Corrosion. Corrosion is defined as the gradual eating away or deterioration of a metal by chemical or electrochemical reactions with its environment.

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1 Corrosion Introduction Metals and alloys are used as construction and fabrication materials in engineering. If the metal or alloy structure is not properly maintained, they deteriorate slowly by the action of atmospheric gases, moisture and other chemicals. This phenomenon of metals and alloys to undergo destruction by the act of environment is known as corrosion. Corrosion is defined as the gradual eating away or deterioration of a metal by chemical or electrochemical reactions with its environment. Due to corrosion the useful properties of a metal such as malleability, ductility and electrical conductivity are lost. The most familiar example of corrosion is rusting of iron when exposed to atmospheric conditions. During this, a layer of reddish scale and powder of oxide (Fe 3 O 4 ) is formed and the iron becomes weak. Another example is the formation of green film or basic carbonate [CuCO 3 + Cu(OH) 2 ] on the surface of copper when exposed to moist air containing CO 2. It has been roughly assessed that the amount of iron wasted due to corrosion is one fourth of world production. The direct loss due to corrosion in India amounts to Rs. 200 crore/annum while the money spent annually in controlling corrosion is of the order of Rs. 50 crore. It is better to control rather than to prevent corrosion, since it is impossible to eliminate corrosion. Cause of Corrosion In nature, most metals are found in a chemically combined state known as an ore. All the metals except gold, platinum and silver exist in nature in the form of their oxides, carbonates, sulphides, sulphates, etc. These combined forms of the metals represent their thermodynamically stable state (low energy state). The metals are extracted from these ores after supplying a large amount of energy. Metals in the uncombined condition have a higher energy and are in an unstable state. It is their natural tendency to go back to the low energy state, i.e., combined state by recombining with the elements present in the environment. This is the main reason for corrosion. Effects of corrosion: The following are the effects of corrosion: [1] Lost production during a shut down [2] Replacement of corroded equipment [3] High maintenance costs such as repainting [4] Loss of efficiency [5] Contamination of the product. Theories of Corrosion: [1] Direct chemical attack or Chemical or Dry corrosion [2] Electrochemical theory or Wet corrosion [3] Differential aeration or Concentration cell corrosion 1

2 Theories of Corrosion [1] Direct Chemical Attack or Chemical or Dry Corrosion Whenever corrosion takes place by direct chemical attack by gases like' oxygen, nitrogen and halogens, a solid film of the corrosion product is formed on the surface of the metal which protects the metal from further corrosion. If a soluble or volatile corrosion product is formed, then the metal is exposed to further attack. For example, chlorine and iodine attack silver generating a protective film of silver halide on the surface. On the other hand, stannic chloride formed on tin is volatile and so corrosion is not prevented. Oxidation corrosion is brought about by direct action of oxygen at low or high temperatures on metals in the absence of moisture. Alkali metals (Li, Na, K, etc.) and alkaline earth metals (Mg, Ca, Sn, etc.) are readily oxidized at low temperatures. At high temperatures, almost all metals except Ag, Au and Pt are oxidized. Alkali and alkaline earth metals on oxidation produce oxide deposits of smaller volume. This results in the formation of a porous layer through which oxygen can diffuse to bring about further attack of the metal. On the other hand, aluminium, tungsten and molybdenum form oxide layers of greater volume than the metal from which they were produced. These non-porous, continuous and coherent oxide films prevent the diffusion of oxygen and hence the rate of further attack decreases with increase in the thickness of the oxide film. The protective or non-protective nature of the oxide film is determined by a rule known as the Pilling-Bedworth rule. The ratio of the volume of the oxide formed to the volume of the metal consumed is called the Pilling-Bedworth rule. According to it, if the volume of the oxide layer is greater than the volume of the metal, the oxide layer is protective and non-porous. On the other hand, if the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is nonprotective and porous. [2] Electrochemical Theory or Wet Corrosion According to the electrochemical theory, the corrosion of a metal in aqueous solution may be a two-step process, one involving oxidation and another reduction. It is known that two metals having different electrode potentials form a galvanic cell when they are immersed in a conducting solution. The emf of the cell is given by the difference between the electrode potentials. When the electrodes are joined by a wire, electrons flow from the anode to the cathode. The oxidation reaction occurs at the anode, i.e. at the anode the metal atoms lose their electrons to the environment and pass into the solution in the form of positive ions. Fe Fe e - 2

