Corrosion Control 28.2.13

8/10/2019 Corrosion Control 28.2.13

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CORROSION CONTROL

It is impractical to eliminate corrosion but corrosion can be controlled and minimized by

several methods. The rate of corrosion can be controlled by either modifying the base

metal/alloy or the environment.

I. Control of corrosion by modifying metal:

a) Modifying the composition of metal/alloy

b) Proper designing.

c) Modifying the surface.

II. Control of corrosion by modifying the environment

a) Removal of harmful constituents

b) Use of inhibitors

CONTROL OF CORROSION BY MODIFYING THE BASE METAL

a) Modifying the composition of the metal:

1. Selecting pure metal to avoid impurities, which cause heterogeneity and

leads to electrochemical corrosion.

Impurities cause heterogeneity and leads to electrochemical

corrosion.

Corrosion resistance of pure metal depends on the nature ofenvironment.

Al in the pure form gives coherent, impervious, protective film. But

in alkaline medium, the oxide film is destroyed.

In many cases it is not practical to produce pure metal because of

high cost and inadequate mechanical properties like softness and

low strength.

2. By alloying with passive metals.

Alloy should be completely homogeneous.

Ex.: Stainless steel:steelalloywith a minimum of 10.5or 11% chromiumcontent by mass.

Cr enhances the corrosive resistance of stainless steel by forming stable CrO3 layer.

Because of the self healing effect of Cr, it enhances the corrosive resistance of

stainless steel.


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3. keeping the metal surface clean with out dust deposits


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b) Proper Designing:

1. Avoiding the contact of dissimilar metals.

Avoid the contact of dissimilar metals.

Choose high anode area than cathodic area. When the ratio of anodic to

cathodic area increases, the rate of corrosion decreases.

Choose the metals which are close in the emf series.

Insulate the fitting of different metals.

2. By providing proper design in fabrication of metallic tools:

3. Avoiding sharp corners, stressed parts and crevices to avoid stress corrosion.


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4. Anodic metal should not be painted.

5. Storage tanks should be supported at the bottoms.


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6) Cathodic Protection:The metallic structure can be protected by forcing it to behave

as cathode thereby corrosion does not occur.

a. Sacrificial anodic protection method: The metallic structure to be protected is

connected by a wire to a more anodic metal like Zn, so that all corrosion is concentrated

at this more active metal.

The more active metal itself corroded slowly while the metallic structure

is protected.

The more active metal is called sacrificial anode or galvanic anodes

and it is replaced by fresh one, when consumed completely.

Galvanic or sacrificial anodes are made in various shapes and sizes using

alloys of zinc, magnesium and aluminium. ASTM International

publishes standards on the composition and manufacturing of galvanic

anodes

Buried pipelines, underground cables, marine structures, ship-hulls,

water-tanks, piers, etc.


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b. Impressed current cathodic protection:

An impressed current is applied to in opposite direction to nullify the

corrosion current and convert the corroding metal from anode to

cathode.

Impressed current is derived from DC source (battery) with an

insoluble anode (graphite).

A sufficient d.c current is applied to insoluble anode, buried in the soil

or immersed in the corroding medium and connected to the metallic

structures to be protected.

Anodes for ICCP systems are available in a variety of shapes and

sizes. Common anodes are tubular and solid rod shapes or continuous

ribbons of various materials. These include high silicon cast iron,

graphite, mixed metal oxide, platinum and niobium coated wire and

others.

The positive DC output terminal would be connected via cablesto the

anode array, while another cable would connect the negative terminal

of the rectifier to the pipeline, preferably through junction boxes to

allow measurements to be taken.

It is useful for large structures for long-term operations. eg. Buried

water or oil pipes, water tanks.


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CONTROL OF CORROSION BY MODIFYING THE SURFACE

Modifying the surface by application of protective coatings.

1. The coated surface isolates the underlying metal from the corroding

environment.

2. The coating applied must be chemically inert to the environment under

particular condition of temp. and press.

