Study On The Corrosion Resistance Of Composite Coating By Micro-arc Oxidation And Electrophoresis On Cast Iron

Cast iron HT250 is the main material for the manufacture of medium speed engine pistons in ships.The piston top is vulnerable to corrosion due to the harsh working environment,which reduces the operating reliability of the engine and increases the cost of equipment maintenance and operation.Especially,the top of the piston of methanol engine is easily corroded by formic acid which is the combustion product of methanol fuel,which limits the further application of methanol engine.Therefore,improving the corrosion resistance of cast iron has important practical significance.Micro-arc oxidation and electrophoresis were used in this paper,a porous corrosion resistant ceramic coating was prepared on the surface of cast iron by micro-arc oxidation,Si O₂ electrophoretic anticorrosion coating was prepared on the surface of ceramic layer by electrophoresis technique,the composite anticorrosive coating was formed by mechanical interlocking of the coatings prepared by combined treatment.The research contents and conclusions are as follows:（1）Research on micro-arc oxidation parameters on cast iron.Using sodium silicate and sodium carbonate solutions as electrolytes,the influences of positive voltage and oxidation time on the thickness,roughness,surface morphology and cross-section morphology of the coating were investigated.The growth process,chemical composition and corrosion resistance of cast iron MAO coating were studied.The results show that with the increase of positive voltage and oxidation time,the thickness and roughness of the coating gradually increase.The coating presents the change rule of looseness-densification-looseness.The coating prepared with 220V and 2.5 min has high density and good bonding state with substrate.The MAO coating is mainly composed of Fe,Si and O and exists in amorphous form.The corrosion potential of cast iron MAO samples prepared by the optimal parameter is positive shifted 0.13 V,and the corrosion current density is decreased by one order of magnitude.（2）Preparation of Si O₂ electrophoretic deposition coating and sealing performance exploration.Using Si O₂ solution and electrophoretic paint as the electrophoresis solution,the effect of electrophoretic sealing and the bonding state of the composite coating were measured by the change of current during the reaction process,the morphology and the element content at the defect.The results show that the reaction current decreased from 1.5 A to 0 A during the electrophoretic process,and the defects of the micro-arc oxide coating were blocked by Si O₂colloidal particles,and the content of C and Si elements increased significantly.The electrophoretic coating and micro-arc oxidation coating form a staggered mechanical bonding.（3）Study on corrosion resistance of composite coating.Electrochemical corrosion test,formic acid soak test,high temperature oxidation resistance and heat shock resistance test were carried out on the samples treated by micro-arc oxidation and electrophoresis.The results show that,after the composite treatment,the corrosion potential of the sample is positive shifted by0.5 V,the corrosion current density is decreased by 3 orders of magnitude,and the polarization resistance is increased by 294.7 times.After soaking in 5%formic acid for 8 h,the surface of the sample is intact,and the weight loss of the sample is almost zero.After repeated high temperature for 2 h and 350℃for 6 times,no cracking occurs in the coating.