Corrosion Science PDF

RUAS, Bangalore- 560058 Corrosion Science Corrosion is defined as the destruction or deterioration of metals or alloys by the environment through electrochemical or chemical reactions. Ex: 1. Rusting of iron, 2. Formation of green scale on Cu metal. Elec...

RUAS, Bangalore- 560058 Corrosion Science Corrosion is defined as the destruction or deterioration of metals or alloys by the environment through electrochemical or chemical reactions. Ex: 1. Rusting of iron, 2. Formation of green scale on Cu metal. Electrochemical theory of corrosion: Air Water film or moisture e (neutral) Anodic area Cathodic area Fe Fe2+ + 2e O2 + 2H2O + 4e 4OH- Iron rod According to electrochemical theory of corrosion, when a metal such as iron is exposed to atmosphere; 1. A large number of minute galvanic cells are formed on the surface of the metal. 2. Oxidation takes place at anode i.e. electrons are released at anode M Mn+ + ne Fe Fe2+ + 2e 3. Reduction takes place at cathode i.e. electrons are accepted at cathode Depending upon the nature of the surrounding environment, following reactions will occur; If the medium is aerated and neutral 2H2O + O2 + 4e 4OH If the medium is deaerated and neutral 2H2O + 2e H2 + 2OH If the medium is deaerated and acidic 2H+ + 2e H2 The metal ions formed at anode and the OH formed at cathode react to form the metal hydroxide that further oxidizes to form the respective metal oxide, which is corrosion product. Fe2+ + 2OH Fe (OH)2 2 Fe(OH)2 + (n-2) H2O + ½ O2 Fe2O3.nH2O ( rust) Galvanic Series: A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater. The relative position of the materials can change in other environments. The farther apart the materials are in this series, the higher the risk of galvanic corrosion. Most cathodic, noble, or resistant to corrosion: Platinum Gold Graphite Titanium Silver 18-8 stainless steel (passive) Chromium steel >11 % Cr (passive) Nickel (passive) Silver solder Copper 1  RUAS, Bangalore- 560058 Brass Tin Lead Cast iron Steel or iron Most anodic or easy to corrode 2024 Aluminium Cadmium Commercially pure Aluminium Zinc Magnesium and its alloys Types of corrosion 1. Differential metal corrosion 2. Differential aeration corrosion 3. Stress corrosion Differential Metal Corrosion: This type of corrosion occurs when two different metals are in contact with each other. One of the metal acts as anode and the other as cathode. The former corrodes. This happens due to the difference in the potential of two electrodes. The metal, which is placed higher in the electrochemical series (having lower reduction potential), acts as anode and undergoes corrosion. Thus, when Fe is in contact with Zn, Zn corrodes whereas if Fe is in contact with Cu, Fe corrodes. Rate of corrosion increases with increase in potential difference between two metals. Fe Cu Zn Fe e e Ex: When screws and nuts are made of different metals, this type of corrosion takes place. Differential Aeration Corrosion: This type of corrosion occurs when different parts of a metal are exposed to different concentrations of oxygen. Thus when an iron is half immersed in water, the part immersed in water is less aerated and acts as anode. The part which is above the surface of water is more aerated and acts as cathode. Thus corrosion begins at the bottom portion of the rod. Ex: When equipments such as transformers are placed on a flat base, this type of corrosion takes place. Hence equipments are placed on legs. Corrosion in barbed wires and corrosion at the joints in cross wires are other examples Half-immersed iron piece Atmospheric air Cathode O2+2H2O + 4e Corroded part-Anode e Fe2+ Aerated solution Fe2+ 4OH- of NaCl Water line corrosion In steel water tanks, the bottom portion is less aerated, which acts as anode (below the waterline). The portion above water, which is more aerated and acts as cathode. Corrosion begins at this portion and moves slowly upwards until the entire tank corrodes. Oxygen Water-line Cathode Corroded part (anode) 2  RUAS, Bangalore- 560058 Pitting corrosion When dust settles on the surface of a metal, the portion of the metal below the dust is less aerated than the rest of the metal. The dust covered portion acts as anode. In a corrosive environment, this portion undergoes corrosion, forming a pit. Cathodic region O2 + 2H2O + 4e 4OH- Rust Dust Iron block Pi t Anodic region FeFe2+ + 2e Stress corrosion When a metal rod is under stress (such as bending), the bent portion acts as anode due to slight displacement of atoms in this region. The remaining portion of the metal acts as cathode and hence the stressed portion undergoes corrosion when the environment is favourable for corrosion of the metal. Thus iron rod under stress undergoes corrosion in the presence of alkali and stressed brass undergoes corrosion in the presence of ammonia. Thus for stress corrosion, (1) the