电镀锌稀土转化膜工艺及机理研究

【作者】 章江洪；

【导师】 张英杰；

【作者基本信息】 昆明理工大学 ， 材料学， 2010， 博士

【摘要】 稀土转化膜处理技术具有无毒、无污染、耐蚀效果好的特点,是无铬转化膜处理技术最具潜力的发展方向之一。目前,该处理技术的研究主要为工艺和性能的研究,研究重点在铝及合金上,已在铝及合金表面制备出具有较好耐蚀性能的稀土转化膜,而锌及合金稀土转化膜的耐蚀性能仍有较大的提升空间；在机理研究方面,目前仍无一个公认的成膜及耐蚀机理,很少涉及到从热力学及动力学角度研究成膜反应机理,未见从微观角度采用量子化学方法研究成膜反应历程,因此,本论文采用添加成膜促进剂和氧化剂的化学浸泡法,针对钢铁材料有效防护性的镀锌层,进行稀土盐转化膜处理的工艺研究，并在工艺研究的基础上,从热力学、动力学、量子化学模拟等多角度探讨稀土转化膜的成膜机理,研究转化膜耐蚀机理,建立稀土转化膜处理技术的基础数据。本研究采用正交试验结合单因素试验确定电镀锌稀土转化膜工艺配方为：Ce(NO3)3·6H2O 30 g·L-1, H2O2 15ml·L-1,辅助成膜促进剂4 g.L-1,成膜促进剂0.5g·L-1,H3BO3 2.5g·L-1,pH为2-3,处理液温度在15-45℃之间,处理时间2-3min,空停时间5-10s。在该工艺配方下,镀锌层与处理液界面的pH值可达9.95,能满足Zn(OH)2、Ce(OH)3、Ce(OH)4在界面上的沉积,所开发出的稀土转化膜外观均匀光亮、耐中性盐雾时间达36hr。工艺配方中成膜促进剂和氧化剂的加入,对有效形成高耐蚀性的稀土转化膜起到至关重要的作用,其中氧化剂H202的加入,通过加快微阴极发生反应,促使界面微阴极区pH值上升,进而有利于金属表面氢氧化物沉淀膜的形成；成膜促进剂的加入是通过降低成膜三个反应阶段的表观活化能,从而实现对转化膜生长的促进。针对开发的工艺配方,基于稀土转化膜的XPS和XRD分析测试结果,结合成膜过程的热力学、动力学分析,以及量子化学模拟成膜反应历程,得出稀土转化膜的成膜机理为：由于镀锌层表面存在晶体缺陷,这些晶体缺陷属于能量较高的电化学不均匀的区域,在含有H202的处理液中易形成众多微电池,发生锌的氧化反应、H202的还原反应及氧的去极化反应,其中H202中的O采用氧端吸附在镀锌层表面,通过发生O-O键断裂,以OH-形式吸附于镀锌表面；而O2吸附于镀锌层表面,首先会促使镀锌层表面吸附H20分子中的O-H键断裂,以HO2-的形式吸附,进而再与吸附的H20发生反应,发生O-O键断裂,在界面上以OH-形式吸附,最终导致镀锌层界面微阴极区产生大量OH-。此外,H202作为氧化剂可将处理液中的Ce3+氧化为Ce4+,故当微阴极区pH达到一定数值后,Zn(OH)2、Ce(OH)3、Ce(OH)4均会沉积于有晶体缺陷的区域,形成晶核,随着沉积物的增多,沉积物向四周扩散形成基膜,基膜在生长过程中与膜的溶解并存,最终长大并覆盖镀锌层表面形成由氢氧化物沉积膜所构成的稀土转化膜。