosion resistance, deposits are pore free, hard and wear resistant.
Applications :
1) Electroless Ni coating are used extensively in electronic application providing a non-magnetic underlay in magnetic components.
2) ABS plastics coated objects are used for decorative applications automotive, electronic, and domestic of industry like knobs in hi-fi equipment, tops on perfume bottles costume jewellery and car decoratives. They are also used in digital and electronic instruments for electromagnetic interference shielding.
Electroless plating of Cu :
The insulators like plastics, printed circuit boards(PCB) and glass are used as base objected and treated by stannous chloride and palladium chloride and electro plated-through-hole technique. The bath solution is made of copper sulphate, farmaldehyde (reducing agent), sodium hydroxide and rochelle salt (buffer), EDTA (complexing and exaltant).
The pH of the bath solution is ll and temperature maintained is 25 °C. The overall reaction is

Applications :
Cu electroless plating is used for making printed circuit boards.

 Metal Spraying 

In this process, the coating metal, taken in molten state is sprayed on the roughened surface of the base metal.  The metal sprayed adheres to the base metal surface.  The sprayed coatings are not continuous but somewhat porous and a sealer oil is applied on such coatings to provide smooth surface.  The adhesion strength of this metal spraying is less when compared with hot dipping or electroplating.  The metal surface is generally roughened by sand blasting process.  There are two techniques followed to apply metallic coating by spraying.
a) Wire gun method is more widely used for common metals.  The coating metal wire is melted by oxyacetylene flame and atomized by a blast of compressed air.
b) Power metal method is limited to low melting metals like Zn, Pb, Sn etc. Finely                      divided powdered metal is sucked from the powder chamber by an aspirator and heated as it passes through the flame of the blow pipe.  The blow pipe disintegrates the metal into a cloud of molten globules which are adsorbed on the base metal surface.
Applications of metal spraying :
a) This method is used to coat fabricated structures and the assembly of the parts.
b) The coating is used to apply large surface areas of irregularly shaped articles.
c) Non metallic articles made of glass, wood, plastic etc. are also coated by metal spraying.

