UNIT – IV

Corrosion and protective coatings

Corrosion

Corrosion is defined as the gradual destruction or deterioration of metals or

alloys by the chemical or electrochemical reaction with its environment.

Causes of Corrosion

Metals have natural tendency to revert back to their compounds which are more

stable.

Hence they react with the agents in the environment and get converted into metallic

compounds.

The attack by agents begins at the surface and slowly travel inward.

Gases like oxygen, SO2, H2S etc are responsible for chemical corrosion.

Oxygen and moisture are responsible for electrochemical corrosion.

Consequences of Corrosion:

Enormous waste of money and materials.

Loss due to corrosion runs into 2 to 3 billion dollars/ year.

Machinery, pipes breakdown.

Replacement cost increases.

Maintenance cost increases.

Fire hazard due to holes in gas pipes.

Constructions become unusable due to corrosion.

The factors affecting corrosion.

1. Nature of the metals

a. Position in the galvanic series:

The extent of corrosion depends upon the position of the metal in the galvanic

series. Greater the oxidation potential, greater is the rate of corrosion. When two

metals are in electrical contact, the metal higher up in the galvanic series becomes

anodic and suffers corrosion. Further the rate and severity of corrosion depends

upon the difference in their positions in the galvanic series. Greater the difference

faster is the corrosion of anodic metal.

b. Relative anodic and cathodic areas:

Corrosion is more severe and localized if the anodic area is smaller and the

cathodic is larger. The reason is that the high demand for electrons by the larger

cathodic area can be met by the smaller anodic area only by undergoing rapid and

severe corrosion.

c. Over Voltage:

The difference between the potential of the electrode when gas evolution is

actually observed and the theoretical value of the reversible electrode potential

for the same electrode is called the over voltage. The reduction in over voltage at

the cathode will accelerate the rate of corrosion of the anode metal.

d. Purity of the metal :

Impurities present in the pure metal causes heterogeneity and form tiny

electrochemical cells at the exposed metallic surface in presence of a conducting

medium. The metal having higher oxidation potential will act as an anode which

dissolves in the medium and corrosion occurs. For example impure Zinc

undergoes corrosion when it is in contact with lead or iron. Therefore corrosion

resistance of a metal depends on its purity.

e. Nature of the corrosion product:

The extent of corrosion depend on the nature of the corrosion products, such

as its stability and solubility. If the corrosion product is insoluble and forms a

film on the surface the corrosion may not proceed further (Pb in storage battery).

If the corrosion product is soluble in medium, corrosion proceeds vigorously.

Nature of the Environment:

1. Temperature:

Rise in temperature of the environment increases the corrosion reaction, due to increase in conduction of the medium and decrease in over voltage.

2. Humidity:

Atmospheric corrosion of iron is slow in dry air but increases rapidly in the

presence of moisture. This is due to the fact that moisture acts as a solvent for the

oxygen in the air to furnish the electrolyte essential for setting up a corrosion cell.

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

3. Impurities:

Polluted atmosphere usually in industrial belts, containing impurities such as

CO2,SO2,H2, fumes of HCl, H2SO4etc, especially in a humid atmosphere, enhance the rate of corrosion.

4. Effect of pH:

Generallyacidic environment is more harmful than alkaline or neutral environment.Therefore corrosion rate of metals by acidic surrounding can beminimized by increasing the PH of the solution.

CLASSIFICATION (OR) THEORIES OF CORROSION Based on the environment, corrosion is classified into

(i) Dry or Chemical corrosion, and (ii) 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, etc. There are 3 main types of dry corrosion.

1. Oxidation corrosion (or) corrosion by oxygen.

2. Corrosion by hydrogen.

3. Liquid-metal corrosion. Oxidation Corrosion (or) Corrosion by Oxygen

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 (Li, Na, K, etc.)

and alkaline-earth metals (Mg, Ca, Sn, etc.) are rapidly oxidised at low temperature. At

high temperature, almost all metals (except, Ag, Au and Pt) are oxidised.

Mechanism of Dry Corrosion (i) 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-

(ii) Oxygen changes to ionic form (O2-) due to the transfer of electron frommetal,which

occurs at the oxides film / environment interface

½ O2 + 2e- −−−−−> O2-

(iii) Oxide ions reacts with the metal ionto form the metal-oxide film.

