Pitting Resistance of Alloy 800 as a Function of Temperature and Prefilming in High-Temperature Water

Pitting behavior of alloy 800 (UNS N08800) was investigated as a function of temperature and prefilming in high-temperature water. The behavior was characterized in terms of pitting potential (U_p) and pit density (n_p). U_p decreased with increasing temperature and chloride activity. Prefilming of test coupons over a period between 100 h and 5,000 h in ammoniated water at 300°C had no apparent influence on U_p at room temperature (RT), 180°C, and 300°C. However, the number of pits in prefilmed coupons was much higher than in coupons covered with an air passive layer. The effect of prefilming on pit nucleation was investigated in detail with regard to a model and test methods developed by Bianchi, et al. Density of pits in prefilmed coupons was at least 1 order of magnitude higher than in air passive coupons. Maximum pit density was measured after a prefilming period of 100 h. The effect was discussed in terms of Bianchi’s model and in terms of features of passive films. The initially amorphous metastable passive film on alloy 800 became crystalline at increased temperatures. Crystallization induced lattice defects, such as dislocations and grain boundaries, in the passive film. The film grew and slowly transformed into a thick oxide layer. The transformation process was associated with enhanced susceptibility to pit nucleation.