Corrosion11 Docx_.docx

CORROSION When metals are exposed to the environment(as dry gas, moisture, liquids etc), the metal surface starts decaying by conversion into more stable metal compounds more or less rapidly. Thus corrosion may be defined as the gradual eating away, disintegration, decaying or deterioration of a metal by chemical or electrochemical reaction with its environment. Or The loss of materials (metals and alloys) or its useful properties, by chemical or electrochemical interaction with its environment is known as corrosion. Example Rusting of iron, formation of green layer on copper surface. The branch of Engineering that deals with the study of corrosion mechanisms and to prevent or control it economically and safely is known as Corrosion Engineering. FACTS ABOUT CORROSION 1. Corrosion is a natural process. 2. Corrosion is an electrochemical process. 3. Corrosion leads to tremendous loss. It can’t be eliminated completely. However, its extent can be minimized. 4. Corrosion is exactly the reverse of extraction of metals and also known as weeping of metals. CAUSE OF CORROSION In nature most metals (except noble metals like Au, Pt etc) are found in combined forms as their oxides, carbonates, sulphides, chlorides etc.. They are reduced to their metallic states by the process of extraction from their ores. During extraction of metals, considerable amount of energy is required. Consequently, isolated pure metals can be regarded in excited state than their excited ores and they have a natural tendency to revet back to combined state(lower energy state). THEORIES OF CORROSION Different theories of corrosion are: 1. Direct Chemical attack theory or Chemical corrosion or dry corrosion 2. Wet or electrochemical corrosion (electrochemical theory) 3. The acid theory 1. DIRECT CHEMICAL ATTACK THEORY OR CHEMICAL CORROSION OR DRY CORROSION Type of corrosion that occurs mainly through direct chemical action of an oxidizing gas e.g. O 2, S, halogens, H2S etc. in absence of a liquid or electrolyte at ambient or elevated temperature, is known as dry corrosion. When a metal is corroded by direct chemical action, an insoluble, soluble or a liquid corrosion product is formed. a. If it is insoluble, it means that a solid film of corrosion product is usually formed on the surface of the metal, which then protects the metal from further corrosion. e.g. Cl2 and I2 attack Ag generating a protective film of AgCl or AgI. b. If it is soluble or is liquid, it goes on dissolving or it is removed as it is being formed. Thus the surface is left exposed for further attack. e.g. During de-tinning of metal cans using dry Cl2 gas at high temperature, the coating of Sn forms volatile SnCl4 and so all the Sn is readily removed from the metal surface. However, the base metal, Fe is very little affected or attacked by dry Cl2 because it reacts with Fe to form solid FeCl3 (non volatile), which protects the rest of the metal. There are 3 main types of chemical corrosion(i) Oxidation Corrosion: It is brought about by the direct action of O2, usually in the absence of moisture. At low T, only alkali and alkaline earth metals are oxidized rapidly, while at high temp, almost all metals (except Ag, Au and Pt) are oxidized. 2M 2Mn+ + 2ne(oxidation) n/2O2 + 2ne nO2(reduction) 2M + n/2O2 2Mn+ + nO2Metal oxide

Mechanism: Oxidation occurs first at the surface of the metal and the resulting metal oxide scale (layer upto 300A thickness is called film, while more than it, is called scale) forms a barrier that tends to restrict further oxidation. For oxidation to continue, a. either the metal ion must diffuse outwards through the scale to the surface b. O2 must diffuse inwards through the scale to the underlying metal.

Both transfers occur but the outward diffusion of metal is generally much more rapid than the inward diffusion of O2, since the metal ion is appreciably smaller than the oxygen ion and consequently, of much higher mobility. Metal + O2 Metal oxide (Corrosion product) Case I: When metal ion diffuses faster outward: In this case oxide layer is formed at the metal oxide – gas or scale – gas interface. Eg. In FeO, CoO, NiO, Cu2O etc.

Case II: When oxygen diffuses inward: In this case oxide layer is formed at the metal- scale interface or metal – metal oxide interface. Eg in ZnO, CdO, TiO2 etc.

