Unit-2 Coorosion & Its Control

8/22/2019 Unit-2 Coorosion & Its Control

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UNIT II Corrosion and its corrosion control

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UNIT II:Corrosion and its corrosion control: Introduction, causes and different types of corrosion and effects ofcorrosion,Theories of corrosion - Chemical, Electrochemical corrosion, corrosion reactions,factors affecting corrosion - Nature of metal - galvanic series, purity of metal, nature of oxide film, nature ofcorrosion product.Nature of environment-effect of temperature, effect of pH, Humidity, effect of oxidant. Corrosion control

methods - Cathodic protection, sacrificial anode, impressed current cathode.Surface coatings - methods of application on metals- hot dipping, galvanizing, tinning, cladding,electroplatingOrganic surface coatings - paints constituents and functions.


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Corrosion and its corrosion control

Introduction:The destruction and consequent loss of a solid metallic material, through an unwanted chemical orelectrochemical attack by its environment, starting at its surface, is called corrosion.Corrosion can be a fast (or) slow process depending upon the metal, and environment in which it is undergoingcorrosion. The familiar examples of corrosion:

Rusting of Iron: A reddish brown scale formation on iron and steel objects. It is due to formation ofhydrated ferric oxide.

Green scales are formed on copper vessel. It is due to the formation of basic cupriccarbonate(CuCO3+Cu(OH)2)

Causes: Most metals, except the nobel metals (Au, Pt etc), occur in nature in the form of their compounds suchas oxides,sulphides,carbonates etc.The metals are extracted from ores in endothermic process, energy beingsupplied in the form of heat (or) electrical energy.

Corrosion of metals occurs either by direct chemical attack (or) by electro chemical attack on the metalby corrosive environment. Direct chemical attack takes place generally in the absence of moisture. This type ofcorrosion is known as dry corrosion in which the constituents of the corrosion medium directly combine withmetal due to their chemical affinity to the metal. Most of the corrosion cases are electrochemical in nature,taking place by the electrochemical attack on the metal in presence of moisture (or) conducting medium. Suchcorrosion is wet corrosion.

Direct chemical or dry corrosion:According to this theory, direct chemical corrosion occurs due to the direct chemical reaction between

the metal and the gases (oxygen, halogens, oxides of sulphur, and oxides of nitrogen, hydrogen sulphide andfumes of chemicals) present in the corrosion environment. This type of corrosion is generally observed inabsence of moisture or conducting electrolyte medium, and therefore, known as Dry corrosion. Directoxidation of metals and alloys on exposure to oxygen in air a common example of chemical corrosion. Twomajor factors deciding the rate of corrosion are the chemical affinity between the metal and the gases, and natureof the corrosion product formed.When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this film decides thefurther action. (Stable, Unstable, Volatile, Porous)

Pilling-Bedworth rule:The protective and non-protective nature of the oxide layer can be predicted on the basis of Pilling-Bed worthruleAccording to this rule an oxide is protective (or) non-porous, if the volume of the oxide is at least as great asthe volume of the metal from which it is formed, and the oxide layer is porous (or) non-protective, if the volumeof the oxide is less than the volume of metal from which it is formed.

For example: Heavy metals such as Aluminium, chromium, lead, Tin, etc form an oxide film. Metals like alkali and alkaline earth metals form a porous oxide films, which allow further diffusion of

oxygen to come in contact with fresh metal surface, thereby acts as a non-protective film.The extant of chemical corrosion by other gases such as CO2,SO2,Cl2,H2S,F2 etc also depends upon the

formation of protective film (or) non-protective film by the corrosion product.Example: 1) The AgCl film from the attack of Chlorine on the silver metal surface is non-porous and protective.2) SnCl4 formed on the surface of Tin by attack of chlorine is volatile, and leaves the fresh Tin surfaces forfurther attack.


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Electro chemical theory of corrosion (or) Wet corrosion:Most of the corrosion problems are best explained on the basis of electro chemical corrosion. These are oftencalled Wet corrosion since aqueous medium or moisture is required for corrosion to takes place.

