| Oxide films are commonly found on metal surfaces.It is necessary to remove the oxide film to deposit protective coatings to improve the wear-resistance and corrosion-resistance or decorative lustrous coatings on metal surface.The removal degree of the oxide film significantly affects the adhesion and morphology of the coating.In general,the adhesion between coatings and substrates and morphology of coatings determine the performance of the coating,which also reflects the nucleation and growth process of deposited atoms on the substrate.Thus,it is of great significance in both engineering and science to find out the influence of the removal degree of oxide film on the adhesion and morphology of the coatings.In this paper,we innovatively proposed that the removal degree of oxide film could be characterized by underpotential deposition of A1 in ionic liquid.And on this basis,the influence of removal degree of oxide film on the adhesion and morphology of Al coatings was further explored.Linear sweep voltammetry,open circuit potential,chronoamperometry,potentiostatic method and the pulse potentiostatic method were used to study removal of oxide film on 316L stainless steel ionic liquid.The micro morphologies of the samples were characterized by SEM and AFM.The information of surface composition after underpotential deposition was acquired by XPS.The adhesion of the coatings was tested by pull-off adhesion testing.The results showed that local and uniform dissolution could occurred on 316L stainless steel in 2:1 AICl3-EMIC ionic liquids,which was influenced by oxide film and potential.The 316L stainless steel dissolved uniformly when there were no oxide films.Interestingly,local dissolution occurred at the potential less than 2.5 V,and uniform dissolution occurred at the potential exceeded 2.5 V when there were oxide films on stainless steel substrates.In a constant current dissolution,local and uniform dissolution occurred as the potential swept from low to high.During the removal process of the oxide films,the first step was the breakdown of the oxide films,which depended on the anodic dissolution potential.Then the general breakdown occurred when the potential exceeded 2.5 V.The general breakdown occurred at many positions across the whole surface instantaneously,followed by a planar dissolution pattern.The oxide films were not removed from the surfaces of substrates after the general breakdown until the electrolyte was stirred.Al can be under:potential deposited on 316L stainless steel where there was no oxide film.The deposited electric quantity increased when the removal degree of oxide films increased.By comparing the deposited electric quantity when the oxide film is completely and partially removed,the removal degree of oxide film could be measured,which illustrates the removal rate.The removal rate of oxide film had great influence on the adhesion of A1 coating.When the removal rate was 0%,the adhesion of A1 coating was very poor.When the removal rate was 10%-60%,the adhesion was slightly improved with the increase of the removal rate,but still not satisfied.The adhesion of A1 coating was quite good when the removal rate exceeded 70%.The removal rate of oxide film had a significant effect on the morphology of A1 coatings.When there were the oxide film on stainless steel substrates,the nuclei of Al is quite few,leading to large grains with heterogeneous and partially-covered surface morphology.When the oxide film was completely removed,the nucleation sites are quite homogeneous,resulting in A1 grains with uniform sizes and coatings with smooth and compact morphologies.When the oxide film was partially removed,the number of abnormal A1 grain growth and holes between grains decreased as the removal rate increases.The influence of removal rates on A1 coatings becomes more obvious when the coatings get thinner.As a result,it is of crucial to assure the complete removal of oxide films to make uniform and defect-free thin Al coatings. |
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