When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this film decides the further action. a. Stable film: A stable layer is fine grained in structure and can adhered tightly to the parent metal surface. So, an impervious layer(which cuts off penetration of attacking O2 to the underlying metal) is formed on the metal surface, which acts as protective coating. Consequently, further oxidation corrosion is checked. e.g. oxide films on Al, Sn, Pb, Cu etc. b.Unstable film: The oxide layer formed decomposes back into the metal and O2 Metal oxide Metal + O2

So, oxidation corrosion is not possible in this case. E.g. in Au and Ag. c.Volatile film : The oxide layer volatilizes as soon as it is formed, thereby leaving the underlying metal surface exposed for further attack. This causes rapid and continuous corrosion, leading to excessive corrosion. e.g. Molybdenum oxide (MoO3) is volatile. d. Porous: i.e. having pores or cracks. In such cases, atmospheric O2 have access to the underlying surface of metal through the pores or cracks of the layer, thereby the corrosion continues unobstructed till the entire metal is completely converted into its oxide. Pilling Bedworth Rule: According to that If volume of metal oxide ≥ volume of metal, then oxide layer is protective or non-porous If volume of metal oxide< volume of metal, then oxide layer is porous. e.g. i. Alkali and alkalime earth metals form oxides of volume less than the volume of metal, hence results in the formation of porous film. ii. Al forms an oxide of greater volume than the volume of Al, resulting in the formation of tightly adhering non-porous layer.

(ii)

Corrosion by other gases: Dry gases CO2, SO2, Cl2, H2S and Fe etc also cause corrosion of metals. The extent of corrosion depends upon the chemical affinity between metal and the gas involved and the nature of the film formed on the metal surface (porous or non-porous). If the film formed is protective or non-porous, the extent of attack decreases becauses the film formed protects the metal from further attack. E.g. AgCl layer formed by the attack of Cl2 on Ag. If the film formed is non-protective or porous, the whole metal is gradually destroyed. E.g. (a) dry Cl2 attacks on Sn forming volatile SnCl4 thereby leaving fresh metal surface for further attack. (b) In petroleum industry, H2S at high temperature attacks steel forming FeS scale, which is porous and interferes with the normal operations. (iii) Liquid metal corrosion: Such type of corrosion occurs due to the chemical action of flowing liquid metal at high temperature on solid metal or alloy. This type of corrosion is common in devices used for nuclear power. The corrosion reaction involves one of the following: (a) Dissolution of a solid metal by a liquid metal (b) Internal penetration of the liquid metal in to the solid metal In both the cases, the solid metal becomes weak.

2. WET OR ELECTROCHEMICAL CORROSION (ELECTROCHEMICAL THEORY) The main hypothesis of this theory are: i) Any electrochemical reaction can be divided into at least one oxidation and one reduction reaction. ii) There is no net accumulation of charge during an electrochemical reaction. This theory is based on Nernst theory, according to which all metals have a tendency to pass into solution. This theory explains the indirect corrosion and is also known as immersed corrosion. The tendency of a metal to pass into solution, when immersed in a solution of its own salt is measured in terms of its electrode potential. The standard electrode potentials of various metals have been determined in comparision with the standard electrode potential of hydrogen taken as zero. Electrochemical corrosion occurs in 2 conditions: (i) When a conducting liquid is in contact with metal (ii) When 2 dissimilar metals or alloys are either immersed or dipped partially in a solution. This corrosion occurs due to the existence of separate anodic and cathodic areas between which current flows through the conducting solution. At anode oxidation occurs, while at cathode reduction occurs. The metallic ions (at anodic part) and non-metallic ions (formed at cathodic part) diffuse towards each other through a conducting medium and form a corrosion product somewhere between anode and cathode. Thus, the essential requirements of electrochemical corrosion are(i) Formation of anodic and cathodic areas (ii) Electrical contact between the cathodic and anodic parts to enable the conduction of eMechanism Electrochemical corrosion involves flow of e- current between the cathodic and anodic areas (i)

Anodic reaction: At anode, metal atoms lose their e- and pass into solution

Mn+(aq) + ne(oxidation) Fe(s) Fe2+(aq) + 2e(oxidation) Fe2+(aq) + 2OH-(aq) Fe(OH)2 This continues as long as e- and ions are removed from the environment. If they are not removed, the corrosion will not proceed further. M(s)

(ii) Cathodic reaction: The e- released at the anode are carried to the cathode and are responsible for the following reactions: (a) Electroplating: The metal ions at the cathode collect the e- and deposit on the cathode surface e.g. Cu2+(aq) + 2eCu(s) (b) Liberatiion of H2: In acidic solution (in the absence of O2), reduction of H+ ions take place and H2 is liberated 2H+ + 2eH2 (Such corrosions in which H2 gas is evolved, is called hydrogen type corrosion )

(b) In acidic solution in the presence of O2, H2O is formed at cathode as a by-product O2 + 4H+ + 4e2H2O (c) In neutral or alkaline medium in the presence of O2 O2 + 2H2O + 4e4OH(such type of corrosion involving O2 is called oxygen type corrosion)

(d)

