5056铝箔表面新型三价铬转化膜的形成过程及影响因素OA北大核心CSTPCD

Aluminum foil is widely used in transportation,building and packaging industries,and is also an important raw material for making current collectors of power batteries.For such applications,surface treatment of the aluminum foil is necessary to ensure sufficient corrosion resistance.In this work,a 5056 aluminum foil was surface-treated in a newly-developed trivalent chromium process(TCP)solution.The structure and performance of the conversion coating were controlled by changing the treatment time,pH and temperature of the solution.The corrosion resistance and surface characteristics of the conversion coating under optimized conditions were characterized by potassium dichromate droplet test,electrochemical test,contact angle test,and neutral salt spray test.The structure and composition of the coating were characterized with an ultramicrotomy-assisted scanning electron microscopy and an X-ray photoelectron spectroscopy.It was found that the corrosion resistance of the new TCP conversion coating on the surface of the 5056 aluminum foil was significantly affected by the treatment time,pH and temperature of the solution.In order to obtain a uniform,continuous,and dense TCP conversion coating,the treatment time should be limited to 50-60 seconds,the pH and temperature of the solution should be maintained at 3.5-4.0 and 40-50℃,respectively.The thickness of the conversion coating on the 5056 aluminum foil was about 80 nm,and it was mainly composed of TiO2,ZrO2,Al2O3,and Cr(OH)3.As a result of the treatment,the surface contact angle of the aluminum foil increased from 16.7° to 106.5°,exhibiting good hydrophobic property.Compared with the bare alloy,the corrosion current density of the alloy sample with typical TCP conversion coating decreased by more than one order of magnitude in a 3.5%aqueous solution.After 1 632 hours of neutral salt spray test,no obvious signs of corrosion attack were observed on the treated alloy.Besides,no transition from trivalent chromium to hexavalent chromium was detected by Raman spectroscopy.The formation process of the TCP conversion coating includes four stages:nucleation,stable growth,cracking and peeling,and dynamic balance between re-growth and cracking/peeling.When the aluminum foil is immersed in the TCP solution,the dissolution of the natural oxide film on the alloy surface and the nucleation of the conversion coating occurs successively.It then quickly comes to the stable growth stage,and its corrosion resistance is best when the conversion coating completely covers the alloy surface.When the coating time is further extended,although the thickness or compactness of the conversion coating may increase,the tendency to crack and peel also increases,which damages the integrity of the coating and reduces its corrosion resistance.Although the conversion coating re-nucleates and re-grows at the cracked and peeled sites,its corrosion resistance is still inferior to the stage when the conversion coating just completely covers the alloy surface.