Applications of carbonic acid solution for developing conversion coatings on Mg alloy

Works on exploring an environmentally clean method for producing an Mg,Al-hydrotalcite(Mg6Al2(OH) 16CO3·4H2O) layer and/or calcium carbonate(CaCO3) layer on Mg alloy in a carbonic acid solution system(aqueous HCO3-/CO3 2-or Ca 2+ /HCO3-) at 50℃ were reviewed.Conversion treatment for the Mg,Al-hydrotalcite conversion coating was as follows.Mg alloy was treated first in acidic HCO3-/CO3 2-aqueous for precursor layer formation on Mg alloy surface and then in alkaline HCO3-/CO3 2-aqueous to form a crystallized Mg,Al-hydrotalcite coating.Duration of an Mg,Al-hydrotalcite coating on Mg alloy surface was reduced from 12 h to 4 h by the conversion treatment.On the other hand,for reducing the formation time of CaCO3 coating on Mg alloy,the aqueous Ca 2+ /HCO3-with a saturated Ca 2+ content was employed for developing a CaCO3 coating on Mg alloy.A dense CaCO3 coating could yield on Mg alloy surface in 2 h.Corrosion rate(corrosion current density,Jcorr) of the Mg,Al-hydrotalcite-coated sample and CaCO3-coated AZ91D sample was 7-10μA/cm 2,roughly two orders less than the Jcorr of the as-diecast sample(about 200μA/cm 2) . No corrosion spot on the Mg,Al-hydrotalcite-coated sample and CaCO3-coated sample was observed after 72 h and 192 h salt spray test,respectively.     

pH值对AZ91D镁合金表面植酸转化膜的影响

在pH值不同的几种植酸转化液中对AZ91D镁合金进行表面转化处理,利用扫描电镜及自带能谱仪、Tafel曲线和阻抗谱等分析手段,结合NaCl水溶液点滴实验,研究了pH值对植酸转化膜表面成分及耐蚀性的影响.结果表明:在酸性条件下形成的植酸转化膜的耐蚀性较好;转化液的pH值会影响植酸螯合物的形成和转化膜的物质组成,进而影响转化膜的耐蚀性能;在酸性条件下,镁合金表面形成的植酸转化膜应属于电子导体膜,它阻碍了腐蚀介质与基体的接触,同时抑制腐蚀产物的扩散,对镁合金起到防护作用.     

成膜温度对AZ91D镁合金表面植酸转化膜的影响

利用Tafel曲线、阻抗谱分析、NaCl水溶液点滴实验和扫描电镜观察等手段,研究了温度对AZ91D镁合金的植酸转化膜表面形貌及耐蚀性的影响.结果表明:当温度为25℃～35℃时转化膜的耐蚀性较好,温度过高或过低都会使转化膜的耐蚀性变差;转化膜表面的裂纹是在干燥处理过程中由于表层转化膜体积收缩而产生的;镁合金表面形成的植酸转化膜应属于电子导体,该层膜的形成阻碍了腐蚀介质与镁合金基体的接触,同时抑制腐蚀产物的扩散,对镁合金起到了较好的防护作用.     

Enhanced corrosion resistance of phytic acid coated magnesium by stearic acid treatment

A green chemical conversion coating for magnesium was obtained with a phytic acid solution. The microstructure and corrosion properties of phytic acid conversion coated magnesium were further improved by soaking in stearic acid solution. The phytic acid conversion coated magnesium after soaking in the stearic acid showed no micro-cracks and the surface became very smooth. The corrosion behavior of the uncoated and coated magnesium samples was studied by electrochemical methods. The corrosion resistance of the stearic acid treated sample was much higher than that of phytic acid conversion coated magnesium or uncoated magnesium. The electrochemical results indicated that the stearic acid treated coating provided effective corrosion protection to the magnesium sample.     

Electrochemical properties and structure of vanadium-based conversion coatings on electro-galvanised steel

The corrosion resistance of vanadium-based conversion coatings on electro-galvanised steel (EG) was examined in terms of the different valence species V⁵⁺ and V⁴⁺ as the raw material in the treatment solution. The effects of mixing phosphoric acid with the vanadium-based conversion coating have also been investigated. Corrosion performance was tested by neutral salt spray test and the mechanism of corrosion resistance was analysed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. It was found that conversion coatings formed from V⁵⁺ or V⁴⁺ did not clearly show different corrosion behaviour, but that the addition of phosphoric acid to the solution including V⁵⁺ improved corrosion protection. These results indicate that mixing phosphoric acid lowers the pH of the solution and improves corrosion resistance. The role of phosphoric acid is also explained by the formation of hydrogen bonds at the interface between the vanadium species and EG substrate.     

