Effect of the metallic substrate and zinc layer on the atmospheric corrosion resistance of phosphatized and painted electrogalvanized steels

In this work, the influence of the metallic substrate and zinc layer on the atmospheric corrosion resistance of phosphatized and painted electrogalvanized steels was evaluated. For this purpose two types of electrogalvanized steels were used, one with drawing quality carbon steel as metallic substrate, and the other one with a substrate of Ni–Cu–Cr added carbon steel. The corrosion resistance was determined by accelerated and non‐accelerated corrosion tests, using cyclic test chambers and field tests in industrial and marine atmosphere. The mean corrosion advance and the maximum corrosion penetration of samples were measured using image analysis techniques. The study showed that the substrate and zinc layer mass play a decisive role on the atmospheric corrosion resistance of the phosphatized and painted electrogalvanized steels. It was also verified that it is possible to use smaller zinc layer masses without compromising the corrosion resistance of the material, provided that the metallic substrate has characteristics of atmospheric corrosion resistance, thus contributing to the improvement of the zinc coated steels.     

Study of role of phosphate coating properties on electrophoretic paint performance by EIS and simulated corrosion tests

The influence of phosphate bath chemistry on phosphate coating properties and, eventually, on corrosion resistance properties of electrophoretic (ED) painted steel samples was examined. Two types of zinc phosphate bath chemistry with different concentration of zinc and nickel ions, and different values of total acid, free acid, and accelerator content were studied. The phosphate bath with higher value of acid ratio (total acid/free acid), nickel ion concentration, and accelerator point produced the best phosphate coating on steel surface with globular microstructure, small crystal size, high P ratio, and uniform coverage. Phosphate coated and only alkali cleaned samples were cathodic ED painted. Their corrosion resistance properties were evaluated by salt spray test, cyclic corrosion test and electrochemical impedance spectroscopy. The relative increase in corrosion resistance of painted sample due to best phosphate coating was 1·4 to 1·8×. Paint adhesion of samples significantly deteriorated when painted without phosphate pretreatment.     

Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part I: Measurement of porosity of phosphate

The corrosion resistance of phosphated and painted steels is associated with the integrity of phosphate and paint layers. In this work, a methodology to measure the porosity of phosphated coatings based on cathodic polarization technique with an electrolyte of a sodium sulfate aqueous solution was developed. The morphology and mass of phosphate layers were determined. Phosphating was performed using the spraying and immersion techniques, varying the refiner content of bath in the immersion operation. The phosphate coatings obtained by immersion with a refiner showed the lowest values of porosities among the samples studied. The phosphate layers, obtained by spraying, immersion without a refiner, and immersion with a half content of refiner, showed the lowest porosity on electrogalvanized steel. The phosphate layer obtained by immersion with a refiner displayed the lowest porosity on the substrate of ungalvanized steel.     

涂膜破坏后膜下镀锌钢板大气腐蚀扩展研究

    通过SEM和EDS研究了带有划痕的涂装镀锌钢板在海洋大气环境下暴露一年半后,划痕周围涂膜下腐蚀产物的扩展行为,并分别对电镀锌、热镀锌、电镀锌预磷化的材料进行了耐蚀性比较.结果表明,在划痕周围涂膜下镀层的耐蚀性与镀层制作工艺无关,而与镀层厚度有关;涂膜前的预磷化对耐蚀性产生了不利影响;虽然涂膜与镀锌层间发生分离,但对镀层的耐蚀性没有产生影响;海洋大气环境存在的氯容易富集在腐蚀产物的前端,而硫则较均匀的分布在整个腐蚀产物转化区.     

Effect of Mg2+ on the microstructure and corrosion resistance of the phosphate conversion coating on hot-dip galvanized sheet steel

The effect of Mg²⁺ in the phosphate solution on the microstructural evolution and corrosion resistance of the coating on hot-dip galvanized steel has been explored. Surface morphology observations reveal that increasing the solution Mg²⁺ concentration increases the population density and refines the grain size of the phosphate grains. In the presence of Mg²⁺, the phosphate coating is composed of mixed Zn phosphate hydrate and (Zn, Mg) phosphate hydrate. Furthermore, elevating Mg²⁺ in the solution reduces the porosity of the phosphate coating. As a result, the corrosion resistance of the phosphate coating is improved.     

