Corrosion protection of aluminium metal-matrix composites by cerium conversion coatings

Effect of cerium pretreatment on the corrosion inhibition characteristics of AA6061 T6 alloy-10% Al2O3 metal-matrix composite was investigated in aerated 3.5% NaCl solution after different immersion times ranging between 1 and 60 days at room temperature. Series of specimens were prepared under the following conditions: as-polished; directly treated in CeCl3; alkaline etching followed by CeCl3 treatment; immersion in boiling water followed by CeCl3 treatment; and alkaline etching followed by immersion in boiling water and then CeCl3 treatment. Electrochemical impedance spectroscopy (EIS) was used to investigate the corrosion behaviour of treated and untreated specimens. The EIS was measured by perturbing the free corrosion potential of the specimens with 10 mV a.c. with a frequency decreasing from 10 mHz to 65 kHz. Polarization tests were used to investigate the perfect passivity domain after corrosion. Optical microscopy and SEM were used to investigate the occurrence of localized corrosion due to chloride ion attack. Specimens untreated, directly treated in CeCl3 and alkaline etched followed by CeCl3 treatment suffered from pitting as well as crevice corrosion after 30 days of immersion in NaCl. Conversely, the specimens after alkaline etching followed by oxide thickening in boiling water before cerium treatment showed promising corrosion protection even after 60 days of immersion in NaCl. The degree of protection depends to a high extent on the specimen preparation prior to cerium pretreatment. Copyright (C) 2002 John Wiley Sons, Ltd.