Anticorrosive behaviour of alkyd paints formulated with ion-exchange pigments

Hexavalent chromium compounds (chromates) have been widely used as inhibitive pigments in the formulation of anticorrosive paints. However, their high toxicity and carcinogenic effects are forcing the development of effective chromate-free organic coatings. One such alternative, which is very attractive from a scientific point of view, is the use of ion-exchangeable pigments (IEPs).

The few studies conducted with this type of pigment are not conclusive about their anticorrosive efficiency and controversy surrounds their functioning mechanisms, interchange capacity and anticorrosive performance.

In the present research, which focuses on the anticorrosive protection of this type of pigment, alkyd paint coatings formulated with vanadate-hydrotalcite (HT/V) (anionic) and calcium/silica (Ca/Si) (cationic) IEPs have been applied on low carbon steel specimens. A traditional zinc chromate pigment has also been used for comparative purposes.

The effect of these non-toxic pigments on the protective properties of coatings has been tested by means of natural and accelerated corrosion tests (humidity, salt spray and Kesternich, 0.2 l SO₂) and electrochemical impedance spectroscopy (EIS).

None of the IEPs equalled the anticorrosive behaviour of the zinc chromate in the different tests. The anionic pigment (HT/V) seems to present good behaviour in chloride environments (salt spray, NaCl solutions, etc.) while the cationic pigment (Ca/Si) performs well in the humidity condensation and SO₂ tests.     

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.     

An organically modified zirconate film as a corrosion-resistant treatment for aluminum 2024-T3

Current coating systems for aircraft corrosion protection are based on a traditional chromate surface treatment, primer, and topcoat. The Air Force is seeking environmentally benign corrosion-resistant surface treatments for aluminum-skinned aircraft as a replacement for environmentally hazardous surface treatments involving chromates. Performance of replacement treatments must be able to satisfy the durability needed for dramatically extended lifetimes, be compatible with present and future environmental requirements, and be easily integrated into current primer/topcoat paint systems.

Organically modified zirconate sol–gel films were investigated as an environmentally compliant replacement for chromated surface treatments, which included functionalized components to tailor the chemistry at both the aluminum oxide substrate/surface treatment interface and the surface treatment/topcoat interface. Sol–gel films were applied to aluminum 2024-T3 alloy coupons, and the resulting film chemistry was investigated. FTIR was used to identify organic components in the film and X-ray photoelectron spectroscopy was used to investigate the interface chemistry. The result of the chemically modified sol–gel synthesis was a coating in which a concentration gradient was formed at the surface, which is discussed. The corrosion protection attributes of these films was also studied and the electrochemical evaluation of sol–gel films will be discussed, both as stand-alone coatings and as part of a full coating system.

Organically modified sol–gels exhibited significantly better protection in terms of barrier properties in comparison to a typical chromate-based processes. The resulting data from evaluations of sol–gel produced coatings show promise towards the goal of producing a robust chemical interaction/bonding of such corrosion-resistant coatings on the surface of aluminum-skinned aircraft without the use of environmentally hazardous chromate agents.     

Inhibitor properties of “green” pigments for paints

Zinc chromate is one of the anticorrosive pigments most frequently used in the formulation of primers. However, its environmental aggressiveness and toxicity severely restrict its use, and different green alternatives have been proposed in order to replace zinc chromate. In the last decade, the behaviour of zinc phosphate as anticorrosive pigment has been intensively researched. During this time, various modifications have been made to this family of pigments to improve its properties, and a “second generation” of phosphate pigments, incorporating elements such as molybdenum, aluminium, or iron, has been produced. In this paper, the inhibitive properties of zinc phosphate and three second-generation phosphates have been investigated, using zinc chromate pigment as a reference. Pigment extract solutions, at different values of pH, have been used as corrosive media. Carbon steel samples were immersed in such solutions and their corrosion rates were measured using electrochemical techniques. The data obtained suggest that zinc chromate provides the highest percentage of inhibition in neutral and basic solutions, but phosphate-based pigments showed better results in acid solutions. Given this performance advantage, together with their less harmful environmental impact, these phosphate-based pigments can be proposed as realistic alternatives to chromates in the formulation of protective paints for use in acidic conditions.     