In neutral or alkaline medium in the absence of O2 2H2O + 2eH2 + 2OHe.g. Corrosion of Fe occurs by O2 in the presence of aqueous solution (rusting of iron) At anode Fe Fe2+ + 2eAt cathode 1/2O2 + H2O + 2e2OHNet reaction Fe + 1/2O2 + H2O Fe2+ + 2OH- or Fe(OH)2 Depending upon the avaibility of oxygen, two types of corrosion products are formed: i) In excess supply of oxygen: In excess supply of oxygen, ferrous hydroxide is easily oxidized to ferric hydroxide. 2Fe(OH)2 + H2O + 1/2O2 2Fe(OH)3 Fe2O3.xH2O Yellow rust ii) In limited supply of oxygen: In limited supply of oxygen, black magnetite Fe3O4 or ferroferric oxide is formed. 6Fe(OH)2 Fe2O3.FeO.6H2O Black Differences between wet corrosion and dry corrosion Wet corrosion Dry corrosion 1. It takes place in presence of water or an 1. It takes place in absence of liquid or electrolyte. electrolyte. Gases and vapours are the corrodants. 2. It is an electrochemical attack. 2. It is a chemical attack. 3. It generally takes place at low 3. It takes place at high temperature. temperature. 4. It is also known as low 4. It is also known as high temperature corrosion. temperature corrosion. 5. It is generally fast. 5. It is generally slow. 6. Eg. Rusting of iron in water. 6. Eg. Attack of steel furnace by gases at high temperature. 3. THE ACID THEORY: According to this theory, corrosion occurs in the presence of an acid (usually H2CO3). This theory is particurly applicable to rusting of iron in the atmosphere. Rusting of iron is due to continued action of O2, CO2 and moisture, converting metal into a soluble ferrous bicarbonate which is further oxidized to basic ferric carbonate and finally to hydrated ferric oxide 2Fe + O2 + 4CO2 + 2H2O 2Fe(HCO3)2 2Fe(HCO3)2 + H2O + [O] 2Fe(OH)CO3 + 2CO2 + 2H2O 2Fe(OH)CO3 + 2H2O 2Fe(OH)3 + 2CO2 This theory is supported by the following facts: (i) Rust analysis generally shows the presence of ferrous and ferric carbonates along with hydrated ferric oxide. (ii) Retardation of rusting in presence of added lime or NaOH to the water in which Fe is immersed. ECONOMIC ASPECTS OF CORROSION Enormous losses occur due to corrosion every year in all countries of the world. In developed countries it amounts to about 3.5% of the GNP i.e. Gross National Product. Loss due to corrosion may be discussed under the following headings: i) Direct loss: It includes a) Repair or replacement of the corroded component or equipment. b) Over-design to allow for corrosion. c) Cost involved in anticorrosive painting and other protective methods. ii) Indirect loss: It may include a) Plant shut down b) Loss of production c) Explosions and possible loss of life d) Contamination of products. The cost of replacement of materials and equipments, lost through corrosion has been currently estimated to be 9 billion dollars per year on account of replacement of pipes, building products, autoparts, water heaters and other materials subject to corrosion. In addition, the life of equipment, plant, bridge etc is very much reduced. Infact, it is very difficult to access

exactly the losses incurred due to corrosion, because these losses can not be measured in terms of the cost of metal alone but the high cost of fabrication into the machine tools / structures / equipments should also be taken into account. The losses due to corrosion varies from country to country, depending upon the climatic conditions prevailing in the country. In India, the corrosion problems are more serious than cold countries because of its tropical climate. According to a rough estimate, direct loss due to corrosion in India are estimated to be Rs 250 crore per year. The money spent on its prevention is about 50-70 crores per year. The indirect loss due to corrosion can not be estimated in terms of money. Taking all the factors into consideration, it is therefore extremely important to know the mechanism of corrosion, so that it can be minimized to a greater extent and the heavy losses incurred due to corrosion may be greatly reduced. TYPES OF CORROSION [I] Galvanic Corrosion When two dissimilar metals are in electrical as well as in electrolytic contact with each other, the more reactive metal undergoes corrosion. This type of corrosion is known as galvanic corrosion. This is also known as bimetallic corrosion. The more reactive metal acts as anode and it undergoes corrosion while the less reactive metal acts as cathode and is protected. In case of galvanic corrosion more reactive metal corrodes much faster as compared to the situation when it is not coupled. E.g. Zinc and copper couple More reactive metal Zn (Corrodes) Less reactive metal Cu 2+ + 2e (protected)

