CORROSION AND CORROSION CONTROL

Slide 1 : 1 Corrosion & Corrosion Control By V.S.Saravanamani Assistant Professor in Chemistry Annapoorana Engineering College,Salem

Corrosion and corrosion control : Corrosion It is the gradual deterioration of metals by chemical, electrochemical or biochemical interaction with the environment. Causes of Corrosion Metals occur in nature as their oxides, sulphides carbonates etc. The chemically combined state is thermodynamically more stable. When we extract a metal from its ore, the metal is in a higher energy state, which is thermodynamically unstable. So it tries to go back to the stable state by chemical or electrochemical interaction with the environment. Corrosion and corrosion control V.S.Saravanamani, AP/Chemistry,AEC,Salem 2

Slide 3 : Consequences or effects of Corrosion Efficiency of the machine decreases. Plant has to be shut down due to failure. Product is contaminated. The toxic products of corrosion cause health hazards. There is a necessity to over design to allow for corrosion. V.S.Saravanamani, AP/Chemistry,AEC,Salem 3

Slide 4 : Classification or Theories of Corrosion I. Dry or Chemical Corrosion II. Wet or Electrochemical Corrosion V.S.Saravanamani, AP/Chemistry,AEC,Salem 4

Slide 5 : I. Dry or Chemical Corrosion It is due to the attack on metal surface by atmospheric gases like O2, SO2, H2S etc. (e.g.) Tarnishing of silver by H2S. There are three types of dry or chemical corrosion. Oxidation Corrosion Corrosion by Hydrogen Liquid Metal Corrosion V.S.Saravanamani, AP/Chemistry,AEC,Salem 5

Slide 6 : (1) Oxidation Corrosion It is due to the direct attack of oxygen on metal surface in the absence of moisture. Alkali and Alkaline earth metals are corroded at low temperatures. At high temperatures, most metals except Au, Pt and Ag are oxidized.   Mechanism Oxidation occurs at the surface of the metal to form M2+ ions. M  M2+ + 2e- Oxygen takes up the electrons. O2 is reduced to O2- ½ O2 + 2e-  O2- O2- ion reacts with M2+ to form metal oxide. M2+ + O2-  MO The metal surface is converted to a monolayer of metal oxide. Further corrosion occurs by diffusion of M2+ ion through the metal oxide barrier. The growth of oxide film is perpendicular to the metal surface. V.S.Saravanamani, AP/Chemistry,AEC,Salem 6

Slide 7 : V.S.Saravanamani, AP/Chemistry,AEC,Salem 7

Slide 8 : Different types of oxide films are formed. (i) Porous and Non-Porous Oxide Film (or) Pilling-Bedworth Rule (a)If the volume of the oxide layer formed is less than the volume of the metal consumed, the oxide layer is porous. (e.g.) The volumes of oxides of alkali and alkaline earth metals are less than the volume of the metal consumed. So the oxide layer is porous and non-protective. (b)If the volume of the oxide layer formed is greater than the volume of the metal consumed, the oxide layer is non-porous. (e.g.) The volumes of oxides of heavy metals such as Pb, Sn are greater than the volumes of the metal consumed. So the oxide layer is non-porous and protective. V.S.Saravanamani, AP/Chemistry,AEC,Salem 8

Slide 9 : (ii) Stable Oxide Layer A stable oxide layer is firmly adsorbed on the metal surface. The layer is impervious and prevents further corrosion. So the layer itself acts as a protective coating. (E.g.) Oxides of Al, Cu etc.   (iii) Unstable oxide Layer This is mainly produced on the surface of noble metals such Ag, Au etc. The unstable oxide decomposes to stable metal and oxygen. Metal Oxide Metal + Oxygen   (iv) Volatile Oxide The oxide film volatilizes as soon as it is formed. It leaves fresh metal surface for further continuous attack. (e.g.) Molybdenum oxide MoO3. V.S.Saravanamani, AP/Chemistry,AEC,Salem 9

Slide 10 : (2) Corrosion by Hydrogen (a) Hydrogen embrittlement Definition It is formation of cracks and blisters on the metal by hydrogen gas when the metal comes into contact with H2S. Iron liberates atomic hydrogen by reacting with H2S. Fe + H2S  FeS + 2H Hydrogen atoms diffuse into the voids of metal matrix. When the pressure of the gas increases, cracks and blisters develop on the metal. V.S.Saravanamani, AP/Chemistry,AEC,Salem 10