According to electro chemical theory, corrosion of metals takes place due to the formation of anodicand cathode regions on the same metal surface (or) when two different metals are in contact with each other inpresence of a conducting medium. At anodic region oxidation reaction takes place and metal gets converted intoits ions, liberation of electrons.Consequently, metal undergoes corrosion at anodic region. At Cathodic region,

reduction reaction takesplace.Since the metal cannot be reduced further, metal atoms at the Cathodic regions areunaffected by the corrosion reactions. Some constituents of the corrosion medium take part in the cathodicreaction.

The electrons liberated at anodic region migrate to cathodic region constituting corrosion current .Themetal ions liberated at the anode and same anions formed at the cathode diffuse towards each other through theconducting medium and form a corrosion product. Somewhere between the anode and cathode.

Corrosion of metals continues as long as both anodic and cathodic reactions take place simultaneously.There cannot be anodic reaction without cathodic reaction vice versa.

To understand the wet theory, let us take the example of corrosion of iron. Oxidation ofmetal takes place at anode while the reduction process takes place at cathode. By taking rustingof iron as an example, the reaction can be explained as that it may occur in two ways: (i) evolutionof hydrogen and (ii) absorption of oxygen

.

At anode: oxidation occurs.

At cathode:Case I: Evolution of H2The hydrogen ions (H+) are formed due to the acidic environment and the following reaction occurs in theabsence of oxygen

2H+ + 2e- H2 (reduction)The overall reaction is Fe + 2H+Fe+2 + H2

In this case, metals react in the acidic environment and are dissolved (undergo corrosion) to release H2 gas. All

metals above hydrogen in electrochemical series can show this type of corrosion. In hydrogen evolution type of


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corrosion, anodic area is large as compared to its cathodic area Fig.b

Case II: Absorption of O2This type of corrosion takes place in neutral or basic medium in the presence of oxygen. The oxide of ironcovers the surface of the iron. The small scratch on the surface creates small anodic area and rest of thesurface acts as cathodic area. The following chemical reactions occur at anode and cathode.

At anode Fe Fe++ + 2e (oxidation)

Ferric hydroxide is actually hydrated ferric oxide, Fe2O3.H2O, which is yellowish rust. Anhydrous magnetite,

Fe3O4 [a mixture of (FeO + Fe2O3)], is also formed, which is brown-black in colour. It is mark able that thecorrosion occurs at anode but the corrosion product is formed near cathode. It is because of the rapid diffusion ofFe++ as compared to -OH (Fig. a). Hence corrosion occurs at anode, but rust is deposited at or near cathode.

Types of corrosion:

(a) Galvanic Corrosion (b) Pitting Corrosion (c) Stress Corrosion

(d) Crevice Corrosion (e) Erosion Corrosion (f) Soil Corrosion(g) Micro-biological Corrosion (h) Water-line Corrosion

(i) Differential aeration Corrosion (j) Intergranular corrosion

Galvanic Corrosion: This type of electrochemical corrosion is also called bimetallic corrosion. When twodissimilar metals are connected and exposed to an electrolyte, they will form a galvanic cell. The anodic metal willbe oxidised and it will undergo corrosion. Zinc and copper metals connected with each other in an electrolytemedium form a galvanic cell. Zinc acts as anode and undergoes corrosion while cathode will be unaffected

At anode: Zn Zn++ + 2e- [Oxidation] corrosion

At cathode: Cu++ + 2e-Cu [Reduction] unaffected

1. Galvanic corrosion can be avoided by coupling metals close to the electrochemical series.2. Fixing insulating material between two metals.3. By using larger anodic metal and smaller cathodic metal.

