Page 1 :
MODULE II- CORROSION METAL FINISHING, CORROSION, Introduction:, Definition, Corrosion is defined as the destruction or deterioration of a metal or its alloy, and consequent loss of metal, caused due to direct chemical action or, electrochemical reactions with its environment., Ex: Formation of reddish brown scale on iron due to formation of Fe2O3 Green scale on copper, vessel due to CuCO3 & Cu (OH)2, Dry corrosion is direct chemical attack in the absence of moisture by dry gases like SO2, CO2,, Cl2, H2S etc., Wet corrosion is common in nature & occurs by electrochemical attack on the metal with the, moisture and oxygen present in the atmosphere., Electrochemical theory of corrosion, According to electrochemical theory, corrosion of metals occurs due to the, following changes, when they are exposed to the environment., 1. A large number of minute galvanic cells are formed which acts as, anodic and cathodic areas., 2. At anode the metal undergoes oxidation and electrons are liberated, which migrates towards cathodic region, 3. Oxygen of the atmosphere undergoes reduction at cathodic area in the, presence of moisture forming hydroxyl ions at the cathode, , Anodic reactions: At anode the metal undergoes oxidation- liberating, electrons, M, Mn+ + neMetal, Metal ions, Ex: when iron is exposed to the environment it undergoes, oxidation as, Fe, Fe2+ + 2e_, Cathodic reactions: The electrons released at anode migrate to the cathodic, area and reduces oxygen to hydroxyl ions., The different cathodic reactions are,, a) In acidic medium: In acidic medium and in the absence of, oxygen, hydrogen ions are reduced to hydrogen gas., 2H+ + 2e H2, b) In alkaline and in the absence of O2: If the solution is alkaline and
Page 2 :
in the absence of oxygen the cathodic reaction is,, 2 H2O + 2e2OH- + H2, c) In neutral and aerated medium: when the solution is neutral and, aerated, hydroxyl ions are formed as follows., 2 H2O + O2 + 4e4OHd) Formation of corrosion product: The hydroxyl ions migrate, towards anode and react with metal ions (Mn+ ions) and forms corrosion, product. In the case of iron OH- reacts with Fe2+ ions and forms, an insoluble hydrated ferric oxide known as brown rust., 2 Fe2+ + 4OH2 Fe(OH)2, 4Fe(OH)2 + O2 + 2H2 O, 2Fe2O3. 3H2O, Rust, , Factors affecting the rate of corrosion:, The main important factors, which affect on the rate of corrosion, are, a) Nature of the metal and, b) Nature of the environment, a) Nature of the metal:, The metals with lower electrode potential (higher up in series)are more reactive, than with metal of higher potential. The more reactive metals are more susceptible, for corrosion. Thus the tendency of a metal to undergo corrosion decreases with, increase in electrode potential. For example K, Na, Mg, Zn etc. with low electrode, potential values are more susceptible to corrosion. The noble metals Ag, Au, Pt, etc. with high electrode potential values are less susceptible to corrosion., Example: The potential difference between copper and iron is 0.78V. The, potential difference between iron and tin is 0.3 V. In these two cases iron, undergoes fast corrosion when it is in contact with copper than when it is, in contact with tin., i) Nature of corrosion product:, If the nature of corrosion product forms, a protective layer on the metal, surface, it prevents the further corrosion of metal. If the corrosion product, is thin, invisible and doesn’t form a protective layer it leads to further, corrosion of metal., Example: In oxidizing environment metals like Al, Cr, Ti etc forms, protective metal oxide films on their surfaces which prevent further, corrosion. Metals like Zn, Fe, Mg, etc do not form protective layer and are, readily under goes corrosion .Therefore the rate of corrosion depends on the, nature of corrosion product., ii) Anodic to cathodic areas:, If a metal has small anodic and large cathodic area the rate of corrosion, increases and vice versa. This is because when anode is small the electrons, liberated during oxidation are completely consumed on large cathodic, surface for the reduction reactions and rate of corrosion increases. If the, cathode is smaller the consumption of electrons will be slower and corrosion will become
Page 3 :
