CORROSION

Any process of chemical or electrochemical decay or destruction of a metal due to the action of surrounding medium is called corrosion

Corrosion is naturally spontaneous process

Metal in combined state (oxides, sulfides etc)

Pure Metal

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Metallurgical processes

Corrosion

So……Why Study Corrosion?• Metals are precious resources• Engineering design is incomplete without knowledge

of corrosion• Corrosion – contaminate products such as

pharmaceutical, food and dairy products or luxury items like soap

• Applying knowledge of corrosion protection can minimize disasters

• Artificial implants for the human body!!!

• Environment/atmosphere surrounding metals can be air, water, sea water, acids, alkalies, steam, gases, other molten metals, soils, etc.

• Metals come in contact with all these corrosion influencing factors at varying temperatures ranging from room temperature to working temperature of the various processes.

• This contact results in gradual transformation of metal into its compounds. This is also known as weathering of metals or weeping of metals

• Due to corrosion,

(i) Metal loses its efficiency and many of its useful properties get

altered.

(ii) Maintenance cost and cost of material increases while

production rate decreases.

(iii) Purity of the product gets affected.

• Knowing these destructive effects of corrosion, it becomes

essential to understand the mechanism of corrosion.

Corrosion

Dry/Atmospheric

Corrosion due to

Oxygen

Corrosion due to other gases

Wet/Electrochemical

Evolution of Hydrogen

Absorption of Oxygen

Dry/Atmospheric corrosion

Presence of Oxygen• Metallic surfaces exposed to air undergoes oxidation 2M + O2 2MO, where M is a metal• Metallic oxide (MO) formed on the surface of the

metal is in form of a thin film • Stable film: Porous– Na2O, K2O; Non-porous–

Oxides of Al, Ni, Cr• Unstable film: Oxides of noble metals• Volatile film: Oxides of Mb

Nature of Oxide film

stable unstable volatile

porousEx. Iron Oxide

(FeO)

Non porousOxides of Al

Ni, Cr

Oxides of noble metals i.e. Au, Ag etc.

Oxide of molybdenum

If oxide film is porous in nature, the rate of further corrosion is

not reduced much.

These pores give free access to oxygen, which attacks the

underlying pure metal. Thus, corrosion does not stop till pure

metal is available.

Gold does not get corroded on oxidation• There is a formation of unstable oxide film• Oxide layer formed decomposes back into metal and

oxygen• Consequently, oxidation corrosion is not possible,

thus Ag, Au, Pt do not undergo oxidation corrosion

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Metal oxide

Volatile oxide film ex. Molybdenum oxide

Corrosion due to other Gases• Corrosive effect of gases depends on chemical

affinity of the metal for gases• Cl2, H2S, SO2, CO2, etc add to the corrosive effect to

the metal• Gases react with metals to form films of

corresponding compounds – get deposited on the metal surface

• This film may be porous (non-protective) or non-porous (protective)– Ag + Cl2 (dry gas) AgCl (protective film)– Ag + H2S Ag2S (corrosive)

Contd…Gases like H2 attacks the metal at ordinary temperature and is

called hydrogen embrittlement Fe + H2S FeS + 2[H] (nascent hydrogen)

Nascent hydrogen diffuses into the metal and form voids/ holesNascent hydrogen atoms combine to form molecular hydrogen

[H] + [H] H2 (g)Molecular hydrogen leaves metal in form of gas leaving a

cavity in the metalWhen maximum number of H2 molecules are evolved, metal

loses its tensile strength, ductility and malleability – becomes weak

Wet/ Immersed/Electrochemical Corrosion

Based on Nernst theory -- Occurs at solid-liquid interfaceOccurs when

Metals are in contact with moist air/ liquid mediumTwo dissimilar metallic articles are dipped in a solution, then

out of two, one acts as the anode and the other acts as the cathode

Anode gives of ions in the solution and gets corroded Cathode accepts ions and forms a protective coating and

does not get corrodedCorrosion brought about by ionic reactions in presence of

moisture or solution (as conducting medium) is called Electrochemical corrosion

Electrochemical Corrosion Evolution of HydrogenSteel tank is filled with acidic waterSmall piece of Cu is placed at

bottom of the steel tankAt the junction where Cu is in

contact with steel tank, metallic Fe gets corroded

Fe Fe2+ + 2e--

Electrons flow from anode to cathode, H+ ion are eliminated as H2;

2H+ + 2e-- H2(g)

Fe + 2H+ Fe2+ + H2(g)

Absorption of Oxygen • Steel plate is coated with an

oxide film• Entire coated steel plate acts

as cathode• Imagine a crack on one point

of oxide film• Fe is exposed to atmosphere

and this point acts as anode Fe Fe2+ + 2 e–

½ O2 + H2O +2 e— 2OH--

Differential Aeration Corrosion• It is the most common type of corrosion, and it occurs when

one part of the metal is exposed to a different air concentration from the other part.