Zn 2+ + 2e Cu

Oxidation Reduction

At anode At Cathode

Factors affecting galvanic corrosion: i) Potential difference between the two metals coupled: The greater the difference between the potential of two metals coupled, the greater is the corrosi on current and the greater is the rate of corrosion. ii) Relative area of cathode and anode: Small anodic area and large cathodic area gives rise to an intense corrosion. e.g. a) Steel pipe connected to copper plumbing. b) Steel screw in brass marine hardware Prevention: a) Use insulating gaskets between the two metals b) If the metals have to be coupled try to select the metals which are close to one another in electrochemical series. DIFFERENCES BETWEEN ELECTROCHEMICAL SERIES AND GALVANIC SERIES Electrochemical series Galvanic series 1. The arrangement of metals and 1. The arrangement of metals and alloys non-metals in increasing order of in decreasing order of their their standard reduction potential is corroding tendency in an unpolluted

known as electrochemical series. 2. It contains metals and non-metals 3. It is an ideal series 4. ECS is based upon the electrode potential which is determined by using Nernst equation 5. Position of metals is fixed in ECS 6. It gives no idea about the position of alloys 7. It gives information about the relative displacement tendencies

sea water wit is known as galvanic series. 2. It contains metals and alloys. 3. It is a practical series 4. This series is based on actual corrosion rate 5. Position of a given metal in Galvanic series may change 6. It gives clear idea about the position of alloys 7. It gives information about the relative corrosion tendencies

[II] Erosion Corrosion It is caused by the combined effect of the abrading action of turbulent flow of gases, vapour and liquids and the mechanical rubbing action of solids over a metal surface. The major cause for this corrosion is the removal of protective surface film. Erosion corrosion can be minimized by using harder metals and design changes to avoid excess of friction and using proper lubrication [III] Crevice Corrosion It is a local corrosion and is usually created by dirt deposits, corrosion products, crack in paint coatings etc. Selection of resistant materials, proper design to minimize crevice and maintaining clean surfaces are the measures taken to control crevice corrosion. [IV] Pitting Corrosion Pitting corrosion is, usually due to the breakdown or cracking of the protective film on a metal at specific points.This gives rise to the formation of small anodic area (at the breakdown point) and large cathodic area (rest of the part on the metal surface). e.g. metals like stainless steel and aluminium, which are normally protected by a thin oxide film, are subjected to pitting corrosion in a chloride environment, thus making it unsuitable for use in a sea water system.

Presence of other external impurities like sand, dust, scale etc. on the surface of metal can also be a cause of this type

of corrosion. In this case, the small part below the impurity function as the anodic area while the rest of the part on the metal function as the cathodic area. Once a small pitis formed, then the rate of corrosion increases leading to more and more corrosive damage. Or Pitting corrosion is a form of extremely localized attack that results in the formation of holes in the metals. Thes e holes may be large or small in diameter but in most cases they are relatively small. Pits are sometimes isolated ar so close together that they look like a rough surface. This is localized form of corrosion because the rate of attack being greater at some areas than others. A pit is a narrow hole whose diameter is much smaller than the depth or length. Pitting corrosion is the most dangerous form of corrosion because it leads to the sudden failure of material due to formation of hole Facts about pitting corrosion i) Pitting corrosion is autocatalytic, self stimulating and self propagating. ii) Stagnant conditions leads to pitting corrosion. iii) It takes place exclusively in chloride and chloride containing environment.

Causes and mechanism Pitting corrosion is the result of breakdown or cracking of the protective film on the metal at specific points. The breakdown of protective film may be caused by i) Surface roughness or non-uniform finish ii) Scratches or cut edges iii) Local straining of metal due to non-uniform stresses. iv) Sliding under load v) Impingement attack vi) Chemical attack Because of breakage of protective film, small anodic area is formed leading to the rapid dissolution of metal within the pit. Therefore within the pit, we have high positive charge density and in order to maintain the electrical neutrality chloride ion migrates into the pit resulting in the formation of metal chloride (MCl). Most of the metal chlorides undergo hydrolysis to form metal hydroxide and HCl. Thus within the pit we have very high concentration of H+ and Cl-. Both H+ and Cl- stimulate the dissolution of metal and local dissolution of metal takes place resulting in the formation of hole, M M+ + e+ M + Cl MCl MCl + H2O MOH + HCl HCl H+ + ClOnce this process starts, it is autocatalytic, self propagating and self accelerating

Prevention i) Design for complete drainage ii) Preventing stagnant conditions iii) Add molybdenum to ordinary stainless steel for better pitting resistance iv) Periodic inspection of the structure and cleaning the deposits frequently. [IV] Differential aeration Corrosion (Oxygen Concentration Cell Corrosion) It occurs when one part of the metal is exposed to a different air/O 2 concentration from the rest of the part. The portion of the metal surface exposed to less oxygen acts as an anodic area and gets corroded, while the more oxygenated part of like cathodic area and thus is protected. e.g. A iron nail inside the wood undergoes corrosion easily. [V] Waterline Corrosion Waterline corrosion results from differential aeration leading to the formation of oxygen concentration cells. It has generally been observed that maximum corrosion takes place in a steel tank containing water along a line, just beneath the level of water, because access of oxygen is much less there. The area above the waterline is highly oxygenated and hence acts as cathodic area.(O2 is having the tendency to undergo reduction, so area with more O2 acts as cathode). Consequently, it is not corroded. However, little corrosion takes place when water is relatively free from acidity.

e.g. Iron corrodes under drops of water (or salt solution). Areas covered by droplets, having no access of O2, becomes anodic with respect to other areas, which are freely exposed to O2. Waterline corrosion is also caused in marine ships and is accelerated by marine plants which are attached to the sides of the ships. This type of corrosion is prevented to a great extent by painting the sides of the ships by special anti-fauling paints.

