Electrochemical effects of silane pretreatments containing cerium nitrate on cathodic disbonding properties of epoxy coated steel

In the recent years, silane materials, because of their environmental friendly nature and ease of application have been attended as an alternative for chromate conversion coatings. Different materials were searched for improvement of the efficiency of silane formulation. In this research, pretreatment of carbon steel substrates was carried out using γ-glycidoxypropyl-trimethoxysilane (γ-GPS) as functionalized silane. Cerium nitrate as a corrosion inhibitor material was introduced into the silane material and epoxy resin was applied on the pretreated steel substrates. Effects of the pretreatment on electrochemical properties, cathodic disbondment, dry and wet adhesion strength, and surface morphology of resultant epoxy coating were investigated. Results showed that pretreatment of steel substrate with γ-glycidoxypropyl-trimethoxysilane (γ-GPS) doped with cerium nitrate leads to improvement of cathodic disbondment and also dry and wet adhesion of epoxy coating. Furthermore, this type of pretreatment reduced the disruption of interfacial bonds at the binder/substrate interface. Addition of 2?wt% cerium nitrate into the silane formulation led to the maximum efficiency of resultant coating.     

Surface chemistry of galvanized steel sheets relevant to adhesion performance

A review is presented on the recent development of surface treatment technologies for hot-dip galvanized steels relevant to adhesion of organic coatings. Applications of surface analytical techniques have elucidated that the surface layers of the nanometer scale dramatically govern the adhesion performance of painting or adhesive bonding. Surface enrichment of aluminium in the zinc layer deteriorates paint adhesion due to the reduction in phosphatability on the galvanized steel sheets and decreases the adhesive strength of the epoxy/dicyandiamide-bonded sheets due to the loss of acid-base interaction at the adhesive-substrate interface. In addition, the co-segregation of Al and Pb into the surface layer is responsible for the intergranular corrosion of zinc and facilitates the formation of a weak boundary layer, resulting in poor bond durability in a wet atmosphere. Improved adhesion performance has been established by developing new technologies that reduce the surface enrichment of minor elements or impurities in the zinc layer on the galvanizing line or that adopt a surface conditioning process prior to pretreatment in subsequent coil coating lines.     

Electrochemical and morphological properties of zirconium conversion coating in the presence of nickel ions on galvanized steel

A hexafluorozirconic acid-based conversion coating was applied on a galvanized steel substrate and the influence of nickel ion from nickel sulfate solution (in zirconium solution and in a separate solution) on the corrosion resistance behavior and morphology of zirconium conversion coating was investigated. Electrochemical impedance spectroscopy and DC polarization were conducted in 3.5 wt% NaCl solution in order to optimize practical conditions of zirconium conversion coating and NiSO₄ solution on the galvanized steel substrate. Field emission scanning electron microscopy and X-ray photoelectron spectroscopy were employed to study the morphology and composition of the coated surfaces. Results revealed that the conversion coating obtained from solution containing zirconium and nickel ions (Zr + Ni) did not improve corrosion resistance and uniformity of the coating in comparison with Zr conversion coating in optimized condition. However, a positive effect was obtained from samples coated with separate solutions of zirconium and nickel (Zr–Ni). Improved corrosion resistance and morphology of Zr-based conversion coating were observed in Ni²⁺ concentration, pH, and immersion time of 10 g/L, 6 and 300 s, respectively. Morphology and surface composition analysis proved that two separate layers of conversion coating containing zirconium, zinc, and nickel oxide/hydroxide compounds were formed in the case samples that were treated by separate solutions. This led to better uniformity and higher thickness of the coating. Finally, adhesion strength of epoxy organic coating on galvanized steel with and without conversion coating was investigated by pull-off measurement. Zr–Ni conversion coating in optimum conditions had a positive effect on adhesion of organic coating in comparison with blank sample and samples pretreated with Zr and Zr + Ni conversion coatings through increased surface roughness and physical interlocking.     