3 Thus, there is a tendency at the anode to destroy the metal by dissolving it as ions. Hence corrosion always occurs at anodic areas. The electrons released at the anode are conducted to the cathode and are responsible for various cathodic reactions such as electroplating (deposition of metals), hydrogen evolution and oxygen absorption: (i) Electroplating: The metal ions at the cathode collect the electrons and deposit on the cathode surface. Cu e - Cu (ii) Liberation of hydrogen: In an acid solution, (in the absence of oxygen) hydrogen ions accept electrons and hydrogen gas is formed. 2H + + 2e - H2 In a neutral or alkaline medium, (in the absence of oxygen) hydrogen gas is liberated with the formation of OH- ions. 2H2O + 2e - H2 + 2OH - (iii) Oxygen absorption: In the presence of dissolved oxygen and in an acid medium, oxygen absorption reaction takes place. 4H + + O2 + 4e - 2H2O In the presence of dissolved oxygen and in a neutral or weakly alkaline medium, OH - ions are formed. 2H2O + O2 + 4e - 4OH - Thus it is clear that the essential requirements of electrochemical corrosion are as follows: (a) Formation of anodic and cathodic areas. (b) Electrical contact between the cathodic and anodic parts to enable the conduction of electrons. (c) An electrolyte through which the ions can diffuse or migrate. This is usually provided by moisture. [3] Differential Aeration or Concentration Cell Corrosion Anodic and cathodic areas may be generated even in a perfectly homogeneous and pure metal due to different amounts of oxygen reaching different parts of the metal which form oxygen concentration cells. In such circumstances, those areas which are exposed to greater amount of air become cathodic while the areas which are little exposed or not exposed to air become anodic and suffer corrosion. Figure shows the part of a metal surface covered with dirt which is less accessible to air than the rest of the metal. 3

4 Hence, the area covered with dirt becomes anodic and suffers corrosion. The anodic reaction is Fe Fe e - The most common reactions taking place at the cathode are which is further oxidized to Fe(OH) 3. Since the anodic area is small and the cathodic area is large, corrosion is more concentrated at the anode. Thus, a small hole is formed on the surface of the metal. This type of intense local corrosion is called pitting. Figure shows a wire fence in which the areas where the wires cross are less accessible to air than the rest of the fence and hence corrosion takes place at the wire crossings which are anodic. In a similar way, iron corrodes under drops of water or salt solution. Areas covered by droplets, having less access of oxygen become anodic with respect to the other areas which are freely exposed to air. Differential aeration corrosion occurs when one part of metal is exposed to a different air concentration from the other part. This causes a difference in potential between differently aerated areas. It is experimentally found that poor oxygenated parts are anodic. Consequently, differential aeration of a metal causes a flow of current. 4

5 Factors influencing corrosion: The rate and extent of corrosion depend mainly on (a) the nature of the metal and (b) the nature of the environment. Nature of the Metal [I] Position in the galvanic series: The extent of corrosion depends upon the position of the metal in the galvanic series. Greater the oxidation potential, the greater is the rate of corrosion. When two metals are in electrical contact, the metal higher up in the galvanic series becomes anodic and suffers corrosion. Further, the rate and severity of corrosion depend upon the difference in their positions in the galvanic series. The greater is the difference, the faster is the corrosion of anodic metal. [2] Relative areas of the anode and cathode: The rate of corrosion is more when the area of the cathode is larger. When the cathodic area is larger, the demand for electrons will be more, and this results in increased rate of dissolution of metals at anodic regions. [3] Purity of the metal: The impurities present in a metal create heterogeneity and thus galvanic cells are set up with distinct anodic and cathodic areas in the metal. The higher the percentage of impurity present in a metal, the faster is the rate of corrosion of the anodic metal. For instance, impurities such as Pb and Fe in zinc lead to till formation of tiny electrochemical cells at the exposed part of the impurity and the corrosion of zinc around the impurity takes place due to local action. It is evident that the corrosion resistance of a metal may be improved by increasing impurity. [4] Physical state of the metal: Metal components subjected to unevenly distributed stresses are easily corroded. Even in a pure metal, the areas under stress tend to be anodic and suffer corrosion. Caustic embrittlement takes place in stressed parts such as bends, joints and rivets in boilers. [5] Nature of the oxide film: Metals such as Mg, Ca and Ba form oxides whose volume is less than the volume of the metal. Hence, the oxide film formed will be porous, through which oxygen can diffuse and bring about further corrosion. On the other hand, metals like AI, Cr and Ni form oxides whose volume is greater than that of the metal and the nonporous oxide film so formed will protect the metal from further corrosion. [6] Solubilities of the products of corrosion: Solubility of the corrosion product formed is an important factor in corrosion. If the corrosion product is soluble in the corroding medium, the corrosion of the metal will proceed faster. On the other hand, if the corrosion product is insoluble, then the protective film formed tends to suppress corrosion. 5

