Effect of Pulse Frequency on the Wear and Corrosion Resistance of Black Micro Arc Oxidation Coatings on Magnesium Lithium Alloy in Marine Environment
Magnesium lithium alloy is a metal structural material with excellent comprehensive performance,which has broad application prospects in fields such as shipborne aircraft. However,the poor wear and corrosion resistance of magnesium lithium alloy in marine environment greatly limits its application and promotion. In addition,the engineering application of magnesium lithium alloy also needs to meet the requirements of decoration,absorption or shielding of stray light. A black micro arc oxidation coating was prepared by adding two colored salts,copper pyrophosphate (Cu2P2O7) and cobalt sulfate (CoSO4) to the electrolyte. The resulting coating exhibited numerous defects such as microcracks. The microstructure and phase composition of the micro arc oxidation (MAO) coating were improved by adjusting the pulse frequency to enhance its wear and corrosion resistance in marine environment. In the experiment,with LA103Z magnesium lithium alloy as the substrate and 5 g/L Na2SiO3,8 g/L KF,12 g/L NaOH,15 g/L KNaC4H4O6·4H2O,3 g/L sodium citrate,5 g/L copper pyrophosphate,and 3 g/L CoSO4·7H2O as the electrolyte,a black MAO coating was prepared on the surface of the magnesium lithium alloy by adjusting the frequency (200,400,600,800,1000 Hz). The electrical parameters used were as follows:voltage of 300 V,duty cycle of 3％ and oxidation time of 10 min. The blackness value of MAO coating was measured with a CR9000 spectrophotometer. TESCAN VEGA 3 scanning electron microscope and its accompanying energy dispersive spectrometer were used to characterize the wear morphology of the coating,as well as the microstructure and element content before and after corrosion. Bruker D8 XRD was used to characterize the phase composition of coatings,with scanning angles and step sizes of 20°-90° and 0.02 (°)/s,respectively. AXIS Ultra DLD XPS was used to characterize the surface composition of coatings. The HT-1000 friction and wear testing machine produced by Lanzhou Zhong Ke Kaihua was used to characterize the wear resistance of coatings in simulated seawater. In the immersion corrosion experiment,simulated seawater was used as the corrosion solution,and Chen Hua Company's CHI660E electrochemical workstation was used to simulate seawater as the corrosion solution. The reference electrode was a saturated calomel electrode (SCE),the counter electrode was a Pt mesh,and the MAO coating sample was used as the working electrode. The corrosion resistance of the coating was characterized by an exposed area of 1 cm2 of the sample. A black MAO coating was successfully prepared on LA103Z magnesium lithium alloy by MAO technology. With the increase of pulse frequency,the content of Cu element in the coating gradually decreased,while the content of Co element gradually increased,transitioning from red brown to black,with a blackness value of 24.95. The number of micropores and microcracks on the coating decreased. At 1000 Hz,the friction coefficient of the coating was relatively small at 0.15,and the narrowest width of the wear mark was 293.05 μm. The coating exhibited good wear resistance. After corrosion,the surface was intact and the corrosion products were minimal,providing good long-term corrosion protection for 168 hours in the marine environment. Electrochemical experiments have shown that as the pulse frequency increased to 1000 Hz,the corrosion current density of the coating was the smallest,only 9.885 × 10-7 A/cm2,and the impedance increased by 9 times compared to 200 Hz. By using pulse frequency to control the proportion of colored oxide components in the micro arc oxidation coating,the coating color is controlled and the microstructure of the coating is improved. The systematic analysis of the effect of frequency on the wear and corrosion resistance of the coating further reveals the corrosion failure behavior of black MAO coatings in the marine environment.
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