由于Ce4+沉积所需的pH值小于Ce3+沉积所需的pH值,故Ce4+优先于Ce3+沉积在界面上,故转化膜中Ce的价态主要为+4。稀土转化膜在干燥、放置的过程中,氢氧化物的沉积物会发生脱水,形成稳定的氧化物,而三价铈也会氧化为四价铈,最终形成由ZnO、CeO2、Ce2O3、Zn(OH)2、Ce(OH)3、Ce(OH)4构成的复合膜层,该膜层主要为非晶态物质,局部存在晶态物质。转化膜的生长分为快速生长、缓慢生长、平稳生长三个阶段,各个生长阶段的单位面积转化膜的质量与生长时间均呈指数关系,其中,第一阶段成膜反应的表观活化能较小,成膜反应速率较快,第二阶段成膜反应的表观活化能最大,成膜反应速率较慢,第三阶段成膜表观活化能下降,成膜反应与膜溶解反应在竞争中趋于平衡,成膜反应具有较低的活化能。对稀土转化膜耐蚀机理的研究是建立在膜层耐蚀性研究的基础上。通过浸泡实验、中性盐雾试验、电化测试等方法,对比考察稀土转化膜、镀锌层及低铬转化膜的耐腐蚀性能,结果表明稀土转化膜可明显提高镀锌层的耐蚀性,其耐蚀性能优于低铬转化膜。结合微观形貌分析、XRD测试及腐蚀动力学数据计算,得出稀土转化膜耐蚀机理为：稀土转化膜是由微小颗粒堆积而成的细密、无裂纹、缺陷较少的紧密完整膜层,该膜层对基体镀锌层的覆盖性较好,阻碍了O2的传输和电子的传递,对腐蚀的阴、阳极反应均有不同程度的抑制,降低腐蚀动力,有效的保护了基体不受腐蚀介质的侵蚀,提高基体的耐蚀性。膜层腐蚀反应的发生是由于转化膜微观上存在着化学和物理上的不均匀性,转化膜首先在这些薄弱区域发生腐蚀,易突破构成腐蚀原电池,随着Zn2+的不断水解,造成微区酸度增大,转化膜在酸性环境中溶解,膜层被破坏；此外,腐蚀介质中带负电的Cl-向微区移动,与Zn2+、Ce3+、Ce4+结合形成Zn(OH)xCly、Ce(OH)xCly等腐蚀产物,腐蚀电池反应加剧,稀土转化膜破坏严重。由于膜层在5%NaCl溶液中的表观活化能大于0.5%H2SO4溶液中的表观活化能,故稀土转化膜在0.5%H2SO4溶液中更易发生腐蚀。作为转化膜要运用于实际生产当中,必须要求其具有优良综合性能,通过对稀土转化膜附着力、硬度及粗糙度的测试,表明在适宜处理时间下所制备的稀土转化膜,由于膜层与基体之间的结合力与膨胀应力达到平衡,结合效果较好,整体附着力最好,表面粗糙度起伏最小,膜层具有较好的均匀性、致密性和平整性。该转化膜的形成可以提高镀锌层的维氏硬度,有利于提高镀锌件的耐磨性能、强度和使用寿命。镀锌层稀土转化膜处理技术可以成功的运用于处理锌铁合金镀层(含铁量为0.4-0.7%),可在锌铁合金镀层表面制备出光亮平整、良好耐蚀性的稀土转化膜,该膜层具有与低铬酸盐转化膜相近的耐蚀性,可明显提高镀层的耐蚀性,能够较好的保护基体抵制腐蚀介质的侵蚀。通过本论文的研究,在电镀锌表面制备出外观均匀光亮、耐蚀性优于传统低铬转化膜的稀土转化膜,建立稀土转化膜成膜及耐蚀机理,提供稀土转化膜处理技术的基础数据，为稀土转化膜处理技术最终能取代铬酸盐处理技术做出有意义的探索。更多还原

