Enhancement of the Ti-6Al-4V alloy corrosion resistance by applying CrN/CrAlN multilayer coating via Arc-PVD method

In this study, the Ti-6Al-4V substrate was coated by CrN-CrN/TiN-TiN and CrN/CrAlN multilayer coatings using the cathodic arc physical vapor deposition (Arc-PVD) method. The results of potentiodynamic polarization (PDP) have shown the lowest and highest corrosion current density belong to the double-layer (0.16 µA/Cm²) and TiN (0.51 µA/Cm²) samples, indicating the higher corrosion resistance of the double-layer coating. The field emission electron microscope (FESEM), X-ray diffraction pattern (XRD), open circuit potential (OCP), PDP, and electrochemical impedance spectroscopy (EIS) analysis were employed in order to characterize the coatings and evaluate their corrosion behavior. Finally, applying the double-layer coating resulted in the significant improvement of the protective behavior of the Ti-6Al-4V alloy, as compared to the sample coated with TiN in corrosive environments.     

Effect of electroless nickel interlayer on the electrochemical behavior of single layer CrN, TiN, TiAlN coatings and nanolayered TiAlN/CrN multilayer coatings prepared by reactive dc magnetron sputtering

The electrochemical behavior of single layer TiN, CrN, TiAlN and multilayer TiAlN/CrN coatings, deposited on steel substrates using a multi-target reactive direct current (dc) magnetron sputtering process, was studied in 3.5% NaCl solution. The total thickness of the coatings was about 1.5 μm. About 0.5 μm thick chromium interlayer was used to improve adhesion of the coatings. With an aim to improve the corrosion resistance, an additional interlayer of approximately 5 μm thick electroless nickel (EN) was deposited on the substrate. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to study the corrosion behavior of the coatings. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the corroded samples. The potentiodynamic polarization tests showed lower corrosion current density and higher polarization resistance (R_p) for the coatings with EN interlayer. For example, the corrosion current density of TiN coated steel was decreased by a factor of 10 by incorporating 5 μm thick EN interlayer. Similarly, multilayer coatings of TiAlN/CrN with EN interlayer showed about 30 times improved corrosion resistance as compared to the multilayers without EN interlayer. The porosity values were calculated from the potentiodynamic polarization data. The Nyquist and the Bode plots obtained from the EIS data were fitted by appropriate equivalent circuits. The pore resistance (R_pore), the charge transfer resistance (R_ct), the coating capacitance (Q_coat) and the double layer capacitance (Q_dl) of the coatings were obtained from the equivalent circuit. Multilayer coatings showed higher R_pore and R_ct values as compared to the single layer coatings. Similarly, the Q_coat and Q_dl values decreased from uncoated substrate to the multilayer coatings, indicating a decrease in the defect density by the addition of EN interlayer. These studies were confirmed by examining the corroded samples under scanning electron microscopy.     

Modification of Cr/CrN composite structure by Fe addition and its effect on decorative performance and corrosion resistance

A CrN ceramic coating is a promising substitution for electroplated Cr (Ⅵ) hard coatings; however, it has not yet replaced a decorative Cr (Ⅵ) layer owing to its low reflectivity of visible light and relatively poor corrosion resistance. A Cr/CrN composite structure can address these shortcomings. Our recent work demonstrated that an addition of Fe can facilitate the densification of hot-pressed Cr sputtering targets and enhance the mechanical properties of Cr/CrN composite coatings. In this work, we focus on reporting optical properties and corrosion resistance of the Fe-modified Cr/CrN layers for decorative applications, and clarifying the effect of Fe on these performances. The results show that Fe can increase the amount of metallic phase in the Cr/CrN composite structure and helps enhance its reflectance in the visible region. The composite coating deposited by Cr₉₀Fe₁₀ (at.%) target/alloy exhibits a greater reflectivity of visible light and a better corrosion resistance than those of a single-phase CrN coating. This is attributed to its fine microstructure, which is beneficial for a dense and smooth surface. The color of the coating can be controlled by both the partial pressure ratio of N₂ during deposition and the Fe content in the Cr-Fe sputtering target. The present results show that the addition of Fe can lead to the reduction in the processing cost of Cr targets, enhance the mechanical properties of the composite coatings, and broaden the deposition window. Such cost-effective Fe-modified Cr/CrN composite coatings are expected to be used in various decorative applications.     