Comparison of the efficiency of inorganic nonmetal pigments with zinc powder in anticorrosion paints

This paper deals with the study of properties of anticorrosion pigments of varying chemical composition in epoxyester paints. Two type lines of paints were prepared. The first line comprised an anticorrosion pigment with a PVC concentration of 10% while the other line comprised an anticorrosion pigment with a PVC concentration = CPVC. The following nontoxic anticorrosion pigments were observed: zinc phosphate, zinc phosphomolybdate, calcium hydrogen phosphate, zinc phosphate modified with an organic corrosion inhibitor, strontium–aluminum polyphosphosilicate, zinc–aluminum polyphosphate, calcium metaborate, calcium ferrite, calcium borosolicate, and strontium chromate. The epoxyester primers were observed for the effect of the type of pigment on the anticorrosion properties. Anticorrosion efficiency was derived from tests in a condenser chamber and in a salt spray cabinet as well as from a test of chemical resistance of pigmented coatings. The evaluation of anticorrosion efficiency of inorganic nonmetal pigments was carried out by means of comparison with anticorrosion efficiency of metal dust.     

Study of the anticorrosive behaviour of epoxy binders containing non-toxic inorganic corrosion inhibitor pigments

The anticorrosive performance of epoxy coatings pigmented with non-toxic corrosion inhibitors pigments was investigated in this work. The coatings used contained the following pigments: zinc phosphate (ZP), zinc phosphomolybdate (ZPM) and zinc calcium phosphomolybdate (ZCPM). For comparative studies epoxy coatings with the following compositions were made up: one only with filler (CRG); one without pigments, varnish (VR) and other with zinc chromate (ZC) pigment. The corrosion inhibitor performance of the coatings was evaluated by immersion tests in 0.01 mol L⁻¹ NaCl aqueous solutions and accelerated tests in a salt spray chamber. The corrosion inhibitor performance of the samples was monitored using open-circuit potential (E_oc) measurements and electrochemical impedance spectroscopy (EIS) technique. Complementary tests were carried out using water vapour permeability of free-standing films and thermogravimetric (TG) analysis. The permeability test showed that the addition of the studied pigments did not modify the barrier properties of the free-films in comparison that pigmented with chromate. Thermal analysis indicated that the addition of the pigments improved the thermal stability of the coatings and it suggested a resin/pigment interaction. The total immersion tests and salt spray tests demonstrated that the barrier properties of the coatings pigmented with the inhibitors were not degrading as much as that pigmented with ZC. Therefore, all the three pigments could replace ZC as an anticorrosive pigment in similar conditions to those described here. The best corrosion inhibitor performance in the total immersion test was presented by the ZPM and ZCPM coatings while in the salt spray test the corrosion inhibitor performance of all the three pigmented coatings was similar, suggesting that only in the less aggressive test is possible to detect any difference between the coatings with the non-toxic pigments.     

The role of chromate in filiform corrosion inhibition

This paper is part of a study on filiform corrosion (FFC) on aluminum alloy 2024-T3 and focuses on the surface characterization of corroded samples. Untreated samples were used as well as samples which had undergone pretreatments including polishing, surface etching and chromated conversion coatings (CCC). These samples were coated with both pigmented and non-pigmented epoxy-based coatings. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis was used to investigate the nature of the surface as well as optical images to gauge the rate of FFC advancement. FFC corrosion rates decreased on samples which had received a surface etching pretreatment and a chromate conversion coating. Pigmented coatings reduced the rate of FFC further and led to two different types of corrosion surface morphology. On pretreated samples, the corrosion appeared deeper and pit-like, possibly due to an enhanced polymer-substrate bond. On untreated samples, widespread FFC developed. SEM and EDX analysis of various intermetallic particles (IMPs) on all samples, inside and outside of corroded regions, revealed that Cr from pigments was found deposited on Cu-containing intermetallics in corroded areas while Fe/Mn-containing particles were free from all pigment traces. These results suggest that the Cr deposition on Cu particles hinders the cathodic reduction of oxygen necessary for FFC advancement. For this reason, the coating pigments proved, under FFC conditions, to be a more effective inhibitor than the Cr originating from a chromate conversion coating.     

