Preparation And Process Optimization Of Micro-arc Oxidation Coating With High Corrosion Resistance On Rivet Steel Of Automobile

In this paper,alumina coating with high corrosion resistance was prepared on the surface of low carbon steel for automobile rivets by micro-arc oxidation（MAO）technique,which reduced the corrosion tendency of substrate itself and coupling with magnesium alloy.Within the scope of this article first to select the optimum substrate roughness and electric parameters in the experiment,then on this basis,the use of sodium molybdate and alpha alumina as additive to the electrolyte has carried on the optimized step by step,finally was prepared on the surface of mild steel in car rivet high corrosion resistant alpha alumina coating,and the film morphology characterization and performance testing,the results showed that:1.NaAlO₂and NaOH were used as electrolytes to study the effects of surface roughness MAO ceramic film microstructure and properties of 10B21 steel substrates.Although different roughness had little effect on the phase composition,microhardness and bonding strength of MAO film,it had great influence on the film thickness and surface quality,which affected its corrosion resistance.In the experimental range,the corrosion resistance of the samples polished to 1500#sandpaper is the best and the roughness of the matrix is 17.038μm.2.The effects of voltage,frequency and duty cycle on the growth of MAO film were investigated by single variable method with the optimal roughness samples and NaAlO₂and NaOH electrolyte formulations.The results showed that with the increase of voltage,low frequency and duty ratio increases,the growth rate of the coating is also faster,thickening of coating but more porous,soluble film tend to be larger.Finally,a group of optimal electrical parameters within the experimental range were obtained through comparison of various characterization means:forward voltage of500V,frequency of 1500Hz,forward duty cycle of 30%.3.On the basis of the optimized substrate roughness and electrical parameters,sodium molybdate and alpha alumina particles were added into NaAlO₂and NaOH based electrolytes for step-by-step optimization,and the high corrosion resistance alumina coating was finally prepared.Experimental results show that:the addition of sodium molybdate,remove iron oxide formed in micro-arc oxidation,increase of alumina content in the membrane layer and make preparation of micro-arc oxidation film performance better.The addition ofα-Al₂O₃particles not only provides the core of heterogeneous nucleation for the formation of alumina ceramic layer,but also plays the role of hole sealing,which greatly increases the thickness of the film and improves the performance.Among the low carbon steel ceramics with boron in E1,E2and E3 prepared in the three electrolytes,the E3 film has the best comprehensive properties,the thickness is 31.43μm,the surface mass is uniform and dense,the film contains highα-Al₂O₃,the binding strength is 138.4MPa.The corrosion potential is increased to-0.3445 V and the corrosion current density is decreased to 7.9μA/cm².After four days of neutral salt spray contact corrosion test,the corrosion rate of AZ91decreased from 42.59g·mm^-2·d^-1to 7.63 g·mm^-2·d^-1,and the galvanic corrosion tendency of low carbon steel and AZ91 decreased.