Synthesis and properties of electrodeposited Ni/ceria nanocomposite coatings

Composite plating is a method of co-depositing fine particles of metallic or non-metallic compounds or polymers in the plated layer to improve material properties such as lubrication, wear resistance and corrosion resistance. In the present study, Ni was chosen as the matrix material and ceria nanoparticles were chosen as the distributed phase. Nanocrystalline ceria powder was synthesized by the solution combustion process and characterized by powder X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nanosize ceria particles were co-deposited with nickel from a nickel sulfamate bath using conventional electrodeposition method. The electrodeposition was carried out at current densities of 0.23, 0.77, 1.55, 3.1 and 5.4 A/dm². The microhardness of the Ni matrix was enhanced by the incorporation of ceria particles. Potentiodynamic polarization, electrochemical impedance spectroscopy and SEM were used to characterize the corrosion behaviour of Ni and Ni/CeO₂ coatings. These studies showed improved corrosion resistance for Ni/CeO₂ when compared to Ni. The microhardness, corrosion resistance and wear resistance of Ni and Ni/CeO₂ were compared.     

Preparation of MgO coatings on magnesium alloys for corrosion protection

MgO coating is formed on magnesium alloy by anodic electrodeposition in 6 M KOH solution, whereas Mg(OH)₂ coating is produced by anodization in 10 M KOH solution, which could be successively converted to MgO by calcination in air at 450 °C. The evolution of morphology, structure and composition of anodic film obtained on Mg alloy is investigated using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray diffraction (XRD). Potentiodynamic polarization measurements show that the as-grown MgO protective coatings are very effective in improving the corrosion resistance of magnesium alloy compared to bare metallic magnesium.     

Low-temperature titania coatings for aluminium corrosion protection

TiO₂ coatings on AA6082 aluminium alloy were obtained at low temperature (80 and 100°C) by the sol–gel dip-coating technique starting from titanium tetra-isopropoxide solution in ethyl alcohol. The preparation was carried out in the presence of acetic acid with both functions of catalyst and chelating agent. The curing temperatures used for these coatings are between 80 and 100°C, low enough to make such coatings suitable to incorporate additives such as organic inhibitors or polymeric nanoparticles. The coated samples were characterised by scanning electron microscopy and energy-dispersive spectroscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy (EIS). Compact coatings with a thickness of 500?nm, consisting of amorphous and nearly stoichiometric titanium dioxide, were obtained. EIS results revealed an effective corrosion protection of the substrate for more than 120?h of immersion in 3.5?wt-% NaCl aqueous solution.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

Assessment of graphene oxide/epoxy nanocomposite as corrosion resistance coating on carbon steel

Graphene and its derivatives are new materials with unique properties. In recent years, various studies about the nanocomposites based on graphene oxide (GO) have been carried out. However, the electrochemical properties of nanocomposite coatings based on GO materials have not been widely studied. In this study, GO/epoxy nanocomposite coatings were prepared by incorporation of different amounts of GO nanosheets into the epoxy matrix via mechanical agitation and sonication process. The dispersion of GO nanosheets in matrix was analysed by optical microscopy, transmission electron microscopy and Fourier transform infrared. The anticorrosive properties of these nanocomposite coatings were investigated by electrochemical impedance spectroscopy tests. Results showed that the corrosion protection of coatings was improved by addition of GO into the coatings material. Furthermore, the best corrosion resistance was achieved in 0·25 wt-%GO/epoxy nanocomposite coatings.     

Formulation and evaluation of nano-structured polymeric coatings for corrosion protection

The effect of the addition of multi-walled carbon nano-tubes (MWCNT) to epoxy and vinyl chloride/vinyl acetate copolymer coatings on their ability to protect the substrates was studied. Coatings were formulated from these resins with and without MWCNT reinforcement. Steel substrates were prepared and coated with each formulated coating and submerged in 5% NaCl solution to study their corrosion resistance by means of Electrochemical Impedance Spectroscopy (EIS). In addition, thin films from these polymers, with and without nano-reinforcement, were cast. Dogbone specimens were cut in order to study their mechanical properties. Some of these specimens were immersed in the NaCl solution for two weeks in order to compare their mechanical properties with samples not exposed to salt water. Optical microscopy was used to capture the progress of sample corrosion. EIS measurements showed that the addition of MWCNTs to epoxy and vinyl chloride/vinyl acetate copolymer (VYHH) coatings increased their charge transfer resistance in comparison with the neat coatings. This is an indication of the enhanced corrosion protection of the nano-coatings. In addition, mechanical strength tests, both before and after immersion in 5% NaCl solution, showed that thin films from both epoxy and VYHH resins containing MWCNTs had improved strength, an indication of an improvement in the coatings' cohesive properties.     

