Fault-tolerant hybrid epoxy-silane coating for corrosion protection of magnesium alloy AZ31

In this work, a hybrid epoxy-silane coating was developed for corrosion protection of magnesium alloy AZ31. The average thickness of the film produced by dip-coating procedure was 14 μm. The adhesion strength of the epoxy-silane coating to the Mg substrate was evaluated by pull-off tests and was found to be higher than 16 MPa both in dry and wet conditions. The hybrid epoxy-silane coating showed high corrosion resistance both when intact and when punched through by a needle. The low frequency impedance of intact coating was higher than 1 GΩ cm² after one month of immersion in 3.5% NaCl solution. Both, artificially induced defects and corrosion sites that appeared on the metal surface did not propagate. Their passivation behavior, that we call fault-tolerance, was observed by EIS, SVET-SIET and SEM-EDS. It was ascribed to the good adhesion, high coating integrity and corrosion inhibiting effect provided by diethylenetriamine used as epoxy hardener.     

The combined role of inhibitive pigment and organo-modified silica particles on powder coatings: Mechanical and electrochemical investigation

The addition of organo-modified silica particles (OSP) to powder coating containing zinc molybdenum phosphate pigment (ZMP) has been investigated. The OSP were directly incorporated at different concentrations (1, 2.5, 3.5 and 4.5 wt%) in a polyester powder monolayer coating with 10 wt% ZMP. The adhesion and anticorrosion properties were evaluated by means of adhesion studies and different electrochemical techniques: electrochemical impedance spectroscopy (EIS) and an accelerated cyclic electrochemical technique (ACET). The optimum quantity of OSP that gave maximum anticorrosive performance of the coating system was determined (1.0%), however two opposite effects of addition on the coatings led to a reduction of anticorrosive properties for higher contents. Finally, the electrochemical behaviour of the optimized combined system was compared to the effects achieved when both additives were separately added to powder coatings by means of an additional electrochemical test.     

Evaluation of corrosion resistance of polypyrrole/functionalized multi-walled carbon nanotubes composite coatings on 60Cu–40Zn brass alloy

Polypyrrole/multi-walled carbon nanotubes (PPy/MWCNT) and its carboxylic functionalized (PPy/MWCNT-COO⁻) composite films were successfully electropolymerized by cyclic voltammetry as protective coating against corrosion on 60Cu–40Zn brass alloy surface. It yielded to strongly adherent and smooth nanocomposite films. Kinetics of the corrosion protection was investigated in 3.5 wt% NaCl solutions by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The results showed that the presence of MWCNT in PPy coat considerably reduces the corrosion rate of 60Cu–40Zn brass alloy. The enhanced inhibition is most likely due to interaction between MWCNT and PPy. This in turn, improves the alloy passivation improvement and alters the permselectivity of the coating from anionic selectivity to the cationic selectivity. Moreover, PPy/MWCNT-COO⁻ functionalized nanocomposite provided higher corrosion resistance coating than PPy/MWCNT alone.     

Effect of silane modified nano ZnO on UV degradation of polyurethane coatings

Nanosized ZnO modified by 2-aminoethyl-3-aminopropyltrimethoxysilane (APS) was prepared using the precipitation method. Modified nano ZnO by silane (ZnO-APS) was characterized by XRD, SEM, TEM and UV–vis measurements. The degradation of the polyurethane coating, the polyurethane coatings containing 0.1 wt% nano ZnO and the polyurethane coatings containing nano ZnO-APS at two concentrations (0.1 and 0.5 wt%) during QUV test was evaluated by gloss measurement and electrochemical impedance spectroscopy. The coating surface after QUV test was observed with SEM. The results show that nano ZnO-APS has spherical structure with particle size around 10–15 nm. Nano ZnO improved the UV resistance of the PU coating and surface treatment by APS enhanced the effect of nano ZnO. The presence of nano ZnO-APS at 0.1 wt% concentration significantly improved the UV resistance of polyurethane coating.     

Effect of mixture ratio on water uptake and corrosion performance of silicone-epoxy hybrid coatings coated 2024 Al-alloy

A silicone-epoxy hybrid coating cured with amino silane was developed to provide corrosion protection on 2024 Al-alloy using air spraying. Water uptake characteristics of the silicone-epoxy hybrid coatings were investigated by electrochemical impedance spectroscopy in a 5 wt% NaCl solution. The effect of mixture ratio of silicone-epoxy and amino-silane on the water uptake (solubility, diffusion coefficient and permeation) was studied by using a single frequency (10 kHz) capacitance method. The glass transition temperature (T_g) was also investigated through differential scanning calorimeter (DSC) before and after immersion in the NaCl solution. Consequently, the excess of silicone-epoxy resin or amino silane improved the solubility of water in the coatings. A low water permeation coefficient was obtained with the mixing ratio 8/2 of silicone-epoxy and amino-silane, in which the T_g value was found to be larger than other three mixing ratios before immersion. After immersion for 750 h, the impedance modulus of EFA 2 coating (mixing ratio 8/2) in the low frequency was still close to 10⁸ Ω cm² that accounts for the good protective performance.     