Protective coatings are

Metallic coatings, Non- metallic coatings and Organic coatings

A. By metal plating (Metallic Coatings):

a. Anodic metal Coatings:

i. Produced from coating-metal which are anodic to the base metal.

ii. If any break or discontinuities occur in such anodic coatings, a

galvanic cell is formed the coating-metal and the exposed part of

the metal.

iii. The coating metal (being anodic) is attacked leaving behind the

underlying base metal unattacked.

Eg. Galvanization (coating of Zn on iron or steel).

Galvanized steel.


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b. Cathodic metal Coatings:

Obtained by coating of noble metal on the base metal.

They protect the base metal because they have higher resistance

than the base metal.

They provide effective protection t o the base metal only when

they are completely continuous and free from pores, breaks and

discontinuities.

If such coatings are punctured, much more corrosion damages can

be done to the base metal.

The exposed base metal acts as anode and intense localized attack

occurs at the small exposure.

Eg. Tin-coating on steel.

C. Electroplating D. Electroless plating (without

electricity)

Electroless plating will give more smooth coatings than electroplating.


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2. Non-metallic coatings:

Inorganic surface barriers produced by chemical or

electrochemical reactions, brought at the surface of the metal.

They are particularly used as an excellent base for organic

coatings.

3. Organic coatings: By painting:

Inert organic-barriers (like paints, varnishes, lacquers and enamels)

applied on metallic surfaces for both corrosion protection and

decoration.


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II.CORROSION CONTROL BY MODIFYING THE ENVIRONMENT

a) By Removal of harmful constituents:

i) Deaeration:

In oxygen concentration corrosion, exclusion of oxygen from the aqueous

environment reduces metal corrosion.

Expulsion of dissolved oxygen is done by the adjustment of temperature,

together with mechanical agitation.

The method also reduces CO2-content of water, thereby decreasing the

corrosion rate of steel pipelines carrying steam condensates from the boiler.

Involves addition of chemicals, capable of combining rapidly with the oxygen

in aqueous solution.

Eg. Sodium sulphite ( Na2SO3)

2 Na2SO3+ O2------------- 2 Na2SO4

Hydrazine hydrate

N2H4+ O2--------------- N2+ 2H2O

Hydrazine hydrate is advantageous over the sodium sulphite because the reaction

products are N2gas and water.

iii) Dehumidification;

Reduces the moisture content of air to such an extent that the amount of water

condensed on the metal is too small to cause corrosion.

Alumina gel or silica gel used in closed area like air-conditioning shop.

iv) Alkaline Neutralisation:

Prevention of corrosion by neutralizing the acidity of corrosive environment

(H2S, HCl, CO2, SO2, etc).

Alkaline neutralizers (NH3, NaOH etc) are injected either in vapour or liquid

form.

Used in controlling the corrosion of refinery equipments.


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b) Use of Inhibitors:

A substance which when added in small quantities to the aqueous environment

effectively decreases the corrosion of metal.

i) Anodic Inhibitors:

They control the corrosion reaction occurring at the anodeby forming a sparingly

soluble compound with the newly produced metal ion.

The compounds are adsorbed on the metal surface forming protective film or

barrier thereby reducing the corrosion rate.

Although this type of control is effective, it may be dangerous, since severe local

attack can occur, if certain areas are left unprotected by the depletion of the

inhibitor.

Eg. Chromates, phosphates, tungstates or other ions of transition elements with

high oxygen content.]

ii) Cathodic Inhibitors:

In acidic solution: Corrosion may be reduced either by slowing down the

diffusion of hydrated H+ ions to the cathode, and/or by increasing the

overvoltage of hydrogen evolution.

The diffusion of H+ ions is considerably decreased by organic inhibitors

like amines, hetrocyclic compounds and urea which are capable of being

adsorbed at the metal surface.

Antimony and arsenic oxides deposit adherent film of metallic arsenic or

antimony at the cathodic areas, (so the hydrogen evolution hindered)

thereby increasing considerably the hydrogen overvoltage.

In neutral solutions: Corrosion can be controlled by eliminating oxygen

from the corroding medium or by retarding its diffusion to the cathodic

areas.

Adding deaerating agent the oxygen can be eliminated.

The inhibitors like Zn, Ni salts react with hydroxyl ions at the cathode

forming corresponding insoluble hydroxide which act as impermeable

self-barriers.

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Corrosion Control 28.2.13