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

Abstract

Polarisation data for alloy 800, alloy 690 and the pure metals Cr, Ni and Fe and stress corrosion cracking (SCC) data from C-ring specimens at controlled potentials for the nickel base alloys 800 and 690 in boiling solutions of 50% NaOH with or without addition of SiO2 and Na2S2O3, are presented. The surface films on the SCC specimens were analyzed by Auger electron spectroscopy (AES). Alloy 800 exhibited a high susceptibility to caustic SCC in the passive region at potentials more noble than ∼−0.30 V (SCE) but was much less susceptible to SCC at potentials lower than ∼−0.45 V (SCE). This difference in SCC susceptibility was attributed to the different compositions of the surface films formed in different potential ranges. At higher potentials the films mainly consisted of oxides of Ni with some Fe and a little Cr, whereas those formed at lower potentials were composed of oxides of Cr, Ni and Fe. Alloy 690 had a lower susceptibility to caustic 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 stimulated anodic dissolution and decreased the stability of the passive films of the alloys due to the effect of S2O32− on the anodic polarisation behaviour of Ni. The addition of S2O32− accelerated caustic SCC in the alloys, the maximum cracking rate being observed at ∼−40 mV (SCE).