Chapter II

CORROSIONand its CONTROL

CORROSION AND ITS CONTROL

Corrosion is defined as the gradual destruction or deterioration of metals or alloys by the chemical or electrochemical reaction with its environment.

DEFINITION

Due to formation of corrosion product over the machinery, the efficiency of the machine gets lost.

The products gets contaminated due to corrosion.

The corroded equipment must be replaced frequently.

Plant gets failure due to corrosion.

Corrosion releases toxic products, health hazard, etc.

CONSEQUENCES OF CORROSION

Based on the environment, corrosion is classified in to 1. Dry or Chemical corrosion 2. Wet or Electrochemical corrosion.

Dry or Chemical corrosion: Dry corrosion is due to the attack of metal surfaces

by the atmospheric gases such as oxygen, hydrogen sulphide, sulphur dioxide, nitrogen, inorganic liquids etc.

There are three main types of dry corrosion;

1. Oxidation corrosion (or) corrosion by oxygen 2. Corrosion by hydrogen. 3. Liquid – metal corrosion.

CLASSIFICATION OF CORROSION

OXIDATION CORROSION

Oxidation corrosion is brought about by the direct attack of oxygen at low or high temperatures on metal surface in the absence of moisture.

Alkali metals like (Li, Na, K, etc) and alkaline-earth metals (Mg, Ca, Sr, etc) are rapidly oxidised at low tempt.

At high temperature, almost all metals (expect Ag, Au and Pt) are oxidized.

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Oxidation occurs first at the surface of the metal resulting in the formation of metal ions (M2+), which occurs at the metal/oxide interface.

M → M2+ +2e-

MECHANISM OF DRY CORROSION

Oxygen changes to ionic form (O2-) due to the transfer of electron from metal, which occurs at the oxide film / environment interface.

½ O2 + 2e- → O2-

Oxide ions reacts with the metal ion to form the metal- oxide film.

M + ½ O2 → M2+ +O2- ≡ MO (Metal-oxide film)

NATURE OF OXIDE FILM

The nature of oxide film formed on the metal surface plays in important role in oxidation corrosion.

(i) STABLE OXIDE LAYER

A stable oxide layer behaves as a protective coating and no further corrosion can develop.Example: oxides of Al, Sn, Pb, Cu, etc., are stable oxide layers.

(ii) UNSTABLE OXIDE LAYER

Unstable oxide layer is mainly produced on the surface of noble metals, which decomposes back in to the metal and oxygen.

Metal oxide → Metal +Oxygen

Example: Oxides of Pt, Ag, etc., are unstable oxide layers.

(iii) VOLATILE OXIDE LAYER

The oxide layer volatilizes as soon as it is formed, leaving the metal surface for further corrosion.

Example: Molybdenum oxide is volatile.

(iv) POROUS OXIDE LAYER Metal oxides having pores and cracks allow penetration of oxygen to the underlying metal, resulting in the complete conversion of metal into its oxide.

PILLING –BEDWORTH RATIO

• The ratio of the volume of the oxide formed to the volume of the metal consumed is called “pilling-bedworth ratio”.

R = Md/nmD

Where M - mass of metal oxidem - atomic weight

d - density of the metalD - density of the scale

n- number of metal atoms in a formula of the scale

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(PILLING-BEDWORTH RULE)

According to Pilling–Bedworth rule,

1. If the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is porous and non-protective.Example: Oxides of alkali and alkaline earth metals.

2. If the volume of the oxide layer formed is greater than the volume of the metal, the oxide layer is subjected to cracking and spalling resulting in poor oxidation resistance and protection.Example: Oxides of heavy metals such as Sb,V, W, etc.

3. If the volume of oxide layer is equal to the volume of the metal then there will be protective oxide film formation.Example: Oxides of Al, Pb, Ni, etc.

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CORROSION BY OTHER GASES

Gases like SO2, CO2, Cl2, H2S etc – induce corrosion

action on metals

Corrosion effect depends on the chemical affinity between the metal and the gas.