M + ½ O2 −−−−−> M2+ + O2- ≡ MO

(Metal-oxide film)

Once the metal surface is converted to a monolayer of metal-oxide, for further

corrosion (oxidation) to occur, the metal ion diffuses outward through the metal-oxide

barrier. Thus the growth of oxide film commences perpendicular to the metal surface.

Nature of Oxide Film The nature of oxide film formed on the metal surface plays an important role in oxidation corrosion. (i) Stable Oxide Layer

A stable oxide layer is a fine-grained in structure, and gets adsorbed tightly to the

metal surface. Such a layer is impervious in nature and stops further oxygen attack through

diffusion. Such a film 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 into 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 (MoO3) is volatile.

(iv) Protective (or) Non-Protective oxide film (Pilling- Bedworth rule)

(a) According to Pilling-Bedworth rule, 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: The volume of oxides of alkali and alkaline earth metals such as Na, Mg, Ca,

etc., is less thanthe volume of the metal consumed. Hence the oxide layer formed is porous

and non-protective.

(b) On the other hand, if the volume of the oxide layer formed is greater than the volume

of the metal, the oxide layer is non-porous and protective. Example :The volume of oxides of heavy metals such as Pb, Sn, etc., is greater than the

volumeof the metal. Hence the oxide layer formed is non-porous and protective. Pilling-Bedworth Ratio

The ratio of the volume of the oxide formed to the volume of the metal consumed is called

“Pilling-Bedworth ratio”

PBR

If O is greater than M, then O/M is high. This is protective or non-

porous layer If M is greater than O, then O/M is low. This is non-

protective or porous layer Corrosion by Hydrogen (a) Hydrogen embrittlement(at ordinary temperature) When metals contact to H2S at ordinary temperature causes evolution of atomic

hydrogen. Fe + H2S −−−−−>FeS + 2H

This atomic hydrogen diffuses readily into the metal and collects in the voids, where

it recombines to form molecular hydrogen.

H + H −−−−−> H2↑

Collection of these hydrogen gases in the voids develop very high pressure, which

causes cracks and blisters on metal. Thus, the process of formation of cracks and blisters

on the metal surface, due to high pressure of hydrogen gas is called hydrogen

embrittlement. (b) Decarburisation(at Higher Temperature)

At higher temperature atomic hydrogen is formed by the thermal dissociation of molecular hydrogen.

H 2 2 H

when 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 decrease in carbon content in steel is termed as “decarburisation” of

steel

Liquid - Metal Corrosion

This is due to the chemical action of flowing liquid metal at high temperature. The

corrosion reaction involves

(i) either dissolution of a solid metal by a liquid metal. (or)

(ii) liquid metal may penetrate into the solid metal.

Mechanism of electrochemical corrosion

Wet or electrochemical corrosion is the more common type of corrosion.

This type of corrosion occurs when a conducting solution is in contact with a metal (or) when two dissimilar metals in contact with each other and also in contact with a conducting solution. Since this type of corrosion can occur only in presence of a liquid it is called as wet corrosion. A number of tiny electrochemical cells are set up on the corroding metal and this is responsible for corrosion. Hence this type of corrosion is called as electrochemical corrosion. Mechanism

Due to setting up of small electrochemical cells, the metal surface is

separated into cathodic and anodic areas. Anodic reaction: The anodic reaction involves the dissolution of the metal to produce metallic ions with the liberation of electrons.

𝐹𝑒 → 𝐹𝑒2+ + 2𝑒− The electrons flow through the metal to reach the cathodic area. Cathodic reaction: These electrons can be given to oxygen atoms if the metal is in contact with atmospheric oxygen and water to form hydroxide ions.

𝑂2 + 𝐻2𝑂 + 2𝑒− → 2𝑂𝐻−

The 𝐹𝑒2+ ions produced at the anode and the 𝑂𝐻− ions produced at the cathode diffuse towardseach other and when they meet ferrous hydroxide is precipitated.

𝐹𝑒2+ + 2𝑂𝐻− → 𝐹𝑒(𝑂𝐻)2

If enough oxygen is present, the ferrous hydroxide is converted to yellow rust.

