Improving corrosion resistance of aluminium metal matrix composites using cerium sealed electroless Ni-P coatings

A conversion coating treatment using cerium salts was developed for the surface sealing of electroless nickel-phosphorus (Ni-P) coatings on carbon fibre reinforced aluminium (Cf/Al) composites. The corrosion resistances of uncoated and coated materials (i.e. the Ni-P coating, the Ce conversion coating and Ce sealed Ni-P coatings) were evaluated in 3.5 wt-%NaCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy. Ce sealed Ni-P coating showed the highest corrosion resistance and clearly improved the overall corrosion resistance of Cf/Al composites. Thus, the Ce sealed Ni-P coating had no obvious microcracks that were generally evident in the more conventional Ce conversion coatings. This is presumed to occur because the electroless nickel surface is relatively homogeneous, compared with the Cf/Al composite surface on which different local coating thicknesses would encourage increased microcrack formation. X-ray photoelectron spectroscopy analysis showed that the Ce conversion coating mainly contained both Ce(3+) and Ce(4+) species; however, Ce(4+) species were the dominant oxidation state on Ce sealed Ni-P coatings.