The degree of attack by gases depends on the formation of protective or non-protective films on the metal surface.

(1) If the film is non-porous or protective, the intensity of

attack decreases. Eg. AgCl film – attack of Cl2 on Ag

(2) If the film is porous or non-portective, the surface of the whole metal is gradually destroyed.

Eg. Dry Cl2 gas attack on Sn – formation of volatile

SnCl4 – fresh surface is exposed for further attack

(a) HYDROGEN EMBRITTLEMENT: Contact of metal with H2S results in the generation

of atomic hydrogenE.g. Fe + H2S → FeS + 2H

H + H → H2 ↑

Corrosion caused by the exposure of metal to hydrogen environment.

Hydrogen in atomic state diffuses into the metal matrix and collects in the voids present in the metal.

The hydrogen atoms combine to form H2 gas. A very high pressure is developed, which results in

cracks and blisters on metal. This process is called hydrogen embrittlement.

CORROSION BY HYDROGEN

(b) DECARBURISATIONAt higher temperature atomic hydrogen is formed by the thermal dissociation of molecular hydrogen.

H2 → 2HWhen steel is exposed to this environment, the atomic hydrogen readily combines with carbon of steel and produces methane gas.

C + 4H → CH4↑

Collection of these gases in the voids develop very high pressure, which causes cracking. Thus the process of decreases in carbon content in steel is termed as “Decarburisation” of steel.

This is due to the chemical action of flowing liquid metal at high temperature. The corrosion reaction involves either,

(i) Dissolution of a solid metal by a liquid metal (or)

(ii) Liquid metal may penetrate in to the solid metal.

LIQUID - METAL CORROSION

WET (or) ELECTRO-CHEMICAL

CORROSION

Wet corrosion occurs under the foll. conditions,

When two dissimilar metals are in contact with each other in the presence of an aqueous solution or moisture.

When two dissimilar parts of the same metal are in contact with an aqueous solution of an electrolyte.

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MECHANISM OF WET CORROSIONAccording to electrochemical theory of corrosion, the wet corrosion involves two steps.a) Oxidation or metal dissolutionb) Reduction

Metal dissolution occurs always at anode leading to the formation of metal ions and electrons

M → Mn+ + ne-

At cathode, electron consumption resulting in either evolution of hydrogen or absorption of oxygen depending on the nature of corrosion environment.

(a) Acidic environment

If the corrosive environment is acidic, hydrogen evolution occurs at cathodic part.

2H+ + 2e- → H2↑

(b) Alkaline or Neutral environment

If the corrosive environment is slightly alkaline (or) neutral, hydroxide ion forms at cathodic part.

½ O2 + 2e- +H2O → 2OH-

Thus the metal ions (from anodic part) and non-metallic ions (from cathodic part ) diffuse towards each other through conduction medium and form a corrosion product between anode and cathode.

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a) Hydrogen evolution type corrosion

Metals have negative reduction potential i.e., below hydrogen reduction potential value in the electrochemical series get dissolved in acidic solution with simultaneous liberation of “hydrogen gas”.

Example: Iron metal in contact with non-oxidizing acid like HCl results in H2 evolution

At anode: Iron (Fe) undergoes dissolution to Fe2+ with the liberation of electrons.

At cathode: The liberated electrons follow from anode to cathode, where H+ ions get reduced to H2.

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b) Absorption of oxygen or formation of hydroxide ion type corrosion

Fe surface usually contains a coating of Iron oxide and if this oxide layer develops, cracking happens.

As a result anodic areas are created on the surface and remaining area acts as cathode.

Example: Iron metal in contact with a neutral solution of electrolyte in the presence of O2, OH- ions are formed.

At anode: Iron (Fe) undergoes dissolution to Fe2+ with the liberation of electrons.

At cathode: The liberated electrons follow from anode to cathode, where dissolved O2 is consumed to form OH- ions.

