Electrochemical deposition and characterization of ZnCo alloys and corrosion protection by electrodeposited epoxy coating on ZnCo alloy

ZnCo alloys electrochemically deposited on steel under various deposition conditions were investigated. The influence of deposition current density, temperature and composition of deposition solution on the phase structure and corrosion properties of ZnCo alloys were studied. It was found that ZnCo alloy obtained from chloride solution at 5 A dm⁻² showed the best corrosion properties, so this alloy was chosen for further examination. Epoxy coating was electrodeposited on steel and steel modified by ZnCo alloy using constant voltage method. The effect of ZnCo alloy on the corrosion behavior of the protective system based on epoxy coating is interpreted in terms of electrochemical and transport properties, as well as of thermal stability.     

Electrochemical and sorption characteristics and thermal stability of epoxy coatings electrodeposited on steel modified by Zn–Co alloy

The corrosion behaviour, transport properties and thermal stability of epoxy coatings electrodeposited on steel and steel modified by Zn–Co alloys were investigated during exposure to 3% NaCl solution. The electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption measurements and thermogravimetric analysis (TGA) were used. Zn–Co alloys were electrodeposited on steel from chloride and sulphate baths, by different current densities. From the time dependence of pore resistance and coating capacitance of epoxy coating, diffusion coefficient of water through epoxy coating and thermal stability it was shown that Zn–Co sublayer obtained from chloride solution significantly improves the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of exposure time, indicating the great stability of this protective system, due to the existence of a passive layer consisting of basic salts.     

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 μm thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.     

Corrosion behaviour of epoxy coatings electrodeposited on galvanized steel and steel modified by Zn–Ni alloys

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on hot-dip galvanized steel and steel modified by Zn–Ni alloys were investigated during exposure to 3% NaCl solution. Zn–Ni alloys were electrodeposited on steel by direct and pulse current. From the time dependence of pore resistance, coating capacitance and relative permittivity of epoxy coating, diffusion coefficient of water through epoxy coating, D(H₂O) and thermal stability, it was shown that Zn–Ni sublayers significantly improve the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of investigated time for epoxy coatings on steel modified by Zn–Ni alloys, indicating the great stability of these protective systems, due to the existence of the inner oxide phase layer and the outer layer consisting of basic salts.     

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.     

Influence of the curing temperature of a cataphoretic coating on the development of filiform corrosion of aluminium

Aluminium alloys are known to be particularly sensitive to filiform corrosion. The initiation of this particular type of corrosion is related to different parameters such as the presence of defects, the permeability of the coating to water and oxygen, the adherence of the paint system and the presence of salts.

In this work, the filiform corrosion resistance of Al 6016 substrates coated with a cataphoretic paint was studied. The curing of the coating was performed at different temperatures (185, 175, 165, 155 and 135 °C) in order to modify its mechanical properties and its permeability to oxygen.

The paint properties were studied by different techniques allowing the estimation of the degradation of the metal–primer system and giving some information about intrinsic paint properties.

The corrosion protection of the coating was evaluated by a normalized filiform corrosion test and by electrochemical impedance spectroscopy on scratched samples. The glass transition value and the internal stresses of the cataphoretic coatings obtained for different curing temperatures were determined by a stressmeter equipment.

This study enabled us to underline the influence of the curing temperature on the intrinsic properties of the coatings such as the glass transition temperature, the internal stresses, the adherence, the permeability and the corrosion protection properties.     

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.     

