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CORROSION, , Syllabus, Concept, , of metallic, , corrosion, Types of corrosion and, electro-chemical theory, factors affecting the, of corrosion,, Oxide film formation, corrosion rate,, rusting, Prevention of corrosion, and, and its, characteristics, tarnishing,Chemical, by various methods., fogging and, , 11.1. Concept of Metallic, Corrosion, With the, , exception, , aluminium, magnesium,, , of noble metals, like gold,, platinum, etc., most metals such, zinc, etc. occur in nature, in the, , as iron,, copper,, carbonates, chlorídes and silicates. When, combined forms as their, oxides,, metals, are, sulphides,, into, the, put, result, exposed surface of the metal, use, they are, exposed to environment. As a, present in the surroundings. Thus, the begins to decay due to its contact with gases, moisture, and liquids, destruction of metal, attack by the environment or, by an electrochemical attack. starts at the surface due to direct chemical, Metallic corrosion, , The process of destruction, , unintentional chemical, , called corrosion., , or, , may be defined, , or, , deterioration, , electrochemical, , of the, , attack, , as, , under, , surface of, , a metal or, alloy due, by environmental gases and moisture to, is, , Examples : Rusting of iron and formation of green, layer on copper., Destruction of metal starts at its surface., Causes of metallic corrosion-Corrosion takes, place due to the following, , conditions, (a) The destruction of metallic surface takes, when it comes in contact with, place, gases such as oxygen,. carton dioxide,, sulphur dioxide, hyarogen sulphide and moisture. atmospheric, (b) The corrosion of surface of the metal starts when it forms, electro-chemical cell in contact with, moist air, water or, any other liquid., Harmful effects of corrosion-Corrosion is a very harmful chemical, process., () When iron is exposed to the atmospheric conditions, a, of, layer reddish, of iron, , (Fe O=Fe0 +Fe03), , iron becomes weak., , is fomred, , at its, , surface. In this way,, , scale of magnetic oxide, rusting of iron takes place and, , (n) When copper is exposed to moist air containing carbon dioxide, a green film of basic, carbonate, ICuCO +Cu(OH)%] is formed on its surface. This tarmishes the colour of copper metal., 1) The metal becomes impure due to corrosion and its utility is decreased., (IV) The value of the metal is reduced due to corrosion., , () The metals used in machines do not remain suitable for use due to corosion., , :

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232, , Applied Chemistry, , 112. Types or Kinds of Corrosion, There are two types of corrosion, 1. Atmospheric corrosion, , 2. Immersed corrosion, , 1. Atmospheric corrosion-It is called dry or chemical corrosion. This corrosion is caused due, to the chemical reaction when the surface of the metal comes in contact with atmospheric gases ike O2,, , SO2, H2S, halogens or anhydrous inorganic liquids. It includes oxidation corrosion by oxygen, corrosion, , by other gases such as SO2, HS, CO2, Cl2, etc. and liquid metal corrosion., 2. Immersed corrosion-It is also called wet or electro-chemical corrosion. This corrosion is, caused when the metal is immersed completely or partially into water or any other liquid. It is due to the, formation of electro-chemical cell when the metal comes in contact with moist air, water or any other, concentration cell corrosion, water line corrosion, This includes biometallic or, , galvanic corrosion,, , liquid., , and underground or soil corrosion., , 113. Factors affecting the Corrosion Rate, well, , as, , electrically in presence of, , an, , include nature of the metal, The factors which influence the rate and extent of corrosion, , as, , nature of the corroding environment., , 1. Nature of metal:, metals are in, ) Position in gulvanic serics-When two, the galvanic series, undergoes more corrosion., electrolyte, the more active metal, which is higher up in, two different metals are in contact, the, (i) Relative areas of anodic and cathodic parts-When, ratio of the areas of cathodic and anodic parts. In, corrosion of anodic part is directly proportional to the, contact, , case, , the, , and highly localised., of anode is smal, the corrosion is severe, more rapid, increases rate and extent of corrosion. It is, Purity of Metal-Increasing impurity in a metal, form tiny electro-chemical cells at the exposed parts and the, in a metal, , area, , (ii), , generally, because impurities, anodic part undergoes corrosion., (iv) Physical, , state of metal-The smaller the, , grain, , size of the metal, the greater will be its, , solubility, , as well as corrosion., , Passive eharacter of metal-Passive metals like Al, Cr, Ni and Co exhibit much higher corroson, resistance. Hence, the corrosion resistance of stainless steel is due to the presence of chromium in it., 2. Nature of the corroding environment:, ()Temperature-Due to rise in temperature ofenvironment, the rate of reaction and hence, rate, of corrosion generally increases., , () Humidity in air-Humid atmosphere enchances the rate of corrosion., , Gi) Presence of impurities in atmosphere-The corrosion is speeded up in the presence of, inpurities in atmosphere. Thus, the corrosive gases like CO2, SO2, H,S and fumes of HCI, HsO4, etc. in, the vicinity of industrial areas cause more corrosion., (iv) Nature of ions present-In presence of chloride ions, the protective and passive surface film, , of the metal is destroyed and the exposed surface undergoes corrosion easily. Anions like silicate form, insoluble products which inhibit corrosion., () Influence of pH-Acidic medium having pH less than 7 causes more, corrosion than neutra, medium with pH 7 and alkaline medium having pH more than 7., The corrosion of metals, which arc, attacked by acid, can be reduced by increasing the pH of the, =, , attacking environment.