1/2O2 + H2O + 2e- 2OH-

Fe2+ + 2OH- Fe(OH)2

Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (rust)

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Chemical Corrosion Electrochemical Corrosion

It occurs only in dry condition

It occurs in the presence of moisture or electrolyte

It is due to the direct chemical attack of the metal by the environment

It is due to the set up of a large number of cathodic and anodic areas

Even a homogeneous metal surface gets corroded

Hetergeneous surface or bimetallic contact is required for corrosion

Corrosion products accumulate in the same place, where corrosion occurs.

Corrosion occurs at the anode, while products formed elsewhere

Chemical corrosion is self-controlled

It is continuous process

It follows adsorption mechanismEg. Formation of mild scale on iron surface

It follows electrochemical reactionEg. Rusting of iron in moist atmosphere

TYPES OF ELECTROCHEMICAL CORROSION

GALVANIC CORROSION

When two different metals are in contact with each other in the presence of an aqueous solution or moisture, galvanic corrosion occurs. Here, the more active metal (with more negative electrode potential) acts as anode and the less active metal (with less negative potential) acts as cathode.

Example for galvanic corrosion

Steel screw in a brass marine hardware corrodes. This is due to galvanic corrosion. Iron (higher position in electrochemical series) because anodic and is attacked and corroded, while brass (lower in electrochemical series) acts as cathodic and is not attacked.

Bolt and nut made of the same metal is preferred. Why? It is preferred in practice, because galvanic corrosion is avoided due to homogeneous metals (no anodic and cathodic part).

Prevention: Avoid unfavorable area effect Selection of metals & alloys Insulating dissimilar metals Using inhibitors Applying cathodic protection

This type of corrosion occurs when a metal is exposed to varying concentration of oxygen or any electrolyte on the surface of the base metal.

Example

• Metals partially immersed in water (or) conducting solution (called water line corrosion).

• If a metal is partially immersed in a conducting solution the metal part above the solution is more aerated and hence become cathodic.

• On the other hand, the metal part inside the solution is less aerated and thus, become anodic and suffers corrosion.

DIFFERENTIAL AERATION CORROSION

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At anode (less aerated) corrosion occurs M → M2+ +2e-

At cathode (more aerated part) OH- ions are produced ½ O2 +H2O + 2e- → 2 OH-

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Examples for differential aeration corrosion

Pitting or localized corrosion Crevice corrosion Soil corrosion. Corrosion on wire fence.

(a) Pitting Corrosion

This is a localised attack, resulting in the formation of a hole around which the metal is relatively unattacked.

Example Metal area covered by a drop of water, sand, dust, scale etc.,

• Consider a drop of water or aqueous NaCl resting on a metal surface.

• The area covered by the drop of H2O acts as an anode due to less O2 conc., and suffers corrosion.

• The uncovered area (freely exposed to air) acts as a cathode due to high oxygen concentration

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The rate of corrosion will be more, when the area of cathode is larger and the area of anode is smaller. A small hole or pit is formed on the surface of the metal.

At anodeIron is oxidized to Fe2+ ions (Fe → Fe2+ +2e-)

At cathodeOxygen is converted to OH- ions. (1/2O2 +H2O +2e- → 2OH-)

Net reaction isFe2+ +2OH- → Fe(OH)2 →Fe(OH)3

This type of corrosion is called pitting.

(b) Crevice corrosion

• If a crevice between different metallic objects or between metal and non-metallic material is in contact with liquids, the crevice becomes the anodic region and suffers corrosion.

• This is due to less oxygen with crevice area. • The exposed areas act as the cathode.

(c) PIPELINE CORROSION• Differential aeration corrosion may also

occur in different parts of pipeline.• Buried pipelines or cables passing from

one type of soil to another, example from clay (less aerated) to sand (more aerated) may get corroded due to differential aeration.

(d) CORROSION ON WIRE-FENCE• A wire fence in which the areas where the wires cross

are less aerated than the rest of the fence and hence corrosion occurs at the wire crossings, which are anodic.

