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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.
Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing stainless steels, using their chemical compositions. The formulae are based mainly on chromium, molybdenum and nitrogen contents. Grades with a PREN of 40 or more are known as 'super' austenitics or duplex types, depending to which basic family they belong. A table of calculated PREN values compares some of the ferritic, austenitic and duplex steel grades.
Specifying mechanically polished, brushed and buffed stainless steel finishes and their applications.