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233, , Corrosion, , 114. Galvanic Series, , Anodic, , An, , arrangement of metals and alloys with their reduction electrode, potential down in an increasing order is called electro-chemical series or, galvanic series. A metal high in series is more anodic and it will undergo, , (Easily Corroded), , 1. Mg, , 2. Zn, , corrosion faster than a metal below it. Therefore, galvanic series gives useful, information to study corrosion of metals and alloys. The galvanic series or, some metals and alloys is given in table 11.., Thus, Mg corrodes faster than Zn and Zn corrodes faster than Fe., , 3. Al, 4. Cd, , 5. Al alloys, , Copper corrodes faster than Ag., , 6. Mild steel, , Table 11.1. Galvanic series, Distinction between Electro-chemical series and Galvanic series, , Electro-chemical series, , 7. Cast iron, , 8. Pb, 9. Sn, 10. Brass, 11. Cu, , Galvanic series, , 1. This series comprises of metals and . This series is related to metals and, , non-metals, , alloys., , 12. Ni, , 2. It predicts the relative displacement 2. It predicts the relative corrosion, tendencies., 3. This series was developed by |3., dipping pure metals in their salt, solution of 1 M concentration., 4. The position of a given metal in it 4., is permanently fixed., , 13. Ag, 14. Au Cathodic, , tendencies., This series was developed by, studying corrosion of metals and, alloys in unpolluted sea-water., , The position of a metal, when in, the form of alloy, is diferent from, that of the pure metal., , 15. Pt (less feactive), Noble metals, , Table 11-1. Galvanic series, , 5. It gives no information regarding 5. This helps in the study ofcorrosion, of metals as well as alloys., , position of alloys., , 115. Theories of Corrosion, There are two main theories of corrosion, 2. Electro-chemical Theory, , 1. Direct Chemical Theory, , 1. Direct Chemical Theory, According to chemical theory, the surface of the metal comes in direct contact of atmospheric gases, , like oxygen, carbon diovide, hydrogen sulphide, sulphur diovide, halogens, etc. and a chemical reaction, ccurs, Consequently, a thin layer of metalic oxide, carbonate, sulphide and hydroxide is formed on the, surface of the metal. This layer formed is compact and stable. Due to the formation of this layer, the, contact between metallic surface and the atmospheric gases disconnects and corrasion of metal stops., Chemical corrosion is of three types, , Oxidation corrosion-It is due to the direct action of oxygen on metals at low or high, , temperatures usualy in the absence of moisture. Metals are generally attacked very slowly at ordinary, temperature. Alkali metals (Li, Na, K, etc.) and alkaline earth metals (Be, Ca, St, ete.) are oxidised rapidly, at low temperatures. All other metals except Ag, Au and gold are oxidised at high temperatures., The chemical reactions that take place during oxidation corrosion are as under, , 2M, , 2M" +2ne, , Metal, , Metal ion, , n12 O +2ne, , >n O (oxide ion), , loss of electrons), gain of electrons)

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234, , Applied Chemistry, , 2M"t+ n0, , 2M +nl2 O, , Metal ion, , Oxideion, , Metal oxide is formed, , combine with the, form the protective or non-protective films on the surface of the metal to cause corrosion., , i), , Corrosion, , by other gases-Gases like CO, SO, HaS, Clh, F,, , high temperature, , to form ferrous, , H2S, , attacks steel at, , H2S, , attacks the surface of silver vessel in open air, , sulphide,, , etc., , metal to, , FeS scale., , of atmosphere, , to form, , silver, , sulphide, Ag2S., , As a result, the surface of silver becomes black., metal, , metal on solid, to chemical action of flowing liquid, due, caused, is, corrosion-It, metal, Liquid, in devices used for nuclear power., at high temperatures. This type of corrosion takes place, , iii), alloy, 2. Electro-chemical Thecory, This theory was put forward by Nernst, , or, , corrosion takes place due, in 1889. According to this theory,, conducting solution. This, cathodic areas in the, , the anodic and, dissimilar, to the flow of electron-current between, metal or when two, is in contact with the, liquid, electro-chemical, a, where, conducting, mechanism of, type of corrosion