Protective properties of cataphoretic epoxy coating on aluminium alloy AA6060 modified with electrodeposited Ce-based coatings: Effect of post-treatment

Ce-based conversion coatings (CeCCs) are a promising alternative to toxic chromate coatings on the metal substrates. In this work the CeCCs were electrodeposited on aluminium alloy AA6060 from aqueous solution of Ce(NO₃)₃ at different potentials (−0.95 V, −1.2 V and −1.4 V). Effect of deposition potential and post-treatment in the phosphate solution on morphology and protective properties of CeCCs with top cataphoretic epoxy coating was studied. To assess the differences between the protective systems, originating from the different CeCCs pre-treatments, electrochemical impedance spectroscopy (EIS), polarization measurements, AFM and SEM/EDS analysis were used. The EIS study was undertaken to follow the evolution of corrosion behaviour of epoxy coating/CeCCs protective systems over prolonged time of exposure to the chloride environment (3 wt.% NaCl). Results suggest significantly improved corrosion stability of epoxy coating on AA6060 with as-deposited CeCCs sub-layers with respect to the same epoxy coatings with phosphate post-treated CeCCs. The far best protective properties, i.e., the greatest value of pore resistance and the lowest value of corrosion current density were provided by the epoxy coating/CeCC protective system with CeCC deposited at −1.2 V and without post-treatment.     

Thermal curing study of bisphenol A benzoxazine for barrier coating applications on 1050 aluminum alloy

Polybenzoxazine coatings were elaborated by dip coating of a solution, prepared from a commercial bisphenol A benzoxazine (BA-a), on a 1050 aluminum alloy. The monomer was dissolved in acetone and the influence of the different application parameters (withdrawal speed and viscosity of the solution) on the wet coating thickness was evaluated. A heat treatment was then performed on the coating to polymerize the benzoxazine monomer by a ring opening mechanism attested for by Fourier Transform Infrared spectroscopy (FT-IR) and followed by Differential Scanning Calorimetry (DSC). Dielectric Analysis (DEA) and Thermogravimetric Analysis (TGA) showed a particular behavior related to a partial decomposition taking place at 180 °C and associated with the creation of intermediary ionic and volatile species. Finally, the barrier protection was evaluated by Electrochemical Impedance Spectroscopy (EIS) for 30 days in sodium chloride solution (0.1 M). The results showed an improvement of the impedance modulus from 10⁴ Ω cm² for an uncoated aluminum to a value as high as 10⁹ Ω cm² with a 10-μm thick polybenzoxazine coating.     

Corrosion performance of waterborne epoxy coatings containing polyethylenimine treated mesoporous-TiO2 nanoparticles on mild steel

Waterborne epoxy coatings were modified by adding mesoporous-TiO₂ nanoparticles (meso-TiO₂). In order to achieve proper dispersion of meso-TiO₂ in the epoxy-based coating and make possible chemical interactions between meso-TiO₂ and polymeric coating, meso-TiO₂ was treated with polyethylenimine (PEI) of various molecular weights. Corrosion performance of mild steel coated specimens was investigated employing electrochemical impedance spectroscopy (EIS) and salt spray test. Coatings with meso-TiO₂/PEI (600 molecular weight) possessed the best corrosion performance among the coating specimens. The EIS results showed that the resistance value of coating with meso-TiO₂/PEI (600 molecular weight) was above 9.87 × 10⁷ Ω cm² which was higher than neat epoxy coating. Possible chemical interactions between polymeric matrix and treated nanoparticles caused high barrier properties and high degree of cross-linking.     

The protective properties of epoxy coating electrodeposited on Zn–Mn alloy substrate

The epoxy coating was cataphoretically deposited on steel and steel modified by electroplated Zn–Mn alloy of different chemical contents. The samples were immersed in 0.5 mol dm⁻³ NaCl solution for 60 days. The electrochemical impedance spectroscopy (EIS) analysis showed that the values of pore resistance for epoxy coating on steel and Zn–Mn alloy with 16 at.% Mn were two orders of magnitude higher, while the capacitance values were two orders of magnitude lower than those for the epoxy coating on Zn–Mn alloy substrates with 5 and 8 at.% Mn. It was assumed that the main reason for such a difference was metallic substrate dissolution during cataphoretic deposition, due to high pH (12.9). This assumption was supported by energy dispersive X-ray spectrometry (EDS) measurements showing that the amount of released Zn in epoxy coatings decreased as Mn percent in the Zn–Mn alloys increased. In addition, Zn–Mn alloy coatings on steel, as well as bare steel, were immersed in 0.1 mol dm⁻³ NaOH solution, pH 12.9, simulating conditions during cataphoretic deposition, and polarization resistance measurement in this solution indicated that Mn inclusions in Zn–Mn alloy substrate prevent Zn dissolution in alkaline medium.     

