Investigation On Phosphate Chemical Conversion Coating About Mg Alloy Based On Interfacial Reaction Control

The surface protection process can achieve the purpose of corrosion protection by isolating Mg alloy from corrosive medium.For the advantages of simple process and low cost,the chemical conversion technology is widely used.Although chromate conversion process is relatively mature in those conversion technologies,it is greatly limited in application.Because it will pollute environment for heavy metal ions contained.It has been widely studied for phosphate conversion technology based on advantage of environmental protection and non-toxic,which is a deposition coating when the ion concentration reaches the critical supersaturation and is controlled by the"metal/solution" interface reaction.In this paper,the main problems existing in the field of conversion coating were studied,starting from the interface reaction control of"metal/solution".The research contents are as follows:1.On the bath side of the "metal/solution" interface,the previous bath design idea is changed,and the design principle of the bath with chromate-and crack-free is proposed.The designed bath contains two types of solutes.The first type of solute can react with the Mg alloy to raise the pH value of the "metal/solution" interface,thus triggering the nucleation of the second type of solute.The second type of solute is in the state of supersaturation,and it is a physical crystallization process to deposition.Even if the generated coating isolates the Mg alloy from the conversion bath,it can continue to be deposited and thickened.The coating formation behavior is controlled by adjusting TA/pH,which is essentially by increasing the Jsp of the second solute so that it is as close as possible to JC,sp(about 1×10-3).The closer to JC,sp,the higher the nucleation density,the thicker and denser the coating.When designing the conversion bath,adding proper amount of complexing agent can establish Mn2+ buffer pair and expand the adjustment range of TA/pH.The adding of an appropriate amount of oxidant can increase the electrochemical reaction rate of the coating formation process and promote the dissolution of Mg matrix.The adding of appropriate amount of catalyst can increase the concentration of cations at the metal/solution interface and increase the deposition rate.The essence of chemical conversion coating deposition is that the solute at the "metal/solution" interface reaches the critical supersaturation state.The calculation of supersaturation and the determination of critical supersaturation can be used to guide the design of conversion bath,so that the corrosion resistance of PCCs can be significantly improved.2.Excessively high supersaturation of the second type of solute will reduce the effective components of the conversion bath,so it has limitations to bath design.The surface state of the Mg alloy at the initial conversion stage determines the quality of the coating at the later stage.Therefore,it is pre-nucleated to control the surface state of the Mg alloy on the "metal/solution" interface.The pretreatment bath for pre-nucleation requires a high supersaturation of the second solute,so the design is carried out by appropriately increasing the concentration of the second solute and the pH of the bath.The designed bath possesses a high supersaturation(Jsp?1.35×10-3)and will form MnHPO4·3H2O clusters,which need to be prepared on the spot.The crystal nuclei generated by the conversion process are closely bonded with the Mg alloy matrix,which can avoid hydrogen evolution blowing away,and the formed crystal nuclei can provide a growth site for PCCs.Even in the low supersaturated reference phosphating bath(Jsp?2.83×10-6),the nuclei of MnHPO4·3H2O can grow rapidly and merge into a more compact coating,which can avoid the problem of reducing the active component caused by the use of high supersaturated conversion bath.Due to the high density of N-MnPCC,its anode and cathode effect is significantly inhibited,so the corrosion resistance is significantly improved.3.The heterogeneous electrochemical properties of the Mg alloy surface will lead to the heterogeneous nucleation and the heterogeneous growth of the coating,resulting in defects.The "nuclei rearrangement" treatment can be carried out on the"metal/solution" interface,so that the obtained crystal nuclei after pre-nucleation can be distributed on Mg alloy surface uniformly.The pretreatment was carried out with reference phosphating bath with low susaturation(Jsp?2.83×10-6),and the the "nuclei rearrangement" treatment was carried out by evenly rubbing Mg alloy surface.AFM observation showed that the surface grain was refined and the nucleation density increased,and the crystal nuclei were uniformly distributed on ? and ? phases.After 10 s conversion process,the a and ? phases were uniformly distributed with ellipsoidal crystal nuclei with a particle size of about 500 nm.Combined with EDS results,the crystal nucleus composition was deduced to be MnHPO4·3H2O.The "nuclei rearrangement" treatment can reduce the surface energy of the Mg alloy,which makes the coating tend to spread out and grow,and the crystal nucleus can grow rapidly and form uniform and compact PCCs eventually.During the corrosion process,R-MnPCC appears"self-repair" phenomenon,because R-MnPCC is uniform and compact,the anode and cathode effect is inhibited,and the corrosion products accumulate at the defect,which hinders the corrosion of the corrosion medium to the Mg alloy matrix.Therefore,R-MnPCC shows the phenomenon of "self-repair" which temporarily improves the corrosion resistance.4.The heterogeneous surface electrochemical properties of Mg alloys not only lead to heterogeneous nucleation,but also forming corrosion microcouple during the corrosion process,which accelerates peeling off of the coating.Impurities on the metal side can be removed by "surface purification" pretreatment.The designed "surface purification" pretreatment bath is of the following characteristics:it is alkaline of bath,which can prevent the Mg alloy matrix from being dissolved.Adding oxidant(NO3-)that can increase the oxidation potential of the bath system and promote the dissolution of metal impurities.Addition complexing agent(EDTA4-)that can reduce the equilibrium potential of impurity metal,promote the dissolution and prevent redeposition of them.The concentration and pH value of the treatment bath can be determined according to the predominance area diagram calculated by thermodynamics.After the "surface purification" treatment,the Alx(Mn,Fe)y cathode phase in the Mg alloy AZ91-T4 is removed,the surface electrochemical properties become relatively uniform,the micro-galvanic effect is reduced,and the nucleation growth rate of SP-MnPCC during the conversion process is significantly reduced.After the removal of Alx(Mn,Fe)y phase,the pits were covered by coating,and the coating was relatively uniform and smooth.The removal of the cathode phase also reduces the micro-galvanic corrosion in the corrosion process,slows down the corrosion rate of the corrosion medium on Mg alloy,provides time for the formation of corrosion products and the accumulation at the defect of PCCs,delays the corrosion process,and improves the protection effect of SP-MnPCC significantly.