Film Growth On Aluminum In High Temperature Water

Film growths on aluminum and two aluminum-1 wt.% nickel alloys in water at 250 and 350 deg C were studied. It was found that oxide growth does not advance on a uniform front but, to the contrary, the advancing surface contains many outcrops in the form of thin platelets, chunky outcrops, and whiskers. With both the pure metal and the alloys considerable intergranular attack was observed. The general corrosion product was usually more uniform in crystal size when formed on the pure metal, but variations in crystal size were observed on both aluminum and alloys with varying features of the metal surface. The roughness of the general oxide surface (including outcrops) was found to increase rapidly to about 0.2 micron and then remain relatively constant with increasing film thickness. The composition of films formed under all investigated conditions, except one, was found to be boehmite ( alpha-Al2O3-H2O). This exception was films carried by the alloy specimens after testing for 32 days at 350 deg C. In this case the main corrosion film was still boehmite, but in addition the outer surface supported long needles of diaspore ( beta-Al2O3-H2O).