• This causes a difference in potential between differently aerated areas.

• Poorly oxygenated parts are anodic and rest are cathodic. Consequently, differential aeration of metal causes a flow of current, called differential current.

• Differential aeration accounts for the corrosion of metal partially immersed in a solution or water just below the level line of corresponding solution or water.

Differential Aeration • Concentration cell corrosion• Due to electrochemical attack on metal surface

exposed to electrolyte of varying concentration/ varying aeration

Zn Zn2+ + 2e– (oxidation)

1/2O2 + H2O + 2e– 2OH– (reduction)

Iron corrodes under the drop of waterAreas covered under the drop of water or salt solution have less access to oxygen and turn anodic than the remaining parts of the metal which is large cathodic areaOxygen concentration cell hence increases corrosion rate

Waterline corrosion

• It is a classic type of differential oxygen concentration corrosion• It is found that the maximum amount of corrosion takes place

along a line just beneath the level of water meniscus. The area above the waterline is more oxygenated and thus acts as a cathode and remains completely un-affected by the corrosion.

• Waterline corrosion occurs in ships at a very high rate. Here, the aquatic plants from sea also play a major role as they get attached to the bottom of the ships.

Contd…

• In ships floating on the sea water, the portion of it in contact with marine water gets corroded, if there is a difference in the materials used in assembling the ship.

• If metal iron or brass alloy is used together to join various parts, due to difference in potential, a galvanic cell gets set, leading to corrosion.

• In such cases all along the surface of metal small galvanic cells are set up, where area of high potential acts as an anode and the one with lower potential acts as a cathode.

• Portion of metal acting as anode, deteriorates and at cathode the cathodic product gets evolved/deposited.

• In such type of environment, on metal surface small anodic and cathodic areas are formed. Here, cathodic area is slightly larger and hence rate of corrosion is comparatively higher. The mechanism of corrosion in slightly alkaline medium, proceeds as,

At Anode M M 2+ + 2e–

At cathode ½ O2 + H2O + 2e – 2OH–

M 2+ + 2OH– M(OH)2

If the metal is exposed to acidic medium the mechanism at

cathode proceeds to form H2 gas, while anodic reactions

remain the same.

To avoid the formation of galvanic cells,

a) Metals should be as pure as possible.

b) The materials used to assemble the different parts should be of same potentials.

c) Moisture and other electrolytic/aqueous medium, if present, other corrosion controlling methods may be used.

Active orAnodic

Noble Or cathodic

Galvanic Series

Pitting Corrosion• Localized accelerated attack • Similar to differential aeration

concept• Resulting in formation of

cavities in metal due to dirt, deposits, cracked oxide film

• Occurs in form of pinholes, pits/ cavities

• Bottom of pit-- less oxygenated• Result in cracking of protective

film on metal at specific points forming small anodic & large cathodic areas

Intergranular Corrosion (IGC)

• Microstructure of metals are made of grains separated by grain boundaries

• Corrosion occurs along grain boundaries (less noble) than grain centres (noble)

• Corrosive attack is localized at these “less noble” areas

• IGC is the result of sensitization of the material due to “inadequate” heat treatment during welding.

• In presence of carbon in steel, chromium reacts with carbon during heat treatment to produce Cr-carbides

23Cr + 6C Cr23C6

Stress Corrosion• Combined effect of metal stresses & corrosive environment• Similar to intergranular corrosion• Localized electrochemical attack• For stress corrosion to occur:

– Presence of stressed metal– Specific corrosive environment is necessary (caustic alkali)

• Presence of stress produces strains (anodic), thereby possess higher electrode potential – becomes chemically active – easily attacked by corrosive environment

