The influence of zinc surface pretreatment on the adhesion of epoxy coating electrodeposited on hot-dip galvanized steel

The adhesion and electrochemical properties of epoxy coatings electrodeposited on hot-dip galvanized steel with and without passive films were investigated during exposure to 3% NaCl. The passive films were formed in hot air, in boiling water and by chromating. Adhesion was measured both by a standardized pull-off method and by swelling in N-methyl pyrrolidone. Pretreatment of hot-dip galvanized steel with passive film formed in hot air increases both dry and wet adhesion strength of the epoxy coating compared to pretreatment with passive film formed in boiling water and chromate coating. The overall increase of wet adhesion for this sample was maintained throughout the whole investigated time period. It was shown that the change in adhesion of epoxy coating on a chromate coating is smallest of all investigated samples, although the initial value of adhesion on this surface had the lowest value. The corrosion stability of coated Zn samples pretreated by different methods, was investigated by electrochemical impedance spectroscopy and in the initial time of exposure to NaCl the highest values of pore resistance were also obtained for the epoxy coating on Zn pretreated in hot air, whereas the epoxy coating on a HDG steel with a chromate coating showed the smallest change in electrochemical properties (pore resistance, coating capacitance, charge-transfer resistance) during prolonged exposure time.     

Corrosion behaviour of epoxy coatings electrodeposited on galvanized steel and steel modified by Zn–Ni alloys

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on hot-dip galvanized steel and steel modified by Zn–Ni alloys were investigated during exposure to 3% NaCl solution. Zn–Ni alloys were electrodeposited on steel by direct and pulse current. From the time dependence of pore resistance, coating capacitance and relative permittivity of epoxy coating, diffusion coefficient of water through epoxy coating, D(H₂O) and thermal stability, it was shown that Zn–Ni sublayers significantly improve the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of investigated time for epoxy coatings on steel modified by Zn–Ni alloys, indicating the great stability of these protective systems, due to the existence of the inner oxide phase layer and the outer layer consisting of basic salts.     

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.     

A comparative study of epoxy and polyurethane based coatings containing polyaniline-DBSA pigments for corrosion protection on mild steel

Organic coatings based on epoxy and polyurethane matrices containing polyaniline doped with dodecyl benzene sulfonic acid (Pani-DBSA) were prepared and applied over steel plates (SAE 1020). The plates were submitted to salt spray chamber for up to 30 days in order to evaluate the corrosion protection of these coatings. The properties of the coated plates were analyzed as a function of time by electrochemical impedance spectroscopy, open circuit potential, optical microscopy, and Raman spectroscopy. In general, results indicate a decrease in the electrical resistance, increase in capacitance and decrease in open circuit potential. Epoxy based coatings have improved performance when Pani-DBSA is used as pigment, whereas for the polyurethane coatings, Pani-DBSA seems to play an adverse effect. Raman spectroscopy indicates a possible chlorination of the epoxy matrix after 30 days exposure to salt spray chamber.     

Thermal aging of painted galvanized steel after mechanical deformation

In the present work the effects of thermo-mechanical degradation on the protective properties of coil coated samples are studied. A cupping test was performed, with penetration depths of 2, 4 and 6 mm and a finite-element model was set to define the local strain for the different depths of penetration. Different thermal stresses were applied to accelerate the natural weathering: a thermal cycle with a maximum temperature of 45 °C was chosen to simulate the stress occurring in the outdoor exposure, a second thermal cycle (maximum temperature 95 °C) was proposed as 1-week accelerated testing procedure. The effects of the thermo-mechanical stress were studied by electrochemical impedance spectroscopy. The ageing procedure and the electrochemical measurements were useful to evaluate the barrier and adhesion properties for the various coating formulations and to define the influence of the different strain levels.     

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.     