2𝐹𝑒(𝑂𝐻)2 + 12⁄ 𝑂2 → 𝐹𝑒2𝑂3. 2𝐻2𝑂

In limited supply of oxygen black rust is formed

2𝐹𝑒(𝑂𝐻)2 + 12⁄ 𝑂2 + 𝐻2𝑂 → 𝐹𝑒2𝑂3. 3𝐻2𝑂

The corrosion products are found in between the cathode and the anode. It is important to note that the loss of metal takes place only at the anodic area and the cathodic area metal is not corroded.

Corrosion control

1. Sacrificial anode protection method and 2. Impressed current cathodic protection.

(i) Sacrificial anode protection method

In this method, the metallic structure which is to be protected is connected

to a more anodic metal (a metal having higher oxidation potential) through a conducting wire. Under thiscondition, corrosion occurs only in the more anodic metal and then it will protect the metallic structure sacrificially. Railway lines, ship hulls, steel structures of offshore petroleum wells, underground pipe lines etc., are protected from corrosion by the use of Mg or Zn as the sacrificial anodes.

Applications of Sacrificial anodic protection

1. This method is used for the protection of ships and boats. Sheets of Mg and

Zn are hung around the hull of the ship. Zn or Mg will act as anode

compared to iron (ship or boat is made of iron), so corrosion concentrates

on Zn or Mg. Since they are sacrificed in the process of saving iron, they are

called sacrificial anodes. 2. Protection of underground pipelines, cables from soil corrosion. 3. Insertions of Mg sheets into the domestic water boilers to prevent the

formation of rust. 4. Calcium metal is employed to minimize engine corrosion.

Impressed current cathodic protection method In this method, an impressed current is applied in the opposite direction

of the corrosion current to nullify it, and thus we prevent the formation of anodic sites in the metallic structures which are responsible for corrosion.

This can be done by connecting negative terminal of the battery to the

metallic structure to be protected and positive terminal of the battery is

connected to an inert anode. Inert anodes used for this purpose are graphite,

platinised titanium. The anode is buried in a “back fill” (composed of gypsm). The

“back fill” provides good electrical contact to anode. Since in this method current

from an external source is impressed on the system, this is called as impressed

current method.

Applications of impressed current protection

Structures like tanks, pipelines, transmission line towers, underground water pipelines, oil pipelines, etc., can be protected by this method.

Corrosion Inhibitor

Corrosion Inhibitor is a substance which when added in small quantities to the

corrosive aqueous environment, decreases the corrosion of the metal.

Inhibitors are of two types. They are Anodic inhibitors and Cathodic Inhibitors

Anodic inhibitors:

The inhibitors such as chromates, Phosphates, Tungstates or ions of transition

element with high oxygen content suppress the corrosion reaction, occurring at

the anode, by forming a sparingly soluble compound with the metal ion. They

form a protective film or barrier on the metal surface by adsorbtion, thereby

reducing the corrosion rate.

This type of control is effective but if certain areas are left unprotected, sever

local attack can occur.

Cathodic Inhibitors:

a) In acidic solutions, the evolution of Hydrogen takes place as the cathodic

reaction.

2𝐻+ (𝑎𝑞) + 2𝑒− → 𝐻2(𝑔)

Corrosion may be reduced either by slowing down the diffusion of H+ ion or by

increasing the overvoltage of hydrogen evolution. Organic inhibitors like amines,

mercaptans, ureas and thioureas decrease the diffusion of H+ ion by getting

adsorbed at the metal surface. Antimony and arsenic oxides increase the

hydrogen overvoltage by depositing an adherent film of metallic arsenic or

antimony at the cathode areas

b) In neutral solutions, the following cathodic reaction takes place

𝐻2 𝑂 (𝑒) + 1

2 𝑂2 + 2𝑒− ↔ 2𝑂𝐻− (𝑎𝑞)

The corrosion may be controlled by eliminating oxygen either by adding reducing

agents like sodium sulphite or by dearation. It may also be controlled by

retarding the diffusion of oxygen into the cathodic area by the addition of

inhibitors like Mg, Zn, Ni salts.These reacts with the hydroxyl ions at the cathode

forming corresponding insoluble hydroxides, which are deposited on the cathode

forming impermeable self- barriers.

Protective coatings

Metallic structures and machine parts undergo corrosion and therebylose their

strength. In order to decrease the corrosion and protect the base metals

protective coatings are applied on the surface of the metal. The protective

coatings act as a physical barrier between the metal and the atmosphere and thus

prevent corrosion. In addition these coatings also possess a decorative value.