takes place, in a solution. The, metals are immersed, , completely, , or, , dipped partially, , corrosion can be as under, , always takes place, , at anodic, , place., compound like, , liberated and oxidation takes, , At anodic area, free electrons, dissolution of the metal, destroyed due to, are, , or formation, , of, , a, , metal is, In this way, anodic, corrosion, oxide. Therefore,, , areas., , At anode:, M, Metal, , M", , Oxidation), , M " t + ne, Metal on, , dissolves in solution, , Metal ion, , forms, , compound such, , as, , oxide, , the anode. Hence, at, due to gain of electrons released at, At cathodic area, reduction takes place, the electrons to form ions like, constituents in the conducting medium gain, dissolved, cathodic part,, OH., oxide, O2and hydroxide,, electro-chemical corrosion involves the following:, , To sum, , up,, in contact with each other., The formation of anodic and cathodic areas, The presence ofa conducting mediunm., Corrosion takes place at anodic areas only., Corrosion product is formed between anodic and cathodic areas., , (i), (ii), (iii), (iv)In cathodic area, the electrons are consumed to evolve hydrogen or by absorption of oxygen, environment., upon the nature of corrosive, anode, For example : (i) Iron undergoes corrosion in acidic medium due to flow of electrons from, , depending, , to cathode through the metal and Ht ions of acidic solution liberate hydrogen gas., > Fe2t + 2e, Fe, At anodic area:, H2 T, 2Ht+2e, At cathodic area:, , Oxidation), (Reduction), , Fe +2Ht, Fe2t + H2 T, is:, All metals above hydrogen in electro-chemical series have a tendency to dissolve in acidic solution, with the evolution of H2 gas., , Theoverall reaction, , (i) Rusting of iron due to absorption of oxygen-In neutral aqueous solution, the rusting of iron, in presence of oxygen can be expressed as follows

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235, , Corrosion, Fe, , At anodic area:, , ... (Oxidation), , Fe2t +2e, , .... (Reduction), , 02+H2O+2e 20H20H, , At cathodic area :, , ions at cathode diffuse and meet to form, , OH, , Fe* ions formed at anode and, , a, , precipitate, , of, , ferrous hydroxide., , Fe2+20HIn presence of enough oxygen, ferric, , Fe (OH), , hydroxide, , 4Fe(OH)> + O2 + 2H,0, , is formed., 4Fe(OH)3, , This yellow product is called rust. Its chemical formula is Fe^03.2H,0, , In limited oxygen, corrosion product is black anhydrous magnetite, FegO4, Electro-chemical Corrosion, Difference between Chemical Corrosion and, , Electro-chemical Corrosion, , Chemical Corrosion, , 1. It, , in, , occurs, , dry, , I. It, , conditions., , occurs, , electolyte, , of, in wet conditions like presence, or, , aqueous solution., of, the formation of a series, , occurs through, attack ofenviron1ment 2. It, 2. It occurs by direct chemical, galvanic cells., on the metal., corrosion, , 3. Uniform corrosion takes, 4., , Corrosion, , product, , place, , 3., , in it., , accumulates at, , the, , same, , place| 4., , where corrosion takes place., 5. It, , can, , occur, , on, , homogeneous, , well, , as, , as| 5., , Non-uniform, , occurs, , Corrosion product accumulates, corrosion occurs at anode., , It, , o c u r s on, , in it., at the cathode and, , heterogeneous metal surfaces, , only., , heterogeneous surfaces., , 116. Galvanic, , higher, , or Biometallic, , Corrosion, , metal, which takes place in the, The type of corrosion,, like zinc, two dissimilar metals, in galvanic series when, connected, , to, , an, , acidic medium, , by, , exposed, , and, , Zinc anode, , Pure Cu cathode, , (Corrodes), , (unaffected), , Electron flow, , are electrically, corrosion. In this, or biometallic, is, electrolyte, called galvanic, series than copper., place in galvanic, case, zinc is at higher, corrosion and, which acts as anode, undergoes, , and copper, , Therefore, zinc,, , copper acts, , as cathode., , takes, Mechanism. Corrosion, , place in, , E, , alkaline, in neutral or slightly, formation process and, process., medium by oxygen absorption, from zinc anode to copper, flows, The electron current, anode., oxidation accurs at, , Zn, , >, , Zn*, , hydrogen, , cathode. Thus,, , Zn, , Zn2* +2e, , ., , place, from attack., cathodic metal remains protected, corrosion are, Some examples of galvanic, conneced to copper plumbing., () Steel pipe, solder around copper wire., (ii) Lead-antimony, hardware., a brass marine, (ii) Steel screws in, shaft in bronze bearing., (iv) A steel propeller, takes, So the corrosion, , (Oxidation), , at anodic metal and the, , e, Electrolyte, , Fig., , 11-1. Galvanic Corrosion