Cementation or Diffusion Coatings

  This type of coatings are obtained by heating the base metal in a revolving drum containing powdered coating metal.  Diffusion of the coating metal into the base metal takes place, resulting the formation of layers of alloy of varying composition.  The layer adjucent to base metal is solid solution and the outer layers are richer in coating metal.
Sherardising (developed by Sherad Cowpercoles in 1890) is the process of cementation, using zinc powder as coating material on the surface of iron. The iron article to be coated is cleaned by acid pickling process and rotated for 2-3 hours in a drum containing zinc dust at 350 - 370 0C. During this process Zn diffuses into iron forming Fe - Zn alloy at the junction of the base metal and coating metal and a thin layer of zinc is deposited on the surface of iron.  Sherardising is used for coating small steel articles like bolts, screws, nuts, threaded parts, washers, valves and gauge tools.  The advantage of sherardising is that the metal coating is uniform and there is no change in the dimension of the article.
Colorizing : Is a method of coating aluminium on the surface of iron by cementation.  The article is cleaned by sandblasting and heated in a drum tightly packed with a mixture of aluminium powder and aluminium oxide together with traces of aluminium chloride as flux in a reducing atmosphere of hydrogen.  The layer formed has the approximate composition of Al2Fe3containing about 25% by weight of aluminium.  Colorizing is used for the protection of furnace parts.
Chromizing :  The base metal heated with a mixture of 55% Cr, and 45% alumina powder to 1300 - 1400 0C for 3 - 4 hours.  A mixture of volatile chromous chloride and hydrogen with steel parts produce the protective layer of chromium diffused into iron surface.  Chromizing is fairly and extensively used for the protection of turbine blades.
Siliconising : Coating silicon on the surface of molybdenum by cementation is called siliconising. Silicon tetrochloride is used as a coating metal.
3.10 organic surface coatings - paints
Organic coatings are inert - barriers, applied as metallic surfaces and other constructional materials for both corrosion protection and decoration.  The following are the factors which determine the protective value of the organic surface coatings.
a) Chemical inhertness to the corrosive environment
b)   Good surface adhesion
c)   Impermeability to water, salts and gases
d)   Proper application method
The most important organic surface coating is paint.  The following are the constituents of paints and their functions.
Paint is a mechanical dispersion mixture of one or more pigments in a vehicle or drying oil.
Constituents of paints :
(a)   Pigment :
Pigment is a solid substituent and an essential constituent of paint.  The functions of pigment are
a) Constitute the body of the paint
b) Give strength to paint
c) Provide desired colour to paint
d) Imparts esthetic appeal (pleasant appearance) to paint
e) Gives protection to paint by reflecting harmful u.v. light
f) Improves the impermeability of paint film to moisture
g) Increases weather resistance of the film.
The following are the common pigments used
Common pigment Colour imparted by the pigment
1. White lead, zinc oxide, lithophone etc. - white
2. Red lead, ferric oxide, chrome red etc. - red
3. Chromium oxide - green
4. Prussian blue - blue
5. Carbon black - black
6. Umbre brown - brown
Characteristics of a good pigment :
A good pigment must be opaque, chemically inert, non toxic, freely mixable and cheap.
b)  Vehicle or drying oil :
The film forming constituent of a paint is drying oil.  The functions of drying oils are
1. The main film forming function is due to the drying of the film. Hence plays a key role in drying of the paint.
2. It acts as a vehicle or medium for the dispersion of the various constituents in it.
3. It provides toughness and adhesion to the paint.
4. It gives durability and water proofness to the paint.
The drying oil contain a number of double bonds in their structure.  The film formation and drying of the film is the major function, which takes place in the following way.  When the paint is applied in the form of a film, the drying oil absorbs oxygen from air, forming peroxides, diperoxides and hydroperoxides at the double bonds.  These peroxides isomerise, polymerise, and condense to form a characteristics tough, coherent, hard elastic, insoluble, infusable, highly cross linked structure polymer film.  Hence as the number of double bonds increase, the protective film forming capacity of the polymer increases.  Drying oils are the esters of higher fatty acids.
Example of drying oils are
Based on the nature of unsaturation the drying oils are classified into three categories.
1. Drying oil which contain high percentage of conjugated fatty acid esters, help in quick drying of the paint.  For example, linseed oil, tung oil, dehydrated castor oil, perilla oil etc.
2. Semi drying oils contain low percentage of conjugated fatty acid esters containing one double bond only, help in slow drying of the paint and used as blending agent with drying oils.  For example, soyabean oil, rosin oil, fish oil etc.
3. Non drying oils contain only saturated fatty acid esters.  They do not dry at all, even on long exposure to air.  For example mustard oil, sunflower oil and rape seed oil.
(c) Thinners :
The functions of thinners are
1. reduce viscosity of the paint
2. helps in dissolving vehicle and the additives
3. Suspend pigments
4. Increase penetration power of the vehicle
5. Increase the elasticity of the paint film
6. Helps in drying of the paint, as it evaporates easily.
Examples :  Common thinners used are turpentine, mineral spirit, benzene, dipentene, naphtha, xylol, kerosene etc.
(d) Driers :
The functions of driers are
1. Driers are oxygen carrier catalysts.
2. Accelerate the drying of the paint film through oxidation, polymerisation and condensation.
3. The main function of the driers is to improve the drying quality of the oil film.
Examples :  The most effective driers are resinates, linoleates, tungstates and naphthates of Co, Mn, Pb and Zn.
(e) Extenders or fillers :
Extenders are low refractive indices materials, generally of white colour the functions of extenders are
1. reduce the cost
2. increase durability of the paint
3. Help to reduce the cracking of dry paint film
4. Serve to fill voids
5. Act as carrier for pigment colour.
Examples :  Barytes (BaSO4), talc, asbestos, ground silica, gypsum, china clay etc.
(f)  Plasticizers :
The functions of plasticizers are
1. To provide elasticity to the film
2. To minimise its cracking.
Examples :  Tricresyl phosphate, triphenyl phosphate, tributyl phthalate.
(g) Anti skinning agents :
The main function of the anti skinning agent is prevent gelling and skinning of the paint film.
Examples :  Polyhydroxy phenols.

