corrosion & prevention mod s.k.g

8/6/2019 Corrosion & Prevention Mod S.K.G

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CORROSION AND ITS PREVENTION

G. Chowdhury PROF.(MET)


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INTRODUCTION: -

Corrosion is an inevitable phenomenon.

In fact our economy would be directly changed if there were no corrosion.

The approximate estimate of loss of metal due to corrosion, 2 to 2.5 billiondollars per annum all- over the world.

In India it is Rs600.00 cores.

Though corrosion is inevitable, its cost can be considerably reduced.

An inexpensive magnesium anode could double the life of a domestic hotwater tank.

Washing a car to remove road- salt is helpful.

Proper selection of materials and good design reduces cost of corrosion.

A good maintenance-painting programme pays for itself many times


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Corrosion Theory

Humans have most likely been trying to understand andcontrol corrosion for as long as they have been using metalobjects.

With a few exceptions, metals are unstable in ordinaryaqueous environments.

Metals are usually extracted from ores through the

application of a considerable amount of energy.

Certain environments offer opportunities for these metalsto combine chemically with elements to form compounds

and return to their lower energy levels.


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Corrosion Theory Contd

Corrosion is the primary means by which metalsdeteriorate.

Most metals corrode on contact with water (andmoisture in the air), acids, bases, salts, oils,aggressive metal polishes, and other solid andliquid chemicals.

Metals will also corrode when exposed to gaseousmaterials like acid vapors, formaldehyde gas,ammonia gas, and sulfur containing gases.


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Corrosion specifically refers to any process involving the deteriorationor degradation of metal components.

The best known case is that of the rusting of steel.

Corrosion processes are usually electrochemical in nature, having theessential features of a battery.

When metal atoms are exposed to an environment containing watermolecules they can give up electrons, becoming themselves positivelycharged ions, provided an electrical circuit can be completed.

This effect can be concentrated locally to form a pit or, sometimes, a

crack, or it can extend across a wide area to produce general wastage.

Localized corrosion that leads to pitting may provide sites for fatigueinitiation and, additionally, corrosive agents like seawater may lead togreatly enhanced growth of the fatigue crack.

Pitting corrosion also occurs much faster in areas wheremicrostructural changes have occurred due to welding operations.


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Corrosion is the disintegration of metal through anunintentional chemical or electrochemical action,starting at its surface.

All metals exhibit a tendency to be oxidized, some moreeasily than others.

A tabulation of the relative strength of this tendency iscalled the galvanic series.

Knowledge of a metal's location in the series is animportant piece of information to have in makingdecisions about its potential usefulness for structuraland other applications


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The corrosion process (anodic reaction) of the metal dissolving as

ions generates some electrons, as shown here, that are consumed

by a secondary process (cathodic reaction).

These two processes have to balance their charges.

The sites hosting these two processes can be located close to each

other on the metal's surface, or far apart depending on the

circumstances.

This simple observation has a major impact in many aspects ofcorrosion prevention and control, for designing new corrosion

monitoring techniques to avoiding the most insidious or localized

forms of corrosion. (more advanced reading)

The electrons (e- in this figure) produced by the corrosion reaction will need to be

consumed by a cathodic reaction in close proximity to the corrosion reaction itself.

The electrons and the hydrogen ions react to first form atomic hydrogen, and then

molecular hydrogen gas.

If the acidity level is high (low pH), this molecular hydrogen will readily become a gas

as it is demonstrated by exposing a strip of zinc to a sulfuric acid solution.


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As hydrogen forms, it could inhibit further corrosion

by forming a very thin gaseous film at the surface of

the metal. This "polarizing" film can be effective in reducing

water to metal contact and thus in reducing corrosion.

Yet it is clear that anything which breaks down this

barrier film tends to increase the rate of corrosion.

Dissolved oxygen in the water will react with the

hydrogen, converting it to water, and destroying the

film.


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1.5 MANIFESTATION (FORMS) OF

CORROSION.