Corrosion inhibition of aluminum by organic coatings formulated with calcium exchange silica pigment

One of the first commercial ion-exchange anticorrosive pigments to be developed was Shieldex^® (Si/Ca). Its proposed corrosion protection mechanism, based on the retention of aggressive cations and the subsequent release of calcium cations, has created certain controversy. A number of studies have focused on the anticorrosive behavior of this pigment on carbon steel and galvanized steel to replace chromates (Cr⁶⁺) as inhibitor pigment, but none has considered its performance on aluminum or aluminum alloys. In this research, alkyd coatings have been formulated with Si/Ca pigment at different concentrations and applied on aluminum 1050 (Al 99.5%) specimens. These specimens have then been subjected to accelerated tests (condensing humidity, salt spray, and Kesternich) and natural weathering in atmospheres of different aggressivity. Corrosion performance has been also evaluated in the laboratory by electrochemical impedance spectroscopy. The study has also considered an organic coating with zinc chromate anticorrosive pigment for comparative purposes. The results obtained with organic coatings formulated with Si/Ca pigments confirm that they provide corrosion protection of the underlying aluminum substrate, even improving the behavior of the reference zinc chromate in some environmental conditions.     

Corrosion resistance of steel treated with different silane/paint systems

This article reports on a comparative study on the corrosion resistance of low-carbon steel substrates pretreated with different silane solutions and painted. The pure silanes used to pretreat the steel panels were 3-aminopropyltriethoxysilane (γ-APS), 3-glycidoxypropyltrimethoxysilane (γ-GPS), and bis(3-triethoxysilylpropyl)amine. The study also considered other silane solutions with ureido, amino, and epoxy organofunctional groups, and two bis-functional silanes: bis(γ-trimethoxysilylpropyl)amine (BAS) and 1,2-bis(triethoxysilyl)ethane (BTSE). A conventional phosphate-type pretreatment was also applied for reference purposes. The pretreated panels were then finished with an alkyd/polyester aminoplast base paint. As a branch test, an acrylic/urethane paint was also applied. Different tests were conducted to evaluate the anticorrosive ability of the different silane/paint systems: outdoor exposure in an atmosphere of moderate aggressivity; accelerated corrosion test (salt fog test); and electrochemical impedance spectroscopy (EIS). The results show that the steel pretreated with certain silanes, especially γ-APS, yields similar results to steel subjected to conventional phosphate pretreatment.     

Optimization of phosphate coating properties on steel sheet for superior paint performance

The adhesion of electrodeposition (ED) paint on steel sheets for automobiles is highly influenced by the properties of the zinc phosphate coating which is used to improve its corrosion resistance. In the present study, a steel surface was pretreated with two types of zinc phosphate formulations followed by ED painting. The surface morphology, crystal plane, and porosity properties of phosphate coating on steel samples were studied by scanning electron microscope, X-ray diffraction, and electron probe microanalyzer, respectively. The corrosion resistance of painted samples was evaluated by an accelerated corrosion test as well as by electrochemical techniques like cathodic disbonding and AC?CDC?CAC tests. The phosphate coating enriched with a phosphophyllite structure showed small globular crystals with less porosity, whereas a hopeite structure showed coarse crystals with high porosity and comparatively thicker coating. The maximum corrosion resistance was observed in the painted sample, where the phosphate coating comprised a phosphophyllite structure. On the other hand, the painted samples phosphated with a predominantly hopeite structure showed inferior corrosion resistance performance. The unphosphated sample showed severe degradation in paint adhesion and corrosion resistance, which substantiates the importance of phosphate pretreatment.     

Corrosion stability of polyester coatings on steel pretreated with different iron–phosphate coatings

The influence of steel surface pretreatment with different types of iron–phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO₂, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron–phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while “dry” and “wet” adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance, R_p, and smaller values of coating capacitance of polyester coating, C_c, on steel pretreated with iron–phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron–phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.     

