Corrosion And Its Control

UNIT – II – CORROSION AND CORROSION CONTROL Syllabus: 9 hrs. Chemical corrosion – Pilling – Bedworth rule – electrochemical corrosion – different types – galvanic corrosion – differential aeration corrosion – factors influencing corrosion – corrosion control – sacrificial anode and impressed cathodic current methods – corrosion inhibitors – protective coatings – paints – constituents and functions – metallic coatings – electroplating (Au) and electroless (Ni) plating. Session No 10 11 12 13 14 15

Topics to be covered Chemical corrosion, corrosion due to Oxygen – Pilling – Bedworth rule on corrosion products electrochemical corrosion – Hydrogen evolution and oxygen absorption cathodic process, different types – galvanic corrosion differential aeration corrosion – factors influencing corrosion corrosion control – sacrificial anode and impressed cathodic current methods corrosion inhibitors protective coatings – paints – constituents and functions

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metallic coatings – Principle and methods of electroplating, electroplating (Au)

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Principle and methods of electroless plating ,electroless (Ni) plating. Revision

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1

Introduction: Corrosion is a general term that refers to the deterioration and ultimate destruction of a metal due to its reaction with the surrounding gaseous or liquid environment. Corrosion is a decay process in which metals exhibit their natural tendency to revert to their native combined state of existence as minerals-e.g. as oxides, sulphates, carbonates etc. All metals and alloys are susceptible to corrosion under different environmental conditions. Only metals such as gold and platinum exist in nature as metals and are not susceptible to corrosion under ordinary atmospheric conditions and hence are called noble metals. Corrosion causes a heavy loss to industries since the modern day domestic and industrial applications uses mainly metals and alloys.

Types of Corrosion: Corrosion may be broadly classified into two types based on the mechanism of corrosion. These include (a) dry corrosion also called as chemical corrosion (b) wet corrosion also called as electrochemical corrosion`` (a) Dry Corrosion (or) Chemical Corrosion: One of the most common ways by which metals get corroded is by direct interaction with atmospheric gases such as oxygen, hydrogen sulphide, halogens, sulphur dioxide, oxides of nitrogen. Oxygen is primarily responsible for corrosion of most metallic structure as compared to other gases and chemicals. Direct attack on metal by oxygen even at ambient temperatures in the absence of moisture leads to oxidation corrosion, that is, the formation of the corresponding metal oxide, which is normally thermodynamically spontaneous process. The oxidative corrosion may be considered to involve the reactions of oxidation of the divalent metal to form the metal ion with the simultaneous release of electrons and the combination of the electrons with oxygen to form oxide ions. M

M2+ + 2e-

2

½O2 + 2e-

O2-

The overall reaction: M2+ O2- (metal-oxide film)

M + O2

Diffusion of oxygen are responsible for continued oxidation and growth of the film into an oxide scale. The growth of an oxide film can occur due to (a) migration of metal ions outwards to the surface (b) migration of oxide ions from the surface to the bulk, (c) simultaneous migration of metal ions and oxide ions, and (d) penetration of molecular oxygen through the metal-oxide interface. In general, outward diffusion of metal ions and electrons is likely to be more rapid due to the fact that cations are smaller in size compared to the oxide ions. M

Mn+ + ne- (oxidation) Mn+ Mn+ O2Mn+ e

Metal-M

-

Atmospheric Oxygen 2-

O

O2½O2 + 2e-

O2-(reduction)

Mechanism of Oxidation of Metal to Metal oxide Nature of the oxide formed plays an important role in oxidation corrosion process i.e. Metal + oxygen

metal oxide (corrosion product)

When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature this film decides the further action. If the film is: (i) Stable: A stable layer is fine-grained in structure and can get adhered tightly to the parent metal surface.