6 Nature of the Environment [1] Temperature: The rate of chemical reactions and the rate of diffusion of ions increase with temperature. Hence, corrosion increases with temperature. A passive metal may become active at a higher temperature. [2] Humidity: Atmospheric corrosion of iron is slow in dry air but increases rapidly in the presence of moisture. This is due to the fact that moisture acts as the solvent for the oxygen in the air to furnish the electrolyte essential for setting up a corrosion cell. Rusting of iron increases when the relative humidity of air reaches from 60 to 80%. [3] Effect of ph: The possibility of corrosion with respect to ph of the solution and the electrode potential of the metal can be correlated with the help of a Pourbaix diagram. The Pourbaix diagram for iron in water is shown in Figure.. The diagram shows clearly the zones of corrosion, immunity and passivity. In the diagram x is a point where ph is 7 and the electrode potential is -0.4 V. It is present in the corrosion zone. This shows that iron rusts in water under those conditions. This is noticed to be true in actual practice also. From figure, it is seen that the rate of corrosion can be altered by shifting the point x into immunity or passivity regions. The iron would be immune to corrosion if the potential is changed to about -0.8 V and this can be achieved by applying external current. On the other hand, the corrosion rate of iron can also be reduced by moving into the passivity region by applying positive potential. The diagram clearly indicates that the corrosion rate can also be reduced by increasing the ph of the solution by the addition of alkali without disturbing the potential. 6

7 [4] Nature of the electrolyte: The nature of the electrolyte also influences the rate of corrosion. If the electrolyte consists of silicate ions, they form insoluble silicates and prevent further corrosion. On the other hand, if chloride ions are present, they destroy the protective film and the surface is exposed for further corrosion. If the conductance of electrolyte is more, the corrosion current is easily conducted and hence the rate of corrosion is increased. [5] Concentration of oxygen and formation of oxygen concentration cells: The rate of corrosion increases with increasing supply of oxygen. The region where oxygen concentration is lesser becomes anodic and suffers corrosion. Corrosion often takes place under metal washers where oxygen cannot diffuse readily. Similarly, buried pipelines and cables passing from one type of soil to another suffer corrosion due to differential aeration, e.g. lead pipeline passing through clay and then through sand. Lead pipeline passing through clay get corroded because it is less aerated than sand. Types of Corrosion The following are the different types of corrosion: [1] Galvanic corrosion: Galvanic corrosion is a type of electrochemical corrosion in which two different types of metals in contact are jointly exposed to corrosive atmosphere. Here the metal with more negative electrode potential will become the anode and goes into solution or corrode, e.g. (a) zinc and copper metals and (b) steel pipe connected to copper plumbing. This corrosion can be minimized by (a) avoiding galvanic couples and (b) providing an insulating material between the two metals. [2] Stress corrosion cracking: Corrosion of metals is also influenced by some physical differences like internal stresses in the metals. Such differences result during manufacture, fabrication and heat treatment. Metal components are subjected to unevenly distributed stresses during their manufacturing process. The electrode potential thus varies from one point to another. Areas under great stress act as the anode while areas not under stress act as the cathode. Various treatments of metals and alloys such as cold working or quenching, bending and pressing introduce uneven stress and lead to stress corrosion. Corrosion takes place so as to minimize the stress. Most of the time it ends in breaking of the components into pieces. Corrosion of head and point portions of a nail indicates that they have been acting as anode to the middle portion. Actually the head and the point portions were put under stress during their manufacture. In the case of iron-wire hammered at the middle, corrosion takes place at the hammered part and results in breaking of the wire into two pieces. Caustic embrittlement takes place in stressed parts such as bends, joints and rivets in boilers. 7

