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Can stainless steel rust ? |
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The corrosion resistance of stainless steel is based on the formation of a very thin passive layer on its surface. Corrosion occurs when this passive layer is destroyed.
The permanence of stainless steel depends on the right choice of steel, the correct processing as well as on the appropiate chemical post-treatment through pickling and surface passivation.
Furthermore, the corrosion resistance depends on the surface, i.e. the smoother and more homogeneous the surface is the higher the corrosion resistance will be. Caused especially by inclusions or depositions, e.g. processing-related impressed rust or dust particles, fast-spreading local corrosion can occur.
The following kinds of corrosion can occur with stainless steel:
With this type of corrosion the surface is attacked evenly; the corrosion is measured as decrease in thickness per year.The corrosion related decrease is determined in laboratory tests for various media and various materials. The results are published in so-called resistance tables. This kind of corrosion can be entirely ruled out through proper choice of material and can be precluded with austenitic materials that are used in the areas water-waste water.
Contact corrosion is a very common form of corrosion that develops when metallic materials with different electrochemical potentials physically touch each other in the presence of an electrolyte. In this situation, the lower-grade metall will be attacked by the electrolyte and will go into solution. The extend of the corrosion depends on the strength of the current in this galvanic cell.
We encounter contact corrosion very often. A commonly known example is the connection between steel and stainless steel flanges.
Contact corrosion is also common when cast flanges are attached with stainless steel screws.
Contact corrosion can be prevented or reduced by insulating the metals at the points of contact, by keeping electrolytes away from the metals (move contact areas into dry environments), through constructive measures that avoid contact between cathode areas and very large anode areas.
Crevice corrosion occurs when the stainless steelīs passive layer is destroyed, for example through aggressive media and a concurrent lack of oxygen. For this reason, crevice corrosion can often be found in narrow gaps and in small cavities, e.g. underneath seals, underneath the heads of screws.
An example in case is the gas hood inside digesters. In the interior of the gas hood the aggressive medium methane gas is present while oxygen is lacking completely in this environment (see picture 2). In a situation as this, crevice corrosion has to be particularly considered.
Crevice corrosion can be prevented by choosing the appropriate material (e.g. high chromium and molybdenum content) and by appropriate constructive measures that avoid gaps and crevisses (see picture 1).
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Picture 1: Gas hood made of stainless steel
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Picture 2: rusted gas hood
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HUBER - WHERE QUALITY AND SERVICE SPRING TO MIND
Huber Technology UK, Units C and D Brunel Park
Bumpers Farm, Chippenham Wiltshire SN14 6NQ
Phone +44 1249 765000, Fax +44 1249 449076, E-mail: rotamat@huber.co.uk |
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