![]() The plating on the fastener is rapidly consumed, leaving the bare fastener metal.Į. The corrosion of the base metal may be markedly increased by the fastener material.ĭ. The corrosion of the base metal is marginally increased by the fastener.Ĭ. The corrosion of the base metal is not increased by the fastener.ī. Guidelines for selection of fasteners based on Galvanic Corrosion Base MetalĪ. The phenomenon is also area related so careful choice of metals can minimise the effect – if the more noble metal has a relatively large area, the less noble will corrode more quickly. The greater the potential difference between the two metals, the faster the corrosion. When the zinc plating is scratched or removed over a discrete area, the zinc, which is less noble than steel, corrodes faster while corrosion of the steel is slowed and thus it is protected (when plating is removed over a large area normal atmospheric corrosion takes place). This effect is one reason why zinc plating is used to protect steel. The metal which is less noble on the galvanic series will corrode faster than it otherwise would have done, while the other is protected. If the metals are dry though, galvanic (bi-metallic) corrosion cannot occur.Įffectively a small cell is set up rather like a very inefficient battery. Often also referred to as “bi-metallic corrosion”, this occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte e.g. The corrosion of zinc, when exposed to the atmosphere, results in zinc carbonate (white rust) which develops at a rate of about one tenth that of red rust.ĭepending on the durability required, and the degree of pollution, for most atmospheric exposure situations stainless steel will be the answer in grade A2 (304) for long term rural and urban exposure with low chloride concentrations or Grade A4 (316) for urban locations with higher chloride concentrations and industrial or coastal exposure. Stainless steel benefits from a similar protection mechanism in the development of a chromium based passive protection layer. The oxidation of aluminium occurs immediately it is exposed to the atmosphere producing a protective layer hence the dull patina and apparent corrosion resistance of this material. In iron and steel “Rusting” is an aggressive phenomenon producing prodigious growth which, in the case of unprotected components contained within a structure, can exert forces sufficient to crack certain building materials. Most metals occur naturally as oxides and oxidation (which occurs in the presence of oxygen and water) and atmospheric corrosion is just the natural tendency to revert to that condition. Rapierstar 400 series Stainless Steel fasteners benefit from a Magni coating that provides an additional level of corrosion protection inherent in Martensitic Stainless Steel, however, Rapierstar would not recommend the use of Martensitic Stainless Steel fasteners into aluminium. They are classed as a "hard" ferro-magnetic group and are considered to have a fair relative resistance to corrosion among the common stainless steel fastener materials. Due to the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels and are capable of drilling and self-tapping into steel reinforcement. Martensitic Stainless Steels (410 grade) are similar to low alloy or carbon steels. It has good resistance to pitting and crevice corrosion at normal temperatures. ![]() industrial coastal and marine environments including total immersion in seawater. This grade of stainless steel is suitable for long term use in the most aggressive conditions normally encountered i.e. The molybdenum improves the resistance to pitting corrosion. The passive layer resists normal atmospheric corrosion in unpolluted rural areas but is susceptible to pitting and crevice corrosion in aggressive environments, such as industrial and coastal locations and may stain in polluted urban atmospheres. ![]() Austenitic stainless steel derives its corrosion resistance from Chromium, Nickel and in some grades, Molybdenum. ![]()
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