Abstract
When iron is oxidized at 500°C in oxygen at 10 Torr, blisters develop in the scale. This does not occur when the iron is oxidized at 500°C in oxygen at 700 Torr, nor when the iron contains carbon as a cementite precipitate. It appears that the iron oxide scale blisters if cation vacancies can accumulate at the oxide‐metal interface. The higher oxygen pressure inhibits scale blistering by promoting more rapid formation of the outer layer of 
, which lowers the rate of vacancy formation and accumulation. The presence of carbon in the substrate inhibits scale blistering by providing additional vacancy sinks, thereby largely preventing the accumulation of vacancies at the oxide‐metal interface.
When the iron‐carbon alloys are oxidized at 500°C in oxygen at 10 Torr, a decarburized zone appears in the alloy substrate near the oxide‐metal interface. This means that, under the experimental conditions, the rate of diffusion of carbon from the alloy substrate exceeds the rate of diffusion of iron into the scale. Decarburization does not occur when the alloy is oxidized at 500°C in oxygen at 700 Torr. This effect of oxygen pressure can be explained if it is assumed that at 10 Torr, carbon oxides are formed at the oxide‐metal interface and escape through pores in the scale. At the higher pressure, oxygen enters the pores and is incorporated into the scale, thereby blocking the pores and preventing the escape of carbon oxides.