1.5.1 UNIFORM, OR GENERAL ATTACK

1.5.2 GALVANIC OR TWO- METAL CORROSION.

1.5.3 CONCENTRATION CELL CORROSION(CREVICE CORROSION).

1.5.4 PITTING CORROSION.

1.5.5 INTERGRANULAR CORROSION.1.5.6 SELECTIVE LEACHING.

1 1.5.7 EROSION CORROSION.

1.1.5.8 STRESS CORROSION. Contd


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1.6 CORROSION PREVENTION.

1.6.1MATERIAL SELECTION.

1.6.2PROTECTION BY DESIGN & FABRICATIONPROCEDURE.

1.6.3 ALTERATION / MODIFYING CORROSIVE

ENVIRONMENT.

1.6.4 CATHODIC PROTECTION.

1.6.5 APPLICATIONS OF PROTECTIVE

COATINGS.

1.7 SUMMARY.


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CORROSION :-

DESTRUCTION OF MATERIALS THROUGH A

CHEMICAL OR ELECTROCHEMICAL ATTACK BY ITS

ENVIRONMENTS STARTING AT ITS SURFACE.

DESRTUCTION OF MATERIALS BY MEANS OTHERTHAN STRAIGHT MECHANICAL.

EXTRUCTIVE METALLURGY IN REVSESE.


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CLASSIFICATION OF CORROSION:-

LOW TEMPERATURE & HIGH TEMPERATURE CORROSION

DIRECT CHEMICAL CORROSION OR DRY CORROSION.

- OXIDATION CORROSION.

- CORROSION BY OTHER GASES.

ELECTRO CHEMICAL CORROSION OR WET CORROSION


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OXIDATION CORROSION:-

2Mp2Mn+

+2ne-

(loss of electron)Metal ion

nO2+ 2ne-p 2nO2- (Gain of electron)

Oxide ion

or 2M+ nO2 p 2Mn+ + 2nO2-

Metal ion Oxide ion

|__________|

Metal oxide


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MECHANISM OF OXIDATION OF METAL


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CORROSION BY OTHER GASES:

SO2, CO2, CI2 H2S, F2 &OTHER GASSES ALSO CORRODETHE METALS.

IF PROTECTIVE FILM FORMED,THE INTENSITY OF

ATTACK DECREASES (c.g.Agcl film)

IN CASE OF NON PROCTIVE OR POROUS FILM THE

WHOLE SURFACE IS GRADUALLY DESTROYED(FORMATION OF VOLATILE SnCI4)


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NATURE OF THE OXIDE FORMED.

STABLE OXIDE:-METALS LIKE Al,Sn,Cu FORMS STABLE

FILMS & RESTRICTS FURTHER OXIDATION.

UNSTABLE OXIDES:-Ag,Au& Pt DO NOT GO OXIDATION

CORROSION AS OXIDE FORMS BUT DECOMPOSES INTO

METALS & OXYGEN.

VOLATILE OXIDE;-OXIDE LAYERS VOLATIZES AS SOONAS IT IS FORMEDEXPOSING SURFACE FOR FURTHER

ATTACK.(Mo-OXIDE)

POROUS OXIDE:-CRACKS AND PORES OF THE FILMS

ALLOWS OXYGEN ACCES TO THE METAL SURFACE &

CORROSION CONTINUES UNABATED.


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WET OR ELECTROCHEMICAL CORROSION.

WHEN A CONDUCTING LIQUID IS IN CONTACT WITH

METALS.

WHEN TWO DISSIMILAR METALS ARE IMMERSED

PARTIALLY IN A SOLUTION.

CONDITIONS FOR ELECTROCHEMICAL CORROSION

EXISTANCE OF SEPARATE CATHODIC & ANODIC

AREAS.

CURRENT FLOW BETWEEN THEM THROUGH

CONDUCTING SOLUTION.

IN ANODIC AREA OXIDATION TAKES PLACE &

METALS IS DESTROYED.