【Abstract】 The rare-earth conversion coatings treatment technology is one of the most potential development directions of non-chromium conversion coatings treatment technology. However, researches on this technology are mainly focused on the aluminum and alloy, the zinc and alloy rare-earth conversion coatings technology has not been investigated systematically. The present study explores mechanisms of coating-formation and anti-corrosion on the basis of process researches, building up essential data of the rare-earth conversion coatings treatment technology.On the basis of the preliminary work, the rare-earth conversion coating was prepared on the surface of zinc coating using immersion method taking Ce(NO3)3·6H2O as main salt and H2O2 as the oxidant in the acidic atmosphere. Through theoretical calculations, it was shown that the pH values of zinc coating and liquid interface were up to 9.95, and could meet the depositions of Zn(OH)2, Ce(OH)3 and Ce(OH)4 at the interface, indicating the feasibility of process study. Process researches were investigated through combination method of orthogonal test and single factor experiments, using test time of the coating resistance to neutral salt spray as the evaluation standard, developing rare earth conversion coating process specification with characteristics of uniform bright appearance, resistance to neutral salt spray time up to 36 hr as following:Ce(NO3)3·6H2O 30g·L-1; H2O2 15ml·L-1, auxiliary film-forming promoting agent 4g·L-1; the promoter 0.5 g·L-1; H3BO3 2.5g·L-1, pH 2.0～3.0; solution temperature 15-45℃; immersion time 2-3 min; slot times 5-10 s.It is of great significance to the efficient formation of rare earth conversion coating with high corrosion resistance through the addition of oxidant and promoter of film-formation in the process formula. In the light of electrochemistry test, morphology analysis and dynamics data calculation, reaction mechanisms of oxidant and promoter of film-formation were elucidated respectively as following:micro-cathode reaction was speeded up by the addition of H2O2, resulting in the increase of pH for interface of micro-cathode region, which was favorable to the formation of metal surface hydroxides precipitation coating; the growth of conversion coating was realized by lowering the apparent activation energy for three stages of coating-formation reaction through the addition of promoter of coating-formation.According to the process formula developed, XPS and XRD analysis results of conversion coatings based on rare-earth, thermodynamics and dynamics analyses of film formation, and film-formation reaction mechanism simulated by quantum chemistry, the film-formation mechanism of rare earth conversion coating was concluded as following: due to the existence of crystal defects on the surface of zinc coating, which were a electrochemical uneven area with high energy, resulting in the formations of micro-batteries easily in the treatment liquid containing H2O2, taking place of reaction of zinc oxidation, H2O2 reduction and oxygen depolarization, the O in H2O2 and Zn on the galvanized zinc surfaces formed chemical bond through adsorption mode, and adsorbed on the surface of galvanized zinc in the form of OH- through the O-O bond rupture; O2 was adsorbed on the surface of zinc coating, giving rise to the breakage of O-H in H2O adsorbed on the surface of zinc coating and adsorbed in the form of HO2-, and furthermore reacting with the adsorbed H2O, occurring O-O bond rupture, and finally adsorbed in the form of OH- at the interface, resulting in massive production of OH- in the micro-cathode area of zinc coating interface. H2O2, as oxidative agent, could oxide Ce3+ to Ce4+ in treatment solution, therefore, when pH in micro cathode area reached a certain value, Zn(OH)2, Ce(OH)3 and Ce(OH)4 would deposit in areas with crystal defects, forming nucleus. With increasing of deposits, they diffused into around to form the basement coating; co-existing with dissolving of coating during the growth of the basement coating, growing up and covering the galvanization zinc surface finally, forming the rare earth conversion coating comprising of hydroxide deposition coating. Due to pH value of Ce4+ deposition is less than that of Ce3+, Ce4+ takes precedence over Ce3+ depositing on the interface, the valence of Ce in Ce conversion coatings is mainly +4. The hydroxide deposit would dehydrate to form stable oxide compounds and Ce3 oxidized to Ce4+ under the conditions of rare earth conversion coating being in dry medium or stocking process, forming composite coatings which were composed of ZnO, CeO2, Ce2O3, Zn(OH)2, Ce(OH)3 and Ce(OH)4. The growth of conversion coating was divided into three stages of the fast growth, slow growth and smooth growth. The apparent activation energy of the first stage was smaller, giving rise to the fast reaction of coating-forming; while the apparent activation energy of the second stage was biggest, resulting in the slower reaction of the coating-forming; the apparent activation energy of the third stage decreased, coating formation reaction and coating dissolving reaction tended to reach equilibrium in the competition, coating-formation reaction had lower activation energy; growth rates for various stages and the quality of conversion coating per unit area met the exponent law.Researches on the corrosion resistance mechanism of rare-earth conversion coating were based on investigations of coating corrosion resistance. By using immersion experiments, the neutral salt spray tests and electro-chemical testing methods, etc; comparison studies of corrosion resistance performances for rare earth conversion coatings, zinc coating and low-chromium conversion coating, it was shown that the rare earth conversion coating could significantly improve the corrosion resistance of zinc plating, whose corrosion resistance was better than that of low-chromium conversion coating. The corrosion resistance mechanism of rare-earth conversion coating was concluded by using microstructure analysis, XRD testing and calculations of corrosion kinetics data as follows: Rare-earth conversion coating was compact film which was formed by the accumulation of tiny fine particles without cracks and less defects, and the coverage of coating on the substrate of galvanized zinc layer, hindering O2 transmission and electronic transmission, inhibiting both reactions of anodic and cathodic of corrosions, lowering corrosion kinetics and protecting matrix from corrosion effectively, improving the corrosion resistance performance of the matrix. The occurrence of corrosion reaction of coating was due to the non-uniformity of the coating on chemistry and physics for microstructure, the apparent activation energy of conversion coating in 5% NaCl solution is greater than in 0.5% H2SO4 solution, showing that it was easier to occur corrosion for rare-earth conversion coating in 0.5% H2SO4 solution.The conversion coating must have good comprehensive performances when being used in the actual production. It was demonstrated through tests of adhesion, hardness and roughness of rare-earth conversion that rare-earth coating prepared under the optimized processing time, binding force and swelling stress between the coating and the substrate reaching equilibrium, having better binding effect, the best overall adhesion and the smallest surface roughness fluctuation, the film has characteristics of good uniformity, compactness and smooth. The formation of the conversion coating could enhance Vickers hardness of the galvanized zinc layer, which was favorable to improving the wear resistance of galvanized zinc parts, strength and service life.Zinc coating of rare earth conversion coating processing technology could be applied successfully to process Zn-Fe alloy coatings (iron content of 0.4-0.7%), rare earth conversion coating with characteristics of luminously smooth and good corrosion resistance could be prepared on the surface of Zn-Fe alloy coating, its corrosion resistance was similar to the low chromate conversion coating.Through present thesis research, it was shown that basic data of rare earth conversion coatings techniques could be built up, and a meaningful exploration for the replacement of the chromate treatment technology was carried out.更多还原