Types of protective coatings:

Protective coatings are of the following three types:

a) Metallic coatings

b) Chemical conversion coatings

c) Organic coatings viz paints and varnishes.

Metallic coating:

Corrosion of the base metal can be prevented by either of the two types of

coatings given below.

a) Anodic coating

b) Cathodic coating

Anodic coating:

If metals anodic to the base metal are coated, it is called as anodic coating.

The metals which have lower electrode potential than the base metals are

employed for providing an anodic coating. For example, metals such as Zn, Al

and Cd have lower electrode potential than iron and hence can be coated to

protect steel.

If Zn metal is coated on steel, Zn becomes the anode because of its lower

electrode potential and steel remains as the cathode. Due to corrosive attack by

the agents in the atmosphere(O2,H2O), zinc starts dissolving, whereas iron is

protected.

Thus zinc coating sacrifices itself to protect steel.

Cathodic coating:

In this method, the base material is protected by coating with a metal

which has higher electrode potential. This means that the coating metal is more

corrosion resistant than the base metal.

Example: coating of tin on iron sheets protects iron from corrosion.

But, the tin coating on iron provides protection as long as the surface of

iron is completely covered with the tin coating. Even a small puncture in the tin

coating exposing the iron sets up an electrochemical cell in which iron is the

anode and tin is the cathode. This results in severe corrosion of the exposed iron

part leading to pitting.

Differences between anodic and cathodic coatings

S.no Anodic coating Cathodic coating

1 It protects the base by sacrificing itself It protects the base metal by being more corrosion resistant

2 Electrode potential of coating metal is lower than that of the base metal.

Electrode potential of the coating metal is higher than that of the base metal

3 Small punctures in the coating does not severely affect the base metal

Even small punctures in the coating results in severe corrosion of the base metal leading to pitting

4 Example: coating Zn on iron Example: coating of tin on iron

COATING PROCESS

The important methods employed in metal coating are

1. Hot dipping

2. Electroplating

3. Electroless plating

4. Metal spraying

5. Metal cladding

6. Cementation

Hot dipping:

Metals with low melting points can be coated on iron or steel by this

method. The base metal sheets(iron or steel) is dipped into a bath of molten Zn or

Sn.

a) Galvanizing:

Coating of zinc or iron or steel is called galvanizing. The steel article(sheet,

pipe or wire) is first cleaned by dipping it in dil.H2SO4 for 15 to 20 mins at

60 to 90oC.This process is called pickling and is done in order to remove

any rust, scale or other impurities. The article is then washed in water and

then dried. It is then dipped in molten zinc at 425-435oC. The surface of

molten zinc is covered by ammonium chloride(flux) to prevent oxidation of

zinc. The zinc coated article is passed through rollers to remove excess zinc

and to give a coating of uniform thickness. Then it is annealed by heating it

to650oC and gradually cooling it.

Uses:Galvanization protects iron sacrificially and also gives it a good

appearance.

Disadvantages: zinc dissolves in dilute oxide to give toxic compounds.

Hence, galvanized cans cannot be used for storing food items.

b) Tinning:

Coating tin over steel sheet is called tinning. The steel sheet is first cleaned

using dilute H2SO4(pickling) and is passed through a bath of zinc

chloride(flux). The role of flux is to make tin adhere to steel strongly.

Then the sheet is passed through two chambers containing molten

tin. The tin coated steel sheet is then passed through a chamber containing

hot palm oil. The palm oil forms a thin layer over tin and protects it from

oxidation. Then the sheet passes through hot rollers to remove excess tin.

Fig. Tinning of Sheet Steel

Uses: Due to non toxic nature of tin the tin coated steel is used for making

contains for storing food materials.

Electro plating:

A easily corroded base metal is protected by a coating of a metal resistant

to corrosion by the passage of electricity.

The base metal is made as the cathode and the coating metal is made as the

anode. The electrolyte used is a salt of the coating metal dissolved in water. When

electricity is applied the coating metal gives a thin coating on the article to be

coated.

Theory of electroplating:

Let us consider copper plating to illustrate the theory behind

electroplating. Copper sulphate solution is taken in a electrolytic cell. Copper is

made as the anode. The steel article which is to be coated with copper forms the

cathode.

