Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests

The formation of zinc phosphate coating by cathodic electrochemical treatment and evaluation of its corrosion resistance is addressed. The corrosion behaviour of cathodically phosphated mild steel substrate in 3.5% sodium chloride solution exhibits the stability of these coatings, which lasts for a week's time with no red rust formation. Salt spray test convincingly proves the white rust formation in the scribed region on the painted substrates and in most part of the surface on unpainted surface. The protective ability of the zinc corrosion product formed on the surface of the coated steel is evidenced by the decrease in the loss in weight due to corrosion of the uncoated mild steel, when it is galvanically coupled with cathodically phosphated mild steel. Potentiodynamic polarization curves reveal that E_corr shifts towards higher cathodic values (in the range of −1000 to −1100 mV versus SCE) compared to that of uncoated mild steel and conventionally phosphated mild steel substrates. The i_corr value is also very high for these coatings. EIS studies reveal that zinc dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the non-metallic nature of the coating is progressively increased due to the formation of zinc corrosion products, which in turn enables an increase in corrosion resistance with increase in immersion time. The zinc corrosion products formed may consist of zinc oxide and zinc hydroxychloride.     

Phenol electropolymerization: a straight route from monomers to polymer coatings

Recent results on phenol electropolymerization aimed at the simultaneous production and deposition of protective coatings are described. Discussion is especially focused on the electrochemical growth mechanism of the non-conducting polymer films (previously unpublished chronoamperometric and chronopotentiometric results are provided and compared with a simple model), incorporation of corrosion inhibitors, the selected inhibitor being 2-benzothiazolylthiosuccinic acid, coating of phosphated mild steel and phosphated galvanized steel via the cathodic deposition of minor Zn amounts in the phosphate layer pores and protective performance of the coated samples, studied by a.c. impedance. It is shown that significant advances have been achieved since 1987, when a comprehensive review was published, in both the fundamental understanding of the process and the protective capabilities of the coatings.     

Novel isocyanate-free self-curable cathodically depositable epoxy coatings: Influence of epoxy groups on coating properties

Novel self-curable cathodically depositable coatings were developed from glycidyl functional epoxy ester-acrylic graft co-polymer (EEAG) without using any external crosslinking agents. The EEAG-amine adducts (EEAGAs) were prepared by reacting EEAG with varying amount of diethanolamine (DEoA) which are neutralized with acid and dispersed in deionised water to give stable dispersion for cathodic electrodeposition (CED) coatings. The dispersions were cathodically electrodeposited on phosphated steel panels and thermally cured to give uniform coating. The coatings were evaluated for different mechanical, chemical and corrosion resistance properties. The coatings were evaluated for their thermal properties using thermo gravimetric analysis (TGA). The final properties of the coatings were found to be affected by the amount of amine reacted with epoxy. The coating films showed good overall performance properties for their use in coating industry.     

Corrosion resistance of phosphate coatings obtained by cathodic electrochemical treatment: Role of anode–graphite versus steel

The formation of phosphate coatings by cathodic electrochemical treatment using graphite and steel anodes and evaluation of their corrosion resistance is addressed in this paper. The type of anode used, graphite/steel, has an obvious influence on the composition of the coating, resulting in zinc–zinc phosphate composite coating with graphite anode and zinc–iron alloy–zinc phosphate–zinc–iron phosphate composite coating with steel anode. The corrosion resistance of the coating is found to be a function of the composition of the coating. The deposition of zinc/zinc–iron alloy along with the zinc phosphate/zinc and zinc–iron phosphate using graphite/steel anodes has caused a cathodic shift in the E_corr compared to uncoated mild steel substrates. The i_corr values of these coatings is very high. EIS studies reveal that zinc/zinc–iron alloy dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the formation of zinc and iron corrosion products imparts resistance to the charge transfer process and increases the corrosion resistance with increase in immersion time. The corrosion products formed might consist of oxides and hydroxychlorides of zinc and iron. The study suggests that cathodic electrochemical treatment could be effectively utilized to impart the desirable characteristics of the coating by choosing appropriate anode materials, bath composition and operating conditions.     