用于钢铁零件工序间防锈的植酸防锈液研制及其性能

以环保特性优良的植酸为基本成分(150 mg/L),通过添加少量生物缓蚀剂、稀土缓蚀剂等成分作为工序间零件的防锈溶液,对机加工零件经过工序间防锈处理的试样进行湿(相对湿度90%)热(50℃)环境下的加速腐蚀、中性盐雾腐蚀试验,考察机加工钢铁零件经过工序间防锈处理后的防锈性能和电化学行为。试验结果表明:工序间防锈液处理的零件在高温高湿环境下的防锈性能优良。工序间防锈液中的植酸与生物缓蚀剂、稀土缓蚀剂在钢铁零件工序间防锈处理中具有协同效应。经工序间防锈液处理的试样其阻抗谱出现两个容抗弧,且腐蚀电位正移400 mV。     

MB8镁合金植酸转化膜的制备及性能

通过单因素实验优化镁合金植酸转化工艺,发现在植酸体积浓度为10 mL/L,pH=2.5,温度50℃,成膜时间20 min条件下,可在MB8镁合金表面制备出均匀一致的植酸转化膜.该膜层微观形貌与铬酸盐转化膜类似,表面呈现出均匀分布的网状微裂纹,类似于"龟裂的土地",膜层主要成分为Mg,O,P,Mn和C;动电位极化曲线测试...     

Study of the formation and growth of tannic acid based conversion coating on AZ91D magnesium alloy

A tannic acid based conversion coating was formed on AZ91D magnesium alloy through a solution containing C₇₆H₅₂O₄₆ (tannic acid), NH₄VO₃, K₂ZrF₆ and H₃PO₄. The growth process and corrosion resistance of the conversion coating were investigated by SEM, FESEM, XPS, IR, potentiodynamic polarization and EIS test. The results showed that the main components of the coatings were penta-hydroxy benzamide–magnesium complex, Al₂O₃ and MgF₂. Zr(HPO₄)₂·H₂O(α-ZrP) was first formed in the time interval of 70 s–120 s, which acted as catalyst to activate the main reaction of gallic acid being oxidized into the penta-hydroxy benzamide–magnesium complex, as well as the formation of MgF₂ and Al₂O₃, which took place in the time interval of 120 s to 600 s. The thickness of coating treated for 300 s and 600 s was 1.1 μm and 1.6 μm, respectively. Polarization measurement and EIS test in 3 wt.% NaCl solution indicated that the conversion coating treated for about 300–600 s shows good corrosion resistance while an excess time treatment (720 s) induces the coating surface coarse and uneven, leading to the reduction of corrosion resistance of the conversion coating.     

转化时间对AZ31B镁合金植酸转化膜的影响

采用析氢试验、Tafel分析方法及SEM、EDS对AZ31B镁合金在植酸溶液中不同时间所形成转化膜的性能、表面微观结构及成分进行了研究。结果表明,当转化时间较短时,转化膜的防腐蚀性能随着转化时间的增加而增加,并在转化时间为40min时,其转化膜防腐蚀性能最佳;随着转化时间的进一步增加,转化膜的防腐蚀性能随之下降。植酸转化膜含有镁,铝,锌,氧,磷元素,转化膜表面存在一定的裂纹,且裂纹处仍有很薄的一层植酸转化膜。此外,析氢试验、电化学试验及动力学分析表明,植酸转化试样相比于未处理试样,具有更好的防腐蚀性能。     

ZK60镁合金磷酸盐及锡酸盐化学转化膜

为了提高ZK60镁合金的耐腐蚀性能,利用磷酸盐溶液或锡酸盐溶液,考察了在ZK60镁合金上形成磷酸盐或锡酸盐化学转换膜的工艺条件及膜层的耐蚀性能。通过改变处理时间和温度,可以得到性能不同的转换膜。通过电化学阻抗谱技术和极化曲线技术研究转化膜的耐腐蚀性能,利用扫描电镜研究其微观结构。结果表明:在磷酸盐溶液中,当处理温度为50℃时,磷化30min后试样的电荷转移电阻(Rct)为224.03Ω·cm2;当温度为80℃,反应时间为45min时,所得转换膜的Rct为377.67Ω·cm2,阻值最大,耐腐蚀性能最好;对于锡酸盐化学转化膜,90℃下,处理60min的膜层耐蚀性能最好,其Rct为388.32Ω·cm2,与磷酸盐化学转化膜相比,两种膜的保护性能相差不大。     

镁合金表面植酸转化膜研究Ⅱ.pH值对镁合金植酸转化膜的影响

通过SEM观察、AES元素深度分析、成膜增重实验、动电位极化曲线和EIS电化学测试，研究了植酸处理液的pH值对AZ91D镁合金转化膜生长速度及耐蚀性的影响.结果表明：溶液在pH=8时，转化膜生长速度最快，膜层较厚，且完整，无碎裂；pH=12时，转化膜生长速度较慢，膜层较薄；pH=5时，转化膜生长速度最慢，有碎裂.当植酸溶液的pH值在5～12时，镁合金表面形成的转化膜均可提高其耐蚀性，且pH=8时形成的膜耐蚀性最好.     