Study on corrosion electrochemical behavior of several different coating systems by EIS

Corrosion electrochemical behavior of chlorinated rubber top coating (single-layer), inorganic zinc-rich primer/chlorinated rubber top coating (double-layers) and inorganic zinc-rich primer/epoxy middle paste/chlorinated rubber top coating (tri-layers) in 3.5 wt% NaCl solution was studied by electrochemical impedance spectroscopy (EIS). A series of impedance spectra of the three coating systems during immersion were measured; and their protective properties were compared according to the spectra. The experimental results showed that, the protective properties of the double-layers coating system were even worse than that of the single-layer coating system; and the tri-layers coating system had the best protective properties in the three coating systems; epoxy middle paste had played a very important role for protective properties of the composite coating system.     

Improved electrochemical performance of trivalent-chrome coating on Al 6063 alloy via urea and thiourea addition

This project aims at improving the electrochemical performance of trivalent-chrome coating through urea and thiourea addition. The electrochemical behaviors of coatings formed with different concentrations of urea and thiourea were investigated in 3.5 wt.% NaCl solution at 25 °C, using potentiodynamic polarization curves and EIS. The corrosion resistance of coatings is improved greatly by adding a small amount of inhibitors, whereas the excessive addition deteriorates the corrosion resistance. Thiourea addition presents better effect than urea. To explain the EIS results of the coatings, a simple equivalent circuit was designed. The EIS parameters were obtained by fitting the EIS plots. The results of the polarization curves and EIS show that the inhibitor-containing coatings present better corrosion resistance than the coating without inhibitor. The morphology and composition and valence state of the conversion coatings were examined by SEM and EDS and XPS, respectively. The results indicated that the trivalent chromium coating was developed on Al 6063 alloy, urea and thiourea inhibitors were also deposited on the substrates, respectively. A noticeable chemical shift was also observed.     

Effect of sub-layer temperature during HFCVD process on morphology and corrosion behavior of tungsten carbide coating

WC coating was deposited on the polished and cleaned 316L stainless steel by Hot Filament Chemical Vapor Deposition (HFCVD) technique at 400°C and 500°C. Field Emission Gun Scanning Electron Microscope (FEG-SEM) was used to study the corrosion morphology of the WC coatings. Energy dispersive spectroscopy (EDS) was used to analyze the chemical composition of the coatings. Coating porosity was measured by immersion in water. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of the coating in the solution of 1 mol/L H₂SO₄. Results showed that the WC coatings have a honeycomb microstructure where its porosity was increased at higher temperature of the sub-layer. Also, the WC coating significantly increases the corrosion resistance of 316L stainless steel. And increasing the sub-layer temperature in the HFCVD method reduces the corrosion resistance of the WC coating. Corrosion morphology was indicative of pitting corrosion of the WC coating.     

On the use of CrN/Cr and CrN interlayers in hot filament chemical vapour deposition (HF-CVD) of diamond films onto WC-Co substrates

CrN/Cr-based films were deposited using PVD-arc technique onto Co-cemented tungsten carbide (WC-Co) substrates and, then, seeded with diamond powder suspension or mechanically treated by Fluidized Bed Peening (FBP) of brittle diamond powders. Multilayered coatings were obtained from the superimposition of 4 µm-thick diamond coatings, deposited on the PVD interlayer using hot filament chemical vapour deposition (HFCVD). The effectiveness of fluidized bed peened CrN/Cr interlayers on the adhesion enhancement of diamond on WC-Co substrates was studied and compared to diamond coated WC-Co substrates with unpeened CrN/Cr or CrN interlayers, or pre-treated with two-step chemical etching (Murakami's reagent and Caro's acid, MC-treatment).In particular, growth, morphology, wear endurance and adhesion of the CVD deposited diamond films onto peened CrN/Cr interlayer were looked into. Diamond coatings on peened CrN/Cr interlayers exhibited a rougher surface morphology than as-prepared CrN/Cr films as a result of the surface roughening of the ductile Cr layer produced by the repeated impacts on it of the diamond powders during FBP. FBP was found to be a necessary step in improving the scarce adhesion of CVD diamond onto CrN/Cr-interlayer.However, the use of FB peened CrN/Cr interlayer did not represent the best way to pre-treat WC-Co substrates, as the unpeened single-layer CrN, or the use of MC pretreatment, was found to ensure better adhesion and wear endurance.     

