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.     

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.     

Ceramic microspheres to improve anticorrosive performance of phosphate paints

Zinc phosphate and related compounds have been proposed as “green” replacements for traditional anticorrosive pigments such as zinc chromate and lead oxides. However, environmental concerns have risen in the last years because the disposal of these materials in the environment increased phosphate levels in water and produce eutrophication of water bodies. The present paper deals with the possible incorporation of ceramic microspheres in alkyd paints in order to diminish phosphate content. The results suggested that paints with low phosphate content can be successfully formulated with a suitable selection of the amount and type of microspheres.     

快干单组分含氯防腐涂料的研制

采用磷酸锌、锶铬黄和绢云母作为防锈颜料,选用含氯树脂作为主体树脂,酚醛改性环氧树脂作为辅助树脂,制备了快干单组分含氯防腐涂料。考察了含氯树脂与酚醛改性环氧树脂配比、防锈颜料配比、颜基比以及增塑剂用量等对涂层性能的影响。实验结果表明:该涂料干燥时间≤7 h,为快干型防腐涂料;含氯树脂与酚醛改性环氧树脂质量比为10∶1,磷酸锌加入量为8%、锶铬黄加入量为2%、绢云母加入量为8%、增塑剂加入量为7.4%时,单道涂层的耐盐雾性可达500 h以上。     

环保型磷酸盐防锈颜料的研究进展

磷酸盐防锈颜料是当前取代红丹和锌铬黄等传统有毒颜料的有效品种,以其突出的耐腐蚀性能和环保特性在涂料工业中引起重视并取得了长足进展。简要论述无毒磷酸盐防锈颜料的种类,以及国内外应用研究情况,并对其组成、应用性能的发展方向提出了建议。     

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.     

Comparative electrochemical studies of zinc chromate and zinc phosphate as corrosion inhibitors for zinc

The anticorrosive performance of two inhibitive pigments, zinc chromate and zinc phosphate, was compared using electrochemical impedance spectroscopy (EIS) and the scanning vibrating electrode technique (SVET) in pigment extracts in 0.1 M NaCl. It was observed that zinc was protected from corrosion in both extracts. In tests using hot dip galvanised steel painted with an epoxy primer incorporating the pigments, the SVET detected the anodic and cathodic distribution along the scribes, although no significant differences were observed among the various primers. On the contrary, EIS was able to distinguish processes occurring on the metal surface exposed by the scribe in different samples. For primers with anticorrosive pigment, a time constant at high frequencies was attributed to a layer of protective nature, probably formed by metal ions from the substrate and inhibitive ions leached from the anticorrosive pigments.     

A new pigment for smart anticorrosive coatings

The purpose of this paper was to evaluate the performance of a modified zeolite as an anticorrosive pigment for paints. A procedure to prepare the pigment was outlined and its anticorrosive properties assessed following the electrochemical behavior of a steel electrode in pigment suspension. In a second stage, alkyd paints were formulated employing different anticorrosive pigments: (1) 30% by volume (v/v) of the modified zeolitic rock, (2) 10% (v/v) of zinc phosphate, and (3) a mixture of 10% (v/v) zinc phosphate plus 20% (v/v) of the modified zeolitic rock. In every case, percentages were referred to the total pigment content. Titanium dioxide, zinc oxide, and barium sulfate were incorporated to complete the pigment formula. The pigment volume concentration/critical pigment volume concentration (PVC/CPVC) ratio was 0.8. The performance of the resulting anticorrosive paints was assessed by accelerated (salt spray and humidity chambers) tests and electrochemical (corrosion potential, ionic resistance, and polarization resistance) essays. It was demonstrated that the modified zeolite is effective in protecting steel from corrosion when it is used in combination with zinc phosphate. There exists a synergism between the modified zeolite and zinc phosphate that allows the zinc phosphate content in anticorrosive paints to be reduced.     

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.     

Challenges of chromate inhibitor pigments replacement in organic coatings

General considerations concerning pigment grade corrosion inhibitors are presented and new characteristic functional parameters proposed. The chemistry, contemporarily practiced for corrosion inhibitor pigment synthesis, as well as known mechanistic considerations relevant to the corrosion inhibitor species available in pigment grades, are reviewed. Inherent limits of chemistry are outlined regarding the feasibility of developing equally effective and versatile non-toxic alternatives for chromates.

Fundamental aspects of corrosion inhibitor pigment behavior are discussed in the context of organic coatings degradation mechanisms. Disclosed experimental data demonstrate that correlation exists between solubility of corrosion inhibitor pigments, leaching from, and osmotic blistering of organic coatings. A mathematical expression is proposed and empirically proven to adequately describe the leaching rate of corrosion inhibitor pigments from organic coatings.