Example: Metals and alloys when subjected to welding and bending• Results in stress of the metal• At the stress portion, atoms in the stressed region

get displaced– becomes anodic– corrosion

Soil Corrosion

Affected Facilities: • Underground storage tanks, pipes, cablesFactors affecting soil corrosion • Aeration: More air results in lesser corrosion• Water retention: More water = More electrolyte = More

corrosion• Dissolved Salt content: More salts = higher conductivity

= greater corrosion

• Soil acidity : Greater acidic soils, greater the rate of corrosion, passive in neutral/ alkaline soils

• Ionic species & microbes : Halide ions and active bacteria produce an acidic environment Greater corrosion of the metals

Microbiological Corrosion • Corrosion influenced due to microorganisms• Microorganisms utilize nutrients for their metabolic activites

that influences corrosion• O2-consuming bacteria (eg, Thiobacillus sp.) present in water

decreases conc of O2 in the medium in contact with the metal – this results in differential aeration – Metal corrodes

• Fe-oxidizing bacteria : Gallionella, Sphaerotilus (nuisance aerobic bacteria) form biofilms – form slime in the pipeline– pitting corrosion

Factors Influencing Corrosion

Nature of the Metal Nature of Corroding Medium

Position in Galvanic series

Relative areas of anodic, cathodic parts

Purity of Metal

Physical state of metal

Nature of surface film

Passive Character

Solubility & volatility of corrosion products

Temperature

Humidity

Impurities in atmosphere

pH

Conductance of corroding medium

(a) Electrode potential and position of metal in the galvanic series : This is a major factor for corrosion of metals. If two dissimilar metals are in corroding environment, the metal having higher electrode potential and position in the galvanic series undergoes corrosion, i.e. it acts as an anode. Amongst the two metals in contact, greater the difference in the electrode potential, higher is the rate of corrosion

(b) Purity of the metal :If the metals are impure, then the

impurities present in them cause heterogeneity, which gives

rise to small electro-chemical cells at the sites where metal

and impurities are exposed, to the corrosive environment and

thus, the corrosion starts, which then affects the entire metal.

Example : Iron Pillar

(c) Areas occupied by the anode and cathode :If two dissimilar metals are in contact, one forming anode while another forming cathode, then the corrosion of the anodic metal (part) is directly proportional to the ratio of the areas occupied by the cathode and anode.

Thus, corrosion at anode = area of cathode/ area of anode

Hence, if cathode is large and anode is small, then corrosion at anode is higher.

• Bcz the current density is much greater in small anodic area & the demand of electrons by large cathodic areas can be met by smaller anodic area only by undergoing brisk corrosion.

(d) Nature of corrosion product : If corrosion product is soluble

in corroding medium , corrosion proceeds at much higher rate.

But if product is insoluble further corrosion is suppressed. For

example formation of PbSO4 in case of Pb in H2SO4 medium.

• Similarly if product is volatile it causes rapid n continuous

corrosion. Ex MoO3.

(e) Physical state: Smaller the grain size of metal or metal alloys

greater will be its solubility & hence corrosion.

• Nature of surface film: In aerated atmosphere practically all metals get coated with thin metal oxide film. The ratio of volumes of metal oxide film to metal is known as specific volume ratio.

• Greater the specific volume ratio lesser is oxidation corrosion rate. For ex for Ni, Cr & W it is 1.6, 2.0, 3.6 respectively.

• Consequently the rate of oxidation corrosion of tungsten is least even at elevated temperatures.

II)Working Condition/Environment of the metal:

Working conditions of metals also govern the rate, extent, mechanism of corrosion.

These conditions can be summarized as :

(a)Percentage of oxygen and humidity in air.

(b) Temperature

(c) pH of the medium

(d) Presence of other gases/suspended particles.

(e) Conductance of corroding medium

(f) Polarisation of electrodes

• In air, if availability of oxygen is higher, the rate of corrosion is higher. This is because of formation of oxygen

concentration cells. Thus, the part of the metal having less

availability of oxygen, becomes anodic and undergoes

corrosion, while the other part with more content of oxygen

acts as a cathode and remains unaffected.

• The corrosion also is found to enhance in humid air as

compared to in dry air. This is because, humid air/moist air has

tendency to dissolve gases such as O2, CO2 etc. and also the

acidic vapours from industries.

• Due to this, it becomes easy to set up an electrochemical cell

on the surface of metal.

• Some metals like Mn, Cr, Fe, Mg etc. get corroded in moist

air, even in the absence of oxygen, forming few other

compounds.

