unit_-_ii corrosion and control

7/29/2019 Unit_-_ii Corrosion and Control

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Unit - II

UNIT II CORROSION AND CORROSION CONTROL

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.

Lecture Session No: 10 Topic: Chemical corrosion, corrosion due to Oxygen

Pilling Bedworth rule on corrosion products

Introduction:

Metals exist in nature in combined state as their oxides, carbonates, chlorides,

sulphides etc. They are reduced to their metallic state during extraction. When metals

are exposed to environment their surfaces begin to decay as they come in contact with

gaseous or liquid environment. This process of deterioration and ultimate destruction

of a metal due to its reaction with the surrounding is called Corrosion. During

corrosion the metals exhibit their natural tendency to revert to their native combined

state of existence as oxides, sulphates, carbonates etc. The most familiar example for

corrosion is Rusting of iron when exposed to atmospheric conditions. During this a

layer of reddish scale and powder of oxide (Fe2O3) is formed. 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 (or) chemical corrosion

(b) Wet corrosion (or) electrochemical corrosion

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(a) Dry 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.

Oxidation corrosion:

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-

O2 + 2e- O2-

The overall reaction:

M + O2 M2+ O2- (metal-oxide film)

Oxidation occurs first at the surface of the metal and the resulting metal oxide

layer acts as the barrier for further reaction. For oxidation to continue either the metal

must diffuse outwards or the oxygen must diffuse inwards. In general, outward

diffusion of metal ions and electrons is likely to be more rapid than inward diffusion,

due to the fact that cations are smaller in size compared to the oxide ions.

M Mn+ + ne- (oxidation)

Mn+

Mn+ O2-

Mn+

e- O2-

Metal-M O2-

O2 + 2e- O2-(reduction)

Mechanism of Oxidation of Metal to Metal oxide

The nature of the oxide formed plays an important role in oxidation corrosion process

i.e. Metal + oxygen metal oxide (corrosion product)

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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

Metal oxide Metal + Oxygen

Consequently, oxidation corrosion is not possible, thus metals like Ag, Pt, Au do not

undergo oxidation corrosion.

Unstable metal oxideExposed area

Metal +O2 Metal metal oxide Metal + 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 Volatile metal oxide

Fresh surface

Metal +O2 Metal metal oxide Metal exposed forFurther 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 porous metal oxide

Metal +O2 Metal Further attack throughpores/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)

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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

respectively. The ratio r indicates the volume of oxide formed from a unit volume of

metal. If R1, 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.

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Lecture Session No: 11 Topic:Electrochemical corrosion Hydrogen evolution

and oxygen absorption cathodic process, different types galvanic corrosion

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.

M M2+ + 2e-

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:

This type of corrosion occurs in acidic environment. Considering metal like

Fe, the anodic reaction is dissolution of iron as ferrous ions with the liberation of

electrons.

Fe Fe2+ + 2e-

These electrons flow through the metal, from anode to cathode, where H+ ions

are

eliminated as hydrogen gas.

Electrolyte

Iron H+Cl-

Fe2+ H+Cl-

2e- H+Cl-

H2 H+Cl-

H+Cl-

Mechanism of Hydrogen evolution

2H+ + 2e- H2

The net reaction is Fe + 2H+ Fe2+ + H2

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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 well-metal 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.

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

electrons.

Fe Fe2+ + 2e-

Iron Aq. Soln.

H2O

Fe2+ H2O

2e- OH- O2

H2O 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:

O2 + 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

This product is called the yellow rust, actually corresponds to Fe2O3.H2O.

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If the supply of oxygen is limited, the corrosion product may be even black anhydrous

magnetite, Fe3O4.

Types of Corrosion:

(i) Galvanic Corrosion:

When two dissimilar metals are electrically connected 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. For example if iron and

copper are connected, here Fe acts as anode i.e. more active metal when compared

to copper which acts as cathode. Thus the corrosion occurs at the anode and the

cathode is protected.