Anticorrosive polyurethane paints with nano- and microsized phosphates

Two types of phosphate fillers (nanosized aluminum phosphate and microsized aluminum–zinc phosphate) were tested as anticorrosive fillers in 2 K solvent-borne polyurethane paints based on commercial acrylic resin with OH groups and an isophorone diisocyanate-type hardener. Three coating compositions containing commercial fillers (mica/quartz, TiO₂, wollastonite, talc) and also mentioned nanosized aluminum phosphate or microsized aluminum–zinc phosphate were prepared using a pearl-mill as well as a laboratory dissolver, applied onto a steel substrate and cured at room temperature for 14 days. An influence of the type and content of a phosphate filler on properties of polyurethane paints and coatings has been investigated. Incorporation of nanosized aluminum phosphate into coating compositions increases their viscosity while cured paints exhibit reduced adhesion to steel substrates. The results of corrosion tests in a salts spray chamber as well as immersion in an aqueous NaCl solution indicated that the paint system with 9.8 wt.% of applied nanofiller had similar protective properties to a polyurethane coat containing a higher dose (i.e. 15.6 wt.%) of commercial microsized aluminum–zinc phosphate.     

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.     

The effect of amorphous chalcogenides on mechanical and anticorrosive properties of protective organic coatings containing high amounts of zinc metal particles

The objective of this paper concerns reduction of zinc metal content in organic coatings while preserving their high anticorrosive efficiency. The two goals can be achieved by using amorphous chalcogenides as components of the protective coating. Special attention was paid to materials containing Ge₂₀Se₈₀, Ge₃₀Se₇₀ and Ge₄₀Se₆₀, which were characterized by physico-chemical properties. An epoxy ester resin was used as binder for the investigated organic coatings. Organic coatings were prepared by combining zinc metal with amorphous chalcogenides. The resistance of the prepared films was evaluated using the results of mechanical tests. The anticorrosive efficiency of the prepared films was evaluated using the results of direct corrosion tests. Thus the following conclusion can be made from the results of anti-corrosive tests: the higher the pigment volume concentration of amorphous chalcogenides in the coatings, the higher the protective performance of the paint against corrosion.     

Composite multilayered coatings on mild steel

A composite multilayered coating which consisted of an electrodeposited Zn–Fe alloy layer, a zinc phosphate conversion layer, and one, two, or three organic layers was deposited on a mild steel substrate. The adhesion between these multilayered coating and the mild steel substrate was studied with the aid of a scratch testing technique. Observation of the worn surface of different multilayered coatings was performed with the aid of metallurgical microscopy. The same multilayered coatings were examined with FTIR spectroscopy and X-ray diffraction techniques. Finally, the corrosion behavior of bare and multilayered coated mild steel in 0.1 M NaCl solution (pH = 5.5, T = 25°C) was studied.     

Solventborne paint systems on carbon steel and hot-dip galvanized steel for a wide range of atmospheric exposures

The paper analyzes the performance of solventborne paint systems applied on carbon steel and hot-dip galvanized steel in a wide range of atmospheric exposures. The study has involved paint systems exposure for 3.5 years in eight natural atmospheres. The atmospheric conditions cover from temperate rural climates to tropical severe marine and Antarctic coastal regions. The paint systems included several alkyds formulated with a variety of pigments (anticorrosive and barrier), epoxies, chlorinated rubber, and zinc-rich (ethyl silicate and epoxy). It has been concluded that in rural and urban atmospheres alkyd systems afford equivalent anticorrosive protection of steel to the epoxy/polyurethane system. The toxic red lead pigment may be replaced in long linseed-oil alkyd primer paints by non-toxic pigments, such as a mixture of micaceous iron oxides (MIO) and black iron oxides or zinc phosphate, without affecting the anticorrosive properties of the paint system. In aggressive atmospheres (industrial, marine), paint systems including zinc-rich primers or applied on galvanized steel must be used, especially in surface regions with coating faults (scribes).     