• The rate of corrosion is greater at higher temperature, because diffusion of gases increases with the rise in temperature.

• In acidic, pH, the rate of corrosion is higher, because the mechanism of electrochemical corrosion proceeds by evolution of hydrogen gas at cathode.

• In alkaline or neutral electrolytic medium the electrochemical corrosion occurs by following the mechanism of absorption of oxygen; thereby forming an oxide film as a cathodic product. Such a film gets adhered to the surface of the metal and further rate of corrosion is governed by nature of the corrosion product.

• If in air certain other gases or suspended particles are present, the rate of corrosion increases.

• Example : Gases like SO2, H2S or fumes of HCl, H2SO4, adjacent to a metal surface tend to increase the electrical conductivity of the metals, thereby increasing corrosion.

• Presence of suspended particles such as NaCl , (NH4)2SO4 which are hygroscopic in nature, absorb moisture and act as a strong electrolyte, thereby increasing the rate of corrosion.

• If charcoal particles are present in atmosphere, they have a tendency to absorb SO2 and H2S or moisture and thereby

increase the corrosion of metal.

Corrosion Control

Protection of CorrosionThe methods for corrosion control vary from condition to condition. Few methods to prevent the corrosion are

1) Selection of the material

2) Proper designing

3) Use of alloys/Pure metal.

4) Modifying the environment

5) Use of inhibitors

6) Cathodic and Anodic protection methods.

7) Application of protective coatings.

Selection of the materiala) Avoid the contact of dissimilar metals especially if the working

environment is corrosive.

Example : Joining of different parts to build a ship. Here if

metal strips are used to join the wooden parts together, and if

screws used are of brass (Cu and Zn alloy) then the localized

corrosion enhances tremendously when ship floats on marine

water, and highly active metal (zinc) starts acting as anode.

b) If it is unavoidable, to choose two dissimilar metals, then area

of anodic metal should be larger than that of cathodic metal.

c) If two dissimilar metals are to be selected, then metals should

be chosen in such a way that they are as close as possible in

the galvanic series.

d) If two dissimilar metals are to be used, the current flow should

be reduced by introducing an insulator to cut off the contact

between these metals.

Proper Designing

• Designing of the parts should be such that it avoids sharp bends, stresses, etc. or as far as possible the use of screws, nuts, bolts should be avoided, rather welding should be preferred; or the material selections should be in such a way that potential difference does not exist to greater extent.

• The surfaces of two joining parts should be as smooth as possible, so that crevices are as thin as possible, which avoids accumulation of the corrosive liquids, suspended particles, dust. dirt, grit stagnation of water, free circulation of air, etc

Use of alloys / pure metal

• If the metal used to manufacture machine parts is 100% pure, the corrosion resistance is more as compared to the impure metal. Thus, use of pure metal or purifying the metals prevents corrosion. This method is useful only if corrosion proceeds by electrochemical mechanism.

• Corrosion resistance of metals can be improved by alloying the metals. Alloying can help to greater extent, if it gives homogeneous product.

• Example : Iron can be alloyed with chromium and carbon to give steel, which has better corrosion resistance. Here, chromium metal has a tendency to form an oxide which gets adhered to the surface. Also, this oxide film, if broken gets healed immediately.

• Thus, steels containing 13% chromium are normally used to make cutlery, surgical instruments, springs, etc. while higher percentage of chromium such as upto 25% are used to prepare turbine brackets, heat resisting parts etc.

Cathodic protection

Sacrificial anodic protection– Metallic structure to be protected is connected by a

wire to a more anodic metal– Structure to be protected is made cathodic– External anode will give off ions & get corroded --

sacrificial anodic protection

Impressed current cathodic protection• We know that metal in electrolyte – anodic and

cathodic areas are developed• Anode– gives off ions & cathode is protected• So anode & cathode has certain effective potential• This potential is the current – local action

current/corrosion current• Local action current is nullified or counter-balanced

by applying current in opposite direction• This is called the impressed current – dc source

• Impressed current from dc source is applied such that anode and cathode will possess the same/similar potential

Application of protective coatings :This method is most easy and effective because it can provide a continuous barrier between the metal surface and the corrosive environment. The problem of localized corrosion starts only when the protective coat gets broken, due to friction or by any other wear and tear of metal surface.

Types of protective coatings : There are two types of protective coatings.