Slide 11 : (b) Decarburisation It is the process of decrease in the carbon content of steel. At high temperature, molecular hydrogen decomposes to atomic hydrogen. When steel is exposed to this environment, carbon in the steel reacts with atomic hydrogen. C + 4H  CH4 Hence the carbon content in steel decreases. Collection of methane gas in the voids of steel develops high pressure and causes cracking.   (3) Liquid Metal Corrosion It is due to the chemical action of flowing liquid metal at high temperature. It involves : dissolution of a solid metal by the liquid metal. Penetration of liquid metal into the solid metal. V.S.Saravanamani, AP/Chemistry,AEC,Salem 11

Slide 12 : II. Wet (or) Electrochemical Corrosion : It occurs under the following conditions. When two dissimilar metals or alloys are in contact with each other in presence of an aqueous solution or moisture. When the metal is exposed to an electrolyte with varying amounts of oxygen. V.S.Saravanamani, AP/Chemistry,AEC,Salem 12

Slide 13 : Mechanism of Wet Corrosion (1) Metal dissolution occurs at the anode. M  Mn+ + ne- (2) Reduction reaction occurs at the cathode in different environments. (a) Acidic environment : Here hydrogen gas is evolved at the cathode. 2 H+ + 2e-  H2  (b) Neutral environment : In neutral or slightly alkaline medium, hydroxide ions are formed at the cathode. ½ O2 + 2e- + H2O  2OH- V.S.Saravanamani, AP/Chemistry,AEC,Salem 13

Slide 14 : Hydrogen Evolution type corrosion (In Acidic Medium) All metals above hydrogen in the electrochemical series tend to get dissolved in acidic solution with simultaneous evolution of H2 gas. V.S.Saravanamani, AP/Chemistry,AEC,Salem 14

Slide 15 : (e.g.) When iron comes into contact with non-oxidising acid like HCl, hydrogen evolution occurs. At anode : Iron is oxidized to Fe2+ Fe  Fe+2 + 2e- At cathode : H+ ion is reduced to H2. 2 H+ + 2e-  H2  V.S.Saravanamani, AP/Chemistry,AEC,Salem 15

Slide 16 : (b) Absorption of Oxygen (or) Formation of hydroxide ion type corrosion (In neutral or weakly alkaline medium) The surface of iron is normally coated with a thin film of iron oxide. But if some cracks develop on the film, anodic areas are created on the surface. The rest of the metal part acts as cathode. V.S.Saravanamani, AP/Chemistry,AEC,Salem 16

Slide 17 : V.S.Saravanamani, AP/Chemistry,AEC,Salem 17

Slide 18 : (e.g.) When iron is in contact with an electrolyte solution in presence of oxygen, OH- ions are formed. At anode : Iron is oxidized to Fe+2 Fe  Fe2+ + 2e- At cathode : O2 is reduced to OH-. ½ O2 + 2e- + H2O  2OH-  Overall Reaction Fe+2 + 2OH-  Fe(OH)2 If enough oxygen is present, Fe(OH)2 is oxidized to Fe(OH)3. 4Fe(OH)2 + O2 + H2O  4Fe(OH)3 V.S.Saravanamani, AP/Chemistry,AEC,Salem 18

Slide 19 : Differences between chemical corrosion and electrochemical corrosion V.S.Saravanamani, AP/Chemistry,AEC,Salem 19

Slide 20 : Types of electrochemical corrosion There are two types: Galvanic corrosion Differential aeration or Concentration cell corrosion V.S.Saravanamani, AP/Chemistry,AEC,Salem 20

Slide 21 : (i) Galvanic corrosion When two different metals are in contact with each other in presence of aqueous solution or moisture, galvanic corrosion takes place. The metal with more negative electrode potential acts as anode. Metal with less negative electrode potential acts as cathode. V.S.Saravanamani, AP/Chemistry,AEC,Salem 21

Slide 22 : In the Zn-Fe couple as shown in the figure, zinc with more negative electrode potential, dissolves in preference to iron. Zn acts as anode and Fe as cathode. V.S.Saravanamani, AP/Chemistry,AEC,Salem 22