METALS DESTROYED EITHER BY DISSOLVING

OR ASSUMING COMBIND STATE.


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REACTIONS:-

AT ANODE:-

M p Mn+ + n e- (oxidation)

(Metal) (Metal ion)

M+p Dissolved in solution.

(Metal ion)

pForms compound such as oxide

AT CATHODE:- REDUCTION REACTION (GAIN OF

ELECTRON TAKES PLACE.)

DISSOVE COSTITUENTS IN THECONDUCTING MEDIUM

ACCEPTS THE ELECTRON TO FORM SOME IONS (OH-,O2-).

THE METALIC & NONMETALIC IONS DIFUSES EACH OTHER

& FORMS CORROSION PRODUCTS.


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MECHANISM OF WET OR ELECTRO CHEMICAL

CORROSION

TWO TYPES OF REACTION:-EVOLUTION OF HYDROGEN.

(ACIDIC MEDIUM)

FepFe2+ +2e- (Oxidation)

2H++ 2e- p H2 oThe overall reaction is: Fe +2H+pFe2+ +H2 o

ABSORPION OF OXYGEN.

RUSTING OF IRON IN ATMOSPHERIC OXYGEN IS ACOMMON EXAMPLE OF SUCH TYPE OF CORROSION.


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MECHANISM OF WET CORROSION BY HYDROGEN EVOLUTION


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ELECRTOCHEMICAL REACTIONS OF ZINC IN AIR FREE HCI

ACID.


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OXIDATION CORROSION:-

AT ANODIC AREAS METALS IONS DISSOLVES AS

FERROUS IONS WITH LIBERATION OF ELECTRONS.

Fep Fe2+ + 2e- (Oxidation)

O2 + H2 O +2 e--p 2OH- (Reduction)

Fe

+

+2OH

-pFe (OH)2IF ENOUGH OXYGEN IS PRESENT, FERROUS HYDROXIDE

IS EASILY OXIDISED TO FERRIC HYDROXIDE.

4Fe(OH)2 + O2 +2H2Op 4Fe (OH)3

THIS PRODUCT, CALLED YELLOW RUST, CORRESPONDS

TO Fe2O3.H2O). IF THE SUPPLY OF OXYGEN IS LIMITED,

THE CORROSION PRODUCT MAY BE EVEN BLACK

ANHYDROUS MAGNETITE, Fe3O4..


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MECHANISM OF WET CORROSION BY OXYGEN ABSORPTION.


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FOUR TYPE OF ELECTROCHEMICAL CELL

* GALVANIC CELL OR DISSIMILAR METAL CELL.

* CONCENTRATION CELL.

* DIFFERENTIAL STRESS CELL.

* DIFFERENTIAL TEMPERATURE CELL.


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Corrosion Cell Definition

Nature can encourage corrosion in many fashions.

The force behind the corrosion, attraction of metals towards the

formation of stable oxides.

Oxidized forms of metals can be divided into three types:

1.Composition Cell

2.Stress Cell

3.Concentration Cell


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1.Composition CellComposition cells (also known as Galvanic cells) arise when

two metals with dissimilar compositions or microstructures

come into contact in the presence of an electrolyte.

The two most common examples :

1.1Dissimilar metals

1.2Multi-phase alloy


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1.1 Dissimilar metals:

Formed by two single-phase metals incontact, such as iron and zinc, or nickel andgold.

The metal that is higher on theElectrochemical Series will be the cathode.

The other metal will suffer anodic reactionsand will corrode.


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1.2 Multi-phase alloy: Formed by a metal alloy composed of multiple phases,

such as a stainless steel, a cast iron, or an aluminum alloy.

The individual phases possess different electrode

potentials, resulting in one phase acting as an anode and

subject to corrosion.


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2. Stress Cell

Stress cells can exist in a single piece of metal where aportion of the metal's microstructure possesses more

stored strain energy than the rest of the metal.

Metal atoms are at their lowest strain energy state when

situated in a regular crystal array.

