Smart self-healing anti-corrosion vanadia coating for magnesium alloys

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 (neutral pH 7 and pH 9) on the corrosion protection performance of a magnesium substrate were investigated. EIS and linear polarization techniques were used to evaluate the electrochemical behavior in 3.5% NaCl. The results showed a marked increase in the localized corrosion resistance after applying vanadia surface treatment of 50 g/l due to self-healing effect. The optimum conditions to obtain protective coatings for AZ31 HP-O with a self-healing ability were determined. Changes in surface morphology, composition and microstructure of the conversion coatings were followed by SEM-EDS and macroscopic imaging techniques.     

Preparation,characterization and properties of Cr-incorporated DLC films on magnesium alloy

Cr-incorporated diamond-like carbon (Cr-DLC) films were deposited on AZ31 magnesium alloy as protective coatings by a hybrid beams deposition system, which consists of a DC magnetron sputtering of Cr target (99.99%) and a linear ion source (LIS) supplied with CH₄ precursor gas. The Cr concentration (from 2.34 to 31.5 at.%) in the films was controlled by varying the flow ratio of Ar/CH₄. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the microstructure and composition of Cr-DLC films systematically. An electrochemical system and a ball-on-disk tribotester were applied to test the corrosion and tribological properties of the film on the AZ31 substrate, respectively. At low Cr doping (2.34 at.%), the film mainly exhibited the feature of amorphous carbon, while at high doping (31.5 at.%), chromium carbide crystalline phase occurred in the amorphous carbon matrix of the film. In this study, all the prepared Cr-DLC films showed higher adhesion to AZ31 than the DLC film. Especially for the film with low Cr doping (2.34 at.%), it owned the lowest internal stress and the highest adhesion to substrate among all the films. Furthermore, this film could also improve the wear resistance of magnesium alloy effectively. But, none of the films could improve the corrosion resistance of the magnesium alloy in 3.5 wt.% NaCl solution due to the existence of through-thickness defects in the films.     

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.     

Corrosion behavior study of AZ91 magnesium alloy coated with methyltriethoxysilane doped with cerium ions

The present work aims to investigate the corrosion behavior of AZ91 magnesium alloy treated with a 4% (v/v) methyltriethoxysilane (MTES) alcohol solution, with and without an alkaline pretreatment. The corrosion resistance was assessed by electrochemical impedance spectroscopy (EIS) and current densities were monitored by potentiodynamic polarization curves during immersion in a 0.1 M Na₂SO₄ solution. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to perform a surface analysis. The electrochemical results showed an improvement of anticorrosion properties of AZ91. Furthermore, alkaline pretreatment enhances adhesion between silane film and substrate surface. This can be attributed to a surface enrichment in hydroxyl groups after the alkaline step, which increases formation of Si–O–Mg covalent bonds. The addition of Ce(NO₃)₃ to the MTES bath was evaluated, and it was found that the electrochemical response depends on the cerium ions concentration used. It was shown that the addition of 6.0 × 10⁻⁵ M of Ce(NO₃)₃ to a MTES bath improves corrosion resistance. Higher concentration of cerium ions lead to destabilizing the siloxane network, decreasing the efficiency of the silane coatings.     

Effect of various additives on morphology and corrosion behavior of ceramic coatings developed on AZ31 magnesium alloy by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) coatings were developed on AZ31 magnesium alloy using alkaline silicate with KOH as a base electrolyte system, and with the addition of sodium aluminate, sodium tetra borate, potassium titanium fluoride, tri sodium ortho phosphate and urea as additives. The phase composition and surface morphology of these multi-phase coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results showed that the anions namely, SiO₃^2?, AlO₂^?, B₄O₇^2?, F^? and PO₄^3?, effectively participated in the coating formation influencing its chemical composition and surface morphology and thereby corrosion resistance. The mechanism of corrosion process of each coating was explained in detail with the help of Electrochemical Impedance Spectroscopy (EIS) analysis and equivalent circuit modeling. It was observed that the sample treated by PEO in the electrolyte solution containing sodium tetra borate as an additive showed higher corrosion resistance which could be attributed to its morphological characteristics.     