Such a film behaves as protective coating in nature, thereby

shielding the metal surface. The oxide films on Al, Sn, Pb, Pt, cu etc., are stable. (ii) Unstable: the oxide layer formed decomposes back into the metal and oxygen 3

Metal oxide

Metal + Oxygen

Consequently, oxidation corrosion is not possible, thus metals like Ag, Pt, Au do not undergo oxidation corrosion. Unstable metal oxide Exposed area

Metal

+O2

Metal

Metal

metal oxide

+

O2

decomposes

(iii) Volatile: the oxide layer volatilizes as soon as it is formed, thereby leaving the underlying metal surface exposed for further attack. E.g. Molybdenum oxide MoO3 Exposed area

Metal

Volatile metal oxide

+O2

Metal

metal oxide

Metal

Fresh surface exposed for Further attack

volatilizes

(iv) Porous: having pores or cracks on the surface of the metal, atmospheric corrosion have access to the underlying surface of metal, through the pores or cracks of the layer thereby the corrosion process continues till entire metal is completely converted into its oxide. Exposed area

Metal

porous metal oxide

+O2

Metal

Further attack through pores/cracks

continues

Pilling and Bedworth rule: According to Pilling and Bedworth, the oxidation resistance of a metal is related to the specific or molar volume ratio of the corrosion product, namely, the metal oxide and the metal. It is expressed mathematically as R = (M/D) x (d/m) Where M and m are the molecular weight and atomic weight of the metal oxide and the metal respectively, D and d are the densities of the oxide and the metal 4

respectively. The ratio r indicates the volume of oxide formed from a unit volume of metal. If R<1, the oxide layer can cover the metal surface only when it is under tension. However, such tension will tend to produce cracks thereby exposing the metal surface to oxygen. On the other hand, if R>1, as in the case of copper-oxygen system the oxide is able cover the metal surface effectively. The strongly adherent non-porous oxide layer protects the metal from further oxidation.

Dry corrosion by other gases and chemicals: Other gases present in the working environment such as chlorine, fluorine, sulphur dioxide, hydrogen sulphide and oxides of nitrogen are also corrosive. Hydrogen embrittlement: Hydrogen sulphide attacks metals forming the corresponding metal sulphide and releases atomic hydrogen. The atomic hydrogen diffuses readily into the metal and collects at the void spaces where it combines with atomic hydrogen to from hydrogen gas. The accumulation of the gas develops a high pressure causing cracks and blisters in the metal. The condition is known as hydrogen embrittlement. Decarburization: The atomic hydrogen on coming into contact with steel combines with the carbon of steel to form methane gas which collects in voids.

As the pressure

increases due to the accumulation of the gas cracks occur in steel, a condition known as decarburization.

Wet corrosion or electrochemical corrosion: The corrosion of metal in aqueous environments is more prevalent than under dry conditions.

Iron undergoes corrosion to form rust. The rust formed on the surface of

iron is loose and does not adhere to the metal surface. According to the electrochemical theory of corrosion, wet corrosion is a two step process which occur simultaneously, namely, oxidation and reduction. The surface of a piece of iron in contact with an aqueous solution of electrolyte becomes a galvanic or a voltaic cell consisting of anodic and cathodic regions. The galvanic cell formed facilitates the flow of positive current from the anodic region to the cathodic region through the electrolyte leading to the dissolution or corrosion of the anodic region. 5

M2+ + 2e-

M

The electrons are utilized at the cathodic region either to form hydrogen or hydroxide ion depending on the pH of the medium. In acidic medium hydrogen is evolved. (a) Evolution of Hydrogen: Considering metal like Fe, the anodic reaction is dissolution of iron as ferrous ions with the liberation of electrons. Fe2+ + 2e-

Fe

These electrons flow through the metal, from anode to cathode, where H+ ions are eliminated as hydrogen gas. Electrolyte Iron

