Corrosion inhibition by chromate and phosphate extracts for iron substrates studied by EIS and SVET (original) (raw)
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Corrosion Science
The anticorrosive performance of zinc chromate and zinc phosphate, used as extracts in 0.1 M NaCl, was studied using electrochemical impedance spectroscopy (EIS), the scanning vibrating electrode technique (SVET) and open circuit potential (OCP) measurements. Neither of the pigments managed to totally prevent corrosion. Phosphate had a much lower inhibiting efficiency and acted at a very low rate, whereas chromate acted immediately after immersion, but lost some of its action after a few hours of continuous immersion. Iron corroded uniformly in the phosphate extract, whereas in the chromate extract nucleation of metastable pits occurred. Evolution of the chromate layer with time was revealed in the EIS spectra in the form of a relaxation constant that developed during immersion, which was interpreted as being due to the formation of a porous healing layer formed on repassivated pits. The joint use of open circuit potential measurements, EIS and SVET is illustrated as a means of assessing different phenomena on the metal surface.
Progress in Organic Coatings, 2005
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.
Efficiency of certain chromate-free pigments for the corrosion protection of galvanized steel
Materials Science
By using electrochemical methods of a.c. and d,c. and electron microscopy, we investigated the corrosion behavior of galvanized steel in a slightly acid medium that simulates atmospheric precipitation and contains extracts of chromate and chromate-free pigments. We established that the joint use of phosphate and calcium-containing pigments can ensure efficient corrosion protection on a par with chromate pigments due to the formation of a phosphate film with high barrier properties on a metal. A galvanized steel sheet painted under laboratory conditions is widely used for the production of roofs and the facing of walls [l]. The varnish-paint coating applied to it consists of a primer and a top layer. To improve the protective properties, anticorrosive pigments (very often, toxic chromates) are added to the primer. For this reason, there is an intensive search for alternatives to chromate corrosion inhibitors of metals. Phosphate pigments are rather promising substitutes for chromates. They belong to nonoxidized inhibitors of the anodic type, and the efficiency of their protection depends on the amount of oxygen in a corrosive medium [2]. However, in the presence of ions of metal, phosphates can work;as cathodic inhibitors by depositing a protective film with barrier properties on the surface of the metal. According to the data presented in [3], bivalent ions of calcium and zinc increase the anticorrosive efficiency of polyphosphates. This is explairied by the formation of a barrier layer on cathodic sections that retards the free ingress of oxygen to the surface of iron. By using the method of potentiokinetic polarization of iron in a diluted aerated orthophosphate solution, one established in [4] that the addition of a small amount of calcium salts leads to inhibition of the cathodic reaction due to the appearance of a film of slightly soluble calcium phosphates on the surface of the metal, which restricts the ingress of oxygen to the surface of the metal. One can form a film with protective properties on the surface of metal in corrosive solutions containing phosphate inhibitors and calcium salts. However, the behavior of inhibitors dissolved in corrosive media considerably differs from the behavior of anticorrosive pigments dispersed in an organic coating. As a rule, they are slightly soluble and their action is screened by a polymeric binder. The aim of the present work is to study the efficiency of the inhibition of corrosion of a hot-galvanized steel by chromate-free pigments that are dissolved in a medium followed by the formation of ions of calcium and phosphate. Materials and Testing Procedure We used electrochemical impedance spectroscopy. Tests were carried out in a three-electrode electrochemical cell with a working area of 4.5 cm 2. We use a calomel electrode as the reference one and a platinum electrode as the operating one. We carried out measurements with an ASM potentiostat and a Solartron frequency response analyzer 1250 at the corrosion potential by applying a.c. with an amplitude of 10mV and a frequency from 10,000 to 0.005 Hz to the specimen. We processed impedance spectra by the Boukamp method [5]. Simultaneously, we recorded potentiodynamic polarization curves with an AutoTafel device. We performed all electrochemical measurements in a corrosion solution with inhibiting pigments that simulates acid rain precipitation in European countries [6]. We used phosphate (A), calcium-containing (B), and control chromate (C) pigments. To make saturated solutions-extracts, we added 2 g of each pigment to 1 liter of an acid rain solution and used a magnetic mixer. After a day, they were twice filtered to remove the undissolved fraction. In a
Progress in organic coatings, 1995
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.
