Comparison of single and multilayer coatings based on ormosil and conversion layers for aluminum alloy corrosion inhibition

Corrosion resistance behavior of single and multilayer coatings based on ormosil, trivalent chromium conversion coating (tccc) and hexavalent chromium conversion coatings (hccc) on the surface of 2024-T3 aluminum alloy (AA) was investigated using potentiodynamic polarization curves and accelerated salt spray testing. The magnitude of the corrosion resistance for single layer coatings increased tccc < ormosil < hccc. Multilayer ormosil/ormosil coatings are subject to phase separation, leading to poor performance in the electrochemical and accelerated salt spray testing. The presence of either hccc or tccc in the multilayer film was found to augment the inherent corrosion resistance of the ormosil barrier film. Multilayer coatings composed of either tccc/ormosil or hccc/ormosil were found to exhibit R_corr values in the range 158–177 kΘcm². This implies that the environmentally-benign and nontoxic trivalent chromium conversion coating used in combination with an ormosil film may be a potential alternative for hexavalent chromium conversion coatings. Environmental Institute, 003, Life Science East, Stillwater, OK 74078.     

Evaluation of durability of nano-silica containing clear coats for automotive applications

In automotive application, multilayer coating systems are typically used to guarantee protection from corrosion phenomena and aesthetic properties. A coating system's appearance and its durability is becoming increasingly important for automotive original equipment manufacturers. Clear coats are required to maintain long term aesthetic appearance, color and gloss stability to weathering and mechanical damage. One key property is scratch and mar resistance. The use of nano-fillers is a promising strategy to increase the abrasion resistance without affecting the optical clarity of high gloss coatings.     

The corrosion protection of mild steel by single layered polypyrrole and multilayered polypyrrole/poly(5-amino-1-naphthol) coatings

Electrochemical synthesis of polypyrrole (PPy) and top coat of poly(5-amino-1-naphthol) (PANAP) on PPy films from oxalic acid solution was achieved on mild steel (MS) by cyclic voltammetry technique. The morphology and the structure of the films were investigated by scanning electron microscopy (SEM). The corrosion performance of this multilayer coating and single PPy coating were investigated in 3.5% NaCl solution by using open circuit potential (E_ocp)–time curves, polarization curves and electrochemical impedance spectroscopy (EIS). It was found that the multilayer PPy/PANAP coating could provide much better protection than single PPy coating for corrosion of MS. It was observed that corrosion performance of coatings was increasing with immersion period. This was explained by auto-undoping properties of PPy coatings during immersion in corrosive solution. The improved corrosion performance in the presence of PANAP top coat on PPy was explained by increase in barrier effect of bilayer films.     

Testing and evaluation of nonchromated coating systems for aerospace applications

The advanced corrosion resistant aircraft coatings program (ACRAC) is part of the Air Force strategy to improve performance and reduce environmental impact of coatings used on Air Force weapon systems. The program addresses the Air Force near and mid-term strategies to eliminate chromate corrosion inhibitors and reduce steps in the outer mold line coating process. Evaluation of the coating process (surface preparation, conversion coating, primer, topcoat) as a system is a key feature of the ACRAC program. Results to date indicate that the current-state-of-the-art nonchromated coating systems are significantly less effective than those with chromate. A chromate conversion coating is required for the nonchromate primer system to meet minimum requirements. Sol–gel-process based conversion coatings can replace chromate conversion coatings provided a chromated primer is used. Several approaches to incorporating inhibitors into sol–gel coatings are discussed. Electrochemical methods for testing coating performance are discussed and a new procedure based on impedance spectroscopy for evaluating active damage repair is presented.     

Effects of pH and temperature on the deposition properties of stannate chemical conversion coatings formed by the potentiostatic technique on AZ91 D magnesium alloy

The effects of pH and temperature of a stannate bath on the quality of stannate chemical conversion coatings formed on AZ91 D magnesium alloy by using the potentiostatic polarization technique at E = −1.1 V were investigated in order to improve uniformity and corrosion protection performance of the coating films. It was found that the uniformity and corrosion resistance of coating films deposited by potentiostatic polarization were closely associated with pH and temperature of the coating bath. The pH and temperature to obtain the best coating film were investigated as a function of corrosion protection performance evaluated by curves of potentiodynamic anodic polarization conducted in borate buffer solution. Scanning electron microscope observation and electrochemical corrosion tests of the stannate-coated samples confirmed significant improvement in uniformity and corrosion resistivity of coating films deposited by the potentiostatic technique by modifying the pH and temperature of the coating bath. It was also found that uniformity and corrosion resistivity of the coating films deposited by the potentiostatic technique were considerably improved compared to those of coatings deposited by the simple immersion method at the best conditions of pH and temperature of the coating bath.     

