An Investigation On Interface Reaction Regularity For Caustic Stress Corrosion Cracking Of Fe-Cr-Ni Alloy

Stress corrosion behaviors of modified alloy 800 (alloy 800M) in hot concentrated caustic solution were investigated by means of polarization measurements, stress corrosion cracking (SCC) tests of C-rings under controlled potentials, analyses with scanning electron miscroscopy (SEM) and Auger electron spectroscopy (AES), and also discussed on the basis of analysis of potential-pH diagrams.The steady dissolution current densities (ist) of alloy 800M polarized potentiostatically were measured. The simplifed steady potentiostatical polarization curves were drawn on the basis of ist. The ratio RPD measured from ist and steady passive current density iP at low potentials(RPD= ist /ip) was designated as a polarization dissolution susceptibility parameter. The values of parameter RPD of both plate and C-ring specimens were similar at sensetive potentials between -30mV/SCE and 40mV/SCE where caustic stress corrosion cracking was found on C-rings of alloy 800M tested in boiling 50%NaOH+0.3%SiO2+0.3%Na2S2O3 solution for 96 hours. Caustic SCC of alloy 800M could be predicted by the electrochemical characters synthesizing polarization curves with fast and slow scan rates, ist and RPD-The mechanism of intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) of alloy 800M in caustic solution was investigated. At -500mV/SCE, general IGA of alloy 800M was attributed to the dissolution rate of grain boundaries(iio) being higher than that of grains(ioc)- At this potential, the relative content of Cr in surface films was low and thus the protective ability was worse, S2" formed by the reduction of S2O32- in the solution and segregated at grain boundaries leading to an increase in the chemical activity of grain boundaries. In the potential range between -40mV/SCE and 40mV/SCE, the occurrence of IGSCC might be attributed to the decrease of the protective ability of surface films with the formation of unstable oxide of Ni resulting in theactive dissolution and propagation along grain boundaries of small cracks under applied stress. IGA in surface of alloy 800M promoted the initiation and propagation of IGSCC. Grain boundaries were the active paths of IGSCC.