S.No Dry Corrosion Wet corrosion

1 Corrosion occurs in the absence of moistureCorrosion occurs in presence of conductingmedium

2 It involves direct attack of chemicals onthe metal surface

It involves formation of electrochemical cells

3 The process is slow It is a rapid process

4Corrosion products are produced at the

site of corrosionCorrosion occurs at anode but rust is depositedat cathode

5 The process of corrosion is uniformIt depends on the size of the anodic part of

metal


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Example of galvanic corrosion: 1. Steel screws in brass marine hardware, 2. steel pipe connected to copperplumbing, 3. steel propeller shaft in bronze bearing, 4. zinc coating on mild steel, 5. lead-tin solder around copperwires.

Pitting corrosion: Due to crack on the surface of a metal, local straining of metal, sliding under load, chemicalattack, there is formation of a local galvanic cell. The crack portion acts as anode and rest of the metal surfaceacts as cathode. It is the anodic area which will be corroded and the formation of a pit is observed. This type ofcorrosion is thus called pitting corrosion. Metals owing to their corrosion resistance to their passive state show

pitting and ultimately result in format ion of passivity. Presence of external impurities such as sand, dust,scale embedded on the surface of metals lead to pitting. For example, stainless steel and aluminium show pittingin chloride solution.

At anode: Fe Fe 2++ 2e- [Oxidation] corrosion

At cathode: O2+H2O +2e-2 OH-[Reduction] unaffected

Stress corrosion: In a metallic structure, if there is a portion under stress, it will act as anode and rest part of

the structure will act as cathode. It is now a galvanic system and hence anodic part which is smallin area will corrode more. Stress corrosions are observed in the following systems:

Crevice corrosion: If surface of painted metal is scratched, it will undergo corrosion. Thescratched portion acts as small anode and the rest part will act as cathode forming a local cell.Crevice corrosion is formed near joints, rivets and bolts. Changes in the concentration ofoxygen/acidic medium causes crevice corrosion.

Crevice corrosion water line corrosionWaterline corrosion: It has been observed in the case of an iron tank containing water, that theportion of iron tankjust below the water level undergoes corrosion. It is due to the differencein oxygen concentration. Corrodingportion is poor in oxygen and acts as anode

Example: When marine plants attach themselves to ocean going ships, water line corrosion is oftenaccelerated and for its checking, use of anti-fouling paints is must.

Inter-granular corrosion: This corrosion is observed in case of alloys. The corrosion product isobserved at the boundaries of grains. Externally, it is not seen. There is a sudden failure ofmaterial due to this Corrosion. For example, during the welding of stainless steel (an alloy ofFe, C, Cr), chromium carbide isprecipitated at the grain boundaries and the region adjacent tograin boundaries becomes depleted of chromium composition and is made anodic with

respect to solid solution within the grains richer in chromium. Rapid quenching after heattreatment of a metal is the remedy of inter-granular corrosion


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Factors affecting corrosion:There are several factors which influence the rate of corrosion to a varying degree. Some of the importantfactors affecting the rate of corrosion are discussed below.1.Nature of the metal: The tendency of a metal to undergo corrosion is dependent on the nature of the metal. Ingeneral, the metals with low electrode potential values are more reactive than the metal with high electrodepotential values. The more reactive metals are more susceptible for corrosion. Thus the tendency of a metal toundergo corrosion decrease with increase in electrode potential.For example: The so called metals like K,Na,Mg,Zn etc with low electrode potential values are highlysusceptible for corrosion. The noble metals such as Ag, Gold, and Pt with high electrode potential values areless susceptible for corrosion.Galvanic series:Galvanic series have been prepared which gives real and useful information regarding the corrosion behaviourof meals and alloys in a given environment.

GALVANIC SERIES2. Hydrogen over voltage: A metal with low hydrogen over voltage on its surface is more susceptible for

corrosion, when the cathodic reaction is hydrogen evolution type. With lower hydrogen voltage, hydrogen gas is

liberated easily and thus the cathodic reaction is faster. This will make the anodic reaction also faster, thereby

promoting overall corrosion reaction. When the hydrogen over voltage on the metal surface is high, cahtodic

reaction is slower and the corrosion of the metal also becomes slower.

3. Purity of metal: Impurity in metals causes heterogeneity and hence small localized galvanic cells and

therefore enhancement of corrosion.


