【作者】 唐鋆磊；

【导师】 左禹；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2010， 博士

【摘要】 不锈钢材料具有高的化学稳定性和优良的综合机械性能,是由于其基体中含有Cr, Ni等合金元素,使不锈钢表面有很强的钝化能力,使其在很多介质中都具有优良的耐腐蚀特性,所以在诸多工业领域得到广泛应用。但是在许多化工生产设备中一些无机酸类如硫酸等的工作温度往往达到85℃以上,由于温度升高,这些无机酸就可能由氧化性转化为还原性,这样不锈钢表面的钝化膜将会溶解掉,并且失去了自我修补的能力。大部分的有机酸,如醋酸和甲酸都具有一定的还原性,在高温条件下也会对不锈钢造成较快的腐蚀。通过一定的表面改性方法的方法对不锈钢进行处理,能够改善其耐蚀性,Pd作为阴极性元素添加入钝性金属后能够起到较好的改性作用,显著提高钝性金属的耐蚀性,但前人的研究多局限于Pd元素对于Ti基金属的改性,而且多为添加入基体中或者利用离子注入等高能物理方法进行表面处理。这些方法都存在着成本较高,工业适用性较差的特点。本文采用化学镀,电镀和电刷镀等技术成熟、工业适用性强的表面处理方法成功地在不锈钢基材上获得了结合力良好、耐蚀性能优异的Pd及Pd系合金膜层,获得的Pd系膜层能够对不锈钢基体起到显著的表面改性作用,通过电化学作用在高温非氧化性酸性介质中获得非常优异的耐蚀性能。这些镀膜技术相对于在不锈钢基体中添加合金元素而改善其耐蚀性的方法,其成本将会大大降低,且基体的机械加工性能不受影响,而且适用于各种形状的不锈钢工件,其中电刷镀技术可以应用于大型设备的现场施工。(1)开发了一种适合于316L不锈钢基材的化学镀Pd工艺,获得了膜层均匀、结合力良好的化学镀Pd膜。通过电子扫描显微镜(SEM)、能谱(EDS)、X射线光电子能谱(XPS)等方法表征了316L不锈钢表面化学镀Pd膜的表面形貌与膜层成分。通过浸泡实验、极化曲线和电化学交流阻抗(EIS)研究了316L不锈钢表面化学镀Pd试样在硫酸介质和甲乙混合酸介质中的腐蚀行为及规律,评价了其在这两种典型非氧化性酸性介质中的使用性能。结果表明：316L不锈钢表面化学镀Pd膜主要由Pd、N、P、O组成,在沸腾稀硫酸中耐蚀性能优异,腐蚀速率较316L不锈钢下降了4个数量级,在甲乙混合酸中腐蚀速率也显著降低。在含卤族离子的沸腾硫酸溶液中,当卤族离子浓度很低时,化学镀钯膜仍具有优异的耐蚀性能,随着卤族离子浓度的增加耐蚀性能下降,溴离子比氯离子对试样的腐蚀作用更强。在甲乙混合酸介质中,随溴离子浓度的增加,化学镀Pd试样的耐蚀性能下降。(2)开发了一种适合于316L不锈钢基体的电镀Pd膜工艺,能够在不锈钢基材上获得结合力良好,表面均一的电镀Pd膜。通过SEM、EDS、XPS、XRD、TEM等方法表征了316L不锈钢表面化学镀Pd膜的表面形貌、膜层成分与结构。通过纳米压痕、显微硬度测量表征了膜层的物理性能。通过腐蚀挂片、极化曲线测量和EIS研究了316L不锈钢表面化学镀Pd试样在硫酸介质和甲乙混合酸介质中的腐蚀的腐蚀行为及规律,评价了在这两种典型非氧化性酸性介质中的使用性能，并和化学镀Pd膜进行了对比。实验结果表明电镀Pd膜膜层晶粒均匀细致,基本为纯Pd,膜层为多晶结构,晶格结构为面心立方体。电镀Pd膜在上述两种介质中的腐蚀行为和规律与化学镀Pd膜相似,但电镀Pd膜因为组成更纯净而比化学镀Pd膜拥有更好的耐蚀性能。(3)首次报道了一种专门针对不锈钢基体的电刷镀Pd膜工艺,在316L不锈钢表面获得了获得结合力良好,在高温非氧化性酸性介质中耐蚀性优良的电刷镀Pd膜。利用SEM、EDS、XRD等方法表征了316L不锈钢表面电刷镀Pd膜的表面形貌、膜层成分与结构,以及电刷镀工艺参数对于膜层的影响规律。结果表明电刷镀Pd膜的形貌与刷镀电压和刷镀速度相关,膜层的微裂纹随刷镀电压的增加而减少,随刷镀速度的增加而增多。膜层的晶格结构也随刷镀电压和速度呈现规律性变化,晶格常数随刷镀电压的增加而增加,随刷镀速度减少而增加。膜层中的共沉积氢对膜层的结构变化作用明显,并且其含量显著影响电刷镀Pd膜的性质,随膜层中共沉积氢的增加,膜层的耐蚀性能下降。(4)开发了一种电镀Pd-Ni合金膜工艺和一种电刷镀Pd-Cu合金膜工艺,在316L不锈钢上制备得到了外观均一,结合力良好的Pd-Ni合金膜和Pd-Cu合金膜。腐蚀挂片和电化学测试表明Pd-Ni合金膜层在高温硫酸介质中的耐蚀性能优异,但却不具备在高温甲乙混合酸介质中对316L不锈钢基体的保护效果,而Pd-Cu合金膜在高温硫酸介质中不具备防护性能,但在高温甲乙酸介质中对316L不锈钢基体有优异的保护效果。通过SEM、EDS、XPS、XRD等研究方法对Pd-Ni合金膜和Pd-Cu合金膜进进行了表征,研究结果表明：Pd-Ni合金膜为面心结构的完全固溶体,膜层中的Ni含量可以通过工艺参数在30-50at.%之间调控,Pd在Pd-Ni膜层表面富集,膜层中Ni含量的增加会降低膜层的耐蚀性。电刷镀Pd-Cu合金膜的晶粒大小随刷镀电压的增大而减少,但刷镀电压对膜层中Pd和Cu的相对含量却影响不大,与Pd-Ni相反,Pd-Cu合金的表面存在Cu的富集。(5)系统地通过动电位扫描技术、电位测量技术、电偶腐蚀实验、交流阻抗技术和X射线光电子能谱研究了不锈钢和Pd系膜层的复合体系在高温硫酸中的耐蚀机理和腐蚀失效机制。Pd系膜覆盖不锈钢表面后,由于电偶作用,不锈钢基体处于钝化区,暴露的不锈钢表面则生成稳定的钝化膜,不锈钢因此而获得了优异的耐蚀性。镀钯体系的腐蚀电位与该偶合体系的阴阳极面积比密切相关,两者的面积比与混合电位之间存在着对应关系。只有混合电位达到或超过基体的钝化电位,体系才能获得稳定的钝态,而阴极和阳极面积比越大,体系越稳定。Pd/不锈钢复合体系腐蚀失效过程基本可以划分为三个阶段：稳定阶段,缓慢腐蚀阶段和腐蚀迅速进行阶段。基体缓慢腐蚀暴露出更多的基体面积与氢去极化反应是镀Pd膜腐蚀失效的两个根本原因,其中基体腐蚀是内在驱动力,而氢去极化起到了加速腐蚀的作用。交流阻抗研究能够佐证耐蚀物理模型。Mott-Schottky曲线的结果和XPS一致,显示出在自腐蚀电位范围内,镀Pd试样的表面钝化膜为Cr2O3和Pd的低价氧化物。更多还原

【Abstract】 It is well known that stainless steels owe their corrosion resistance largely to the formation of passive films on the alloy surface. Stainless steels show good corrosion resistance in oxidizing corrosive mediums where the passive films formed on the surface are stable. However, in many reductive corrosion mediums such as boiling dilute sulfuric acid solutions or boiling acetic plus formic acids, passivity can not be steadily established on the surface and active corrosion happens for stainless steels. For the alloys with passive ability such as titanium alloys or stainless steels, if corrosion potential of the alloy is raised from active potential