[VI] Micro-Biological Corrosion Corrosion caused by the metabolic activity of various micro-organisms, is called microbiological corrosion.It occurs because such activities can i) Produce a corrosive environment ii) Create electrolytic concentration cells on the metal surface iii) Alter the resistance of metal films iv) Can affect the rate of cathodic or anodic reaction The micro-organisms can develop in an environment with or without oxygen and are classified aerobic or anaerobic. Anaerobic bacteria such as microspira or vibrio desulfuricans reduce sulphates to sulphur which is used to prepare their protoplasm. When these organisms die, sulphur is liberated as H2S which cause sulfide stress cracking. In the presence of O 2 , some bacteria directly oxidize Fe to iron oxides and hydroxides (corrosion). [VII] Stress-Corrosion Cracking In this, the metal under stress becomes more anodic and tend to increase the rate of corrosion. For stress corrosion to occur presence of tensile stress and a specific corrosive environment are necessary.The stress can be due to non uniform deformation by unequal cooling from high temperature as in weldingand by internal structure rearrangements

involving volume changes. This type of corrosion involves in a localized electrochemical corrosion, occurring along narrow paths, forming anodic areas with respect to the more cathodic areas at the metal surface. Presence of stress produces strains, which result in localized zones of higher electrode potential. These become so chemically active that they are attacked, even by a mild corrosive environment, resulting in the formation of a crack. FACTORS AFFECTING CHEMICAL CORROSION [I] Nature of the Metal (i) Oxidation potential : The extent of corrosion depends upon the position of the , metal in the electrochemical series and galvanic series. When two metals are in electrical 'i contact in presence of an electrolyte, the metal higher up in the galvanic series becomes anodic and suffers corrosion. Further, the more the two metals are apart in the galvanic '' series the greater will be the corrosion of the anodic metal. (ii) Relative areas of anode and cathode : When two dissimilar metals or alloys are in contact, the corrosion of the anodic part is directly proportional to the ratio of areas ; of the cathodic part and the anodic part. Thus corrosion becomes severe and rapid when the anodic area is small and cathodic area is large. Small anodic area and large cathodic area gives rise to an intense localized corrosion. This is because the demand for electrons by large cathodic areas (reduction, gain of electrons) can be met by smaller anodic areas (oxidation, loss of electrons) by undergoing corrosion more rapidly. e.g. Steel rivet in Cu sheet. Steel is more anodic and has smaller area. Therefore steel undergoes corrosion more briskly (iii) Purity of metal : Impurities in a metal, generally, cause heterogeneity and form minute/tiny electrochemical cells (at the exposed parts) and the anodic parts get corroded. For example, the impurities such as Pb, Fe or C in zinc lead to the 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. The rate of corrosion increases with the increasing exposure of the impurities. (iv) Physical state of the metal : The rate of corrosion is influenced by the physical state of metal. The smaller the grain size of the metal or alloy, the greater will be its solubility and hence, greater will be its corrosion. Moreover, areas under stress, even in a pure metal, tend to be anodic and corrosion takes place at these areas. (v) Nature of surface film : In aerated atmosphere, practically all metals get covered with a thin surface film of metal oxide having a thickness of a few angstroms. The film may contain one or more forms of the metal oxide and its thickness depends upon the nature of the metal and temperature. The ratio of the volumes of the metal oxide to the metal, is known as a specific volume ratio. Greater the specific volume ratio, lesser is the oxidation corrosion rate:, For example, the specific volume ratios for W, Cr and Ni are 3•6, 2.0 and 1.6, respectively, which indicates that the rate of oxidation at elevated temperatures is least for W (vi) Passive character of metal : Metals like Tl, Al, Cr, Mg, Ni and Co are passive and they show much higher corrosion resistance than expected from their positions in galvanic series, due to the formation of highly protective, but very thin film (of oxide) on the metal or alloy surface. Moreover, the film is of such a self-healing nature, that if broken, it repairs itself, on re-exposure to oxidising conditions. Thus, corrosion resistance of stainless steel is due to the passivating character of chromium present in it. (vii) Solubility of corrosion pro ducts : If the corrosion product is soluble in the corroding medium, then corrosion proceeds at a faster rate. On the contrary, if the corrosion product is insoluble in the medium or it interacts with the medium to form another insoluble product (e.g., PbSO4 formation in case of Pb in H2SO4 medium), then the corrosion product acts as a physical barrier, thereby suppressing further corrosion. (viii) Volatility of corrosion products : If the corrosion product is volatile, it volatilizes as soon as it is formed, thereby Leaving the underlying metal surface exposed for further attack. This causes rapid and continuous corrosion, leading to excessive corrosion. For example, molybdenum oxide (MoO3), which is the oxidation corrosion product of Mo, is volatile. [II] Nature of Environment (i) Temperature : The rate of chemical reactions and the rate of diffusion increase with temperature, hence, corrosion increases with temperature. (ii) Presence of moisture : Humidity of air is the deciding factor in atmospheric corrosion. Critical humidity is defined as the relative humidity above which the atmospheric corrosion rate of metal increases sharply. The value of critical humidity depends on the physical characteristics of the metal as well as nature of the corrosion products. The reason why corrosion of a metal becomes faster in humid atmosphere is that gases like CO 2, O2 etc. and vapours, present in atmosphere furnish water to the electrolyte, which is essential for setting up an electrochemical corrosion cell. (iii) Presence of impurities in atmosphere : Atmosphere, in the vicinity of industrial areas, contains corrosive gases like CO2 , H2S, SO 2 and fumes of HC1, H2SO4, etc. In presence of these gases, the acidity of the liquid adjacent to the metal surfaces increases and its electrical conductivity also increases, thereby corrosion is increased.

