The Study On The Dependence Of Corrosion Process Of Magnesium Anodic Films On PH

Magnesium and its alloys have many excellent properties, however, magnesiumand its alloys have several drawbacks, including poor resistance to corrosion, whichseverely limit their extensive applications in many industry fields. For the sake ofimproving corrosion resistance of magnesium alloys, Magnesium alloys are usuallyrequired surface treatment, in numerous kinds of the magnesium alloy surface treatmentmethods, the anodic oxidation method is performance and market competitivenesswhich is one of the most important methods. In this paper, anodic oxidation film onAZ31, AZ91and AM60series magnesium alloys was investigated. The Mott-Schottkyanalysis with AC impedance was used to study magnesium alloys anodic film corrosionmechanism, provided theoretical basis to improve the corrosion resistance ofmagnesium alloy.In3.5%NaCl solution which pH=5, the carrier density （ND） of AZ31, AZ91andAM60increased from10¹⁷to1020cm³, the flat-band potential of anodic oxidation filmswas ranged from-1.54V to-1.77V, the anodic films showed a N type semiconductorproperties, in pH=53.5%NaCl solution, the carrier density changes rapidly, flat bandpotential is positive, corrosion rate is faster, compared to which in pH=6.8and pH=123.5%NaCl solution. In the soaking time at1h, the low frequency impedance value ofAZ31, AZ91and AM60anodic oxidation films was maximum, before12h, the lowfrequency impedance value decreased, the corrosion resistance the films decreased,immersed at12h, the low frequency impedance values appeared repeatedly trend,related to the process of oxidation corrosion product coverage dissolving, withimmersion time increased, the corrosion rate increased.In3.5%NaCl solution which pH=6.8,AZ31, AZ91and AM60magnesium anodicfilms showed a N type semiconductor properties, the carrier density （ND） increasedfrom10¹⁷to10¹⁹cm³, magnesium alloy carrier density keeps on10¹⁹cm³, immersed inthe prophase, the carrier density of magnesium anodic films was two orders ofmagnitude smaller than that of the substrate, the flat-band potential of anodic oxidationfilm has ranged from-1.52V to-1.83V, flat band potential of magnesium alloy rangefrom-1.44V to1.62V, the flat band potential of anodic films was negative shiftcompared to that of magnesium alloy, oxidation corrosion resistance was better, theprocess of corrosion damage of anodic oxide film has included four stages, which are self-repair period, induction period of pitting corrosion, pitting corrosion and quickcorrosion period respectively.In3.5%NaCl solution which pH=12, the carrier density （ND） increased from10¹⁷to10¹⁹cm³, the flat-band potential of anodic oxidation film was ranged from-1.60V to-1.84V, the anodic films showed a N type semiconductor properties, in pH=12alkalineenvironment, the corrosion resistance of the film was the best. The AZ31, AZ91andAM60oxide films in pH=12alkaline environment, the carrier density isAZ91AM60AZ31, the flat band potential is AZ91AM60AZ31, the corrosionresistance is AZ91AM60AZ31. At12h, the flat band potential is the most negative.AZ31, AZ91and AM60magnesium alloys and their anodic films were analyzedby X-ray diffraction （XRD）. AZ31Magnesium alloy was composed of α-Mg, AZ91andAM60Magnesium alloys were composed of α-Mg and β-Mg¹⁷Al12, and the maincomponent of anodic films included MgO and Mg2SiO4. Al, Mn and other elementswere involved in films.Different links of PASP molecules were calculated using Quantum chemistrymethod, PASP molecular charge were mainly distributed in the N, O atoms, with theincrease of molecular chain,,, increased gradually,△E, S decreased gradually, thecorrosion resistance increased with the increase of molecular chain.