INHIBITION EFFECT OF THE BORATE ION ON INTERGRANULAR STRESS-CORROSIONCRACKING OF SENSITIZED TYPE-304 STAINLESS-STEEL

The intergranular stress corrosion cracking (IGSCC) behavior of sensit ized type 304 (UNS S30400) stainless steel (SS) in dilute sodium berat e (Na2B4O7) solutions at 95 degrees C was examined by slow strain rate testing (SSRT) using a dynamic observation system. The borate ion (B4 O72-) suppressed IGSCC susceptibility of sensitized type 303 SS by del aying the crack initiation time and reducing the crack initiation freq uency (CIF). The inhibition effect of the berate ion (B4O72-) on crack initiation may have resulted from its buffer effect on local acidific ation at crack nuclei and its inhibitive nature in reacting with metal ions to form a protective film. However, B4O72- concentration provide d no remarkable inhibition effect on crack velocity (CV), which depend ed mainly upon solution conductivity, within the scattered values. Hyd roxyl ions (OH-) also suppressed initiation of IGSCC, but not as effec tively as B4O72-. The crack initiation time obeyed the exponential pro bability distribution, and the CV obeyed the Weibull probability distr ibution.