Copper sulphate ionizes as follows

CuSO4 ↔Cu2+ + SO42-

When current is passed cn2+ ions moves towards the cathode and gets

deposited on the cathode.

Cu2+ +2e- Cu

The SO4 2-ions moves towards the anode and combines with Cu2+ ions

which are produced by dissolution of copper anode. Thus, the Cu2+ lost from the

electrolyte is replenished.

Cu + SO4 2- CuSO4 + 2e-(at anode)

Thus the copper anode dissolves away and has to be changed.

Instead of copper anode an inert electrode like graphite can be used. But,

CuSO4 has to be continuously added to make up for the loss of Cu2+ ions lost in

making the coating.

Electroplating of nickel:

Nickel plating provides a less porous, corrosion resistant and wear

resistant surface .

Anode: nickel (or inert electrode)

Cathode: article

Temp: 40-70oC

Current: 20-30mA/cm2

Electroplating of chromium:

It gives a shiny metallic coat to the base material. Since, chromium plating

is porous, the article is given an undercoat of nickel before chromium plating.

Anode: graphite electrode

Medium: acidic

Cathode: article to be coated

Plating bath: H2CrO4 and H2SO4 in 100:1 proportion by volume.

Current: 100-200mA/cm2

Cleaning of articles before electroplating:

The surface of the articles must be cleaned thoroughly to obtain uniform

and adherent coating. Solvent cleaning of the surface is done by organic solvents

to remove oil, grease etc. this is followed by cleaning with steam or with alkali

solution. Acid cleaning is used to remove oxides and other metals on the surface.

For better results sand blasting method is used.

Factors affecting electroplating:

1) Low electrolytic concentration gives better coatings.

2) Thin deposits are more adherent than thick ones.

3) Minimum current density should be used.

4) pH should be maintained at the correct level throughout.

5) Mild stirring is helpful.

Electroless plating:

A noble metal is uniformly coated on a base metal(like iron, steel) without

the use of electricity. The salt solution of the noble metal is reduced using a

reducing agent.

Noble metal ions + reducing agent metal + oxidized products

The metals thus produced gives a uniform coating on the article placed

inside the solution. The surface of the article (base metal) must be catalytically

activated for such coating to be made.

Electro less nickel plating:

1) The surface to be coated is degreased by using organic solvents and alkali.

2) This is followed by acid treatment to remove oxide deposits.

3) Metals and alloys of Al, Cu, Fe, brass etc can be directly nickel coated

without any activation.

4) Stainless steel surface should be activated by dipping it in a hot solution of

50% H2SO4.

5) Activation of non-metallic articles (like glass, plastics, quartz etc) is carried

out first by dipping in SnCl2 solution containing HCl. This is followed by

dipping in palladium chloride solution. On drying a thin active layer of Pd

is formed on the surface.

Method:

A plating bath solution of the following composition is made.

a) NiCl2 solution 20g/l

b) Buffer (sodacetate)- 10g/l

c) Complexing agent cumexaltant- sodium succinate

d) Optimum pH- 4.5

e) Optimum temp- 93oC

The article to be coated is placed in this path and the reducing agent

sodium hypophospite(20g/l) is added and stirred.

The complete cell reaction is

Ni 2+ + 2e- Ni

HPO2- + H2O HPO3- + 2H+ + 2e-

By adding the above equations we get,

Ni 2+ + 2e-+ HPO2- + H2O Ni+ HPO3- + 2H+ + 2e-

The Ni produced ion the reaction gets coated on the article uniformly.

Advantages:

1. Even article of intricate shapes can be coated uniformly.

2. Nickel deposits are non porous and better than electro plated nickel

coating.

3. Gives harder surface and better wearability.

Copper coating on printed circuit board:

PCB’s are made out of epoxy or phenolic polymers.

These phenolic sheets are activated by first dipping in SnCl2 containing

HCl followed by dipping in palladium chloride solution and drying, the surface is

found to have a thin layer of Pd.

The bath solution for copper plating consists of

1. Copper sulphate 12g/l

2. Buffer – NaOH + Rochelle salt

3. Complexing agent cum exaltantEDTA (20 g/l)

4. Optimum pH – 11.0

5. Temperature – 25oC

6. Reducing agent - Formaldehyde

Reduction occurs and Cu metal is produced.