Active metal-based corrosion protective coating systems for aircraft requiring no-chromate pretreatment

There has been much R&D effort expended to develop pretreatments and coatings that allow the replacement of toxic, carcinogenic, mutagenic, environmentally hazardous chromates used as pretreatments and pigments in aircraft coating systems. There have been many claims for chromate replacement in primer and pretreatment systems for aircraft, but no systems presently are in use that can function and meet specifications without some form of chromate used in the pretreatment and/or primer. The Mg-rich primer technology developed at North Dakota State University and now in final commercial development at AKZO Nobel Aerospace Coatings shows that finally aerospace Al alloys can be protected against corrosion. With simple cleaning only or a non-chromate pretreatment, the Mg-rich primer (MgRP) + aerospace topcoat provides an aircraft protection system that give corrosion protection that equals or exceeds any system using chromate in any form. About 18 peer reviewed papers have been published and at least twice that many presentations at technical meetings describing this new aircraft primer technology. There are an extensive number of samples in outdoor exposure, and in exposure on small parts of aircraft like port-hole covers and doors, and the coating system has been in accelerated exposure cabinets of all sorts. In the first versions of the magnesium primer premature blistering was noted during immersion or B117 continuous salt spray testing which may be due to hydrogen generation from water contact at a particle. Efforts to control the level of activity of the magnesium have been successfully accomplished since the testing reported in this paper which controls this phenomenon. Current formulations meet and exceed the ASTM B117 test results of full chromate primer systems. Open circuit potential (OCP) measurements indicate an extended period of cathodic protection of the Al aircraft alloys such as AA 2024 T-3 and AA 7075 T-6. After this stage of protection, a combination of Mg oxide, hydroxide and carbonate compounds seem to give protection to the system, as we have seen corrosion protection given to samples for greater than 10,000 h of cyclic exposure for Mg-rich primers with good aerospace topcoats. Preliminary data are presented for other Mg alloys as pigments in metal-rich coating systems. All data indicates that the Mg-rich (or Mg-alloy pigment-rich) primer + aircraft topcoat system gives excellent corrosion protection by mechanisms entirely different from the modes of protection for aircraft alloys given by the toxic, carcinogenic chromate compounds now in use in all corrosion protection systems for aircraft. In most cases thus far examined, the protection, especially in cyclic exposure, exceeds the present chromate-based systems.     

Release of cerium dibutylphosphate corrosion inhibitors from highly filled epoxy coating systems

Carcinogenic chromates are phased out as corrosion inhibitors in organic coatings, and are replaced by benign alternatives. Cerium-based compounds are excellent corrosion inhibitors in an aqueous environment. However, whether they are effective as corrosion inhibitor in an organic coating also depends on their interaction with the coating matrix, which should result in a sufficient release of inhibitors over a desired period of time. In this work we report on the leaching of cerium dibutylphosphate, Ce(dbp)₃, from an epoxy coating system as a first step toward a fundamental understanding of the leaching behavior. To this end, coating systems containing various levels of inhibitor loading, ranging from 1 up to 50 wt%, were fabricated. These coatings were characterized using Fourier transform spectroscopy (FTIR), and Scanning Electron Microscopy (SEM). Leaching experiments were performed by exposing the coating films to water, while the concentration of dissolved Ce(dbp)₃ in this water was monitored in situ using ultraviolet–visible (UV–vis) spectroscopy. The leaching of inhibitors showed a clear dependence on the initial inhibitor loading of the coating system. For highly loaded coatings (>25 wt%) all inhibitors leach out, whereas for a low loading level (<10 wt%) no leaching was observed. The difference between the high and low loading levels might be the formation of a porous network in the highly loaded coatings due to the dissolution of large Ce(dbp)₃ clusters. Using Magnetic Resonance Imaging (MRI) we visualized the penetration of a water front into this porous network during the leaching experiments. This water-filled porous network provides an excellent pathway for transport of Ce(dbp)₃. For low levels of inhibitor loadings such a network is absent, and the transport of Ce(dbp)₃ is blocked. The samples containing moderate levels of inhibitor loading showed an increasing amount of leaching for decreasing values of pH. We believe that this pH dependence is the result of the concurrent leaching of significant amounts of Bisphenol-based coatings residues at low pH since the removal of these residues will also result in an increased porosity of the coating.     