EXERCISE

I.  Short Answer Questions :

1. What is meant by rusting of iron ?
2. What is dry corrosion ?
3. What is Pilling - Bedworth rule ?
4. What is electrochemical corrosion ?
5. The rate of metallic corrosion increases with increasing temperature. Why ?
6. Why a wire mesh corrodes faster at the joints ?
7. Give reasons why corrosion of water filled steel tanks occurs below the water level.
8. Name any four metals whose specific volume of oxides is greater than metals.
9. Why rusting of iron is quick in saline water than in ordinary water ?
10. What is mean by ‘critical humidity’ of corrosion environment ?
11. Explain the affect of grain size of the metal on the corrosion ?
12. Why small anodic area results in intense corrosion ?
13. What is impressed current cathodic protection ?
14. Give any three factors which enhance the process of corrosion.
15. Why does corrosion occur to steel pipe connected to copper plumbing ?
16. Why bolt and nut made of same metal is preferred in practice ?
17. Why does addition of amines protect iron again corrosion ?
18. What is meant by sacrificial anode ?
19. Which gases in the atmosphere are likely to accelerate rusting of iron ?
20. What are the units of corrosion ?
21. A pure metal half immersed vertically in water starts corroding at the bottom. Give reasons.
22. What is galvonic series ?
23. Why impure metal corrodes faster than pure metal under identical condition ?
24. What is differential aeration corrosion ?
25. What is meant by the term passivity ?
26. Why is chromium used for coating of iron ?
27. What is sherardizing ?
28. What is cementation ?
29. Why brass stensils are usually tinned ?
30. Name any three corrosion resistant metals used for metal cladding.
31. Why are drying oils used in paints ?
32. Give any three functions of pigments in paints .
33. Why Iron faster than Aluminium?
34. What is the chemical composition of rust?
35. Why does part of a nail inside the wood undergoes corrosion easily?

iI.  Essay  Answer questions :

1. Describe briefly
a) pitting corrosion   b) stress corrosion
c) inter grennueal corrosion   d) dry corrosion
2. How do the following factors influence the rate of corrosion?
a) position in galvanic series b) nature of surface film
c) humidity in air d) amount of oxygen in atmosphere
3. a) Define metallic corrosion ? Explain the mechanism of electrochemical corrosion by hydrogen evolution and oxygen absorption.           [JNTU MID]
b) Describe briefly cathodic protection by sacrificial anode ?
4. a) State and explain Pilling-Bedworth rule.
b) Explain differential aeration corrosion by illustrating with one example.
5. Give reasons for the following statements.