Other examples for differential aeration corrosion (i) Corrosion occurring under metal washers, where

oxygen cannot diffuse easily.

(ii) Lead pipeline passing through clay to cinders undergo corrosion. Since the pipeline under cinders is more aerated, it gets corroded easily.

FACTORS INFLUENCING CORROSION

The rate and extent of corrosion dependsmainly oni. Nature of the metalii. Nature of the environment

Nature of the metal

A) Position in EMF series

The extent of corrosion depends upon the position of the metal in the EMF series.

Greater the oxidation potential, greater is the rate of corrosion.

When two metals are in electrical contact, the metal higher (-) ve electrode potential in the EMF series becomes anodic and suffers corrosion.

Further, the rate and severity of corrosion depends upon the difference in their positions in the EMF series.

Greater the difference, faster is the corrosion of anodic metal.

(b) Relative areas of the anode & cathode

The rate of corrosion is more when area of the cathode is larger.

When cathodic area is larger, the demand for electrons will be more and this results in an increased rate of dissolution of metals at anodic regions.

(c) Purity of the metal

The 100% pure metal will not undergo any type of corrosion.

But, the presence of impurities in a metal create heterogeneity and thus galvanic cells are sets up with distinct anodic and cathodic area in the metal.

Higher the percentage of impurity, faster is the rate of corrosion of the anodic metal.

(d) Physical state of the metal Metal components subjected to unevenly

distributed stresses are easily corroded.

Even in a pure metal, the areas under stress tend to be anodic and suffer corrosion.

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(f) Nature of the Corrosion Product

If the corrosion product is soluble in the corroding medium, the corrosion of the metal will proceed faster.

On the other hand, if the corrosion product is insoluble, then the protective film formed will tend to suppress corrosion.

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Metals such as Mg, Ca, Ba, etc., form oxides whose volume is less than the volume of the metal.

(G) Nature of the oxide film

Hence, the oxide film formed will be porous, through which oxygen can diffuse and bring about further corrosion.

On the other hand metals like Al, Cr, Ni etc. form oxides whose volume is greater and the non-porous oxide film so formed will protect the metal from further corrosion.

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NATURE OF THE ENVIRONMENT

(A) Temperature The rate of chemical reaction and the

rate of diffusion of ions increases with rise in temperature

Hence, corrosion increases with temperature.

A passive metal may become active at a higher temperature.

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(B) Humidity

The rate of corrosion will be more, when the humidity in the environment is high.

The moisture acts as a solvent for the oxygen in the air to produce the electrolyte, which is essential for setting up a corrosion cell.

Rusting of iron increases when the relative humidity of air reaches from 60 to 80 percent.

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(C) Effect of pH

The rate of corrosion is maximum when the corrosive environment is acidic.In general, acidic environment is more corrosive than alkaline or neutral medium.Corrosion rate can be reduced by increasing the pH of the medium.But metals such as Al, Zn dissolves in alkaline medium.

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(D) NATURE OF THE ELECTROLYTE

If the electrolyte consists of silicate ions, they form insoluble silicates and prevent further corrosion.

On the other hand if chloride ions present, they destroy the protective film and the surface is exposed for further corrosion.

If the conductance of electrolyte is more, the corrosion current is easily conducted and hence the rate of corrosion is increased.

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(E) Conductance of the corroding medium

The corrosion current depends upon the internal resistance of the short circuited cells, which in turn depends on the conductance of the electrolytic medium.If the conductance of the soil is more, the corrosion of underground structure will also be more.Conductance in clayey, soils and mineralized soils is more.

CORROSION CONTROL

The rate of corrosion can be controlled by either modifying the metal or the environment.

1. Proper DesigningA major factor in the corrosion failure of a component is a faulty geometrical design.Some important design principles are:1) Avoid crevices2) Avoid residual moisture3) Avoid galvanic corrosionGalvanic corrosion can be prevented by the following methods,a) Use an electrical insulatorsb) Introduce an easily exchangeable corroding places4) Avoid protruding parts.

