Corrosion

The process of deterioration of metals and their properties, following a reaction with surrounding environment.

Corrosion usually occurs first in a tank around its corners, welds and edges of the structure.

- Active corrosion – Active corrosion means gradual chemical or electrochemical attack on a metal producing loose scale by atmosphere, moisture or other agents.

Anaerobic corrosion – Corrosion occurring in the absence of oxygen.

Bacterial corrosion – Corrosion caused by certain bacteria, particularly sulphate reducing anaerobic bacteria. Such corrosion develops under corrosion deposit product, or in deaerated seawater circuits.

Crevice corrosion – An intense localised form of corrosion, usually associated with small volumes of stagnant solution resulting from holes, masked surfaces or crevices.

Edge corrosion – Local corrosion at the free edges of stiffeners, brackets, flanges, manholes etc.

Electrochemical corrosion – Corrosion due to the passage of an electric current. If the current is produced by the system itself the corrosion is called galvanic and if it results from an impressed current it is called electrolytic corrosion.

Erosion corrosion – A combined action involving corrosion and erosion in the presence of a moving corrosive fluid, leading to the accelerated loss of material. Erosion corrosion is characterized by grooves, gullies, waves, valleys etc., usually with directional pattern and with bright surfaces free from corrosion products.

Galvanic corrosion – This type of corrosion occurs when two metals or alloys are electrically in contact in a corrosive environment (electrolyte). The less noble metal is attacked; it is called anode. The most noble metal is protected; it is called cathode.

Fretting corrosion – Surface damage, usually in an air environment, between two surfaces, one or both of which are metals, in close contact under pressure and subject to a slight relative motion.

Friction corrosion – Corrosion of metal surfaces that is caused by frictional forces.

 - General corrosion, overall corrosion – Corrosion evenly distributed on the surface.

Grooving corrosion – Local corrosion normally close to welding joints along abutting stiffeners and at  stiffener or plate butts or seams.

Localised corrosion – More or less localised corrosion attacks such as pitting corrosion, crevice corrosion, corrosion on welds and on edges.

Pitting corrosion – Local, randomly scattered corrosion mainly on horizontal surfaces and at structural details where water is trapped, particularly at bottom of tanks. Onr coated areas the attack produces deep and small-diameter pits, which may lead to perforation. Pitting of uncoated areas in tanks, as it progresses, forms shallow but very wide scabby patches (e.g. 300 mm in diameter) and the appearance resembles general corrosion.

Stress corrosion – In this type of corrosion, fractures occur in the material exposed to a corrosive environment and subject to stresses. For example these stresses can be residual stresses of welding or thermal stresses.

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