a) rusting of iron is faster in saline water
b) presence of  impurities in metal enhance the rate of corrosion
c) a copper equipment should not possess a small steel bolt
d) cathode coating if punctured causes accelerated corrosion of the base metal.
6. What is paint ? How do paints differ from varnish ? Explain the different constituents of paints and their functions with suitable examples.
7. Distinguish the following with suitable examples.
a) cathodic and anodic  coatings     b) galvanic series and electro chemical series
c) galvanizing and sherardizing     d) corrosion and erosion
8. What are metallic coatings ? Describe the electroplating method of coating and the factors which influence the thickness of the coating.
9. Mention the different methods adopted for cleaning the metal surface before applying protective coating.
10. a) What is pickling ? Write the composition of pickling bath used for Zn and Fe.
b) What is meant by metal cladding ? Explain the process and its significance.
11. a) Describe briefly important parameters involved in electroplating.
b) Explain the mechanism of tinning and its application.
12. a) How are metals protected by impressed current method ?
b) Give a brief account on hot dipping.
c) Explain the cementation process for protection of metals.
13. Write short notes on :
a) anodizing b) electroplating
c) metal cladding d) tinning
14. Differentiate the following with suitable examples
a) galvanizing and tinning b) anodic coating and cathodic  coating
c) drying oil and semi drying oil d) sherardizing and colorizing
15. Give an account of the composition functions of the following constituents of points.
a) pigment b) plasticizers
c) drying oil d) fillers
16. Explain following with suitable examples.
a) passive character   of metal   b) electrodeposition
c) intergranular corrosion   d) galvanic series.
17. a) Give the reasons for failure of paint.
b) Explain the methods of cleansing articles before electrode position.
c) Write a brief account on metallised coatings.
d) Explain the industrial applications of anodizing.
18. a) Give the requirements of a good point.
b) Explain critical humidity and self leading film of metals.
c) Write brief account on pigments.
19. Explain the principle involved in
a) anodic protection b) cathodic protection
c) galvanization   d) cementation
20. Give a brief account on the following
a) sacrificial anode
b) impressed current  
c) corrosion inhibitors
d) prevention of corrosion by modifying the environment
21. a) Describe briefly
i) pitting corrosion ii) stress corrosion
b) How do the following factors influence the rate of corrosion?
i) over voltage ii) presence of impurities in atmosphere
22. Explain the following methods of corrosion control
a) cathodic protection b) organic coatings
23. a) State and explain Pilling-Bedworth rule.
b) Why corrosion occurs in steel pipe connected to  copper plumbing.
c) Write short notes on differential aeration corrosion.          [GITAM - 2014]
24. a) What is pitting corrosion? Explain its mechanism with suitable examples.
b) Describe the mechanism of electrochemical corrosion. Discuss the rate of oxygen in corrosion cells?
25. a) What is corrosion? What are the units to express corrosion and explain the causes  of corrosion? [JNTU MID EXAM]
b) What are the affects of corrosion in metals?
26. What happens and why it happens:
a) iron sheets riveted with copper bolts
b) an iron pole is partly burried underground
c) a Zn article under stress
d) Zn plate fixed below the ship
27. Explain the following statements with suitable examples?
a) carry over in boilers cause corrosion to turbine blades
b) the presence of sodium carbonate in boiler feed water causes caustic embrittlement to  boiler plate
c) a pipeline buried under water logged soils undergoes corrosion
d) fabricated articles by alloys like high Zn brasses and Ni brasses undergo stress corrosion.
28. a) What are corrosion inhibitors? Classify different types of inhibitors with examples.
b) Explain how faulty design of the equipment produces corrosion.
29. Explain the following with suitable examples
a) the role of modifying environment in prevention of corrosion
b) use of inhibitors in prevention of corrosion
c) proper designing of equipment in prevention of corrosion
30. a) How does the nature of metal influence rate of corrosion?
b) What is the affect of nature of corroding environment on rate of corrosion ?
31. a) Explain the electrochemical theory of corrosion of metals with special reference  to rusting of iron .                                          [Jntu-2010]
  b) Write a note on galvanizing and metal cladding.
32. a) Explain the process of galvanizing and tinning.     [Jntu-2010]
b) What are organic paints ? Describe their constituents.
33. Discuss the electrochemical theory of corrosion. [JNTU 1994]
34. What is a paint? What are the different constituents of paint and explain their functions.      [JNTU 1992, BIT Mecra 1993]
35. a) Write a note on sheradizing and cladding.
b) What is meant by ‘drying of oil’? What are its characteristics? [Anna 1996]
36. Write a brief account on cathodic protection. [JNTU 1993]
37. How are metals protected by impressed current method? [JNTU 2012]
38. Discuss the differential aeration and pitting corrosion.                  [JNTUK 2014]
39. Give in detail the various factors influencing the rate of corrosion.                        [GITAM 2014]