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Example: If Zn metal is contaminated with impurities like Pb or Sn which are at higher level than Zn they use

Zn as sacrificial anodes and hence more corrosion is seen in comparison to pure metal.

4. Nature of oxide film: The smaller the specific volume ratio [vol of metal oxide/vol of metal] greater is the

oxidation corrosion, because oxide film formed will porous, through which oxygen can diffuse and bring above

further corrosion.

Example: The specific volume ratios of W, Cr, and Ni are 3.6, 2.6, and 1.6 respectively.Consequently; the rate

of corrosion W is least, even at high temperatures.

5. Nature of corrosion product: If the corrosion product deposited in insoluble, stable, uniform and porous, it

acts as a protective film, preventing the further corrosion of metal. A thin, invisible, continuous film formed on

the surface acts as barrier between the fresh metal surface and the corrosion environment. The fresh metal

surface and the corrosion product is soluble, unstable, on-uniform and porous, the corrosion continuous

unabated. In such cases the fresh metal surface is continuously exposed to the corrosion environment and

corrosion of the metal surface takes place continuously.

6. Nature of environment:

Effect of temperature: The rate of a chemical reaction, in general increases with rise in temperature.

Corrosion process (redox reaction) is one such chemical reaction.Therefore, the rate of corrosion

increase in temperature increase the conductance of the corrosion medium, which also contributes to

the increase in corrosion rate.

Effect of p H: In general, lower pH of the corrosion medium, higher is the corrosion medium.However;

some metals like Al, Zn e.t.c undergo fast corrosion in highly alkaline solution. The pH of the solutions

also decides the type of cathodic reaction.

Humidity: The greater is humidity, the greater is the rate and extent of corrosion. This is due to the fact

that moisture acts as solvent for O2,H2S,SO2, and NaCl etc to furnish the electrolyte essential for setting

up a corrosion cell.Example:1.Atmosphere corrosion of iron is slow in dry air compared to moist

air.2.Gases like H2S,SO2 etc increases the acidity of medium by their dissolution in water and hence

increases the corrosion rate.3.Dissulution of NaCl in water leads to increased conductivity and thereby

increased corrosion rate.

Effect of oxidant: The presence of oxidizing agents increase the corrosion rate of the metal. Systems

handling water like boilers, heat exchangers etc are faced with serious corrosion problem permissible.

Even Nobel metals undergo corrosion in presence of oxidizing agents.

S.NO GALVANIC SERIES ELECTROCHEMICAL SERIES

1It predicts the corrosive tendencies ofmetal alloys

It predicts the relative displacement tendencies

2Calomel electrode is used as a referenceelectrode

Standard hydrogen electrode is used asreference electrode

3Positioning of metal or alloy may change Position of metal is fixed. That cannot be

changed

4The metals and alloys are immersed in thesea water for study

concentration of salts of the same metal thatwas being used

5

Electrode potentials are measured for both

metals and alloys

Electrode potentials measured only for

metals and non-metals


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Corrosion control:-

Cathodic protection: - Cathodic protection is a method of protecting a metal (or) alloy from corrosion by

converting it completely in to cathodic and of it is allowed to act as anode.

Principle:-Metals normally undergo corrosion by electrochemical process with the formation of anodic and

cathodic regions in contact with each other. The corrosion of metal takes place at the anodic region whereas at

the cathodic the metal is unaffected.Therefore, corrosion can be prevented by eliminating the anode sites and

converting the entire metal into cathodic area.

Cathodic protection can be achieved by the following methods

*Sacrificial anode method *Impressed current method

Sacrificial anode method:-In this method, the protected metal surface is converted into a cathode by connecting

it to a more active metal. This active metal acts as an auxiliary anode.Zn, Mg, Al, are the common auxiliary

anodes used in this method. These metals, being more active, acts as anode and undergo preferential corrosion,

protecting in the metal structure. Since the anodic metals are sacrificed to protect the metal structure, the method

is known as Sacrificial anode method.

