Study On Plasma Electrolytic Oxidation Of Aluminum Alloy In Tungstate-containing Electrolyte

Plasma electrolytic oxidation is an emerging metal surface protection method developed from conventional anodizing.This technology uses a protected metal as an anode and stainless steel or platinum as a cathode to grow a dense oxide coating which is mainly composed of the substrate metal oxide on the surface of protected metal.Compared with traditional anodizing,the electrolyte used in plasma electrolytic oxidation is more environmentally friendly;also,the resultant coating is larger in thickness and shows excellent wear and corrosion resistance.However,the reaction mechanism of plasma electrolytic oxidation is very complicated,and the selection of different electrical parameters,electrolytes and substrate metals has a great influence on the whole reaction process.This article mainly studies the plasma electrolytic oxidation behavior of aluminum alloy in the electrolyte containing tungstate and discusses the reaction mechanism.The main contents of this paper are as follows:(1)A compact and smooth black oxide coating with different colors was prepared on the surface of aluminum alloy in tungstate-containing electrolyte by plasma electrolytic oxidation,and the aggregation state of tungsten element in the coating was characterized by a variety of modern detection and analysis technologies.It was found that tungsten was mainly distributed at the interface between the coating and the substrate;Further analysis showed that there were two valence states of tungsten in the prepared coating,namely 0 and + 6.Based on a large number of existing experimental results and other researchers’ research in this field,we consider that free state tungsten is formed by the substitution reaction of melted substrate metal and tungsten oxides,which are formed in the discharge channels under the action of high temperature and high voltage.(2)The composition and distribution of various phases in the prepared coating were detected and analyzed.By XRD and TEM,the composition of crystal and amorphous in the coating and their position distribution were determined.It was found that plasma electrolytic oxide coating was a complex oxide coating which was mainly composed of the oxide of substrate metal and mixed with crystal and amorphous.Besides,in addition to the oxides of substrate metal,the crystalline material also contained the single substance and oxide generated by the components in the electrolyte under the action of high temperature and high voltage;the amorphous phase in the coating mainly came from the deposition of the electrolyte on the surface of the coating,as well as the complex structure which contained both substrate metal elements and elements in the electrolyte.It was also found that in the plasma electrolytic oxidation coating the amorphous phase was mainly formed in the wall of the large inner pore,and a reasonable formation mechanism was put forward in this paper.(3)By designing different frequencies and d uty cycles,the plasma electrolytic oxidation of aluminum alloy under different electrical parameters was studied.The results showed that the increase of the frequency and the decrease of the duty cycle would lead to the increase of the voltage and the de crease of the uniformity of the coating structure.In addition,the negative pulse could not lead to the spark during plasma electrolytic oxidation process;however,at high frequency,discharge channels did not cool down completely and the next discharge cycle started again,so the optical signal could be observed with negative pulse or without pulse in this case.Besides,under bipolar conditions,the increase in frequency will cause the surface roughness of the PEO coatings to become larger and the corro sion resistance to deteriorate;while under unipolar conditions,the corrosion resistance and surface roughness of the coatings are little affected by the frequency change.