冷轧钢表面植酸-硅烷双膜复合层的耐蚀性能

为了研究植酸-硅烷复合膜对冷轧钢腐蚀行为的影响,采用浸泡法在冷轧钢板表面分别制备了植酸化学转化膜、硅烷膜层及植酸-硅烷复合膜层。采用扫描电子显微镜观察冷轧钢板表面经植酸转化膜和植酸-硅烷复合膜层处理之后的形貌;运用红外光谱和X-射线光电子能谱方法研究植酸转化膜及植酸-硅烷复合膜层的化学组成;采用极化曲线、电化学阻抗谱、盐雾试验研究植酸-硅烷复合膜层的耐蚀性能及在3.5%氯化钠腐蚀溶液中的稳定性。结果表明:植酸-硅烷复合膜层的耐腐蚀性能及在氯化钠溶液中的稳定性明显高于单一的硅烷膜层。其作用机理在于植酸转化膜一方面可以提高硅烷膜层与金属基体的结合力,另一方面可以与硅烷膜层协同作用提高复合膜的耐腐蚀性能。     

铝合金表面钼酸盐改进植酸转化膜的耐蚀性能

目的通过钼酸钠(SM)添加剂、SM前处理、SM后处理三种方案对铝合金表面植酸转化膜进行改进研究,以进一步提高其耐蚀性。方法通过动电位极化测试研究改进后铝合金在3.5%(质量分数)Na Cl溶液中的耐蚀性。结果随着SM添加剂浓度的增加,铝合金表面植酸转化膜的耐蚀性先增强再减弱,SM质量浓度为30 g/L时的腐蚀保护效率Pe最大,达95.5%,而不含SM时的Pe仅为86.8%。p H值太大(p H=8.0)或太小(p H=3.0)都不利于形成耐蚀性更好的膜层,p H值为6.0时的Pe达98.6%。SM后处理会严重影响植酸转化膜的耐蚀性,腐蚀电流密度Jcorr大幅增大;SM前处理可提高植酸转化膜的耐蚀性,Pe达98.2%;SM前处理与添加剂同时应用时,植酸转化膜耐蚀性提高幅度更显著,Jcorr仅为0.042μA/cm2,极化电阻Rp达222 kΩ·cm2,Pe达99.5%。结论 SM添加剂和SM前处理均可明显提高铝合金表面植酸转化膜的耐蚀性,且复合作用时的效果更显著,而SM后处理不能提高铝合金表面植酸转化膜的耐蚀性。     

Chromate conversion coating treatments for electrodeposited zinc-iron alloy coatings from an acidic sulphate bath

The composition, microstructure and morphology of a zinc-iron alloy deposit obtained from an acid sulphate bath have been investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that under the optimised conditions, the bath produced smooth, uniform, bright, fine-grained zinc-iron alloy deposits having the desired 0.4–0.8 wt.% Fe. The zinc-iron alloy deposit was identified as h.c.p. η—phase and showed an highly preferred orientation. A low concentration Chromate treatment has been successfully developed and applied to electrodeposited zinc-iron alloy coatings with 0.4–0.8 wt.% Fe. The corrosion resistance of the Chromate conversion coatings on zinc-iron alloy deposits has been assessed using the neutral spray tests and electrochemical measurements. The results indicated that the corrosion resistance of the zinc-iron alloy deposit was significantly improved and strongly depended on iron content of the alloy deposit after Chromate passivation treatment. The composition depth-profile of the Chromate conversion coating was measured by sequential use of X-ray photoelectron spectroscopy (XPS) and sputter-etching. It was found that the major constituents of the Chromate conversion coating were zinc, chromium and oxygen, and a small amount of iron was also detected. According to analysis of the XPS data, the Chromate conversion coating on the zinc-iron alloy deposit is mainly composed of oxides and hydroxides of zinc and chromium. SEM investigation indicated that the surface morphology of the Chromate conversion coating was characterised by some ‘dried riverbed’ microcracks.     

Corrosion resistance,composition and structure of RE chemical conversion coating on magnesium alloy

Golden yellow rare earths chemical conversion coating was obtained on the surface of magnesium alloy by immersing in cerium sulfate solution.The corrosion resistance of RE conversion coating was evaluated using inmersion test and potentiodynamic polarization measurements in 3.5%NaCl solution.The morphologies of samples before corrosion and after corrosion were observed by SEM.The structures and compositions of the RE conversion coating were studied by means of XPS,XRD and IR.The results show that,the con...     