(a) Metallic coatings

(b) Non-metallic coatings (Organic Coatings)

Metallic Coatings : This type of coatings are either anodic or cathodic depending upon the electrode potential of base metal and coating metal.

For example, metals like Zn, Al or Cd, if coated on iron or steel, serve as anodic coatings, because their electrode potentials are higher than the base metal.

Here, as long as coat layer is intact, the corrosion of base metal is totally under control, but in case any pores, cuts or breaks occur, there is a formation of galvanic cell, between the coated metal portion and the exposed base metal portion.

Metallic coatings

Hot dipping• Metals like Zn, Sn, Pb have low melting points Aim: Base metal X has to be coated with any one of the above metals• Above metal (say Zn) is placed in a furnace which is

maintained at temperature just above their melting points

• We get molten form of the metal (say Zn)• Base metal (X) is dipped in molten form (Zn) and one

gets the desired coating

Contd..

• Metal coatings anodic to base metal is called anodic coating (Zn on Iron)

• Metal coatings of a more noble metal wrt base metal is cathodic coating (Sn on Iron)

Anodic coating: In case of galvanized steel, where Zn is coated on iron (steel), if coating of zinc is broken due to stress or friction, galvanic cell originates.

As Zn is anodic to iron, it dissolves and hence iron is protected even though cell is set up and iron remains protected till entire amount of zinc gets dissolved.

• In case of cathodic coatings, a coat of more noble metal is applied on the base metal surface. As the electrode potential of the noble metal is lower, they protect the base metal.

• e.g. Application of Sn coating on iron surface.• Cathodic coating can provide complete protection to the base

metal till the coat layer is intact. But the moment this layer is broken, the base metal get exposed.

• Since the electrode potential of base metal is higher than that of the coated metal, a galvanic cell is set up where anode is small (exposed base metal) and cathode is larger (coated metal). This gives rise to severe corrosion of base metal.

• Example : Tin coating on iron surface. In this case, tin coating can provide protection to iron surface only till the coated layer is without any breaks.

• If, due to any external factors, the coated layer is punctured, immediately there is formation of galvanic cell where base metal (iron) acts as an anode and the coated metal (tin) acts as a cathode.

• Since, cuts are normally minute, the anode is small and cathode is large. Thus, in corroding environment, the rate of corrosion is very high.

• This continues, till coated layer is repaired to be intact again.

Galvanizing • Application of a coat of Zn metal on the surface of

the base metal X

• Galvanising :• The process involves simply dipping of base metal articles in

the molten mass of zinc, i.e. the coating metal. • To avoid irregular coating, normally the base metal surface is

cleaned to wash away by treating it with H2SO4, by picking

method, or by using dil. H2SO4, for removing any scales, rust (oxide) and any other impurities.

• Such surfaces are washed with water and dried properly, followed by dipping into a molten mass of metal, which is covered with flux solution generally NH4Cl which prevents

oxide formation (because clean metal surface immediately tends to form oxide by reacting with atmospheric oxygen).

• The coated layer is levelled to desired thickness by passing article (pipe, sheet or wire) through the pairs of hot rollers, fixed at desired thickness apart, thereby removing excess of zinc coated unevenly.

• The coated metal is then subjected to annealing at lower temperatures and finally cooled slowly.

• Galvanised iron is used for making water pipelines, roofing sheets, wires, nails, screws, bolts, nuts, tubes, etc.

• Galvanised iron sheets cannot be used to make containers to store food stuffs (especially acidic food items) because coated metal (zinc) gets dissolved in acidic medium forming zinc compounds which are highly poisonous, if consumed.

Tinning :• It is similar to the previous process i.e. the base metal say iron

is cleaned by dil. H2SO4, then washed clean with water and

dried properly. Then the article is passed through ZnCl2 flux

and dipped in the molten mass of tin (m.p. 260 C).• After the dipping process, the article is taken out and then

layer of coated tin is levelled by passing the article through a pair of hot rollers set at particular thickness.

• Excess of tin coated on base metal is scraped off. Later the coated article is annealed slowly to bring down to the room temperature.

The rollers are normally kept under the layer of palm oil. The oil protects the freshly applied layer of tin from oxidation.

Tinning is advantageous, because it is resistant to atmospheric corrosion and action of organic acids.

Tinned sheets are used to manufacture containers to store food – stuffs, oils, ghee, pickles, pharmaceuticals, because there is no danger of poisonous product formation as in case of galvanized articles.