Hybrid nanocomposite coating by sol–gel method: a review

The development of environmentally friendly process for pretreatments of metallic substrates is a field of growing research due to the ban against chromates used as protective pretreatments. Among the possible candidates for environmentally friendly pretreatments of aluminum alloys are the silica-based sol–gel coatings. Such coatings are able to form an Si–O–Al conversion layer providing a stable alumina/sol–gel film interface, which inhibits the onset of corrosion. Sol–gel technology offers a wide range of chemical mechanisms and exhibits high potential substitutes for the environmentally unfriendly chromate metal-surface pretreatment. Sol–gel derived organo-silicate hybrid coatings, preloaded with organic corrosion inhibitors, have been developed to provide active corrosion protection when integrity of the coating is compromised. The incorporation of organic corrosion inhibitors into hybrid coatings has been achieved as a result of physical entrapment of the inhibitor within the coating material at the stage of film formation and cross-linking. Sol–gel derived coatings, especially the hybrid films, provide a dense barrier against electrolyte uptake, and offer a wide range of applications as corrosion protective, hydrophilic coatings, hydrophobic anti-reflective coatings, migration barriers against liquid and volatile compounds, antibacterial modification of textiles and water-repellent antistatic textiles. In this paper, the novel applications of the sol–gel derived coatings are presented and discussed.     

Ion-exchange pigments in primer paints for anticorrosive protection of steel in atmospheric service: Cation-exchange pigments

Hexavalent chromium compounds (chromates) have been widely used as inhibitor pigments in the formulation of anticorrosive primers. However, their high toxicity and carcinogenic effects are forcing the development of effective chromate-free organic coatings. One such alternative is the use of ion-exchangeable pigments. Studies of these pigments have not been conclusive about their anticorrosive efficiency and controversy surrounds their functional mechanisms, exchange capacity and anticorrosive performance.     

汽车涂料与有机颜料

汽车涂料中最重要的装饰性材料是汽车面漆,有机颜料作为展色材料是汽车面漆一定会选择的一个非常重要的材料品种。该文出于汽车涂料的专门要求,展开了从涂料角度观测对有机颜料选择的性能要求,对应汽车涂料对有机颜料的性能要求,探讨了有机颜料的表现力、耐受力、稳定性和经济性等的适用性。     

钢的环保型防锈抑断颜料的研制

概述了研制的钢的环保型防锈颜料(替代铬酸盐)的防锈抑断性能。在3．5％(质量分数)NaCl的中性和酸性水溶液中，对5种常用钢的防锈性能比铬酸盐好；在3．5％NaCl的碱性水溶液中的防锈性比铬酸盐略低；在3．5％NaCl的中性水溶液中，抑制两种常用钢应力腐蚀的能力比铬酸盐高，而抑制腐蚀疲劳断裂(CF)的能力与研制颜料的配方有关。     

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz f 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.     

[V10O28]6-柱撑纳米水滑石在AZ31镁合金有机防腐涂层中的应用

研制了一种以钒酸盐阴离子([V10O28]6-)柱撑纳米水滑石防腐颜料替代铬酸盐,用于AZ31镁合金腐蚀防护的有机涂层.研究了水滑石在不同浓度的NaCl溶液里的吸附和离子交换性能,以及钒酸盐缓蚀剂的极化曲线:考察了该水滑石防腐颜料的添加比例对镁合金环氧防腐涂层性能的影响,并通过电化学交流阻抗(EIS)测试技术对各试样进行了性能检测.结果表明,添加了20%(质量分数)水滑石的环氧涂层对镁合金具有较好的防腐作用.     

Evaluation of eco-friendly anticorrosive pigments for paints in service conditions

The performance of different replacements for chromates was studied. The anticorrosive properties of seven phosphate-based pigments, a calcium-exchange silica and a ferrite were assessed by electrochemical techniques and formulating solvent-borne paints with epoxy and alkyd resins. Paints contained 30% by volume of anticorrosive pigment, with respect to the total pigment content.     