Polarization and impedance studies on zinc phosphate coating developed using galvanic coupling

Galvanic coupling technique is capable of producing coatings of desired thickness. Good quality coatings can be produced at low temperature. Galvanic coupling of mild steel (MS) with the other cathode materials such as titanium (Ti), copper (Cu), brass (BR), nickel (Ni), and stainless steel (SS) accelerates iron dissolution, enables quicker consumption of free phosphoric acid and facilitates an earlier attainment of point of incipient precipitation, resulting in a higher amount of coating formation. In the present investigation, potentiodynamic polarization and electrochemical impedance spectra on MS substrates phosphated using galvanic coupling are studied. This study reveals that MS substrates phosphated under galvanically coupled condition possess better corrosion resistance than the substrates phosphated under uncoupled condition.     

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.     

Study on an elaborated method to improve corrosion resistance of zinc phosphate coating

Coating with dense and fine particles containing fewer cracks and lower porosity shows more improved protective properties due to limiting pathways between the environment and base metal. The main aim of present research is to introduce an innovative method that is called rephosphating to achieve this morphology. The outstanding point of the present investigation is to highlight the significant effect of surface pretreatment by secondary grinding of phosphated surface and then rephosphating of this surface to obtain a coating with appropriate properties. The SEM observations showed that this method has an obvious influence on the formation of a very uniform zinc phosphate coating on the plain carbon steel compared with the traditional method of phosphating. Furthermore, the protecting properties of phosphated and rephosphated samples were described and compared using the neutral salt spray and the electrochemical polarization tests. The results showed that rephosphating method had a beneficial effect on improving the corrosion resistance. As well, improved paint adhesion of rephosphated sample was observed compared with that of the phosphated sample. Finally, it was concluded that when rephosphating method can be used to repair damaged phosphated areas, the coating with more compact morphology and improved properties can be achieved.     

Nano zinc oxide as a UV-stabilizer for aromatic polyurethane coatings

Nano zinc oxide (ZnO) was co-deposited together with a cathodic electrodeposition paint onto phosphated normal steel panels. The films containing nano zinc oxide were compared with blank films regarding their stability against UV radiation. SEM micrographs show that nano-ZnO can stop the formation of cracks in the film. On the other hand, AFM, surface roughness and loss of gloss studies showed that the presence of nano zinc oxide particles reduces the photo-degradation of the aromatic polyurethane binder. It was also found that the presence of the nano-sized ZnO particles in the films reduces the tendency of the films to yellowing.     

Reduction of cathodic delamination rates of anticorrosive coatings using free radical scavengers

Cathodic delamination is one of the major modes of failure for anticorrosive coatings subjected to a physical damage and immersed in seawater. The cause of cathodic delamination has been reported to be the result of a chemical attack at the coating–steel interface by free radicals and peroxides formed as intermediates in the cathodic reaction during the corrosion process. In this study, antioxidants (i.e., free radical scavengers and peroxide decomposers) have been incorporated into various generic types of coatings to investigate the effect of antioxidants on the rate of cathodic delamination of epoxy coatings on cold rolled steel. The addition of <5 wt% free radical scavengers to epoxy coatings improved the resistance toward cathodic delamination by up to 50% during seawater immersion, while peroxide decomposers had a limited effect. Testing using substrates prepared from stainless steel, copper, aluminum, galvanized steel, and brass also showed a reduction in the rate of cathodic delamination when the coating was modified with a free radical scavenger. The protective mechanism of free radical scavengers investigated for the primers are similar to that of antioxidants used for protection against photochemical degradation by UV-radiation of top coatings. Both substrate corrosion and degradation of a coating exposed to UV-radiation lead to the formation of free radicals as reactive intermediates.     

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.     

Effect of nano-ZnO particles on the corrosion resistance of polyurethane-based waterborne coatings immersed in sodium chloride solution via EIS technique

Nano-composite coatings were formed by incorporating 3 wt% nano-ZnO in a polyurethane-based waterborne coating. The nano-ZnO based composite coatings were applied on standard phosphated steel panels by cathodic electrodeposition. The electrodeposited nano-composite coatings were then baked for 20 min at 165 °C. To investigate the corrosion resistance of the coatings, the coated panels were immersed in 3.5 wt% NaCl solutions for 2880 h (120 days). The improvement in corrosion performance of the composite coatings was evaluated using electrochemical impedance spectroscopy technique. It was found that the films containing nano-sized ZnO particles show a corrosion resistance of 2 orders of magnitude higher than that of the neat films.     

