Effect of cerium pretreatment on the corrosion inhibition characteristics of AA6061 T6 alloy-10% Al 2 O 3 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 CeCl 3 ; alkaline etching followed by CeCl 3 treatment; immersion in boiling water followed by CeCl 3 treatment; and alkaline etching followed by immersion in boiling water and then CeCl 3 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 CeCl 3 and alkaline etched followed by CeCl 3 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.
The effect of surface preparation prior to cerium treatment on the corrosion protection of AA6061 T6-10% Al 2 O 3 in NaCl solution was investigated using electrochemical impedance spectroscopy and DC polarization techniques. A new surface preparation method consisted of pre-etching followed by oxide-thickening is proposed. Optical microscope and scanning electron microscopy showed a marked decrease in the number and depth of pits for the pre-etched as compared to the as-polished specimens. X-ray photoelectron spectroscopy revealed that pre-etching eliminated the active sites where the chloride ions can adsorb. Simultaneously, the adsorption of cerium ions is uniformly on the surface. The oxide-thickening step has a vital role in the protection mechanism. It was found that presence of a thick oxide layer completely changed the mechanism of protection.
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