Cu 2+ + 2e- Cu

2 HCHO + 4 OH- 2HCOO- + 2 H2O + H2 + 2e-

By adding the above equations we get,

Cu 2+ + 2 HCHO + 4 OH- Cu + 2HCOO- + 2 H2O + H2

The PCB is electroplated with copper. Then, the selected areas are

protected by employing electroplated image(or photoresist). Then the reminder

of Cu coating is etched away so as to get the required type of circuit pattern (or

track). Finally, the connection of two sides of PCB is made by drilling holes

followed by electro less copper plating through holes.

Metal spraying:

In this process, the coating metal in the molten condition is sprayed on the

surface of the article to be coated. The coated metal adheres to the metallic

surface.

The sprayed coatings are continuous but porous. Hence, a sealer oil or

paint is applied on such a coating to provide a smooth surface.

Advantages of the method:

1. Greater speed of working

2. Applicability to large surfaces

3. Ease of application

Method used:

Wire gun method:

A wire of the coating metal is melted by oxy-acetylene flame and atomized

and sprayed by a blast of compressed gas.

Powder metal method:

A finely powdered coating metal is sucked from the powder chamber. It is

heated and melted when it passes through the flame of the blow pipe. The blow

pipe breaks the metal into a cloud of molten globules which are sprayed over the

article. This is absorbed by the surface of the article and a fine coating is

obtained. This method can be employed for low melting metals like Zn, Pb, Sn

etc.

Metal cladding:

In this process a base metal sheet like steel is covered on one side or on

both sides by a thin sheet of noble metal. The sandwiched base metal is passed

through sheet rollers under heat and pressure. The sheets bond with each other

and a corrosion resistant metal sheet is obtained.

Base metals thus protected are mild steel, Al, Cu, Ni and their alloys. Cladding

metals – corrosion resistant metals like Ni, Cu, Pb, Ag& Pt.

Use:In aircraft and automobile industry.

Example: Al clad – in this duralium is sandwiched between two layers of 99.5%

pure aluminum.

Cementation or diffusion coating:

A rotating drum is filled with articles to be coated and the powder of the

coating metal. The drum revolves and is continuously heated to a suitable

temperature.

The coating metal diffuses into the surface of the base metal and forms

layers of alloys of varying composition. The layer adjacent to the base metal may

be an intermediate compound or solid solution. The outer layers are rich in

coating metal. The coating thickness is controlled by varying time of treatment

and temperature. This process is suitable for coating small articles( like bolts,

screws and valves). The coating metals used are those which can alloy with iron

(like Zn, Cr & Al).

Sherardizing- (developed by sherard) is a cementation process in which Zn

powder used as the coating metal.

Colorizing:The steel articles are sand blasted and tightly packed in a drum with

Al powder, Al oxide & NH4Cl flux. A reducing atmosphere is maintained using

hydrogen gas. The drum is made to revolve while heating it. This method is

mainly used for protection of furnace parts.

Chromizing:It is carried out by heating metal articles with a mixture of 55%

chromium powder& 45% of alumina to 1300-1400oC for 3 to 4 hours. This is

mainly used for protection of gas turbine blades.

CHEMICAL CONVERSION COATINGS

By chemical or electro chemical reactions the surface of the base metal is

converted into its inorganic compounds which act as barrier for corrosion. These

surface coating act as a good base for paints, lacquers and oils.

Important conversion coatings are

1) Phosphate coating

2) Chromate coating

3) Chemical oxide coating

4) Anodized coating

Phosphate coating:

This is done by immersing the base metal or articles into a bath of mixture

of phosphate and phosphoric acid. It can also be applied by spraying or brushing.

A chemical reaction occurs at the surface and a corrosion resistant coating is

crated at the surface.

The phosphate used are iron, manganese and zinc along with

accelerators(like copper salts). As these reactions are slow, accelerators are used.

The surface film consist of zinc-iron or manganese iron-phosphate.

Phosphate coatings are porous and do not offer complete protection

against corrosion. However they serve as excellent primer coat for applying

paints, lacquers and oils.

Chromate coating:

It is done by dipping the article in a bath of acidic potassium chromate

followed by its immersion in neutral chromate solution. The protective chromate

film formed on the surface is due to the presence of trivalent and hexavalent

chromates.