metal should be stressed such as due to welding, riveting and (2) the presence of specific corrosive environment for the metal is necessary. Ex: Caustic embrittlement occurs in boilers operating at high temperature and pressure. Here the stressed portion acts as anode. If a small hole or pit is present in this portion, the alkali will seep into the hole and due to boiling the concentration of alkali increases. When the concentration increases beyond 10%, it will lead to the formation of sodium ferroate, which further undergoes decomposition. Intense corrosion occurs at this part. The final product will be magnetite. Na2CO3 + H2O 2 NaOH + CO2 Fe + NaOH Na2FeO2 3 Na2FeO2 + 4 H2O 6 NaOH + Fe3O4 + H2 6 Na2FeO2 + 6 H2O + O2 12 NaOH + 2 Fe3O4 (magnetite) Factors that affect the rate of corrosion: Nature of corrosion product or passivity: If the corrosion product is non-porous, non conducting, insoluble and stoichiometric, the corrosion rate decreases because the product forms a protective coating over the surface of the metal and prevents further corrosion. Ex: Al, Cr, Ti. If the corrosion product is porous, conducting, soluble and non-stoichiometric, corrosion proceeds uninhibited. Ex: Iron, Zn, Mg. Electrode potential: If two different metals are in contact corrosion takes place due to potential difference. Thus if the two metals are placed closer in the electrochemical series, the potential difference is less and the corrosion is slower. If the two metals are placed farther apart in the electrochemical series, the potential difference is high and corrosion rate is high. Thus iron corrodes faster when in it is contact with Cu than with Tin. Anodic and cathodic areas: Smaller the anodic area and larger the cathodic area, faster is the corrosion. A large cathodic area can consume more electrons (cathodic reaction becomes faster which in turn increases the anodic reaction) and since this is supplied by a small anodic area, the corrosion will be fast. Thus when tin is coated on iron, (iron is anode and tin is cathode), even if a pinhole is formed, results in small anodic area and large cathodic area. Therefore corrosion will 3  RUAS, Bangalore- 560058 be intense. If zinc is coated on iron, zinc being anodic to iron, even if the coating peels off at certain places, corrosion would not take place on iron till Zn is present. pH: In general, lower the pH i.e., more acidic the conditions, higher is the rate of corrosion. If the pH is greater than 10 i.e., in highly alkaline medium, corrosion practically ceases due to the formation of protective layer of hydroxides. If the pH is between 3 and 10, corrosion takes place in the presence of oxygen. If the pH is less than 3, corrosion is intense due to the evolution of H 2. Temperature: As the temperature increases, conductance and diffusion of the medium increases and hence corrosion becomes intense. High temperature decreases passivity and increases corrosion rate. Nature of the metal: Corrosion also depends on the position of the metal in the galvanic series. A metal which is placed higher in the galvanic series has a lower electrode potential, is more reactive and hence corrodes easily. Physical state of the metal: The rate of corrosion also depends on the physical state of the metal which includes grains size, stress etc. If the grain size is small, the surface contact is more, and the corrosion rate is high. A stressed metal (due to bending, riveting, welding etc.) undergoes corrosion faster because the stressed portion is anodic with respect to the remaining portion of the metal. Hydrogen overvoltage: A metal with low hydrogen overvoltage (OV) is more susceptible to corrosion, when the cathodic reaction involves hydrogen evolution. The decrease in the over voltage of the corroding metal/alloy, accelerates the corrosion rate. Ex: when Zn metal in contact with 1N H2SO4, it undergoes corrosion by the evolution of hydrogen gas. The rate of the reaction is very slow, because its O.V. is high (~0.7V). If a few drops of CuSO4 solution is added the rate of corrosion increases since, Cu gets deposited on Zn forming minute cathodes, where the hydrogen OV value is only 0.33V. Anodic and cathodic polarizations: Larger the difference in the potentials at anode and cathode faster is the corrosion. As corrosion current flows, some irreversible reactions occur near anode and cathode, and these reactions oppose the flow of current that leads to the variation of potentials at the anode and cathode. This is called polarization. Polarization decreases potential difference and hence corrosion rate decreases. Anodic polarization: If the anode alone undergoes polarization, corrosion rate depends on anodic polarization. A plot of current density against potential shows that anode polarization curve is steeper. Ec Ec Ec Ecurr Ecorr Ecorr Ea Ea Ea icorr icorr icorr Current density Current density Current density Cathodic polarization: if the cathode alone undergoes polarization, corrosion rate depends on cathodic polarization. A plot of current density against potential shows that cathode polarization curve is steeper. When both anode and cathodic polarization occur, it is called as mixed polarization. 