| Magnesium alloys have received extensive attention in recent years due to their low density,high specific strength,specific stiffness,and excellent casting properties.In the automotive industry,Mg alloys are considered one of the candidates for the next generation of automotive engines.However,due to the poor corrosion resistance of the Mg alloy,the corrosion problem in the coolant greatly limits its practicality.In response to this problem,this paper first study the corrosion behavior of NZ30K Mg alloy in FD-1 commercial coolant,and then prepares a new type of phytic acid coating with high corrosion resistance on NZ30K Mg alloy,then a silane coating layer was further prepared on the new phytic acid conversion coating by heat curing to improve the protection life.The main conclusions are shown as follow:Firstly,the corrosion area of NZ30K Mg alloy in FD-1 multi-effect coolant is relatively large,and the corrosion progresses in the depth direction;the corrosion rate of NZ30K alloy in FD-1 coolant is 6.818mg/(cm2·week),which is bigger than the corrosion rate of 0.67mg/(cm2·week),the standard of Mg alloy engine material.Secondly,the novel phytic acid conversion coating layer prepared in this paper is even and flat,with some microcracks and small particle agglomerates on the surface.The optimum process obtained by orthogonal test are:Mn(H2PO4)2 25g/L,phytic acid 10g/L,reaction temperature 95°C,reaction time 10min.Increasing the reaction temperature will reduce the film thickness,and decrease the number of cracks in the coating.The addition of Mn2+ion can greatly reduce the width of the film crack and increase the flatness of the film layer.When Mn2+ion was absent,the film layer becomes chopped.When Mn2+ion was present,the film layer change to uniform.the self-corrosion current density NZ30K Mg alloy before and after the phytic acid coating was decreased from 9.279?A/cm2 to0.337?A/cm2,the protection efficiency of Mg alloy is 96.37%;Thirdly,the curing temperature and time of the phytic acid-silane composite coating can significantly affect the corrosion resistance of the coating.When the curing temperature is 120°C and the curing time is 30min,the composite coating has the best corrosion resistance,the self-corrosion current of the composite coating is 0.14×10-4?A/cm2.It can reduce the corrosion current of NZ30K Mg alloy in FD-1 commercial coolant by 6 orders of magnitude,greatly improving the corrosion resistance of Mg alloy in FD-1 multi-effect coolant.performance.By comparing the VTES film and the phytic acid-VTES composite coating in FD-1 coolant,it can be found that the phytic acid chemical conversion film can prolong the protective life of the VTES film layer as a pretreatment,and the single VTES film can protect the Mg alloy from corrosion at 576h,while the composite coating increasing the protection time of 120h to 696h. |
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