TYPES OF CORROSION

& OTHERS DEFINITION

Md.Habibur Rahman Chowdhury

Applied Chemistry & Chemical Engineering.

Bangabondhu Sheikh Mujibur Rahman Science & Technology University

Gopalgonj-8100

TYEPS OF CORROSION:

There are basically eight types of corrosion

I. Uniform corrosion

II. Pitting corrosion

III. Transgranular and Intergranular (Intercrystalline ) corrosion

IV. Exfoliation corrosion

V. Stress corrosion

VI. Crevice corrosion

VII. Galvanic corrosion

VIII. Erosion

Uniform corrosion:

Uniform corrosion develops as pits of very small diameter, in the order of a micrometer,

and results in a uniform and continuous decrease in thickness over the entire surface area of

the metal. The rate of uniform corrosion can be determined by measuring the mass loss, or

the quantity of released hydrogen.

Uniform corrosion includes the commonly rusting of iron and other metal.The most

common form of corrosion and the greater proportion metal deterioration in terms of metal is

converted to corrosion products.

Pitting corrosion:

Pitting corrosion localized form of corrosion is characterized by the formation of irregularly

shaped cavities on the surface of the metal. Their diameter and depth depend on several

parameters related to the metal, the medium and service conditions. The rate of pitting

corrosion can be determined neither by determining the mass loss nor by measuring released

hydrogen.

Q: What is shallow?

Pitting corrosion is a localized type of attack, with the rate of corrosion is greater at

some areas than at others.If understandable attack is confined to a relatively small ,fixed areas

of metal ,acting as anode.If the area of attack is relatively larger and not so deep,the pits are

called shallow.

Q: What is pitting factor?

The ratio of deepest metal penetration to average metal penetration as determined by

the weight loss of the sample is called pitting factor. Depth of pitting is sometimes expressed

by the pitting factor.

Q: What is Crevice corrosion?

Crevice corrosion is a localized corrosion in vacation. overlapping zones for riveting,

bolting or welding, zones under joints and under various deposits. These zones also called

crevice and it is very tiny .This type of corrosion is called Crevice corrosion and this type of

corrosion is also known as deposit attack.

Q: What is the difference between crevice and pitting corrosion ?

The difference between crevice and pitting corrosion is given below :

Crevice corrosion Pitting corrosion 1.Geometric configuration determine the site of attack in crevice corrosion.

Crystal defects of various structural heterogeneity determine the site of attack in crevice corrosion.

2.Crevice corrosion can take place in most corrosive electrolytes.

Pitting corrosion can take place only if certain anions are present in solution.

3.All the systems which show pitting attack are particularly capable to crevice corrosion.

But the reverse is not always correct.

4.Crevice corrosion is less dangerous. Pitting corrosion is more dangerous than crevice corrosion.

Q: Why pitting corrosion is more dangerous than the general corrosion?

Geometric configuration determine the site of attack in crevice corrosion. Crystal defects of

various structural heterogeneity determine the site of attack in pitting corrosion. Crevice

corrosion can take place in most corrosive electrolytes. Pitting corrosion can take place only if

certain anions are present in solution. All the systems which show pitting attack are

particularly capable to crevice corrosion. But the reverse is not always correct. Crevice

corrosion is less dangerous. Pitting corrosion is more dangerous than crevice corrosion. For

above reasons Pitting corrosion is more dangerous than general corrosion.

Q: What is trans granular corrosion?

Transgranular corrosion is the corrosion within the metal, at the level of the grain. It

spreads in all directions, corrosion affects all the metallurgical constituents, there is no

selective corrosion. This is called transgranular or transcrystalline corrosion.

Q: What is Galvanic corrosion ?

When two dissimilar metals are in direct contact in a conducting liquid, experience

shows that one of the two may corrode. This is called galvanic corrosion. The other metal will

not corrode. It may be protected by this way.

Galvanic corrosion does not depend on the metal’s texture, temper, etc. Galvanic

corrosion may occur with any metal, as soon as two are in contact in a conductive liquid. It

works like a battery. The appearance of galvanic corrosion is very characteristic. It is not

dispersed like pitting corrosion, but highly localized in contact zone with the other metal.

Q: What is the pre requisits of Galvanic corrosion ?

All the three of the following conditions must be met for galvanic corrosion. If can eliminate

one pre requisite condition,galvanic corrosion can not occur.

1) Dissimilar metals

2) Metal to metal contact

3) Exposure to an electrolytes

Dissimilar metals : In generally,every metal has corrosion potential,referred to as its anodic

index .The farther apart to metals are in this rating,the stronger the rate of galvanic corrosion.

Metal to metal contact: The two different metals must be in direct contact with each other for

corrosion to transfer from cathode to anode.In the transportation industry,different metals

are adjoined on hinges steps,doorframes,mounting brackets.

Exposure to an electrolytes: An electrolytes is an electrically conductive substance. It

transfers corrosion from the more resistant metal to the weaker metal .Rain and road splash

are naturally occuring electrolytes but the most destructive electrolytes facing the industry are

road de-icers.Road salt accelerate corrosion because they are excellent conductors.

Sodium chloride :Sodium chloride spread on ice and snow.It is used as a solid,but dissolved to

a liquid in snow and ice.

Calcium chloride:Calcium chloride salts is also solid at room temperature, but it works more

effectively in lower temperatures than sodium chloride.

Q:what is the effect of distance and area of galvanic corrosion?

Galvanic corrosion occurs in the atmosphere. The severity depends on the type and amount of

moisture.

Distance effect:Accelerated corrosion due to galvanic effects is the greatest near the

junction,while attack decreasing with increasing distance from that point.The distance

affected depends on the conductivity.

Area effect:Area effect or the ratio of cathodic to anodic area is the important factor of

galvanic corrosion.An unfavourable area ratio cnsists of a large cathode and a small anode.In

the current flow cell, the current density is greater for a small electrode and greater the

corrosion rate than for a larger. Corrosion of the anodic area may be 100 or 1000 times

greatest than if the anodi and cathodic areas were equal size.