Examples: 1) A Mg block connected to a buried oil storage tank.

2) Mg blocks are connected to bury pipe lines.

SACRIFICIAL ANODE METHOD IMPRESSED CURRENT METHOD

Impressed current method:-Another method of providing cathodic protection is by applying a direct current

larger than the corrosion. The protected metal is made Cathodic by connecting it to cathode of external source of

current .The anode of the source is connected to an inert electrode.The metal structure being cathode does not undergo corrosion. Anode being inert remains unaffected.

Graphite is widely used as inert anode in this method.

Platinum, Silicon, Iron also used as anodes.

Advantages:1) One installation can protect large of metals

2) Low maintains cost.

Surface coatings methods of applications on metals:

The following are the methods of applications of the metallic coatings.

Hot dipping: This is a method of coating a low melting metal such as Zn,Sn,Pb,Al etc on iron ,steel and copper

which have relatively higher melting points. The base metal is dipped in the molten bath of the coating metal,

which covered by a molten flux layer which cleans the base metal surface and prevents the oxidation of coating metal. For

adhesion of the coating metal on the surface of the base metal, the base metal surface must be very clean.

The most widely used hot dipping methods are (a) Galvanisation (b) Tinning

Galvanisation: It is a process of coating iron or steel sheets with a thin coat of Zinc to prevent iron from rusting.

The base metal iron or steel is cleaned by acid pickling method with dilH 2SO4 for 15-20 min at 60-900

C.The sheet is then washed well and dried .It is dipped in a bath of molten Zinc maintained at 425-4300C .The

surface of the bath is kept covered with ammonium chloride flux to prevent oxide formation. The sheet is taken

out and excess of Zinc is removed by passing it between a pair of rollers. Then the sheet is subjected to

annealing process at 6500C and cooled slowly. An alloy of iron and zinc were formed at the junction of the base

metal and coating metal.


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Applications: It is mostly used to protect iron used for roofing sheets, wires, pipes, nails, bolts, screws,

bucketsand tubes. Galvanised utensils cannot be used for preparing and storage food stuffs especially acidic in

nature, because zinc dissolves to form highly toxic or poisonous compounds

I r ons h e e t

D i l.H2S O

4

A t 6 0 - 9 0 0

W a s h in gbath

D r y in gc h a m b e r

H o ta ir

M o l te n Z i n c a t

4 2 5 - 4 3 0 0 CE x ce s so f Z i n c

A n n e a l i n gc h a m b e r

P a i r o f h o tro l lers

G a l v a n iz i n gs h e e t

N H 4C l

f lux

Galvanisation of steel sheet

Tinning: Coating tin over iron or steel articles is called tinning. In this process the surface of the base metal

(iron sheet) is cleaned by acid pickling with dil.H2S04 and passed through a bath of zinc chloride flux. The flux

helps the molten tin bath and pressed between two rollers from a layer of palm oil. Palm oil helps to protect the

tin coated against oxidation. The rollers remove excess of tin and produce a thin film of uniform concentration.An alloy of the base metal and coating metal at their junction is produced.

Applications: 1.The metals possess good resistance atmospheric corrosion. Tin non toxic and widely used for

coating steel, copper, and brass sheets.2.The containers coated with tin is used for storing food stuffs,Ghee,oils

and packing food materials.

Rolers

Tin platedsheet

Palmoil

Molten TinAcidpicklingTank

ZnCl2

Flux

Metal cladding: In this process a dense, homogenious layer of coating metal is bonded (cladded)

firmly and permanently to the base metal on one or both sides .The metal cladded is cladded is called

cladding metal. The choice of cladding metals depends on the corrosion resistance required for any

particular environment.

The corrosion resistance metals like Ni, Cu, Pb, Ag, Pt etc and alloys like stainless steel,

Nickel alloys, copper alloys are used as cladding materials. The common base metals cladded are mile

steel, aluminium, copper, nickel and their alloys. 99.5% pure aluminium is subjected to cladding to

protect duraluminium to produce Alclad which is widely used in aircraft industry.