H+ClFe2+

2e-

H+ClH+ClH+Cl-

H2 H+Cl-

Mechanism of Hydrogen evolution

2H+ + 2eThe net reaction is

Fe + 2H+

H2 Fe2+ + H2

This type of corrosion causes displacement of hydrogen ions from the acidic solution by metal ions. Consequently, all metals above hydrogen in the electrochemical series have a tendency to get dissolved in acidic solution with simultaneous evolution of hydrogen. In neutral or a weakly alkaline medium, hydroxide ions are formed by the reduction of absorbed oxygen (b) Absorption of oxygen: Rusting of iron in neutral aqueous solution of electrolytes in the presence of atmospheric oxygen is a common example of this type of corrosion. The surface of iron is, usually, coated with at thin film of iron oxide. However, if this iron oxide film develops some cracks, anodic areas are created on the surface; while the wellmetal parts act as cathodes. It follows that the anodic areas are small surface parts; while nearly the rest of the surface of the metal forms large cathodes. 6

At anodic areas of the metal dissolves as ferrous ions with liberation of electrons. Iron

Aq. Soln. H 2O

Fe2+ H2O 2e-

O 2

OHH 2O

O2

Mechanism of oxygen absorption The liberated electrons flow from anodic to cathodic areas, through iron metal, where electrons are intercepted by the dissolved oxygen as: ½O2 + H2O + 2e-

2OH-

The Fe2+ ions and OH- ions diffuse and when they meet, ferrous hydroxide is precipitated. Fe2+ + 2OH-

Fe (OH)2

If enough oxygen is present, ferrous hydroxide is easily oxidized to ferric hydroxide. 4Fe(OH)2 + O2 + 2H2O

4Fe(OH)3

Types of Corrosion: (i) Galvanic Corrosion: When two different metals are in electrical contact in the presence of an electrolyte, the metal higher up in the electrochemical series becomes anodic and suffers corrosion because of its higher oxidation potential. Examples: plain carbon steel in contact with stainless steel and brass. The Figure represents Fe-Cu couple, in which iron dissolves to copper here Fe acts as anode i.e. more active metal when compared to copper which acts as cathode. Fe

Cu

More active

Less active

Galvanic corrosion 7

The galvanic corrosion may be avoided by a proper selection of metals and alloys based on their position in galvanic series. (ii) Differential aeration corrosion: This type of corrosion occurs, when one part of metal is exposed to a different air concentration from the other part.

The parts of the metal exposed to a higher

concentration of oxygen become cathodic while parts of the metal exposed to a relatively lower concentration of oxygen become anodic and get corroded.

Example: If a metal

like Zn is partially immersed in a dilute solution of a NaCl solution is not agitated properly, then, the parts above are more strongly aerated and hence, become cathodic. On the other hand, parts immersed to greater depth show a smaller oxygen concentration and thus, become anodic. So a difference of potential is created, which causes a flow of current between the two differentially-aerated areas of the same metal. Zinc will dissolve at the anodic areas, and oxygen will take up electrons at the cathodic areas to form hydroxyl ions. Zn

Zn2+ + 2e-

½O2 + H2O + 2e-

2OHZn rod

Cathode ½O2 + H2O + 2e-

2OH-

Anode NaCl solution

Differential aeration corrosion caused by partially immersion of Zn rod

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(iii) Pitting Corrosion: Pitting corrosion is a localized accelerated attack; it is usually the result of breakdown or cracking of the protective film on a metal at specific points. Breakdown of the 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. More oxygenated Cathodic part ½O2 + H2O + 2e-

Iron

2OH-

Pit (Anodic) Fe

Fe2+ + 2e-

Pitting corrosion on the surface of Iron The presence of the extraneous impurities like sand, dust, scale etc., embedded on the surfaces of metals also lead to pitting. Owing to the differential amount of oxygen in contact with the metal, the small part underneath the dust or sand becomes the anodic areas and the surrounding large parts become the cathodic area.

Intense corrosion

therefore starts underneath the impurity. Once a small pit is formed, the rate of corrosion will be increased. (iv)Crevice corrosion: Crevice corrosion is formed between different metallic objects or between a metal and non-metallic material joined by blot, nuts, rivets and washers. The crevice on coming into contact with a liquid becomes anodic region as the oxygen supply to this area is less compared to other parts and gets corroded preferentially.

====

Anode

Crevice Corrosion

9

Factors influencing the corrosion The rate and extent of corrosion mainly depends on 1. Nature of the metal 2. Nature of the environment.

