Lec 13 Corrosion1

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CORROSION OF METALS

Introduction

Definition

Economic impactTypes of Corrosion

Combating corrosion

Anti corrosive coatingsand Paints

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http://en.wikipedia.org/wiki/Image:Rust_and_dirt.jpg


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The diffuse dispersion of metals in the society

has many sources.

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Corrosion - IntroductionAny material which serves the humans needs such as

housing, food, clothing, communication,

transportation, energy, socioeconomic development

has the inherent tendency to decay.

Virtually no engineering material is stable. The rate of

decay varies depending upon the material and its

environment.

The basic cause of corrosion is the instability of metals

in their refined forms. The metals tend to revert to their

natural states through the process of corrosion..1/23/2013 3WEC


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No material is inert!

All metals/alloys interact with the environment.

SO2

Metal runoff/releaseMe, Men+

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Corrosion process

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ContdCorrosion - IntroductionHow to stop this reversion?

In order to stop reversion to the natural state,engineering materials require

protective coatings,

inhibitors,

alloy additions,

design procedures,

maintenance and refurnishing.

It is good to extend the life of materials keep themcorrosion free as possible.

Extending the life of a 1000 megawatt nuclear power plant

for one day would mean a saving of 0.5 mill dollars.1/23/2013 6WEC


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What is Corrosion ?

Corrosion is the gradual physico

- chemical destruction of

materials by the attacking actionof the environments. The word

corrosion originated from the

Latin word corrodereto

gnaw away.

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Corrosion may be defined as the

destruction of material by chemical,

electrochemical, or metallurgical

interaction between the environmentand the material.

The term corrosion is sometimes alsoapplied to the degradation of plastics,

concrete and wood, but generally refers to

metals.

Definition of Corrosion

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Impacts of corrosion

Losses are economic and safety:

Reduced Strength

Downtime of equipment Escape of fluids

Lost surface properties

Reduced value of goods

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Impacts of corrosion

The consequences of corrosion are many and

varied and

the effects of these on the

safe, reliable and efficient operation of

equipment or structures

are often more serious than the simple loss of a

mass of metal.

Failures of various kinds and the need for

expensive replacements may occur even though

the amount of metal destroyed is quite small.

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Economic Impact

Corrosion a most serious problems of

industry

Corrosion causes damage in the billions

of dollars each year. Apporx. 10% worldmetal production is completely lost due to

corrosion each year.

According to estimate national annualcosts of corrosion to a country vary from

1 % to 4 % of GNP.1/23/2013 11WEC


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A study showed that During 1998 in the US, the total

annual direct cost of corrosion was

$276 billion ( 3.1% of the US GDP.

15% to 25% of this loss can be

saved if available knowledge and

techniques are used.

Economic Impact

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How much does corrosion really cost?

Before After

Silver Bridge Collapse

December 15, 1967 in Kanauga, OH

Highways and Bridges

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Highways & Bridges1: $8.3 Billion per year + safety issues

How much does corrosion really costin USA ?

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Economic losses

Costs of corrosion are divided into direct & indirect losses

Direct losses

include costs of replacement of corroded structures &

machinery / their components, e.g., condenser tubes,

pipelines, metal roofings, repainting of structures againstrusting.

These losses also include the extra cost of using

- corrosion-resistant material in place of Carbon steel or

other cheaper materials,

- adding inhibitors to enclosed systems,

- protective systems for metal structures.1/23/2013 15WEC


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Economic lossesIndirect losses

Loss of product

Loss of production

Loss of efficiency

Product contamination

Over design

Maintenance of stand-by Machinery &

Equipment

General loss e.g., loss of health & life which

can not be computed in terms of money.1/23/2013 16WEC


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Types of Corrosion

Dry corrosion or

oxidation of metals

Wet corrosion or

electrochemical

oxidation

Galvanic Corrosion

Electrode

Potential Examples of

Galvanic

Corrosion

Atmospheric

corrosionPitting Corrosion

Erosion / Cavitation

corrosionFretting corrosion

Crevice Corrosion

Stress Corrosion

Intergranular

Corrosion

Microbial Corrosion1/23/2013 17WEC


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Dry Corrosion or Oxidation of Metals(scaling of iron at high temperature)

Many metals tend tooxidize to some extentat all temp. When ironheated in the presenceof oxygen it becomescoated with a film ofblack scale as per

following reaction:2Fe + O2 = 2FeO

Iron is oxidized as itsatoms lose electrons.

