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Stainless steels can be susceptible to certain localised corrosion mechanisms, namely crevice corrosion, pitting, intercrystalline corrosion, stress corrosion cracking and bimetallic (galvanic) corrosion. Localised corrosion is often associated wuth chloride ions in aqueous environments. Corrosion resistance relies on a good supply of oxygen. Higher levels of chromium, nickel, molybdenum and nitrogen increase resistance to localised corrosion.
Bimetallic corrosion can only occur when two dissimilar metals are in ‘electrical’ contact and are bridged by an electrically conductive liquid. The risk of corrosion and staining when stainless steels are in contact with either galvanised steel or aluminium are described. (143)
In certain aggressive environments some grades of stainless steel will be susceptible to localised attack. Six corrosion mechanisms are described in this article, namely pitting corrosion, crevice corrosion, bimetallic (galvanic) corrosion, stress corrosion cracking (SCC), general (uniform) corrosion and intergranular (IGC), sometimes known as intercrystalline or IC) or weld decay attack.
The 316 types are used widely in marine applications, but their corrosion resistance in contact with seawater is limited. They cannot be considered ‘corrosion proof’ under all situations. These grades are susceptible to crevice and pitting corrosion, which limits there use in seawater applications. The affects of water chloride levels, flow rates, temperature and oxygen levels are noted and cathodic protection that can be derived from contact with less noble metals such as carbon steels and aluminium. The 304, and more especially the free machining 303 types, should not be considered for seawater service.
Finding an unusual grade of stainless steel can be a difficult prospect. This guide is intended to help the enquirer to find anything from the low Cr ferritic grades to the highly alloyed austenitic and duplex steels.
Welding austenitic stainless steels to carbon and low alloy steels are established methods in the process and construction industries. Over-alloyed fillers are used to avoid dilution of the parent stainless steel in the fusion zone. Filler type 308 can be used for joining a 304 type ‘parent’ to a carbon steel but more highly alloyed fillers, such as the 309 type are preferable. There should be no risk of post weld bimetallic ( galvanic) corrosion, if the joint is repainted.