Slide 23 : Example : Steel screw in a brass marine hardware easily undergoes corrosion. Iron has E0 = -0.44V. For Cu E0 = +0.34 V. Iron corrodes in preference to Cu.   Prevention Galvanic corrosion is minimized by providing an insulation between the two metals. V.S.Saravanamani, AP/Chemistry,AEC,Salem 23

Slide 24 : (ii) Differential aeration (or) concentration cell corrosion V.S.Saravanamani, AP/Chemistry,AEC,Salem 24

Slide 25 : Let a metal be partially immersed in a conducting solution. The part of the metal above the solution is more aerated and acts like cathode. The less aerated metal part inside the solution acts as anode and corrodes. At anode : Corrosion occurs (less aeration) M  M2+ + 2e- At cathode : Production of OH- ions (more aeration) ½ O2 + 2e- + H2O  2OH- V.S.Saravanamani, AP/Chemistry,AEC,Salem 25

Slide 26 : Water line corrosion Let us consider metal tank partially filled up with water. The metal area above water line is exposed to higher concentration of oxygen (cathode) than the metal below water level. The metal less exposed to O2 acts as anode and corrodes. This is called water line corrosion. Examples of differential aeration corrosion Pitting or localized corrosion Crevice corrosion Pipeline corrosion Corrosion on wire fence V.S.Saravanamani, AP/Chemistry,AEC,Salem 26

Slide 27 : (i) Pitting Corrosion It is the localized attack resulting in the formation of a hole due to corrosion. Example : Metal area covered by a drop of water, sand, dirt etc. V.S.Saravanamani, AP/Chemistry,AEC,Salem 27

Slide 28 : The area covered by the drop or dirt acts as anode and corrodes. The uncovered area exposed to air or O2 acts as cathode. The rate of corrosion is more if the cathodic area is larger and anodic area is smaller. Thus more material is removed from the same area and a pit is formed. At anode : Iron is oxidized to Fe+2 Fe  Fe2+ + 2e- At cathode : O2 is reduced to OH-. ½ O2 + H2O + 2e-  2OH- Overall reaction : Fe2+ + OH-  Fe(OH)2 V.S.Saravanamani, AP/Chemistry,AEC,Salem 28

Slide 29 : (ii) Crevice Corrosion Let a crevice or crack between two different metallic objects be in contact with a liquid. The crevice acts like anode due to less oxygen availability and corrodes. The exposed area acts as cathode. (e.g.) rivets, joints. V.S.Saravanamani, AP/Chemistry,AEC,Salem 29

Slide 30 : (iii) Pipeline Corrosion Buried pipelines or cables passing from one type of soil (clay, less aerated) to another type (sand, more aerated) get corroded due to differential aeration. V.S.Saravanamani, AP/Chemistry,AEC,Salem 30

Slide 31 : (iv) Corrosion on wire fence In a wire fence, the wires at the crossings are less aerated than the rest of the fence. So corrosion takes place at the wire crossings, which become anodic. V.S.Saravanamani, AP/Chemistry,AEC,Salem 31

Slide 32 : Factors influencing corrosion Nature of the metal (i) Position in emf series Metals above hydrogen in the electrochemical series corrode easily because they have negative reduction potential. When two metals are in contact, the more active metal with a higher negative potential corrodes. (ii) Areas of anode and cathode Corrosion will be severe if the anodic area is smaller and cathodic area is larger. The larger cathodic area demands more electrons. So the anodic area corrodes faster. V.S.Saravanamani, AP/Chemistry,AEC,Salem 32

Slide 33 : (iii) Purity 100% pure metal will not corrode. (e.g.) Pure Zn does not corrode. If the metal has trace amount of impurity, it corrodes. (e.g.) Zinc metal with iron or copper impurity forms an electrochemical cell. The base metal Zn acts as anode and corrodes. (iv) Over Voltage Corrosion rate is inversely proportional to the over voltage of the metal in a corrosive surroundings. (e.g.) The hydrogen over voltage of Zn in 1M H2SO4 is 0.7V. So the rate of corrosion is low. But when some Cu impurity is present, the over voltage is reduced and corrosion rate increases. V.S.Saravanamani, AP/Chemistry,AEC,Salem 33

