How Does Corrosion Happen And What Are Its Different Types?

Corrosion has always been a fierce nemesis for many engineering industrial sites. It’s most famous form of course is iron rust, but it can be a major problem for metal equipment, structures, and pipelines. It’s a problem that cannot be neglected, and special measures must always be taken to prevent it from happening.

What is corrosion?

Corrosion is a natural process. It is a result of an electrochemical reaction between a metal and its surroundings which causes the metal to breakdown.

Rust for example, occurs when iron combines with the air and water from its surrounding environment to form iron oxide. This iron oxide (although a solid) is more porous, weaker, and is much more brittle compared to the original metal.

Corrosion in a pipeline can occur either on the outer shell of the pipe, due to its contact with the surrounding environment, or internally; due to the contact of the pipe’s inner metal material with the fluids passing through it. Internal corrosion can take place due to sludge, build up on the pipeline wall, or the nature of the fluid itself (some fluids are highly corrosive by nature). Internal corrosion can usually be prevented by including corrosion inhibitors in the fluids that are being transported in the pipeline.

Types of Corrosion

1) Uniform or General Corrosion

Uniform or General Corrosion

Uniform or General Corrosion (credits: corrosion college)

The metal loss is uniform from the surface.

Often combined with high-velocity fluid erosion, with or without abrasives.

2) Pitting Corrosion

Pitting Corrosion

Pitting Corrosion (credits: EMEC)

The metal loss is randomly located on the metal surface.

Often combined with stagnant fluid or in areas with low fluid velocity.

3) Galvanic Corrosion

Galvanic Corrosion

Galvanic Corrosion (commons)

Occurs when two metals with different electrode potential is connected in a corrosive electrolytic environment.

How Galvanic Corrosion Takes Place

How Galvanic Corrosion Takes Place

The anodic metal develops deep pits and groves in the surface.

4) Crevice Corrosion

Occurs at places with gaskets, bolts and lap joints where crevice exists.

Crevice corrosion creates pits similar to pitting corrosion.

5) Concentration Cell Corrosion

Occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies.

Often combined with stagnant fluid or in areas with low fluid velocity.

6)Graphitic Corrosion

Cast iron loosing iron in salt water or acids.

Leaves the graphite in place, resulting in a soft weak metal.

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