Process And Coating Characterization In Plasma Electrolytic Oxidation Of Magnesium Alloy In Two Group Electrolytes

Magnesium is the lightest of all metals used as the basis for constructional alloys, which is regarded as one of the most prospective metals in 21st century. Unfortunately, its poor corrosion resistance is the one of the main properties that has hindered its widespread use in many fields, particularly for outdoor applications. So it is valuable to strengthen its anti-corrosion performance through suitable surface treatment methods. Plasma electrolytic oxidation (PEO) is a new electrochemical surface treatment technology developed on the basis of the conventional anodizing, which employed higher energy density. The surface of valve metals, such as Al, Mg, Ti etc., can convert to the ceramic-type coatings in situ under the conditions of plasma chemical, electrochemical, thermal-chemical effects and their anti-corrosion, wear-resisting and electrical-insulating performances can be improved greatly. At present, there are many studies mainly for aluminium and titanium that centered on performance evaluation, characterization and testing aspects of PEO coatings. The in-depth insights into the process fundamentals, consequent process-property relationships as well as the mechanisms of the coating formation and corrosion failure are lacked, which often hold back further technological developments of PEO and further applications of magnesium and its alloys. Based on the developed PEO process by our own endeavor, this paper systematically studied the relations among the main parameters involved in PEO process. The phase composition and formation mechanism of PEO coatings were discussed. According to the correlations between the appearance of microdischarge population and the morphology of PEO coatings, the growth mode of the coatings under sparking was established. The anti-corrosion performance and the corrosion mechanism of PEO coatings were studied in detail. Further studies included the effects of PEO on mechanical properties and high temperature oxidation-resisting of the metal substrate. These achievements can widen the application range of magnesium alloys and deepen the understandings about the fundamentals of PEO process. The main research results are the following: Five factors were proposed for electrolyte determination through systematic analysis for the properties of electrolytic constituents used in different countries. The final two better type recipes were determined according to the performances, such as anti-corrosion, microhardness, and process stability etc. One is the phosphate recipe and its constituent was 10⁶0g/L sodium hexametahposphate ((NaPO₃)₆), 6³0g/L potassium hydroxide (KOH), 5²0g/L fluoride and suitable stabilizer; the other is the silicate recipe and its constituent was 15⁶0g/L sodium silicate (Na2SiO3), 4³0g/L potassium hydroxide (KOH), 5²0g/L fluoride and suitable stabilizer. The results of polarization curve/total immersion/salt spray experiments all showed that both P-film and Si-film could enhance the corrosion resistance of AZ91D alloy significantly. An equivalent circuit model was established for PEO system and a corresponding equation was derived for describing the intrinsic relations among the main parameters involved in PEO process. This equation has the following features compared with others: the physical meaning is explicit; the calculation procedure is simple and the applicability is well. Under the conditions of constant voltage and constant current, the discussions about this equation demonstrated that the controlling mode of constant current was superior to the one of constant voltage. Moreover, the formation mode of PEO coating through step-down current method was firstly presented. The step-down current can regulate the magnitude of electric current in different stages, which affects the status of the microdischarge on the surface of the electrode and is helpful to acquire the optimized and dense PEO coatings. New systemic equation and step-down current method can greatly facilitate the further improvements of the PEO power supply and the actual operation. The XRD patterns of PEO coatings formed under different processing time in two group electrolytes were labeled individually and quantitative estimations for every phase were achieved through the simplified K value method. The results showed that the mass percent of the phase composition was 33.5%Mg, 35.6%MgO, 30.9%MgAl2O4 for the P-film and 41%MgO and 59%Mg2SiO4 for the Si-film; the content of MgO in PEO coatings increased with the longer processing time. By virtue of the model of cylinder discharge channel, a temperature field distribution inside the discharge channel was established, which could approximately gave a theory basis for the phase formation, such as MgO,MgA2O4,Mg2SiO4 etc. At the same time, the chemical reaction steps for the MgAl2O4, Mg2SiO4 phase were proposed. The results and theory analysis of phase formation give an important example for the potential applications of PEO process. The essence of PEO lies in the phenomenon of microdischarge. The appearance of microdischarge population experienced apparent changes in size, spatial density and color and could be roughly divided into three stages. Firstly a large number of sparks were in the form of moving discrete white microdischarges. Then, with the voltage increasing, some of microdischarges became yellow, larger and slower moving. Secondly the microdischarge became yellow completely and the microdischarge spatial density decreased apparently. In the end, the micro-arc was seen on the certain areas and the maximum cross-section area of a single microdicharge increased. The causes for these changes may be related with the evolution of the type and quantity of the disintegrated gas bubbles formed in the interface between the electrolyte and substrate, that is, from thehydrogen plasma in the initial stage to oxygen plasma in the terminal one. Corresponding to the transition of microdischarge, the morphology of PEO also experienced distinct changes. Firstly the coating surface appeared very small and intensive micropores with net-like microcracks. Then a uniform cellular structure of the oxide ceramic layer appeared. The diameter of the micropores became larger than that in the first stage. At the same time, the net-like fine microcracks also existed. In the stage of the arc discharge, the partial micropores began to be sealed and there existed a track of plasma gas spout. The further studies revealed a barrier layer with a polycrystalline and combined amorphous structure. Moreover, there clearly existed a fluoride-enriched area in the barrier layer. A non-uniform growth mode of PEO coatings was proposed and the trap of the gas bubbles during the course of coating growth was the main cause which the PEO coating contained a lot of pores. This paper studied the corrosion failure mechanism of the PEO coatings in 5wt.% NaCl aqueous solution by use of OCP and EIS measurement technologies. The results could be concluded the following four continuous steps: the penetration of corrosive medium towards the pores; the pores filling with the corrosion products; the barrier layer adjacent the bottom side of pores corroded progressively; the final failure of the barrier in certain spots and the corrosion process tended to a steady state. The tensile properties of magnesium alloy with PEO treatment were determined preliminarily, then the fractography and morphology of the coating were observed using SEM. The results showed that the tensile properties of the substrate experienced slightly influence after it had undergone PEO treatment. The thermal stability of stripped powder of the two type coatings exhibited rather well. When the samples with PEO coatings exposed in 410℃at atmosphere with different time, the area of thermal corrosion spots tended to be increased. The new developed PEO process has been applied successfully to die-castings of magnesium alloy. The process was stable and the subsequent coatings were uniform and dense. The controlling mode and the process parameters applied in these experiments are great valuable to the subsequent production.