Slide 34 : (v) Nature of the Film Nature of film formed on the metal surface determines extent of corrosion. (e.g.) In the case of alkali and alkaline earth metals, the oxide film formed is porous. The corrosion continues. In the case of heavy metals, the oxide film is non-porous. The film acts as a protective layer. (vi) Nature of corrosion product If the corrosion product is soluble in the corroding medium, corrosion rate is faster. Similarly if the corrosion product is volatile (e.g. MoO3), corrosion will be more. V.S.Saravanamani, AP/Chemistry,AEC,Salem 34

Slide 35 : 2. Nature of Environment (i) Temperature Increase of temperature increases corrosion rate because the rate of diffusion of ions increases. (ii) Humidity Rate of corrosion is more, if humidity of environment is high. Moisture acts as solvent for O2, CO2 etc, to produce electrolyte necessary for formation of corrosion cell. V.S.Saravanamani, AP/Chemistry,AEC,Salem 35

Slide 36 : (iii) Corrosive gases Acidic gases like CO2, SO2, H2S etc, produce electrolytes and increase corrosion.   (iv) Presence of suspended particles Particles like NaCl, (NH4)2SO4 along with moisture are powerful electrolytes and increase rate of corrosion. V.S.Saravanamani, AP/Chemistry,AEC,Salem 36

Slide 37 : Generally in alkaline medium, the rate of corrosion is less compared to acidic medium. The effect of pH on the corrosion of iron in water is shown in the Pourbaix diagram as indicated in the figure The figure shows zones of corrosion, immunity and passivity. Z is the point at which pH=7 and corresponding electrode potential is E= -0.2V. This is in the corrosion zone. So iron rusts under these conditions. V.S.Saravanamani, AP/Chemistry,AEC,Salem 37

Slide 38 : The rate of corrosion can be altered by shifting the point Z to different regions. If the potential is changed to -0.8V by applying external current, iron becomes immune to corrosion. If the potential applied is positive, iron becomes passive. If the pH is increased to more than 7, corrosion rate decreases. If the pH is reduced to less than 7, rate of corrosion increases. V.S.Saravanamani, AP/Chemistry,AEC,Salem 38

Slide 39 : CORROSION CONTROL The rate of corrosion can be controlled by modifying the metal or environment. Some control methods are proper selection of metals Use of pure metals Use of metal alloys Cathodic protection Sacrificial anode protection Impressed current cathodic protection Changing the environment Use of inhibitors Anodic inhibitors Cathodic inhibitors Applying protective coatings V.S.Saravanamani, AP/Chemistry,AEC,Salem 39

Slide 40 : 1) Proper selection of metals Noble metals are used in ornaments and in surgical instruments, because they do not corrode. Contact of dissimilar metals far away from each other in electrochemical series should be avoided. 2) By using pure metals Pure metals have high corrosion resistance. Even a trace of impurity will lead to corrosion, the base metal becoming anode.   3) Use of alloys Use of metal alloys is a good method of protection against corrosion. (e.g.) Stainless steel containing chromium forms a coherent oxide film which protects steel against further attack. V.S.Saravanamani, AP/Chemistry,AEC,Salem 40

Slide 41 : 4) Proper designing Complicated designs with more angles, sharp edges and corners should be avoided. Direct contact of dissimilar metals lead to galvanic corrosion. So insulating material between the two metals should be inserted. Smaller area for cathode and larger area for anode must be provided. Tanks and containers should be designed such that the liquid should be drained off completely. Crevices should be avoided or they should be filled using fillers. Bendings should be smooth. Annealing minimizes corrosion. V.S.Saravanamani, AP/Chemistry,AEC,Salem 41

Slide 42 : V.S.Saravanamani, AP/Chemistry,AEC,Salem 42

Slide 43 : V.S.Saravanamani, AP/Chemistry,AEC,Salem 43

Slide 44 : 5) Cathodic Protection The metal to be protected is made to act like a cathode. This is achieved in two ways.   a) Sacrificial anodic protection Here the metal to be protected is made cathode by connecting it to a more active metal (anodic metal) called sacrificial anode. Only the more active metal will be corroded, protecting the parent metal. Since the anodic metal is sacrificed, the method is called sacrificial anodic protection. Mg, Zn are used as sacrificial anodes. V.S.Saravanamani, AP/Chemistry,AEC,Salem 44

Slide 45 : Metal to be protected Applications Protection of buried pipelines, cables Protection of ships and boats Calcium metal is used to minimize engine corrosion Magnesium sheets are inserted into domestic water boilers to prevent rust formation. V.S.Saravanamani, AP/Chemistry,AEC,Salem 45

