Caustic stress corrosion cracking of nickel-rich, chromium-bearing alloys

Polarisation data for alloy 800, alloy 690 and the pure metals Cr, Ni and F e and stress corrosion cracking (SCC) data from C-ring specimens at control led potentials for the nickel base alleys 800 and 690 in boiling solutions of 50% NaOH with or without addition of SiO2 and Na2S2O3, are presented. Th e surface films on the SCC specimens were analyzed by Auger electron spectr oscopy (AES). Alloy 800 exhibited a high susceptibility to caustic SCC in t he passive region at potentials more noble than similar to -0.30 V (SCE) bu t was much less susceptible to SCC at potentials lower than similar to -0.4 5 V (SCE). This difference in SCC susceptibility was attributed to the diff erent compositions of the surface films formed in different potential range s. At higher potentials the films mainly consisted of oxides of Ni with som e Fe and a little Cr, whereas those forced at lower potentials were compose d of oxides of Cr, Ni and Fe. Alloy 690 had a lower susceptibility to caust ic SCC than alloy 800 under the same testing conditions, presumably due to the much higher Cr content in the surface films on alloy 690. Na2S2O3 stimu lated anodic dissolution and decreased the stability of the passive films o f the alloys due to the effect of S2O32- on the anodic polarisation behavio ur of Ni. The addition of S2O32- accelerated caustic SC-C in the alloys, th e maximum cracking rate being observed at similar to -40 mV (SCE). (C) 2001 Elsevier Science Ltd. All rights reserved.