Improving the corrosion performance of epoxy coatings by chemical modification with silane monomers

In this communication, commercial epoxy resins were chemically modified with various silane monomers under the catalysis of organotin compound, aiming to enhance the corrosion resistance of epoxy coatings on 2024-T3 aluminum substrates. Immersion studies conducted in 3.5 wt.% NaCl solution showed that the coating capacitance (C_c) decreases significantly after the silane modification, as measured by electrochemical impedance spectroscopy (EIS), indicating the higher resistance to water permeation. EIS measurements also indicated an enhancement in protectiveness of silane-modified epoxy coatings against substrate corrosion, which was characterized by higher charge transfer resistances (R_ct) and lower double layer capacitance (C_dl) at substrate/electrolyte interface. The adhesion of epoxy coatings was also found to improve after the modification with silane components. The best performance was observed for coating system modified by 3-glycidoxypropyltrimethoxy silane (GPTMS).     

Polybenzoxazine/SiO2 nanocomposite coatings for corrosion protection of mild steel

A series of nanocomposite coatings (PBS) consisting of silane functional polybenzoxazine (PB-TMOS) and SiO₂ nanoparticles were developed for corrosion protection of mild steel. The influence of silica content on corrosion resistance of PBS coatings was investigated by electrochemical measurements. The surface chemistry of nanoparticles and its effect on morphology of the PBS coating was also studied utilizing Fourier Transforms Infrared Spectroscopy, ²⁹Si Nuclear Magnetic Resonance and Scanning Electron Microscopy analyses. The results indicate that the presence of the covalent bond between nanoparticles and PB-TMOS, greatly improves the interfacial interactions at the polymer/filler interfaces resulting in a better corrosion performance.     

Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings

Electrochemical techniques for the assessment of porosity in electrodeposited metal coatings are reviewed. The determination of porosity and corrosion, resistance is illustrated by electrochemical data from three coating/substrate systems namely: electroless nickel on aluminium and steel and immersed gold coatings on an electroless copper-plated ABS polymer. Nickel coatings were up to 24 μm thick while gold deposits had thickness between 75 and 190 nm. Tafel extrapolation and linear polarisation resistance methods were used to determine the corrosion rate of the coated substrates. The aluminium samples were tested in 5% w/v (0.85 mol dm^− 3) NaCl, while coated steel and ABS samples were immersed in 0.125 mol dm^− 3 H₂SO₄ and 0.1 mol dm^− 3 NaBH₄, respectively, at 295 K. Current vs. time curves and anodic polarisation behaviour have also been considered.     

Corrosion protection of steel by epoxy nanocomposite coatings containing various combinations of clay and nanoparticulate zirconia

Epoxy-based nanocomposite coatings containing various amounts of nano-clay and aminopropyltrimethoxy silane (APS) treated zirconia nanoparticles were prepared via slurry method. Morphology and dispersion of nanoparticles within the nanocomposites were evaluated using XRD and TEM analyses. Corrosion performance of mild steel coated specimens was investigated using EIS and EN techniques. The results showed that the simultaneous addition of the spherical ZrO₂ and layered clay nanoparticles promotes the exfoliation of the clay nanoparticles and in so doing improves the corrosion performance of nanocomposite coatings via enhancing the barrier properties and ohmic resistance.     

The nanostructure, wear and corrosion performance of arc-evaporated CrBxNy nanocomposite coatings

The composition, nanostructure, tribological and corrosion behaviour of reactive arc evaporated CrB_xN_y coatings have been studied and compared to CrN. The CrB_xN_y coatings were deposited on a commercial Oerlikon Balzers RCS coating system employing 80:20 Cr:B targets. To vary the composition, the nitrogen fraction was adjusted (N₂ fraction = N₂/(Ar + N₂)) and a moderate bias voltage of − 20 V was applied during coating growth. The coating composition and nanostructure was determined using time-of-flight elastic recoil detection analysis (TOF-ERDA), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Ball-on-disc dry sliding wear tests were conducted using an alumina ball counterface both at room temperature and at 500 °C with the relative humidity controlled at 20%. Potentiodynamic corrosion tests were undertaken in 3.5% NaCl aqueous solution. The wear tracks were examined using optical profilometry and scanning electron microscopy (SEM); the surface composition inside and outside of the wear tracks were investigated using Raman spectroscopy and XPS. All coatings exhibit nanocomposite structures and phase compositions which are in fair agreement with those expected from the equilibrium phase diagram. The lowest wear rate at room temperature and 500 °C was found for CrB_0.14N_1.14, which was shown to exhibit the highest hardness and possesses a nanocomposite nc-CrN/a-BN structure. CrB_0.12N_0.84 coatings showed the lowest passive current density in potentiodynamic corrosion tests.     

