Intrinsic stresses in anodic films on aluminium

Abstract

Stresses in anodic barrier layers on aluminium were investigated by measuring the isothermal deformation of clamped membrane electrodes galvanostatically anodised on one side. At open circuit, the residual stresses were always tensile, independent of thickness within 250 nm and decreasing with current density from ca. 85 N/mm 2 at 1 mA/cm 2 to ca. 40 N/mm 2 at 0.05 mA/cm 2. The growing layers on the other hand exhibited compressive stresses predominantly caused by ion migration processes. The constant orders of magnitude were −20 N/mm 2 at 0.05 mA/cm 2 and −60 N/mm 2 at 0.2 mA/cm 2. At 1 mA/cm 2, however, a decrease from initially ca. −320 N/mm 2 to ca. −160 N/mm 2 at 240 nm was observed. This and the microscopically confirmed plasticity of growing oxides led to the outline of a growth model which was further tested by some preliminary measurements of the intrinsic forces present during the pore initiation in a phosphoric acid electrolyte.