Tinning• Application of a coat of Sn metal on surface of the base

metal X

260C

Electroplating: In this method, freshly cleaned base metal is made cathodic to set up a galvanic cell is suitable bath (electrolytic solution) containing solution of the compound of the metal to be coated on base metal.

Anode is the coating metal. The method is highly useful to produce coatings of metals such as Cu, Zn, Sn, Ni, Cr, Cd, Pb, Ag, Au, Pt and various alloys.

To achieve highly uniform coating, certain precautions are to be taken

i) Current density should not fluctuate.

ii) Composition of electrolytic solution should be consistent.

Electroplating

Cathode: Base metal (to be electroplated)Anode: Coating metalElectrolyte: Soluble salt of coating metal in the electrolytic cell

iii) Temperature should not fluctuate.

iv) Rates of anodic and cathodic reactions should be same• It is experienced that during electroplating, maintaining

consistency in above parameters is very tedious and it requires skilled supervision.

• Hence, electroplating method is used for the articles where it is unavoidable, such as anodizing a particular metal or plating of noble metals where layer has to be as thin as possible without wastage etc.

Organic coatings • Organic coatings are liquid formulations of different

ingredients mixed in appropriate proportions. • Any organic coating can give high protection from corrosion

depending upon its chemical inertness in the corroding medium, adhesion with the underlying surface non-porous nature, etc. These qualities are influenced by the method used for these coatings to be applied.

• Generally, the thickness of coating should be adequate, particularly at bends, crevices or edges, which helps to sustain the wear and tear due to abrasion or friction.

• The adhesion of the coating with underlying surface can be enhanced by using properly cleaned surfaces for application of coatings.

• If the surface is covered with certain impurities, contact with the coating is interrupted; and thus leads to poor adhesion. Subsequently, such a coating undergoes cracks at an early stage.

• Organic coatings such as paints, varnishes, enamels, lacquers etc. provide good corrosion resistance against temperature.

Organic Coating

Definition Constituent Functions

Paints Mechanical dispersion mixture of one/more pigments in a vehicle

Drying oil/ vehicle(linseed oil, fish oil)

Help pigments to hold on surface, Provide dry film by oxidation,Provide tough, durable, water resistant film

Pigments(white pigments: ZnO, TiO2; colored: chromes)

Provide opacity, color strength, protection, resistance to abrasionMinimize cracking on drying

Thinners(turpentine, spirit, kerosene)

Increase elasticity of paint film, reduce viscosity of paint to suitable consistency, helps drying of paint

Driers (oxygen carrier catalysts like resinates & tungstates of Co, Mn, Zn)

Improve quality of oil film, accelerate drying process of the oil film thro’ oxidation, polymerisation and condensation

• Extenders/Fillers– Inert materials that improve the properties of the

paint, eg, gypsum, chalk, talc, silica, etc– They serve to fill voids in the paint film– Reduce cracking of paint after drying & improve

durability of the paint film

• Plasticizers: To give elasticity to the paint film and prevent cracking

Characteristics of a good paint

• Should be fluid enough to spread easily over the protected surface

• Possess high covering power• Form tough, uniform, adherent and impervious film• Film should not get cracked on drying• It should protect painted surface from corrosion effects of the

environment• Color of the paint should be stable to effects of the atmosphere• Consistency should be adequate so that it can be spread easily

Organic Coating

Definition Constituent Functions

Varnish Colloidal dispersion of natural/ synthetic resins in oil

Resin(shellac, phenol, urea, polymers)

Provide high resistance to weathering, chemical action, waterProvides hardening to dried films

Drying oil/ vehicle(linseed oil, fish oil)

Helps in drying varnish films by oxidation & polymerization

Thinners(turpentine, spirit, kerosene)

Adjust/ reduce viscosity of varnish to suitable consistency

Driers (Pb, Co, Mn, linoleates)

Improve quality of oil film, accelerate drying process of the oil film thro’ oxidation, polymerisation and condensation

Antiskinning agents(tertiary amyl phenols)

Helps film to get adhered to underlying surface

Characteristics of good varnish

• Produce protective film – hard, tough, durable and resistant film to wear and tear

• Dry quickly• Aesthetic appeal of the film• Does not crack/ peel off on drying• Color stable to exposure to atmosphere

