Corrosion behavior of sulfuric acid anodized layer of cold sprayed pure aluminum coating
In order to explore anti-corrosion effect of sulfuric acid anodic oxide layer on cold sprayed pure alumi-num coating,sulfuric acid anodic oxide coating was prepared on the surfaces of coatings with two kinds of poros-ity. The morphology,chemical composition and corrosion resistance were evaluated by SEM,EDS,neutral salt spray test and electrochemical test. The results show that for the low-porosity (0.8％) coating,although the thick-ness of anodic oxide layer is slightly lower than that of substrate due to large specific surface area,the density of anodic oxide layer is high,which can form a dense passive film to prevent further corrosion of materials. Howev-er,the high porosity (1.5％) coating has poor continuity of coating surface because of more open pores on the sur-face and the mechanical process. The electroplating solution penetrates into the coating structure along the pores on the coating surface,causing an anodic oxidation reaction inside the coating and damaging the coating struc-ture. This phenomenon results in localized pores larger than 20 μm in the coating structure,the coating fails to form a dense passivation layer. It causes that the passivation zone of high porosity (1.5％) coating is narrow,and the passive current density increases obviously with the increase of passivation potential. Compared with the alu-minum alloy substrate,the self-corrosion current density and dimensional passive current density of the coating anodic oxide layer are higher,and the corrosion resistance is inferior to the substrate. Besides,the corrosion resis-tance decreases with the increase of porosity. However,the anodic oxide layer of the coating can still withstand neutral salt spray corrosion for 336 hours,meeting the anti-corrosion requirements of aircraft metal parts. In order to make the sulfuric acid anodized layer of the coating have a similar corrosion resistance to that of substrate and ensure the coating density,it is recommended to reduce the coating porosity as much as possible.
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