8 Corrosion Control Corrosion can be controlled by the following ways: [1] By selection of the material: Selection of the right type of the material is the main factor for corrosion control. Thus, noble metals are used for surgical instruments and ornaments as they are most immune to corrosion. [2] By using pure metals: Pure metals have higher corrosion resistance. Even minute amount of impurities may lead to severe corrosion, e.g. 0.02% iron in aluminium decreases its corrosion resistance. [3] By alloying: Both corrosion resistance and strength of many metals can be improved by alloying, e.g. stainless steels containing chromium produce a coherent oxide film which protects the steel from further attack. [4] By annealing: Heat treatment like annealing helps to reduce internal stresses and reduces corrosion. [5] By eliminating galvanic action: If two metals have to be in contact, they should be so selected that their oxidation potentials are as near as possible. Further, the area of the cathode metal should be smaller than that of the anode, e.g. copper nuts and bolts on large steel plate. The corrosion can also be reduced by inserting an insulating material between the two metals. [6] By cathodic protection: The principle involved in cathodic protection is to force the metal behave like a cathode. Since there will not be any anodic area on the metal, corrosion does not occur. There are two types of cathodic protection. (a) Sacrificial anodic protection. (b) Impressed current cathodic protection. (a) Sacrificial anodic protection: In this technique, a more active metal is connected to the metal structure to be protected so that all the corrosion is concentrated at the more active metal and thus saving the metal structure from corrosion. This method is used for the protection of sea going vessels such as ships and boats. Sheets of zinc or magnesium are hung around the hull of the ship. Zinc and magnesium being anodic to iron get corroded. Since they are sacrificed in the process of saving iron (anode) they are called sacrificial anodes. The corroded sacrificial anode is replaced by a fresh one, when consumed completely. Important applications of sacrificial anodic protection are as follows: [i] Protection from soil corrosion of underground cables and pipelines (Figure a). [ii] Magnesium sheets are inserted into domestic water boilers to prevent the formation of rust water (Figure b). 8

9 Sacrificial anodic protection Figure a Figure b (b) Impressed current cathodic protection: In this method, an impressed current is applied in an opposite direction to nullify the corrosion current and converting the corroding metal from anode to cathode. This can be accomplished by applying sufficient amount of direct current from a battery to an anode buried in the soil and connected to the corroding metal structure which is to be protected. The anode is in a backfill (composed of gypsum) so as to increase the electrical contact with the soil. Since in this method current from an external source is impressed on the system, this is called impressed current method. 9