The three most common examples:

2.1 Grain boundaries

2.2 High localized stress2.3 Cold worked


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2.1 Grain boundaries:By definition, metal atoms situated along grain boundaries are not located in a regular crystalarray (i.e. a grain).

Their increased strain energy translates into anelectrode potential that is anodic to the metal inthe grains proper.

Thus, corrosion can selectively occur along grainboundaries


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2.2 High localized stress:

Regions within a metal subject to a high local stress will contain metal

atoms at a higher strain energy state.

As a result, high-stress regions will be anodic to low-stress regions and

can corrode selectively.

For example, bolts under load are subject to more corrosion than

similar bolts that are unloaded.

A good rule of thumb is to select fasteners that are cathodic (i.e.

higher on the Electrochemical Series) to the metal being fastened in

order to prevent fastener corrosion.


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2.3 Cold worked: Regions within a metal subjected to cold-work

contain a higher concentration of dislocations, and

as a result will be anodic to non-cold-workedregions.

Thus, cold-worked sections of a metal will corrode

faster.

For example, nails that are bent will often corrode

at the bend, or at their head where they were

worked by the hammer


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3. Concentration CellConcentration cells can arise when the concentration of

one of the species participating in a corrosion reaction

varies within the electrolyte.

The two most common examples

3.1 Electrolyte concentration

3.2 Oxygen concentration


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3.1 Electrolyte concentration:

Consider a metal bathed in an electrolyte containing its own ions.

The basic corrosion reaction where a metal atom losses an electron andenters the electrolyte as an ion can proceed both forward andbackwards, and will eventually reach equilibrium.

If a region of the electrolyte (adjacent to the metal) were to exhibit adecreased concentration of metal ions, this region would become anodicto the other portions of the metal surface.

As a result, this portion of the metal would corrode faster in order toincrease the local ion concentration.

The net affect is that local corrosion rates are modulated in order to

homogenize reduction ion concentrations within the electrolyte.


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3.2 Oxygen concentration:

Perhaps the most common concentration cell affectingengineered structures is that ofdissolved oxygen.

When oxygen has access to a moist metal surface,corrosion is promoted.

However, it is promoted the most where the oxygen

concentration is the least . As a result, sections of a metal that are covered by dirt or

scale will often corrode faster, since the flow of oxygen tothese sections is restricted.

An increased corrosion rate will lead to increased residue,

further restricting the oxygen flow to worsen the situation.Pitting often results from this "runaway" reaction.


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EMF SERIES

POTENTIAL DIFFERENCE BETWEEN METALS EXPOSED

TO SOLUTUION CONTAINING UNIT ACTIVITY ARE

MEASURED AT CONSTANT TEMPERATURE.

IT IS PRESENTED IN TABULAR FORM KNOWN AS EMF

SERIES.

ALL POTENTIALS ARE REFERRED AGAINST HYDROGEN

ELECTRODE.

POTENTIAL BETWEEN METALS ARE DETERMINED BY

TAKING THE ABSOLUTE DIFFERENCES BETWEEN THEIR

STANDARD EMF POTENTIALS.


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Table 10-1 Standard EMF Series for Metals

Metal-metal ion

equilibrium

Electrode potential vs.

normal hydrogren

electrode at 250C

(volts)

Noble or cathodic Au-Au

+3

+1.498

Pt-Pt+2 +1.2

Pd-Pd+2 +0.987

Ag-Ag+1 +0.799

Hg-Hg3+2 +0.788

Cu-Cu+2 +0.337

H2-H+ 0.000

Pb-Pb+2 -0.126

Sn-Sn+2 -0.136

Ni-Ni+2 -0.250

Co-Co+2 -0.277

Cd-Cd +2 -0.403

Fe-Fe+2 -0.440

Cr-Cr+2 -0.744

Zn-Zn+2 -0.763

Al-Al+3 -1.662

Mg-Mg+2 -2.363

Na-Na+1 -2.714

Active or anodic K-K +1 -2.925

Source: A.J. deBethune and N.A.S. Loud, "Standard Aqueous Electrode Potentials and

Temperature Coefficients at 250C", Clifford A. Hampel, Skokie, 1964


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GALVANIC SERIES

MOST ENGINEERING MATRRIALS ARE METALS OR ALLOYS.

