Lanthanide salts as thermal conversion coatings on silicon carbide reinforced AA5Ao6 aluminium metal matrix composite

In this study, cerium and lanthanum chloride binary mixtures conversion coatings were investigated as corrosion inhibitors for silicon carbide particulates reinforced AA5A06 aluminium composite and AA5A06 aluminium alloy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests revealed that cerium (III) chloride additions markedly improved the pitting corrosion resistance as compared to lanthanum (III) chloride additions, with maximum increase noted for samples treated with 750 ppm cerium (III)+ 250 ppm lanthanum (III) chlorides. Scanning electron microscopy, energy dispersion spectroscopy and X-ray photoelectron spectroscopy examinations revealed the existence of a heterogeneous layer formed by precipitation of cerium/lanthanum oxide/hydroxide on the cathodic intermetallics and an aluminium oxide film on the rest of the metal matrix. Improved corrosion resistance was attained for SiC particulates reinforced aluminium composite by means of the cerium/lanthanum conversion treatment, due to the presence of mainly cerium (IV) species as a result of oxidation of cerium (III), which provides auto-protection properties.