CAUSTIC STRESS CORROSION CRACKING OF NICKLE-RICH, CHROMIUM-BEARING ALLOYS

摘要

The electrochemical behavior and susceptibility to stress corrosion cracking (SCC) ofrnNi-rich, Cr-bearing alloys 800 and 690 in boiling solutions of 50% NaOH with orrnwithout additions of SiO_2 and Na_2S_2O_3 were studied. The information on correlation ofrnactivation and passivation behavior of the alloys with the electrochemical behavior ofrnmajor alloying elements in the alloys was gained by means of the anodic polarizationrncurves of alloys 800 and 690, as well as pure metals Cr, Ni and Fe. The SCC behaviorrnwas tested using C-ring specimens at controlled potentials. The surface films formed onrnSCC specimens were analysed by Auger Electron Spectroscopy (AES) after SCCrntesting. The correlation of SCC behavior with the results of electrochemicalrnmeasurements and AES analyses is discussed. Alloy 800 exhibited high susceptibilityrnto caustic SCC in the passive region at potentials more noble than ca.-0.30V(SCE) andrnwas much less susceptible to SCC at potentials lower than ca. –0.45V(SCE). Therndifference in SCC susceptibility was associated with the composition of surface filmsrnformed in different potential ranges. The surface film formed at higher potentialsrnconsisted of oxides of mainly Ni and some Fe with little Cr, while that formed at lowerrnpotentials was composed of oxides of Cr, Ni and Fe, and the latter showed much higherrnresistant to caustic SCC. The susceptibility to caustic SCC of alloy 690 was much lowerrnthan that of alloy 800 under same testing conditions and this might mostly be attributedrnto the much higher Cr content in the surface films of alloy 690. Na_2S_2O_3 stimulated thernanodic dissolution and retarded the passivation of the alloys and this was mainlyrnattributed to the effect of S_2O_3rn2- on the anodic polarization behavior of Ni. Addition ofrnS_2O_3rn2- gave rise to caustic SCC acceleration of the alloys, the maximum cracking raternwas observed at ca. –40mV (SCE). It is also indicated that the electrochemicalrnmeasurements of alloys and pure metals are informative for studying SCC and itsrnmechanism.