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.     

Investigation of the corrosion protection behavior of natural montmorillonite clay/bitumen nanocomposite coatings

The influence of clay particles on the corrosion properties of bituminous coating was studied. Different percentages of natural montmorillonite clay (Cloisite Na⁺) were added to emulsified bitumen in water to make 2 wt.%, 3 wt.% and 4 wt.% of clay/bitumen nanocomposite coatings. The coatings were applied on steel 37. Optical microscopy and transmission electron microscopy (TEM) were employed to study the structure of nanocomposite. To investigate the anti-corrosion properties of the coated panels, electrochemical impedance spectroscopy (EIS) was used. The findings indicated that the addition of clay nanolayers improved corrosion resistance of the coatings. Moreover, increasing clay loading up to 4 wt.%, increased the corrosion resistance.     

Corrosion inhibition by poly(N-ethylaniline) coatings of mild steel in aqueous acidic solutions

Poly(N-ethylaniline) (PNEA) coatings on mild steel have been electrodeposited from 0.1 to 0.5 M aqueous oxalic acid solutions containing 0.1 M N-ethylaniline (NEA) using potentiodynamic synthesis technique. The effect of oxalic acid concentration on the corrosion behavior of PNEA coated mild steel surfaces were investigated by DC polarization and electrochemical impedance spectroscopy (EIS) techniques in 0.1 M HCl and 0.05 M H₂SO₄ solutions. Corrosion test results showed that corrosion resistance of PNEA coatings decreases with increasing concentrations of oxalic acid in polymerization solution. Decreasing acidity of the polymerization solution causes more effective protection against corrosion in aqueous acidic corrosive medium.     

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 synthesis of poly-2-aminothiazole on mild steel and its corrosion inhibition performance

2-Aminothiazole (AT) was polymerized by electrochemical technique on a mild steel (MS) electrode from 0.01 M monomer containing 0.3 M ammonium oxalate solution. Cyclic voltammetry was used for the synthesis. Poly-2-aminothiazole (pAT) film with a light-brownish color was obtained on the MS surface. The effectiveness of polymer film in preventing corrosion of MS was tested in 0.5 M HCl solution. For corrosion tests, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques were utilized. The results obtained indicated that, the polymer film adherent to the steel surface. The polymer film gives a good corrosion protection against the attack of corrosive environment.     

Structure and surface properties of Al2O3–TiO2 ceramic coated AZ31 magnesium alloy

Magnesium and its alloys are the engineering materials which have the potential ability to be able to used widely particularly in the automotive, aerospace and in the biomedical sectors, especially thanks to their features such as lightness, specific strength that they have and biocompatibility. However, due to their poor wear resistance and corrosion resistance, the areas of usage are being restricted. This situation prevents Mg alloys to be used without any surface protection despite their good mechanical properties such as high strength/weight ratio. In this study, plasma spraying method is used to improve the poor corrosion resistance of AZ31 Mg alloy. Al₂O₃–13 wt% TiO₂ (AT13) and Al₂O₃–40 wt% TiO₂ (AT40) composite ceramic coatings were coated successfully on the surfaces of AZ31 Mg samples. The wear properties of the AT13 and AT40 coated samples were investigated for tribological applications. Surface morphology and microstructure of the duplex treated samples were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microhardness value of the uncoated AZ31 Mg alloy is 40±3 HV_0.1, while the microhardness values of the AT40 and AT13 coatings are enhanced to 800±39 HV_0.1 and 1500±35 HV_0.1, respectively.     

Thermomechanical and corrosion inhibition properties of graphene/epoxy ester–siloxane–urea hybrid polymer nanocomposites

The effect of graphene on the corrosion inhibition properties of a hybrid epoxy–ester–siloxane–urea polymer was investigated. The weight fraction of graphene was varied from 1 to 2 wt%. Direct current polarization (DCP) and electrochemical impedance spectroscopic (EIS) techniques were used to measure the polarization and coating resistance of the coated aluminum alloy substrate. The grapheme/hybrid polymer composite coatings showed much higher corrosion inhibition property when compared to the neat hybrid polymer coating. An increase in glass transition temperature and rubbery region modulus was also observed for composites containing 1–2 wt.% of graphene. A direct correlation between the rubbery plateau modulus of free standing composite thin films and corrosion resistance of the composite coatings was made, indicating that the corrosion protection mechanism is due to restriction of the polymer chain motion by graphene which causes a decrease in coating permeability.     