聚氨酯改性环氧富锌防腐涂料的制备

通过正交实验法,以涂膜的吸水率为依据,确定了制备聚氨酯改性环氧共聚物时,聚氨酯的最佳用量,适当的反应时间和反应温度,固化剂的最佳添加量;并通过浸泡腐蚀失重法,确定了锌粉的最佳含量。对涂膜进行了红外光谱、极化曲线和扫描电镜分析,对比检测了未改性环氧富锌防腐涂料、聚氨酯改性环氧富锌防腐涂料的各方面性能。结果表明,聚氨酯改性环氧富锌防腐涂料具有较高的机械强度、低吸水率、良好的附着力和耐化学腐蚀性能。     

Silane and epoxy coatings: A bilayer system to protect AA2024 alloy

This work has proved that a good combination of a simple and fast metal pre-treatment, followed by the deposition of a thin layer of an organic–inorganic silane coating and further layer of epoxy coatings, are able to protect the aluminium alloy AA2024-T3 against corrosion in high concentrations of NaCl solution. The alloy AA2024 is one of the most employed aluminium alloy in structural applications due to its good mechanical properties. However, AA2024 alloy series commonly presents galvanic corrosion due to the rich content of copper element. The influence of different surface pre-treatments, the presence of a silane layer as pre-coating treatment and the influence of phosphonic acids combined with the silane layer on the corrosion protection and adhesion to the aluminium alloy have been examined using accelerated corrosion tests. High roughness and the presence of a pre-coating film between the metal surface and the organic coating were essential for a good protection and resistance to blistering appearance in the surface of AA2024-T3.     

Effects of carbon nanotube content on adhesion strength and wear and corrosion resistance of epoxy composite coatings on AA2024-T3

The effects of multiwalled carbon nanotube (MWCNT) content on the adhesion strength and wear and corrosion resistance of the epoxy composite coatings prepared on aluminum alloy (AA) 2024-T3 substrates were evaluated using atomic force microscopy (AFM), blister test, ball-on-disk micro-tribological test and electrochemical impedance spectroscopy (EIS). The adhesion strength of the epoxy composite coatings improved with increasing MWCNT content. Increased MWCNT content also decreased the friction coefficient and increased the wear resistance of the epoxy composite coatings due to improved solid lubricating and rolling effects of the MWCNTs and the improved load bearing capacity of the composite coatings. Finally, EIS indicated that increased MWCNT content increased the coating pore resistance due to a decreased porosity density, which resulted in an increase in the total impedance of the coated samples.     

Preparation of epoxy–clay nanocomposite and investigation on its anti-corrosive behavior in epoxy coating

An epoxy–clay nanocomposite was synthesized using a quaternary ammonium-modified montmorillonite clay and diglycidyl ether of bisphenol A (DGEBA) type epoxy resin, in order to produce anti-corrosive epoxy coating. Anti-corrosive properties of the nanocomposite were investigated using salt spray and electrochemical impedance spectroscopy (EIS) methods. The results showed an improvement in the barrier and anti-corrosive characteristics of epoxy-based nanocomposite coating and a decrease in water uptake in comparison with pure epoxy coating. Wide-angle X-ray diffraction (WAXD) patterns and transmission electron microscopy (TEM) analysis showed that the interlayer spacing of clays increased after addition of epoxy resin along with applying shear force and ultrasound sonicator. The best performance of this coating was achieved at 3 and 5 wt.% clay concentration.     

Investigation of corrosion protection properties of an epoxy nanocomposite loaded with polysiloxane surface modified nanosilica particles on the steel substrate

In this study, it has been aimed to investigate the corrosion protection properties of an epoxy/polyamide coating loaded with different concentrations (ranged from 3 to 6% (w/w)) of the polysiloxane surface modified silica nanoparticles (nano-SiO₂). The nanocomposites were applied on the steel substrates. Field emission scanning electron microscope (FE-SEM) and UV–vis techniques were utilized in order to investigate the nanoparticles dispersion in the coating matrix. The effects of addition of nanoparticles on the corrosion resistance of the coating were studied by an electrochemical impedance spectroscopy (EIS) and salt spray test. The coating surface degradation was studied by optical microscope and Fourier transform infrared radiation (FT-IR) spectroscopy. Results obtained from UV–vis and FE-SEM analyses revealed proper and uniform distribution of surface modified nanoparticles in the epoxy coating matrix. It was shown that the coating corrosion protection properties were significantly enhanced in the presence of 5 wt% silica nanoparticles. Less degradation occurred on the surface of the coatings loaded with 5 wt% nanoparticles.     

