Study On Corrosion Of Pure Mg, Plasma Electrolytic Oxidation Film And Composite Coating Of Mg Alloy

Magnesium and its alloys have a number of applications in automobile and computer parts, aerospace components, mobile phones, and household equipment etc for their excellent physical and mechanical properties, such as low density, high strength and high stiffness. Unfortunately, magnesium and its alloys are highly susceptible to corrosion, which greatly restricts their further application, especially in some harsh service conditions. Therefore, it is very important to research the corrosion mechanism of Mg alloy and explore the surface modification process to prevent corrosion of magnesium alloys.The influence of surface film structure onto the potentiodynamic polarization behaviour and the electrochemical impedance behaviour was studied in this paper. The dynamic discussion and numerical simulation of EIS was carried out according to the integrated mechanism of magnesium which was advanced by Guangling Song. In recent years, the plasma electrolytic oxidation technique had a great adcance which was developed based on the anodic oxidation. The affect of the electrolyte composition onto the film properties was studied on the base of a bipolar pulse power supply. The process of electroless plating on plasma electrolytic oxidation film was investigated in which the nickel sulfate was used as the main salt. The properties of the composite coating were tested in the paper also.Potentiodynamic polarization curves of pure Mg were tested in 0.1mol/LNa3BO3 solution with different pH value. When the pH of Na3BO3 solution was 7, the self-corrosion potential and the self-corrosion current density was-1.51V and 5.63×10-4 A/cm2 separately. After the pH of solution raised up to 11, the self-corrosion potential imcreased to -1.42V, the self-corrosion current density decreased to 3.99×10-4 A/cm2. The corrosion process of pure Mg in silicate sodium solution was influenced by the anodic polarization and cathodic polarization. The content of Na3BO3 influenced the polarization characteristics little.The EIS of pure Mg in silicate sodium with pH7 was different in immersion period. The Nyquist plots showed two parts of capacitive loops and one inductive loop after the Mg immersed for 1h. The inductive loop of Nyquist plots disappeared after 5 hours immersion. One capacitive loop and one inductive loop formed the Nyquist plots of pure Mg in silicate solution for 12 hours. The norminized Nyquist plots of different immersion time were different which meaned that the corrosion mechanisms were different. The normizied EIS of pure Mg in different contents of Na3BO3 changed little. The nominized EIS plots had litted change according to immersion time. These results showed that the corrosion mechanism was same in different contents of Na3BO3 and different immersion time.On the base of formed tested results and the integrated corrosion mechanism of Guangling Song, the dynamic behaviour of pure Mg in Na3BO3 solution with pH value of 11 was discussed in the paper. The pure Mg was coated by an discontinuous MgO and uniform Mg(OH)2 layer in the pH11 silicate sodium solution. The corrosion process was controlled by the reaction of Mg to Mg2+ on the surface of pure Mg without MgO area. It can be described as the reaction of two electrons in two successive electrochemical steps. Firstly, the Mg changed to the intermediate state Mgad+ on the pure Mg sites which were not coated by MgO. Secondly is the transformation of Mgad+ to Mg2+. The second step had two parallel paths, one was a chemical path and give rise to the NDE and the other is electrochemical leading to Mg2+.A bipolar pulse power supply was used to prepare plasma electrolytic oxidation films on magnesium alloy AZ91D. The main constutuents of films prepared in electrolyte with 15～20g/L Na2SiO3 and 5g/L KOH were MgO and MgSiO3. After prepared a continuous and polorious coating, magnesium alloy AZ91D had less surface area which exposed to corrosive medium with 3.5wt.% NaCl. After the phosphate was soluted into the electrolyte, the amount and the size of pores was decreased which made the self-corrosion potential positive and the self-corrosion current density decreased by one magnitude.The fluorid was soluted into the oxidation electrolyte to prepare the oxidation film. The plasma electrolytic oxidation film had a nice and uniform microstructure which can be divided into a transition layer and a compact layer. The loose layer can not been observed. The F- can be detected in the plasma electrolytic oxidation film because the affect of the reverse pulse. The composition of film were MgF,KMgF3 and some MgO by the high temperature and high pressure effect in the plasma oxidation process. The density of film was enhanced by F-. The self-corrosion potential of PEO film increased to -1.4V. The AZ91D alloy with PEO film had obvious passive behaviour. The EIS results showed that the corrosion resistance of the AZ91D alloy was improved by the PEO film greatly because the flurid was dissolved into the silicate sodium solution. The compact film can prevent the transmittsion of corrosion medium and product across the film onto the magnesium substrate. After the immersion of 250 hours, the inductive loop of EIS appeared which mean the pitting corrosion of magnesium substrate. The coating failed after 300 hours immersion in 3.5wt.% NaCl solution.The electroless plating process on the PEO film prepared in the silicate sodium solution was studied by experiments. In order to decrease the corrosion of magnesium substrate in the plating process, ammonium bifluoride was dissolved into the plating solution and the content was determined by comparitive experiments. The composition of plating solution, pH value and temperature was studied by the experiments. The electroless plating layer grew from the pores of the PEO film and got into a uniform layer which made the electroless plating layer and the PEO film have a mechanical occlude state. The electroless layer was amorphous structure and had a traditional cell surface structure. The adhesion of the composite coating was nice according to the thermal shock experiments.The self-corrosion potential of the composite coating on magnesium alloy AZ91D was-0.4 VSCE- The passive behaviour was observed in the experiment of potentiodynamic polarization test. The EIS results showed that magnesium alloy AZ91D with composite coating had excellent corrosion resistance compared with the bare magnesium alloy. In the earlier and intermediate immersion period, the impedance increased as the immersion time was prolonged because the composite coating can prevent the transmitting of corrosion medium and products. The micro crack introduced the localized corrosion of magnesium substrate which formed because the accumulation of corrosion product in the coating. Magnesium substrate lost the protection of the composite coating after 300 hours immersion in 3.5wt.% NaCl solution.