PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE ZINCATE IMMERSION PROCESS FOR ALUMINUM AND ALUMINUM ALLOYS.

Abstract

A detailed experimental study has been carried out to investigate the zincate immersion deposition process for 99.99%, 6061, and 356-T6 aluminum samples. In particular, the effect of iron and tartrate in the immersion bath, the aluminum surface preparation, and the relationship of the first immersion step to the second immersion step were investigated by chemical, electrochemical (polarization and rest potentials), and surface analytical scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) techniques. Eh-pH diagrams were constructed to determine the most stable zinc, iron, and aluminum species in solution. These diagrams predict that ferrous and ferric ions, as well as aluminum should form stable complexes with tartrate at the typical immersion deposition conditions (Eh -0.9 to -1.0 and pH 14 to 15). Experimentally, tartrate was found to enhance the dissolution rate of aluminum in highly caustic solutions. The addition of ferric chloride to the immersion bath produced coatings that were more crystalline, and also decreased the amount of hydrogen gas evolved in the second immersion step. The deposition of zinc and iron during the second immersion step was considerably less than that during the first immersion step. The second immersion coating became more adherent as the initial surface roughness decreased, and as grain size was increased the second immersion coating became thicker. For increasing grain size the micrographs for the first and second immersion coatings showed that the coatings became more localized. The second immersion coating thickness and morphology were also dependent upon several first immersion variables, such as bath temperature, immersion time and bath composition. Increased dissolution rates of aluminum in the first immersion produced thinner coatings with a finer crystallite growth. Increased bath temperature and increased first immersion time enhanced the dissolution rate of aluminum. The zinc coating slowed the dissolution rate of aluminum. When zinc was absent from the first immersion bath, the aluminum dissolution was much faster and resulted in thinner coatings upon subsequent second immersion. The molar ratio of zinc deposited to aluminum dissolved was a constant value of 1.1 for both first and second immersions; the molar ratio was also constant for the different aluminum substrates examined in this investigation.