Slide 46 : b) Impressed current cathodic protection method Here an impressed current is applied in an opposite direction to annul the corrosion current. Thus the corroding metal is converted to cathode from anode. The negative terminal of battery is connected to the metal to be protected. The positive terminal is connected to an inert electrode like graphite. The anode is buried in a ‘back-fill’ (containing a mixture of gypsum, coke breeze and sodium sulphate) to increase electrical contact. V.S.Saravanamani, AP/Chemistry,AEC,Salem 46

Slide 47 : Mg M Metal to be protected V.S.Saravanamani, AP/Chemistry,AEC,Salem 47

Slide 48 : Applications Protection of tanks, transmission line towers, underground water pipes, oil pipe line, ships etc.   Limitations It is costly It fails when current is switched off. V.S.Saravanamani, AP/Chemistry,AEC,Salem 48

Slide 49 : Corrosion inhibitors A corrosion inhibitor is a substance that reduces corrosion, when added to the corrosive environment. There are three types of inhibitors. Anodic inhibitors - chromate, nitrate Cathodic inhibitors – amines Vapour phase inhibitors - benzonitrile. V.S.Saravanamani, AP/Chemistry,AEC,Salem 49

Slide 50 : i) Anodic inhibitors (e.g.) chromate, nitrate, phosphates, tungstate. The inhibitors form insoluble compound with the newly produced metal ions and prevent corrosion. This compound is adsorbed on the metal surface to form a passive film. Anodic inhibitors are used to repair the crack of oxide film on metal surface pitting corrosion porous oxide film on metal surface V.S.Saravanamani, AP/Chemistry,AEC,Salem 50

Slide 51 : ii) Cathodic inhibitors There are two types depending on the nature of cathodic reaction in an electrochemical reaction. a) In acidic solution Example : amines, thiourea, mercaptans act as inhibitors. Here evolution of H2 is the cathodic reaction. 2 H+ + 2e-  H2  The corrosion is controlled by slowing down the diffusion of H+ ions to cathode by addition of the inhibitor which is adsorbed on the metal surface V.S.Saravanamani, AP/Chemistry,AEC,Salem 51

Slide 52 : b) In neutral solution Example : hydrazine, sodium sulphite act as inhibitors. Here OH- ions are formed at cathode. H2O + ½ O2 + 2e-  2 OH- Corrosion is controlled by eliminating O2 from the corroding medium by adding Na2SO3. The OH- ions can be eliminated by salts of Mg, Zn etc. V.S.Saravanamani, AP/Chemistry,AEC,Salem 52

Slide 53 : iii) Vapour phase inhibitors (e.g.) benzotriazole, dicyclohexyl ammonium chromate act as inhibitors. These organic inhibitors readily vapourise and form a protective layer on the metal surface. Applications To prevent corrosion in closed space, storage containers, sophisticated equipment etc. V.S.Saravanamani, AP/Chemistry,AEC,Salem 53

Slide 54 : PROTECTIVE COATINGS Metal surface is covered by a protective coating to prevent corrosion. The coating acts as a physical barrier between the metal surface and the environment. The coating gives a decorative appeal and also imparts hardness, oxidation resistance and thermal insulation to the surface. The main types of coating are: Metallic coating Chemical conversion coating Organic coating Non-metallic coating V.S.Saravanamani, AP/Chemistry,AEC,Salem 54

Slide 55 : PAINTS Paint is a mechanical dispersion of one or more fine pigments in a medium (thinner + vehicle). When a paint is applied to metal surface, the thinner evaporates. The vehicle undergoes slow oxidation to form a pigmented film.   Requirements or requisites of a good paint A good paint should, have good covering power spread easily on the surface not crack on drying adhere well to the surface give a glossy film be corrosion and water resistant have stable colour V.S.Saravanamani, AP/Chemistry,AEC,Salem 55

Slide 56 : Constituents of Paint Pigment Vehicle Thinner Drier Filler Plasticizer Antiskinning agent V.S.Saravanamani, AP/Chemistry,AEC,Salem 56

Slide 57 : 1. Pigment It is a solid that gives colour to the paint. Functions: To give colour and opacity to the film. To provide strength to the film. To protect film by reflecting U.V. rays. To provide resistance to abrasion and weather. Example: White pigment - White lead, TiO2 Blue pigment - Prussion blue Green pigment - Chromium oxide Red pigment - Red lead, Fe3O4 V.S.Saravanamani, AP/Chemistry,AEC,Salem 57

