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Corrosion in Petrochemical and Refinery Industries

Mon, 27 June, 2022

Corrosion is defined as the degradation of a metal due to chemical or electrochemical reactions with its environment.

Common types of corrosion

1) General Corrosion

2) Intergranular corrosion

3) Galvanic corrosion

4) Pitting Corrosion

5) Crevice Corrosion

6) Stress Corrosion Cracking

1) General Corrosion

General corrosion attack proceeds uniformly over the entire surface and there is a uniform loss of wall thickness (mm/year). This is common in steel in water or seawater or stainless steel in acidic or strong alkaline solutions. A corrosion allowance (mm) may be specified in the design in order to take this into account.

2) Intergranular Corrosion

Intergranular corrosion attack is localised at and adjacent to the grain boundaries and occurs in stainless steel due to chromium (Cr) carbide precipitation (sensitisation) and for welded stabilised grades (eg 321, 347) it can take the form of knife line corrosion in the HAZ. Resistance to intergranular corrosion in stainless steel is improved by lowering the carbon content, controlling the welding procedure, or adding titanium (Ti) or niobium (Nb) to stabilise the stainless steel.

3) Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are placed in contact and their different redox potentials cause corrosion. Practical implications of the galvanic series are that the more anodic metal will be the metal that corrodes. The farther apart the two metals, the faster will be the corrosion rate. In order to protect against galvanic corrosion, you need to electrically insulate dissimilar metals from each other and/or eliminate the electrolyte (eg water).

4) Pitting Corrosion

Pitting corrosion is a highly destructive non-uniform attack due to the localised breakdown of passivity. The pit becomes the anode whilst the surface becomes the cathode; this is the effect of differential aeration which means that oxygen levels become reduced in the pit compared to the oxygen levels outside of the pit. Pitting corrosion results in holes in the metal. In stainless steel, it occurs most commonly in chloride-containing environments or oxidising salts. Resistance against this type of corrosion is improved by increasing Cr and Mo content; N has also a favourable influence.

5) Crevice Corrosion

Crevice corrosion attack occurs in narrow crevices filled with liquid and where the oxygen level is very low (differential aeration) eg gasket surfaces or under bolt/rivet heads, and intrusions at weld toes. Under-deposit corrosion is a form of crevice corrosion and is where corrosion occurs under non-metallic deposits or coatings on the metal surface. Materials resistant to pitting corrosion are also resistant to crevice corrosion.

6) Stress corrosion cracking

Stress corrosion cracking or SCC occurs when metal is subjected to tensile stress and exposed to a corrosive environment. Material can remain unattacked generally while fine branched cracks progress through it (cracking can be either intergranular or transgranular). In austenitic stainless steels, SCC occurs in chloride-containing solutions. The risk of SCC increases with increasing salt concentration, tensile stress and service temperature (it is seldom found below about 60°C). In carbon steel SCC occurs in sour (H2S containing) environments at low temperatures.

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