In-situ phosphatizing coatings III: A water-reducible alkyd baking enamel

We report the successful formulation of stable and compatible in-situ phosphatizing coatings (ISPCs) for a waterborne alkyd-amino baking enamel applied on bare cold-rolled steel (CRS), iron phosphated Bonderite 1000 (BD), and iron phosphated plus Parcolene 60 chromated (BD+P60) coupons. The enhanced coating adhesion of water-based ISPCs is confirmed by the cathodic delamination measurements. After 100 hr of salt spray (fog) test, the corrosion resistance performance (measured by the corrosion disbondment across the “X” scribe, d in mm) of the water-based ISPC on CRS panel (d=4.0−7.0 mm) outperformed that of the control alkyd paint on B-1000 (d=26 mm) and also on BD+P60 (d=14 mm) coupons. The superior coating performance of water-based ISPCs is believed to result from the in-situ metal surface phosphatization as detected by the reflectance FTIR technique. Department of Chemistry and Biochemistry, DeKalb, IL 60115-2862.     

Protective properties of organic phosphate-pigmented coatings on phosphated steel substrates

Investigations have been carried out on properties of coatings, differing by their pigmentation and binder, and applied on different chemical pre-treatments of the steel surface. Paints based on alkyd and alkyd-melamine binders, pigmented with zinc phosphate and modified basic zinc phosphate were applied on amorphous and crystalline phosphated steel surface and, for the comparison purpose, on degreased steel surface. The effect of the binder, the pigment and the pre-treatment of the steel surface on the protective properties of the coatings were determined by measurements of adhesion, water absorption and water permeability and by results obtained in salt spray and Prohesion tests. Coatings based on alkyd binder show a lower damage degree and good retention of adhesion in corrosion conditions, in spite of a higher water absorption and water permeability and a lower initial adhesive strength. Protective properties of coatings have been found to be highly dependent upon the substrate pre-treatment. Chemical pre-treatment of the steel substrate increases the protective properties of the system, which is particularly evident in the case of crystalline phosphating and the coating pigmented with modified basic zinc phosphate. This phenomenon can be explained by the synergism between this phosphate pigment with crystalline phosphate layer.     

The improvement of anticorrosion properties of zinc-rich organic coating by incorporating surface-modified zinc particle

The corrosion behaviors of zinc-rich coating with various zinc contents, ranging from 0 to 60 volume percent, in thin organic coatings (below 5 μm) were characterized by electrochemical impedance spectroscopy (EIS), free corrosion potential (E_corr) measurement and cycle corrosion test (CCT). It was verified that both coatings with 60 volume percent of zinc powder and without zinc powder showed good corrosion resistance mainly due to the cathodic protection and barrier effect, respectively. On the other hand, coatings with an intermediate concentration (10–40 vol.%) of zinc powder was not successful in protecting a steel substrate efficiently. To improve anticorrosion property of zinc-rich coating, the surface modification of zinc particle was carried out with derivatives of phosphoric and phosphonic acid in the aqueous solution. The effects of the surface modification of zinc particle on corrosion resistance of the coating were investigated with scanning vibrating electrode technique (SVET) and X-ray photoelectron spectroscopy (XPS). The best anti-corrosion performance was achieved when the incorporated zinc particle was treated with phosphoric acid 2-ethylhexyl ester and calcium ion simultaneously, which induced the formation of alkyl-phosphate-calcium complex layer of 190 nm in thickness on zinc particles. Corrosion resistance was improved by the decreased zinc activity and the increased compatibility between the formed complex layer on zinc surface and polymer binder matrix.     

Critical effect of pH on the formation of organic coatings on mild steel by the aqueous electropolymerization of 2-vinylpyridine

Coatings of poly(2-vinylpyridine) have been formed on mild steel substrates in aqueous medium by electrochemical polymerization of the 2-vinylpyridine monomer. The pH of the solution has been found to be critical for this electropolymerization coating process. At low pH (below 3.5), even with an efficient initiation reaction, the propagation process was impeded and no substantial polymer film was formed. At high pH (above 6.0), only a thin and irregular film formed due to the lack of an effective initiation reaction. Only when the solution pH is in the range of 4 to 5.5 can good quality coatings be formed on mild steel substrates. The detailed effects of the pH on the electropolymerization are discussed in terms of a proposed free radical polymerization mechanism. This research has also resolved the issue of some of the non-reproducible experimental results reported in the literature and confirmed the feasibility of forming poly(2-vinylpyridine) coatings on a mild steel substrate by electropolymerization of the monomer.     