10 [8] By modifying the environment: The corrosion rate can be reduced by modifying the environment. The environment can be modified by the following: (a) Deaeration: The presence of increased amounts of oxygen is harmful since it increases the corrosion rate. Deaeration aims at the removal of dissolved oxygen. Dissolved oxygen can be removed by deaeration or by adding some chemical substances like Na 2 SO 3 (b) Dehumidification: In this method, moisture from air is removed by lowering the relative humidity of surrounding air. This can be achieved by adding silica gel which can adsorb moisture preferentially on its surface. (c) Inhibitors: In this method, some chemical substances known as inhibitors are added to the corrosive environment in small quantities. These inhibitors substantially reduce the rate of corrosion. (i) Anodic inhibitors: Anodic inhibitors include alkalis, molybdates, phosphates. chromates, etc. When these inhibitors are added, they react with the ions of the anode and produce insoluble precipitates. The so formed precipitate is adsorbed on the anode metal forming a protective film thereby reducing corrosion. (ii) Cathodic inhibitors: In an electrochemical corrosion, the cathodic reactions are of two types depending on the environment. In acidic solution, the cathodic reaction is 2H + + 2e - H2 In acidic solution; the corrosion can be controlled by slowing down the diffusion of H + ions through the cathode. This can be done by adding organic inhibitor like amines and pyridine. They adsorb over the cathodic metal surface and act a protective layer. In neutral solution, the cathodic reaction is H2O + 1/2O 2 + 2e - 2OH - The formation of OH - ions is only due to the presence of oxygen. By elimination the oxygen from the medium, the corrosion rate can be reduced. Oxygen can be removed by adding some reducing agents (Na 2 SO 3 ) or by deaeration. (iii) Vapour phase inhibitors: Vapour phase inhibitors (VPIs) are organic inhibitor which readily sublime and form a protective layer on the metal surface, e.g., dicyclohexylammoniumnitrite. They are used in the protection of machinery, sophisticated equipment, etc. which are sent by ships. The condensed inhibitor can be easily wiped off from the metal surface. [9] By Passivation: Passivation is a phenomenon of converting an active surface of a metal into passive i.e. more corrosion resistant by forming a thin, non-porous and highly protective film over the surface. When the metals like aluminium and tin are exposed to the atmosphere or to the oxidizing environment, their surfaces rapidly get converted into oxides. The non-porous nature of these oxide layers prevents further corrosion. In other words, the metals are passivated. 10

11 Similarly, metals which are susceptible to corrosion are made passive by alloying with one or more metals which are passive or resist corrosion. For example, iron is rendered passive by alloying it with any of the transition metals such as chromium, nickel and molybdenum. [7] By applying protective coating: The surface of engineering material can be protected from corrosion by covering it by a protective coating. This coating may be of organic or inorganic material. Protective Coatings Introduction Protective coatings are used to protect the metals from corrosion. The main types of protective coatings are classified as follows. The protective coatings must be chemically inert to the environment and also sufficiently thick. Besides protection from corrosive conditions, such coatings can also give decorative appearance to the base metals. To be more effective, these coatings should adhere well to the surface. Pretreatment of the surface or preparation of materials for coating: The outermost surface of the base metal (which is to be protected) usually contains impurities like rust, scale and grease. These substances, if present at the time of coating, will give porous and discontinuous coatings. In order to get a uniform, smooth and coherent protective coating, these substances are removed by the following methods. 11

12 Degreasing: Oil and grease may be removed by cleaning with organic solvents such as chloroform, toluene and acetone. Immersion in hot alkaline solutions is the most commonly used cleaning technique. For example, sodium carbonate and sodium hydroxide are used for this purpose. Removal of Oxide Scales or Descaling: Removal of the oxide scales and corrosion products (rust) from the surface is called descaling. In this process, the base metal is dipped inside the acid solution at higher temperatures. The acid penetrates through cracks and pores of the scales and then their dissolution takes place. Acids like sulphuric acid, hydrochloric acid and nitric acid are used under dilute conditions. Mechanical Cleaning: Oxide scales, rust and corrosion products are also removed by abrasion such as grinding, wire brushing and polishing. Electrochemical Method: The electrochemical method is used to remove oxide scales which are not removed by other methods. The base metal is made either anode or cathode with an electrolyte (acid or base). At the anode the oxide scale is dissolved in the electrolyte and leaves the base metal, whereas at the cathode the metal oxides are reduced to metal. Metallic Coatings Surface coatings made up of metals are known as metallic coatings. These coatings separate the base metal from the corrosive environment and also function as an effective barrier for the protection of base metals. Metallic coatings are mostly applied on iron and steel because they are cheap and commonly used construction materials. Metallic coatings are usually imparted by the following methods. Hot Dipping In the process of hot dipping, the metal to be coated is dipped in the molten bath of the coating metal and the thickness of the coating is adjusted by squeezing out the excess of the coating metal with rollers. Such hot dip coatings are generally non-uniform. The common examples of hot dip coatings are galvanizing and tinning. [1] Galvanizing: The process of coating a layer of zinc on steel is called galvanizing. The steel article is first pickled with dilute sulphuric acid to remove traces of rust, dust, etc, at C for about minutes. Then this metal is dipped in a molten zinc bath maintained at 430 C. The surface of the bath is covered with ammonium chloride flux to prevent oxide formation on the surface of molten zinc. The coated base metal is then passed through rollers to correct the thickness of the film. It is used to protect roofing sheets, wires, pipes, tanks, nails, screws, etc. 12

