Formation Mechanism of Two‐Layered AlO(OH) Films on Aluminum Alloys via the Steam Coating Process

Abstract The steam coating process forms a corrosion‐resistant two‐layered structured film composed of γ‐AlO(OH) (boehmite) on an aluminum alloy substrate by the direct reaction between steam and the substrate. Because this treatment does not require any chemicals, it is considered an environmentally friendly surface treatment. This study investigates the reaction field, growth rate, and two‐layer formation mechanism of AlO(OH) films to understand the mechanism of AlO(OH) film formation in a steam atmosphere. Consequently, the film thickness significantly increases for a specific amount of the steam source, suggesting that subcritical water in the liquid phase contributes to film formation. In addition, the decrease in the slope of the log‐log plot of the film thickness with increasing temperature suggests the inhibition of AlO(OH) film formation by the concentration of solute atoms at the substrate/dense layer interface. Finally, the crystalline/dense layer interface corresponds to the original surface position of the substrate. Therefore, the two‐layered structure comprises the AlO(OH) formation at the crystalline layer surface and substrate/dense layer interface simultaneously due to the inward and outward diffusion of OH ⁻ and Al ³ ⁺ ions, respectively. The crystalline layer is considered to have crystallinity due to its growth from crystal nuclei on the substrate surface.