Preparation And Corrosion Properties Of A Novel Type Of Chrome-Free Conversion Iflm On Copper

Copper was widely used in generator, cable, transformer, pipe and solarheating devices etc. However copper was easy to be corroded under the media suchas humid atmosphere and corrodent with the presence of oxidants, and to a largedegree the lifetime of copper was reduced. Conversion film was an effectivetechnique to protect the copper surface from being corroded. Currently, traditionalchromate conversion film was forbidden because of the severe environmental issuescaused by the presence of Cr（VI）. Chrome-free conversion film with benzotriazoledisplayed inferior corrosion resistance on copper. Consequently, it was essential todevelop a novel chrome-free conversion film.In this paper, a novel type of chrome-free conversion film on copper wasprepared through adding the sodium molybdate and lanthanum nitrate into solutioncontaining benzotriazole （BTA）, sulfosalicylic acid and citric acid. The effect ofprocessing parameters on the corrosion resistance of conversion film were studiedby orthogonal experiment and single-factor，including content, temperature andtime，and the optimum process was obtained.The corrosion resistance behavior of the conversion film in different corrosivemedium was studied via immersion test, weight loss experiment and neutral saltspray test. And the changes of morphology and structure of conversion film aftercorrosion tests were observed by scanning electron microscopy （SEM）. Polarizationcurves and electrochemical impedance spectra （EIS） measurements were adopted tofurther investigate the electrochemical behavior of conversion film in corrosivemedium. The changes of corrosion current density and the cathodic and anodicpolarization curve were analyzed. By fitting the EIS plots with equivalent circuits,the EIS paraments were obtained and the inhibiting efficiency of conversion filmwas also calculated. Moreover, field emission scanning electron microscopy（FESEM）, atomic force microscopy （AFM） and X-ray photoelectron spectroscopy（XPS） were employed to analyze the morphology and valence state of the elementsof conversion film, and the growth mechanism of conversion film were alsodiscussed preliminarily.The optimum process parameters determined by orthogonal experiments andsingle factor test were as follows: BTA12g/L, Na₂MoO₄8g/L, La（NO₃）₃6H₂O 4g/L, C₆H₈O₇10g/L, C₇H₆O₆S2H₂O4g/L, at50℃for5min. The results ofimmersion test and neutral salt spray test showed that the chrome-free conversionfilm prepared by optimum process can prevent copper substrate from beingcorroded. The results of the polarization curves and EIS showed that with theaddition of the sodium molybdate and lanthanum nitrate corrosion current densityof conversion film was sharply reduced and the polarization resistance wasobviously increased，indicating that the conversion film exhibited better barrierproperties. FESEM and AFM showed that the surface morphology of the conversionfilm was integrated and homogeneous. XPS analysis indicated that the sodiummolybdate and lanthanum nitrate participated in film formation process， andimproved the structure of the conversion film.