Assessment of a one-step intelligent self-healing vanadia protective coatings for magnesium alloys in corrosive media

Eco-friendly vanadia based chemical conversion coating was applied for improving the corrosion resistance of a newly developed magnesium AZ31 HP-O alloy. The effect of vanadia solution concentrations (10, 30 and 50 g/l) and pH on the corrosion protection performance of magnesium substrate were investigated. EIS and cyclic voltammetry techniques were used to evaluate the electrochemical behavior in 3.5% NaCl. Results showed a marked increase in the localized corrosion resistance after applying vanadia surface treatment of 50 g/l as measured by EIS and polarization techniques. The optimum conditions to obtain protective coatings for AZ31 HP-O were determined. The surface morphology, composition and microstructure of conversion coatings were followed by AFM, SEM-EDS and macroscopic imaging techniques.     

Self healing coatings containing dual active agent loaded urea formaldehyde (UF) microcapsules

Urea formaldehyde (UF) microcapsules loaded with linseed oil and mercaptobenzothiazole (MBT) as core materials have been synthesized by in situ emulsion polymerization. The capsules were characterized by FTIR. Surface morphology of microcapsules was analyzed using scanning electron microscope. The thermal stability of the microcapsules is in the temperature range around 600 °C as confirmed by TG analysis. The open circuit potential measurements have shown that the coatings with microcapsules maintain the potential in the noble range (≈−0.390 V vs. SCE) while the coating without microcapsules exhibit potentials in the active range. EIS studies at the artificial defect area have shown that the coating containing microcapsules is able to protect steel in neutral media since the impedance values remained at 10⁷ Ω cm² even after 15 days exposure where as the coatings without microcapsules have lost their protection ability. The self healing ability of the coating containing microcapsules was studied by SVET.     

Effects of Zr content on microstructure and corrosion resistance of Ti–30Nb–Zr casting alloys for biomedical applications

Titanium alloys show attractive properties for biomedical applications where the most important factors are biocompatibility, corrosion resistance, low modulus of elasticity, very good strength-to-weight ratio, reasonable formability and osseointegration. The aim of this study was to investigate the effects of Zr content (7.5 and 15 wt%) on microstructure and corrosion resistance of Ti–30Nb–Zr as-cast alloy samples. The corrosion tests were carried out in a 0.9% NaCl (0.15 mol L^?1) solution at 25 °C and neutral pH range. A horizontal centrifuged solidification set-up was used to obtain as-cast samples with a fine dendritic arrangement. In order to evaluate the corrosion behavior, electrochemical impedance spectroscopy (EIS), polarization curves and an equivalent circuit analysis were used. It was found that the as-cast microstructure tends to have the dendrite arm spacings reduced with increasing Zr content. The results have shown that the addition of Zr to a Ti–30Nb alloy has been efficient to stabilize the beta phase. The resulting impedance parameters and passive current densities have shown that the Ti–30Nb alloys sample without Zr addition presents a higher surface reactivity which can induce a better osseointegration than those containing Zr.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics,adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

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.     

The effect of TiO2 as a pigment in a polyurethane/polysiloxane hybrid coating/aluminum interface based on damage evolution

The effect of titanium oxide as an additive on the performance of a polyurethane/polysiloxane hybrid coating was characterized by an electrochemical approach. The performance evolution was quantified by exposing the hybrid coating on an aluminum substrate to NaCl solution at pH 5 over time. Real-time measurements were performed to quantify and correlate the mechanisms that occur at the coating/substrate interface. Electrochemical impedance spectroscopy (EIS) quantified the hybrid coating/substrate interface performance over the course of the 263 days of exposure, and electrically passive elements described and characterized the degradation/performance stages upon exposure to the acidic NaCl solution.The addition of TiO₂ produced hydrophobicity functionality, and TiO₂ acted as a physical barrier layer that influenced the initial damage stage. Different exposure times were associated the different stages of damage evolution for the hybrid coating and coating/substrate interfaces. Electrochemical testing with high-resolution techniques such as AFM (atomic force microscope) and IFM (infinite focus microscope) characterized the coating surface and the interface performance and resolved the surface and defect formation observed with different levels of TiO₂ content. Of the systems tested, the system with 10 wt% TiO₂ provided the best corrosion inhibition.     

