摘要
目的探明不同氢氧化钠/氟化钾质量之比与硅酸钠浓度的关系对A356铝合金微弧氧化膜层耐蚀性的影响机制,寻找电解液中电解质之间的配比需求。方法利用SEM、EPMA、XRD分析所制得膜层的微观结构、元素分布及物相组成,并通过动电位极化曲线及电化学阻抗谱(EIS)研究膜层的电化学耐蚀性和腐蚀行为。结果当电解液中NaOH/KF之比(均为质量比)为0∶1、1∶2和2∶1时,均无法形成表观质量良好的微弧氧化膜层。而将NaOH/KF之比调整为1∶1时,便在基体表面得到了覆盖完整、均匀连续、光滑平整的膜层。但因此时电解液中主成膜剂Na_(2)SiO_(3)的不足,致使所获膜层与NaOH/KF之比为1∶0时相较而言,膜层表面的微孔尺寸增大,大尺寸的微孔个数也增加,在一定程度上削弱了膜层的抗腐蚀能力。继而保持NaOH/KF之比为1∶1但将主成膜剂Na_(2)SiO_(3)的浓度翻倍后,所制得膜层表面缺陷减少,厚度增加,致密度得到改善,膜层中α-Al_(2)O_(3)、γ-Al_(2)O_(3)、Mullite及AlF_(3)等优质耐蚀物相的含量也明显增多,进而提高了膜层在酸性腐蚀介质和中性Na Cl腐蚀介质中的耐蚀性。结论在硅酸盐电解液中,NaOH和KF都是微弧氧化成膜过程中的重要组分。NaOH促使A356铝合金基体表面形成充分的钝化膜,并显著提高了溶液的电导率,为微弧氧化反应的击穿发生创造了先决条件。KF可增进微弧氧化击穿发生后的放电效应,引发更多的放电火花产生,提高膜层的生长速率,加快成膜物质的沉积速率。NaOH与KF浓度相近,且二者浓度均小于Na_(2)SiO_(3)浓度时,SiO_(3)^(2-)、OH^(-)、F-三者之间交互作用对提高膜层的耐蚀性最为显著。
In this study,the micro-arc oxidation coatings are fabricated on A356 aluminum alloy substrates in silicate solutions by varying the ratio of NaOH concentration to KF concentration of the electrolytes.The work aims to explore the mechanism of the effect of the relationship between the ratio of NaOH/KF concentration and Na_(2)SiO_(3)concentration on the corrosion resistance of the micro-arc oxidation coatings,and find out the desired matching among the electrolytes applied in micro-arc oxidation treatment solution.Eddy current thickness meter,scanning electron microscope(SEM),electron probe microanalyzer(EPMA)and X-ray diffraction(XRD)were employed to determine and characterize the thickness,surface morphology,cross-sectional morphology,elemental composition,phase composition of the coatings,respectively.The corrosion behavior and electrochemical resistance of the coatings were assessed by electrochemical impedance spectroscopy(EIS)and dynamic potential polarization curves with an electrochemical workstation.In addition,the anti-corrosion ability of the coatings in acidic situation was checked with spot test.The results showed that both NaOH and KF were important components in silicate electrolytes for the forming process of micro-arc oxidation coatings.NaOH promoted the formation of a sufficient passivation film on the initial surface of the A356 aluminum alloy substrate and dramatically improved the conductivity of the solution,which in turn created the precondition for the breakdown of the micro-arc oxidation reaction.KF could enhance the discharge effect after the electric breakdown on the substrate surface during the micro-arc oxidation treatment,initiate more discharge sparks,improve the growth rate of the coatings and promote the deposition rate of the coating-forming substances.When the ratios of NaOH/KF in the electrolyte were 0∶1,1∶2 and 2∶1,respectively,the coatings with good appearance could not be obtained.When the ratio of NaOH/KF was adjusted to 1∶1,the micro-arc oxidation coating with complete coverage,uniform continuity and smoothness was achieved on the substrate.However,due to the shortage of Na_(2)SiO_(3)as the primary coating-forming agent in the electrolyte,the thickness of obtained coating increased slightly compared with that in the situation when the NaOH/KF ratio was 1:0,but both the size of micropores on the coating surface and the number of large micropores(>3μm)also increased,which weakened the corrosion resistance of the coatings to some extent.Then,when the NaOH/KF ratio was kept at 1∶1,doubling of the concentration of Na_(2)SiO_(3)as the primary coating-forming agent promoted the movement of discharge sparks on the substrate surface.At this time,the surface defects of the obtained coating were reduced,the coating thickness was increased,the coating compactness was improved,and the contents of the phases with good anti-corrosion ability,such asα-Al_(2)O_(3),γ-Al_(2)O_(3),Mullite,and AlF_(3)obtained in the coatings were obviously increased,which contributed to improving the anti-corrosion ability of the coatings in the acid corrosive medium and the neutral NaCl corrosive medium.Therefore,during the micro-arc oxidation process on A356 aluminum alloy,when the concentration of Na_(2)SiO_(3)in the silicate electrolyte is higher than the concentrations of NaOH and KF,and the concentrations of NaOH and KF are similar,the interaction between SiO_(3)^(2-),OH-,and F-in the micro-arc oxidation reaction exhibits a pronounced effect on increasing the coating thickness,decreasing the defects on the coating surface,and promoting the deposition rate of anti-corrosion phases in the coatings.Consequently,this significantly enhances the corrosion resistance of the coating.
作者
衡志丹
马颖
欧凯奇
梁志龙
李正强
安晓丽
王晟
HENG Zhidan;MA Ying;OU Kaiqi;LIANG Zhilong;LI Zhengqiang;AN Xiaoli;WANG Sheng(State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,Lanzhou University of Technology,Lanzhou 730050,China)
出处
《表面技术》
EI
CAS
CSCD
北大核心
2024年第18期100-115,共16页
Surface Technology
基金
甘肃省创新研究群体计划(1111RJDA011)。