EIS study of environmentally friendly coil coating performances

The corrosion protection of metal structures by the application of organic coatings is obtained using complex paint cycles. The metal pretreatment as well as primer, intermediate and top coat are the usual layers composing the protection system. Although chromate pretreatment processes and chromate containing primers are widely used, they require highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems. This fact pushed many research laboratories to develop new chromium-free pretreatments and primer inhibitors. Electrochemical impedance spectroscopy (EIS) proved to be a very useful technique to study the protection properties of the organic coating because of the possibility of evaluating the corrosion process on the metal substrate and to measure the electric and dielectric characteristics of the coating. In this study, new chromium-free pretreatments and new primers containing ecological inhibitors were applied to steel substrates coated using an Al-Zn alloy. The results were compared with those obtained using traditional protection systems. The characterization of the complete system (pretreatment and primer) in sodium sulfate solution clearly showed the different water uptake behavior of the two primers, which highlights the better barrier properties of the one containing phosphate. However the corrosion protection of the primer containing chromates is also due to the inhibitive action of the chromates. Hence such a type of primer better withstands the presence of defects in the coating. Moreover the presence of environmentally friendly pretreatments do not worsen the corrosion protection which appears comparable to that observed when chromatation is employed.     

Nanostructured coatings approach for corrosion protection

Nanostructured surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach were investigated as potential replacement for chromate-based surface treatments on aircraft aluminum alloys. In the traditional sol–gel method, hydrolysis-condensation processes are followed by condensation polymerization upon film application. This process sequence provides a low temperature route to the preparation if thin coatings which are readily applied to most metallic substrates. The recent discovery of a method of forming functionalized silica nanoparticles in situ in an aqueous sol–gel process, and then cross-linking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This Self-assembled Nanophase Particle (SNAP) process can be used to form thin, dense protective organic surface treatment coatings on Al aerospace alloys. The ability to design coating components from the molecular level upward offers tremendous potential for creating multifunctional coatings.

The important components of Al alloy corrosion inhibition by chromate are storage and release of Cr^VI species, inhibition of cathodic reactions (primarily oxygen reduction), and inhibition of attack at active sites in the alloy. Unlike chromate-based treatments, current SNAP coatings provide barrier-type corrosion resistance but do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. In this study, organic inhibitors were tested for corrosion protection of aluminum alloys in combination with the (SNAP). Scanning Vibrating Electrode Technique, anodic polarization, electrochemical impedance spectroscopy, and salt spray test were used to study this new approach for chromate replacement.     

应用于塑料和涂料中的有机颜料

在生产塑料时,选择正确的颜料取决于颜料的分散能力,迁移特性,热稳定性,光稳定性以及价格。这些性能与颜料的浓度,塑料的厚度,助剂的使用有很大的关系。当颜荆粉末与高分子树脂混合,在溶剂中高速分散,制成涂料。所有的涂料者含有四个主要成分：颜料、高分子/树脂连接剂、溶剂、助剂。此文阐述有机颜料在塑料与涂料中的性能并且总合其在商业上的一些应用。     

Mg-rich coatings: A new paradigm for Cr-free corrosion protection of Al aerospace alloys

Environmentally acceptable alternative coatings to chromate pigments and pretreatments for the corrosion control of Al alloy 2024 T-3, commonly used in aircraft, were designed, formulated, and tested as primer coatings to provide protection using particulate Mg-rich pigmentation. The system was designed by analogy to pigmented Zn-rich primer coatings used for the protection of steel. In the current study, four coating polymer systems were examined as possible candidates as polymer matrices for Mg-rich cathodic protect coatings. Mg-rich primers were formulated with ∼50-micron average particle size magnesium powder, near to the critical pigment volume concentration (CPVC) for this system. Top-coated scribed coatings systems have been subjected to Prohesion^™ exposure in dilute Harrison’s solution for up to 5000 hr. These coatings are the first nonchromated coatings to satisfy 3000 hr of such exposure and remain shiny and undamaged in the scribe area, only showing damage at about 4800 hr. The corrosion byproducts generated in the scribe areas during Prohesion exposure were examined by energy dispersive X-ray analysis (EDXA), and the local pH of the coating determined by the nature of the salt formed as a function of exposure conditions and time, did not cause Al corrosion. Presented at the 81 st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, 2003, in Philadelphia, PA.