2. Cathodic Protection• The principle involved in cathodic

protection is to force the metal to be protected to behave like a cathode.

• Since, there will not be any anodic area on the metal, corrosion does not occur.

There are two types of cathodic protection.• Sacrificial anodic protection method• Impressed current cathodic protection

method.

SACRIFICIAL ANODIC PROTECTION METHOD

• In this method, the metallic structure to be protected is made cathode by connecting it with more active metal (anodic metal).• So that all the corrosion will concentrate only on the active metal.• The artificially made anode thus gradually gets corroded protecting the original metallic structure.• Hence this process is otherwise known as sacrificial anodic protection.

Examples of sacrificial anode• This method is used for the

protection of ships and boats.• Sheets of zinc and magnesium are

hung around the hull of the ship.• Zinc and magnesium being anodic to

iron get corroded.• Since they are sacrificed in the

process of saving iron (anode), they are called sacrificial anodes.

• Protection of underground pipelines and cables from soil corrosion.

• Magnesium rods are inserted in to domestic water boilers or tanks to prevent the formation of rusty water.

• Calcium metal slag's are employed to minimize engine corrosion.

Applications of Sacrificial Anode

IMPRESSED CURRENT CATHODIC PROTECTION

In this method, an impressed current is applied in the opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode.This can be done by connecting negative terminal of the battery to the metallic structure to be protected.Positive terminal of battery is connected to an inert anode. inert anode used for this purpose is graphite or platinised titanium.

The anode is surrounded by ‘backfill’ (containing mixture of gypsum, coke, breeze, sodium sulphate) to improve the electrical contact between the anode and the surrounding soil.

APPLICATION OF IMPRESSED CURRENT PROTECTION

This type of cathodic protection is applied to open water-box coolers, water tanks, buried oil and water pipes, condensers, marine piers, transmission line towers, etc.,

Comparison of sacrificial anode and impressed current cathodic method

Sacrificial anodicmethod

No external power supply is necessary.This method requires periodical replacement of sacrificial anode.Investment is low.

Impressed current method

External power supply must be present.Here anodes are stable and do not disintegrate.Investment is more.

Soil corrosion effects are not taken in to account.This is most economical method especially when short-term protection is required.

Soil corrosion effects are taken in to account.

This method is well suited for large structures and long term operations.

Control of corrosion by modifying the environment

DEAREATIONThe presence of increased amount of oxygen is harmful and increase the corrosion rate.Deareation involves removal of dissolved oxygen by increase of temperature with mechanical agitation.It also removes dissolved CO2 of water.

In this method, moisture from the air is removed by lowering the relative humidity of the surrounding air.This is done by adding silica gel (or) alumina, which adsorbs moisture preferentially on its surface.

DEHUMIDIFICATION

ALKALINE NEUTRALISATION The acidic character of the corrosive

environment (due to presence of H2S, HCl, CO2, SO2, etc) can be neutralized by spraying alkaline neutralisers (like NH3, NaOH, lime etc).

CORROSION INHIBITORS

DEFINITIONA corrosion inhibitors is ‘a substance which when added to in small quantities to the aqueous corrosive environment effectively decreases the rate of corrosion of the metal’.

Inhibitors are classified in to three types, ANODIC CATHODIC VAPOUR PHASE

Chromates, phosphates, tungstates of transition elements, inhibit the anodic corrosion reaction by forming sparingly soluble compound with a newly produced metal ion.

They are absorbed on the metal surface forming a protective film or barrier there- by reducing corrosion rate.

This kind of corrosion rate is not fully reliable since certain areas left uncovered by the film can produce severe corrosion.

ANODIC INHIBITORS

In acidic solution, the main cathodic reaction is evolution of hydrogen.

2H+(aq) +2e- → H2 (g)

• In an acidic solution, the corrosion can be controlled by slowing down the diffusion of H+ ions through the cathode.

• This can be done by adding organic inhibitors like amines, pyridine, azoles, etc.They absorb over the cathodic metal surface and act as a protective layer.