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Applied Chemistry, , 236, , 117. Concentration Cell Corrosion, The type of corrosion that is due to electro-chemical attack on the metal surface exposed to an, , electrolyte of varying, , concentration, , or, , of, , varying aeration, , is called concentration cell corrosion. This, , occurs when a part of metal is exposed to a different air concentration from the other part. Poor oxygenated, below the, becomes anodic which causes corrosion of metal partially immersed in a solution, just, , part, water line. The electrons flow from anode to cathode. The medium acts as an electrolyte. For example, When zine metal is partially immersed in a dilute solution of, , neutral salt like NaCl without agitating the salt properly, the, , Zinc rod, Cathode (more, , Water line, , part immersed to a greater depth has low Oxygen concentration oxygenated), , and hence, becomes anodic. The parts above and closely, and, adjacent to water line have high oxygen concentration, , at anodic, hence, become cathodic. Thus, zinc will dissolve, areas and electrons will be taken up by oxygen to form hydroxy, , Electron, , flow, , Corroded anode, , ions at cathodic areas., Zn, , Znt +2e, , (less oxygenated, , Oxidation), , part), , Zn47 + ,, , O2+H20+2e, Due to, , NaCl solution, , ...(Reduction), , 20H-, , the, migration of ions through the electrolyte,, to Fig, anode, from, flow, , I12., , electrons, Circuit is completed and the, cathode through the metal., , Concentration, , cell, , corrosion, , corrodes under drops, concentration is lower. Iron, occurs where oxygen, corrosion, that, shows, This, have low concentration of oxygen and, Areas covered by droplets, of water or salt solution in a similar way., to the air., the areas which are freely exposed, become anodic with respect to, , 118. Pitting Corrosion, covered with dust, dirt, rust or any, When the surface of a metal is, concentration, acts as anode due to low, other substance, the covered surface, surface acts as cathode due to higher, of oxygen. The uncovered, formed and a, a concentration cell is, concentration of oxygen. As a result,, due to corrosion. Thus, a localized, is formed on the covered surface, , cavity, , of cavities and, , attack, resulting into the formation, metal, is called pitting corrosion., and scale, The presence of impurities like dust, sand, accelerated, , pits in, , Corrosion, product, , Cathode, , athode, , Anode, , the, , (Corroded metal), , Fig., , 11-3., , Pitting corrosion, , on the surface, characteristic pitting in chloride solution., show, steel and aluminium, of the metal lead to pitting. Stainless, The, corrosion increases considerably and the depth of pit increases., Once a small pit is formed, the rate of, of concentration cell., pitting corrosion of iron is an example, film on a metal at specific points. Ths, causes cracking or brealkdown of the protective, , results Pitting, into the formation of snmall anodic and large cathodic areas. This produces corrosion current unaer, the suitable environment., , 11-9. Oxide Film Formation and its Characeteristics, Alkali metals like Li, Na, K and alkaline earth metals like Be, Ca, Sr, when come in contactw, oxygen of air, form, , a, , thin, , layer of metallic, Metal, , oxide, , +Oxygen, , on, , its surface., , Metal oxide, , (corrosion product)

