Effects of W-alloying and aging treatments on the localized corrosion of Fe-29Cr superferritic stainless steels were investigated by measuring the influences of these treatments on the pitting potentials determined potentiodynamically and the repassivation rate in deaerated 4 M NaCl solution and also by observing the microstructural changes that ocurr during aging. The results were compared with the effect to Mo on the Fe-29Cr alloys.
The pitting potential of 29Cr-xW(x=0 to 4) ferritic stainless steel gradually increased with W-content from 465 $mV_{SCE}$ for 29Cr-0W to 680 $mV_{SCE}$ for 29Cr-4W in deaerated 4 M NaCl at 80℃, a result of beneficial effect of W on the localized corrosion of Fe-29Cr alloys. We have a less effect in enhancing the resistance to localized corrosion than does Mo when compared at equivalent weight percent. However, at equivalent atomic percent, W and Mo have a similar effect on the resistance to localized corrosion of Fe-29Cr alloys. Thus, the pitting potential of 29Cr-4W alloy, though it is 260 $mV_{SCE}$ more active than that of 29Cr-4Mo alloy, is similar to that of 29Cr-2.13Mo alloy having the same atomic percent. However, it is striking that 29Cr-3Mo-1W alloy is more resistant to the localized corrosion than is the 29Cr-4Mo alloy in spite of its lower atomic percent(W+Mo), suggesting a combined and beneficial effect of W and Mo in enhancing the resistance to localized corrosion of Fe-29Cr alloys.
A large difference was observed in precipitation behavior between 29Cr-4W and 29Cr-4Mo alloys when they had been aged at 850℃. For 29Cr-4W alloy, σ phase precipitated initially in the form of continuous film along grain boundary, then developed into discrete and long plates, and eventually χ phase grew out from both sides of some σ precipitates. On the other hand, for 29Cr-4Mo alloy, χ phase precipitated in the form of coarse and continuous poly grains primarily along grain boundary. The thin and continuous σ phase was observed at the centerline of χ phase. These differences in precipitation behavior result in a significant difference in the effect of aging on the resistance to localized corrosion between the two alloys. 29Cr-4W alloy exhibited a rapid drop in pitting potential from about 680 $mV_{SCE}$ for the solution treated one to -100 $mV_{SCE}$ when aged for less than half hour at 850℃, and then the pitting potential gradually decreased to about -210 $mV_{SCE}$ with aging for 25 h. In contrast to this, the pitting potential of 29Cr-4Mo alloy gradually reduced from about 945 $mV_{SCE}$ for the solution treated to about 480 $mV_{SCE}$ for the alloy aged for 25 h. This large difference in the influence of aging on the localized corrosion between the two alloys was found to be associated with the degree of depletion of Cr and/or Mo adjacent to the precipitates formed during aging. It was demonstrated from a compositional analysis of the precipitates that the precipitation of σ phase, a major precipitate for 29Cr-4W alloy, caused elements of Cr and Mo to be more depleted around the precipitate than did the χ phase, a major precipitate of 29Cr-4Mo alloy. Therefore, the rapid drop in pitting potential for 29Cr-4W alloy with aging is due to the severe depletion of Cr and Mo adjacent to σ precipitates.
Electrochemical potentiokinetic reactivation(EPR) test was modified and applied to aged alloys to confirm the differences in the degree of depletion of Cr and Mo adjacent to the precipitates of the alloys. The modified double loop(DL) EPR tests were conducted in deaerated 0.5 M $H_2SO_4$ + 0.01 M KSCN solution, 50℃ and at a scan rate of 1.67 mV/s. The results of EPR tests showed that for the same aging treatments, the susceptibility to intergranular corrosion of 29Cr-4W alloy was greater than that of 29Cr-4Mo alloy or even 29Cr-2.13Mo alloy. SEM micrographs of the etched structures of samples subjected to the EPR test demonstrated that 29Cr-4W alloy revealed a deeper intergranular attack than did 29Cr-4Mo alloy, confirming that the σ precipitate produced more severely depleted region for Cr and Mo elements than did χ precipitate.
Scratch tests, conducted in deaerated 4 M NaCl at 50℃, showed that the repassivation rate of Fe-29Cr alloys was closely related to the (Mo+W) atomic percent. Thus, the repassivation rate of 29Cr-4W alloy is slower than that of 29Cr-4Mo alloy but comparable to that of 29Cr-2.13Mo alloy.