GALVANIC SERIES GIVESS MORE ACCURATE PREDICTION

OF GALVANIC RELATIONSHIPTHAN THE EMF SERIES.

THIS SERIES IS BASED ON POTENTIAL MEASUREMENTS &

GALVANIC CORROSION TESTS IN UN POLUTED SEA WATER.


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GALVANIC SERIES

The relative nobility of a material can be predicted by measuringits corrosion potential.

The well known galvanic series lists the relative nobility of certainmaterials in sea water.

A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small

anodic area.

Rapid thickness loss of the dissolving anode tends to occur underthese conditions.

Galvanic corrosion problems should be solved by designing toavoid these problems in the first place.

Galvanic corrosion cells can be set up on the macroscopic level oron the microscopic level.

On the microstructural level, different phases or other

microstructural features can be subject to galvanic currents


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GALVANIC SERIESContd

Galvanic series relationships are useful as a guide forselecting metals to be joined, will help the selection ofmetals having minimal tendency to interactgalvanically, or will indicate the need or degree ofprotection to be applied to lessen the expected potentialinteractions.

In general, the further apart the materials are in thegalvanic series, the higher the risk of galvaniccorrosion, which should be prevented by design.

Conversely, the farther one metal is from another, thegreater the corrosion will be.

However, the series does not provide any informationon the rate of galvanic corrosion and thus serves as abasic qualitative guide only.

T bl 10 2 G l i S i f S C i l M t l d All i


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Table 10-2 Galvanic Series of Some Commercial Metals and Alloys in

Seawater

Noble or cathodic Platinum

Gold

Graphite

TitaniumSilver

18-8 stainless steel (passive)

Nickel (passive)

Silver solder

Cupronickels (60-90Cu, 40-10Ni)

Bronzes (Cu-Sn)

Copper

Brasses (Cu-Zn)

Nickel (active)

Tin

Lead

Lead-Tin solders

18-8 stainless stel (active)

Steel or iron

2024 Aluminum (4.5Cu 1.5Mg 0.6Mn)

Cadmium

Commercially pure aluminum

Zinc

Active or anodic Magnesium and magnesium alloys


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MANIFESTATION OF CORROSION:-

THE BASIS FOR THIS CLASSIFICATION BEING THE

APPREANCE OF THE CORRODED METAL.

*UNIFORM OR GENERAL ATTACK.

*GALVANIC OR TWO-METAL CORROSION.

*CONCENTRATION CELL CORROSION

(CREVICE CORROSION)

* PITTING CORROSION.

* INTERGRANULAR CORROSION.

* SELECTIVE LEACHING.

* ERROSION CORROSION.

* STRESS CORROSION


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FORMS OF CORROSION:-

1) UNIFORM CORROSION:

THE MOST COMMON FORM OF CORROSION CAUSED DUE TO

CHEMICAL OR ELECRTROCHEMICAL REACTION & THE

ENTIRE EXOPESED SURFACE BECOMES THINNER &

EVNETUALLY FAILS.

2) GALVANIC CORROSION:

WHEN TWO DISSIMILAR METALS ARE IMMERSED IN

CORROSIVE SOLUTION POTENTIAL DIFFERENCE EXISTS

BTWEEN THEM.

THE LESS CORROSION RESISTANT METAL (ANODE) ISCORRODED.

THE MORE CORROSION RESISTANT METAL (CATHOD) IS

NOT CORRDED.


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3)CONCENTRTION CELL CORROSION

(CREVIS CORROSION)

INTENSE LOCALIZED CORROSION OCCURES WITHIN

CRIVICS AND SHIELDED AREAS OF METAL SURFACES

EXPOSED TO CORROSIVE MEDIUM.