4  RUAS, Bangalore- 560058 Corrosion control 1. Metal coatings a. Anodic Metal Coating: In this type of coating the base metal is coated with the metals which are anodic to it. Ex: Zn or Mg coated on Iron Galvanizing (Hot Dipping): Coating of iron with zinc (anodic to iron) is called galvanizing. The base metal is first degreased with organic solvents and then treated with sulphuric acid to remove any oxide that may be present on the surface. The metal is washed with water to remove the acid. The metal is treated with a solution of a zinc chloride and ammonium chloride. This acts as flux. The metal is finally dipped in molten zinc at 4500C when zinc gets coated on the metal. It is rolled to remove the excess zinc from the surface. Water Iron sheet Drier Galvanized iron Rollers Organic dil H2SO4 ZnCl2+NH4Cl Molten Zn (450 0C) Excess Zn solvent b. Cathodic coating: In this method of metal coating, the base metal is coated with a metal, which is cathodic to it. Tinning of iron is an example. Tinning: The base metal iron is degreased, treated with dilute sulphuric acid and washed with water. It is immersed in a solution of Zinc chloride and ammonium chloride. It is dipped in molten tin and finally in palm oil to avoid the oxidation of molten tin. It is finally passed through rollers to remove the excess tin. Tinning is employed for cans used for storing food. This is because tinning prevents corrosion, is nontoxic and is more economical than electroplating. Water Iron sheet Drier Tin coated iron Rollers Palm Oil Organic dil H2SO4 ZnCl2+NH4Cl Molten Tin Excess Tin Solvent (230 -240 0C) 2. Inorganic Coatings a. Anodizing: Anodizing is the process of formation of thin and compact layer of metal oxide over base metal through electrolysis. It is done generally on non-ferrous metals such as Al. The object (Al) to be protected is made as anode and an inert electrode is made as cathode (copper, stainless steel or lead). These are dipped in a solution of chromic or sulphuric acid. When electrolysis is carried out with a potential of 40V for about 20 minutes, a thin layer of oxide is formed on the surface. The oxide formed is porous and it is converted into non porous layer by treating with boiling water or treating with Nickel or Cobalt acetate (alumina is converted to Alumina monohydrate). Finally surface is dyed by dipping in aqueous dye solution at 50-600C. (Net reaction) 2Al + 3H2O Al2O3 + 3H2 a. Phosphating: A phosphate undercoating (phosphating) is generally given before the finishing. Phosphating offers improved corrosion resistance and also imparts the surface a good 5  RUAS, Bangalore- 560058 paint adhesion quality. Steel, zinc and aluminium objects which are to be provided with a paint finish are generally given a phosphate undercoat. Modern phosphating bath solutions contain a mixture of phosphoric acid, a metal phosphate and an accelerator. Accelerators are used to speed up the reaction (coating rate) and reduce the crystal size. Nitrite accelerators are most widely used. The pH is usually maintained at about 1.8 to 3.2 and electrolysis is carried out at a temperature of 35 0C. The mechanism of phosphating involves the following steps; (i) Dissolution of the metal as metal ions. (ii) Metal ions reacting with the phosphate ions to form a metal phosphate. (iii)Deposition of the metal phosphate on the surface of the metal. 3Fe + 2H3PO4 Fe3(PO4)2 + 2 H2 The coating of mixed iron and zinc phosphates on steel sheet fabrications such as in refrigerators, washing machines and car bodies provide a better paint adhesion in addition to improved corrosion resistance. 3. Organic Coating: One of the methods for preventing corrosion is to have a thin coat of paint, enamel on the metal surface to be protected. Organic coatings are the barriers introduced between the metal surface and the environment for protection form corrosion and also to impart decorative finish. Organic coatings are inert to environment and possess a good adhesion property. The requirements of a good organic coating are: * It should adhere tenaciously to the metal surface and improve its physical appearance. * The film formed should be continuous, uniform and impervious to air and moisture. * Should be chemically inert to corrosion environment. * Should have proper application methods. The performance of paint coatings depends to a large extent on the application technique and even a slight negligence at any stage may cause failure of the coatings. The application of coating involves the following sequence: Surface preparation, priming, filling, sanding