A study on corrosion resistance characteristics of PVD Cr-N coated steels by electrochemical method

The corrosion behavior of Cr-N coated steels with different phases (-Cr, CrN and Cr₂N) deposited by cathodic arc deposition on AISI H13 steel was investigated in a 3.5% NaCl solution at ambient temperature. Potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were the techniques applied to characterize the corrosion behavior. It was found that the CrN coating had a lower current density from potentiodynamic polarization tests than others. The porosity, corresponding to the ratio of the polarization resistance of the uncoated and the coated substrate, was higher in the Cr₂N coating than in the other Cr-N coated steels. EIS measurements showed, for most of the Cr-N coated steels, that the Bode plot presented two time constants. Also, the Cr₂N coating represented the characteristic of Warburg behavior after 72 h of immersion. The coating morphologies were examined in planar view and cross-section by SEM analysis and the results were compared with those of the electrochemical measurement. The CrN coating had a dense, columnar grain-sized microstructure with minor intergranular porosity. From the above results, it is concluded that the CrN coating provided a better corrosion protection than the other Cr-N coated steels.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291-2303], first-order model used to simulate the localized degradation experienced in Zn-4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corrosion Science 47 (2005) 2740-2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.     

Corrosion and corrosion test methods of zinc coated steel sheets on automobiles

The effect of zinc and zinc alloy coated steel sheets on perforation corrosion in actual automobiles and the relevant accelerated corrosion test methods were studied. The main factor affecting corrosion in the crevice of lapped panels was the coating weights of zinc and zinc alloys rather than the type of coating. Perforation corrosion process of galvanized steel in the crevice of lapped portion in automobiles was divided in four stages. Based on the analysis of corrosion in actual automobiles, a Perforation Corrosion Index, PCI for lapped steel panels was proposed. Assuming PCI for various accelerated corrosion test methods, the corrosion resistance of various types of coated steel sheets in actual environments was evaluated.     

Microstructure and corrosion behaviour of plasma‐nitrocarburized sintered steel

Powder characteristics and manufacturing processes determine the microstructure, and therefore, the physical, chemical, and mechanical properties of sintered steels. In particular, porosity and corrosion resistance are intimately related, since the contact area between substrate and electrolyte significantly affects the corrosion resistance of sintered steels. This study addresses the effect of powder characteristics and pressing parameters on the microstructure and corrosion resistance of low‐carbon sintered and sintered/plasma‐nitrocarburized steel. The results indicated that the corrosion resistance increased with increasing density and decreasing specific surface area. Additionally, plasma‐nitrocarburizing was highly effective in coating open pores of the material.     

Synergistic corrosion protection for galvanized steel by phosphating and sodium silicate post-sealing

To improve the corrosion resistance of phosphate coatings, the phosphated hot-dip galvanized (HDG) sheets were post-sealed with sodium silicate (water glass) solutions. The morphology and chemical composition of the composite coatings was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The effect of sodium silicate post-sealing treatment on the corrosion behaviors of phosphate coatings was investigated by neutral spray salt (NSS) tests and electrochemical measurements. The results show that after the silicate post-treatment the pores among zinc phosphate crystals are sealed with the films containing Si, P, O and Zn, leading to the formation of the continuous composite coatings on the surface of HDG steel. The corrosion resistance of the composite coatings depending on concentration of sodium silicate and post-sealing time is greatly improved by the silicate post-treatment. The optimum concentration of silicate and post-sealing time are 5 g/L and 10 min, respectively. Both the anodic and cathodic processes of zinc corrosion on the samples are suppressed conspicuously, and the synergistic protection effect of the single phosphate coatings and the single silicate films is evident. Moreover, the low frequency inductive loop in electrochemical impedance spectroscopy (EIS) is disappeared and the electrochemical impedance values are increased for more than one order of magnitude. The corrosion protection of the composite coatings is comparable to that provided by the chromic acid post-treatment.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

