Preparation And Properties Of Mo/Mn Composite Conversion Coating On AZ31B Magnesium Alloy Surface

Magnesium alloys possess excellent metal characteristics such as low density,high specific strength,and light weight,which have broad applications in aerospace,aviation,marine,automotive,3C electronics,and other fields.However,magnesium has strong chemical activity and is prone to corrosion in the usage environment,which severely limits the application of magnesium and its alloys in the industrial field.Chemical conversion is one of the most commonly used methods for surface corrosion protection of magnesium alloys.The traditional chromate conversion coating can effectively improve the corrosion resistance of the magnesium alloy surface,but the conversion solution and coating contain carcinogenic hexavalent chromium,which is prohibited from use.Therefore,developing environmentally friendly conversion technologies has become a hot topic in the research of chemical conversion treatment for magnesium alloy surfaces.This article used green environmentally-friendly（NH?）?Mo O?as the main salt of conversion coating,preparing a chromium-free and Mo/Mn composite conversion coating（Mo-Mn-P-TA conversion coating）.The process formulation of Mo-Mn-P-TA conversion coating was optimized through orthogonal experiment,single factor experiment,resistance drip experiment,electrochemical test experiment and confirmed the Mo-Mn-P-TA solution system is:10～30 g/L ammonium molybdate（（NH?）?Mo O?）,1～5 g/L sodium dihydrogen phosphate（Na H₂PO?）,1～2 g/L potassium permanganate（KMn O₄）,1～3 g/L tartaric acid（TA）,conversion temperature is 25～30°C,conversion time is 0～20 minutes with p H value of 3～4.Based on Mo-Mn-P-TA conversion process,the substitution of sodium dihydrogen phosphate for sodium paranoid acid,successfully preparing a weakly acidic composite conversion coating（Mo-Mn-V-TA conversion coating）.The Mo-Mn-V-TA conversion solution system is:20～30 g/L ammonium molybdate（（NH?）?Mo O?）,1～2 g/L sodium metavanadate（Na VO₃）,1～2g/L potassium permanganate（KMn O₄）,1～3 g/L tartaric acid（TA）,conversion temperature is25～30°C,conversion time is 0～25 minutes with p H value of 5.5～6.5.Corrosion-resostant experimental results of Mo-Mn-P-TA conversion system showed that the best Mo-Mn-P-TA conversion coating’s resistance drop time was 65 seconds greater than the substrate;the corrosion current density of the best Mo-Mn-P-TA conversion coating was reduced by two quantitative grades compared to the substrate,and the coating resistance R_Cwas approximately four times as high as the substrate.Through FT-IR and XPS analysis,it was found that the main components of the Mo-Mn-P-TA conversion coating are Mo O₂,Mo O₃,Mn O₂,metal phosphate,Mg（OH）₂,Mg O and metal complexes.The experiment results of Mo-Mn-V-TA conversion system showed that the best Mo-Mn-V-TA conversion coating has increased the resistance drop time by 67 seconds compared to the substrate;the corrosion current density of the best Mo-Mn-V-TA conversion coating was reduced by two degrees compared with the substrate,and the coating resistance R_C was approximately four times the substrate.By analyzing the components of the Mo-Mn-V-TA conversion coating,it was known that the conversion coating is composed mainly of Mg（OH）₂,Mg O,Mn O₂,Mo O₂,Mo O₃,VO₂,V₂O₅ and metal complexes（Mo,Mn,V,Mg,etc.）.The addition of tartaric acid causes metal ions and carboxylic acid group of tartaric acid reacts to form the absorption of metals in the conversion coating.