Study On The Morphological Evolution And Electrochemical Behavior Of The Initial Stage Of Micro-arc Oxidation Coating Of Pure Aluminum

The micro-arc oxidation(MAO)process includes anodizing stage,micro-arc oxidation stage and arc discharge stage.As the starting point of the micro-arc oxide coating growth,it is of great significance to study the formation-growth of the anodic oxide coating during the micro-arc oxidation process and its influence on the subsequent establishment of plasma discharge and the formation of the micro-arc oxide coating.This article selects the three most commonly used electrolytes for micro-arc oxidation: silicate(sodium silicate),aluminate(sodium aluminate)and phosphate(sodium phosphate,sodium hexametaphosphate,sodium tripolyphosphate and sodium pyrophosphate)as experimental electrolytes and AA1060 pure aluminum as experimental substrate.The initial phase composition,electrochemical behavior,signals recorded during the experiment and changes in electrolyte properties of MAO are analyzed in different types of electrolytes.And the influence of electrolyte type on coating type,coating growth process,the difference of plasma discharge process and electrochemical behavior of different types of coating,etc.are explored.And on the basis of the research conclusions obtained,taking sodium pyrophosphate as the starting point,the formation mechanism of porous anodic oxidation coating in the MAO anodic oxidation stage in the polyphosphate electrolyte is investigated in detail.Studies have shown that the composition of the electrolyte has a fundamental impact on the type of coating in the anodization stage and the establishment of plasma discharge in the coating.The coating prepared in the silicate and aluminate electrolyte is a dense "barrier" type oxide coating during its anodization stage.This kind of coating is more prone to discharge breakdown,and the transformation from anodized aluminum(AAO)coating to MAO coating is completed in a relatively short time.The coating prepared in the electrolyte of three poly-type phosphates(sodium hexametaphosphate,sodium tripolyphosphate and sodium pyrophosphate)are thick and loose “honeycomb-like”oxide coating in the anodizing stage.This type of coating has been in a hybrid state where AAO coating and MAO coating coexist for a long time.The coating prepared in the single-chain structure phosphate(sodium phosphate)electrolyte always presents a loose "honeycomb-like" coating,but plasma discharge cannot be established under sufficient MAO time,which is mainly related to the fact that the coating is seriously eroded by the electrolyte and cannot reach the critical voltage.There are two reasons why the coating prepared in the polyphosphate electrolyte is in a loose "honeycomb" state.One is that the initial pore nucleus is first formed in weak areas such as substrate impurity accumulation areas or scratches,and coating cracks,and it gradually develops into a tubular structure with smooth edges.Second,the oxygen ions in the oxide coating are oxidized to oxygen during the MAO process and appear as oxygen-filling holes in the coating.The enlargement and rupture of the holes form a tubular structure with rough tube walls and deposits at the openings finally.