unit 3 Protective measures against corrosion.ppt

7/27/2019 unit 3 Protective measures against corrosion.ppt

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Protective measures against

corrosion


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Modification of the environment

Modification of the properties of the metal

Prevention of corrosion by materialselection & design

Cathode protection

Other methods


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Modification of the environment


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De aeration

De activation

Dehumidification

Alkaline neutralization


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De aeration

consider oxygen concentration type of

corrosion

Here elimination of oxygen from aqueous

solution reduces metal corrosion


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removal of dissolved oxygen is done by

adjustment of temperature & by mechanical

aeration

This method also reduces the CO2


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Deactivation

This involves the addition of chemicals , which

can combine with the oxygen in aqueous

solution.

Sodium sulphite (Na2SO3)


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2 Na2SO3+O2 2Na2SO4

Hydrazine is good over sodium sulphiteN2H4+O2 N2 +2H2O


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Dehumidification

This process reduces the moisture content

of air to such a level

that condensation of water on surface of

the metal should be so small.


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Alumina & silica gels , preferably adsorb

the moisture from their surfaces


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Alkaline neutralization

This is meant to control acidic condition to

avoid corrosion

Acidity includes the presence of H2S, HCl,

CO2, SO2etc


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Alkaline neutralizers are used to control

corrosion

They are ammonia , sodium hydroxide etc


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Inhibitors

Anodic inhibitors

Cathodic inhibitors


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Anodic inhibitors

Alkalies ,molybdates ,phosphates , chromates

are used as inhibitors

The above chemicals react with the ions of the

anode &form insoluble precipitates


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These precipitates form as a protective

layer on metal surface

This layer acts as barrier


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Cathodic inhibitors

In acidic solutions the reaction is

2H+

+2e-

H2

Corrosion can be reduced by slowing down

the diffusion rate of hydrogen ion


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In natural solutions ,the cathodic reaction

is of absorption of oxygen type

1/2O2

+ H2

O+2e- 2OH-


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Modification of the properties ofthe metal


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1.Alloying :

The corrosion resistance property will beincreased by alloying suitable element

Chromium is the best metal for alloying steelor iron

Because the film made is self healing


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2.Refining :

Asstainless possess P,S & C

There is a chance of corrosion

Refining is a process to lower quantities of

above elements


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3. Annealing :this is a process to relieve the metal from

stresses by heating


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3. Use of protective coatings :

Anodic coatings

Cathodic coatings


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ANODIC COATINGS

In this ,the metal used is for coating is

more acidic

Anodic coatings protect the underlying

base metal

For ex: coating of Al, Cd & Zn on steel

surface


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zinc is being more anodic will get attacked

It protects iron /steel under it

Here steel works as cathode


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even if pores or breakages are seen ,still

steel will not get corroded

Because of coating of zinc


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Cathodic coatings

These coatings protect the underlying

base metal by their

Noble character &

higher corrosion resistant character


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Here the good protection is possible only

with plane surface

If pores are seen ,the corrosion will be

speeded up


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Because the coated metal behaves ascathode

Exposed metal acts as anode


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Galvanic cell will set up and localized

corrosion is seen

This results in severe pits


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Prevention of corrosion bymaterial selection & design


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Design & material selection

principles

Avoid L, T, & U shaped profiles in

constructionBecause this may cause corrosion


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weldingwelding


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welding


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Water stands here

Water slips from here


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General facts about designing

Avoid the contact of dissimilar metals in

the presence of corroding solution.

If at all 2 dissimilar metals are in contact,

they should be as close as possible in

electrochemical series.


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When ever direct contact of 2 dissimilar

metals is must

An insulating fitting may be made to avoiddirect contact


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Others

Metal cladding

Electroplating

Painting

Plastic coating

unit 3 Protective measures against corrosion.ppt

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