The influence of pulse plating parameters on microstructure and properties of Ni-W-Si3N4 nanocomposite coatings

Nanosized silicon nitride (Si₃N₄) particles reinforced Nickel-tungsten composite coatings were deposited on the surface of C45 steel sheet by pulse electrodeposition. The effect of duty cycle, frequency, current pattern and presence of Si₃N₄ nanoparticles on microstructure, phases and corrosion resistance and mechanical properties of the coatings were investigated. The Si₃N₄ phase was incorporated into Ni-W alloy matrix uniformly and the inclusion content of in the coating was analyzed by energy dispersive x-ray spectrometer (EDS). The structure, microhardness and surface roughness of the coatings was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers micro-indenter and atomic force microscopy (AFM). The corrosion protection of steel by the coatings was evaluated by weight loss and electrochemical impedance spectroscopy (EIS). Corrosion rates of the coatings were determined using the Tafel polarization test. The results indicated that the duty cycle of 60%, pulse frequency of 1000 Hz, average current density of 5 A/dm⁻², and Si₃N₄ nanoparticles concentration of 30 g/L were the optimal plating conditions. The amount of Si₃N₄ particles incorporated into the coating that were produced under the optimum plating conditions was 2.1 wt%, and the microhardness was 1031 Hv as well as the crystallite size of this coating was 27 nm.     

Corrosion inhibition of magnesium using biocompatible Alkyd coatings incorporated by mesoporous silica nanocontainers

In the present study, mesoporous silica nanocontainer powders with and without inhibitor (fluoride) were dispersed in the Alkyd coatings to improve corrosion resistance of Mg metal. Then, the corrosion behavior of these coatings was studied in comparison with Mg in 2 g dm⁻³ NaCl solution. Electrochemical tests showed that these coatings could protect the surface from chloride attack. Moreover, release of fluoride from mesoporous silica nanocontainers leads to the formation of MgF₂ as an inhibitive compound at the interface.     

Thermal shielding performances of nano-structured intumescent coatings containing organo-modified layered double hydroxides

Cone calorimetry tests performed at 50 kW/m² heat flux have been exploited for assessing the fire resistant properties of nano-structured intumescent coatings containing modified layered double hydroxides (hydrotalcites, LDHs) and deposited on steel plates. The effects of different types of modified hydrotalcites (i.e. magnesium–aluminum lactate hydrotalcite, magnesium–aluminum gluconate hydrotalcite, magnesium–aluminum hydrotalcite modified with a fatty acid, magnesium–aluminum hydrotalcite modified with rosin) on the thermal shielding performances of the intumescent coatings and their intumescent degree have been thoroughly discussed and compared with the pristine unfilled counterparts.More specifically, the coatings containing organo-modified LDHs showed better thermal shielding performances with respect to the reference intumescent coating; on the contrary, the use of unmodified hydrotalcite in the intumescent formulations was found detrimental. The thermal shielding performances of the coatings filled with modified LDHs were found to be strictly related to the intumescent degree developed during the cone calorimetry tests. In addition, it was possible to compare the thermal shielding performances of the nanofilled coatings by evaluating the temperatures achieved after 2000 s exposure to the 50 kW/m² heat flux of the cone: the thermal shielding performance sequence was LDH-GL > LDH-RS > LDH-LA > LDH-FA > LDH).Finally, the intumescent degree of the modified coatings was found to decrease with increasing the hydrotalcite content, hence lowering their thermal shielding performances.     

