Inhibition Effect of the Borate Ion on Intergranular Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel

The intergranular stress corrosion cracking (IGSCC) behavior of sensitized type 304 (UNS S30400) stainless steel (SS) in dilute sodium borate (Na₂B₄O₇) solutions at 95°C was examined by slow strain rate testing (SSRT) using a dynamic observation system. The borate ion (B₄O₇^2–) suppressed IGSCC susceptibility of sensitized type 304 SS by delaying the crack initiation time and reducing the crack initiation frequency (CIF). The inhibition effect of the borate ion (B₄O₇^2–) on crack initiation may have resulted from its buffer effect on local acidification at crack nuclei and its inhibitive nature in reacting with metal ions to form a protective film. However, B₄O₇^2– concentration provided no remarkable inhibition effect on crack velocity (CV), which depended mainly upon solution conductivity, within the scattered values. Hydroxyl ions (OH^–) also suppressed initiation of IGSCC, but not as effectively as B₄O₇^2–. The crack initiation time obeyed the exponential probability distribution, and the CV obeyed the Weibull probability distribution.