and finishing or top coats. Surface preparation includes degreasing the surface, and ensuring dry and scale free surface. A high degree of protection is attained by the application of several coats of paints of different composition. The first coat is the primer such as phosphate coating which must be strongly adherent to the surface of the articles to be subsequently painted. Fillers such as nitrocellulose, epoxides, etc., are applied on the well dried primed surface in order to improve the external appearance of the paint and to produce a smooth coating. After drying, the roughness and irregularities on the surface are smoothened by means of emery paper (sanding) before applying the finishing or top coats. Organic coatings are applied by different methods such as brushing, spraying, dipping, roller coating, etc. Break down of the organic coating may cause severe corrosion of the exposed metal parts. 4. Cathodic protection In this method corrosion of a metal is controlled by supplying electrons externally so that the base metal acts as cathode. This can be achieved by two methods: Sacrificial anode method: In this method, the base metal is connected to more active metal, which is more anodic than the base metal. E.g., iron is combined with zinc. Zinc provides electrons and thus iron becomes cathode so that zinc corrodes and iron is protected. However, zinc has to be replaced from time to time. Ship hulls and buried pipes are protected from corrosion by this method. 6  RUAS, Bangalore- 560058 Steel rod Zn strips Anode mesh Hull of ship Concrete RCC COLUMN Ground Zinc metal in contact with oil pipe line Underground oil pipe Sacrificial anode method Impressed voltage or impressed current method: In this method, electrons are supplied to the base metal by an external D.C. source. The negative terminal of the source is connected to the metal. Water tanks and oil pipe lines are protected from corrosion by this method. The structure to be protected should be made negative (connected to the negative terminal of a d.c. source), otherwise it corrodes rapidly. Resin bonded graphite rod, high silicon-iron alloy (Fe containing 14%Si and 3%Mo) or platinised Ti or Ta is used as an inert anode and is connected to the positive terminal. - DC power supply + Ground - ve +ve Inert metal Electrical contact Oil or gas pipe line Anodic protection This method is applicable to metals like Al, Ti, and Ta etc which form a protective oxide layer on their surface. Principle: Growth of oxide film by the application of an anodic current on the metal specimen in a suitable oxidizing environment. The potential that is required to protect the metal can be obtained from potential- current curve as shown in Figure. In the initial stages (AB) the current increases indicating the dissolution (corrosion) of the metal. When the current reaches a critical point, icrit, passivation, that is, the development of oxide layer sets in. The potential at i crit is called the passivating potential (Ep). Above Ep (along BC) the current flow decreases to a very small value called the passivating current, ip. It is the minimum protective current density to maintain passivation. At this stage (at C), an increase in potential will not corrode the metal since it is in a highly passive state. A schematic arrangement for anodic protection is shown in Fig. It consists of a generator, an auxiliary platinum electrode, a reference calomel electrode, and a potentiostat. The metal specimen kept in a suitable oxidizing atmosphere acts as the working electrode (anode). The potential is first increased to E p for the initial corrosion to occur. The potential is then slightly increased to E f (Flade potential) for passivation. 7  RUAS, Bangalore- 560058 Finally the potential corresponding to Ep and the current corresponding to ip is maintained. Potentiostat In Out Passive C E Ep Reference electrode B Active Pt A Oxidizing medium IP Icrit Metal CURRENT Corrosion inhibitors These are chemicals, which inhibit anodic and cathodic reactions. Anodic inhibitors These inhibit anodic reactions by forming a protective coating on the surface of the metal. These are oxidizing agents such as chromates and molybdates. These form protective layer of oxides on the surface and thus prevent corrosion of the metal. Phosphates, borates are non-oxidizing inhibitors. These form insoluble metal phosphates, borates and deposit as protective coating on the surface of the metal. They are used in IC engines, internal surface of pipelines. 