slower., For example when tin is coated on iron in such a way that some pin holes are left results in, small anode area and large cathode area. Since tin is cathodic to iron corrosion occurs at the, small anode area exposed. On the other hand when zinc coated on iron leaving behind small, pin holes results large anodic and small cathodic areas. Since zinc is anodic to iron corrosion, of iron will not occur., b) Nature of the environment:, i) Temperature: Increases in temperature results in an increase in the, conductance of the aqueous medium and rate of corrosion also increases and, vice versa., ii) pH: Generally acidic media are more corrosion than alkaline and neutral media., However some metals like Al, Zn etc. undergoes fast corrosion in highly alkaline medium., iii) Con du ct ing medi u m, , Types of corrosion:, The corrosion can be mainly classified as follows, 1. Differential metal corrosion(Galvanic corrosion), This type corrosion occurs when two different metals are in, contact with each other due to the formation of galvanic cell. The metal having, less standard reduction potential value under goes oxidation and liberates, electrons, which migrates to the cathode. The other metal having high, standard rate pressure ( SRP) value acts as cathode and reduction reaction, takes places on its surface forming OH- ions or any one kind of reduction, reactions. The rate of corrosion depends on the potential difference, between the two metals. If the difference is more corrosion occurs faster and, vice versa. The anodic metal undergoes corrosion and cathodic metal is, unaffected ., , The reactions that occurs are, At anode:, M, Mn+ + neAt cathode: Depending on the nature, cathodic reaction are as fallows, i) 2H+ + 2eii) H2O + O2 + 4e, iii) 2H2O + 2e-, , -, , of, , corrosion, , environment, , the, , H2 (in acidic environment), OH(in neutral environment), 2OH- + H2 (in alkaline environment), , Example: Iron metal in contact with Copper metal, Brass tap in contact with, Iron pipe etc.
Page 4 :
2. Differential aeration corrosion, This type of corrosion occurs when a metal is exposed to different, concentrations of Oxygen. The part of metal which is more exposed to air act as, cathode and unaffected. The other part of the metal, which is less, exposed to, air act as anode and undergoes corrosion., For example when an iron strip is partially immersed in aerated solution of, sodium chloride as shown in figure. The metal part inside electrolyte get corroded due to, anodic reaction and the more oxygenated area of the metal(cathode) is protected, Ex.: Nail inside the wall corrodes, Window frame inside wall corrodes, Example i) Water-line corrosion:, It is observed in water storage tanks, ship water lifting, pipes etc. For example in water storage tank the metal, Tank part inside water level exposed only to dissolved, oxygen get corroded(anode) while the part above water, line is exposed to higher oxygen concentration is, protected(cathode)., , Example ii) Pitting corrosion:, Pitting corrosion occurs when small particle like dust, mud etc get deposited on, metals surface. The portion of metal covered by the dust or other particles is, less aerated and acts as anode. The other portion of the metal exposed to more, oxygen of the environment act as cathodic region. Corrosion takes place at the, portion below dust and a small pit is formed. Then the rate of corrosion, increases due to small anodic area and large cathodic area. All the electrons, liberated at anode (pit) are absorbed by large cathode area for reduction reaction., , Corrosion control method, Inorganic coating :, Anodizing of Al:, The process of formation of a film of metal oxide on the surface of a metal by, electrochemical oxidation is known as anodizing., The aluminium article to be anodized is degreased followed by electropolishing to remove, impurities present on the metal surface. It is connected to positive terminal i.e. made as, anode and steel or copper is made as cathode., Bath composition:, Electrolyte:, 5-10 % Chromic acid/10% H2SO4/Oxalic acid, Temperature: 350 C, Current density: 10-20 mA/cm2
Page 5 :
Electrode reaction:, , Finally the object is treated with nickel or cobalt acetate followed by boiling water, treatment to improve corrosion resistance., Uses: Used for soap boxes, Tiffin carriers ,window frames etc., Metal coating, Deposition of a protective metal over the surface of a base metal is called metal coating. It is, divided in to two types: anodic and cathodic coating, Anodic coating(Galvanization), Galvanization is a process of coating a base metal surface with zinc metal by hot dipping, process. It involves the following steps:, The metal surface is washed with organic solvents to remove grease and oil., Rust and other deposits are removed by washing with dilute H2SO4., Finally the article is washed with water and dried., The article is then dipped in a bath of molten zinc maintained at 425-4300C and, covered with NH4Cl flux to prevent the oxidation of molten zinc., The excess zinc on surface is removed by passing through a pair of hot rollers, which, wipes out excess of zinc coating and produces a thin coating., Uses: It is used to protect iron from corrosion in the form of roofing sheets, fencing wire,, buckets, bolts, nuts, nails ,screws, pipes etc., They are not used for preparing and storing food stuffs, since zinc dissolves in dilute