Influence Of The Ratio Of Total Acidity To PH Value Of Conversion Bath And Surface Condition Of Magnesium Alloy On The Corrosion Resistance Of Phosphate Conversion Coating On Magnesium Alloy

Magnesium?Mg?and its alloys are promising engineering materials for transportation,aerospace,and electronics industries,which has been praised as"green engineering materials in the 21st century".However,their relatively low corrosion resistance is the main issue limiting Mg from having a wide range of applications.Over the past decades,many kinds of protective coating technologies have been developed to improve the corrosion performance of Mg alloys.Among the above,chemical conversion coating are widely used owing to their high protective effectiveness,low-cost and eases in operative procedure.The bath formulation is the key topic in the research filed of chemical conversion coating.However,the designing rule of bath formulation is absence and the fabrication of conversion coating of Mg alloys usually referees the experience of chromate conversion coating on aluminum alloys.Therefore,in this thesis,the concept of"TA/pH"?i.e.,the ratio of total acidity of solution to pH value?was proposed according to the metal/solution interface reaction.The effect of TA/pH on the microstructure and corrosion resistance of the phosphate conversion coating?PCC?of Mg alloy was investigated,and then a designing rule of bath formulation was established.In addition,the influence of surface roughness,size and distribution of second phase and the preheating pretreatment on the microstructure and corrosion resistance of the PCC were also investigated in the view point of the metal/solution interface.To some degree,the series of works promote the transformation of conversion coating technique from basic research in laboratory to application in industry.A new concept,total acidity/pH value?TA/pH?was proposed to guide the design of Mn-PO₄ based the formulations for fabrication of a PCC on Mg alloy with a desirable microstructure and corrosion protection performance.A solution with a low TA/pH facilitated the formation of a PCC with a homogeneous microstructure and improved corrosion performance,whereas a high TA/pH led to a porous layer with a poor corrosion resistance.The effect of TA/could be summarized as following:first,the increase of pH led to the higher concentration of HPO₄^2-and the higher ion-product of MnHPO₄,which benefited to exceed the critical supersaturation degree of MnHPO₄ and promoted the nucleation density.Second,the higher TA/pH indicated stronger the shielding effect and convection effect of hydrogen bubble,which suppressed the nucleation of the compounds to develop the outer layer.Finally,TA/pH influenced the formation of(HPO₄^2-)_n clusters.This can increase the supersaturation degree of MnHPO₄ on the bath/metal interface,resulting in a high growth rate.Surface pretreatment is generally applied before application of protective coatings on Mg alloys,which influences surface microstructure and electrochemical activity of the substrate and has an effect on the coating properties.The effect of various pretreatment processes?sand-blasting,grinding and polishing?on the microstructure and corrosion protection performance of phosphate conversion coating?PCC?on AZ91D Mg alloy was investigated in the present study.The morphology and electrochemical activity of the Mg alloy surface has been greatly modified by sand-blasting pretreatment process,which induces severe micro-galvanic corrosion between the newly formed?phase and?-Mg and thus accelerates the corrosion rate of the uncoated and coated Mg alloy.As for the ground/polished alloy,the conversion coating becomes more uniform and denser with the decrease of the surface roughness.The findings are of great importance to select a proper surface pretreatment process before application of conversion coatings and are highly relevant for industrial surface treatments of Mg alloys.The influence of the size and distribution of the second phase??phase?on the microstructure and corrosion resistance of PCC of AZ91 alloy was studied by means of electrochemical measurement and microscopy observation.Results reveal that T6 treatment led to the refined and more evenly distributed?phases,which increases the electrochemical homogeneity of AZ91D magnesium alloy.After conversion treatment,a uniform,compact and dual-layer PCC with structure forms on the surface of T6-AZ91D alloy,and exhibits high corrosion performance.On the contrary,T4 treatment decreases shows low electrochemical homogeneity of AZ91D magnesium alloy,resulting in a single-layer and poor corrosion resistance PCC.It is attributed to the high electrochemical homogeneity lead to a uniform distribution of galvanic current density in the formation process,resulting in the decrease and more homogenous distribution of the interfacial total acidity/pH?TA/pH?.It indicates that the interfacial reaction quotient of MnHPO₄(J=[Mn²⁺]×[HPO₄^2-])increases and is easy to exceed the critical supersaturation degree,in turn fabricates a compact and uniform PCC.The effect of preheating pretreatment on the microstructure,corrosion resistance and formation of PCC of Mg alloy was studied by microscopy observation,electrochemical measurement and numerical simulation.If the preheating temperature was higher than that of bath,the bath of boundary layer would be heated,which led to the increase of the concentration of HPO₄^2-.It indicated the increase of ion-products of MnHPO₄ in the Mg/bath interface,resulting in the rapidly exceeding its critical supersaturation.Thus,the nucleation rate of MnHPO₄ was accelerated,and then a homogeneous PCC with a good corrosion resistance was obtained.On the contrary,when the preheating temperature was lower than that of conversion bath,the bath of boundary layer would be cooled,leading to the decrease ion-products of MnHPO₄ in the Mg/bath interface.As a result,the PCC becomes more inhomogeneous and porous with poor corrosion performance.