The barrier function of some high performance organic coatings, such as aircraft or coil, is negligible and requires the contribution of an effective corrosion inhibitor pigment, which, typically, is strontium chromate. In this context, the feasibility of replacing chromate inhibitor pigments is assessed and the “gap” observable between the inhibitor performance of chromates and traditional non-chromate pigments is noted.

Experimental data are displayed regarding the corrosion inhibitor performance of a novel, organic–inorganic, hybrid type corrosion inhibitor pigment in typical aircraft primer application on aluminum and plated steel. The presented data demonstrate the feasibility of chromate replacement in this specific case.     

Evaluation of anti-corrosive pigments by pigment extract studies, atmospheric exposure and electrochemical impedance spectroscopy

The inhibition efficiencies of zinc chromate, barium metaborate, calcium silicate, amino carboxylate, calcium barium phosphosilicate, aluminum triphosphate and a modified zinc phosphate on the corrosion of steel and zinc were determined by polarization experiments on pigment extracts. Zinc phosphate and zinc chromate were the best and were studied further to determine the effect of pH and chloride concentration on their inhibition of steel. Zinc chromate is adversely affected by high concentration of chloride ions, which effect seems to be less pronounced on zinc. A low pH, although increasing the solubility of zinc phosphate, does not increase its efficiency. The pigments were also incorporated into an epoxy-poly(amide) binder, applied to cold-rolled steel and galvanized steel, exposed at a marine exposure station and the degradation monitored by electrochemical impedance spectroscopy. There was a general correlation between the results of pigment extract studies and atmospheric exposure except in the case of phosphate pigments on cold rolled steel.     

水性环氧防腐涂料的研制

以自制的环氧树脂乳状液为漆基、磷酸锌作为活性防锈颜料合成了水性环氧防腐涂料。研究了涂料中颜基比、固化剂与环氧树脂的当量比等对涂膜性能的影响。结果表明,颜基比小于1,固化剂CEN-537与环氧树脂E-44的当量比为1或稍大于1时,所得涂膜的各项性能指标可达到较好的平衡;涂料的防绣性能与以硅铬酸铅为防绣颜料的传统涂料相当。新研制的水性环氧防腐涂料性能达到了国外同类产品的水平。     

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.     

环保型亚磷酸钙防锈涂料的研制

用无毒亚磷酸钙作为主要防锈颜料,磷酸盐作为辅助防锈颜料,研制了一种环保型亚磷酸钙防锈涂料。按国标测定了该涂料的性能指标,并与传统的红丹、锌铬黄、磷酸锌等防锈涂料进行对比试验。试验结果表明,亚磷酸钙涂料的防锈性能与红丹防锈涂料相当,优于锌铬黄、磷酸锌防锈涂料。     

集装箱用水性聚氨酯外面漆的研制

以物理性能优异的水性聚氨酯树脂为主要成膜物质,并利用防腐性能优异的苯丙乳液对其进行共混改性,优选了无毒防锈颜料,制备出了一种综合性能优异的集装箱专用水性聚氨酯外面漆。结果表明:当丙烯酸树脂B的混入量为20%,以磷酸锌为防锈颜料,颜填料体积浓度(PVC)为25%时,外面漆的性能满足JH/T E01—2008集装箱涂料行业标准。并系统地考察了外面漆施工时易出现的流挂、针孔等漆膜弊病的影响因素及防治措施。     

Zinc hypophosphite: a suitable additive for anticorrosive paints to promote pigments synergism

In this investigation, the synergism promoted by zinc hypophosphite in an anticorrosive pigment mixture is reported. This paper describes the anticorrosive behavior of a commercial pigment mixture containing zinc hypophosphite, reduced levels of zinc phosphate, and zinc oxide. The anticorrosive performance of the pigment mixture was assessed by electrochemical techniques (corrosion potential and linear polarization measurements) employing pigment suspensions. The behavior of each separate component of the mixture was also studied in the same way. The nature of the protective layer was investigated by scanning electron microscopy (SEM). In a second stage, the anticorrosive properties of the pigment were assessed by incorporating it into alkyd and epoxy paints that were evaluated by accelerated (salt spray and humidity tests) and electrochemical measurements. Experimental results showed that improved anticorrosion protection is achieved in paints with reduced zinc phosphate contents as a consequence of the synergistic interaction between zinc hypophosphite and the other components of the pigment mixture.     