acids, producing toxic zinc compounds., CATHODIC PROTECTION:, Cathodic protection is a method of protecting a metal or alloy from corrosion by, converting base metal completely into cathodic and no part of it is allowed to act as anode. The, principle of cathodic protection is to reverse the flow of electrons during corrosion. This, can be achieved by providing electrons from external sources that the specimen always, remains cathodic. The electrons can be provided by two methods., Sacrificial anodic method:, In this method the more reactive metals (sacrificial anodes) like Zn, Mg which provide, electrons connected to the metal structure to be protected. The base metal will become, cathodic &more reactive metal become anode, preferentially corrosion occurs at anode., The sacrificial anode which gets corroded has to be replaced to protect base metal, continuously., Example: A Mg block connected to a buried oil storage tank.
Page 6 :
IMPRESSED CURRENT METHOD:, , In this method the electrons for cathodic protection are supplied by a source of direct, current .The specimen to be protected should be made –ve otherwise it corrodes. This is, used in buried oil pipe lines, water storage tanks.
Page 7 :
METAL FINISHING, Definition: It is defined as a process carried out in order to modify the surface properties of a, metal by deposition of a layer of another metal, polymer or by formation of oxide layer., Technological importance of metal finishing:, It is the addition of certain properties to the materials to increase the utility of the material., These include, to increase corrosion resistance, to improved wear resistance, to impart electrical and thermal conducting surface, imparting thermal resistance, imparting hardness, to offer the surface thermal or optical reflectivity, manufacture of electrical and electronic components such as PCBs,capacitors, contacts, electroforming process, electropolishing,electrochemical etching, The important techniques of metal finishing are electroplating and electro less plating, Electroplating:It is a process of electrolytic deposition of metal,alloy or composite on the, base metal by means of electrolysis., The three factors governing the electrolysis are polarisation, decomposition potential and over, voltage., , Factors depending on polarisation are, i., Nature of electrode (size &shape) ii. Concentration of electrolyte and its conductance, iii. Temperature iv. Products formed at electrode v. Rate of stirring of the electrolyte., To decrease the polarisation effect the electrode surface must be large and low concentration, of electrolyte with high conductance.
Page 8 :
Decomposition potential:The minimum external voltage applied in order to cause, continuous electrolysis of an electrolyte is called decomposition potential. It is measured, using an electrolytic cell., , Determination of decomposition potential :, The cell consists of two platinum electrodes immersed in the electrolyte. The voltage is, varied by moving the contact C along the wire AB and the current passing through the cell is, measured with ammeter. When dilute solution of acid or base is taken in the cell, at low, voltage i.e. Less than 1.7 V no reaction is found to occur because the inert platinum, electrodes are converted in to active hydrogen and oxygen gas electrodes. The secondary cell, formed produces back emf opposing the applied emf. When the voltage is increased above, 1.7V sudden evolution of H2 and O2 gas takes place at the respective electrodes. The, abrupt increase in current also observed at this point. The decomposition potential is, calculated by plotting current versus applied voltage as shown in the graph., , The intersection of these two straight lines gives the decomposition potential mathematically, Eback emf = Ecathode - Eanode, The decompositon potential values enables the i.Calculation of emf required for continuous, electrolysis ii.used in separation of metal ion mixture by electrolysis, Over voltage: The excess voltage that has to be applied above the theoretical decomposition, potential to start the continuous electrolysis is called Over voltage., ED = Ecathode - Eanode + Η, where Η -over voltage The factors influences the over voltage are, i.Electrode surface ii.Current density iii. Temperature iv. Rate of stirring of the electrolyte, v.Nature of substance discharged., For example the reversible electrode potential of oxygen gas with smooth platinum electrode, surface under standard condition is 1.23 V. But actual gas evolution takes place at a, potential of 1.68V.This excess 0.45 V is the over voltage of oxygen on smooth platinum, surface., The knowledge of over voltage is used in the electrolysis for deposition of metals. Higher the, electrode potential cations will be easily discharged at the electrode surface.