Chemical synthesis of TiO2 nanoparticles and their inclusion in Ni–P electroless coatings

The work addresses the preparation of Ni3P3TiO₂ nanocomposite coatings on mild steel substrate by the electroless technique. Nanosized TiO₂ particles were first synthesized by the precipitation method and then were codeposited (4 g/l) into the Ni3P matrix using alkaline hypophosphite reduced EL bath. The surface morphology, particle size, elemental composition and phase analysis of as-synthesized TiO₂ nanoparticles and the coatings were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD). Coatings with 20 µm thickness were heat treated at 400 °C for 1 h in argon atmosphere. The morphology, microhardness, wear resistance and friction coefficient characteristics (ball on disc) of electroless Ni3P3TiO₂ nanocomposite coatings were determined and compared with Ni3P coatings. The results show that as-synthesized TiO₂ nanoparticles are spherical in shape with a size of about12 nm. After heat treatment, the microhardness and wear resistance of the coatings are improved significantly. Superior microhardness and wear resistance are observed for Ni3P3TiO₂ nanocomposite coatings over Ni3P coatings.     

Pulsed electrodeposition of carbon nanotubes-hydroxyapatite nanocomposites for carbon/carbon composites

Carbon nanotubes-hydroxyapatite (CNTs-HA) composite coatings, which behaved like single composites, were synthesized by a combined method composed of electrophoretic deposition and pulsed electrodeposition. The phase compositions and the microstructure of the composite coatings were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectrometry (FTIR). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies showed that the CNTs-HA composite coatings protected the bare carbon/carbon composites from corrosion in simulated body fluid (SBF) solution. The adhesion strength of CNTs-HA composite coating prepared by the combined method is 14.57±1.06 MPa achieved at the CNTs EPD time of 10 min. Compared to the other CNTs-HA composite coatings with different content of CNTs, the CNT-HA composite coating with the electrophoretic deposition of 10 min showed the best corrosion resistance. The morphology of CNTs-HA composite coatings immersed in SBF solution rendered the formation of HA crystallites. In addition, in vitro cellular responses to the CNTs-HA composite coatings were assessed to investigate the proliferation and morphology of mouse cells 3T3 cell line.     

Graphene oxide/hydroxyapatite composite coatings fabricated by electrophoretic nanotechnology for biological applications

Graphene oxide (GO) was firstly employed as nanoscale reinforcement fillers in hydroxyapatite (HA) coatings by a cathodic electrophoretic deposition process, and GO/HA coatings were fabricated on pure Ti substrate. The transmission electron microscopy observation and particle size analysis of the suspensions indicated that HA nanoparticles were uniformly decorated on GO sheets, forming a large GO/HA particle group. The addition of GO into HA coatings could reduce the surface cracks and increase the coating adhesion strength from 1.55 ± 0.39 MPa (pure HA) to 2.75 ± 0.38 MPa (2 wt.% GO/HA) and 3.3 ± 0.25 MPa (5 wt.% GO/HA), respectively. Potentiodynamic polarization and electrochemical impedance spectroscopy studies indicated that the GO/HA composite coatings exhibited higher corrosion resistance in comparison with pure HA coatings in simulated body fluid. In addition, superior (around 95% cell viability for 2 wt.% GO/HA) or comparable (80–90% cell viability for 5 wt.% GO/HA) in vitro biocompatibility were observed in comparison with HA coated and uncoated Ti substrate.     

Corrosion behaviour of carbon materials exposed to molten lithium chloride–potassium chloride salt

The corrosion behaviour of four carbon materials namely low density graphite, high density graphite, glassy carbon and pyrolytic graphite were investigated in molten LiCl–KCl electrolyte medium at 600 °C for 2000 h under high pure argon atmosphere. Structural and microstructural changes in the carbon materials after exposure to molten chloride salt were investigated from the weight change and using scanning electron microscopy, atomic force microscopy, X-ray diffraction and laser Raman spectroscopic techniques. Microstructural analysis of the samples revealed the poor corrosion resistance of high density and low density graphite and severe attack was observed at several places on the surface. On the other hand, glassy carbon and pyrolytic graphite were relatively inert, while pyrolytic graphite showed the best corrosion resistance to molten salt attack. In the order of increasing corrosion resistance to molten salt, the carbon materials were found to follow the sequence: low density graphite < high density graphite < glassy carbon < pyrolytic graphite.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.