Effect of substrate temperature on adhesion strength of plasma-sprayed nickel coatings

We plasma-sprayed nickel coatings on stainless steel and cobalt alloy coupons heated to temperatures ranging from room temperature to 650 °C. Temperatures, velocities, and sizes of spray particles were recorded while in-flight and held constant during experiments. We measured coating adhesion strength and porosity, photographed coating microstructure, and determined thickness and composition of surface oxide layers on heated substrates. Coating adhesion strength on stainless steel coupons increased from 10–74 MPa when substrate temperatures were raised from 25–650 °C. Coating porosity was lower on high-temperature surfaces. Surface oxide layers grew thicker when substrates were heated, but oxidation alone could not account for the increase in coating adhesion strength. When a coupon was heated to 650 °C and allowed to cool before plasma-spraying, its coating adhesion strength was much less than that of a coating deposited on a surface maintained at 650 °C. Cobalt alloy coupons, which oxidize much less than stainless steel when heated, also showed improved coating adhesion when heated. Heating the substrate removes surface moisture and other volatile contaminants, delays solidification of droplets so that they can better penetrate surface cavities, and promotes diffusion between the coating and substrate. All of these mechanisms enhance coating adhesion.     

AZ31镁合金钕基转化膜研究

通过化学转化成膜在AZ31镁合金表面制备了钕基转化膜。利用称重实验和OCP测试研究了钕基转化膜的成膜过程,利用SEM,EDS和XPS分别研究了膜层形貌,微观结构和组成成分。研究了空白样品与镀膜样品在3.5 wt.% NaCl 溶液中浸泡不同时间后样品表面形貌和成分,结果表明钕基转化膜可以有效的降低AZ31镁合金在NaCl溶液中的腐蚀速率。利用XRD和电化学测试研究了经过后处理以后的钕基转化膜的成分和耐蚀性能,结果说明后处理可以进一步改善钕基转化膜的腐蚀性能。     

镁合金表面新型高耐蚀性植酸化学转化膜的制备

目的 提高镁合金的耐蚀性能。方法 通过在单一植酸转化膜基础上,利用植酸的强络合作用,在反应溶液中引入金属离子,获得新型高耐蚀性植酸转化膜层。通过正交实验、SEM、EDS和电化学方法,对新型植酸转化膜层的工艺配方、微观形貌、元素组成和耐蚀性进行分析。结果 正交试验研究得到了新型植酸转化膜层的最佳工艺参数为：25 g/L Mn(H2PO4)2+10 g/L植酸,反应温度95 ℃,反应时间10 min。在此工艺下,得到的膜层均匀,和未添加Mn离子的膜层相比,裂纹明显减小。EDS结果表明,膜层的主要元素为Mg、P、O、Mn、C,转化膜的成分可能是锰、镁与植酸形成的络合物。动电位极化曲线的实验结果表明,新型植酸转化膜可以极大地降低镁合金的腐蚀电流密度（Jcorr=0.337 ）,对镁合金的保护效率为96.37%。结论 通过在植酸转化膜中添加Mn元素,不仅可以减小植酸膜层的裂纹宽度,还可以极大地提高镁合金的耐蚀性。     

Mono‐carboxylate conversion coatings for AZ31 Mg alloy protection

Conversion coatings on a magnesium alloy were obtained by dipping AZ31 specimens in aqueous solutions of sodium salts of mono‐carboxylic acids (stearic, palmitic, myristic, lauric, mono‐carboxylate ion concentration from 1 to 5 mM, depending on the salt solubility) for 24 and 72 h at room temperature, or 24 h at 50 °C. The influence exerted by the treatment time, bath temperature and alkyl chain length on the efficiency of these coatings was studied. The performances of the coatings were evaluated by potentiodynamic polarization curve recording after 1 h immersion in 0.05 M Na₂SO₄ solution, while their temporal evolution was monitored by electrochemical impedance spectroscopy (EIS) spectra during 24 h. Further and long lasting tests were carried out also in 0.1 M NaCl solution. The efficiency of the coatings depended on the aliphatic chain length, and increased as the treatment time and the bath temperature were increased. The coating of lower homologue only hindered the cathodic process, while those of the higher homologues markedly inhibited the anodic process too. The best performances were displayed by 24 h‐50°C stearic conversion coating, which maintained a very high efficiency for over 800 h immersion in 0.05 M sulphate solution.     

Electrodeposition of zinc on AZ91D magnesium alloy pre‐treated by stannate conversion coatings

A stannate chemical conversion process followed by an activation procedure was employed as the pre‐treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre‐treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X‐ray diffraction (XRD). The electrochemical corrosion behavior of the coatings in the corrosive solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the activated stannate chemical conversion coating provided a suitable interface between zinc coating and the AZ91D magnesium alloy substrate. The corrosion resistance of the AZ91D substrate was improved by the zinc coating.