改性800合金碱性SCC机理研究

通过对改性800合金（800M）在沸腾的50％NaOH 0.3%SiO2 0.3%Na2S2O3溶液中发生的晶间腐蚀（IGA）和沿晶应力腐蚀破裂（IGSCC）试样的扫描电镜（SEM）观察，结合阳极化曲线、表面膜俄歇电子能谱（AES）分析和恒电位极化的稳定溶解电流密度ist等测试研究了800M合金在碱液中的IGA形成及IGSCC破裂机理，并探讨了IGA对IGSCC的促进作用。发现在－500mV/SCE整个表面发生的IGA是表面膜中Cr的相对含量低，保护性差，以及溶液中S2O3^2-还原成S^2-在晶界偏析，使晶界的化学活性增加，导致C形环晶界的溶解速率iIG＞晶粒的溶解速率iGC所致。在电位E＝－40及－20mV/SCE发生IGSCC，可能是由于Ni形成不稳定的氧化产物，使合金表面膜保护性能下降，在应力作用下，活性大的晶界溶解，形成的微裂纹沿晶界扩展而导致沿晶应力腐蚀破裂。800M合金表面形成的IGA对IGSCC的萌生与扩展有促进作用，晶界是合金发生IGSCC的活性通路。

AN INVESTIGATION ON CAUSTIC SCC MECHANISM OF ALLOY 800M

The mechanism of intergranular attack (IGA) and intergranular stress corrosion cracking (IGSCC) of modified alloy 800(alloy 800M) in boiling 50%NaOH+0.3%SiO 2+0.3%Na 2S 2O 3 solution was investigated by means of scanning electron miscroscopy(SEM), Auger electron spectroscopy (AES), measurements of anodic polarization curves and the steady dissolution current density (i st). At -500mV/SCE, general IGA of alloy 800M was attributed to the dissolution rate of grain boundaries(i IG) being higher than that of grains(i GC). At this potential, the relative content of Cr in surface films was low and thus the protective ability was worse, S 2- formed by the reduction of S 2O 2- 3 in the solution and segregated at grain boundaries leading to the increase of the chemical activity of grain boundaries. At -40 and -20 mV/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 the active dissolution of grain boundaries under applied stress and propagation of small cracks along grain boundaries. IGA in the surface of alloy 800M promoted the initiation and propagation of IGSCC, and grain boundaries were the active paths of IGSCC.