Varnishes are of two types :• Oil varnishes• Spirit varnishes

Oil Varnishes Spirit Varnishes

These are formed by dissolving resins in oil

These are formed by dissolving resins in spirits

They dry very slowly They dry very quickly

They leave a tough film They leave brittle film

Organic Coating

Definition Constituent Functions

Enamel Pigmented varnish that on drying gives lustrous, hard, glossy finish

Drying oil/ vehicle(linseed oil, fish oil)

Help carrying pigments & other constituents

Pigments(TiO2+CaSO4-white; black: asphalt+linseed)

Helps to get color shades as desired

Thinners(turpentine, spirit, kerosene)

Adjust consistency of enamel

Driers (oleates of Co, Mn, Zn)

Accelerate drying process

Organic Coating Definition Constituent Functions

Lacquers Colloidal suspension of cellulose derivatives, resins, and plasticizers in solvent & diluents

Cellulose derivatives Provide water proofness, hardness, durability to the film

Resins (phenol aldheyde) Increase thickness of film, retain original gloss, increase adhesion of film

Plasticizers (castor oil, tricresyl phosphate)

Improve adherence, reduce brittleness of film

Solvents (ethyl acetate, butyl acetate)

Helps to dissolve film forming substances like cellulose derivatives

Diluents (benzol, naphtha) Adjust consistency/ viscosity of lacquers

Modifying the environment/Electrolyte-Corrosion control

• Removal of harmful constituents or neutralization of the harmful effects of any such constituents from the electrolyte.

• Generally, the percentage humidity, oxygen, other gases, liquids, suspended particles etc. are responsible for the environmental corrosion. Thus, depending upon the affecting constituents, methods to modify the same can be selected.

• Few modifications are discussed here.a) De-humidificationb) De-aeration c) Neutralization of acids present in environment.

De-humidification • It is a process by which percentage humidity in the

environment/corrosive medium can be controlled. In the atmospheric corrosion, if percentage of moisture in air is minimized, then the amount of water which might get condensed on the surface of the metal can be negligible.

• To achieve this, the substances capable of absorbing the moisture are kept in vicinity. Like silica gel or alumina can be used.

• Such methods can be adopted in small closed areas in shops, foundaries, etc. By frequently changing silica gel or alumina corrosion can be minimised.

De-aeration • It is a process in which only percentage of oxygen from the

corrosive environment is controlled. Due to this, the electrochemical corrosion by absorption of oxygen can be controlled.

• The reduction in oxygen content can be achieved by adjusting the parameters like temperature or pressure, or with mechanical stirring/agitating to expel dissolved gases like O2 and CO2 etc.

• Example : In steel pipelines carrying steam from boilers, by de-aeration method, dissolved O2 as well as CO2 can be expelled out, and thus corrosion can be reduced.

Alkaline neutralisation :• Prevention of corrosion by neutralization of the acidic

character of corrosive environment. (due to the presence of

gases such as H2S, HCl, CO2, SO2 etc.)

• This can be achieved by using NH3, NaOH, lime, etc which

are generally injected in the system.

Use of inhibitors• Substance which inhibits the corrosion, when added in the

corrosive environment.• Inhibitors are generally organic or inorganic substances,

soluble in the corrosive environment, but capable to form a protective layer on either anodic or cathodic areas. Inhibitors are anodic or cathodic.a) Anodic inhibitors :

• Chromates, phosphates, tungstates or other ions of transition elements generally with high oxygen content are capable of suppressing the anodic reaction and thereby preventing the dissolution of metal acting as an anode.

• These inhibitors normally form a sparingly soluble compound with metal ion formed at anode due to anodic reaction (i.e. loss of electrons).

• This compound gets adsorbed on the surface of metal and forms a protective film; thereby reducing corrosion rate.

• This method is highly effective, but for only one drawback, that sometimes if entire surface does not get covered by the protective film formed, the unprotected area forms a small anode, which invites severe local attacks.

• Hence the method is rarely used.

b) Cathodic inhibitors : This type of inhibitors are used in both, acidic as well as neutral solution.

• In acidic solutions the cathodic reaction is evolution of hydrogen.

2H+ + 2e– H2 • In neutral solution, the cathodic reaction is due to the

absorption of oxygen gas and formation of OH– ions asH2O + ½ O2 + 2e– 2OH–

• Mg and Zn salts combine with hydroxide ions forming corresponding metal hyroxides which are insoluble barriers towards corrosion at the cathode.