Electrochemical impedance spectroscopy; a tool for organic coatings optimizations

Electrochemical impedance spectroscopy (EIS) was applied to the optimization of automotive electrodeposited coatings, container interior coatings and industrial maintenance coatings. The electrochemical impedance data were used to predict corrosion protection, film porosity, solution absorption into the coatings and film delamination properties. Variables such as resin contents, crosslink densities, cure temperatures, and solvent types and contents were evaluated for these various types of coatings. In general the electrochemical impedance data correlated well with conventional exposure tests results such as salt fog, cyclic scab corrosion and delamination tests. The impedance spectra permits a rather rapid (15–75 min per sample) assessment of the film's characteristics even when no visually observable changes have occurred. Electrochemical impedance spectroscopy provides a technique to optimize coatings while reducing the time of coating evaluations and gives insight into the chemical and physical properties of the coatings.     

Electrochemical and sorption characteristics and thermal stability of epoxy coatings electrodeposited on steel modified by Zn–Co alloy

The corrosion behaviour, transport properties and thermal stability of epoxy coatings electrodeposited on steel and steel modified by Zn–Co alloys were investigated during exposure to 3% NaCl solution. The electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption measurements and thermogravimetric analysis (TGA) were used. Zn–Co alloys were electrodeposited on steel from chloride and sulphate baths, by different current densities. From the time dependence of pore resistance and coating capacitance of epoxy coating, diffusion coefficient of water through epoxy coating and thermal stability it was shown that Zn–Co sublayer obtained from chloride solution significantly improves the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of exposure time, indicating the great stability of this protective system, due to the existence of a passive layer consisting of basic salts.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

Electrochemical impedance studies of corrosion protected surfaces covered by epoxy polyamide coating systems

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of different types of commercial quality epoxy polyamide dry coatings on mild steel, with thickness between 150 and 250 μm, which were previously weather accelerated in a wet chamber. The following data were established on the basis of impedance curves and corresponding equivalent circuits: the coating capacity that relates to the coating dimension, the pore resistance that represents conductive paths through the pores, and Warburg coefficients, which are the measure of ion diffusion through the coating. EIS data were compared with a criterion in the European standard, and samples were classified on the basis of their quality, also receiving a final ranking by summing-up all the individual rankings. Suggestions have been made, namely that the use of anticorrosive active pigments is obsolete, when the thickness of the coatings exceeds 200 μm.     

A comparative study of the available electrochemical methods for the investigation of epoxy and fluoropolymer coatings deterioration

The corrosion behaviour of phosphatized galvanized steel coated with both epoxy films of different thickness and fluoropolymer films has been studied by means of a.c. impedance spectroscopy (EIS), break-point frequency, potentiodynamic measurements and faradaic distortion methods as well as by the salt spray test. It was observed that the degradation of protective films appears after a long-lasting initial period but once the process starts, the area of defects increases with exposure time. The rate of degradation depends both on type of polymer and of film thickness for the same type of polymer. The same behaviour can be observed from the decrease in pore resistance and charge-transfer resistance (EIS) and increase in double-layer capacitance (a.c. impedance measurements) and corrosion current (potentiodynamic measurements and harmonic analysis).     

热镀纯锌钢板电泳漆膜不良问题探讨

为解决汽车车身使用的热镀纯锌板在前处理、电泳过程中发生的漆膜不良──白斑,通过"倒推排查法"查找问题原因。用显微镜、扫描电镜观察缺陷部位的表面形貌,并用能谱仪表征了元素组成,确定了造成白斑的原因并给出了解决方案。     

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 μm thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.     

热镀锌板环保涂层的高湿环境适应性研究

针对热镀锌普通环保钝化板不能满足华南高湿环境的应用要求,开展了不同表面涂层的热镀锌板在南风天的交变湿热和高温高湿环境下的适应性,并对涂层表面微观形貌和水接触角进行分析,确定了提高表面处理涂层的封闭性可显著改善热镀锌板的高湿环境适应性,找到了一种适用于高湿环境使用的热镀锌自润滑涂层。通过微观形貌和中红外光谱分析,探索了该自润滑涂层的膜层结构,诠释了它高湿环境适应性良好的原因。实际应用表明,该热镀锌自润滑板在华南地区使用情况良好。     

Corrosion protective performances of commercial low-VOC epoxy/urethane coatings on hot-rolled 1010 mild steel

The protective properties of low-VOC epoxy/urethane paint systems of commercial grade have been investigated using a variety of techniques such as electrochemical impedance spectroscopy (EIS). One epoxy-polyamide mastic/urethane, three high-solid epoxy-amine/urethane coatings, one solvent-free epoxy-amine/urethane, one water-based epoxy-amine and one high-VOC alkyd paint system (used as paint reference system) were applied on hot-rolled 1010 mild steel panels and exposed for up to 2000 h in the salt spray cabinet (SSC) or for 1 year at an outdoor marine test site. These paints were tested for their barrier properties, corrosion-induced adhesion loss and visual defects, as well as for their flexibility and resistance to direct impact. The barrier properties increased in the following order: alkyd相似文献   

Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.     

