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Chrome‐free pretreatment for aluminium composites

The effect of manganese pretreatment on the corrosion behaviour of AA6061 T6–10% Al2O3 metal–matrix composite was investigated in aerated aqueous 3.5% NaCl solution using electrochemical impedance spectroscopy (EIS). The corrosion inhibition characteristics were studied for two series of manganese‐pretreated specimens after 30 days of immersion in NaCl solution at different pH, pretreatment times and different surface preparation prior to manganese pretreatment. Polarization tests were used to investigate the perfect passivity domain after 60 days of immersion in NaCl. Results showed that the specimens treated in KMnO4 + NaCl, in pure KMnO4 solution for a long time (1–7 days) at neutral pH and in pure KMnO4 solution at pH 8 for a short time (<1 h) suffered from pitting, crevice or general corrosion. Conversely, the specimens treated in pure KMnO4 solution for 3 h at pH 8 or at neutral pH did not show any sign of corrosion. X‐ray photoelectron spectroscopy revealed the presence of a high percentage of Al2O3 in addition to Mn oxide as a compact surface film. A promising corrosion resistance was obtained by sealing in a silicate solution for 2 min after treatment in pure KMnO4 solution. Microscopic examinations showed that silicon plays an important role in improving the protective power of the passive film by repairing the pitting results from chloride ion attack. Copyright © 2002 John Wiley & Sons, Ltd.