III. Multiple Choice Questions :

1. The rusting of iron is catalyzed by one of the following
a) Fe b) O2 c) Zn d) H+
2. Corrosion is an example of
a) oxidation b) reduction c) electrolysis d) erosion
3. For the corrosion of iron one of the following factor is essential
a) presence of moisture b) presence of both moisture and oxygen
c) presence of hydrogen d) presence of strong acid
4. The buried pipeline is protected from corrosion by connecting to Mg block. It is called
a) impressed voltage protection b) sacrificial cathodic protection
c) sacrificial anodic protection d) any of these
5. During wet corrosion
a) the anodic part undergoes oxidation  
b) the cathodic part undergoes oxidation
c) the anodic part undergoes reduction
d) neither cathodic nor anodic part undergoes any change
6. The metal at the top of electrochemical series is
a) most stable b) best protective
c) most noble d) most active
7. Electrochemical corrosion can occur only if
a) the liquid medium in contact with metal
b) O2  in contact with metal
c) air in contact with metal
d) CO2  in contact with metal
8. During the electrochemical corrosion in acidic environment
a) O2  evolution occurs b) O2 absorption occurs
c) H2 evolution occurs d) H2 absorption occurs
9. During galvonic corrosion the more noble metal acts as
a) anode b) cathode
c) anode as well as cathode d) corroding metal
10. The process of cementation with Zn powder is known as
a) galvanizing b) zincing
c) sherardizing d) tinning
11. The rate of corrosion of iron in atmosphere depends upon
a) humidity of atmosphere b) degree of pollution in atmosphere
c) frequency of rainfall d) all the above factors
12. In water line corrosion the maximum amount of corrosion takes place
a) along the line just above the level of water mensicus
b) along a line at the level of the water meniscus
c) along a line just below the level of water meniscus
d) at the bottom of the vessel
13. Addition of hydrazine - hydrate to corrosive environment
a) retards anodic reaction
b) prevents diffusion of protons to cathode
c) retards cathodic reaction by consuming dissolved O2
d) increases hydrogen over voltage
14. Anodic coating protects the underlined  metal
a) due to its noble character b) sacrificially
c) due to its higher electrode potential d) none of the above
15. The film forming constituent of paint is
a) pigment b) plasticizer
c) thinner d) drying oil
16. The function of ammonium chloride used as flux in galvanization is to
a) prevent oxide formation
b) prevent deposition of impurities
c) reduce the contact of base metal and coating metal
d) none of the above
17. The process of coating steel with tin to prevent it from corrosion is called
a) galvanizing b) tinning c) metal cladding d) electro plating
18. Sand blasting is used for removing the following from the metal surface
a) oxide scale b) oils c) greases d) old paint
19. Acid pickling of steel is carried out by dipping in
a) dilute HCl b) warm dilute HCl c) warm dilute H2SO4 d) dilute H2SO4
20. The following reagents are used for solvent cleaning of the metal surfaces
a) naptha b) acid c) alkali d) Na2CO3
21. Electroplating is a process of depositing a thin layer of a
a) superior metal over inferior base metal
b) inferior metal over superior base metal
c) superior metal over superior base metal
d) inferior metal over inferior base metal
22. Anodic coating protects underlined metal
a) due to noble character b) sacrificially
c) due to its lower oxidation potential d) due to its higher reduction potential
23. Cathodic coatings if punctured
a) have no affect on the base metal
b) causes less corrosion of the base metal
c) causes accelerated corrosion of the base metal
d) cathodic coating corrodes first followed by the corrosion of base metal
24. The oxygen carriers of the paint is called
a) drier b) pigment c) thinner d) drying oil
25. Opacity and desired colour to paint is provided by
a) pigments b) extenders c) driers d) thinners
26. The following metal is used for the cladding of aluminium
a) 99.5% pure Al b) 100% pure Al c) 98.5% pure Al d) 99% pure Al
27. The metal at the top of the electrochemical series is
a) most stable b) most noble c) least active d) more active
28. During corrosion of iron in aqueous solution
a) corrosion occurs at cathode
b) corrosion product is deposited at anode
c) corrosion occurs at anode
d) corrosion occurs at cathode with deposition of rust at cathode
29. The deciding factor in atmospheric corrosion is
a) presence of oxygen in air b) presence of gases like SO2
c) humidity of air d) frequency of rainfall
30. In electrochemical corrosion
a) anode undergoes oxidation b) cathode undergoes oxidation
c) anode undergoes reduction d) both cathode and anode undergo oxidation
31. An inhibitor which when added in small quantities to aqueous corrosive environment
a) effectively decreases the corrosion of the metal
b) increases the corrosion of a metal
c) no effect on corrosion of metal
d) increases the corrosive nature of the environment
32. Which of the following metals give cathodic protection to iron?
a) Zn b) Cu c) Ni d) Ag
33. One of the following gas in the atmosphere do not accelerate the rusting of iron.
a) CO2 b) SO2 c) NO2 d) N2
34. One of the following factors enhance the rate of corrosion
a) Dry air b) Presence of gas like CO2 c) Pure metal d) Passive character of a metal
35. Corrosion is a process of
a) reduction b) electrolysis c) reverse of metal extraction d) neutralisation

