Quality criteria of powder coated HDG steel in corrosive environment

The performance of powder coatings on hot-dip galvanized steel has been studied by testing in 5 years field exposure in marine and industrial atmospheres, as well as in accelerated tests. The aim was to assess the quality of such coating systems in corrosive environments and to establish criteria for how to obtain good quality. The test data demonstrates that polyester powder coatings are a good alternative to wet paint as top coat on hot-dip galvanized steel. The phosphate layer is identified as the most crucial step to ensure high quality. Recommendations for how to obtain good quality are provided.     

Corrosion protection of AA6061 T6-10 % Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite by molybdate conversion coatings

The corrosion protection performance of aluminum composite materials was investigated in 3.5% NaCl solution for time ranging between 1–60 days. Series of specimens were treated in molybdate solutions under different conditions. Free immersion and electrochemical activation techniques were used to apply the molybdate layer. The specimens which were directly treated or pickled before molybdate treatment showed severe pitting corrosion. Conversely, the specimens which were subjected to an oxide thickening step before the molybdate treatment showed outstanding corrosion protection. XPS revealed a higher amount of molybdate as MoO₄^2– for the specimens prepared by electrochemical activation technique than for the free immersion technique.     

氯化钾镀锌工艺的发展

介绍了国内第一、二、三代氯化钾镀锌工艺添加剂的主要组分及性能特点.第三代氯化钾镀锌采用低泡载体光亮剂,除了具有第一、二代氯化钾镀锌的优点外,还具有以下特点:镀液可以采用压缩空气搅拌;允许电流密度大,生产效率高;镀层配合三价铬钝化,耐中性盐雾腐蚀性能超过氰化镀锌和碱性无氰镀锌.     

不锈钢槽体表面钝化工艺的优化

以50%(体积分数)的硝酸溶液对304和316化学镀镍用不锈钢槽体进行钝化处理,采用单因素试验和正交试验方法考察了钼酸铵及氧化钇的添加量、钝化时间和钝化温度对304和316不锈钢槽体表面钝化膜孔隙率的影响,确定了优化的钝化工艺条件如下:钼酸铵和氧化钇的添加量分别为硝酸质量的4.0%和0.4%,304和316不锈钢的钝化温度分别为40°C和35°C,钝化时间5 h。通过扫描电镜观察了钝化前后不锈钢的表面形貌,测定了钝化膜的组成。结果表明,钼酸铵和氧化钇的加入使钝化膜的孔隙率显著降低,提高了钝化膜的耐蚀性。优化工艺制备的钝化膜主要由Cr、Fe、Ni和Mo的氧化物组成,膜层平整致密。     

镀锡钢板钼酸盐钝化膜的耐蚀性能

采用硫酸铜点滴试验和电化学测试方法,研究了在钼酸盐溶液中加入植酸和添加剂对镀锡钢板钝化膜的影响。结果表明,采用钼酸钠10 g/L、磷酸5 mL/L、植酸5 g/L和添加剂1.5 g/L,能够获得具有高耐蚀性能的钝化膜。     

Defect dimension evaluation in organic coated galvanized steel by electrochemical impedance spectroscopy

The protective effectiveness of organic coatings, in controlling corrosion processes by the barrier effect, is dominated by the absence of defects passing through the coating and reaching the substrate. It is, however, difficult in general to identify and quantify the presence of defects. This work is an effort to reach a more precise quantification of the size of defect in organic coatings by means of electrochemical impedance spectroscopy (EIS) measurements. Artificial defects with controlled dimensions between 60 and 200 m were produced on organic coated galvanized steel (coil coating). After optimization of the experimental procedure for EIS data acquisition, the parameters obtained, according to a classical electrical model, were correlated with the defect dimensions. The results show that the double layer capacitance (C _dl) values depend in linearly on the defects area, while this is not true for the pore resistance (R _p) values, as the electrolyte resistivity inside the defects is a function of the defect size. Further work is necessary to extend the results to smaller defects and different systems, that is, different organic coatings and substrates.     

Strength of brittle materials in moderately corrosive environments

The strengths of four brittle materials―cordierite glass ceramic, fused silica, silicon, and polycrystalline alumina were measured after exposure to weakly corrosive water and moderately corrosive buffered HF (BHF) solution. Exposure to water did not alter the strengths in subsequent inert strength tests and decreased the strengths in reactive strength tests. Exposure to BHF increased the strengths in both tests, but only after an incubation exposure time. Prior to the incubation time, the BHF had the same effect as water, suggesting that the bond rupture kinetics were unaffected. Examination of strength‐controlling indentation flaws after the incubation time showed clear corrosive effects on the flaw geometry indicative of reductions in the indentation residual stress fields. The implication is that moderately corrosive environments increase the strength or lifetime of a brittle component by reducing the crack driving force via flaw alteration and do not, as perhaps expected, decrease the strength or lifetime through enhanced chemical reactivity.     

