lec corrosion 13

7/30/2019 Lec Corrosion 13

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CORROSION OF METALS

Introduction

Definition

Economic impact

Types of Corrosion

Combating corrosion

Anti corrosive coatingsand Paints
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 ofdecay varies depending upon the material and its

environment.

The basic cause of corrosion is the instability of metals intheir refined forms.

The metals tend to revert to their natural states through

the process of corrosion..


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No material is inert!

All metals/alloys interact with the environment.

SO2

Metal runoff/release

Me, 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, engineeringmaterials require

protective coatings,

inhibitors,

alloy additions,

design procedures,

maintenance and refurnishing.

It is good to extend the life of materials, keep them corrosion

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.


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What is Corrosion ?

Corrosion is the gradual physico -

chemical destruction of materials by the

attacking action of 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 environment andthe material.

The term corrosion is sometimes also applied 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 ofindustry

Corrosion causes damage in the billions of

dollars each year. Apporx. 10% world metal

production is completely lost due to

corrosion each year.

According to estimate national annual costs

of corrosion to a country vary from 1 % to 4 %

of GNP.


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Economic Impact

A study showed that for 1998 the totalannual direct cost of corrosion in the US

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.


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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 cost

in 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.


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Economic losses

Indirect 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.


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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 corrosion

Pitting Corrosion

Erosion / Cavitation

corrosion

Fretting corrosion

Crevice Corrosion

Stress Corrosion

Intergranular

Corrosion

Microbial Corrosion


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

Many metals tend tooxidize to some extent at

all temp. When iron

heated in the presence of

oxygen it becomes coated

with a film of black scale

as per following reaction:

2Fe + O2 = 2FeO Iron is oxidized as its

atoms lose electrons.


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Wet Corrosion or Electrochemical Oxidation

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 soon begins to

develop, as :

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


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Contd Wet Corrosion or

Electrochemical Oxidation

The fundamental principle is that atoms

of iron in contact with oxygen and water

oxidize, i.e., they lose electrons and

enter into solution as ferrous ions (Fe++)

which are further oxidized to ferric ions

(Fe+++).


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Ranks the reactivity of metals/alloys in seawater

PlatinumGold

Graphite

Titanium

Silver

316 Stainless Steel

Nickel (passive)

Copper

Nickel (active)

Tin

Lead

316 Stainless SteelIron/Steel

Aluminum Alloys

Cadmium

Zinc

Magnesium

8

Galvanic Series


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Electrode Potential for some metals


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Galvanic Corrosion

Galvanic Corrosion occurs when two

dissimilar metals electrically contacteach other and are immersed in an

electrolyte.

In order for galvanic corrosion to

occur, an electrically conductive path

and an ionically conductive path are

necessary.

This affects a galvanic couple wherethe 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

Where

Corrosion

Occurs!!!!


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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 corrosion

an examples of bad plumbing


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Atmospheric Corrosion

Most common & economically most costly form. The atmospheric corrosion process depends not

only on the moisture content of air but also on the

gaseous impurities, dust particles and othercontaminants which favour condensation of

moisture on the metal surface.

Atmospheric corrosion rates vary markedly all over

the world depending upon the climatic region,

season of the year, time of the day etc.


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Contd Atmospheric Corrosion

Approaching the sea coast, air is laden with increasing

amount ofsea salt.

At industrial areas, appreciable amounts ofSO2which is

converted into H2SO4, and to lesser amounts ofH2S, 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.

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 the

flowing fluid drops below

the vapour pressure of the

liquid.

It is observed on ship

propellers, pump impellors

etc.


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Fretting

corrosion

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 relative movement. Fretting corrosion ruins bearings, destroys dimensions, and reduces

fatigue strength.

This type of corrosion is a mechanical-chemical phenomenon.


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Crevice Corrosion

It is a intense

localized corrosion at

a crevice under a

bolt head or flanged

connections usually

due to concentration

cells.


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Crevice Corrosion

St C i


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Stress Corrosion

Stress corrosion is caused by

simultaneous presence of tensilestress 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 dependson the corrosive medium and on

the structure of base metal.

Intergranular Corrosion


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It 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, nickel some Alalloys

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 causing

sulfide stress cracking.

In the presence of oxygen, some

bacteria directly oxidize iron to

hydroxide.

Titanics bow exhibiting

microbial corrosion damage in

the form of rusticles


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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 has sufferedenvironmental 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.


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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.

Erosion corrosion


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Erosion corrosion

Erosion-Corrosion

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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stainless 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 thepoints 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.


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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.


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House Drain andDrain Cap

METAL: Cast Iron

ENVIRONMENT: Residential basement water exposure

FORM OF CORROSION: General

METHOD TO CONTROL! Surface is painted for protection. Note the 1 year old cap shows significant

corrosion already!

1 year old

cap

30 year oldcap

i


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Corrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel. Thisship 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.


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The current trend for

aluminium vehicles is not

without problems. This

aluminium alloy chassis

member shows very

advanced corrosion due to

contact with road salt

from gritting operations or

use in coastal / beach

regions.

Corrosion: an extensive expensive


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Corrosion: an extensive, expensive

materials degradation process

Corrosion in a hi h tem erature i eline

A t d hi k


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A rusted shipwreck

Forms of Corrosion


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Forms of Corrosion

Stainless screw v cadmium plated steel washer

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 to

Supercritical Water Oxidation (SCWO)

environment

Exfoliation in aluminum aircraft alloy


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Corrosion control / prevention

C b ti C i


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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

Cathodic protectionUse of Inhibitors

Surface coatings


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Proper Design


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Sketch of Cathodic Protection


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Sketch of Cathodic Protection

showing pipe, auxiliary anode and rectifier

i i b i hibi


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Reactive Coatings by using Inhibitor

If the environment is controlled in re-circulating

systems, corrosion inhibitors (chromates, phosphates,

long change organic molecules) are added to it.

These form an electrically insulating and / or chemically

impermeable coating on exposed metal surfaces, to

suppress electrochemical reaction. Thus corrosion is

reduced or eliminated.

Inhibitors are added to the antifreeze mixtures used in

automobile radiators.

Anti corrosive surface Coatings


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Anti-corrosive surface Coatings

Metal surface can be protected by the provision of

following surface coatings:

Paints,

Oxide films and

Metallic coatings

Note: Prior to applying coatings, pretreatments(such as shot/sand blasting, cleaning, pickling,

rinsing, drying etc) of substrate is essential.

Paints


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Paints

Paints and other organic coatings are primarily used

to improve to appearance of the surfaces andstructures. Its use for corrosion protection only is

secondary.

Paints [which are mixture of pigments (metallicoxides e.g., TiO2, Pb3O4, Fe2O3 or other compounds

ZnCrO4, PbCO3, BaSO4 etc) suspended in organic or

aqueous vehicle] provide a protective film to themetal 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 theend 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 area

Metallic Coatings


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Metallic Coatings

Metallic coating may be obtained by a variety of

methods such as:Diffusion (Carburizing, Cyaniding,

Nitriding, Chromizing)

Hot dipping (mainly coating of Zn, Sn, Pb, Al &Cd on steel)

Galvanizing / Znic coating; used for the

protection of nuts, bolts,

screws, nails etc.

Electroplating

Metallic Coatings


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Metallic Coatings

Cladding 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.


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Thanks


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Corrosion prevention

lec corrosion 13

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