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Wet Corrosion or ElectrochemicalOxidation

Iron does not rust

in a completely dry atmosphere,

nor will it rust

in completely pure,O2-free water,

but in a moist atmosphere, as a result,

reddish-brown deposit of ferric hydroxide soonbegins to develop, as :

4Fe + 6H2O + 3O2 = 4Fe(OH)3

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Contd Wet Corrosion orElectrochemical Oxidation

The fundamental principle is that

atoms of iron in contact with oxygen

and water oxidize, that is they lose

electrons and enter into solution as

ferrous ions (Fe++) which are further

oxidized to ferric ions (Fe+++).

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Electrode Potential for some metals

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Ranks the reactivity of metals/alloys in seawater

PlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium

8

Galvanic Series

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Galvanic Corrosion Galvanic Corrosion occurs when two

dissimilar metals electrically contact

each other and are immersed in an

electrolyte.

In order for galvanic corrosion to

occur, an electrically conductive pathand an ionically conductive path are

necessary.

This affects a galvanic couple where

the more active metal corrodes at an

accelerated rate and the more noble

metal corrodes at a retarded rate

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Requirements for Corrosion:

In order for galvanic corrosion to occur, an electrically conductive

path and an ionically conductive path are necessary.Ionic Current Path

Electronic Path

ANODE CATHODE

WhereCorrosionOccurs!!!!

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Anodic & Cathodic Reactions

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Examples of Galvanic corrosion

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Galvanic corrosion of pearlite

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Galvanic corrosionan examples of bad plumbing

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Atmospheric Corrosion Most common & economically most costly form.

The atmospheric corrosion process depends

only on the moisture content of air but also on the

gaseous impurities,

dust particles and

other contaminants which favour condensation of moisture on

the metal surface.

Atmospheric corrosion rates vary markedly all over the

world depending upon the

climatic region,

of the year,

time of the day etc.

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Contd Atmospheric Corrosion

Approaching the sea coast, air is laden withincreasing amount ofsea salt.

At industrial areas, appreciable amounts of SO2

which is converted into H2SO4, and to lesseramounts of H2S, NH3, NO2 and various suspended

salts are encountered.

A metal resisting one atmosphere may lack effective

resistance elsewhere and hence relative

performance of metals changes with location.

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Pitting Corrosion Pitting is a localized attack, in the

form of pits, such as observed on

stainless steel immersed in

chloride ion solutions.This causes

premature failure.

The main causes of pitting are

galvanic corrosion,

low or high oxygen concentrations

and

applied stresses.

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E i i


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Erosion corrosion

Erosion is acceleration of

corrosion because of

relative movement

between corrosive fluid

and material as may occur

in pumping equipment.

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Cavitation corrosion

Cavitation corrosion is a

form of velocity effect

caused by the explosion of

bubbles formed where the

local pressure in theflowing fluid drops below

the vapour pressure of the

liquid.It is observed on ship

propellers, pump impellors

etc.1/23/2013 33WEC


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Frettingcorrosion

Fretting corrosion is a surface damage produced by vibration

which results in striking or rubbing at the interface of close

fitting, highly loaded surfaces.

It is common at surfaces of clamped or press fits, splines,

keyways, and other close-fitting parts subject to minute relativemovement.

Fretting corrosion ruins bearings, destroys dimensions, and

reduces fatigue strength.

This type of corrosion is a mechanical-chemical phenomenon.1/23/2013 34WEC


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Crevice Corrosion

It is a intenselocalized corrosion

at a crevice under

a bolt head or

flanged

connections

usually due to

concentration cells.

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Crevice Corrosion

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St C i


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Stress Corrosion

Stress corrosion is caused by

simultaneous presence of tensile stress

and a particular corrosive medium e.g.,

cracking of cold worked Cu or brass in

ammonia or

stainless steel in chloride-containing water.

The magnitude of stress necessary to

cause failure depends on

the corrosive medium and

on the structure of base metal.

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Intergranular Corrosion


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Intergranular Corrosion occurs in the vicinity of grain boundaries

Due to elevated temperature Chromium Carbides can form in the

grain boundaries of SS.

This Chemical reaction depletes the Cr in the HAZ near the grain

boundary, making those areas much less resistant to corrosion.