(iv) Effect of pH : The hydrogen ion concentration of the medium is another important factor in corrosion reactions as well as corrosion control. Acidic media are generally more corrosive than alkaline and neutral media. However, amphoteric metals (like Al, Zn, Pb etc.) dissolve in alkaline solution as complex ions. The corrosion rate of iron in oxygen-free water is slow, until the pH is below 5.0. The corresponding corrosion rate in presence of oxygen is much higher. Consequently, corrosion of metals, readily attacked by acid, can be reduced by increasing the pH of the attacking environment, e.g., Zn (which is rapidly corroded, even in weakly acidic solutions such as carbonic acid) suffers minimum corrosion at pH = 11. (v) Nature of ions present : Chloride ions present in the medium destroy the passive film and corrode many metals and alloys. On the other hand, the presence of anions like silicates in the medium leads to the formation of insoluble reaction products, e.g., silica gel, which inhibits further corrosion. (vi) Concentration of oxygen and formation of oxygen concentration cell : The rate of corrosion increases with increasing supply of oxygen. Less oxygen concentration, e.g., oxide-coated part or lessexposed parts become anodic, while the more oxygenated regions or parts more exposed to oxygen become cathodic. This leads to the formation of oxygen-concentration cell in which the anodic part suffers corrosion. At cathode : 2H 2O +O2 + 4e4OH(Electrons are consumed) Moist air At anode : Fe Fe 2+ + 2e(Electrons are supplied) Thus, oxidation concentration cell promotes corrosion, but it occurs where the oxygen concentration is lower. CORROSIVE AGENTS Though there are a large number of corrosive agents which are capable of causing corrosion, but air is possibly the most active and the most prevalent. O2, N2, corrosive gases such as SO2, SO3, H2S, chlorides, oxides etc. present in air are responsible for the corrosion action. Sometimes these gases are not corrosive themselves, but in the presence of moisture, they form solution of corrosive nature. PROTECTION.FROM CORROSION OR CORROSION CONTROL [I] Design and Material Selection The design of the material should be such that corrosion even if it occurs, is uniform and does not result in intense and localized corrosion. Important design principles are :

(1) The contact of different metals in the presence of a corroding solution should be avoided. In the absence of this principle,

corrosion is localized on the more active metal, while the less active metal remains protected. (2) If an active metal is used, it should be insulated from more cathodic metals. (3) If two metals are to be in contact, they should be so selected that their oxidation potentials are as near as possible. (4) When two dissimilar metals are to be in contact, the anodic material should have as large area as possible and the cathodic material should have as small area as possible. (5) When contact of dissimilar metals is unavoidable, suitable insulator should be inserted between them to reduce current flow and attack on the anode.

(6) If two metals have to be in contact, a protective coating over both the metals will reduce the chance of pitting. Special care should be taken to coat the anode completely because any scratch or crack on the anode coating can lead to intense attack at the exposed area. (7) When a structure consists of two dissimilar metals, it is beneficial to use a more active third metal in contact so that the structure is saved from corrosion at the expense of' the third metal. (8) The design should allow for adequate cleaning and flushing of the critical parts (i.e. susceptible to dirt, deposition etc.) of the equipment, sharp corners and recesses should be avoided, because they favour the formation of stagnant areas and accumulation of solids.