into passive region by applied anodic current or by alloying with elements with higher oxidation/reduction potentials, corrosion resistance would be improved. Different techniques were reported for palladium deposition on titanium, including vacuum evaporation, ion beam mixing, etc.. The excellent corrosion resistant Pd and Pd alloy films were prepared on 316L stainless steel by electroless plating, electroplating and electronic brush plating in this paper. The palladium plated stainless steel obtained very good corrosion resistant in boiling dilute sulfuric acid solutions and boiling acetic plus formic acids. These processes provided the possibilities of these films used in many industry conditions. (1) An uniform palladium film on 304 stainless steel was obtained by electroless plating. Scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), weight loss tests and electrochemical measurements were used to character the properties of the film. The palladium plated stainless steel samples showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling dilute sulfuric acid solutions and boiling acetic/formic acids, corrosion rates of palladium plated 316L stainless steel samples were 4 and 2 orders of magnitude lower than the original 316 stainless steel samples. In these acid solutions with Cl- or Br- concentration less than 0.1mol/L, the palladium plated samples also showed better corrosion resistance. The Pd plated samples also showed very good corrosion resistance in boiling mixtureof 90% acetic acid + 10% formic acid + Br- solution. However, the corrosion resistance of Pd plated samples decreased with the increase of halogen ions concentration.(2) Palladium films with good adhesive strength were deposited on 316L stainless steel by electroplating. SEM, EDS, XPS, weight loss tests and electrochemical methods were used to study the properties of the films. The electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solution, the corrosion rates of the palladium plated 316L stainless steel samples were 4 orders of magnitude lower than that of the original 316L stainless steel samples. In the solution with 0.1mol/L Cl- or Br-, the palladium plated samples also showed better corrosion resistance. The Pd plated samples also showed very good corrosion resistance in boiling mixtureof 90% acetic acid + 10% formic acid + Br- solution. However, the corrosion resistance of Pd plated samples decreased with the increase of halogen ions concentration. In comparison, the electroplated samples showed slightly better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former.(3) Palladium films with good adhesive strength were deposited on 316L stainless steel by brush plating. SEM, EDS, XPS, weight loss tests and electrochemical methods were used to study the properties of the films. The brush plated palladium film was mainly consisted of palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples showed excellent corrosion resistance in boiling 20% H2SO4 solution and boiling acetic/formic acids with 0.005mol/L Br- ions added. The corrosion rates of the palladium plated 316L stainless steel samples were about two orders of magnitude lower than that of the original 316L stainless steel samples. This method provides a possibility to prepare protective palladium