13 [2] Tinning: The coating of tin on iron is called tin plating or tinning. In tinning, the base is first pickled with dilute sulphuric acid to remove surface impurities. Then it is passed through molten tin covered with zinc chloride flux. The tin coated article is passed through a series of rollers immersed in a palm oil bath to remove the excess tin. Tin-coated utensils are used for storing foodstuffs, pickles, oils, etc. Galvanizing is preferred to tinning because tin is cathodic to iron, whereas zinc is anodic to iron. So, if the protective layer of the tin coating has any cracks, iron will corrode. If the protective layer of the zinc coating has any cracks, iron being cathodic does not get corroded. The corrosion products fill up the cracks, thus preventing corrosion. Cementation In cementation, the base metal is heated with the coating metal in the form of fine powder in order to promote the diffusion of the coating metal into the base metal. The coatings obtained are of uniform thickness. The base metal is generally steel and the coating metals used is zinc, chromium and aluminium. When the coating metal is zinc, the process is called sherardizing. When the coating metal is chromium, the process is called chromizing. When the coating metal is aluminium, the process is called calorizing. [1] Sherardizing: Cementation with zinc powder is called sherardizing in honour of Sherard Cowpercoles who developed the process in The base metal is heated with zinc dust in a metal drum maintained at a temperature of C. The drum is closed tightly and rotated with constant heating for two to three hours. During this process zinc gets diffused into iron forming an alloy of Fe-Zn on the surface. Sherardized coatings are used for protecting small steel parts such as nuts and bolts against atmospheric corrosion. [2] Chromizing: The base metal is heated with a powdered mixture of 55 per cent chromium and 45 per cent alumina at a temperature of C for about 3-4 hours in a closed drum. The purpose of using alumina is to prevent the coalescing of chromium particles. The outermost surface of the base metal is converted into a chrome alloy which protects the metal against corrosion. This method is used to protect gas turbine blades. [3] Calorizing: Here the base metal is heated with a powdered mixture of aluminium and alumina in a drum at a temperature of C for 4-6 hours. Calorized steel is used for making furnace parts. 13

14 Electroplating or Electrodeposition Electroplating is a process in which metals are deposited or plated on base metals from solutions containing metallic ions by means of electrolysis. The objectives of electroplating are as follows: (1) To obtain improved resistance to corrosion and chemical attack (2) To get better appearance (3) To get increased hardness (4) To change the surface properties of metals and non-metals. In the electroplating process, the freshly cleaned base metal which is to receive the coat is made the cathode in a suitable electrolyte bath containing (a) a solution of the salt of the metal 10 be electrodeposited, (b) buffer solution to control the ph, and (c) additional reagents to enhance conductivity and to aid the formation of smooth, dense and coherent coating. The concentration of the salt solution is maintained by the addition of the metal salt at regular intervals or by the use of continuously dissolving anode of the metal. The plating is usually done at a high current density. The nature of the deposit depends upon the current density, ph and the concentration of the bath. A typical electroplating process is shown in Figure. Organic Coatings Organic coatings are inert organic barriers applied to the surface of base metals for corrosion resistance and decoration. Paints, varnish, lacquers and enamels are the main organic coatings. Paints Paint is a viscous suspension of finely divided solid pigment in a fluid medium which on drying yields an impermeable film having considerable hiding power. When paint is applied to a metal surface, the thinner evaporates, while the drying oil slowly oxidizes forming a dry pigmented film. 14