Slide 58 : 2. Vehicle (or) Drying Oil It is the film-forming liquid. It holds the ingredients of the paint. It is a non-volatile high molecular weight fatty acid of vegetable or animal. Function To hold the pigment on the surface. To form a protective layer by oxidation and polymerization. To impart water repellency, toughness and durability of film. To improve adhesion of film. Example Lin seed oil, Castor oil. V.S.Saravanamani, AP/Chemistry,AEC,Salem 58

Slide 59 : 3. Thinner It is the volatile portion of paint. It is added to reduce the viscosity of the paint for easy application on the surface. It easily evaporates after paint is applied. Functions To reduce viscosity of paint. To dissolve vehicle and other additives. To suspend the pigments. To increase elasticity of film. To increase penetration of vehicle. To improve drying of film. Example Turpentine, Dipentine, Xylol. V.S.Saravanamani, AP/Chemistry,AEC,Salem 59

Slide 60 : 4. Drier It is a substance used to speed up drying of the paint. Functions To act as oxygen carrier or catalyst. To provide oxygen essential for oxidation and polymerization of drying oil. Example Metallic soap, linoleate and resinate of Co, Mn etc. V.S.Saravanamani, AP/Chemistry,AEC,Salem 60

Slide 61 : 5. Extender or Filler These are white pigments that form bulk of the paint. Functions To reduce cost of paint To prevent shrinkage and cracking of film To modify shades of pigment To retard settling of pigments in paint. Example Talc gypsum, china-day. V.S.Saravanamani, AP/Chemistry,AEC,Salem 61

Slide 62 : 6. Plasticizer It is added to the paint to provide elasticity to the film and prevent its cracking. Example Triphenyl phosphate, Tricresyl phosphate   7. Antiskinning agent It is a chemical added to the paint to prevent gelling and peeling of the paint. Example Polyhydroxy phenols. V.S.Saravanamani, AP/Chemistry,AEC,Salem 62

Slide 63 : Pigment Volume Concentration (P.V.C.) The P.V.C. of a paint is calculated using the equation. P.V.C. = If P.V.C. is high, durability, adhesion and consistency of the paint will be low. V.S.Saravanamani, AP/Chemistry,AEC,Salem 63

Slide 64 : Failure of Paints A paint may fail due to any one of the following reasons: Chalking : It is the gradual powdering of the paint film on the painted surface. This happens due to improper dispersion of pigment in vehicle. Cracking : A paint film cracks due to unequal expansion or contraction of paint coats. Evasion : This is very quick chalking. Blistering : It is due to improper surface exposure of paint to strong sunshine. V.S.Saravanamani, AP/Chemistry,AEC,Salem 64

Slide 65 : Metallic Coating Electroplating or Electro-deposition It is the deposition of coat metal on the base metal by passing direct current through an electrolytic solution of a soluble salt of the coat metal. The base metal to be electroplated acts as cathode. The coat metal or an inert electrode forms anode. The electrolyte is a soluble salt of coat metal. V.S.Saravanamani, AP/Chemistry,AEC,Salem 65

Slide 66 : Objectives or uses or applications of Electroplating: To enhance resistance to corrosion of base metals. To give a decorative appearance. To enhance resistance to chemical attack. To improve hardness and wearing resistance. To obtain polished surface. V.S.Saravanamani, AP/Chemistry,AEC,Salem 66

Slide 67 : Theory If the coating metal itself forms the anode, the concentration of electrolyte bath does not change during electrolysis. The metal ions deposited on the cathode are replenished continuously by dissolution of the anode.   Example Electroplating of Gold V.S.Saravanamani, AP/Chemistry,AEC,Salem 67

Slide 68 : V.S.Saravanamani, AP/Chemistry,AEC,Salem 68

Slide 69 : The object to be gold plated is treated with organic solvent like acetone, CCl4 to remove grease, oil etc. It is then washed with dil H2SO4 to remove scales, oxides etc. The cleaned object is made cathode of electrolytic cell. Anode is a gold plate. AuCl3 solution is the electrolyte. When current is passed into the solution, gold ions migrate to the cathode, get reduced and deposit on the object. V.S.Saravanamani, AP/Chemistry,AEC,Salem 69