4)INTERGRANULAR CORROSION

LOCALISED ATTACK AT AND ADJACENT TO GRAIN

BOUNDERIES, WITH LITTLE CORROSION OF THE

GRAINS.

5)SELECTIVE LEACHING

REMOVAL OF ONE ELEMENT FROM THE SOLID SOLUTION

BY CORROSION PROCESS.

6)


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6)EROSION CORROSION

RATE OF DETERIORATION OR ATTACK ON A METAL IS

ACCELERATED BECAUSEOF RELATIVE MOVEMENT BETWEEN A

CORROSIVE FLUID AND THE METAL SURFACE.

7)STRESS CORROSION

BY SIMULTENEOUS PRESENCE OF TENSILE STRESS AND A

SPECIFIC CORROSIVE MEDIUM CAUSES STRESS CORROSION

CRACKING.


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8) PITTING CORROSION

A LOCALISED ACCELERATED ATTACK RESULTING IN THE

FORMATION OF CAVATIES AROUD WHICH THE METAL

ISRELATIVELY UNATTACHED.

GENERAL FACTS ABOUT CREVICE CORROSION


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GENERAL FACTS ABOUT CREVICE CORROSION:-

CORROSION IS ACCELERATED IN APPARENTLY

INACCESSIBLE AREAS.

CORROSION IS ACCELERATED UNDER ACCUMULATION OF

DIRT, SAND, OR OTHER CONTAMINANTS.

CONTACTS BETWEEN METALS & NON METALLIC

SURFACES ( GASKETS, WOODS, PLASTICS, GLASS,

ASBESTOS etc.)


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MECHANISM OF DIFFERENTIAL AERATION ATTACK

CAUSED BY PARTIAL IMMERSION OF METAL.


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MECHANISM OF DIFFERENTIAL ATTACK CAUSED BY

THE PRESENCE OF DROP OF SALT SOLUTION ON THE

IRON SURFACE.


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ANODE IS FORMED AT INACCESSIBLE

LOCATION,WHERE OXYGEN IS LESS AVAILABLE.

FACTORS INFLUENCING CORROSION:


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FACTORS INFLUENCING CORROSION:-

NATURE OF METALS.( POSITION IN GALVANIC SERIES)

RELATIVE AREAS OF ANODIC & CATHODIC PARTS.

PURITY OF METALS.

PHYSICAL STATE OF METALS.

SOLUBILITY OF CORROSION PRODUCTS.

VOLATILITY OF CORROSION PRODUCTS.

NATURE OF THE CORRODING ENVIRONMENT


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NATURE OF THE CORRODING ENVIRONMENT.

TEMPARATURE.

HUMIDITY OF THE AIR.

PRESENCE OF IMPURITES IN ATMOSPHERE.

PRESENCE OF SUSPENDED PARTICLES IN

AIR.

INFLUENCE OF PFORMATION OF PH.

FORMATION OF OXYGEN CONCENTRATION

CELL.


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PREVENTION OF CORROSION:-

- MATERIAL SELECTION.

- DESIGN & FABRICATION PROCEDURE.

- ALTERATON OF ENVIRONMENT.

- CATHODIC PROTECTION.

* SACRIFICIAL ANODE METHOD.

* EXTERNAL POWER SUPPLY.

APPLICATION OF PROTECTIVE COATING:


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-APPLICATION OF PROTECTIVE COATING:-

* ELECTRO PLATING.

* FLAME SPRAYING.

* CLADDING.

* HOT DIPPING.

* VAPOUR DEPOSITION.

* DIFFUSION.

*CHEMICAL CONVERSION.(ANDISING.

PHOSPHATING, CHROMATIZING.)

*ORGANIC COATING.(PAINTS,VARNISHES,ENAMELS, LACQUERS, etc.)


