The Electrochemical Behavior Of Anodic Aluminum Oxide Film Growth Mechanism Of The Pits

Firstly,the electrochemical behavior of anodic aluminium film formed on L2 in neutral NaCl solution is investigated. According to laboratory work, it is found that the corrosion-resistant behavior of anodic aliminium film increases with longer anodization time. The corrosion resistance of film begins to decline when temperature of NaCl solution increases.With temperature of NaCl solution increasing ,the open circuit potential of anodic aluminium film increases firstly up to a maximum point and then decreases .When anodic aluminium film is under high polarization potential of about l0V/sce and high solution temperature , serious corrosion happens. Short pre-immersion treatment of aluminium anodic film in NaCl .solution will strengthen its corrosion-resistant behavior, when pre-immersion time in NaCl solution inceeases , the corrosion resistance begins to decrease.Secondly, the pitting behaviour of L2 anodic film in NaCl solution is mainly investigated .In order to produce corrosion pit in anodic polarization in NaCl solution, the pre-treatment of anodic aluminium film in lower cathodic polarization potential is operated before anodic polarization in NaCl solution. In the process of anodic polarization , anodic aluminium film is penetrated at a high potential and pitting begins to produce and develop. Then potential scanning turns to cathodic direction. In scanning process, the mechanism of pitting development is deeply investigated. In the process of corrosion pit development, the evolution of hydrogen from pit is found and so leads to current fluctuation . the potential for hydrogen evolution is very high and does not conform to common sense .In order to elucidate the unusual phenomenon, we formulate the special mechanism about hydrogen evolution in pitting of anodic aluminum film.,i.e electron translates from Al+ to H+. According to SEM study , we find the pit of anodic aluminum film is hemispherical and is covered by anodic film, so in the process of pitting development it is under occluded situation. In the central point of anodic aluminum film covering pit ,there is a small pit and its diameter is smaller than that of corrosion pit. The small pit constitutes the diffusion tunnel of pitting and governs the form of pit.Lastly, the electrochemical behavior of LY12 aluminum alloy anodic film is studied. The corrosion-resistant behavoir of LY12 anodic film is lower than that of L2 anodic film. According to SEM study it is found that the face of LY12 anodic film is very rough ,and under the same anodization procedure, the thickness of LY12 anodic film is smaller than that of L2 anodic film, this is because of evolution of oxygen in process of anodization of LY12,so current efficiency for anodic aluminum film development decreases.In addition, it is found that the depth of pit for LY12 covered with anodic aluminum film is shallow.