What is A4 Stainless Steel?
A4 grades of stainless steel are usually reserved for use in highly caustic environments where improved resistance to pitting corrosion is required, i.e Marine, exhausts. Pitting corrosion occurs once the chromium becomes exhausted through self-passivation after caustic attack - the effects as usually local to the exposed areas. The pitting forms as the Iron atoms become exposed to Oxygen and form 'rust'. The improved resistance comes from the addition of a higher proportion of Chromium, which helps to form a protective oxide layer on the iron - the more chromium you have the longer the protection it offers lasts.