化学转化处理时间对2198铝锂合金硝酸铈转化膜耐蚀性的影响

Corrosion Resistance of Ce(NO_(3))_(3) Conversion Film on 2198 Al-Li Alloy with Different Chemical Conversion Time

选用2198铝锂合金作为实验材料,以Ce(NO_(3))_(3)为成膜主盐、H_(2)O_(2)作为氧化剂,在其表面制备了一种环保型稀土化学转化膜。采用化学浸泡法,在30℃的温度下对合金进行不同时间的化学转化处理,研究化学转化时间对合金硝酸铈转化膜耐蚀性的影响。测量化学转化膜形成过程中Ce(NO_(3))_(3)溶液的pH变化,试样的开路电位和重量变化;利用扫描电镜和能谱(SEM/EDS)以及X射线光电子能谱(XPS)对化学转化膜层表面形貌进行观察,并对膜的组成进行分析;利用极化曲线和电化学阻抗谱(EIS)研究转化膜的耐蚀性能。结果表明:2198铝锂合金表面生成的化学转化膜呈深黄色,生成的化学转化膜能够覆盖合金表面,含有Al,Ce,O和少量的Cu元素。XPS分析表明,2198铝锂合金的化学转化膜的主要成分为Ce(OH)_(4),Ce(OH)_(3),CeO_(2),Ce_(2)O_(3)和Al_(2)O_(3),少量Cu以单质的形式存在。动电位极化和EIS结果表明,与原始2198铝锂合金相比,处理不同时间获得的化学转化膜试样自腐蚀电流密度(J_(corr))都降低了两个数量级,阻抗增加;且随着处理时间的增加,J_(corr)先减后增,阻抗先增后减;30 min化学转化处理的试样具有最高的耐蚀性。

In order to improve the corrosion resistance of 2198 Al-Li alloy,a cerium-based chemical conversion film—cerium nitrate conversion film,was prepared on the surface of 2198 Al-Li alloy by chemical immersion method.2198 Al-Li alloy had many high-quality characteristics of light weight,high elastic modulus and high specific stiffness.Meanwhile,its corrosion resistance was poor.It was necessary to improve its corrosion resistance and protect the alloy surface correctly.Compared with the traditional chromate conversion film,cerium nitrate conversion film had the advantages of non-toxic and environmental protection,and the conversion film obtained on the surface of aluminum alloy had good corrosion resistance.In the experiment,Ce(NO_(3))_(3) ·6H_(2)O was configured with 0.015 mol·L^(-1)Ce(NO_(3))_(3) solution,0.029 mol·L^(-1)H_(2)O_(2)(30%,volume fraction)was added to the solution,and immediately put into the sample after adding.The samples were immersed in the above solution at 30℃for 10,20,30,40 and 60 min,respectively,and then cleaned with deionized water and dried for 24 h to prepare the cerium nitrate chemical conversion film.2198 Al-Li alloy was chemically transformed at different time periods to explore the effect of treatment time on the corrosion resistance of conversion film.The film-forming mechanism was explored by measuring the changes of pH value of Ce(NO_(3))_(3) solution,open circuit potential and weight change during the film-forming process.The surface morphology and composition of the film were characterized by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and X-ray photoelectron spectroscopy(XPS);the corrosion resistance of the conversion film was studied by polarization curve and electrochemical impedance spectroscopy(EIS).By analyzing the pH value change of the solution,open circuit potential and weight change,it was concluded that the membrane forming process was a microbattery composed of hydrogen evolution from cathode and dissolution from anode.The addition of H_(2)O_(2) electrolysis produced a large number of hydrogen ions to promote the hydrogen evolution of the cathode,resulting in the alkalinity near the cathode phase to produce hydroxide,which combined with the cerium ions in the solution and caused the continuous deposition of cerium nitrate conversion film.A dark yellow chemical transformation film was formed on the surface of 2198 Al-Li alloy.SEM and EDS results showed that a transformation film of cerium nitrate was formed on the surface of Al-Li alloy,which contained Al,Cu,Ce and O elements,but did not contain active element Mg.With the increase of treatment time,the transformation film on the surface of the sample was gradually formed.After 30 min treatment,the film layer on the surface of the sample was almost uniformly covered with metal scratches,and then the new uneven granular material was formed on the surface of the transformation film.XPS results showed that the content of Ce(Ⅲ)in the film was always higher than that of Ce(Ⅳ),the material on the surface of the film mainly existed in the form of hydroxide,and the material in the film mainly existed in the form of oxide.The results of electrochemical tests showed that the self-corrosion current density of the treated samples decreased by two orders of magnitude compared with that of 2198 Al-Li alloy,which showed that the corrosion resistance of the sample after film forming was significantly improved.With the increase of treatment time,the self-corrosion current density decreased first and then increased,and the self-corrosion current density of 30 min was the lowest,which showed that the sample with treatment time of 30 min had the best corrosion resistance.EIS results showed that there was an inductance arc in the low frequency region of the matrix,but there was no inductance arc in the cerium nitrate transformation film samples,and the contain arc radii of the film were obviously larger than that of the matrix.With the increase of treatment time,the impedance of the film layer increased first and then decreased,and the impedance of the sample treated for 30 min was the highest.The results of EIS were consistent with those of polarization curve.The corrosion resistance of 2198 Al-Li alloy could be improved by adding H_(2)O_(2) into Ce(NO_(3))_(3) solution.The surface morphology,composition,film forming mechanism and electrochemical corrosion behavior of cerium nitrate chemical transformation films were analyzed with different chemical transformation treatment time.The cerium nitrate chemical conversion film on the surface of 2198 Al-Li alloy mainly contained Al,O,Ce and a small amount of Cu elements.It was speculated that the main components of the film were Ce(OH)_(4),Ce(OH)_(3),Al(OH)_(3),CeO_(2),Ce_(2)O_(3) and Al_(2)O_(3),etc.The corrosion resistance of cerium nitrate coating on the surface of 2198 Al-Li alloy after different chemical transformation time increased first and then decreased with the extension of transformation time,and the corrosion resistance of sample treated for 30 min was the best.