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Corrosion Performance of MetalsThe following numbers refer to the corrosion rates, expressed in terms of depth of metal

removed in unit time, for mild steel and various stainless steels in

different acids. (reference)

Penetration rates (mm/year) for various steels in different acids

Steel 70% HNO3 (60oC) 20% HCl (20 oC) 80% H2SO4 (20

oC)

Mild Steel Very high 38 0.4

13% Chromium 0.15 120 4.5

12% Chromium 12% Nickel 0.05 5 0.5

17% Chromium 0.1 35 0.7

18% Chromium 8% Nickel Nil 25 1.5


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Corrosion Performance of Metals contd..

Clearly any of the stainless steels would provide better life than mildsteel in hot concentrated nitric acid, but in hydrochloric acid only

the 12 per cent Chromium 12 per cent Nickel steel is significantly

better than the mild steel, which in the sulfuric acid is better than

any of the stainless steels, which in the sulfuric acid is better than

any of the stainless steels, costing about an order of magnitude moreper unit weight.

These differences are largely related to the differing oxidizing

characteristics of the acids, the corrosion resistance of stainless

steels mainly depending upon the formation of a protective

chromium oxide film upon their surfaces, so that in the absence of

an adequate supply of oxygen, as with the hydrochloric acids, the

chromium addition is of little benefit.


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Essentially the same point can be seen in relation to the atmospheric corrosion of

metals, that good corrosion resistance in a particular atmosphere is no guarantee

of good resistance in other circumstances, as the following figures indicate.Penetration rates (mm/year after 10 year exposure) of various metals exposed to

different atmospheric conditions

Metal Atmosphere

Industrial Marine Rural

Aluminum 0.81 0.71 0.0025

Copper 1.19 1.32 0.58

Lead 0.43 0.41 0.48

Zinc 5.13 1.60 0.86

Mild Steel 13.72 6.35 5.08

Cor-Ten Steel (0.4 Cu 1 Cr 0.1 P) 2.54 3.81 1.27

(C T i i i l d l d b U i d S


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(Cor-Ten is a proprietary material developed by United States

Steel Company, and is one of a series of low alloy steels known as

weathering steels.)

The weathering steel, Cor-Ten, owes its improvedcorrosion resistance over mild steel in industrial and ruralatmospheres to the development of an adherent rust filmdue to the incorporation of low-concentrations of certain

alloying elements, but the relative improvement in thoseatmospheres is not nearly so marked in a marineatmosphere.

When immersed in sea water Cor-Ten is not significantlybetter than mild steel and its use is not recommended insuch conditions, although there are examples of it havingbeen so used, no doubt because of its corrosion resistance inother situations and the assumption that this will be goodirrespective of the environment


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The same point an be made in relation to other formsof corrosion; thus molybdenum additions to ferriticsteels are beneficial with respect to stress corrosioncracking resistance in carbonate environments, but aredetrimental in hydroxide solutions, and so on.

The general point to be grasped is that in specifying theoperating conditions for a component or structure it isvital that the environmental conditions be defined withas much precision as any other parameters, such as

stress, temperature etc. Only by so doing can the selection of a material from

the corrosion resistance viewpoint be madesatisfactorily.

M A T E R I A L S E L E C T I O N


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M A T E R I A L S E L E C T I O N.

-THE MOST COMMON METHOD OF CORROSION PREVENTION IS THE

SELECTION OF PROPER METAL & ALLOYS FOR A PARTICULAR

CORROSIVE SERVICE.-STAINLESS STEEL IS NOT STAINLESS,IT IS NOT MOST CORROSION-

RESISTANT, & IT IS NOT A SPECIFIC ALLOY.

- 30 DIFFERENT ALLOYS ( 11.5-30%Cr, 0-22% Ni).

- STAILESS STEELS ARE LESS RESISTANT ALLOYS IN CHOLIRADE-

CONTAINING MEDIUMS & STRESSED STRUCTURES THAN ORDINARY

STRUCTURAL STEEL.