Deposition of zinc–zinc phosphate composite coatings on steel by cathodic electrochemical treatment

The present work aims at the development of an energy-efficient and eco-friendly approach for the deposition of zinc phosphate coatings on steel. The study describes the possibility of preparing zinc–zinc phosphate composite coatings by cathodic electrochemical treatment using dilute phosphoric acid as an electrolyte and zinc as an anode. The methodology enables the preparation of coatings with different proportions of zinc and zinc phosphate by suitably varying the applied current density, pH, and treatment time. Adhesion of the coating on mild steel and adhesion of paint film on the phosphate coating were found to be good. The surface morphology of the coatings exhibited platelet-type features and small white crystals (agglomerated at some places) which represented zinc and zinc phosphate, respectively. An increase in current density (from 20 to 50 mA/cm²) increased the size of the zinc crystals, and coatings prepared at 40 and 50 mA/cm² resembled that of electrodeposited zinc. Since the proportions of zinc and zinc phosphate could be varied with applied current density, pH, and treatment time, it would be possible to use this methodology to prepare coatings that would offer different degrees of corrosion protection.     

Electrochemical and anticorrosion performances of zinc-rich and polyaniline powder coatings

In this work, hydrochloride polyaniline (PANI-Cl) powder was incorporated as a conductive pigment into powder zinc-rich primer (ZRP) formulations in order to enhance the electronic conduction paths between zinc particles inside the coating and the steel substrate (i.e. percolation). Coatings were applied onto steel substrates and immersed in a 3% NaCl solution at ambient temperature.The protective properties and electrochemical behaviour of coatings were investigated by monitoring the free corrosion potential versus time and by using EIS. It was found that corrosion potential remains cathodic and constant for a long time up to 100 days of immersion. From EIS results, it was shown that the coatings exhibit larger impedance values than those observed with liquid or other zinc-rich powder formulations containing carbon black. From Raman spectroscopy results, it may be proposed that zinc particles in contact with PANI-Cl pigments were passivated. Other zinc particles remain still active which ensures the cathodic protection of the substrate. Moreover, coatings exhibit good barrier properties.     

Cyclic potential sweep electrolysis for formation of poly(2-vinylpyridine) coatings

A cyclic potential sweep (CPS) technique has been used to form coatings of poly(2-vinylpyridine) on mild steel substrates by electropolymerization of the monomer. This method can produce thick and uniform coatings of much higher quality than can be formed by other electrochemical methods such as galvanostatic electrolysis, constant cell-potential electrolysis and chronoamperometry. The range and rate of the potential sweep during the CPS are important for successful coating formation. Potential sweeps between –1.0 and –2.2 V vs SCE at rates from 10 to 50 mV s^–1 have been found to be most suitable for the formation of poly(2-vinylpyridine) coatings. The essential reason for the successful application of the CPS technique to the electropolymerization process is the compatibility of the nature of the CPS process and the mechanism of 2-vinylpyridine electropolymerization.     

Studies of arenediazonium salts as a new class of electropolymerization initiator

Electropolymerization holds great potential as a novel process for applying surface coatings onto a variety of substrates. Cathodic electropolymerization has been conducted successfully on metal substrates using initiation systems, such as sulfuric acid and potassium persulfate. However, each of these initiation systems has its own deficiency. This provided the motivation for investigating arenediazonium salts as a new class of cathodic electropolymerization initiators. Our studies found that arenediazonium salts can be easily reduced at reduction potentials lower than that of water to generate initiating free radicals. The reduction efficiency is very high. A copolymer of methyl acrylate and acrylonitrile has been polymerized onto steel surfaces using one of these salts, 4-methyl benzenediazonium tetrafluoroborate, as initiator. Cyclic voltammetry and reflection absorption infrared spectroscopy (RAIRS) studies suggest a strong interaction exists between the initiator and the substrate, which can potentially enhance coating adhesion. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2265–2272, 1999     

Electrochemical monitoring of electrogalvanizing solutions

The corrosion resistance of steel is vastly improved when a zinc cathodic protective layer is applied. In recent years there has been renewed interest in producing this galvanized coating electrolytically. As with many deposition processes, the control of the operating parameters is very important in order to produce good quality zinc coatings. The homogeneity, ductility and smoothness of the zinc layer are critical. These factors are usually influenced by the electrochemical nature of the solution and organic additives are often used to assist in control.Studies on the polarization behaviour of solutions for electrogalvanizing electrolytes were made. Cyclic voltammetry techniques were used to develop methods which might be applicable in monitoring the active organic content of liquorice in solution. The morphologies and orientations of the zinc were studied using SEM and X-ray diffraction techniques.