The effect of organosilane on corrosion resistance of epoxy coating containing cerium nitrate

Structure and corrosion resistance behavior of epoxy coating containing cerium nitrate in different amounts of γ-glycidoxypropyltrimethoxysilane (γ-GPS) were investigated. Corrosion resistance ability of coating systems was characterized by using electrochemical impedance spectroscopy and polarization techniques. Also atomic force microscopy was used to evaluate dispersion of cerium nitrate in the presence of γ-GPS. Furthermore, the effect of γ-GPS on flexibility of coatings was studied by using the cupping test. Increment of 10?wt.% of γ-GPS into the coating formulation led to the superior dispersion of cerium nitrate and also highest corrosion protection performance.     

Corrosion protection coating containing polyaniline glass flake composite for steel

Corrosion protection of steel by glass flake (GF) containing coatings is widely used in marine atmosphere. Even though, the coatings containing glass flake are highly corrosion resistant, their performance is decreased due to the presence of pinholes and coating defects. It is well established that polyaniline containing coating is able to protect the pinhole defects in the coatings due to passivating ability of polyaniline. Hence a study has been made on the corrosion protection ability of steel using polyaniline-glass flake composite containing coating with 10% loading of glass flake in epoxy binder. The polyaniline glass flake composite (PGFC) was synthesized by chemical oxidation of aniline by ammonium persulphate in presence of glass flake. The corrosion protection ability of GF and PGFC containing coating on steel was found out by salt spray test and EIS test in 3% NaCl. In both the tests, the resistance value of the PGFC containing coating has remained at 10⁸-10⁹ Ω cm² where as for the GF containing coating, the resistance values decreased to 10⁵ Ω cm². The enhanced corrosion protection ability of the PGFC containing coating is due to the passivation ability of the polyaniline present in the coating.     

Multilayer architecture design to enhance load-bearing capacity and tribological behavior of CrAlN coatings in seawater

Steel materials employed in severe conditions including strong corrosion, high load and multi-factor coupling damages can easily cause incredible degradation until failure, and the protective CrN-based coatings should be one of promising candidates to relieve those damages for the steel equipment or components. In present paper, the monolayer CrAlN and multilayer Cr/CrAlN coatings were successfully deposited on steel substrates by multi-arc ion technology, and their microstructure, mechanical, tribological and corrosion performances were systematically investigated. The results show that the special multilayer Cr/CrAlN coating could possess much better load-bearing capacity and wear resistance than that of monolayer CrAlN coating, which was due to the facts that the multilayer architecture can effectively release the internal stress and inhibit the expansion of defects. Particularly, the multilayer interfaces could effectively prevent the aggressive medium in seawater infiltrating into the inside of coating, and thus the multilayer Cr/CrAlN coating could have higher corrosion resistance compared to monolayer CrAlN coating. As a result, this multilayer Cr/CrAlN coating could achieve excellent combined performances, indicating that it has greatly potential application as protective coating in seawater.     

阴极电弧离子镀ZrN梯度膜和Zr/ZrN多层膜的腐蚀特性

采用阴极电弧离子镀技术在1Cr13不锈钢表面制备了ZrN梯度层和Zr/ZrN多层膜,并用电化学腐蚀方法和中性盐雾法检测了1Cr13基体、ZrN梯度层和Zr/ZrN多层膜的耐腐蚀性能.结果表明:Zr/ZrN多层膜和ZrN梯度层均能提高1Cr13基体的抗腐蚀能力,而Zr/ZrN多层膜的效果更明显:镀层的内部缺陷(如微孔)和液滴导致薄膜发生孔蚀、隙缝腐蚀和电偶腐蚀;镀层保护的实质是物理屏障作用,细化晶粒、减少膜层中的液滴及针孔等缺陷能显著提高薄膜的抗腐蚀性能.     

ZrO2 sol–gel pre-treatments doped with cerium nitrate for the corrosion protection of AA6060

Sol–gel coatings represent an alternative corrosion protection method to the chromate based systems which must be replaced. Recently, it was shown that ZrO₂ based sol–gel coatings deposited on AA6060 can provide a good corrosion protection to AA6060. However, ZrO₂ based sol–gel systems are not able to provide the self-healing effect which is instead the peculiar property of chromate conversion coatings. The structure of the ZrO₂ based sol–gel systems does not contain species able to restore the barrier properties when defects or damages impair the coating protection.     