Comparison of the tribological and antimicrobial properties of CrN/Ag, ZrN/Ag, TiN/Ag, and TiN/Cu nanocomposite coatings

Nanocomposite coatings including CrN/Ag, ZrN/Ag, TiN/Ag and TiN/Cu with varying silver or copper contents were produced by co-deposition in a dual pulsed magnetron sputtering system. The compositions and structures of the coatings were characterised using energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM), and the physical and tribological properties were assessed by means of nanoindentation and thrust washer wear testing. Although increasing silver or copper content provided a reduction in the coefficient of friction, this was accompanied by reductions in hardness for all the coatings and wear resistance for some of the coatings. Zones of inhibition were used to determine the extent of silver ion release from the coating surfaces, and a NBT (nitro-blue tetrazolium) redox dye was used to determine the antimicrobial effectiveness of the coatings following incubation. The microorganisms tested were Pseudomonas aeruginosa and Staphylococcus aureus. For the NBT assays, significant reductions in the number of viable cells were observed with increasing Ag or Cu content, compared to the ‘pure’ nitride surfaces. Whilst no zones of inhibition were observed for S. aureus, on any of the surfaces, the diameter of the ‘kill’ zones generally increased with increasing silver content for P. aeruginosa.     

Characterisation of corrosion and wear behaviour of nanoscaled e-beam PVD CrN coatings

The present paper analyses the performance of CrN single-layers produced by electron beam PAPVD (EBPAPVD), finding that both corrosion and wear resistance are directly dependant on the structure and stoichiometry of the nitride. The nanolayer structure of the coatings is formed by periodically varying the nitrogen pressure during deposition resulting in layers with higher and lower N-content. This fact, which has not been described in the literature, causes different structure and morphology of the individual films providing excellent properties to the coating. For corrosion resistance, the CrN layer's greater compactness impedes penetration of the electrolyte and thus prevents the formation of a galvanic couple between the coating and the substrate. Moreover, a good wear resistance is obtained, retarding its delamination.     

Thermal behaviour of hard nanocomposite coatings within the W-Si-N system in oxidant and protective atmospheres

This paper reviews the current knowledge on the thermal annealing of W-Si-N sputtered films in both protective and oxidant atmospheres. Firstly, sputter deposited W films are presented as a particular case of single nc-metallic films and their thermal behaviour is analyzed. Afterwards, the thermal stability and the oxidation resistance of W-Si-N system are considered. Special attention is paid to amorphous films. In a protective atmosphere, it is shown that after crystallization the hardness can be higher than those of as-deposited crystalline films with similar nanocomposite structure. The hardest films present a nc-W/a-Si₃N₄ nanocomposite structure type. The key factor for thermal oxidation resistance is the Si content. The higher the Si content the lower the oxidation rate is. The excellent thermal behaviour in oxidant atmospheres is attributed to either the formation of a protective surface layer of SiO₂ or the hindering of oxygen diffusion along the grain boundaries due to the Si-N phase.     

Preparation and characterisation of aluminium pigments coated with silica for corrosion protection

Aluminium pigments with a layer of silica were prepared by a sol–gel method using tetraethoxysilane as precursor and ethylenediamine as catalyst. Under the optimum conditions, the corrosion protection factor can reach 99.3% and average grain almost remains the same size after coating, indicating that the coated aluminium pigments have excellent chemical stability and good dispersibility. FTIR and EDS analyses demonstrate that a layer of silica coating has been formed on the flaky aluminium particle. SEM, AFM and BET analyses show that a smooth and dense silica coating layer has been formed.     

Preparation and characterisation of electrophoretically deposited hydroxyapatite coatings on type 316L stainless steel

Hydroxyapatite (HAP) coatings were developed on type 316L stainless steel (SS) by electrophoretic deposition at various deposition potentials from 30 to 90 V using the stoichiometric HAP (Ca/P ratio 1.67) powder in a suspension of isopropyl alcohol. The optimum coating parameters were established at 60 V and 3 min, after vacuum sintering at 800 °C. The phase purity of the coated surface was confirmed by XRD and secondary ion mass spectrometry confirmed the presence of both Ca and P on the coated layers. The electrochemical corrosion parameters E_corr (open circuit potential) and pitting potentials, evaluated in Hank’s solution shifted towards noble direction for the HAP coated specimens in comparison with uncoated type 316L SS. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values after immersing the coated specimens in Hanks solution for 200 h. This indicates the stable nature of the coatings formed.     

