Microstructure and cracking behavior of hydroxide films formed on aluminum-alloy sheets prepared by steam coating

To elucidate the adhesion properties of anticorrosion hydroxide films formed on aluminum (Al)-alloy sheets prepared by the steam coating process, cracks that propagated inside the hydroxide film formed on Al–Mg–Si alloy (6061 grade) sheets were characterized. Continuous hydroxide films with different thicknesses of 0.7 μm and 1.4 μm formed at different temperatures of 180 °C and 200 °C, respectively. Microstructural characterizations revealed that these hydroxide films exhibited a dual-layer structure consisting of a polycrystalline γ-AlO(OH) layer (on the surface side) and a continuous amorphous layer (on the Al-alloy side). Bending tests caused numerous cracks inside the hydroxide films under tension and compression. On the tensile-strained surface, numerous vertical cracks penetrated through the hydroxide films, although their delamination was not observed. On the compressive-strained surface, local delamination of the upper γ-AlO(OH) layer occurred. However, cracks were not observed in the lower amorphous layer on the Al-alloy substrate deformed under compression. These results indicate that the dual-layer-structured hydroxide film prepared by steam coating exhibits remarkable adhesion to the Al-alloy substrate.