A HSAB concept applied to inhibition effect of anions on IGSCC of sensitized type 304 stainless steel

Abstract The effect of concentration and type of anions on stress corrosion cracking of sensitized type 304 stainless steel was investigated by slow strain rate test (SSRT) with a dynamic cracking observation system. It was found that difference of anion type added into pure water exerted a significant effect on crack initiation time and mean crack initiation frequency. Some anions, such as OH− and B4O72−, can suppress, while others, such as S2O32− and SO42−, accelerate crack initiation. The effect of anion type on the crack initiation behavior was explained on the basis of the passive film model proposed by Okamoto and Shibata and a HSAB (hard and soft acids and bases) concept. The anion having a larger hardness value was found to show a longer crack initiation time or to delay the crack initiation. The anion with a larger hardness, that is expected to show a stronger affinity to proton, is considered to accelerate deprotonization, and to enhance the repassivation process. Consequently, the harder anion is expected to induce a more perfect passive film more rapidly, and then to retard the crack initiation more effectively.