The effect of mixture of mercaptobenzimidazole and zinc phosphate on the corrosion protection of epoxy/polyamide coating

The protective performance of solvent-borne epoxy/polyamide coatings formulated with zinc phosphate anticorrosion pigment was improved through the addition of 2-mercaptobenzimidazole as an organic corrosion inhibitor. In addition to determining the optimum percentage of mercaptobenzimidazole, the electrochemical impedance spectroscopy data could show the influence of inhibitor concentration on the epoxy behavior within 35 days of immersion in 3.5 wt% NaCl solution. The improved corrosion protection and adhesion strength in the presence of the pigment and inhibitor were connected to the deposition of a protective layer at the coating/substrate interface which might limit active zones for electrochemical reactions. The precipitation was confirmed using electrochemical impedance spectroscopy, polarization curves and SEM/EDX surface analysis.     

Synthesis and characterization of CuO-hybrid silica nanocomposite coatings on SS 304

Pure and CuO-dispersed hybrid silica nanocomposite coatings were generated using sols synthesized from acid catalyzed hydrolysis and condensation of n-propyl trimethoxysilane and tetraethoxysilane in combination with copper nitrate. Coatings were initially deposited on soda lime glass substrates by dip coating followed by heat treatment at 150, 250 and 350 °C for 2 h in air and characterized. Coatings were subsequently deposited by dip coating on stainless steel 304 substrates. An optimized heat treatment temperature of 250 °C was chosen based on the contact angles of coatings on soda lime glass substrates and results of thermogravimetric/differential thermal analysis on the dried gels obtained from the sol synthesized from the combination of n-propyl trimethoxysilane and tetraethoxysilane. Gels heat-treated at 250 °C were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy for crystallinity. Characterization of the coatings was carried out with respect to thickness, water contact angle and adhesion. Corrosion testing of coatings on SS 304 was studied by potentiodynamic polarization measurements and electrochemical impedance spectroscopy after 1 h and 24 h exposure to 3.5% NaCl. The corrosion resistances of CuO-dispersed hybrid silica coatings after 1 h and 24 h exposure to 3.5% NaCl solution were higher than that of pure hybrid silica coatings, both of which had thicknesses ranging from 140 nm–200 nm.     

Effect of carbon nano-tubes on the corrosion resistance of alkyd coating immersed in sodium chloride solution

Effect of carbon nano-tubes (CNTs) on corrosion protection of carbon steel coated by alkyd resin and tested after immersion in 3.5% NaCl solution for different periods was evaluated by electrochemical impedance spectroscopy (EIS) measurements and scanning electron microscopy (SEM) investigations. Changes in the impedance characteristics of the systems were found to be greatly affected by the percentage of CNTs. Degradation of alkyd resin film without CNTs was observed after 72 h. On other hand no blisters, pin-holes and delamination were observed for alkyd resin containing 0.5% CNTs. It was found that CNTs improved the corrosion resistance and the adhesion strength of alkyd resin.     

Fabrication of corrosion-resistant Al2O3–CeO2 composite coating on AA7075 via plasma electrolytic oxidation coupled with electrophoretic deposition

Al₂O₃–CeO₂ composite coating was fabricated on AA7075 by combining plasma electrolytic oxidation (PEO) with electrophoretic deposition (EPD). CeO₂ nanoparticles are electrophoretically incorporated into the plasma electrolytic oxidized Al₂O₃ coatings by the synergetic effect of PEO and EPD processes. The passivation behavior of Al₂O₃ and inhibiting nature of CeO₂ have been studied by the electrochemical corrosion analysis in 3.5 wt% NaCl solution and salt spray corrosion test (SSCT) in 5 wt% NaCl as per ASTM standards. The results showed that the Al₂O₃–CeO₂ composite coating via PEO coupled EPD significantly improved the corrosion resistance (~10³ times higher R_p) compared to the plasma electrolytic oxidized Al₂O₃ coating.     

In-situ AFM and EIS study of a solventborne alkyd coating with nanoclay for corrosion protection of carbon steel

A solventborne alkyd composite coating containing modified montmorillonite (MMT) nanoclay was made on carbon steel, and its corrosion protection was investigated by in-situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) measurements in 3 wt.% NaCl solution. X-ray diffraction (XRD) analysis indicated intercalation of the MMT sheets in the composite coating. Thermo-gravimetric analysis (TGA) demonstrated improved thermal stability of the composite coating due to the modified nanoclay. Scanning electron microscopy (SEM) and AFM examination revealed dispersion and also some aggregation of the nanoclay particles in the coating. In-situ AFM images show a stable coating surface at nano-scale during relative long time exposure in the NaCl solution, indicating an enhanced stability of the composite coating. The EIS results confirmed that the composite coating provides an enhanced barrier type corrosion protection for carbon steel in the corrosive solution, which could be attributed to the intercalated lamellar MMT sheets in the coating that block the defects and decrease the transport of water and corrosive species.