Effect of curing characterization on the corrosion performance of polyester and polyester/epoxy powder coatings

In this study, effect of curing program on the corrosion performance of un-pigmented polyester and epoxy polyester powder coatings was evaluated, using various test methods. Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) studies were carried out to determine suitable time –temperature curing program. Protective behavior of cured powder coatings in optimum situation was investigated, using different test techniques such as electrochemical impedance spectroscopy (EIS), water vapour transmission (WVT), and open circuit potential (OCP) variation as a function of time.EIS studies and OCP evaluation revealed that the polyester/epoxy powder coated samples demonstrated a better performance than polyester powder coated samples, due to higher cross-linked density of epoxy based powder. It is argued that these studies may provide sufficient evidence to show that protective characteristics of coatings is definitely influenced by cross-linking density so that more efficient coatings are ones with higher cross-link density.DMTA revealed that the different curing regimes do not affect the glass temperature of the given powder coatings.     

Influence of curing temperature, silica nanoparticles- and cerium on surface morphology and corrosion behaviour of hybrid silane coatings on mild steel

This work is aimed at developing and investigating silane based organic-inorganic hybrid coatings possessing unique properties, which can be used to improve the performance of steel structures subjected to marine corrosion. These silane based sol-gel coatings were prepared by dip coating planar samples of mild steel in solution of an organically modified silica sol made from hydrolysis and polycondensation of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES) in acid catalysis condition. Crack-free coatings were obtained on curing at 200 °C. On increasing the curing temperature to 400 °C, however, cracks developed in the plain organic-inorganic hybrid coatings. This observation was consistent with the visual observations where appearance of the coated specimen changed from colourless metallic to brownish grey on curing from 200 °C to 400 °C temperature. The coatings were further modified using SiO₂ nanoparticles and cerium. The effect of change in the - temperature as well as - composition on the microstructural properties of the coatings was determined using optical microscopy, scanning electron microscopy and atom force microscopy. Additionally, Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR/FTIR) was carried out to show the formation of the Si-O-Si structural backbone of the hybrid material with the organic CH₃ group incorporated into the silica network. The corrosion protection performance of these coatings was examined using potentiodynamic polarisation technique and electrochemical impedance spectroscopy in aerated 3.5 wt.% NaCl solution. The polarization curves and corrosion resistance as measured by the bode plots suggested that the plain hybrid coatings offer good protection against corrosion. However, the SiO₂ and cerium modified nano hybrid coatings exhibited superior performance to that displayed by plain hybrid coatings.     

Preparation of ultraviolet-cured nanocomposite coatings for protecting against corrosion of metal substrates

A cycloaliphatic di-epoxy monomer was used to prepare ultraviolet-cured coatings, in the presence of montmorillonites, either commercially available, or modified on purpose (Cloisite Na⁺, Cloisite 30B), dispersed at two different concentrations (5% and 10% w/w).The corrosion resistance of the ultraviolet-cured films coated on a metal substrate was investigated with electrochemical techniques and compared to the behaviour of the neat ultraviolet-cured epoxy resin films. The coatings showed different stability as revealed by the measurements of the barrier properties depending on the type of nanoclay used.Changing the modifier employed the coatings exhibited intercalated or exfoliated morphologies, as assessed by electron microscopy analysis and confirmed by X-ray diffraction results; the prevention of corrosion was proved dependent on the morphology.     

Cohesive and adhesive properties of polycaprolactone/silica hybrid coatings on poly(methyl methacrylate) substrates

Organic–inorganic hybrid materials were prepared using the sol–gel process in order to improve the scratch resistance of poly(methyl methacrylate) (PMMA) substrates. Sol–gel solutions based on tetraethoxysilane and ,ω-triethoxysilane terminated polycaprolactone with two different organic / inorganic weight ratios (5 / 5 and 7 / 3) were applied and cured on the substrate. The final coatings consisted of interconnected nanodomains of silica and polycaprolactone, which increased in size and resulted into microdomains when the content of organic phase increased. The surface energies of coated and uncoated PMMA, and the cohesive and adhesive properties of the hybrid coatings were examined using contact angle measurements, and fragmentation tests, respectively. The surface energy of the hybrid coatings and their work of adhesion to the PMMA substrate were found to be similar, with a higher polar contribution at increasing silica content. In contrast, the strain to failure, cohesive strength, toughness and adhesion to PMMA of the coating with a 5 / 5 organic / inorganic ratio were considerably higher, compared to the coating with a 7 / 3 organic / inorganic ratio. These results demonstrate that the adhesion strength is the result of a series of complex phenomena that probably involves diffusion of reactants in PMMA and their crosslinking, as suggested by electron energy loss spectroscopy analyses, other than effects deriving from residual stresses and from the morphology produced during the formation of the coating.