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237, , Corrosion, , metal oxide., , The, , n this way, oxidation corrosion occurs due to the oxidation of metal to thin layer of, oxide film formed may be stable, unstable, volatile and porous. The nature of oxide film, , decides, , further action. The characteristics of oxide film are as under, , ) Stable-A stable layer has fine grained structure. It adheres tightly to the parent metalurtace, , Therefore, a stable oxide film of metal is impervious and oxygen cannot attack the underlying metal.Such, a film serves as protective coating on the metal and prevents further oxidation. For example, , Oxide film, , on Al, Pb, Sn and Cu are stable, tightly adhering and impervious in nature., , (i) Unstable-The oxide film, which decomposes back into the metal and oxygen, is, , Metal oxide, , Metal, , =, , called unstable., , +Oxygen, , Due to unstable nature of oxide film formed, oxidation corrosion does not take place in such cases., , Hence, Ag, Au and Pt do not undergo oxidation corrosicn., metal, (ii) Volatil2-If the oxide film volatilizes as and when it is formed, it leaves the underlying, continuous corrosion, surface exposed for further attack. Due to volatile nature of oxide film, rapid and, volatile in, , nature., , is, leading to excessive corrosion takes place. For example: molybdenum oxide (Mo03), The atmospheric oxygen, iv) Porous-If the metal oxide film is porous, it has pores and cracks., a result, the corrosion, As, cracks., can attack the underlying surface of metal easily through these pores and, , continues without any obstruction and the entire metal is converted into its oxide completely., , 11-10. Tarnishing, in presence of, Most metals except noble metals (Ag, Au and Pt) undergo oxidation corrosion, of metallic surface is disfigured, atmospheric gases. As a result, the surface of metal is tarnished. The shape, of metallic surface by atmospheric, by the activities like abrasion, erosion and cavitation on it. The disfiguring, For example : copper and silver are tarnished when they come in, gases is called tarnishing of metals., fornms a green film of basic carbonate,, contact with SO2, SO3 and HS etc. Moist air containing CO2, silver becomes black by the action of H2S gas, on the surface of copper. The surface of, , CuCO3.Cu(OH)2, , to form silver sulphide (Ag2S)., , surface. Due to rise in temperature and in, The metallic lustre is lost due to tarnishing of metallic, of the metal is tarnished more and more., presence of light, the surface, , 11-11. Fogging, formed on the surface of the metal when the relative, It is observed that a creamy white film is, is called fogging. Fogging is controlled by the relative humidity., humidity exceeds the critical value. This, in presence of sulphur dioxide. In fogging at the, Above critical value of humidity, fogging takes place, into SO3 takes place. SO3 combines with moisture or, surface of the metal, catalytic oxidation of SO2, water to form sulphuric acid which reacts with the metal to form its basic sulphate., , 3 SO2, , SOg +H2sO, , 2, , SO3 4+ S, , HSO4, , When humidity exceeds critical, Relative humidity controls fogging., remains almost unaffected. If the, more due to the attack of SO2 but copper, little but very much on copper., value, the attack of H,S on nickel is very, , value, fogging of nickel is, , humidity is, , less than critical, , 11-12. Rusting, and moisture is called rusting of iron., The corrosion of iron in presence of atmospheric oxygen, brown layer of a substance, called rust, is formed, When iron comes in contact with moist air, a reddish, at its surface. The chemical formula of rust is Fe03. x H2O. Rust is a mixture of Fe203 and Fe(OH)3

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238, , Applied Chemistry, , which is brown. The rust formed due to a mixture of, , Low conc. of, , drops of, , a ferrous oxide (Fe0) and ferric oxide (Fe^0,) called, , Oxygen (Anode), , Water, , magnetic oxide of iron or ferrosoferric oxide of iron, , oxygen (cathode), , gh conc. of, , ++, , Oxygen (cathode), , (FeO) which is black in eolour, , High conc. of, , Fe, , Fe, , *, , 4 Fe + 30, +3H,0, Iron, , Fe,O3 + 2 Fe (OH), , Electron, , Rust, , Moist air, , Corroded, , iron, , floW, , Rusting of iron is an oxidation reaction., Rusting involves the following chemical equations:, , Fig., , anode, , Flow, , Iron, 11-4. Rustingg of, , ., , Fe3+ + 3 e, , Fe, , Electron, , (Oxidation), ... (Reduction), , 202-, , O2 + 4eF, , (Reduction), , 02 +H20+2e, , > 20H, Fe203 (Ferric oxide), , 2Fe3 +302, Fe3 +30H, , Fe(OH), , (Ferric hydroxide), , Therefore. rust can, , be, , humidity, , or, , contact, , of, , iron tighty., the surface of, completely destroyed, does not adhere on, It, soft., and, and iron metal is, Rust is porous, a continuous process, is, from the metal