Influence of Magnesium on SCC Susceptibility of Duplex Stainless Steels in Boiling Solutions

摘要

This paper deals with some problems associated with stress corrosion cracking assessment of duplex stainless steels in boiling concentrated solutions. Electrochemical polarization experiments conducted on UNS S32304 lean duplex stainless steel in NaCl, CaCl_2 and MgCl_2 boiling solution indicate that the presence of Mg~(2+) cations has a strong impact on its passive layer properties. This influence has been attributed to a possible incorporation of Mg into the passive film, as evidenced by depth profile analysis using glow discharge optical emission spectroscopy. These findings contrast with those obtained on standard S32205 duplex stainless steel where Mo alloying content appears to prevent Mg insertion behavior and this is supported by theoretical calculations of substitution energies of Mg in Cr_2O_3 oxide film. This specific interaction between Mo and Mg is regarded to be a good explanation of the negative influence of Mo in boiling concentrated solutions of MgCl_2, and suggests that the standard 45 % MgCl_2 155 °C boiling solution is unfortunately unsuitable for stress corrosion cracking resistance assessments of stainless steels with different Mo alloying level.