Stainless steels are defined as being characterized by particularly high resistance to chemical attack by aqueous media. In general, they contain at least 12% by weight of chrome and a maximum of 1.2% carbon. The reason for their high resistance to corrosion is a passive layer that forms on the surface. This consists of a metal oxide or hydroxide layer rich in chrome, only a few Ångstrom units thick, separating the actual metal from the attacking medium. After sufficient time has passed, the passive layer of a stainless steel exhibits a constant composition and remains in a state of equilibrium with the surrounding medium. Once formed, such a layer cannot therefore be transferred to another medium. Following any mechanical damage of the surface, a new layer can generally be expected to form spontaneously at that point. If in some medium a satisfactory passive layer cannot form, or if an existing layer is locally damaged or completely destroyed, corrosion can occur. The decisive element responsible for the formation of a passive layer is chrome. A chrome content above the quoted value of some 12% inhibits rusting under normal atmospheric conditions. Further increases in the chrome content and, according to the application, the addition of molybdenum and other alloys permit corrosion resistance to be extended to much more aggressive media. Only those alloy contents dissolved in the metal are effective in achieving passivation. The highest resistance to corrosion is thus given with a segregation-free matrix whose chrome or molybdenum contents are not reduced by precipitations of the formation of non-metallic phases. The right heat treatment for achieving an ideal structure is described in the particular material sheets. Stainless steels may suffer general corrosion or various types of localised corrosion. Resistance to general corrosion is usually classified as follows:
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