Electrochemical behaviour of ZrO2 sol-gel pre-treatments on AA6060 aluminium alloy

ZrO₂ pre-treatments applied with the sol-gel technique are a possible replacement of chromium based pre-treatments on aluminium alloys. The thickness and homogeneity of the films deposited on AA6060 alloy are strongly related to the process parameters like preparation of the surface, number of dips and thermal treatment of the film.ZrO₂ films were prepared using the dip-coating technique in sol obtained from metal-organic precursors in an organic solvent (0.1 M Zr(OBuⁿ)₄ in anhydrous n-butanol with addition of acetic acid as complexing agent). Different layers were applied on AA6060 changing number of dips and thermal treatment (150 °C for 1 h or 250 °C for 4 min). The typical thickness of the deposited layers was in the range 70-180 nm depending on process parameters. The electrochemical behaviour of the pre-treated alloy in diluted Harrison solution (0.05 wt% NaCl + 0.35 wt% (NH₄)₂SO₄) was investigated by means of potentiodynamic polarization, open circuit potential measurements, and electrochemical impedance spectroscopy. In addition, the electrochemical behaviour of ZrO₂ sol-gel films was compared with that of chromatized AA6060 and fluotitanated/fluozirconated AA6060. In order to evaluate the adhesion properties of the films, ZrO₂ pre-treated AA6060, chromatized AA6060 and fluotitanated/fluozirconated AA6060 were painted with a polyester resin and subjected to thermal cycles in 0.05 wt% NaCl. Each thermal cycle consisted of heating the samples at 90 °C, permanence at 90 °C for 6 h, cooling at room temperature and permanence at room temperature for 18 h. Impedance measurements were performed at the end of each cycle.Potentiodynamic polarization curves and impedance spectra indicate that ZrO₂ pre-treatments have similar barrier properties to those of chromatized AA6060. However, no self-healing ability is observed for ZrO₂ films.The barrier properties of ZrO₂ films are strongly dependent on process parameters. In particular, the number of dips determines the amount of defects in the film and its homogeneity. The electrochemical behaviour strongly improves increasing the number of dips in the deposition bath. Thermal aging cycles evidence good adhesion properties for ZrO₂ pre-treatments.     

Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy

The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce₂(SO₄)₃, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al₂O₃ nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al₂O₃ nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts.     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

Anticorrosion performances of epoxy coatings modified with polyaniline: A comparison between the emeraldine base and salt forms

The protection against corrosion imparted by epoxy paints modified by the addition of polyaniline emeraldine base (0.3% w/w) and Zn₃(PO₄)₂ (10% w/w) has been investigated and compared. For this purpose, accelerated assays through corrosion cycles, which simulate the marine conditions, have been performed using a home-made robotized equipment. Results were compared with those recently obtained for a formulation constituted by the same epoxy paint modified with polyaniline emeraldine salt (0.3% w/w) [E. Armelin, R. Pla, F. Liesa, X. Ramis, J.I. Iribarren, C. Alemán, Corr. Sci. 50 (2008) 721.]. A detailed analysis of the results using the ASTM standard method D-1654-79 allows to conclude that the coating modified with polyaniline emeraldine base performs better than both the unmodified coating and the coatings modified with the inorganic corrosion inhibitors. This behaviour should be attributed to a mechanism based on the ability of polyaniline emeraldine base to store charge. On the other hand, as the epoxy paint modified with the latter polymer provides better results than the formulation containing Zn₃(PO₄)₂, we conclude that inorganic corrosion inhibitors, which may have detrimental effects on both the environment and the human health, can be replaced by a small concentration of environmentally friendly organic polymers.     