1. Nature of the metal (i) Purity of metal: Impurities present in the metal generally form minute or tiny electrochemical cells and the anodic parts get corroded. For example Zinc metal containing impurity such as Pb or Fe undergoes corrosion of Zn due to the formation of electrochemical cells. Consequently, corrosion resistance of a metal may be improved by increasing its purity. (ii) Oxidation potential of the metal: The position in the electrochemical series is indicative of the natural tendency of the metal to undergo corrosion. When two metals are in contact with each other and simultaneously with an electrolyte, a galvanic cell is set up and the metal higher in the series undergoes corrosion. (iii) Overvoltage: When a metal, which occupies a high positon in galvanic series for example Zn is placed in 1N H2SO4, it undergoes corrosion forming a film and evolving hydrogen gas, the initial rate of reaction is quite slow, because of high overvoltage of zinc metal, which reduces the effective electrode potential to a small value. However, if a few drops of copper sulphate are added, the corrosion rate of zinc is accelerated, because some copper gets deposited on the zinc metal, forming minute cathodes, where the hydrogen overvoltage is only 0.33 V. Thus reduction in overvoltage of the corroding metal accelerates the corrosion rate. (iv) Passivity of the metal: Iron dissolves readily in very dilute nitric acid. However at higher concentration acid directly oxidizes the metal to its oxide on the surface. The layer of the oxide formed on the surface makes iron resistant to dissolution, a phenomenon known as passivity. Passive iron is not easily corroded as the oxide film is self-healing, that is, a ruptured film repairs itself on re-exposure to oxidizing conditions. 10

(v) Physical state of metal: The rate of corrosion is influenced by physical state of the metal such as grain size, orientation of crystals, stress etc. The smaller the grain-size of the metal or alloy, greater will be the corrosion. Also, the area under stress, tend to act as anodic and corrosion takes place at these areas. (vi) Relative areas of cathodic and anodic regions: The rate of corrosion is more with the combination of a large cathodic region and a small anodic region, because the greater demand for electrons at the larger cathodic region has to get a greater current density which is supplied by the smaller anodic region.

2. Nature of the corroding environment (i) Temperature: With increase of temperature of environment, corrosion rate is generally enhanced. (ii) Humidity in the atmosphere: Humidity of the air surrounding the metal influences corrosion, the greater the humidity higher being the rate of corrosion. Critical humidity is the humidity of the air above which the rate of atmospheric corrosion of the metal increases sharply and depends on the nature of the metal and the nature of the corrosion products. (iii) Presence of impurities in atmosphere: In presence of gases like CO2, H2S, SO2 the acidity of he liquid, adjacent to the metal surfaces, increases and its electrical conductivity also increases, resulting in higher rate of corrosion. (iv) Presence of suspended particles in atmosphere: If the suspended particles are chemically active in nature (NaCl) they absorb moisture and act as strong electrolytes, thereby causing enhanced corrosion. (v) Effect of pH: The pH of the surrounding medium plays an important role in influencing the rate of corrosion. In general acidic media are more corrosive compared to neutral or mildly alkaline media.

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Corrosion Control 1. Proper designing: (i) Avoid the contact of dissimilar metals in the presence of a corroding solution. (ii)When two dissimilar metals are to be in contact, the anodic material should have as large area as possible. (iii) Whenever the direct joining of dissimilar metals is unavoidable an insulating fitting may be applied in between them to avoid direct metal-metal electrical contact. (iv) A proper design should avoid the presence of crevices between adjacent parts of the structure. (v) It is desirable that the design allows for adequate cleaning and flushing of the critical parts of the equipment. Sharp corners and recesses should be avoided as they favor the formation of stagnant areas and accumulation of solids. 2. Using pure metal: Impurities in a metal cause heterogeneity, which decreases corrosion-resistance of the metal. Thus, the corrosion resistance of a given metal may be improved by increasing its purity. 3. Using metal alloys: Corrosion resistance of most metals is best increased by alloying them with suitable elements, but for maximum corrosion resistance, alloy should be completely homogeneous. Chromium is the best suitable alloying metal for iron or steel. 4. Cathodic protection: In this method the metal to be protected is forced to behave like a cathode, thereby corrosion does not occur. There are two types of cathodic protections: (i) sacrificial anodic protection method (ii) Impressed current cathodic protection (i) Sacrificial anodic protection method: The metallic structure to be protected is connected by a wire to a more anodic metal, so that all the corrosion is concentrated at this more active metal. The more active metal itself gets corroded slowly, while the parent structure is protected. The more active metal so-employed is called “sacrificial anode”. The corroded sacrificial anode block is 12