15 Requirements of a good paint: A good paint should essentially have the following: [1] A good paint should fonn a good impervious and uniform film on the metal surface. [2] It should have a high hiding (covering) power. [3] The film should not crack on drying. [4] A good paint should adhere well to the surface. [5] It should spread on the metal surface easily. [6] It should give a glossy film. [7] It should be corrosion resistant. [8] A good paint should give a stable and decent colour on the metal surface. Constituents of paint and their functions: The important constituents of paint are as follows: (1) Pigments (2) Vehicle or drying oils or medium (3) Thinners (4) Driers (5) Fillers or extenders (6) Plasticizers (7) Anti-skinning agents (1) Pigments: A pigment is a solid and colour-producing substance in the paint. (2) Vehicle or drying oils or medium: The liquid portion of the paint in which the pigment is dispersed is called a medium or vehicle. This is the film forming constituent of the paint. Vehicles are high molecular weight fatty acids present in animal and vegetable oils, e.g. linseed oil, dehydrated castor oil, soyabean oil and fish oil. (3) Thinners: Thinners are added to paints to reduce the consistency or viscosity of the paints so that they can be easily applied to the metal surface. Thinners are volatile in nature and evaporate easily after application of the paint, e.g. turpentine and petroleum spirit (4) Driers: Driers are used to accelerate or catalyze the drying of the oil film by oxidation, polymerization and condensation, e.g. naphthenates, borates and tungstales of lead, cobalt and manganese. (5) Fillers or extenders: Fillers are used to reduce the cost and increase the durability of the paint, e.g. talc, china clay, calcium sulphate and calcium carbonate. (6) Plasticizers: Plasticizers are chemicals added to paints to give elasticity to the film and to prevent cracking of the film, e.g. triphenyl phosphate and tricresyl phosphate. (7) Anti-skinning agents: They are chemicals added to the paint to prevent skinning of the paint, e.g. polyhydroxy phenols. Mechanism of drying oils Drying oils are the film forming constituent of the paint. These are fatty oils which are extracted from plants or animals. The fatty oils are tri-esters of glycerol The oil film after it has been applied to the protected surface absorbs oxygen from air at the double bonds forming peroxides which isomerizes, polymerize and condense to form a tough and highly cross linked macromolecular film. 15

16 Failure of paint: Paint may fail due to several reasons: (1) Chalking: Chalking is the gradual powdering of paint. It is caused by continuous destructive oxidation of the oil after the original drying of paint. (2) Erosion: It is very quick chalking. (3) Flaking: Flaking is caused due to poor adherence of a paint film to the surface because of the presence of grease on the surface. (4) Checking: It is a very fine type of surface cracking. (5) Alligatoring: The centre portion of the film remains attached to the surface, whereas the portion around the centre peels off. The failure of paint can be prevented by the following ways: (1) Carefully preparing the surface before application of paint. (2) Applying a suitable primer coat. (3) Applying the paint evenly. (4) Allowing each paint coat to dry sufficiently before the next coat is applied. 16

17 Syllabus Corrosion and its control theories of corrosion dry corrosion and wet corrosion galvanic series - corrosion of iron in acidic, neutral and basic conditions Differential aeration corrosion, stress corrosion galvanic corrosion Factors influencing corrosion. Corrosion protection self protecting corrosion products Pilling Bed worth rule Coatings Organic (Paints and polymers) - Inorganic Metallic (galvanizing, tinning, electroplating, cementation) Nonmetallic (phosphate, chromate, anodising, chemical oxide). Passivation of metals by chemical treatment protection by sacrificial anode Impressed current. Previous University Questions [1] How is corrosion caused? What are the conditions of dry corrosion? {5 marks} [2] Describe the various factors influencing the corrosion {5 marks} [3] What is direct corrosion? {2 marks} [4] What is meant by differential aeration corrosion? Illustrate with suitable examples {10 marks} [5] Write a note on galvanic corrosion {5 marks} [6] Briefly discuss on galvanic series and galvanic corrosion {10 marks} [7] Write a note on electrochemical corrosion {5 marks} [8] Outline the different types of corrosion {10 marks} [9] Explain the mechanism of wet corrosion. Give details of corrosion protection through sacrificial anodic method and impressed current method {10 marks} [10] State and explain Pilling-Bedworth rule {5 marks} [11] How are metals protected from corrosion by electroplating? {5 marks} [12] What is cathodic and anodic protection for controlling corrosion? Discuss their merits and demerits {10 marks} [13] Write a note on galvanizing and anodisation of aluminium {5 marks} [14] Discuss the different types of inorganic coatings {5 marks} [15] Explain the process of electroplating {5 marks} ietchemistry@gmail.com 17