-STAINLESS STEEL ARE MOST SUSCEPTIBLE TO LOCALISED

CORROSION- INTERGRANULAR CORROSION, STRESS CORROSION

CRACKING, & PITTING THAN ORDINARY STEELS.

- A LARGE NUMBER OF CORROSION FAILURE CAN BE DIRECTLY

ATTRIBUTED TO THE INDISCRIMIATE SELECTION OF STAINLESSSTEEL.

-STAINLESS SEELS REPRESENTS A CLASS OF HIGHLY CORROSION

RESISTANCE STEEL.

COMBINATIONS OF METAL & CORROSIVE REPRESENT THE


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COMBINATIONS OF METAL & CORROSIVE REPRESENT THE

MAXMIMUM AMOUNT OF CORROSION RESISTANCE FOR

THE LEAST AMOUNT OF MONEY.

1. STAINLESS STEEL- NITRIC ACID.2. NICKEL & ICKEL ALLOYS- CAUSTIC.

3. MONEL- HYDROFLUORIC ACID.

4. HASTELLOYS ( CHORIMETS)- HOT HYDROCHLORIC ACID.

5. LEAD-DILUTE SULFURIC ACID.

6. ALUMINUM-NONSTAINING ATOMSPHERIC EXPOSURE.

7. TIN-DISTILLED WATER.

8. TITANIUM-HOT STRONG OXIDIZING SOLUTIONS.

9. TANTALUM-ULTIMATE RESISTANCE.

10. STEEL-CONCENTRATED SULFURIC ACID.

DESIGN & FABRICATION PROCESS:-


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DESIGN & FABRICATION PROCESS:

- WELD RATHER THAN RIVET TANKS & OTHER

CONTAINERS.

- BOTTOM OF TANK & CONTAINERS SHOULD BE

SLOPPED TOWARDS DRAIN HOLES FOR EASY

CLEANING.

- EASY REPLACEMENT SYSTEMS OF CORROSION

PRONE COMPONENTS.

- AVOID EXCESSIVE MECHANICAL STRESSES IN

COMPONENTS EXPOSED TO CORROSIVE

MEDIUM.

- AVOID ELECTRICAL CONTACT BETWEEN

DISIMILAR MATERIALS TO PREVENT GALVANIC

CORROSION.

- AVOID SHARP BEND IN PIPING SYSTEM.


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AVOID SHARP BEND IN PIPING SYSTEM.

- AVOID HOT SPOT DURING HEAT TRANSFER SYSTEM.

- DESIGN TO EXCLUDE AIR.

-AVOID HETEROGENETY, DISIMILAR METALS, VAPOUR

SPACES, UNEVEN HEAT, & STRESS DISTRIBUTION.


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CATHODIC PROTECTION OF AN UNDERGROUND TANK

USING IMPRESED CURRENT.


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PROTECTION OF AN UNDERGROUND PIPELINE

WITH A MAGNESIUM ANODE.


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CATHODIC PROTECTION OF A DOMESTIC HOT-WATER

TANK USING A SACRIFICIAL ANODE.


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THANK U


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Corrosion Cell


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OBJECTIVES.

y BECOME FAMILIAR WITH THE PHENOMENON OF

CORROSION.

y APPRECIATE DIFFERENT TYPES OF CORROSION.

y KNOW THE MECHANISM OF CORROSION.

y UNDERSTAND THE MANIFESTATION (FORMS) OF

CORROSION.

y KNOW DIFFERENT METHODS TO CONTROLCORROSION.

STRUCTURE


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STRUCTURE.

1.1 INTRODUCTION.

1.2 DEFINATION OF CORROSION.

1.3 CLASSIFICATION OF CORROSION.

1.3.1 DRY OR DIRECT CHEMICAL CORROSION

1.3.2 WET OR ELECTROCHEMICAL CORROSION.

1.3.2 (A) EVOLUTION OF HYDROGEN.

(B) ABSORPTION OF OXYGEN1.4 CELL TYPES OF CORROSION.

contd


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corrosion & prevention mod s.k.g

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