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

The effect of concentration and type of anions on stress corrosion cracking of sensitized type 304 stainless steel was investigated by slow strain rat e test (SSRT) with a dynamic cracking observation system. It was found that difference of anion type added into pure water exerted a significant effec t on crack initiation time and mean crack initiation frequency. Some anions , such as OH- and B4O72-, can suppress, while others, such as S2O32- and SO 42-, accelerate crack initiation. The effect of anion type on the crack ini tiation behavior was explained on the basis of the passive film model propo sed by Okamoto and Shibata and a HSAB (hard and soft acids and bases) conce pt. The anion having a larger hardness value was found to show a longer cra ck initiation time or to delay the crack initiation. The anion with a large r hardness, that is expected to show a stronger affinity to proton, is cons idered to accelerate deprotonization, and to enhance the repassivation proc ess. Consequently, the harder anion is expected to induce a more perfect pa ssive film more rapidly, and then to retard the crack initiation more effec tively. (C) 2000 Elsevier Science Ltd. All rights reserved.