Evaluation of the scale integrity in the Al2O3/MA956 system at different polarisation by using EIS

In this work anodic polarisation test and electrochemical impedance spectro scopy (EIS) are combined to evaluate the integrity of the alumina layer gen erated "in situ" at high temperature (1100 degreesC/ 3 h) on MA95 alloy. Th e SEM observations did not show the presence of voids or porosity in the al umina laver and a good adherence was revealed with the metallic substrate. The electrochemical response of the scale/metal system has been evaluated a t different points of the anodic polarisation curve. In general. a polarisa tion applied to the sample, below the breakdown potential, did not affect t he EIS results. irrespective of the polarisation withstood by the system. I n other cases. besides the slightly higher current density measured during the anodic polarisation test, a decrease in the impedance modulus was obser ved. This behaviour could be related to the presence of microdefects in the scale. not visible by SEM. which become increasingly and/or connected duri ng the polarisation test. The very low corrosion current densities in the p assive region, about 10(-9) A/cm(2). and the high polarisation that the alu mina laver withstands till breakdown occurs confirm the excellent corrosion protection provided by the scale. Once breakdown of the alumina layer occu rs. a good repassivation of the system is observed due to the high chromium content of the alloy.