IV.  FILL IN THE BLANKS :

  1. An example of anodic corrosion inhibitor is __________.
  2. When the ratio of anodic to cathodic area decreases the rate of corrosion __________.
  3. The chemical composition of the corrosion product of iron (rust) is __________.
  4. In acidic environment lower the value of hydrogen over voltage __________  is the rate of corrosion.
  5. In galvonic corrosion the metal having relatively ____ EO  value will undergo corrosion.
  6. Formation of __________  type of metal oxide film causes rapid and continous corrosion.
  7. Pickling method is used for the removal of __________  deposits on the metal surface.
  8. Galvanization means coating of __________  on iron and steel objects.
  9. In Copper plating the electrolytic solution contains __________  as electrolyte
10. __________ sheeting consists of a plate of duraluminium sandwiched between 2 layers of aluminium of 99.5% pure.
11. An example of cathodic coating is __________.
12. Corrosion is a gradual decay of metal by  the attack of __________.
13. Soil corrosion is purely __________  in character.
14. The phenomenon of a metal or an alloy exhibiting a much higher corrosion resistance than expected is called as __________.
15. The corrosion that results in the formation of pin holes, pits and cavities in the metal is __________.
16. The type of corrosion which occurs along grain boundaries is called __________.
17. The rate of corrosion increases with __________  in pH.
18. Impurities in metals causes __________.
19. The mechanical dispersion of mixture of one or more pigments in a vehicle is called __________.
20. __________  oils are used as vehicles in paints.
21. The oxygen carrier catalyst in paints are called __________.
22. __________  coatings are produced from coating metals which are anodic to the base metal.
23. Cathoding coatings are obtained by coating a __________  metal than the base metal.
24. The process by which coating metal is deposited on the base metal by passing a direct current through an electrolyte solution containing soluble salt of the metal is _________.
25. __________  is used to remove oils, greases, buffing compounds and fatty substances from the base metal surfaces.
26. Sand blasting is used for removing __________  scales.
27. __________  method is more widely used for common metal spraying.
28. During colorising the composition of the protective layer formed is __________.
29. __________  is produced by interaction of a mixture of volatile chromous chloride and hydrogen with steel parts at 1050 0C.
30. __________ is used to prevent decomposition of both during electroless plating.
31. When the ratio of anodic to cathodic area decreases, the rate of corrosion __________
32. In acidic environment, lower the value of hydrogen over voltage __________  is the rate of corrosion.
33. In galvanic corrosion the metal possessing relatively __________ Eo value will undergo corrosion.
34. The process of cementation of Fe with Zn powder is called __________.
35. Addition of hydrazine hydrate to corrosive environment retards cathodic reaction by consuming __________.
36. The corrosion of metals is expressed in the units of _________.

