| Magnesium alloys have a number of desirable features, including low density, high strength-to-weight ratio, relatively high thermal and electrical conductivity and reclamation property. It was applied widely in automobile industry and electronic products and therefore it was recognized as the green engineering materials in 21st century. However, Magnesium alloy is very active and susceptible to corrosion when exposed in practical application environment. The limited use of magnesium alloys in engineering applications results mainly from its poor corrosion resistance.An excellent anti-corrosion performance conversion coating was obtained on the surface of AM60 and AZ91 magnesium alloys from manganese phosphate bath by adding an accelerant towards the treating solution. Corrosion behavior of the film in 3.5%NaCl was investigated by electrochemical methods. Surface morphology of film was observed using SEM. Composition and content of films were analyzed by EDAX. The formation mechanism of manganese phosphate conversion film was also studied.The best technological conditions were confirmed on the surface of AM60 in the condition of adding accelerant A and B respectively (A was fluorid, B was the mixture of fluorid and organic acid as well as calcium salt). The electrochemical methods tests showed that corrosion potential of the film increased 1.5 V(SCE) comparing with that of basic bath and the corrosion current density decreased obviously comparing with that of the AM60 magnesium alloy substrate, which demonstrated that the film presented outstanding corrosion resistance property, and the film could provide magnesium alloys effectively. EDAX analysis result showed that conversion coating formed by adding A addictive was mainly consisted of three variety elements of oxygen, phosphor and manganese. And the conversion coating deposited by adding B addictive was mainly consisted of four kinds of elements of oxygen, phosphor, manganese and calcium. The potential versus time curve of film fomation process on AM60 magnesium alloys was measured by electrochemical method. It was found that conversion coating formation included three steps which were the initial stages of film formation, dissolution of the metal matrix and the main stage of film formation respectively.The best technological conditions were confirmed on the surface of AZ91 in the condition of adding accelerant C and D respectively (C was organic acid, D was the mixture of fluorid and organic acid). The electrochemical methods tests showed that corrosion potential of the film increased 1.3 V(SCE) comparing with that of basic bath and the corrosion current density decreased obviously comparing with that of the AZ91 magnesium alloy substrate, and the corrosion process was controlled by electrochemical reaction.EDAX analysis result showed that the conversion coating formed by adding different addictive(C and D) was mainly consisted of the same three variety elements of oxygen, phosphor and manganese.The surface morphology of the growth process for the conversion coating formed by adding C addictive was observed using SEM. It was found that initial deposition occurred in the interior of a phase, only a small quantity of deposition deposited onβphase. The crystal grains growed gradually with time, and the surface was covered completely after about 20 minutes.
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