AbstractThe sizes of crystals on aluminium oxide films formed by exposing commercial aluminium (2S) and two nickel‐ and iron‐containing alloys to pure water at high temperature were determined by measuring and counting crystals on electron micrographs of surface replicas. The mean crystal size was determined for eight different times at five temperatures over the range 150–350°. The increase in crystal size followed a law of the typeD= Ainand the values ofnfound were in the range 0.18–0.26. The size distributions for various times at 300° and for various temperatures after 7 h. of exposure are