replaced by a fresh one, when consumed completely. Metals commonly employed as sacrificial anodes are magnesium, zinc, aluminium and their alloys.

Important

applications of sacrificial anodic method include protection of buried pipelines, underground cables, marine structures, ship-hulls, water tanks etc. e>

Mg Iron pipe Sacrificial anode Sacrificial anode –cathodic proctection (ii) Impressed current cathodic protection: In this method the object to be protected is made the cathode of an electrolytic cell by connecting it to the negative terminal of a DC source. The positive terminal of the DC source is connected to scrap iron, platinum, graphite, nickel or lead anode and buried or immersed in a conducting medium adjacent to the metal to be protected. The anode is usually in a backfill so as to increase the electrical contact with the surrounding soil. This type of cathodic protection has been applied to open water-box coolers, water tanks, buried oil or water pipes, condensers e-

Iron pipe

Impressed current-cathodic protection 13

Corrosion Inhibitors Inhibitors are inorganic or organic chemical substances which when added a small quantity to the corrosive environment, effectively decrease the corrosion rate. Inhibitors are classified into (i) Anodic inhibitors (ii) Cathodic inhibitors (iii) Vapour phase inhibitors (i) Anodic inhibitors: Oxidizing agents such as sodium chromate and sodium nitrite function as inhibitors of corrosion by repairing the protective oxide film or by oxidation of corrosion products to less soluble chemicals, which plug anodic sites. These are known as anodic inhibitors because they inhibit anodic oxidation of the base metal. Anodic inhibitors such as chromates, phosphates, tungstates or other ions of transition elements with high oxygen content retard the corrosion of metals by forming a sparingly soluble compound with newly produced metal cations at the anodic sites. This compound will then adsorb on the corroding metal surface forming a passive film or barrier thereby reducing the corrosion rate. (ii) Cathodic inhibitors: (a) In acidic solutions, the main cathodic reaction is evolution of hydrogen. 2H+ + 2e-

H2(g)

Consequently, corrosion may be reduced either by slowing down the diffusion of hydrated H+ ions to the cathode. The diffusion of H+ ions is considerably decreased by organic inhibitors like amines, mercaptans, heterocylic nitrogen compounds, substituted ureas and thioureas, which are capable of being adsorbed at the metal surfaces. (b) In neutral solutions, the cathodic reaction is ½O2 + H2O + 2e-

2OH-

The corrosion can be controlled either by eliminating oxygen from the corroding medium by adding reducing agents like Na2S or Na2SO3 or by retarding its diffusion to the cathodic areas by adding Mg, Zn or Ni salts. Vapor Phase Inhibitors (VPI): These inhibitors readily vaporize forming a protective layer on the metal surface. VPIs are used in the protection of storage containers, sophisticated equipment etc. Ex : Dicyclohexyl

ammonium nitrate, benzotriazole (BTA) etc., 14

Protective coatings Pretreatment of metal surface to be plated or coated: For proper adhesion of plating or coating the metal surface to be plated should be free from greases, oils, rusts and other corrosion products. Removal of theses impurities are carried out in the following ways: (i) Degreasing: Organic solvents such as CCl4, acetone, trichloro-ethylene are used to remove oils, greases present on the metal surface. (ii) Alkali cleaning: This method is used to remove old paint coating.