Protective properties of organic phosphate-pigmented coatings on phosphated steel substrates

Investigations have been carried out on properties of coatings, differing by their pigmentation and binder, and applied on different chemical pre-treatments of the steel surface. Paints based on alkyd and alkyd-melamine binders, pigmented with zinc phosphate and modified basic zinc phosphate were applied on amorphous and crystalline phosphated steel surface and, for the comparison purpose, on degreased steel surface. The effect of the binder, the pigment and the pre-treatment of the steel surface on the protective properties of the coatings were determined by measurements of adhesion, water absorption and water permeability and by results obtained in salt spray and Prohesion tests. Coatings based on alkyd binder show a lower damage degree and good retention of adhesion in corrosion conditions, in spite of a higher water absorption and water permeability and a lower initial adhesive strength. Protective properties of coatings have been found to be highly dependent upon the substrate pre-treatment. Chemical pre-treatment of the steel substrate increases the protective properties of the system, which is particularly evident in the case of crystalline phosphating and the coating pigmented with modified basic zinc phosphate. This phenomenon can be explained by the synergism between this phosphate pigment with crystalline phosphate layer.     

The effect of chromated and organic layers on corrosion resistance of galvanized steel sheets

Chromated zinc steel sheets have been used widely for corrosion resistance performance in various waters. The coating system includes a chromated layer and both a primer and topcoat, a commonly applied system for better corrosion performance. Electrochemical impedance spectroscopy results show that the corrosion resistance of chromated zinc is due to its passivity after water uptake during the exposure. In galvanized steel sheets with a primer, the electrolyte solution can only reach the metallic substrate through the coating pores; thus, a double layer could build up locally at the bottom of the pores, resulting in a good corrosion resistance for the sheet. On the other hand, the high-pore-resistance system of primer and topcoat provides no evidence of corrosion even after 8 weeks’ exposure.     

Formation and characteristics of Al−Zn hydrotalcite coatings on galvanized steel

In this study, a process for depositing hydrotalcite (HT) coatings on galvanized steel was developed and the resulting coatings were characterized. Results showed that coatings formed spontaneously on galvanized surfaces upon exposure to ambient temperature alkaline aluminate solutions. Anodic polarization and electrochemical impedance spectroscopy experiments showed clear evidence of surface passivation. Scanning electron microscopy showed the formation of a continuous and conformal surface film comprised on a compact mass of crystallites. X-ray diffraction confirmed that the coating contained an Al−Zn hydrotalcite compound. Coating formation was enhanced by oxidizer and ammonium salt additions. Coatings formed by using best practices were deposited in less than 10 minutes and demonstrated good surface coverage and good organic coating adhesion. HT coatings formed by using best practices showed excellent organic coating adhesion compared to zinc phosphate control coatings. In salt spray testing, the presence of a hydrotalcite conversion coating under an epoxy neat resin was found to delay the onset of red rusting compared to control samples that were epoxy coated, but not conversion coated.     

热镀锌板的镀层成分对其表面电泳漆膜抗石击腐蚀性能的影响

研究了热镀锌板的镀层成分对其表面电泳漆膜抗石击腐蚀性能的影响。结果表明,经过合金化退火得到的GA板镀层中含有一定量的Fe,镀层较未经合金化退火的GI板硬。GA板镀层表面疏松多孔,电泳漆膜能够充满其缝隙或凹陷之处,固化后在界面区产生了类似勾键作用的机械作用力。因此,GA板与电泳漆膜的附着力优于GI板,石击处的扩蚀速率小于GI板。