Protection of stainless steel by polyaniline films against corrosion in aqueous environments

Polyaniline (PANI) thin films were electrochemically deposited by cyclic voltammetry on stainless steel electrode previously covered by a thin film of polyvinyl acetate (PVAc). The corrosion resistance of PANI covered stainless steel substrates was estimated by using potentiodynamic polarization curves and its linear polarization resistance (LPR) was measured in 0.5 M H₂SO₄, 0.5 M NaCl and 0.5 M NaOH aqueous solutions at room temperature. The results indicate that the PANI-PVAc films did improve the corrosion resistance of the stainless steel in NaOH, behaving even worst, in the case of PANI film, than the uncoated substrate. In H₂SO₄ both PANI and PANI-PVAc coatings gave good protection for the stainless steel electrode, with a slightly better performance of PANI-PVAc than PANI. In NaCl solution both PANI and PANI-PVAc films provided a good protection against corrosion. The better performance of PANI-PVAc coatings for corrosion protection in basic media may be due to its major chemical stability compared to simple PANI films, which lose their conductivity in high pH solutions. The E _corr (free corrosion potential) value of the coated substrate was in the passive region of the uncoated substrate in acidic environment but in the active region in neutral or basic environment.     

An EXAFS investigation of molybdate-based conversion coatings

The composition of molybdate-based conversion coatings on zinc–nickel alloy electrodeposits was examined by extended X-ray absorption fine structure (EXAFS). The absorption spectra from appropriate standards showed distinct differences between the Mo(IV) and Mo(VI) oxidation states. A pre-edge peak was apparent for the various molybdate species and molybdenum trioxide, with the energy and intensity dependent on the coordination of the molybdenum. A shift in the molybdenum K-edge spectra was also evident between Mo(IV) and Mo(VI) compounds. A qualitative analysis revealed that both Mo(IV) and Mo(VI) species were present within the molybdate-based conversion coating, with a significant prevalence of hexavalent molybdenum.     

镀锌层钼酸盐钝化工艺研究

利用对比试验和正交试验对镀锌层钼酸盐钝化工艺进行了研究,通过中性盐雾试验、湿热试验及盐水浸泡试验,研究了钼酸盐钝化工艺参数对钝化膜耐蚀性的影响.通过X-射线光电子能谱对镀锌层钼酸盐钝化膜层进行了初步分析.结果表明:该处理工艺简单、成本较低,钝化膜主要元素为Zn、Mo、O.镀锌层采用钼酸盐钝化液处理后,耐蚀性明显提高,在3.5%的NaCl溶液中浸泡48 h无白锈生成.     

热浸镀Zn-Al合金钝化工艺及耐蚀性能研究

在钢板热浸镀Zn-5％Al合金表面制备了钼酸盐钝化膜,探讨了溶液pH、θ及t对钝化膜性能的影响.采用电化学方法对钝化膜耐腐蚀性能进行测试.结果表明,Zn-5％Al合金钝化膜阳极极化曲线呈现钝化特征.用扫描电镜对钝化膜的表面形貌进行观察,发现钝化膜呈片层状结构,用环境扫描电镜配套的能谱仪对钝化膜成分进行分析,结果表明钝化膜由Mo、P、O、Zn和Al等元素组成.     

新型氯化钾镀锌三价铬钝化剂和膜层耐蚀性

通过测定Tafel极化曲线、交流阻抗谱和进行盐雾试验,研究了新型三价铬彩色钝化剂SF-571和蓝白钝化剂SF-572对镀锌层耐蚀性能的影响,并与进口钝化剂进行对比。氯化钾镀锌工艺为:KCl200g/L,ZnCl250g/L,H3BO335g/L,柔软剂30mL/L,光亮剂1.5mL/L,25°C,25min,电流密度2A/dm2。电化学分析表明,所有镀锌层钝化膜在1%NaCl溶液中均存在钝化现象,采用自制钝化剂SF-571和SF-572所得膜层的耐腐蚀性能达到或优于用进口钝化剂所得膜层。对比膜层的表面形貌可知,自制钝化剂钝化所得膜层表面更平整、致密。     

In situ studies of conversion coated zinc/polymer surfaces during exposure to corrosive conditions

This work investigates the hidden interface between a conversion-coated zinc surface and a polymer coating upon exposure to an electrolyte by simultaneous in situ ATR-FTIR and EIS. Various system properties were distinguished, such as the ingress of electrolyte constituents, and an active process of water-induced alterations of the conversion layer. The interface between a polymer film and a surface treated metal surface is of considerable fundamental and technical interest in many areas of application, and the results obtained open up the use of this method for a wide range of important applications.     