Inhibition of corrosion by a mixture of nonchromate pigments in organic coatings on galvanized steel
Materials Science, 2004
We consider several possible substitutes for toxic chromate pigments. Among the most promising compositions for the corrosion inhibition of galvanized steel with a protective organic coating, we should mention a mixture of phosphate-and calcium-containing pigments. As was shown earlier, such a mixture possesses a high efficiency under conditions of the corrosion of zinc and galvanized steel sheet; however, it is less efficient for the protection of carbon steels. This pigment mixture forms an adsorbed phosphate film on the metal and hinders the course of anodic and cathodic reactions. Zinc ions play an important role for obtaining a synergistic protective effect, increasing the coherence and decreasing the porosity of the deposited film. Traditional and local electrochemical methods (EIS and SVE) were used for studying the corrosion of galvanized steel with coatings in acid rain solution. The data obtained corroborated the high anticorrosive efficiency of a pigment mixture in an organic coating on galvanized steel. In addition, we established some specific features of its inhibiting action near defects in the coating.
Materials Today: Proceedings, 2019
This study reports the use of zinc heptanoate (ZC 7) and zinc decanoate (ZC 10) as pigments for the corrosion inhibition of mild steel in NaCl medium. Their improvements compared to the widely used zinc phosphate (ZP). These pigments synthesized by co-precipitation method. X-ray-diffraction characterizations confirm the obtaining of these compounds. Stationary and dynamic electrochemical measurements (EIS) revealed that the both carboxylate pigments exhibit inhibition efficiency higher than that of reference ZP pigment. Besides, the EIS analysis along with XRD characterizations, carried out on substrates surfaces after electrochemical tests, showed the development of a protective layer based on iron carboxylate.
Corrosion inhibition of galvanized steel with inorganic pigments
Materials Science
Lacquer coatings on a metal are, as a rule, multiple and consist of a base coating, several layers of the basic coating, and a decorative layer [1]. The base coating is a promoter of adhesion for such coatings and contains mainly inorganic inhibiting pigments which electrochemically protect the metal base after the formation of through defects in the coating and penetration of the corrosion medium into the metal. Chromate pigments are widespread anticorrosive modifiers for lacquer base coatings [ 1]. They efficiently protect steels against corrosion in neutral and weakly acid media. However, their application is limited by ecological requirements. At the same time, one intensively searches for substitutes of less harmful pigments for chromates. Before proceeding to the investigation of polymeric coatings containing anticorrosive pigments (possible substitutes for strontium chromate), it is necessary to estimate the protective function of extracts of these pigments without polymeric binder. Having analyzed the information on pigments produced by industry, we took strontium chromate, zinc phosphate modified with zinc molybdate, and aluminum triphosphate.
Progress in Organic Coatings, 2006
The effect of the inhibitive pigment ZAP on waterborne paint systems was studied by means of impedance technique and Raman spectroscopy. The protective mechanism given by the action of ZAP in saline media onto low-carbon steel panels is attributed to the formation of a thin zincsodium-phosphate layer under cathodic delamination conditions. The chemical interactions between the waterborne paint systems and the metal substrate at the curing time and during the delamination tests were investigated by Raman spectroscopy. The layer developed on the delaminated area is thought to be a phosphate layer known as ␣-hopeite. This film is though to arise from a dissolution-precipitation mechanism in alkaline media. 13 14 15 16 17 18
Corrosion Engineering, Science and Technology, 2004
Mixtures of zinc, calcium and phosphate ions have been found to be effective (.90%) in inhibiting the corrosion of steel and zinc during immersion in acid rain solution and when used as pigment additions to organic coatings. The presence of calcium ions, in addition to zinc ions, appears to be fundamental in the inhibition mechanism since, in the absence of calcium, the inhibition efficiency is 50% or less. Although some anodic inhibition occurs, the predominant mechanism appears to be classical cathodic inhibition caused by the precipitation of a sparingly soluble, thin, but persistent, solid film. Surface analysis, by XPS, confirms that phosphorus is present in the form of phosphate species containing zinc and calcium. Also, the atomic ratios of zinc:calcium:phosphorus present in the inhibitive film strongly suggest that it consists of a mixture of CaZn 2 (PO 4) 2 (Scholzite) and Zn 3 (PO 4) 2 (Hopeite) in a 1 : 1 ratio. CEST/2126
2015
In the first part of this study, the effect of addition of various cations (Cu 2+ , Mn 2+ , Ca 2+ , Zn 2+ ) and the aminotris(methylenephosphonic) acid (ATMP) on the inhibition of iron corrosion in 1M HCl were studied using electrochemical methods (polarisation curves and EIS). Results indicate a significant synergistic effect between ATMP and Zn +2 . In the second part, the corrosion behaviour of iron in 1M HCl in the absence and presence of different ratios Zn/ATMP was studied using weight loss test and electrochemical techniques. The immersion solutions were analyzed using the spectroscopy induction coupled plasma. The results confirm the presence of synergistic effect between ATMP and Zn 2+ , which can be was attributed to the formation of Zn and ATMP complex. The maximum protection efficiency was observed at the ratio Zn/ATMP: 50%/50%.