The base metal

containing old paint coating is removed by keeping it in an alkali cleaning agent. This treatment is always to be followed by a thorough rinsing with water. (iii) Sand blasting: In this method oxide scale present on the steel surface is removed by introducing sand into air stream under the pressure o 25-100 atmosphere. (iv) Pickling: In this process the base metal is immersed in an acid solution. This treatment dissolves any corrosion products present on the surface. Organic coatings – Paints: Paint is a mechanical dispersion mixture of one or more pigments in a vehicle. The vehicle is a liquid, consisting of non-volatile, film-forming material, drying oil and a highly volatile solvent, thinner. When a paint is applied to a metal surface oxidizes forming a dry pigmented film. Requisites of a good paint: 1. It should be fluid enough to spread easily over the protected surface. 2. It should possess high covering power 3. It should form a quite tough, uniform, adherent and impervious film 4. Its firm should not get cracked on drying. 5. It should protect the paint surface from corrosion effects of environment. 6. It should form film and the colour remain quite stable. 7. Its film should be glossy.

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Constituents of paints: A paint essentially consists of the following ingredients (i) pigments (ii) vehicle or drying oil (iii) thinner (iv) driers (v) fillers or extenders (vi) plasticizers and (vii) antiskinning agents. 1. Pigment: Pigment is a solid substance, which is an essential constituent of paint. Its functions are to (i) provide capacity to paint (ii) provide strength to paint (iii) provide desire colour to paint (iv) gives aesthetical appeal to the paint film. Examples: Pigments used are whites- white lead, zinc oxide, titanium oxide Red – red lead, ferric oxide, chrome red Blue-Prussian blue, Black – carbon black Brown – Brown umbre 2.Vehicle or drying oil: Drying oil is a film-forming constituent of the paint. These are glyceryl esters of high molecular weight fatty acids, generally present in animal and vegetable oils. Functions of drying oil, it’s a main film forming constituent, acts as a medium, gives toughness, adhesion, durability and water-proofness. Examples: The most widely used drying oil, are linseed oil, soyabean oil and dehydrated castor oil. 3.Thinners: It reduces viscosity of the paint, suspend the pigments, increase the elasticity of the paint film, help the drying of the paint film as they evaporate. Examples: Turpentine, xylol, kerosene etc. 4. Driers: Driers are oxygen-carrier catalysts. They accelerate the drying of the oil-film through oxidation, polymerization and condensation. Examples: resonates, linoleates, tungstates and naphthenates of Co,Mn, Pb and Zn. 5. Extenders or fillers: it increases durability of the paint, help to reduce the cracking of dry paint film. Examples: BaSO4, talc, asbestos, china-clay, magnesium silicate, calcium sulphate etc. 6. Plasticizers: it provides elasticity to the film and to minimize its cracking. Examples: tricresyl phosphate, triphenyl phosphate, tributly phthalate. 7. Antiskinning agents: it prevents gelling and skinning of the paint film. Example: polyhydroxy phenols. 16

Mechanism in drying of oils: The oil film, after it has been applied on the protected surface absorbs oxygen at the double bonds, forming peroxides, diperoxides and hydroperoxides which isomerise, polymerize and condense to form a characteristic tough, coherent, hard elastic, infusible highly cross linked structured macromolecular film. Wet paint (oil + pigment + extender + drier + thinner)

film of oil + pigment + drier Evaporation of thinner

pigmented film crossed linked structure oxidation and Polymerization of

Base material

Base material

Base material

Conjugated double bonds

CH2COO(CH2)7-CH=CH-CH2-CH=CH-(CH2)4-CH3 n

CH COO(CH2)7-CH=CH-CH2-CH=CH-(CH2)4-CH3 CH2COO(CH2)7-CH=CH-CH2-CH=CH-(CH2)4-CH3 Glyceride of linolenic acid (drying oil) Air oxidation and polymerization CH2COO(CH2)7-CH - CH-CH2-CH=CH-(CH2)4-CH3 O

