Corrosion and Resistance
The corrosion resistance of stainless steel arises from a chromium-rich oxide film that forms naturally on the surface of the steel. Although extremely thin, this protective layer is strongly adherent and chemically stable - i.e. ‘passive’.
Although extremely thin, this protective layer is strongly adherent and chemically stable, i.e. ‘passive’, providing the environment remains non-contaminated. If this film is damaged it will normally self repair in a non-contaminated, oxygen-rich, dry environment under conditions that provide sufficient oxygen to the surface.
Stainless steels cannot be considered corrosion proof as the passive state can be broken down under certain conditions.
Surface contamination with iron particles, grease and dirt occurs during fabrication processes such as welding, cutting, grinding as well as during general handling. These contaminants prevent the formation of the passive layer and initiate a galvanic reaction resulting in either pitting & crevice corrosion or tea-staining.
Various types of corrosion exists:
- Galvanic reactions
Occurs when 2 different metals are in contact, with an electrolyte (e.g salt water) providing a micro-electrical current - Pitting corrsosion
Usually caused by the chloride iron (as present in e.g. salt water) resulting in small holes - Abrasive erosion
This is a mechanical process; e.g abrasive liquids through pipes - Micro-biological
The formation of a bacterial film has an effect comparable with crevice and pitting corrosion. - Stress corrosion/cracking
Can occur with a combination of tensile stress, high temperatures and the presence of e.g chloride ions. - Inter-granular
Occurs when, after heating to at least 450°Celcius, Carbon molecules bind with Chrome which preferentially attacks the grain boundaries - Crevice corrosion
Occurs under screws, washers, marine growth etc where stagnant solutions and the lack of oxygen induce corrosion
ElectroRefinement™ (electropolishing) is the treatment of choice in the combat against corrosion.
- Electro-Shine
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