CATHODIC INHIBITORS

In a neutral solution, the cathodic reaction is,

H2O + ½ O2 + 2e- → 2OH-(aq)

The formation of OH- ions is only due to the presence of oxygen.By eliminating the oxygen from the medium, the corrosion rate can be reduced.O2 can be removed by adding some reducing agents like Na2SO3 or by deaeration.Salts of Zn, Mg, Ni are employed as they form insoluble metallic hydroxide which forms impermeable self barriers.

VAPOUR PHASE INHIBITORS

Vapour phase inhibitors are organic inhibitors which readily sublime and form a protective layer on the metal surface.

Example : Dicyclohexyl ammonium nitrite, Benzotriazole.

Vapour phase inhibitors are used in the protection of machineries, sophisticated equipments, etc. which are sent by ships.

The condensed inhibitor can be easily wiped off from the metal surface.

PROTECTIVE COATING

INTRODUCTION Protective coatings are used to protect

the metals from corrosion.

It acts as a physical barrier between the coated metal surface and the environment.

They impart some special properties such as hardness, electrical properties and thermal insulating properties to the protected surface.

Protective coatings

Inorganic coating• Metallic coating• Chemical Conversion

Organic coating 1. Paints2. Varnishes3. Enamels4. Ceramic

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• Mechanical cleaning – To remove loose scale and rust, using hammer, wire-brushing, grinding and polishing.

• Sandblasting – To clean large surface areas in order to produce enough roughness for good adherence of protective coating, using sand with air stream at 25-100 atm.

• Solvent Cleaning – To remove oil, grease, rust using organic solvents like alcohol, xylene, toluene, hydrocarbons followed by cleaning hot water or steam.

Sample Preparation

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Sample Preparation (contd..)

• Alkali Cleaning – To remove old paints that are soluble in alkaline medium using chemicals like NaOH, Na3PO4 etc. After cleaning, the metal is washed with 1% chromic acid solution.

• Acid pickling and etching – Base metal is dipped inside acid solution at a higher tempt for a long duration. Acids used are HCl, H2SO4, H3PO4, HNO3, under dilute conditions.

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Metallic coatings

• Anodic coating – Galvanization:

It is produced by anodic coating metals (Zn, Al, Cd) on the surface of base metal (Fe) based on the relative negative electrode potential.

• Cathodic coating:

It is produced by cathodic coating metals (Sn, Cr, Ni) on Fe surface based on the relative positive electrode potential of coat metal.

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Methods of application of metallic coating

• Hot dipping

• Metal cladding

• Electroplating – Cu, Cr, Ni, Au, Ag

• Cementation

• Vacuum metalizing

• Metal spraying

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Hot Dipping

• It is one of the common method of applying metallic coating on the surface of base metals.

• Hot dipping is a process of coating the base metal by immersing it in the molten coat metal.

• Examples: Galvanizing and Tinning

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Galvanizing: Fe or steel is coated with a thin coat of Zn by immersing in molten Zn to prevent rusting.

• Fe or steel base metal is cleaned by acid pickling using dil. H2SO4 (60-90 0C) for 15-20 min.

• Base metal dipped in molten Zn (430 0C) and then passed through rollers to correct the thickness of the film.

• NH4Cl flux used to protect the surface of molten Zn from oxide formation.

• Annealed at 250 0C and cooled slowly• Coating of Iron pipes, screws, bolts, wires, etc.• Poisonous for utensils that store food stuffs

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Tinning: In this process tin is coated over mild steel sheets immersed in molten tin (Sn).

• The sheet is subject to acid pickling and passed through a bath of molten tin covered with a flux of ZnCl2.

• After coating, the sheet is passed through palm oil to protect from oxidation

• Finally the sheet is passed to roller to get uniform thickness.

• It is used for the coating of steel, Cu and brass sheets that store food stuffs.

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Metal Cladding

• It is the process of sandwitching the base metal between two thin layers of coating metal by hot-rolling the composite to produce a firm bonding.