Morphology and properties of UV-curing epoxy acrylate coatings modified with methacryl-POSS

In this work, the morphology and properties of UV-curing epoxy acrylate (EA) coatings modified with methacryl polyhedral oligomeric silsesquioxanes (M-POSS) were studied. The M-POSS nanocages were introduced into EA UV-curing system via copolymerization at loadings between 0% and 10 wt%. The XRD and FTIR analysis indicated that M-POSS chemically incorporated into the hybrid materials and formed a cross-linked network between M-POSS and EA. The morphological analysis showed that the discrete spherical POSS-rich particles were dispersed in the EA matrix uniformly, and both of the number and mean diameter of POSS-rich particles increased with the increasing M-POSS loadings. The influence of M-POSS on the kinetics of the photopolymerization was determined by real time FTIR spectroscopy and the result showed that the addition of POSS enhanced both of the UV-curing rates and final double bond conversion. The DMA analysis showed that increasing the amount of M-POSS nanocages caused an increase on the nanocomposite's T_g. TGA curves showed that at the later period of degradation process, the thermal stability of nanocomposites was enhanced by M-POSS. With respect to the mechanical properties, the most remarkable trend was an improvement on the impact resistance of nanocomposites with the increasing POSS contents. Because both of the craze and plasticity deformation caused by POSS nanocages would absorb impact energy, hinder the growth of craze.     

Effects of different surface cleaning procedures on the superficial morphology and the adhesive strength of epoxy coating on aluminium alloy 1050

This study reports the effects of different cleaning procedures on the surface characteristics of the aluminium alloy 1050 substrates and on the adhesive strength of the epoxy coating to this alloy's surface. The cleaning procedures used in this study were (1) degreasing by acetone, (2) alkaline etching by 5 w/w% NaOH solution and (3) alkaline etching by 5 w/w% NaOH solution followed by acid cleaning by 50 v/v% HNO₃ solution. The surface morphology, chemical composition and topography of the cleaned substrates were investigated by field-emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS) and atomic force microscope (AFM), respectively. The effectiveness of the cleaning procedures was also studied by polarization test and open circuit potential (OCP) measurements. The surface free energy and work of adhesion were obtained on the cleaned samples using contact angle measuring device. Pull-off test was conducted to evaluate the adhesion strength of the epoxy coating on the aluminium substrates. Results revealed that the surface cleaning of aluminium alloy by alkaline etching followed by acid cleaning method was the most efficient procedure for removing the oxide layer from the surface of aluminium compared to other cleaning procedures. The surface roughness, surface free energy, electrochemical activity and adhesion strength of the epoxy coating to the aluminium surface were significantly increased using this surface cleaning procedure.     

The influence of ultra-fine glass fibers on the mechanical and anticorrosion properties of epoxy coatings

The effects of different contents of ultra-fine glass fiber on mechanical and anticorrosion properties of epoxy coatings have been investigated. The FTIR and SEM have been used to analyze the surface nature and microstructure of the coatings. Electrochemical impedance spectroscopy (EIS) and a salt spray test have also been used to characterize the contents of ultra-fine glass fibers on the impedance of the coatings. When 10%, 20%, 30% of ultra-fine glass fibers are added to the coatings, their hardness and adhesion increases by 67%, 67%, 200% and 21.6%, 39%, 40%, respectively, compared with the properties of the pure coating. But the anticorrosion properties of the coatings containing high ultra-fine glass fiber content decreased with respect to the pure coatings properties.     

A study of the corrosion resistance of a waterborne acrylic coating modified with nano-sized titanium dioxide

The incorporation of nano-sized inorganic pigment particles into organic coatings may offer the potential for improving many of their properties, including corrosion resistance, at relatively low loadings. In the present research, titanium dioxide with a crystallite size of 5-10 nm was added to a waterborne organic primer formulation at loadings from 0.1 to 5% (w/w) and applied to hot-dip galvanized steel (HDG) panels. The corrosion resistance of the modified coatings was measured by neutral salt spray corrosion testing and electrochemical impedance spectroscopy (EIS), with an unpigmented film tested for comparison. 3% (w/w) TiO₂ appeared to produce an optimum improvement in the corrosion resistance.