Performance of phosphate–alumina pigments in waterborne paints for protection of cold-rolled steel

Core–shell theory presents a new easy method to obtain high performance, economic, and eco-friendly anticorrosive mixed pigments. The core–shell pigments in this work are prepared by depositing a surface layer of an expensive efficient anticorrosive pigment (phosphates) on a bulk of less expensive extender pigment (alumina). The combination of these two compounds led to the production of new pigments with improved properties different from each of its individual components; and consequently these improved properties led to changing the efficiency of protection properties of paint films containing these new pigments. The new pigments were characterized using XRD, SEM, TEM, and EDAX analysis to elucidate their structure and prove the presence of phosphate ions on the surface of alumina. These pigments were also evaluated according to ASTM measurements. Then they were incorporated in three groups of emulsion paint formulations based on styrene acrylic emulsion copolymer, and comprising different concentrations of Zn, Mg, and 1Zn·1Mg phosphates/alumina core–shell pigments; in addition to a control formulation free of these pigments, to evaluate their efficiencies in anticorrosive paints for protection of cold-rolled steel. Chemical, physical, and mechanical measurements were carried out, besides corrosion resistance tests and weight loss of steel panels under paint films which were determined in 5% NaCl solution for 28 days. The results proved that the prepared formulations containing the prepared pigments could protect steel efficiently from corrosion compared to the control specimen. Groups II and III containing higher weight percentages of prepared pigments showed the best performance in corrosion protection, and the best performance among these two groups was paint films containing magnesium phosphate/alumina pigments.     

Corrosion studies on tailored Zn·Co aluminate/kaolin core-shell pigments in alkyd based paints

Core-shell theory presents a new easy route to obtain high performance, and eco-friendly anticorrosive pigments with concomitant savings. This theory is based on depositing a surface layer of expensive efficient anticorrosive pigment on a cheap extender expressing the bulk. The combination of both core and shell compounds led to the production of new pigments with improved properties different from each of its individual components; and consequently these improved properties lead to change in the efficiency of protection properties of paint films containing these new pigments.The objective of the present work is to prepare a new ecologically friendly inorganic pigment, based on the solid-solid interaction of cobalt oxide with zinc oxide on surface of kaolin. The species of CoO and ZnO are susceptible to react with alumina part in the kaolin forming Zn·Co aluminate/kaolin. The new pigment will be incorporated in medium oil alkyd-based paints. The effect of cobalt aluminate/kaolin, zinc aluminate/kaolin individually and zinc·cobalt aluminate/kaolin on the protection performance of paint films containing them will be compared and studied. Another comparison will be held out between films containing nano- and micro-zinc·cobalt aluminate/kaolin to determine the best among the prepared pigments for better corrosion prevention. The comparison will be done using accelerated laboratory test for 28 days in artificial sea water and electrochemical impedance spectroscopy. The results revealed that nano-pigments and pigments containing higher concentration of zinc were the best in protecting steel substrates.     

Electrochemical assessing corrosion inhibiting effects of zinc aluminum polyphosphate (ZAPP) as a modified zinc phosphate pigment

Undesirable anti-corrosion performance of zinc phosphate pigment, the classical chromate replacement, has led researchers to take modification into account. Polyphosphate-based anti-corrosion pigments as a result of modification of zinc orthophosphate have been found to function much more efficiently. This study aimed to evaluate performance of steel samples immersed in 3.5% NaCl aqueous solution-containing zinc aluminum polyphosphate (ZAPP) pigment extract compared to those involving conventional zinc phosphate (ZP) pigment extract and also no pigment (blank) using electrochemical tests such as electrochemical impedance spectroscopy (EIS) and linear polarization (LP) as well as surface analysis. Impedance spectra and polarization curves revealed two different trends, showing the superiority of ZAPP pigment. Based on the results of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), presence of a precipitated layer on the surface was confirmed when steel sample was immersed into the solution-containing ZAPP.     

The effect of benzimidazole and zinc acetylacetonate mixture on cathodic disbonding of epoxy coated mild steel

Electrochemical behavior of mild steel in the presence of zinc acetylacetonate (Zn(acac)₂) and benzimidazole (BIMIDA) was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution and compared to zinc phosphate (ZP) pigment and zinc potassium chromate (ZPC) pigment extracts. Results showed superior performance of Zn(acac)₂ and BIMIDA mixture (ZBM) compared to ZP pigment, while it introduced inferior inhibitive action compared to ZPC pigment. The epoxy coatings were formulated with ZBM as anticorrosive additive, ZP and ZPC pigments. The disbonding rate of coated steel follows the order: Blank > ZP > ZBM > ZPC. EIS results showed a complex film could be formed at the hole area for the coatings formulated with ZP, ZBM and ZPC. It is concluded that the stronger the complex film on the surface, the lesser the cathodic disbonding rate would be.