Film formation and stress corrosion cracking of duplex stainless steel in boiling chloride solution

Abstract

The stress corrosion cracking (SCC) behaviour of a duplex stainless steel, U–50, has been studied in boiling 10M CaCl2 solution as a function of pH using the constant strain, constant load, and slow strain rate test methods. The susceptibility to SCC, in the constant strain and constan load tests, decreased with increasing pH in the range 1–6. However, in slow strain rate tests, SCC took place only in solutions with pH <2, Electrochemical tests, such as measurement of variations in open circuit potential with time and current decay tests, indicated the involvement of film formation in the SCC process. Cracks were always seen to be associated with regions of film breakdown. During anodic polarisation, deep round edge pits were seen to form in solutions with pH <2, whereas in higher pH (∼6) solutions, there was localised surface attack which spread downwards with time. The results obtained in the slow strain rate tests and electrochemical tests, and the metallographic evidence supported the film rupture—anodic dissolution model as the mechanism of stress corrosion failure ofU–50 in boiling CaCl2 solution.