V.  Indicate TRUE OR FALSE for the following :

1. In water  line corrosion, maximum amount of corrosion takes place along the
line just above the level of the water meniscus. [ T / F ]
2. To offer sacrificial protection the protective metal must be readily oxidised than
base metal. [ T / F ]
3. Rusting of iron is quicker in saline water than oridinary water. [ T / F ]
4. Impure zinc corrodes faster than pure zinc when placed in salt solution.            [ T / F ]
5. A copper equipment containing a small steel bolt does not undergo corrosion. [ T / F ]
  6. Fe2O3.3 H2O is the chemical formula of rust. [ T / F ]
  7. The part of nail inside the wood undergoes corrosion easily. [ T / F ]
  8. The decay of metal by environmental attack is called corrosion. [ T / F ]
  9. Direct chemical action of atmospheric gases on metal surfaces is called dry corrosion. [ T / F ]
10. If the volume of oxide film is more than the volume of metal no corrosion
occurs. [ T / F ]
11. The metal higher in the electrochemical series does not undergo corrosion.      [ T / F ]
12. Smaller the grain size of the metal or alloy, greater is its corrosion.         [ T / F ]
13. Corrosion speed up in the presence of CO2. [ T / F ]
14. Electrochemical corrosion always takes place at cathodic area. [ T / F ]
15. The lower the pH, lower is the corrosion. [ T / F ]
16. Brass utensils are usually coated with tin. [ T / F ]
17. The ability of the plating system to produce even deposit on a irregular object is called throwing power of the bath.          [ T / F ]
18. Colourising is used for the protection of furnace parts          [ T / F ]
19. The process of coating steel or iron sheets with Zn is called galvanizing.        [ T / F ]
20. A process by which a dense homogeneous layer of a coating metal is bonded firmly and permanently to the base metal on both sides is
called metal cladding. [ T / F ]
21. Pigment provides desired colour and protection to paint film. [ T / F ]
22. Drying oils contain high percentage of saturated fatty acid esters. [ T / F ]
23. Linoleic and linolenic acids do not contain conjugated double bonds.       [ T / F ]
24. Chromium is not used for coating iron by galvanisation. [ T / F ]
25. Plasticizers provide elasticity to the paint film. [ T / F ]
26. The chemical effect of the atmospheric gases like H2S, SO2  on paint film,  brings a  change in the colour of the paint film.         [ T / F ]
27. Anodizing is best suited for aluminium. [ T / F ]
28. Acid pickling is a method not applicable for the removal of scale from the  surface of metal.         [ T / F ]
29. A paint contains a pigment and drying oil only. [ T / F ]
30. White lead in paints acts as a thinner. [ T / F ]

ANsWERS

iii.  multiple choice Questions :
1) d 2) a 3) b 4) c 5) a 6) d 7) a 8) c 9) b 10) c
11) d 12) c 13) c 14) b 15) d 16) a 17) b 18) a 19) b 20) a
21) b 22) b 23) c 24) a 25) a 26) a 27) d 28) c 29) a 30) a  
31) a 32) a 33) d 34) b 35) c 36) b
IV. FILL IN THE BLANKS :
1)  chromate / phosphate  2) increases 3) Fe2O3. 3H2O
4)  higher  5) lower 6) volatile and porous
7)  scale / rust  8) zinc 9) CuSO4
10) Alclad 11) tinning 12) environment
13) electrochemical 14) possivity / passivation
15) pitting corrosion 16) intergranular 17) increase
18) heterogenity 19) paint 20) Drying oils
21) driers 22) Anodic 23) noble
24) electroplating / electrodeposition 25) Solvent cleaning
26) oxide 27) Wiregun 28) Al2F3
29) Chromising 30) Stabiliser
31) increases 32) higher 33) higher
34) sherardizing 35) dissolved oxygen
36) milli inches/year or mm/year
V.  TRUE OR FALSE QUESTIONS :
 1) F   2) T  3) T  4) T 5) F 6) T  7) T  8) T  9) T   10) T
11)  F 12) T 13) T 14) F 15) F 16) T 17) T 18) T 19) T 20) F
21) T 22) F 23) F 24) T 25) T 26) T 27) T 28) T 29) F 30) F