What Forms of Corrosion are there?

Mrs B. Goode, Aberdeen

Corrosion can take many different forms. These can be classified as: general corrosion, pitting corrosion; erosion corrosion; galvanic corrosion; selective corrosion; stress corrosion cracking and fatigue corrosion. Apart from general corrosion, all the others are types of localised corrosion.

A good example of general corrosion is rusting of steel. This is often unsightly but generally is a slow process and can be predicted and taken into account during design. Localised corrosion, on the other hand, is usually unpredictable and often occurs in a sudden and sometimes catastrophic manner.

Pitting corrosion can occur with all common construction metals. It is often responsible for pinholing of water pipes made out of copper, mild steel or even stainless steels. There are also many types of pitting corrosion – some can cause pinholing in a matter of months, while other types take many years for this to happen.

Erosion corrosion is caused principally by too high water flow rates resulting in turbulent flow conditions down a pipe. It can also with most materials, although the critical flow rates required vary greatly from metal to metal. For instance, erosion corrosion can occur in copper at flow rates as low as 1m/s but does not occur in stainless steels until a flow rate of perhaps 20m/s.

We touched on galvanic corrosion in another question here. Selective corrosion is a form of galvanic corrosion within an alloy, whereby the more active phase is preferentially dissolved leaving a porous matrix. The most common forms are dezincification of brasses and graphitisation of grey cast irons.  

Stress corrosion cracking and corrosion fatigue require the combined effects of a tensile stress, a specific corrosive species or medium and a sensitive alloy. The stresses can be applied stresses or residual stresses from the manufacturing process. Typical examples of SCC are austenitic stainless steels in chloride-containing waters and alpha-beta brasses in ammonia-containing waters. Corrosion fatigue is similar to SCC, except that the applied stresses are cyclic, which often speeds up the process.

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