Adhesion properties of hydroxide films formed on the Al-Mg-Si aluminum alloy sheets prepared by steam coating process

In this study, in order to evaluate the adhesion properties of the hydroxide film formed on 6061 alloy (Al-1Mg- 0.6Si (wt.％)) sheets prepared by steam coating, we examined the microstructure and cracking behavior of hydroxide film on Al alloy sheets prepared by steam coating at 220°C for 24 h. EBSD analyses revealed the steam-coated sample exhibits microstructure of a-Al matrix consisted of equiaxed grains with a mean grain size of approximately 20 mm. The studied sample exhibits high hardness of 79 HV due to the strengthening by the precipitates of nanoscale particles (Mg₂Si). The observation for the surface morphologies revealed the hydroxide film consisted of a number of particles with a mean size of approximately 300 nm and cross-sectional microstructural observations identified a continuous hydroxide film with a thickness of approximately 1.2 mm. The XRD measurement demonstrated the formed hydroxide film consisted of the g-AlO(OH) phase. Minute microstructure analyses revealed that the hydroxide film exhibits a dual layer structure consisting of g-AlO(OH) crystalline layer on the surface and a continuous Al-rich layer on the Al alloy. The cracking and delamination behavior of the hydroxide film were examined by bending test. It was found that the local delamination of g-AlO(OH) phase would occur under compressive stress rather than tensile stress, whereas the delamination of the hydroxide film from the Al alloy substrate was not observed. These results indicate that the hydroxide film prepared by steam coating has a remarkable adhesion strength on the Al alloy substrate.