O

CH COO(CH2)7-CH - CH-CH2-CH - CH-(CH2)4-CH3 O

O

CH2COO(CH2)7-CH - CH-CH2-CH - CH-(CH2)4-CH3 ether crosslink

O

O

CH2COO(CH2)7-CH - CH-CH2-CH - CH-(CH2)4-CH3 O

O

CH COO(CH2)7-CH - CH-CH2-CH - CH-(CH2)4-CH3 O

O

CH2COO(CH2)7-CH - CH-CH2-CH - CH-(CH2)4-CH3

17

O

O

Metallic coatings – Principle and methods of electroplating Principle of Electroplating: This process involves coating of a thin layer of one metal over another metal by passing direct current through an electrolytic solution. The base metal to be plated is made of cathode whereas the anode is made of either coating metal itself or an inert material in the electrolytic cell. Procedure: The article to be electroplated is first treated with organic solvent to remove oils, greases etc. then, it is made free from surface scales, oxides, etc., by treating with dil. HCl or H2SO4. The cleaned article is then made cathode of an electrolytic cell. The anode is either the coating metal itself or an inert material of good electrical conductivity. The electrolyte is a solution of a soluble salt of the coating metal. The electrolytic solution is kept in an electroplating tank. The anode and cathode are dipped in the electrolytic solution. When direct current is passed, coating-metal ions migrate to the cathode and get deposited there. Thus, a thin layer of coating-metal is obtained on the article, made as the cathode.

For brighter and smooth deposits, favourable conditions such as low

temperature, medium current-density and low metal-ion concentration are used. Electrolyte to replenish the loss

(+)

(-) Direct current source

Cathode Anode Electroplating

18

Factors affecting the electrodeposit: 1. Cleaning of the article to be plated: Pretreatment of the surface of any material to be electroplated is essential. Maximum coating adhesion can be obtained only, if the base metal surface is free from dirt and grease. 2. pH of the bath liquid: For a good electrodeposit the pH of the bath must be properly maintained. 3. Thickness: For decorative purposes, thin deposit is done, while for corrosion resistance, thick plating is required. 4. Composition of the electrolytic bath: Low metal ion concentrations re preferred, since they give rise to very adherent coating films. 5. Throwing power: Throwing power is the ability of electrolytic cell to give a deposit of uniform thickness over the entire cathodic area. When the cathode is regular in shape maximum throwing power is exhibited by an electrolytic system 6. Temperature: Most of the electroplating bath solutions should be used at room temperature. However, warm baths are also used to increase the solubility of electrolyte thereby increases the concentration and current density of the bath.

Electroplating of gold: Gold plating is a method of depositing of thin layer of gold on the surface of other metals, most such as copper or silver. Copper or silver is first electroplated with a suitable barrier metal like Sn, Ni or bronze to provide leveling and brightening to the substrate and to inhibit the migration of copper or silver into the gold layer. Process: The electroplating of gold is carried out by using either neutral cyanide bath or acid cyanide bath. Anode: inert metal

Cathode:Cu or Ag or Cd.

Electrolyte: Gold potassium cyanide K[Au(CN)2] Temperature: 70-80˚C pH= 6-8 19

Current density : 1-40 mA/cm2 Additives / Salts /Acids: Citrate, phosphate, phosphoric acid. Mechanism: during electrolysis, the electrolyte is decomposed into Au+ ions. K+ + [Au(CN)2]-

K[Au(CN)2] [Au(CN)2]-

Au+ + 2 CN-

At cathode, deposition of Au+ occurs Au+ + e-

Au

Applications : Gold plating is often used in electronic industries for making printed circuit boards, semiconductor lead-out connection because of high electrical conductivity (ii) gold plating of silver is used in the manufacture of jewelry.