Enhancing corrosion resistance of aluminum composites in 3.5% NaCl using pigmented epoxy fluoropolymer

The corrosion protection performance of AA6061 T6–10% Al₂O₃ composite, coated with four different fluoropolymer paint systems, were investigated in 3.5% NaCl aqueous solutions under uniform and scratched conditions. A comparison between epoxy-treated clear and pigmented fluoropolymer coatings with newly developed vanadate-based fluoropolymer coatings was performed from corrosion protection, adhesion and durability points of view. The corrosion rates of epoxy-treated clear FLBZ 1074 increased dramatically after less than one month of exposure in 3.5% NaCl aqueous solutions. The addition of iron oxide or titanium oxide as pigments to the fluoropolymer decreased the porosity of epoxy and hence, improved the corrosion resistance. The newly developed vanadate based-clear FLBZ 1074 system showed outstanding corrosion resistance even after two months of immersion in 3.5% NaCl aqueous solutions under scratched coating conditions. The durability of such new coating based on salt spray test results was very promising (>2000 h without any sign of corrosion). The vanadate-pigmented FLBZ 1074 showed a dramatic increase in the corrosion rates. Moreover, the presence of pigments affected negatively the adhesion performance as well as the durability of the coating. A general model has been used to analyze the impedance data in terms of reactions occurring during the interaction of the coated metal with the environment. It was found that the vanadate-treated specimens not only improve the protective power against filiform corrosion as a factor of time, but also maintain the adhesion performance within the acceptable ranges.     

High performance reaction-induced quasi-ceramic silicone conversion coating for corrosion protection of aluminium alloys

Regulations posed by the Environmental Protection Agency on the use of chromate conversion coatings have triggered the need to find alternative materials for the corrosion protection of aluminium and its alloys. A simple four-step process of creating a quasi-ceramic coating has been developed. The coating, which is made of environmentally safe chemicals, should be easily and safely manufactured for bulk production. Here, we describe the synthesis, characterization and evaluation of a silicone conversion coating for aluminium metal and its alloys. The spectroscopic techniques utilised in this study have shown the mode of the bonding mechanism between the metal and the coating. Thermal analysis of the material was conducted to evaluate its stability, while nano-mechanical properties were determined and correlated with the surface morphology of the coating. Results obtained from FTIR and XPS spectroscopic techniques suggested that the coating adhered to the metal substrates through active surface functionalities, and thermal analysis showed that the coatings contained volatile solvents including water that evaporated at lower temperatures. Nano-mechanical tests suggested that the coating had elastic properties. Accelerated corrosion and immersion tests were also performed on coated and uncoated aluminium alloys. Coated and uncoated aluminium alloys were also exposed to soil containing sulphate-reducing bacteria. Coated coupons showed excellent corrosion protection and antifouling characteristics indicating that the coatings were impervious and of high integrity.     

铝合金表面钛酸盐化学转化膜研究

基于含铬处理对环境影响的考虑,世界范围内都在积极开发有效的替代技术,用于铝合金腐蚀保护从而代替铬酸盐转化处理。研究了铝合金钛酸盐处理技术。通过循环极化曲线和盐雾试验测定,钛酸盐和铬酸盐化学转化膜都有非常宽的钝化区,在NaC l(c=0.5 mol/L)溶液中2种氧化转化膜均没有击穿电位;钛酸盐转化处理的样品经336 h盐雾试验表面无点腐蚀发生。钛酸盐转化膜可以为铝合金提供良好的腐蚀保护作用。     

锌及镀锌层稀土转化膜研究进展

锌及镀锌层表面稀土转化膜具有良好的耐蚀性能和无毒、无污染等优点,是最有希望替代铬酸盐处理的转化膜之一,具有良好的发展前景.本文综述了国内外关于锌及镀锌层上单一稀土转化膜的成膜工艺及其机理,并概述了国内外稀土复合膜的研究成果.     