This creates a galvanic cell which leads to weld decay.

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Preferential attack alonggrain boundaries

Results from localized

differences in chemistry

Common in SS, nickelsome Al alloys

Sensitive Regions

precipitates

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Microbial Corrosion

Microbial or Bacterial Corrosion is

caused or promoted bymicroorganisms.

It can apply to both metals and

non-metallic materials, in both the

presence and lack of Oxygen.

Sulfate reducing bacteria are

common in lack of oxygen; they

produce hydrogen sulfide causingsulfide stress cracking.

In the presence of oxygen, some

bacteria directly oxidize iron to

hydroxide.

Titanics bow exhibitingmicrobial corrosion damage in

the form of rusticles1/23/2013 40WEC


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Corrosion of plastics

This dished end of a

vessel is made of glass

fibre reinforced PVC.

Due to internal stresses

and an aggressive

environment it hassuffered environmental

stress cracking.

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Some examples of corrosion

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Galvanic corrosion

This rainwater guttering is made of aluminium andwould normally resist corrosion well. Someone tied

a copper aerial wire around it, and the localised

bimetallic cell led to a knife-cut effect.1/23/2013 43WEC


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Galvanic corrosion

This polished Aluminium

rim was left over

Christmas with road salt

and mud on the rim.Galvanic corrosion has

started between the

chromium plated brass

spoke nipple and the

aluminium rim.

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Galvanic corrosion

Galvanic corrosion

can be even worse

underneath the

tyre in bicycles.

Here the corrosion

is so advanced it

has penetrated

the rim thickness.

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Erosion corrosion


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Erosion corrosion

Erosion-Corrosion

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I l C i


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Occurs in specific alloysprecipitation of corrosive

specimens along grain boundaries and in particular

environments

e.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr

Solutions: High temp heat treat to redissolve carbidesLower carbon content (in SS) to minimize carbide

formation

Alloy with a material that has stronger carbide

formation (e.g., Ti or Nb)

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Intergranular Corrosionstainless steel

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Influence of corrosion on value

A very slight amount of corrosion may not interfere with the

usefulness of an article, but can affect its commercial value. At

the points where these scissors were held into their plastic case

some surface corrosion has occurred which would mean that

the shop would have to sell them at a reduced price.1/23/2013 49WEC

M hi l i d f


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Motor vehicle corrosion and safety

The safety problems associated with corrosion of motor

vehicles is illustrated by the holes around the filler pipe of this

petrol tank. The danger of petrol leakage is obvious. Mud and

dirt thrown up from the road can retain salt and water for

prolonged periods, forming a corrosive poultice.1/23/2013 50WEC


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House Drain andDrain Cap

METAL: Cast IronENVIRONMENT: Residential basement water exposureFORM OF CORROSION: GeneralMETHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significantcorrosion already!

1 year oldcap

30 year oldcap

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C i t


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Corrosion at sea

Sea water is a highly corrosive electrolyte towards mildsteel. This ship has suffered severe damage in the areas which

are most buffeted by waves, where the protective coating of

paint has been largely removed by mechanical action.1/23/2013 52WEC


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Aluminium alloy chassis member shows very

advanced corrosion due to contact with road salt from

gritting operations or use in coastal / beach regions.

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Corrosion: an extensive expensive


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Corrosion: an extensive, expensivematerials degradation process

Corrosion in a hi h tem erature i eline1/23/2013 54WEC

A t d hi k


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A rusted shipwreck

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Forms of Corrosion


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Forms of Corrosion

Stainless screw v cadmium plated steel washer

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Forms of Corrosion
http://www.corrosion-doctors.org/Aircraft/galvseri-sea.htmhttp://www.corrosion-doctors.org/Aircraft/galvseri-sea.htm


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Forms of Corrosion

Stress Corrosion Cracking (SCC) in a U-bendsample of 316 stainless steel exposed toSupercritical Water Oxidation (SCWO)environment

Exfoliation in aluminum aircraft alloy1/23/2013 57WEC


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Corrosion control / prevention

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Combating Corrosion


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Combating Corrosion

Following are the most important methods

used in industry for protection of corrosion:

Use of high purity metals

Use of alloy additions

Use of special heat treatments

Proper design & fabrication procedure

Cathodic protectionUse of Inhibitors

Surface coatings1/23/2013 59WEC

Proper Design


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Proper Design

Use of dissimilar metals be prevented

In design, avoid presence of cracks

Sharp corners & recesses be avoided

Welded joints be preferred over rivetted joints

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Cathodic Protection


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Cathodic Protection In an electrochemical type of corrosion, a current flows

from anode to cathode. Thus a metal which acts as cathode is protected & the

metal which acts as anode loses some weight & is

corroded

Thus it is obvious that the corrosion may be controlled by

making the whole surface as cathodic with respect of

some other metal, which acts as anode

Two techniques are usedi. Sacrificial anode method

ii. External voltage method

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Cathodic Protection


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Cathodic Protection

A technique to control the metal surface by making that

surface the cathode of an electrochemical cell.

By means of an externally applied electric current, corrosion

is reduced virtually to zero, and metal surface can be

maintained in a corrosive environment without deterioration

for an indefinite time .

It is most commonly used to protect steel, water and fuelpipelines and tanks, ships and offshore oil platforms.

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Sketch of Cathodic Protection


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Sketch of Cathodic Protectionshowing pipe, auxiliary anode and rectifier

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Reactive Coatings by using Inhibitor


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g y g

In re-circulating systems, corrosion inhibitors (organic &

inorganic) are used to control corrosion:

chromates, phosphates (Anodic)Mg & Ca Salts (Cathodic)

long chain organic molecules

These should dissolve in the corroding medium & be able to

form insulating layer

Electrically insulating and / or chemically impermeable

coatings on exposed metal surfaces suppress

electrochemical reaction. Thus corrosion is reduced or

eliminated.

Anodic inhibitors are used in radiators, steam boilers & other

containers1/23/2013 64WEC

Anti-corrosive surface Coatings


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Anti-corrosive surface Coatings

Metal surface can be protected by the provision

of following surface coatings:

i. Metallic Coating

ii. Non-Metallic Coating

Note: Prior to applying coatings, pretreatments

(such as shot/sand blasting, cleaning,

pickling, rinsing, drying etc) of substrate is

essential.

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Metallic Coating


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Metallic Coating Electroplating:- depositing thin layer of metal (Cu, Ni, Ag, Au, Cr,

Cd, W) coating by passing a direct current through an electrolyte

solution containing some salt of the coating metal Dipping:- depositing layer of Zn, Sn, Pb, Al & Cd on metal parts

Galvanizing:- providing layer of zn on steel & iron components,e.g.,nuts, bolts, screws, nails etc used for the protection ofcorrosion

Tinning:-providing thin layer of Sn coating mostly on metal sheets,used to make containers for storing ghee or some chemicals

Spraying:- used for coating of Al, brass, Cu, zn Sn &offers

greater working speed Uniform coating for irregular surfaces

Wire Gun method:- coating metal is melted by C2H2 flame

Powdermethod:metal powder is suckedfrom the chamber & isheated as is passed through the flame of blow pipe

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Metallic Coating


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Metallic Coating Cladding

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Metallic Coatings


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Metallic CoatingsCladding

Coating becomes the integral part of the metal

In this process, sheets of cladding material are strapped to

an ingot of the base material.

After heating to the rolling temp, the straps are removed

and the entire assembly is rolled.

The heat and pressure during rolling weld the two

materials together, e.g.,Alclad or GMCS.

Cementation Sheradizing; layer of Zn

Chromizing; Layer of Cr

Calorizing

:Layer of Al1/23/2013 68WEC

Non Metallic Coating


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Non-Metallic Coating

Paints & Lacquering

Oxide films

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Paints


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Paints

Paints and other organic coatings are primarily used to

improve to appearance of the surfaces and structures.

Its use for corrosion protection only is secondary.

Paints [which are mixture of pigments (metallic oxides

e.g., TiO2, Pb3O4, Fe2O3 or other compounds ZnCrO4,

PbCO3, BaSO4 etc) suspended in organic or aqueous

vehicle] provide a protective film to the metal and is

effective only as long as the film is unbroken

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Anodizing

Aluminum alloys often undergo a surface treatment.

Electrochemical conditions in the bath are carefully adjusted so the uniform

pores several nanometers wide appear in the metals oxide film.

These pores allow the oxide to grow much thicker then passivating.

At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer.

If the coating is scratched, normal passivation processes take over to the

protect the damaged area1/23/2013 71WEC


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Thanks

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Corrosion prevention

Lec 13 Corrosion1

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