Studying the corrosion protection properties of an epoxy coating containing different mixtures of strontium aluminum polyphosphate (SAPP) and zinc aluminum phosphate (ZPA) pigments

Epoxy/polyamide coatings were loaded with different mixtures of strontium aluminum polyphosphate (SAPP) and zinc aluminum phosphate (ZPA) pigments. Moreover, a coating containing zinc phosphate (ZP) was prepared as a reference sample. The coatings were applied on St-37 steel substrates and then were exposed to 3.5 wt% NaCl solution up to 35 days. The corrosion inhibition properties of the pigments extracts were studied on bare steel samples by a potentiodynamic polarization technique after 24 h immersion. The morphological properties and corrosion resistance of the coatings were investigated by scanning electron microscope (SEM), optical microscope, electrochemical impedance spectroscopy (EIS) and salt spray tests.     

The post-annealing effect on tribological and corrosion behaviors of CrN/AlCrN multilayered coating applied by CAE-PVD

The present study investigated the wear and electrochemical behaviors of CrN/AlCrN multilayered coatings post-annealed at 300, 450, and 600°C temperatures. The cathodic arc evaporation technique has been utilized to deposit the coatings. Scanning electron microscope, field emission SEM, energy-dispersive X-ray, grazing incidence X-ray diffraction, and Rockwell-C indenter methods were used to characterize the coatings and to investigate the interdiffusion between the multilayered CrN/AlCrN and the H13 base metal. The results showed that the sharp interface of the CrN and AlCrN layers was blurred by the annealing process supporting the interdiffusion of the layers. The reciprocating wear test and the microhardness tester were used to evaluate the coatings’ mechanical behavior. The hardness and roughness of the coatings were increased by increasing the post-annealing temperature. The smallest wear rates were observed for the samples treated at 300 and 450°C, which were approximately 17 times and 12 times smaller than the wear rate of the sample annealed at 600°C. Electrochemical testing was used to study the corrosion behavior of the coatings. The results showed that by increasing annealing temperature, corrosion resistances of the coatings are improved. As a result, the corrosion current density of the 600°C annealed coating was approximately 434 times smaller than as-deposited coatings.     

SiAlON nanoparticles effect on the corrosion and chemical resistance of epoxy coating

Effect of incorporating SiAlON nanoparticles at different loading levels (0?C12?wt%) on chemical resistance of epoxy coating was investigated by immersion in basic (Na₂CO₃, pH?=?11) and salty (NaCl 3.5?wt%) (environments at 85?°C for 60?days. Epoxy resin chemical resistant coating grade based on bisphenol A was used with polyamine hardener as a curing agent. In these testes, surface morphology changes of the samples were studied and compared owing to initiation and propagation of cracks. Results indicate an enhancement in the epoxy nanocomposite chemical resistance due to the addition of small fraction of SiAlON nanoparticles. Samples containing 3 and 5?wt% of SiAlON nanopowders were considered as optimum samples compared to all the other samples, because they showed more resistances to initiation and propagation of cracks and lower permeability in chemical environment in comparison with neat resin and other samples. Also, epoxy coatings containing SiAlON nanoparticles were successfully coated on steel substrates and corrosion electrochemical behavior of these nanocomposite coatings were characterized by electrochemical impedance spectroscopy (EIS). The electrochemical monitoring of the coated steel over 35?days of immersion in 3.5?wt% NaCl solution at room temperature suggested the positive role of nanoparticles in improving the corrosion resistance of the coated steel.     

