Improving the grain structure and adhesion of Ni-P coating to 3004 aluminum substrate by nanostructured anodic film interlayer

Two different types of interlayers were produced on aluminum alloy specimens prior to electroless nickel coating in an attempt to improve the structure and adhesion of such coatings to aluminum substrates. The amorphous Ni-P layer of medium phosphorous content was deposited on 3004 aluminum alloy sheet either without interlayer or over an interlayer of a zinc coating (zincate process) or an oxide coating (anodizing process). Formation of the nickel electroless coating and the effect of heat treatment were evaluated by x-ray diffraction (XRD) and microanalysis (EDS). The surface morphology and topography of the coatings were characterized by scanning electron microscope (SEM) and atomic force microscope (AFM). Adhesion of the coatings to aluminum substrate was qualitatively evaluated by the bend test. It was observed that application of a nano-porous aluminum anodic oxide as an interlayer caused a much finer morphology of Ni-P electroless coating. In addition, this interlayer effectively increased the adhesion of coating to aluminum surface; the crack density calculated after the bend test was about four times that obtained in the specimen with zincate interlayer and six times more than without interlayer.