Investigation Of The Conversion Coating With High Corrosion Resistance And Low Electrical Contact Resistance On Magnesium Alloy

Magnesium alloys have promising applications in aerospace,automotive,and 3C electronic products due to its very interesting engineering properties such as its low density,high specific rigidity and good electromagnetic compatibility.Unfortunately,the poor corrosion resistance of magnesium alloys still remains to be the biggest challenge for its wider application in industrial engineering products.Coating technology is one of the most effective ways to protect the magnesium alloys from corrosion,thus efforts have been made by researchers to develop variety of coating technologies for magnesium alloy corrosion protection.A new challenge for the coating technologies rises along with the extensive use of magnesium alloys in 3C electrical products,i.e.the coated surface is mandatory to satisfy the requirements of a lower electrical contact resistance(ECR)to ensure good electrical conductivity between integrated circuit broads and internal surface of magnesium alloy casing while maintaining high corrosion resistance.This electrical conductivity is essential to guarantee the continuous electrical contact,grounding and electromagnetic shielding of electronic devices.Chemical conversion treatment is an effective,comparatively low-cost,and easily implemented method,which has been widely adopted in industrial processes.Over the past few decades,chromate conversion treatment is the main methods to satisfy such kind of requirements for avionics applications on aluminum alloys.It is also considered to be viable on magnesium alloys.However,due to the toxic and carcinogenic effects of hexavalent chromium compounds,there has been increasingly stringent legislation regarding their use and waste disposal.It is on the agenda for the investigation of an environment-friendly conversion coating that can satisfy the demand of 3C industry.Based on the concept of electrical contact resistance(ECR),a new designing idea for the chemical conversion coating with high corrosion resistance(CR)and low ECR on AZ91D Mg alloy was presented in the present work and the failure mechanism of the coating was also investigated.In order to enhance the CR of the coating further more,the defects especially cracks in the coating were studied carefully and a new crack mechanism of the coating—hydrogen induced crack was presented.Also,the effect of magnesium substrate microstructure(grain size,second phase size and microstructure homogeneity)on the microstructure and corrosion resistance of the conversion coating was investigated in this thesis.The desired performances are attributed to the unique microstructure of conversion coating,i.e.,β phase protruded from the surface and covered by an extremely thinner passive film,contributing to low ECR,meanwhile,a thicker and compact coating was deposited on a-phase,providing a good corrosion resistance.Five key factors for this designing idea were summarized as:(1)An acidic solution.Due to micro-galvanic effect,β phase acts as cathodic phase and accelerates the corrosion rate of a phase,resulting in the protrusion of β phase.(2)The addition of coating agent such as Ca2+.The presence of coating agents accelerates the coating rate and leads to form a compact coating by co-deposition of Ca and Mg phosphate on a-Mg.(3)The addition of strong oxidizing agent.The presence of strong oxidizing agent suppresses the hydrogen reduction reaction due to their higher redox potential,retarding the growth of flower-like phosphate crystal.As a result,the negative effect of flower-like phosphate crystal could be minimized and an extremely thin passive film forms on the protruded β phase,which acts as conducting spots.(4)Determination of the pH value and the cation concentration for coating formation based on thermodynamic calculation.The pH value is determined according to the boundary between the Mg region and MgHPO4·5H2O region,while,the Ca2+concentration is optimized by considering the boundary between the region of CaH2PO4 and CaHPO4·2H2O.In this work,the bath should contain 0.21 mol/L Ca2+,0.21 mol/L PO43-(total concentration),and the pH value ranges 3.0-3.5.(5)Control the immersion time reasonably.Increasing the conversion time may enhance the corrosion resistance slightly due to the increase of coating thickness,it also leads to the failure of the conductivity in electrical contact due to the generation of coarse CaHPO4·2H2O crystal.Micro cracks in the film take major responsibility for the corrosion failure of the coating.A large number of hydrogen bubbles are detected in the coating by TEM observation.A new mechanism of crack formation "Hydrogen Induced Crack" was presented:Hydrogen evolution is the main cathodic reaction during the conversion process,which leads to the generation of a porous film due to the co-deposition of the bubbles and phosphate precipitant at the initial stage.As the reaction continues,hydrogen evolution mainly occurs on film/substrate interface.These hydrogen bubbles generated at the coating/substrate interface have to pass through the existing film into the bath by diffusion and local severe hydrogen evolution may lead to large amount of hydrogen atoms or molecules diffuse into the existing bubbles or microvoids in the coating.This will result in rapid growth of some bubbles and finally leads to the film crack untill the pressure reaches the critical strength.These cracks will be the convenient channel for further reactions in return and lead to broaden of the cracks.Based on this mechanism,depressing the hydrogen evolution by increasing the concentration of strong oxidizing agent is found to be effective to reduce the formation of cracks and enhance the corrosion resistance.In this study,the effect of the microstructure of AZ91D substrate on the conversion coating was conducted by comparing the microstructure and corrosion resistance of the coating on ingot-cast and die-cast AZ91D.Comparing to ingot-cast,die-cast AZ91D possesses a finer microstructure in which the size of both α and βdecreased by one order of magnitude and a larger area fraction of β phase which is 37.7%more.These two factors enhance the microgalvanic effect which accelerates the coating formation and reduces the hydrogen evolution on a phase.Thus,coating formed on die-cast AZ91D possesses a smaller porosity and is crack-free.Meanwhile,a phase in die-cast AZ91D owns a much more homogenous composition distribution which makes the coating more uniform in thickness.The absence of(α+β)eutectic also avoids the transverse cracks in the coating.