Fe Cu

More active Less active

Galvanic corrosion

The galvanic corrosion may be avoided by a proper selection of metals and

alloys based on their position in galvanic series.

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Lecture Session No: 12 Topic: Differential aeration corrosion factors

influencing corrosion

(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-

O2 + H2O + 2e- 2OH-

Zn rod

Cathode

O2 + H2O + 2e- 2OH-

Anode

NaCl solution

Differential aeration corrosion caused by partially immersion of Zn rod

(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.

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More oxygenated Cathodic part

O2 + H2O + 2e- 2OH-

Iron 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

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

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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 evolvinghydrogen 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.

(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.

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2. Nature of the corroding environment

(i) Temperature:

With increase of temperature of environment, corrosion rate is generallyenhanced.

(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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Lecture Session No: 13 Topic: Corrosion control sacrificial anode and

impressed cathodic current methods

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 replaced by a fresh one, when consumed completely. Metals

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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

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Lecture Session No: 14 Topic: Corrosion inhibitors Anodic, cathodic and VPI

Inhibitors are inorganic or organic chemical substances which when added in small

quantity to the aqueous corrosive environment, effectively decrease the corrosion rate.

Inhibitors are classified as

(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

O2 + 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.

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Lecture Session No: 16 Topic:Protective coatings Paints constituents and

functions

Protective coatings

Protecting the surface of an object by the application of coating is a common

procedure for corrosion protection. A coated surface isolates the underlying metal

from the corroding environment. The coating applied must be chemically inert and it

must prevent the penetration of the environment to the material which they protect.

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

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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.

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, its 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.

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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.

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 + film of oil + pigment pigmented filmextender + drier + thinner) + drier crossed linked

structure

Evaporation of thinner oxidation andPolymerization of

Base material Base material Base material

Conjugated double bonds

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

n CH2COO(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 oxidationand polymerization

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

O O

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

O O


Peroxide crosslink O O


O O


O O


O O

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Unit - II

Lecture Session No: 17 Topic: Metallic coatings Principle and methods of

electroplating, electroplating (Au)

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

Factors affecting the electrodeposit:

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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: 70-80C

pH= 6-8

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]-

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[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.

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Lecture Session No: 18 Topic: Principle and methods of electroless plating

,electroless (Ni) plating

Electroless plating:

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

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.

(ii) Non-metals like glasses, ceramic, plastics are activated by dipping in SnCl,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 Function Examples

Coating metal ion To provide metal ion for

deposition

NiCl2, NiSO4, CuSO4 etc.

Reducing agent To liberate electrons for

the reduction of metal ion

Sodium hypophosphite,

sodium boron hydride

Buffer To maintain pH Sodium acetate, NaOH,

Roschelle salt

Complexant To improve the quality of

deposit.

Sodium acetate, sodium

citrate

Exaltant To increase the rate of

deposition

Succinate, Maleate,

Lactate

Stabilizer to prevent decomposition

of the plating bath

Cations like Pb, Ca etc.,

Brightener To improve the brightness

of the deposit

Thiourea, Sodium

benzoate

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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 20g/l - coating solution

NaH2PO2H2O 20 g/l - reducing agent(Sodium hypophosphite)

Sodium acetate 10 g/l - Buffer

Sodium succinate 15 g/l - complexing agent

Temp 85-95C

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 platingof Ni-P alloy.

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Unit - II

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.

Applications:

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

2. Electroless nickel deposition on polymers find preferred decorative as well

as functional applications.

3. Heat treated electroless nickel coatings finds applications in hydraulic

compressors, pressure vessels, pumps and fuel injection assemblies.

4. Plastic cabinets coated with copper and nickel finds applications in digital

as well as electronic instruments.

Advantages of electroless plating:

1. Does not require electrical power source.

2. It has better throwing power.

3. Intricate parts with irregular shapes can be uniformly coated.

4. By adding complexes and additive agents the quality of electroless deposit

can be improved.

5. 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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unit_-_ii corrosion and control

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