Effect of substrate roughness on the corrosion behaviour of the Al2O3/MA 956 system

This paper presents the influence of substrate roughness on the corrosion b ehaviour of the Al2O3/MA 956 system. An alumina layer of thickness 1-5 mu m was generated of the MA956 alloy by thermal oxidation at 1100 degrees C us ing different exposure times. This Al2O3/MA 956 system with a polished subs trate has shown excellent corrosion behaviour in a physiological fluid, due to the fact that the alpha-Al2O3 layer formed is dense, continuous and fir mly adhered to the substrate, irrespective of the scale thickness. This goo d adherence allows it to withstand potentials above 1.7 V. Specimens with r ough finish substrate and treatment times above 10 h present spallation of the alumina layer at the crests of the roughness profile. In this case a mi xed corrosion behaviour between an alumina coated material and one with a p assive layer is observed. In both types of specimens, rough and smooth, onc e the passivation layer is broken the repassivation capacity of the substra te is ensured due to the high chromium content of the alloy, under oxygenat ion conditions. (C) 1999 Elsevier Science Ltd. All rights reserved.