大块涂层缺陷对碳钢腐蚀特性及阴极保护效果的影响

采用了环氧煤沥青作为涂层材料，研究了大块涂层缺陷对碳钢腐蚀特性及阴极保护效果的影响，测定了具有面积比例为4．91％大块涂层缺陷的Q235钢于3．5％NaCl水溶液中的交流阻抗谱．结果表明，随着浸泡时间的延长，在自然腐蚀电位下，存在剥离涂层缺陷时碳钢的腐蚀程度高于破损涂层缺陷时碳钢的腐蚀程度；在阴极极化条件下，具有剥离涂层缺陷的碳钢阴极保护效果随时间延长逐渐降低直至最后消失，而具有破损涂层缺陷时阴极保护效果随时间延长优于剥离涂层缺陷；电解质溶液向涂层内部的渗透以及涂层缺陷与钢基体间的缝隙腐蚀是导致具有大块涂层缺陷碳钢腐蚀的主要原因．     

Influence of cathodic protection on the lifetime extension of painted steel structures

For corrosion to occur on a coated metal surface, an electrochemical double layer must be established. Hence, the adhesion between the substrate and the coating must be weakened to enable a separate thin layer of water to be formed at the interface from water that has permeated the coating. To prevent the failure of a painted coating, we applied a cathodic protection method. This method has been controversial for a few years because cathodic protection can induce cathodic delamination of the entire surface, especially near the anode, and also because it is not effective on a not-wetted surface from the anode. We therefore evaluated the efficiency of cathodic protection for 700 days in an atmospheric environment and performed surface observation, AC impedance measurements and corrosion tests. In the case of a noncathodic protected painted steel specimen, blisters formed after 100 days and grew in number for the remainder of the test. However, cathodic protection of the painted steel increased the coating resistance and extended the lifetime of the coating. According to our calculation with the BEASY program, the thickness of the water film under a rain condition barely influenced the protection potential. The high voltage of the cathodic protection and the subsequent cathodic delamination caused the paint to peel off near the anode. Hence, the protection voltage should be controlled in accordance with changes to the environmental condition.     

Effect of Nano-oxide addition on corrosion performance of hot dip zinc coating

Corrosion performance of hot dipped zinc coating on low carbon steel was studied at the presence of different nanoand micronsize oxide particles in the liquid zinc bath. Nano-silica, nano-alumina and micro-alumina powders were loaded to the different liquid baths, in the range of 0.05–0.2 wt %. Low carbon steel specimens were immersed in the baths for a constant time of 10s. It was evident that the presence of oxide particles in the liquid bath increased the coating thickness at a constant immersing time; micro-alumina particles provided the thickest coating among the others. Salt spray and potentiodynamic polarization tests were conducted to evaluate corrosion performance of the galvanized coatings, including oxide bearing ones. The results confirmed improvement in corrosion resistance of the nano-oxide bearing zinc coatings; while incorporation of micro-alumina in the bath declined its corrosion resistance. It was shown that incorporation of nano-silica powder in the liquid bath yielded superior corrosion resistance of the zinc layer, in comparison to the other ones. The optimum corrosion performance of zinc coating was achieved via loading 0.1 wt % nanosilica to the liquid zinc bath in this work.     

The characterization and corrosion resistance of cerium chemical conversion coatings for 304 stainless steel

A golden yellow-colored cerium conversion coating was obtained on 304 stainless steel surface by immersing the steel into a solution containing cerium (III), KMnO₄ and sulfuric acid. The corrosion resistance of the coatings was evaluated by electrochemical methods, potentiodynamic polarization experiments and electrochemical impedance spectrum. The experimental results indicated that the corrosion resistance for the conversion coated 304SS in 3.5% NaCl solution increased markedly. The corrosion potential of the treated steel increased to a more noble level, the pitting corrosion potential increased also, the passive potential range was enlarged markedly and the passive current density decreased about one order compared to that of the untreated steel. The cathodic and anodic reaction were both inhibited to some extent. The chemical state of the elements in the coatings was investigated by XPS. The cerium element was in the form of tetravalent state. And AES depth profile analysis suggested that the thickness of the conversion coatings was less than 66 nm. The mechanisms of coatings formation and corrosion resistance are discussed.     