iron easily. Rusting, , separated, , in it., , of rusting, Two main factors, air., in, , are, , (i), , he presence of oxygen, , water vapour, , Rusting-These are, affecting rate of, and moisture in air., Availability of oxygen, , as, , in air, and, , (ii), , moisture o r, , follows, , Factors, , (i), (ii), , Contact, , which, of metals with iron, , are, , less reactive than iron., , salt (NaCI) in water., water., ii), like CO and SO2 in, Dissolution of acidic gases, iron is not allowed to, (iv), order to prevent rusting,, Rusting--In, methods, Prevention of, be controlled by the following, moisture., can, Rusting, and, article., oxygen, on iron sheet or, Dissolution, , of, , com1mon, , come, , in, , vanish or grease, (i) By applying paint,, zinc on iron (Galvanisation)., a coating of, depositing, By, (i), nickel or chromium on iron., a layer of tin,, (iii) By deposing, carbon and chromium with iron., stainless steel alloy on mixing, making, iv) By, of Corrosion by Various Methods, 11-13. Prevention, , control corrosion may be described, Some methods to, ., Occurs,, , as, , under, , material should be designed in such a manner that even if the corrosion, Proper designing--The, not, localised and inter.se. The main principles regarding designing are:, and, it should be uniform, , Contact of dissimilar metals in presence of a corroding solution should be avoided., i ) When two dissimilar metals are to be in contact, the anodic material should have area as large as, possible., , cii) Two dissimilar metals in contact to be used should be as close as possible to each other in, electro-chemical series., (iv) The anodic metal should not be painted or coated to avoid rapid localized corrosion.

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239, , Corrosion, , n c equipment should be supported, , on, , legs, , to allow free circulation, , of air and prevent, , formation, , of damp areas., (vi) The flow a of corrosion liquid should be uniform., (Vi1) There should be no sharp corners and cracks to accumulate dirt and other solids, , increasing, , pure, g, purity. Impurities, in metal, , itsS, , of a given metal can be improved by, cause heterogeneity which decreases the resistance of a metal towards corosto., , meta-Corrosion resistance, , increased, , alloys-Corrosion resistance of metals is generally, . USing, for maximum, Suitable elements. The alloy should be completely homogeneous, alloying iron or steel, chromium is the best alloying metal., metal, , to be, protection-It is based on the principle that the metal, as a cathode so that corrosion does not occur., 4. Cathodic, , by alloying, , them, , corrosSion resistance., , protected, , witn, ror, , should behave, , environment should be reduced, environment-The corrosive nature of the, substances which neutral1ze the, or (ii) by adding specific, constituents, harmful, removal, of, the, ) by, as under, environment can be modified, corosive effect of constituents of the environment. The, from aqueous environment by adjusting, (i) Deaeration-The dissolved oxygen can be expelled, in decreasing, content of water which helps, and doing mechanical agitation. It also reduces CO,, 5., , Modifying the, , temperature, corrosion of steel pipe lines., i) Dcactivation-It is done, , by adding chemicals, , like sodium sulphite, , or, , hydrazine (N>H4), , which, , combine with oxygen., , 2 Na,SO4, , 2 Na,S03+02, , Sod. sulphate, , Sod. sulphite, , Use of hydrazine is, , preferred, , over, , sodium, , sulphite, , because it, , gives nitrogen, , and, , water as reaction, , products., , NaH4 +02, Hydrazine, , N1 +2H,0, Nitrogen, , Water, , surface. It reduces, silica gel is used to absorb moisture on its, (i), reduces rate of corrosion., moisture content of air and hence, corrosion of a metal by adding its, substance which is used to decrease the, 6. Use of inhibitors--A, inhibitor. The inhibitors may be anodic or, corrosive environment is called, small quantity to an aqueous, form a protective film which reduces corrosion., on the metal surface to, cathodic. Inhibitors are adsorbed, in, protective film on the surface of an object is applied, 7. Application of protective coating--A, conditions, enviromental, inert to the, The coating applied should be chemically, order