An investigation of the growth mechanism of scale formed on chromium containing low amounts of sulfur, chlorine, and phosphorus

The growth mechanism of the flat and adherent scale formed at 900 degreesC in 0.1 atm oxygen on chromium, which contained 1 ppm (wt) sulfur, 0.08 ppm (wt) chlorine, and 0.2 ppm (wt) phosphorus, has been found to be very diffe rent in adjacent parts of the scale, consistent with the results of previou s tracer studies of the growth mechanism of chromia formed on chromium. rn some places, it grew primarily by chromium transport, while in others it gr ew primarily by oxygen transport and in still others if grew by a mixture o f both. New oxide formed within the outer part of the scale and, in some ca ses, throughout the scale. A tentative hypothesis is proposed to explain wh y growth mechanisms were different in different par ts of the scale on the same specimen. The scale formed at 950 degreesC on Fe-20%Cr -0.11%Si, which contained 15ppm sulfur, grew predominantly by cation transport, with only a small amount of oxygen transport. rn this case also, new oxide formed wit hin the outer part of the scale.