Role of chromate, molybdate and tungstate anions on the inhibition of aluminiumin chloride solutions

The effect of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions on the inhibition of aluminium corrosion in 0.5 M NaCl solution was investigated. The study comprised potentiodynamic polarization, potentiostatic current–time measurements complemented by SEM–EDAX and XPS investigations. It was found that, the pitting potential of an Al electrode in 0.5 M NaCl solution shifts in the positive direction by addition of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions, and the shift in potential increases with increase in concentration. A pronounced inhibiting influence was achieved on addition of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions to the electrolyte during potentiostatic current–time measurements. Chromate anions exhibit a great passivating influence during I/t measurements. This can be explained by the fact that the chromate anion, as a powerful oxidizing agent, is capable of oxidizing the corrosion sites to give a stable Al₂O₃ film. The inhibition observed on addition of molybdate anions is attributed to the adsorption and reaction of MoO₄ ^2– anions on the electrode surface forming a molybdate layer which selectively impedes the ingress of Cl^– ions and hence inhibits the pitting attack. The adsorption of WO₄ ^2– anions at flawed areas and developing pits was found to be the main factor for the observed inhibition.     

Investigation of strength degradation of adhesive-bonded aluminum joints exposed to corrosive environment using electrochemical methods

Interfacial corrosion is responsible for the strength degradation of adhesive-bonded aluminum joints (ABJs) exposed to corrosive environment. In this study, electrochemical noise and electrochemical impedance spectroscopy (EIS) measurements were performed on the aluminum alloy X610-T4PD covered with adhesive (ACA) to understand the interfacial corrosion. And environmental simulation tests (i.e. neutral salt spray (NSS) and hot humidity environment) for ABJ were carried out to investigate the joint strength degradation. Test results indicated that the variations of current and potential in the EN measurement were closely related to the initiation of corrosion in the samples. The Nyquist plots in the EIS measurement for various immersion times showed that the corrosion of ACA accelerated after about 140 h. Furthermore, a linear relationship between the residual strength of ABJ exposed to NSS and the reciprocal of interfacial corrosion resistance (R_t) of ACA was found, which was verified by ABJ exposed to hot humidity environment.     

铝及其合金无铬钝化的研究进展

综述了国内外铝及其合金的无铬钝化体系的研究状况及其特点,为铝和铝合金无铬钝化的研究提供参考。并对铝及其合金无铬化学转化膜的研究方向提出展望。     

Behaviour of intact and scratched phosphate coatings on zinc, zinc–nickel and mild steel in dilute sodium phosphate solution

Impedance measurements were performed at room temperature in a 0.005 M Na₂HPO₄ solution on steel and on electrodeposited Zn and Zn–12%Ni before and after phosphating. It was found that potential and impedance parameters (R _ct and C _d) for phosphated materials attained nearly steady values within about 20 min, indicating that this solution did not cause meaningful changes in phosphate coatings and that it could therefore be used for the quality evaluation of these coatings. On scratches in phosphate coatings on Zn or Zn–12%Ni phosphate deposits rapidly formed, probably owing to enhanced dissolution of the locally exposed substrates at the nobler potentials of the coated materials. This demonstrates that damaged coatings can easily recover on Zn substrates in a non-aggressive phosphate solution. It is suggested that the formation of phosphate deposits on bare metal amidst phosphate coatings should be taken into account in the porosity determination by chemical or electrochemical methods.     

Model epoxy powder coatings and their adhesion to steel

Model epoxy powder coatings differing in crosslink density were applied to steel. Their adhesion loss upon immersion in water at 35 and 55°C, as well as recovery of the adhesion upon drying were determined. Permeability of free films to gaseous water and oxygen were measured. Impedance measurements were used to study the diffusion characteristics of water into the coatings on steel. The permeability data of the formulations were similar. However, the adhesion behavior was very different. The adhesion in the dry state proved highly dependent on the rheology during the cure process. The adhesion recovery after immersion was apparently determined by the effectiveness of water as a plasticiser in the polymer network.     

不同镀锌工艺三价铬钝化耐腐蚀性研究

采用中性盐雾试验和电化学阻抗谱,研究了酸性氯化钾镀锌、碱性无氰镀锌和碱性氰化物镀锌3种工艺所得镀锌层经三价铬彩色钝化后钝化膜的耐蚀性.结果表明:钝化后,酸性氯化钾镀锌层的耐蚀性最好,碱性无氰镀锌层次之,碱性氰化物镀锌层最差.在质量分数为1%的氯化钠溶液中,镀锌层钝化膜电阻的大小顺序为:酸性氯化钾镀锌>碱性无氰镀锌>碱性氰化物镀锌.酸性氯化钾镀锌钝化膜的耐蚀性较好可以与所得镀层中有机杂质夹杂较少有关.     

镀锌蓝白无铬钝化工艺的研究

采用钼酸钠与有机酸复配,研究了镀锌蓝白无铬钝化工艺.该钝化液稳定性好且成本较低,钝化处理工艺简单,能够在镀锌钢板上形成光亮、均匀的蓝白钝化膜.通过CuSO4点滴实验、盐水浸泡实验及电化学测试对钝化膜的耐蚀性能进行了研究.结果表明:钼酸钠与有机酸复配形成的钝化膜均匀、细致、不存在微孔及裂纹,其耐蚀性与铬酸盐钝化膜的相当....