to save it from corrosion., to be protected. The methods of, material, of the environment to the, and it should prevent penetration, Dehumidification-Alumina or, , are, application of metal coating, metal such as zinc, tin,, method is used to deposit a coating of low-melting, )Hot dipping-This, which have relatively high melting points., Pb or Al on iron, steel or copper, them from rusting is called, iron or steel wiuh a thin coat of zinc to prevent, The process of coating, , galvanization., thin layer of tin metal on iron or steel article is called tinning., The process of coating a, of coating metal is, is a process by which a dense and homogeneous layer, Metal, cladding-It, (i), which cladding, base metal on one or both sides. The base metals on, bonded firmly and permanently to the, nickel and their alloys. The cladding material may be nickel,, is done are inild steel, aluminium, copper,, and their alloys., copper, lead, silver, platinum

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240, , Applied Chemstry, , (ii) Displacement or Immersion plating-It is a process in which a base metal is, in, an, electrolytic solution containing a salt of the cathodic coating metal. The deposition of aimmersed, metaln nis, process takes place due to simple displacement., (v) Sherardizing-This process involves the use of zinc powder as coating material. This process, of concentration was developed by Sherard Cowpercoles in 1890. Sherardizing is used for small steel, , articles like screws, nuts, bolts, washers, valves, gauge tools and threaded parts., ()Electroplating-The depositing of a superiormetal on an inferior metal electrolytucay, called electroplating. It is done mainly, ) to save the metal article from corrosion, (ii) for decoration, (ii) to improve physical appearance and hardness, , Cathode, , Anode +, , to increase the commerical value of the metal., , (iv), , corrosion by, Electroplating is done to save iron and steel from, is, or zinc. Brass article, depositing a layer of nickel, chromium, The ornaments of silver are, electroplated with nickel or silver., and Polassum, film of gold to enhance their beauty, electroplated with a very thin, arguntocyanide, solution, , to increase their cost., , KAg{N, , Fig. 11.5. Electroplating, electroplated is cleaned, anode is, cathode in the electrolytic bath. The, salt of anodic, thoroughly. It is made, is the solution of a, The, electrolyte, for, electroplating., used, which is, made up of the metal, passing electric current in, is deposited on copper article (cathode) by, silver, (anode), :, metal. For example, Procedure-The, , article, , to, , be, , potassium argentocyanide solution., , Important Points to Keep in Memory, , S-Summary, , the surface of a metal, , alloy due to unintentional chemical, example : rusting of iron., , or an, , 1., , Corrosion is a process of destruction of, attack by environmental gases, or electro-chemical, , 2., , corrosion, There are two types of corrosion atmospheric, or wet or electro-chemical corrosion., , 3., , The factors which influence the rate and extent of corrosion are nature of the metal and nature of the, , :, , or, , moisture. For, or, , dry, , or, , chemical corrosion and immersed, , corroding environment., , 4. Oxidation corrosion is due to the direct action of oxygen on metals at low or high tenmperatures., 5. Electro-chemical corrosion always occurs at anodic areas., 6. A metal high in galvanic series is more anodic and hence, undergoes corrosion faster., 7. Corrosion of iron is called rusting of iron. The chemical formula of rust is, , Fe,03r H0., , 8., , Metals, when come in contact with oxygen or air, form a thin oxide film on their surface., , 9, , The metal oxide film may be stable, unstable, volatile and, porous., The process of coating iron or steel shets with a thin coat of, zinc to, , 10., , galvanisation., 11., , The process of coating tin, , iron, , steel articles is called, process depositing a superior metal on, i) to save the metal from corrosion, , 12 Electroplating is a, (ii), (ii), , to, to, , over, , or, , of, , increase its hardness and brightness, increase the commercical value of the metal., , protect them from rusting is called, , tinning., an, , inferior metal. It is done