The chromate coatings are non porous and more corrosion resistant than

phosphate coating. Chromate coatings are mainly used in the protection of zinc,

cadmium plated parts, aluminium and magnesium.

Chemical oxide coating:

This is made by treating base metal with alkaline oxidizing solution or gas. This

treatment increases the thickness of the original oxide layer on the surface and

thereby increasing the protection.

Oxide coating form a good primer base for paints, lacquers and oils.steel

products are obtained in a range of colors yellow to light blue by heating steel in

air at 200-450oC.

Anodized coating:

Anodized coatings are generally produced on non-ferrous metals like Al,

Zn, Mg and their alloys by anodic oxidation process.

Anodized aluminium:

The aluminium article is made the anode in a cell containing a bath

consisting of sulphuric, chromic, oxalic or phosphoric acids. DC is passed at

moderate current densities at 35-40oC. Progressive oxidation occurs at the

surface of the anode and a thick layer of the oxide coating forms on the surface.

These films are further sealed by exposing them to boiling water. Anodized

coatings are more resistant to corrosion.

ORGANIC COATINGS

Organic materials which can form a protective film on a metallic surface is

called an organic coating.

Paints, varnishes, lacquers and enamels are organic coatings used both for

protection of metals as well as for decoration.

Paints:

A paint is a dispersion of one or more pigments in a liquid medium which

consists of a vehicle and thinner. The ‘vehicle’ is a liquid consisting of a non-

volatile, film forming material. The ‘thinner’ is a volatile solvent.

When a paint is applied to a metal surface the thinner evaporate while the

vehicle(drying oil) slowly oxidizes and polymerizes forming a dry pigmented film.

Requisites of a good paint:

1) Good spreadability or applicability

2) Should stick to the surface strongly

3) Should possess high covering power

4) Should not crack on drying

5) Should posses a stable color

6) Should be corrosion and water resistant

7) Should give a glossy film

Constituents of paints and their functions:

Pigments: pigments are solid colored substances and the most essential part of

a paint.

Functions:

1) Gives opacity to the film

2) Gives color to the paint

3) Protects the metal surface from the effects of weather

4) Protects the film from UV radiations which can crack the film

Examples:

Black pigment: lamp black, carbon black

White pigment: white lead(2PbCO3.Pb(OH)2)

Red pigment: Indian red Fe2O3

Green pigment: chromium oxide

Blue pigment: Prussian blue

Vehicles or drying oils:This is the film forming liquid in the paint. These are

glyceryl esters of high moleculer weight fatty acids. That is, they are vegetable or

animal oils.

Functions:

1) They form adherent and protective film on the surface by oxidation and

polymerization.

2) They impact water repellency, toughness and durability to the film.

Eg: linseed oil, soyabean oil, dehydrated castor oil.

The oil after application on a surface absorbs O2 at the double bonds. This results

in the formation of peroxides which polymerizes to form a tough, cross linked

film.

Thinners:Thinners are solvents which are very volatile. They make the paint

less viscous so that the paint can easily be applied on the surface. After

application the thinner dries out quickly leaving behind the film.

Functions:

1) Dissolve the additives added to the paint.

2) Increase the penetration power of the vehicle.

3) Increase the elasticity to the film.

Eg: Turpentine, mineral spirits from petroleum.

Extenders or fillers:These are white or colorless pigment which forms the

bulk of the paint.

Functions:

1) Reduces the cost of paint

2) Prevents shrinkage of paint

Eg: Talc, china clay, gypsum, silica etc

Driers: These are used to accelerate the process of drying.

Functions: They act as oxygen carriers or catalyst.

Eg: metallic soaps of Co, Mn, Pb

Plasticizers: Added to paints to provide elasticity to the film and to prevent

cracking of the film called plasticizers.

Eg: triphenyl phosphate, tricresyl phosphate etc

Antiskinning agents: These are chemicals added to prevent skinning of the

paint.

Eg: polyhydroxy phenol

Failure of paints:

1) Chalking: It is the gradual powdering of the applied paint. This is due to

improper dispersion of the pigment in the vehicle.

2) Cracking: Paint film can crack due to unequal expansion and contraction

and also due to action of UV radiation. This is avoided by giving a hard

primer coat.

3) Color change:The color of the paint changes due to action of light and

atmospheric action.