Advance in Plasma Electrolytic Oxidation on Non-valve Metals

Plasma electrolytic oxidation (PEO), also known as micro arc oxidation (MAO), is a surface modification technology for the in-situ growth of high-performance ceramic coatings with strong adhesion on the surface of valve metals (Al, Mg, Ti, Zr, etc.). It can process workpieces of any shape and has high production efficiency, so it is widely used in marine, aviation, military, chemical and other fields. However, for the so-called non-valve metals, such as carbon steel, copper, brass, zinc and their alloys, it is difficult to establish a high-resistance insulating film on the metal and form a stable arc plasma discharge state during the PEO process, resulting in high energy consumption, low coating formation efficiency and weak adhesion between coating and substrate. As a result, PEO technology is generally considered not suitable for carbon steel, copper, brass, zinc and their alloys. As those non-valve metals are also important in modern civilization and industry, the incapability to treat the non-valve metals is regarded as one of the major disadvantages of the PEO technology. In recent years, some scholars have tried to prepare a layer of aluminum on the non-valve metal surface through magnetron sputtering, plasma spraying, thermal spraying, and other spraying techniques, and then performed PEO treatment on the sample. However, this method is the PEO of the valve metal (aluminum) rather than the non-valve metals. The direct modification of the surface of non-valve metals by PEO is still challenging and highly desirable to prepare multifunctional coatings to improve their friction and corrosion resistance, thermal shock resistance, catalysis and other properties, finally benefit to expand its application fields. Fortunately, in recent years, domestic and foreign researchers have made some progresses in the PEO treatment of non-valve metals. The scope of their research has expanded from the initial carbon steel to brass and zinc and other non-valve metals, and even PEO of the non-metallic silicon. This paper mainly introduces the latest progress in PEO treatment of non-valve metals such as carbon steel, copper, zinc and their alloys. Firstly, the properties of PEO coatings on carbon steel prepared under different processing parameters such as electrolyte composition, electrical parameters, and oxidation time are listed. In addition, it is also described that in the process of PEO of carbon steel, the mechanism of the dielectric breakdown of insulating film may be superior to the theory that the breakdown of gas vapour occurs first to form a coating. Then, the PEO behaviors of copper and its alloys in silicate, aluminate, phosphate and their mixture electrolytes are analyzed, and the corrosion resistance, wear resistance and formation mechanism of the coating are studied. The corrosion resistance, wear resistance, gas sensitivity and biodegradation properties of zinc and its alloys under different process parameters are also discussed. The difference between the PEO coating formation mechanisms for valve metals and non-valve metals has also been discussed. Finally, based on the current status of the PEO technology for non-valve metals, the future development in the PEO treatment of non-valve metals is predicted in terms of the optimization of processing parameters, the expansion of applicable metal substrates, improvement in coating quality, and coating formation mechanisms. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.