镀锌板上丙烯酸树脂复合膜的制备和表征

在镀锌钢板表面制备了丙烯酸树脂复合膜,用扫描电子显微镜观察膜层的微观形貌.用傅里叶变换红外光谱表征膜层的分子结构,用中性盐雾试验和电化学方法测试其耐蚀性,并用划痕浸泡实验测试膜层的自修复性能.结果表明:丙烯酸树脂复合膜表面致密平整;耐中性盐雾腐蚀达72 h;阻抗值和极化电阻值均较大,说明丙烯酸树脂复合膜能有效抑制腐蚀电化学反应;划痕浸泡试验证明丙烯酸树脂复合膜具有自修复功能.成膜过程中碳酸锆铵能够和丙烯酸树脂分子上的羟基和羧基发生交联反应,形成互穿网络结构,提高膜层内部的交联密度,有效地阻挡外界环境的侵蚀,当膜层破损时钼酸盐和磷酸盐与锌反应形成难溶盐吸附在破损处,起到自修复作用.     

Development of chromate-free coated steel sheet with high corrosion resistance and conductivity

The ideal coating structure, which can achieve both high corrosion resistance and high conductivity, was examined by SEM analysis of conventional chromate-free coated steel sheets. Based on the results of SEM analysis, it was found that high conductivity was achieved by maintaining the convex parts of zinc plating crystals at the coating surface. High corrosion resistance was also achieved by combining a novel silicate binder and metal salt. The results of electrochemical analysis revealed that the silicate binder contributed to suppressing the anodic reaction, while a composite coating involving epoxy resin and phosphoric acid was effective for suppressing the cathodic reaction and maintaining a high barrier property for an extended time.     

热镀锌层上磷酸锌转化膜的生长与耐蚀性

热镀锌钢板在pH3.0、45℃的磷酸锌溶液中磷化2～600s,用扫描电镜、能谱仪和X射线衍射仪分析磷化膜的组织形貌和成分,并探讨膜层的生长行为。结果表明:磷酸锌晶体在锌晶粒内及晶界处均可成核,开始是以接近平行的片状生长,并逐渐向多方向生长成扇骨状的晶片。随着磷酸锌晶体的成核和生长,磷化膜的覆盖率增加,但晶体之间的孔隙难以完全消除;长大的磷酸锌晶片容易折断脱落,导致磷化后期膜层的质量增量减小;磷化膜主要由Zn3(PO4)2.4H2O组成。热镀锌钢板经磷化处理后,耐蚀性显著提高,磷化膜的耐蚀性随磷化时间和膜层覆盖率的增加而提高。     

Effect of silicate pretreatment, post-sealing and additives on corrosion resistance of phosphated galvanized steel

Sodium silicate （water glass） pretreatment before phosphating, silicate post-sealing after phosphating and adding silicate to a traditional phosphating solution were respectively carried out to obtain the improved phosphate coatings with high corrosion resistance and coverage on hot-dip galvanized（HDG） steel. The corrosion resistance, morphology and chemical composition of the coatings were investigated using neutral salt spray（NSS） tests, scanning electron microscopy（SEM） and energy dispersive spectroscopy（EDS）. The results show that pretreatment HDG steel with silicate solutions, phosphate coatings with finer crystals and higher coverage are formed and the corrosion resistance is enhanced. Adding silicate to a traditional phosphating solution, the surface morphology of the coatings is nearly unchanged. The corrosion resistance of the coatings is mainly dependent on phosphating time. Phosphating for a longer time （such as 5 min）, the corrosion resistance, increasing with concentration of silicate, is improved significantly. Post-sealing the phosphated HDG steel with silicate solutions, the pores among the zinc phosphate crystals are sealed with the films containing Si, P, O and Zn and the continuous composite coatings are formed. The corrosion resistance of the composite coatings, related to the pH value, contents of hydrated gel of silica and Si2O^2- 5 and post-sealing time, is increased markedly. The improved coatings with optimal corrosion resistance are obtained for phosphating 5 min and post-sealing with 5 g/L silicate solution for 10 min.