Slide 70 : Ionisation : AuCl3  Au+3 + 3Cl- At Cathode : Au+3 + 3e-  Au At anode : Au  Au+3 + 3e- Au+3 + 3Cl-  AuCl3 To achieve a strong adherent and smooth deposit, glue or gelatin is added to the electrolyte bath. To enhance the brightness of the deposit, brightening agents are added to the bath.   Conditions Temperature : 600C Current density : 1 to 10 mA/cm2 Low metal ion concentration Buffer solution to maintain pH. V.S.Saravanamani, AP/Chemistry,AEC,Salem 70

Slide 71 : Applications or Uses or Objectives To give a decorative appearance. Electrical and electronic applications To get a thin coating of gold on cheap jewellery To achieve oxidation resistance, corrosion resistance etc. V.S.Saravanamani, AP/Chemistry,AEC,Salem 71

Slide 72 : Electroless Plating It is the deposition of a noble metal (from its salt solution) on a catalytically active metal surface using a reducing agent without use of electric current. The reducing agent reduces the metal ions. The metal atoms get deposited over the surface to give a thin uniform coating. Metal ions + reducing agent  metal (deposited) + oxidation product Example Electroless nickel plating V.S.Saravanamani, AP/Chemistry,AEC,Salem 72

Slide 73 : The various steps are: Step I : Pretreatment and activation of the surface: The surface to be plated is degreased by using organic solvents or alkali and then accompanied by acid treatment. The surface of stainless steel is activated by dipping in hot solution of 50% H2SO4. Mg alloy surface is activated by giving a thin coating of zinc and copper over it. Al, Cu, Fe, brass etc, do not require activation. Plastic, glass etc, are activated by dipping in a solution of SnCl2/HCl and then in PdCl2 solution. On drying a thin layer of palladium is formed on the surface. V.S.Saravanamani, AP/Chemistry,AEC,Salem 73

Slide 74 : Step II : Preparation of plating bath: The plating bath consists of: Coating Metal : A solution of NiCl2 2gm/lit. Reducing agent : Sodium hypophosphite 20g/lit. Exaltant to accelerate coating rate and complexing agent : Sodium succinate 15g/lit. Buffer to maintain pH at 4.5 : Sodium acetate 10g/lit. Temperature 93oC The pretreated object is immersed in the plating bath for required time. The following reactions occur and nickel is coated on the object. Cathode : Ni2+ + 2e-  Ni Anode : H2PO2- + H2O  H2PO3- + 2H+ + 2e- Overall Reaction : Ni2+ + H2PO2- + H2O  Ni + H2PO3- + 2H+ V.S.Saravanamani, AP/Chemistry,AEC,Salem 74

Slide 75 : Uses of Nickel Plating For decorative coating of jewellery, decorative items and automobile spares For coating of polymers for decorative purpose. For electronic appliances. Advantages of electroless plating over electro plating Electricity is not necessary Complicated parts are uniformly coated Plastics, glass etc, are easily coated Good mechanical, chemical and magnetic properties are obtained. V.S.Saravanamani, AP/Chemistry,AEC,Salem 75

Slide 76 : Differences between Electroplating and Electroless plating: V.S.Saravanamani, AP/Chemistry,AEC,Salem 76

Slide 77 : Control of corrosion by modifying the environment: There are five methods 1. Deaeration : Presence of oxygen increases corrosion rate. Deaeration involves removal of dissolved oxygen by increasing the temperature together with the mechanical agitation. This also removes dissolved oxygen. 2. Deactivation: It is the removal of dissolved oxygen by adding chemicals in aqueous solution. (E.g.) 2Na2SO3 + O2  2Na2SO4 V.S.Saravanamani, AP/Chemistry,AEC,Salem 77

Slide 78 : 3. Dehumidification: It is the removal of moisture from the air by reducing the relative humidity of the surrounding air. It can be achieved by adding silica gel or alumina which absorbs moisture.   4. Alkaline neutralization: The acidic nature of the corrosive environment is due to the presence of HCl, SO2, CO2 etc. They are neutralized with alkali spray. E.g. NaOH, lime etc. 5. Using corrosion inhibitors : A corrosion inhibitor is a substance that reduces the corrosion of a metal when added to corrosive environment. V.S.Saravanamani, AP/Chemistry,AEC,Salem 78