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March 2013
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Pitting is a particularly dangerous form of corrosion as it can go undetected, due to the localised nature of the damage. –
Pitting is often the result of specific circumstances or environments, including: • chlorides on stainless steels • galvanic corrosion – dissimilar metals • bacterial corrosion. Stress corrosion cracking This is different to most types of corrosion as the SCC process results in cracking throughout the structure of the material, typically resulting in an unexpected brittle fracture. SCC occurs in materials due to the combination of an applied tensile stress and a material specific environment. Examples of alloys and the environments that result in SCC are: • Stainless steel in the presence of chlorides (salts). • Carbon steel in the presence of nitrates (dyes). • Brass in the presence of ammonia (fertiliser). Both stress and the specific environment are necessary for SCC. Some materials become susceptible to SCC when a stress is applied in what would otherwise be an inert environment. The result of SCC is that small cracks form in the material, which then propagates until failure occurs. The cracking is typically fine in nature and does not cause gaping cracks in the surface. As such SCC can often go unnoticed until a premature, unexpected failure occurs.
Other types of corrosion There are many different types of corrosion. Below is an overview of the main types of corrosion not already described that occur in metals: • Crevice Corrosion – This is where corrosion occurs within a localised, restricted environment when metal is exposed to an electrolyte. After oxygen is depleted within the crevice corrosion occurs in the crevice site, this can often result in pitting corrosion. • Intergranular Corrosion – This occurs along grain boundaries (within the microstructure of the metal) in certain alloys in specific environments. The grain boundary is preferentially attacked as they represent weaknesses in the structure of the metal. A common example of this is sensitisation of stainless steels which results in the precipitation of chromium to grain boundaries in the form of carbides, leaving the chromium-depleted regions susceptible to oxidation and corrosion. This can give problems in chemical tankers and chemical processing plant. • Selective Leaching – This is where one element within an alloy is preferentially removed through corrosion. An example of this is dezincification of brass, where zinc is selectively leached from the brass alloy, leaving a porous structure that is very weak. • Erosion-Corrosion – This is where combined chemical attack and wear through fluid flow leads to an enhanced rate of corrosion. It typically occurs when surface oxides (rust) is eroded from a metal surface leaving the bare metal exposed, this bare metal then oxidises again. The continued oxidation causes an enhanced rate of corrosion. • Microbiologically Induced Corrosion – This is where a micro-organism produces compounds which are corrosive to an underlying material. A common example of this is Sulphur Reducing Bacteria which can produce corrosive sulphides which cause destructive localised corrosion in low oxygen conditions.
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