films on stainless steel facilities with large areas in industrial sites. The brush plating parameters have obvious influences on electrochemical properties of the palladium films. As the brush plating voltage and the brushing speed increasing, the open circuit potential of the plated samples decreased. The effects of brush plating parameters on corrosion behaviors of the films are mainly attributed to the effect of co-deposited hydrogen in the films. When hydrogen is removed from the Pd films by heat treatment, corrosion resistance of the films turns better.(4) Pd-Ni alloy film was plated on stainless steel by electroplating and Pd-Cu alloy film was plated by brush plating. Immersion tests show that the film Pd-Ni alloy films have good corrosion resistance performance in boiling sulfuric acid, but they are not corrosion resistant in boiling acetic/formic acids. Pd-Cu alloy film does not have corrosion resistance in boiling sulfuric acid, but they showed excellent corrosion resistance in boiling acetic/formic acids with 0.005mol/L Br- ions added. SEM, EDS, XPS, X-ray diffraction and other methods were used for studying these films. The crystal lattice of Pd-Ni alloy film is face centered cubic structure and the Pd-Ni alloy film is complete solid solution. The Ni content can be controled from 30 to 50 at.% by plating parameters. The surface of the film is almost constituted of Pd element. The corrosion resistance of film decreases with the increasing of Ni content. The grain size of brush Pd-Cu alloy plating film decreased with increase of brush voltage, but the relative content of Pd and Cu was not affected by the brush voltage. The Cu element is enrichment on surface of Pd-Cu alloy film.(5) The corrosion mechanism and corrosion failure mechanism of stainless steel and Pd composite system in high-temperature sulfuric acid were studied in detail. The potentiodynamic scanning technique, potential measurement, galvanic corrosion test, EIS, XPS are used in research. Due to the galvanic effect, stainless steel substrate shifts to the passive zone by the covering of Pd, and stable passive film is produced on stainless steel surface where exposed to acid solution. Thus, the stainless steel obtained excellent corrosion resistance. The corrosion potential of galvanic Pd/stainless steel is closely related to Cathode/anode area ratio. There is a corresponding relationship between the area ratio and galvanic potential. Only the galvanic potential reached or exceeded the passive potential of stainless steel, the system can be stable passive. While the larger of the cathode/anode area ratio, the system is more stable. The corrosion failure process of Pd/stainless steel system can be basically divided into three stages:stabilization stage, the slow erosion stage and rapid corrosion stage. The more of the area exposed by slow corrosion of the substrate and hydrogen reduction are the two most fundamental reasons of corrosion failure of the Pd plated stainless steel. The EIS measurement results fit the physical model well. The Mott-Schottky curves and XPS analysis show that the surface of Pd-plated sample is p-type oxide semiconductor at open circuit potential. It indicated the surface passivation film is composed of the low-state oxides of Cr2O3 and PdO.更多还原