• The coat metals are usually metals of least reactivity (Cu, Ni, Ag, Pt, Ti)

• The cladding layer should be very thin and its thickness is only 5% of the total composite metal.

• Duraluminium sandwiched between Al sheets and hot rolled to produce Alkad composite which is free from stress corrosion

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ELECTROPLATING

PRINCIPLE

Electroplating is the process in which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution containing the soluble salt of the coating metal.

Electroplating is probably the most important and most frequently applied industrial method of producing metallic coatings. The metal film produced is quite uniform with little or no pinholes per unit area.

When the thickness of the deposit increases, the number of pinholes decreases.

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• The base metal to be plated is made cathode of an electrolyte cell, whereas the anode is either made of the coating metal itself or an inert material of good electrical conductivity.

THEORY If the anode is made of coating metal

itself in the electrolytic cell, during electrolysis, the concentration of electrolytic bath remains unaltered, since the metal ions deposited from the bath on cathode are replenished continuously by the reaction of free anions with the anode.

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Objectives of electroplating:

(i) To increase the resistance to corrosion and chemical attack of the plated metal.

(ii) To obtain a polished surface(iii) To improve hardness and wear resistance

Example: Electroplating of Cu, Au, Ag, Cr, Ni, Sn etc.

Uses of electroplating:

(i) It is often used in electronic industries for making printed circuit boards, edge connectors, semiconductor lead-out connection

(ii) It is also used in the manufacture of jewelery, refrigerator, electric iron etc.

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Electroplating of Cu• For electroplating of Cu on metal surface,

• Electrolyte: (3-5%)H2SO4 / (15-30%) CuSO4

• Anode: Pure Cu metal or Graphite (inert)• Cathode: Metal to be coated• Additive: Boric acid or gelatin

Ionization reaction of electrolyte is observed,

CuSO4 Cu2+ + SO42-

On passing current, Cu2+ + 2e- Cu (at cathode)

SO42- SO4 + 2e- (at anode)

H2SO4 2H+ + SO42-

Due to common ion effect, the ionization rate of Cu2+ is controlled and the deposition process can also be controlled, with a current density of 0.5 to 1.5 ampere/dm2.

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Factors affecting electroplating

• Surface cleaning – for strong adherent• Concentration of electrolyte.• Conductivity and stability of electrolyte • Thickness of the deposit – for decorative purpose

thin coating and for corrosion protection multiple coating.

• Current density (current per unit of the base metal) should be low for uniform controlled deposition

• Additives: Ensure strong adherence and mirror smooth coating.

• pH of the electrolytic bath

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Thin-Film Coatings

• PVD Coating (Physical Vapor Deposition)• CVD Coating (Chemical Vapor Deposition)

DEFINITION

Paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner + vehicle).

When a paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.

PAINTS

CHARACTERISTICS OF A GOOD PAINT

It should spread easily on the metal surface.It should have high hiding power.It should not crake on drying.It should adhere well to the surface.The colour of the paint should be stable.It should be a corrosion and water resistant.It should give a glossy film.

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CONSITITUENTS OF PAINTS

Paint essentially consists of the followingIngredients,

1. Pigments2. Vehicle or drying oil3. Thinners4. Drier5. Fillers or extenders 6. Plasticisers7. Antiskinning

Pigment is a solid and colour producing substance which is an essential constituent paint,

The functions of a pigment are To provide capacity, strength, desired colour

and give aesthetical appeal to the paint film. To give resistance against UV-light,

abrasion, wear, etc To improve the impermeability of paint film

from moisture and increase weather resistance.

PIGMENTS

THE MOST COMMONLY USED PIGMENTS ARE,

• White pigments: white lead, Zn, lithophone.

• Black pigments: Lamp black, Carbon black

• Red pigments: Venetian red, Indian red, Chrome red and Red lead.

• Blue pigments: Prussion blue.• Green pigments: Chromium oxide.• Yellow pigments: Chrome yellow and

Zinc yellow.