(9) Impurity in a metal causes heterogeneity, which decreases corrosion resistance of the metal. Thus the corrosion resistance of a given metal can be improved by increasing its purity. Pure metals such as Al, Mg provides a coherent and impervious protective oxide film on their surfaces, which prevents those from corrosion. (10) Uniform flow of a corrosive liquid is desirable, since both stagnant areas and highly turbulent flow and high velocities can cause accelerated corrosion. So, highly impingement conditions of flowing liquid should be avoided as much as practically possible.

(11) Noble metals are more immune to corrosion but they cannot be used for general purposes for economical reasons. The next choice is to use the purest possible metal. Even minute amount of some impurity may lead to severe corrosion. For example, minute quantities of iron in magnesium or lead in zinc die casting alloys may be highly

deterimcntal. (12) Both corrosions resistance and strength of many metals can be improved by alloying. Several corrosionresistance alloys have been developed for specific purposes and environrnent. For example, stainless steel containing chromium produces a layer of Cr2O3 which protects the steel from further attack, Al, Be, Mg etc. are added to Cu to improve its oxidation resistance. (13) For maximum corrosion resistance, alloy should be completely homogenous. [II] Cathodic Protection The basic principle involved is to force the metal/structure to be protected to behave as or function as cathode. If e- enters through the external circuit to the structure then it is protected or if current enters to the structure from electrolyte then the structure is protected. It is effected by either of the following ways: (i) By appropriate galvanic coupling (ii) By impressed current (i)

By appropriate galvanic coupling: The structure which is to be protected is connected with a more active metal by a wire. The more active metal undergoes corrosion as it acts as an anode while the structure is protected because it acts as a cathode. This type of anode is known as sacrificial anode. Mg, Zn, Al and their alloys are generally used as sacrificial anode. This type of protection is used for underground pipe lines, ship hulls, boat hulls, water tanks. The sacrificial anode block is replaced by a fresh one, when consumed completely.

(ii)

By impressed current: In this method, an impressed current is applied in opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode.Usually the impressed current is derived from a direct current source. Impressed current cathodic protection is useful for large structures for long term operations,

This method has the following advantages over sacrificial cathodic protection: (i) It is controlled from outside. (ii) No anode has to be replaced. This method is used for the protection electrical cables, transmission towers, condensers etc. Anodic Protection If the potential of the metal is so adjusted that the corrosion is appreciably suppressed because the metal is rendered passive, then it is called anodic protection. In anodic protection, metal is passivated by applying current in a direction that renders it more anodic. [III] Modifying the Environment The corrosive nature of the environment can be reduced either by. (i) the removal of harmful constituents or (ii) the addition of specific substances, which neutralize the effect of corrosivc constituents of the environment. (1) Deaeration : In oxygen concentration type of corrosion, exclusion of oxygen from aqueous environment reduces metal corrosion. Expulsion of dissolved oxygen is done by adjustment of temperature together with mechanical agitation. The method also reduces the CO2 content of water, thereby decreasing the corrosion rate of steel pipelines carrying steam condensates from boilers. (2) Deactivation : It involves the addition of chemicals, capable of combining rapidly with the oxygen in aqueous solution, e.g., sodium sulphite. 2Na2SO 3 + O2 2Na2SO4 (3) Dehumidification : It reduces the moisture content of air to such an extent that the amount of water condensed on metal is too small to cause corrosion. Alumina or silica gel which absorbs moisture referentially on their surfaces, are used only in closed areas like air-conditioning shop. (4) Alkaline neutralization : It consists of preventing the corrosion by neutralizing the acidic character of corrosive environment (due to the presence of HCI, H 2 S, SO2 , CO2 etc.). Such alkaline neutralizers (like NH3, NaOH, lime, etc.) are generally injected either in vapour or liquid form to the corroding system or to its parts. This method has been widely used in controlling the corrosion of refinery equipments. (5) Use of inhibitors: Substances which are added from outside to reduce the rate of corrosion are known as inhibitors. Inhibitors reduce the rate of corrosion either (i) By forming a layer in between which acts as a barrier between the material and environment. (ii) Or by retarding the anodic or cathodic or both processes

