Formation of chemical conversion coatings with high corrosion-resistance on aluminum in strontium hydroxide by secondary dipping treatment

摘要

It is known that the coatings produced by MBV method and Alojin method are inferior with point of anticorrosion because those coatings have characteristic of thin amorphous coatings and much cracks on the surface of coating. Manufacture of the thick coating which was superior in corrossion-resistance is needed, and this study has attempted to obtain chemical conversion coatings in 0.01 mol/l Sr(OH)2 bath with boehmite process. The major component of first coating was Al(OH)_3, because Al(OH)_3 was remade in supersaturated sodium aluminate solution and the first coating were examined by secondary treatment. The chemical Structure, and its composition of each chemical coatings were investigated by X-ray diffraction analysis (XRD), scanning electron microscope (SEM), thermal analysis and secondary ion mass spectrometry. It is found that the first coating formed by dipped at 70deg C for 60 min was thickness of about 10#mu#m. The structure of first coating has two layers: one crystal layer was growing from uniform amorphous layer and another amorphous layer was thickness of about 3 #mu#m. crystallinity of the crystal layer (Gibsite) was enhanced by secondary treatment, the amorphous layer was double thickness compared with first coating, which was confirmed by measure of XRD and SEM. This secondary treatment coatings showed excellent corrosion-resistance (10 times or more). The mechanism of corrosion protection by secondary coating had been attributed to reason that the dissolution of aluminum decreased with coating of thick and uniform amorphous layer.