Fabrication And Corrosion Failure Of PVD Coatings On Magnesium Alloy

Magnesium alloy has been increasingly applied in the automobile industry, electron industry and aerospace industry, but its poor anti-corrosion property limits its further application in these fields. Physical vapor deposition as a green anti-corrosion technique has received more and more attention in surface treatment of magnesium alloy.Magnetron sputtering and ion implantation were used to prepare several protective coatings on AZ31 magnesium alloy in this study. Field emission scanning electron microscope (FESEM), Atomic force microscopy (AFM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction meter (XRD) were applied to characterize the surface morphology, composition and phase structure of the coating/substrate system. Electrochemical methods and salt water immersion were used to study the corrosion behavior of these systems. In addition, their corrosion mechanism was analyzed according to the corrosion electrochemical theory. The main results include:(1) The oxide layer, which was formed on the surface of AZ31 magnesium alloy during the polishing process, consisted of MgO, Mg(OH)₂ and a little Al₂O₃ and MgCO3. It had a loose structure with some dispersive holes on its surface.(2) Both aluminum coating and titanium coating, which were prepared with sputtering technique, had a developed columnar microstructure. The top surface of Al coating took on a pyramid-like morphology with a (111) plane preferential orientation. The top surface of Ti coating took on a round roof-like morphology with a (002) plane preferential orientation. There were some pinholes in all coatings.(3) Aluminum coating had a better protection for magnesium alloy than titanium coating, while titanium coating had a trend to accelerate the corrosion of the substrate. The through thickness poles in the PVD coating/the oxide layer duplex structure were the main sites for galvanic corrosion. The evolution of hydrogen, which took place at these defects, accelerated the damage of the coating. Finally, it led to the failure of the coating/substrate system.(4) AZ31 substrate was dissolved in NaCl solution as an anode. It induced the surface basification effect of magnesium alloy. Then, Al coating suffered a cathodic attack because aluminum is an amphoteric metal.(5) Each monolayer of Al/Ti multilayer presented a developed columnar structure. The (111) texture of Al layer was strongly strengthened in Al/Ti multilayer coating compared to that in aluminum coating. The top surface of the Al/Ti-coated sample was smoother than that of the Al-coated sample. The Al/Ti multilayer could, to some extent, improve the corrosion resistance of substrate, but its durability was very poor in NaCl solution.(6) Al₂O₃/Metal duplex coatings were dense in a microcosmic scope. Two kinds of duplex coatings could, to some extent, improve the surface microhardness of the substrate. Al₂O₃/Ti had a better effect of improvement than Al₂O₃/Al. Al₂O₃/Al decreased the corrosion current density of the substrate greatly, while Al₂O₃/Ti increased that of the substrate. Although the anti-corrosion property of Al₂O₃/Al was better than that of Al₂O₃/Ti in salt water, the endurance of Al₂O₃/Al was not enough strong due to the poor adherence between coating and substrate.(7) Ti-Al-N/Ti-Al duplex coating was prepared by reactive sputtering. Both ceramic layer and metallic layer presented a developed columnar structure. Ti-Al-N layer consisted of titanium nitride, aluminum nitride and a little titanium oxide, aluminum oxide. It is shown that Ti-Al-N/Ti-Al duplex coating could greatly improve the anti-corrosion ability of the substrate. Because low-dose nitrogen ion implantation was used to modify the surface prior to sputtering, the interface adherence was strengthened, which improved the endurance of the coating in salt water. In addition, it also enhanced the surface microhardness of the substrate, which was helpful for improving the ability against mechanical damage.(8) The intrinsic defect caused by columnar structure had a little influence on the corrosion failure of coating/substrate system. The random defect formed in the preparation process was the main factor to induce the corrosion failure of the system. The exposed substrate and the coating around the defect formed a corrosion source in NaCl solution. The dissolution of the substrate under anodic polarization and the hydrogen reaction behavior at the interface of coating/substrate became an impetus for the enlargement of a corrosion source. The defect in PVD coating was a short cut for corrosion and the failure of the system was determined by the enlargement of corrosion sources. Therefore, the corrosion failure of the system was considered as a kind of short circuit corrosion behavior.(9) High-dose nitrogen ion implantation induced the formation of Mg3N2 and AlN in the original oxide layer of AZ31 magnesium alloy. The oxide layer became dense and turned thick. Nitrogen ion implantation improved the anti-corrosion ability of the substrate greatly. Pitting corrosion was the main corrosion mechanism of the substrate after ion implantation. Its anti-pitting ability was enhanced in comparison with that of the untreated sample.(10) Ion implantation was used to modify the surface of AZ31 substrate before the deposition of Ti coating. Thus, the Ti coating/N⁺-implanted layer composite structure was obtained by this method. Deposition of Ti coating didn't change the composition of the implanted layer. The sample treated by the combined technique had a better corrosion resistance than that protected only by pure Ti coating. It was closely due to the improvement of the interface adherence of coating/substrate and the increase of the anodic dissolution resistance.(11) Bias sputtering was used to improve the density of Ti coating successfully. Under bias voltage of -100V, the grain size was minimized to nanometer. When bias voltage increased to -200V, the grain size turned large again and the preferred orientation changed from plane (002), which was formed without bias voltage, to plane (100).(12) Ti coating prepared under bias voltage of -100V had a better protective effect for AZ31 substrate than that prepared without bias voltage. It is found that the corrosion products, both on the substrate and on the coating, were Mg(OH)₂. However, the corrosion products on the substrate took on a honeycomb-like morphology, while those on the coating took on a leave-like morphology. Short circuit corrosion failure was the main corrosion mechanism of the nanocrystalline Ti coating on AZ31 magnesium alloy.Physical vapor deposition is a good substitute for those traditional surface treatment techniques. Appropriate coating materials, coating structure and preparation technique are helpful for improving the corrosion resistance of the coated magnesium alloy.