VEHICLE OR DRYING OILS

1. It is film forming constituents of he paint . These are glyceryl esters of high molecular weight fatty acids present in animal and vegetable oils.

FUNCTIONS:They form a protective film by the oxidation and polymerisation of the oil.They hold the pigment particles together on the metal surface.They impart water repellency, toughness and durability to the film.

Examples : Linseed oil, Soyabeen oil, Dehydrated castor oil, Tung oil, etc.

Thinners are added to the paint to reduce the consistency or viscosity of the paints, so that they can be easily applied to the metal surface. Thinners are highly volatile solvent.

FUNCTIONS They reduce the viscosity of the paint. They dissolve the oil, pigments, etc. and produce a homogeneous mixture. They increase the elasticity of the film. They evaporate readily and help the drying of the film. They increase the penetrating power of the vehicle.Ex: Turpentine, petroleum sprit, kerosene, xylol, etc.

THINNERS (or) SOLVENTS

Driers are used to accelerate or catalysing of the oil film by oxidation, polymerization and condensation.

The main functions of drier is to improve the drying quality of the oil-film.

Examples: Resonates, Linoleates, Tungstates and Napthenates of Cobalt, Lead, Manganese and Zinc.

DRIERS

FILLERS (or) EXTENDERS

Fillers are added to the paint to reduce the cost and increase the durability of the paint.

FUNCTIONS1. They reduce the cost and increase the

durability of the paint.2. They serve to fill the voids of the

paint film.3. They help to reduce the cracking of

paint. Examples: Calcium carbonate, China

clay, gypsum etc.

PLASTICISERS Plasticisers are chemical added to

the paint to give elasticity to the film and to prevent cracking of the film.

Ex: Triphenyl phosphate, Tributyl phthalate, etc.

ANTISKINNING AGENTS

They are chemicals added to the paint to prevent gelling and skinning of the paint film.

Important antiskinning agents are Polyhydroxy phenols.

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Special Paints• Fire retardant paints• Water repellant paints• Temperature-indicating paints• Heat resistant paints

Surface Preparation for Metallic Coatings

• Solvent cleaning• Alkali cleaning• Acid cleaning• Mechanical cleaning• Flame cleaning• Sand-blasting

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Electroless Plating• Electroless plating is defined as the

controlled auto catalytic deposition of a continuous film by the interaction of a solution of a metal salt and a chemical reducing agent.

• The reducing agent reduces the metallic salt to metal, which gets deposited over the catalytically activated surface giving a uniform thin coating.

Metal salt + Reducing agent Metal + Oxidized products

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Various Steps of Electroless Plating

Step 1: Pretreatment and activation of the surface:

(i) Treatment with an organic solvent followed by conditioning or etching by dipping, in a bath of acid.

(ii) Activation by immersion in a colloidal solution of tin or palladium. This yields a thin layer of Pd or Sn on the treated surface.

(iii) A thin layer of the metal to be plated or anyother suitable metal is electroplated on the surface of the object.

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Step 2: Preparation of plating bath:The ingredients in the plating bath are,(i) Coating metal: Soluble salt of the metal to be

deposited. (E.g., chlorides and sulphates)(ii) Reducing agent: It reduces the metal salt to

metal (E.g., Formaldehyde, hyppphosphite etc.)(iii) Exaltant: It enhances the plating rate. (E.g., Succinate, fluoride etc.)(iv) Complexing agent: It improves the quality of the

deposit. (E.g., Tartrate, citrate, succinate etc.)(v) Stabilizers: Stabilizers prevent the decomposition

of the plating bath solution.(vi) Buffer solution: It is added to control the pH of

the bath. (E.g., CH3COONa, NaOH + rochelle salt)

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Step 3: Procedure:

The object to be placed is cleaned and activated and then immersed in the bath containing the salt of the metal, reducing agent, exaltant, complexing agent, stabilizer and buffer solution. The metal salt is reduced to the corresponding metal.

Example: Electroless Nickel Plating