铝合金表面电解沉积稀土转化膜工艺研究

研究了一种通过电解沉积方法在防锈铝LF21表面上生成铈盐转化膜的工艺,应用正交实验研究了有关因素对成膜过程的影响并获得了最佳的技术参数用极化曲线、交流阻抗和中性盐雾试验等方法测试了该工艺形成膜层的耐蚀性能及其组成一结果表明：经过电解沉积稀土转化膜处理后,防锈铝的阳极腐蚀过程受到了阻滞,自然腐蚀电位负移;与经过化学转化膜处理后相比,其耐蚀性能有显著提高,可通过400h的中性盐雾实验,亲水性能亦有明显提高。     

HMMM cured corrosion resistance waterborne ormosil coating for aluminum alloy

Due to the exceptional advantages of sol–gel process, organically modified silane (ormosil) coating has been applied to study the corrosion protection of aluminum alloy. In the present work, GPTMS/MTMS sol–gel solution was prepared by hydrolysis and condensation of 3-glycidoxypropyltrimetoxysilane (GPTMS) and methyltrimethoxysilane (MTMS) in aqueous solution of 0.05 M acetic acid in molar ratios 3:1. To prepare the ormosil coating solutions, a crosslinking agent hexamethoxymethylmelamine (HMMM) and a blocked acid catalyst p-toluenesulphonic (p-TSA) were combined with the sol–gel solution. Aluminum substrates were dip coated and cured at 130 °C for 45 min. Effectiveness of HMMM as a crosslinking agent was analyzed by evaluating corrosion resistance, chemical resistance and hydrophobicity of coating using potentiodynamic polarization method, immersion test and water contact angle, respectively. UV stability of sol–gel solution, ormosil coating formulation and coated substrate was also studied using UV–vis spectroscopy and UV-weatherometer to see performance of coating in outdoor application. Thermal behavior of ormosil was characterized using TGA and DSC. Surface morphology and structural characteristics were also characterized using SEM, AFM and FT-IR-RAS (reflection absorption spectroscopy). Pencil hardness, impact test and bend test were carried out to determine the adhesion, hardness and flexibility of coating.     

钛纳米聚合物涂料在NaCl溶液中的电化学阻抗谱

采用电化学阻抗谱方法对钛纳米聚合物涂料的防腐性能进行了研究,并通过与环氧树脂清漆、环氧玻璃鳞片涂料的对比,说明钛纳米聚合物涂料的防腐性能显著优于上述两种涂料的防腐性能。     

化学镀制备高耐蚀耐磨Ni-P-SiC复合镀层

研究了Ni-P-SiC复合镀层的制备工艺和性能以及SiC含量对镀层性能的影响。采用Taber试验机对Ni-P-SiC复合镀层的磨损性能进行了测试,并用VHX-100型三维视频显微镜对磨损形貌进行了观察,分析了复合镀层的磨损机理。结果表明:SiC颗粒的加入能有效地降低摩擦副之间的犁沟效应及摩擦表面发生粘着的面积,从而减少镀层的磨损。采用电化学实验等手段研究了Ni-P-SiC复合镀层的耐蚀性能。当复合镀层均匀一致,能起到一个良好的屏蔽作用时,耐蚀性十分优异;而镀层缺陷的存在将导致耐蚀性能降低。     

Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys

Lithium salts are being investigated as leachable corrosion inhibitor and potential replacement for hexavalent chromium in organic coatings. Model coatings loaded with lithium carbonate or lithium oxalate demonstrated active corrosion inhibition and the formation of a protective layer in a damaged area during neutral salt spray exposure. The present paper provides an abridged overview of the initial studies into this novel inhibitor technology for the active corrosion protection of aluminum alloys. Coating defects were investigated by microscopic techniques before and after exposure to corrosive conditions. Scanning electron microscopy analysis of cross-sections of the coating defect area demonstrated that the protective layer comprises a typical three-layered structure, which included a dense layer near the alloy surface, a porous middle layer, and a flake-shaped out layer. Potentiodynamic polarization measurements obtained with a microcapillary cell positioned in the coating defect area and electrochemical impedance spectroscopy confirmed the corrosion protective properties of these protective layers. The long-term corrosion inhibition of the lithium-based coating technology was tested in industrial coating systems.     

热浸镀锌层表面钛盐转化膜研究

利用钛盐成膜工艺在热镀锌层表面获得了色泽光亮、耐蚀性能优良的银白色转化膜层。采用扫描电镜、能谱仪、电化学极化和盐水浸泡方法研究了钛盐转化膜层的表面形貌、元素组成和耐蚀性能。分析了钛盐溶液成分及工艺参数对热镀锌层表面转化膜的耐蚀性能影响。确定的最佳工艺条件为:Ti(SO4)21g/L,H2O260mL/L,pH0.5～1.0,处理温度25～30°C,处理时间10min。热镀锌层经此工艺处理后,耐蚀性能明显提高。