Corrosion of AISI 316 Ti in 50% KOH due to deformation induced martensite

During 3 months storing of 50% KOH in a 40-m3 cistern the content of iron in KOH increased above the allowed limit of 10 mg/l. The analysis of circumstances revealed that the only possible source for iron is corrosion of basic material and/or corrosion of welds. Chemical analysis of base material and weld confirmed that both corresponded to declared stainless steel AISI 316 Ti. Metallographic analysis showed the presence of martensite in the outer and inner surface layers with pitting and intercrystalline corrosion. Shotblasting caused plastic deformation and the appearance of deformation induced martensite in both surface layers of the cistern. The presence of martensite diminished corrosion resistance and enabled the start of the corrosion process which increased the content of iron in 50% KOH during the storing period.