3Fe2+ + 2 PO43- Fe3(PO4)2 Cathodic inhibitors: These inhibit cathodic reactions and prevent formation of hydroxides and hydrogen. In acidic medium, the cathodic inhibitors used are; (1) Amines, mercaptans and thiourea etc. They form a protective layer on the surface of the cathode and preventing the diffusion of H+ ions. (2) Compounds of arsenic and antimony: These compounds get reduced to elemental metal on cathode and slow downs liberation of hydrogen because hydrogen over voltage on these metals is very high. In neutral and alkaline medium Sulphates of magnesium, nickel: These react with the hydroxyl ions liberated at the cathode and deposit as insoluble hydroxides on the surface of the cathode. Thus further cathodic reaction is prevented. Inhibition of Oxygen Absorption: Rate of cathodic reaction can be reduced by adding deaerator such as Hydrazine, sodium sulphate. These substances remove oxygen from the corrosion environment by reducing it. N2H4 + O2 N2 + 2H2O 2Na2SO3 + O2 2Na2SO4 -------- 8  RUAS, Bangalore- 560058 Multiple Choice Questions 1. The reaction that takes place during corrosion of a metal is a) Oxidation b) Reduction c) Complex formation d) None of the above 2. Rate of corrosion increases with a) Increase in temperature b) Increase in the conductivity of the medium c) Increase in humidity d) All the above 3. In the presence of CO2, the rate of corrosion a) Increases b) decreases c) Depends on the humidity d) All the above 4. Corrosion of steel screws in brass hinges (exposed to moist air) is an example of a) Differential metal corrosion b) Stress corrosion c) Differential aeration corrosion d) None of the above 5. Corrosion of immersed portion of zinc rod in water is an example of a) Differential metal corrosion b) Stress corrosion c) Differential aeration corrosion d) None of the above 6. Corrosion of steel boiler at high temperature and pressure along the riveted portions is an example of a) Differential metal corrosion b) Stress corrosion c) Differential aeration corrosion d) None of the above 7. Corrosion in barbed wires is due to a) Differential metal corrosion b) Stress corrosion c) Differential aeration corrosion d) None of the above 8. In which of the following situations, corrosion rate is lesser? a) Larger anodic area b) Larger cathodic area c) Higher temperature d) Higher humidity 9. In water line corrosion, maximum corrosion takes place a) along the level of water b) above the level of water c) below the level of water d) at the bottom 10. During electrochemical corrosion, in acidic medium a) hydrogen is evolved b) oxygen is evolved c) oxygen is absorbed d) oxygen is adsorbed 11. According to electrochemical theory of corrosion, for corrosion to take place a) presence of oxygen is sufficient b) presence of hydrogen is sufficient c) presence of both air and moisture is essential d) presence of a strong acid is essential 12. Corrosion protection of steel water tank by connecting it to Mg-block is an example of a) Sacrificial anodic protection b) Sacrificial cathodic protection c) Impressed voltage protection d) Anodizing 13. Galvanizing is an example for a) Cathodic metal coating b) Anodic metal coating c) Sacrificial anode method of corrosion control d) Anodizing 9  RUAS, Bangalore- 560058 14. Which of the following gives chemical conversion coating? a) Anodizing b) Galvanizing c) Electroplating d) Cathodizing 15. Due to polarization, the potential at anode a) Increases b) Decreases c) Remains constant d) Initially increases and then decreases 16. Due to polarization, the potential at cathode a) Increases b) Decreases c) Remains constant d) Initially decreases and then increases 18. Electrode polarization a) Results in increase in rate of corrosion b) Results in decrease in rate of corrosion c) Has no effect on rate of corrosion d) Results in decomposition of electrolyte 19. Formation of protective coating is the principle behind a) Anodizing b) Phosphating c) Both a and b d) Neither a nor b 20. If the hydrogen overvoltage is high, rate of corrosion a) Increases b) Decreases c) Increases initially and then decreases d) Is not affected 21. Anodic protection is applicable to a) Al b) Fe c) both Al and Fe d) neither Al nor Fe Question Bank 1. Define the term corrosion and explain the electrochemical theory of corrosion taking iron as the corroding metal. 2. Discuss (i) Differential aeration corrosion (ii) Galvanic corrosion (iii) Stress corrosion with examples. 3. Discuss the following factors affecting the rate of corrosion: (i) Relative areas of anode and cathode (ii) Temperature (iii) and Polarization at electrodes (iv) Nature of corrosion product 4. Explain the use of (i) Anodic inhibitors & (ii) Cathodic inhibitors for control of corrosion 5. Illustrate the processes, (i) Galvanization & (ii) Tinning 6. Explain (i) Water line corrosion and (ii) Pitting corrosion 7. Explain following corrosion control methods: (i) Cathodic protection (ii) Inhibitors (iii) Metal coatings (iv) Inorganic coatings 8. Discuss the factors influencing the corrosion of metals. 9. Why metals undergo corrosion? 10. What is dry corrosion? Explain dry corrosion mechanism of metals in oxygen. 11. Write notes on (i) Anodizing and (ii) Phosphating 10