Page 9 :
Electroplating process:, It is a process of electrolytic deposition of metal, alloy or composite on the base metal by, means of electrolysis. The deposits are Single metals such as Sn, Cu, Ni, Cr, Ag, Au, Pt etc., Alloys like Cu-Zn, Cu-Sn, Sn-Ni, Ni-Co, Ni-Cr etc., Composites such as metals containing solids like WC, SiC. The principal components of, electroplating are, 1. An electroplating bath containing conducting salt, metal to be plated in soluble, form, complexing agent, additives, buffering agent., 2. Cathode i.e. the article to be plated, 3. Anode:Coating metal itself or inert electrode, 4. An inert container to hold all above components., The object to be plated is made as cathode and the possible reactions at cathode is, Mn+ + ne, M, , The dissolution of metal at anode takes place and the possible reactions at anode is., M, Mn+ + ne, If the inert anode is used the oxygen evolution takes place and metal ions are added in the form, of metal salt., , Electro-plating of chromium (Decorative & Hard ):, Bath composition :, 250 g of chromic acid + 2.5 g H2SO4(100:1), Temperature, :, 45-60 ΟC, Current density, :, 45-430 A/ft2(Decorative purpose) & 290-580 A/ft2(Hard purpose), Current efficiency :, 10-15% (Decorative purpose) & 17-21% (Hard purpose), Anode, :, Insoluble anodes like Pb-Sb or Pb-Sn coated with PbO2, Cathode, :, Object to be plated., At anode:, 2 H2O, O2 + 4H+ + 4e, At Cathode:, , Cr+3+ 3e-, , Cr
Page 10 :
Sulphuric acids acts as catalyst in converting Cr(VI) to Cr(III) by complex anodic reaction. To, maintain the concentration of Cr(III) ions insoluble anodes like Pb-Sb, Pb-Sn, alloys are used, covered PbO2 which oxidize Cr(III) to Cr(VI) & controls the concentration of Cr(III) ions., 1) Cr metal passivates strongly in acid sulphate medium., 2) It gives Cr(III)ions on dissolution. In presence large Cr(III) ions a block Cr deposit is obtained., Uses: Used in decorative and corrosive resistant finish. Decorative & durable finish like, cycles ,automobiles ,furniture, house hold fittings, surgical and dental instruments, others like, optical instruments ,machine tools, electronics parts, piston rings, cylinder liners of diesel, engines& aircrafts cylinders., , H, , (Since H+ ions are produced during the reaction, the p decreseses. To maintain the pH at 4.5, buffer salt(sodium acetate) is added during the reaction., Applications:, 1. Used to obtain a wear resistance surface., 2. Ni coating on aluminium improves the solderability., 3. Ni coating on polymers is used to obtain decorative in Hi-fi equipment, tops on bottles etc.
Page 11 :
Electroless plating of copper, 4., , Pretreatment and activation of surface:, The surface is treated to remove grease and other impurities. In case of insulators, the surface, is activated by dipping in stannous chloride and then in palladium chloride. The surface is dried., Then the electroless-plating is done under the following conditions., Plating bath:, Bath, Reducing agent, Buffer, Complexing agent, pH, Temperature, , : Copper sulphate (CuSO4 ) 12 g per litre, : Formaldehyde (HCHO) 8 g per litre, : NaOH 15 g/litre & Rochelle salt 14 g/litre, : EDTA 20 g /litre, : 11, : 250 C
