CRITICAL CRACKING POTENTIALS OF 26CR-1 MO FERRITIC STAINLESS-STEELS IN BOILING 42-PERCENT LICL SOLUTION

Abstract

The critical cracking potentials (E(cc)) for 26Cr-1 Mo ferritic stainless steels (UNS S44627), above which stress corrosion cracking (SCC) does occur, have been measured at constant load in a hot chloride solution. Various factors affecting E(cc) are examined to determine the concept of E(cc). E(cc) for the low interstitial 26Cr-1 Mo alloy (E-Brite(dagger)) is shown to be a potential for crack initiation and is determined by the competing rates of generation of new surface by slip-induced film breakdown and repassivation. E(cc) for E-Brite is very sensitive to the microstructural conditions developed by prior thermal and mechanical treatments; varying in the range of -485 mV for the mill annealed to -625 mV for the grain coarsened. On the other hand, the minimum potential permitting crack growth is insensitive to these treatments and corresponds to the most active value of E(cc), -625 mV. When strained at a constant strain rate (2.5 x 10(-6)/s), the critical potential above which E-Brite is susceptible to SCC corresponds to the most active value of E(cc) measured at constant load. Thus, it appears that the most active value of E(cc) (-625 mV) is a repassivation potential for growing cracks, and E(cc) approaches that for crack propagation as a limiting condition.