Applications: This method is widely adopted in aircraft industry and automobile industry.


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Electroplating: The process of depositing the coating metal on the surface of the base metal/non

metal by electrolysis is called electroplating.

Applications: Electroplating is a most important and frequently used technique in industries to

produce metallic coating. Both metals and non metals can be electroplated. In metals electroplating

increases resistance to corrosion, chemical attack, hardness and hardness and surface properties. In

non metals electroplating increase strength and decorates the surface of non metals like plastics,

wood, glass etc.Organic surface coatings

It is an important established to protect a metal surface and other constructional materials from corrosion by

using paints.Varnishes, enamels and lacquers. They are collectively come under the category of organic

coatings.

Two important functions of organic coatings

To impart decorative and aesthetic appeal and

To protect the metal from corrosion

Paints:

Wood, metal, cement, bricks etc are well known as constructional materials for our homes, offices, industries

etc.Over a period of time

Metal surface gets scratched and warped

Wooden surface gets scratched and warped Walls crack and chip

Paints forms a protective layer over these surfaces and keep them looking as good as new.

Essentially paint is a combination of following

Pigments:-Which give a paint its colour and opacity

Solvents:-Which give a paint its flow and enable it to be brushed on a surface

Additives:-Which give a paint special properties like resistance to fungus ,rust etc

Requirements of a good paint:-

It should be fluid enough to be spread on the metal surface easily

It should have a high covering power

It should form a quite tough,uniform,adherent,and impervious film

The film should not crack on drying

It should give glossy film

It should adhere well to surface

Constituents of paints and their functions:

The important constituents of paint are

P aint

P igments Vehic les Th inners Driers F illers P lasticisers Anti-S kinning agents

Pigments: - Pigment is a one of the essential constituent of paint

Functions:-

To give aesthetical appeal to the paint film To provide desired colour, opacity and strength to paint

To give protection to the paint film by reflecting harmful UV light

To improve impermeability of paint film to moisture.

Characteristics of good pigments:

Chemically inert

Non toxic

Easily available


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Freely mixable with oil

Cheap, it should also posses higher covering power

Pigment types:Natural (Ex: Talc, clay), Synthetic (Ex: Titanium oxide, Barium sulphate),

Reactive (Ex: Red lead and ZnO)

Vehicle (or) drying oil: - It is a film forming constituent of the paint. The liquid portion of the paint in which

the pigment is dispersed called medium (or) vehicle (or) drying oil

Functions:

They hold the pigment on the metal surface

They form the protective film

These give better adherence to the metal surface

Thinners:-Viscosity of the paints is reduced by the addition of thinners. This is essential so that the paints can

be easily applied on the metal surface.

Example: Turpentine, Benzene, Kerosene

Functions:

They suspended the pigments in the paint

They increase the elasticity of the paint film

They evaporate rapidly and help in the drying of the paint film

They also increase the penetration power of vehicles.

Driers:-The drying of the oil film is accelerated (or) catalyzed by driers. They do this by oxidation,

polymerisation, and condensation.Infact driers are oxygen-carrier catalysts

Example: Borates, Naphthalene, Resinates.

Functions:

The main function of drier is to improve the drying quality of the oil-film. They acts as oxygenated

carrier catalyst which help the absorption of oxygen and catalyze the drying of the oil film.

Filler:-Fillers are inert materials which are used to improve the properties and reduce the cost of paint.

Example: Talc, Chalk, Silica, Calcium carbonate, Calcium sulphate, Barium sulphate

Functions:

They reduce the cost of paint

They serve to fill the voids in the film

Plasticizer:-Plasticizers are added to the paint film to give elasticity to the paint film and to prevent cracking of

the film.

Example: Tricresyl phosphate, Dibutyl tartarate

Anti-skinning agents:-Anti-skinning agents prevent the gelling and skinning of the paint film.

Example: Poly hydroxyl phenols.

Unit-2 Coorosion & Its Control

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