Über den Einfluß des Substrates auf die Haftung und Korrosionsbeständigkeit von Lacküberzügen,die durch Elektrotauchlackierung oder elektrostatische Pulverbeschichtung hergestellt wurden

Influence of the Substrate on Adhesion and Corrosion Resistance of Paint Coatings that are Obtained by Electroimmersion Painting or by Electrostatic Powder Coating The adhesion of paint coatings depends on the size of the forces bonding the paint components to the metal surface. Already very thin films of impurities are adapted to reduce the extent of these bonding forces. The various cleaning – and pretreatment methods do not only change the number of active centres at the metal surface to varying extents, but also the chemical composition and the physical structure of the metal substrate. The various pretreatment methods for steel, aluminium, zinc and magnesium do not only influence the adhesion, but also the corrosion resistance of paint films. Varying steel qualities and different types of paints also have a distinct influence.     

石化防腐涂装工程表面预处理要求评述

石化设备与钢结构防腐涂装前须进行表面处理,表面处理对涂层使用寿命至关重要。探讨了涂层下金属腐蚀、涂层附着力、表面预处理与各类底漆相互关系。认为采用锈面底漆与多功能环氧涂料是较好办法。     

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.     

压铸铝合金防护技术在装备中的应用

介绍了压铸铝合金镀锌彩钝和无色化学氧化工艺.分析了镀锌层附着力差、耐蚀性差的原因,并给出了解决办法.经测试,镀锌彩钝膜附着力和耐蚀性均合格,压铸铝合金无色化学氧化膜的耐蚀性合格.     

Effect on temporary protection and adhesion promoter of silane nanofilms applied on electro-galvanized steel

In this paper, the effectiveness of γ-mercaptopropyltrimethoxysilane (MTMO) films as temporary corrosion protection and adhesion promoter on electro-galvanized steel was studied. The films were synthesized from hydrolyzed MTMO with ethanol or methanol, applied by immersion on electro-galvanized steel and cured under different conditions. The porosity of the coating was evaluated by cyclic voltammetry, the corrosion behavior by polarization curves and the protection degree by exposure in the humidity and prohesion chambers. The films were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). From the obtained experimental results, it was concluded that this protection is markedly affected by the coating curing conditions and the alcohol used in the hydrolysis. MTMO is a good temporary protector against electro-galvanized steel corrosion in high humidity environment but it is less effective in environments containing aggressive ions such as Cl⁻ and SO₄⁻².In a second stage, pretreated samples were coated with an alkyd anticorrosive paint and exposed to the salt spray chamber. Dry adhesion tests as well as SEM and EDS studies were also done in order to know the nature of the disbonding fails (adhesion or cohesion).     

Improvement of corrosion protective performance of organic coating on low carbon steel by PEO pretreatment

Employing pretreatment is a pressing need for preparing anticorrosive coatings on carbon steels. However, conventional pretreatments are usually based on Cr, P and some other toxic elements, which are harmful to human body. For this reason, green and environmental techniques attract more and more attention. In this paper, plasma electrolytic oxidation (PEO) process was used as a pretreatment to fabricate an underlayer for the organic coating on low carbon steel. The anticorrosive performance of the organic coated samples with and without PEO pretreatment was studied by potentiodynamic polarization, ac/dc/ac electrochemical impedance spectroscopy, salt spray and immersion tests, respectively. Results show that the PEO process produces an oxide layer with porous and rough surface structure on the low carbon steel substrate. The porous and rough PEO layer is beneficial for enhancing the adhesion strength and thickness of the organic topcoatings. The organic coated sample with PEO pretreatment exhibits improved corrosion resistance and longer service life in corrosive environment compared to that without PEO pretreatment.     

预磷化电镀锌钢板粉末喷涂前处理工艺

对一种用于汽车和通讯行业的新型预磷化镀锌钢板的粉末喷涂前处理工艺进行了研究,通过晶相、膜质量及盐雾试验,对预磷化板的现有工艺进行了改进。酸洗工艺对预磷化板是不利的,酸洗工艺完全破坏了预磷化膜层。盐雾试验结果表明,预磷化镀锌钢板经过弱碱性脱脂剂清洗,经表调、磷化然后和粉末涂料结合,可以得到良好耐蚀性和附着力强的涂层。     

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.