热浸镀锌层表面钛盐转化膜研究

利用钛盐成膜工艺在热镀锌层表面获得了色泽光亮、耐蚀性能优良的银白色转化膜层。采用扫描电镜、能谱仪、电化学极化和盐水浸泡方法研究了钛盐转化膜层的表面形貌、元素组成和耐蚀性能。分析了钛盐溶液成分及工艺参数对热镀锌层表面转化膜的耐蚀性能影响。确定的最佳工艺条件为:Ti(SO4)21g/L,H2O260mL/L,pH0.5～1.0,处理温度25～30°C,处理时间10min。热镀锌层经此工艺处理后,耐蚀性能明显提高。     

Investigation on corrosion and wear resistance of MgO-Al2O3 composite coating prepared by plasma electrolytic oxidation

Ceramic coatings were prepared on 6061 Al alloy in a mixed electrolyte with/without MgO powders at different treatment durations. The results of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) showed that MgO powder was incorporated into the coatings, and Mg species gradually aggregated into coating inside as prolonging the oxidation time. Scanning electron microscopy (SEM) showed that MgO additive had a certain effect on the microstructures and coating thickness. The corrosion behavior tests evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution suggested that at the same treatment time, the addition of MgO powders can improve the corrosion resistance of the coating, and the Mg-rich layer can affect the corrosion resistance of the coating. The tests of mechanical properties showed that the addition of MgO powders improved the stability and hardness of the coating.     

Sand erosion and crack propagation mechanism of Cr/CrN/Cr/CrAlN multilayer coating

Herein, we have deposited Cr/CrN/Cr/CrAlN multilayer coatings with various modulation ratios on TC11 alloy substrate using cathodic arc system. The influence of various modulation ratios on microstructure and Al₂O₃ sand erosion behavior of coatings is systematically studied. Results reveal that the coatings are about 200 nm per cycle and total thickness is 8 μm. Five groups of coatings exhibit high hardness (>3000 HV_0.025). The coating with modulation ratio of 12 adhesion can reaches 55 N. The residual stress increases with the decrease of the modulation ratio, but the increase is generally low (less than ?2 GPa). In addition, according to sand erosion test, it is found that sand erosion resistance of multilayer coating is significantly around 5 times higher than TC11 alloy matrix. The erosion morphology shows that a large number of irregular cracks and layered spalling appear on the surface of the coating, indicating that the cracks are constantly initiated under the continuous impact of the sand and gravel，and finally gather together and then spalling. Moreover, dynamic response and stress field of the coating under the impact of single sand (Al₂O₃) are studied by numerical simulations. It is determined that coating cracking is caused by high tensile stress under CrAlN layer. In addition, according to crack propagation morphology and influence of different interfaces between multilayered structures on crack tips, propagation/termination mechanism of cracks is analyzed in detail. Cracks are easy to initiate in hard CrAlN layer and consume a lot of energy after propagating into soft Cr layer, thereby ending at next soft and hard interfaces. These results provide experimental and theoretical support for the study of high tenacity and anti-erosion coating.     

Development of HA-CNTs composite coating on AZ31 Magnesium alloy by cathodic electrodeposition. Part 2: Electrochemical and in-vitro behavior

The carbon nano-tubes (CNTs) reinforced hydroxyapatite (HA), with various functionalized CNTs concentration ranging from 0 to 1.5?wt%, were deposited on AZ31 magnesium alloy by direct and pulse cathodic electrodeposition methods. The corrosion resistance of the coatings was tested in simulated body fluid (SBF) using different electrochemical methods such as open circuit potential, polarization and electrochemical impedance spectroscopy. The in-vitro behavior, changes in solution pH as well as the amount of evolved hydrogen of these coatings were also evaluated during five days immersion in SBF. The results indicated that the pulse deposited HA having 1% CNTs coating was the optimum condition which decreased the corrosion current density of AZ31 magnesium alloy from 44.25?µA/cm² to 0.72?µA/cm². Moreover, it stabilized the alkalization behavior of AZ31 alloy and caused a tenfold decrease in the amount of hydrogen generation in SBF. Additionally, the formation of new hydroxyapatite layer on the surface of the pre-exist coatings after five days immersion in SBF was confirmed by SEM characterization.     

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.     

Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc

A trivalent chromium based conversion coating (CCC), based on chromium nitrate solution with Co(II) ions, was developed on Zn substrate. The corrosion resistance of the trivalent CCC, measured in deaerated pH 8.0 borate buffer + 0.01 M NaCl solution using anodic polarization and electrochemical impedance spectroscopy (EIS), was very sensitive to both immersion time and bath pH. Micro-cracks were found on the surface of the CCC. Besides, the density of micro-crack and the coating thickness also depended on immersion time and bath pH. With increasing the coating thickness its pitting potential increased and passive current density decreased. The trivalent CCC formed on Zn for 40 s in pH 1.7 bath showed the best corrosion resistance, and the pitting potential increased significantly from −355 mV_SCE for Zn to 975 mV_SCE for the trivalent CCC on Zn. To explain the corrosion behavior of the trivalent CCC using EIS analysis, a modified equivalent circuit, which considered the micro-cracks in the coating and chromium corrosion product (CCP) deposited in the micro-cracks, was designed and the variation of each electrical parameter was examined. Especially, its corrosion behavior was well described by the variation of the resistance of CCP (R_ccp).     

Synergic enhancement of the anticorrosion properties of an epoxy coating by compositing with both graphene and halloysite nanotubes

The aim of this research was to improve the corrosion resistance of metal surfaces with polymer coatings. Both graphene and halloysite nanotubes (HNTs) were introduced together into the epoxy resin coating for the enhanced barrier protection of the metallic surface. The anticorrosion behaviors of different coatings were comparatively evaluated by the potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests. The potentiodynamic polarization curves showed that the coating containing 0.5 wt % HNTs and 0.8 wt % graphene (H05G08EP) together had the most positive corrosion potential and the minimum corrosion current density. The EIS results revealed that graphene endowed the composite coatings with excellent electrochemical performance for anticorrosive purposes. The NSS tests indicated that H05G08EP endured the longest NSS time. These results suggest that H05G08EP had the best corrosion resistance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47562.     

Corrosion protection by epoxy/poly(aniline-co-pyrrole)/ZnO nanocomposite coating

Anticorrosion behavior of epoxy/poly(aniline-co-pyrrole)/ZnO (EPAPZ) coating on stainless steel 304 alloys is investigated using the electrochemical impedance spectroscopy (EIS) method, and the coating is compared with epoxy/polyaniline/ZnO (EPAZ) and pure epoxy (EP) coatings. Scanning electron microscopy images are used for structural characterization and to compare the particle size of nanoparticles. EIS result showed that coating resistance for EPAPZ, EPAZ, and EP coatings after 90 days of immersion in 3.5% NaCl was 1.18 × 10⁷, 1.08 × 10⁶, and 4.28 × 10⁴ Ω cm⁻², respectively. In addition, the volume percentage of water absorbed by the coating, which could be obtained by coating capacitance, is 2.81, 4.21, and 9.11, respectively. Immersion tests showed 0.063, 0.194, and 0.752% of weight loss in the metals under EPAPZ, EPAZ, and EP coatings, respectively. These results show that the EPAPZ coating has superior anticorrosive performance compared with EPAZ and EP coatings. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48265.     

Corrosion Investigation of Chromium Nitride and Chromium Carbide Coatings for PEM Fuel Cell Bipolar Plates in Simulated Cathode Condition

Transition metals nitrides and carbides are used as coatings on bipolar plates for proton exchange membrane fuel cells (PEMFCs) due to their suitable electrical conductivity and corrosion resistance. Chromium electroplated AISI 316L stainless steel bipolar plates were treated by plasma nitriding and solid carburizing to form chromium nitride and chromium carbide, respectively. The presence of CrN/Cr₂N and Cr₇C₃/Cr₂₃C₆ was verified by X‐ray diffractometry (XRD) in chromium nitride and chromium carbide coatings, respectively. The corrosion behavior of coatings was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) in simulated cathode condition. Coated samples showed better corrosion behavior than untreated bare sample. EIS results indicated decrease in corrosion current density after 500 hours, however coatings acted as barrier against solution access to substrate. Corrosion current densities of coatings were close to targets of United Stated department of energy (DOE).     

Corrosion protection by epoxy coating containing multi-walled carbon nanotubes

Epoxy coatings that contained multiwalled carbon nanotubes (MWCNTs) were prepared. Further, the effect of the MWCNTs on the hydrophobicity and water transport behavior, and hence, on corrosion resistance provided by the epoxy coating were examined using hygrothermal cyclic tests and electrochemical impedance spectroscopy (EIS). The water transport behavior of epoxy coatings with higher MWCNT content decreased to a larger extent for coatings with higher surface hydrophobicity. The corrosion protection of carbon steel coated with epoxy coating that contained MWCNTs correlated well with water transport behavior and hydrophobicity.