Cathodic inhibitors: theyretard the rate of cathodic reaction and hence the overall rate of corrosion. In acidic medium the cathodic reaction taking place is 2H+ + 2eH2 Reduction At cathode Organic inhibitors like amine, mercaptans, substituted ureas having lone pair of e- reduce the rate of cathodic reaction, considerably by reacting with H +. Anodic inhibitors: They generally form a layer in between material and environment.e.g. chromates, phosphates, tungstates etc. [IV] Metallic Coatings Metallic coatings are mostly applied on iron and steel because they are the cheap and most commonly a.sed construction materials and are also the most susceptible ones for corrosion. The metallic coatings often used are of Zn, Sn, Ni, Cu, Cr, A1 and Pb. Generally, the following methods are used for metallic coatings. (1) Electroplating : Noble metals such as Au, Ag, W, Pb etc. protect the base metal by virtue of their noble character. Tin plating and nickel plating are generally used. The electroplating of zinc is called galvanising. In electroplating, the object to be plated is made the cathode of the cell. The electrolyte is a salt of the metal to be deposited. The anode may be of the metal to be deposited or it may be an inert electrode such as graphite. When the anode is that of the metal to be. deposited, the anode dissolves to replenish the metal in the solution. Using this method metals like Au, Ag, Cr, Ni, Cu, Zn, Sn etc. may be electroplated. (2) Hot dipping : Hot dipping is used for coating metals with films of metals having low melting point such as Zn, Su, Pb etc. In this process, the metal to be coated is dipped in the molten bath of the coating metal for sufficient time and then removed out along with the adhering film. The process of providing a zinc coating is called galvanising and the one providing a tin coating is called tinning. (3) Vapourising : Some metals can also be deposited as surface layers by allowing their vapours to strike metallic surfaces with which they undergo alloying. Zn and Al can be plated by vapourisation. The deposition of zinc by vapourisaion is known as sheradising or dry galvanising and deposition of Al is called calorising. (4) Metal spraying : In this method, the molten metal is sprayed on the cleaned base metal with the help of a spraying gun or pistol which can be held in hand to direct the molten metal stream as required. This process is used only when hot dipping is not possible. Metal spraying is utilized for huge structures such as bridges. (5) Cementation : In this method, the base metal articles are packed in the powdered coating metal, or a mixture of the powdered metal and a filler and are heated to a temperature just below the melting Point of the more fusible metal. Generally, an inert or reducing atmosphere is usually maintained during Waa-process. This method is used for producing alloy layer on iron and steel surfaces with Zn, At, Cr, Si etc. Steel may be case-hardened by cementation with carbonaceous materials in the pack carburizing Process. (6) Metal cladding : Many processes for cladding a base metal with another metal or alloy have been developed recently to resist corrosion and wear resistance. In one of the methods, a duplex ingot is cast with the coating material on the outside and subsequently the ingot is rolled into a plate, sheet or bar or drawn into a wire form. Steel sheets clad with stainless steel, copper covered steel articles and tin-cladded lead foils are prepared by this method. Other methods of cladding include : (a) Rolling the clean sheets or plates of the two material together (b) Applying the coating sheet by spot welding or resistance welding (c) Fusing the cladding material over the surface of the base metal. Metallic coating are of two types: a) Sacrificial coating b) Noble coating

Sacrificial coating 1. Base metal is coated with a metal

Noble coating 1. Base metal is coated with a metal

which is more reactive than the base metal. 2. Protects the underlying base metal sacrificially. 3. This is known as anodic coating as the reduction potential of coating metal is less than that of the base metal.

2.

3.

4. 4. Zn, Cd, Al are generally used as sacrificial coating 5. E.g. Galvanised iron i.e. coating of Zn on Fe.

5.

which is more noble than the base metal. Protects the underlying base metal due to tis noble character and higher corrosion resistance. This is known as cathodic coating as the reduction potential of coating metal is more than that of the base metal Ni, Ag, Cr, Pb, Au etc. are generally used as noble coating E.g. coating of Sn on Fe

[V] Inorganic Non-Metallic Coatings The inorganic non-metallic protective coating includes surface conversion or chemical dip coating, anodized oxide coating and vitreous enamel coating. (1) Chemical dip coating or surface conversion : These coatings arc produced by covering the surface of a metal or alloy by chemical or electrochemical methods. The metal is immersed in a solution of a suitable chemical which reacts with the metal surface producing an adherent coating. These coatings afford good protection of the base metal from corrosion in some environments and sometimes, are of decorative value. The most commonly used surface conversion coatings are chromate coatings, phosphate coatings oxide coatings. (2) Anodized oxide coatings : Protective oxide films are produced on A1 and its alloys in air spontaneously. A more protective, thicker and stronger oxide film can be produced by making Al as the anode in an electrolytic bath containing chromic acid or oxalic acid or sulphuric acid. After anodizing, the oxide coating is sealed by immersing in boiling water. This treatment decreases the porosity and increases corrosion resistance of the film. [VI] Organic Coatings Protection of a metal surface from corrosion by using organic protective coatings is an established practice. Important organic protective coatings include paints, varnishes, enamels and lacquers. When applied on cleaned metal surfaces, they act as effective inert barriers which protect the metal from corrosion.