Electroless plating: Definition: The process of producing a thin, uniform and hard deposit of metal on an activated substrate (Metal or non-metal) by using suitable soluble reducing agents without any electrical energy, and the driving force for the deposition is auto catalytic redox reactions. The reducing agent reduces the metallic ions to metal, which gets plated over the catalytically activated surface giving a uniform thin coating. Metal ions + Reducing agent

Metal + Oxidised products

Process: The process involves 1. Pretreatment or activation of work piece to be plated. 2. Preparation of bath composition. 1. Pretreatment or activation of work piece to be plate (i) Metals like Cu, Ag etc. are known as non-catalytic metals. Surface of such metals need activation. They are activated by using steel or iron pieces for initiating the reactions. 20

(ii) Non-metals like glasses, ceramic, plastics are activated by dipping in SnCl2, PdCl2 in HCl. This process produces a thin film of palladium coating on non-metal surfaces which in turn causes the work piece to get activate for electroless plating. 2. Preparation of bath composition Name of the ingredient Coating metal ion Reducing agent Buffer Complexant Exaltant Stabilizer Brightener

Function

Examples

To provide metal ion for NiCl2, NiSO4, CuSO4 etc. deposition To liberate electrons for the Sodium hypophosphite, reduction of metal ion sodium boron hydride To maintain pH Sodium acetate, NaOH, Roschelle salt To improve the quality of Sodium acetate, sodium deposit. citrate To increase the rate of Succinate, Maleate, Lactate deposition to prevent decomposition of Cations like Pb, Ca etc., the plating bath To improve the brightness Thiourea, Sodium benzoate of the deposit

Electroless plating of nickel: 1. Pretreatment and activation of surface The surface to be plated is first degreased by using organic solvents or alkali, followed by acid treatment. Example (i) The surface of the stainless steel is activated by dipping in hot solution of 50% dil H2SO4 (ii) Metals and alloys of Aluminum, copper, Iron etc., can be directly Nickel plated without activation. 2. Preparation of plating bath composition NiCl2,.6H2O NaH2PO2H2O (Sodium hypophosphite) Sodium acetate Sodium succinate Temp

20g/l 20 g/l

- coating solution - reducing agent

10 g/l 15 g/l 85-95˚C

- Buffer - complexing agent 21

pH

4-6

Mechanism: The pretreated object is immersed in the plating bath for the required time. At Anode: The reducing agent, NaH2PO2 in solution, moves toward the activated substrate and liberates electrons by anodic oxidation. H2PO2- + H2O

H2PO3- + 2 H+ + 2e-

At Cathode: The salt solution containing Ni2+ ions gain electrons and get deposited Ni2++ 2e-

Ni

The over all reaction at the surface of work piece is Ni2+ + H2PO2- + H2O

Ni + H2PO3- + 2 H+

Condition during Electroless plating: (a) During the redox reaction, both Ni ions and sodium hypophosphite are consumed, so these are replenished periodically. (b) Maximum plating obtained at 93C, still higher temperature may cause the decomposition of the bath. (c) H+ ions are liberated in the redox reaction so pH of the bath solution decrease during the process, so addition of buffer is essential to get quality plating. Advantages of electroless nickel plating: 1. It gives rise to harder surface with better wear resistance due to plating of Ni-P alloy. 2. Free from pores and possess better corrosion resistance property. 3. Due to excellent throwing power the object having intricate part with irregular shapes can be plated.

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Applications: (i)

Electroless Ni-P coatings are used in various electronic applications.

(ii)

Electroless nickel deposition on polymers find preferred decorative as well as functional applications.

(iii)

Heat treated electroless nickel coatings finds applications in hydraulic compressors, pressure vessels, pumps and fuel injection assemblies.

(iv)

Plastic cabinets coated with copper and nickel finds applications in digital as well as electronic instruments.

Advantages of electroless plating: (i)

Does not require electrical power source.

(ii)

It has better throwing power.

(iii)

Intricate parts with irregular shapes can be uniformly coated.

(iv)

By adding complexes and additive agents the quality of electroless deposit can be improved.

(v)

Electroless deposit is less porous hence it gives better chemical, mechanical and magnetic properties.

References: (a) Engineering Chemistry, Jain and Jain, Dhanpat Rai Publishing Co., 15th Edition. (b) Engineering Chemsitry, B. Sivasankar, Tata McGraw Hill Co.,

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