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.     

Application of PEO75PPO30PEO75 stabilised polymeric micelles for improved corrosion resistance of composite zinc coatings

The influence of stabilised polymeric micelles (SPM) on the structure, corrosion properties and durability of electrodeposited composite zinc coatings is presented and discussed. The core-shell-type SPM used are based on poly-propylene oxide (core) and poly-ethylene oxide (shell). The deposition and dissolution processes of zinc in the presence or absence of SPM have been investigated by the cyclic polarisation method. The corrosion behaviour and protective ability of the composite coatings have been evaluated in a model corrosion medium of 3% NaCl solution by means of polarisation resistance measurements. The changes in the zinc matrix as a result of the embedded SPM were investigated by an X-ray diffraction method.     

Electrochemical evaluation of the corrosion protectiveness and porosity of vacuum annealed CrAlN and TiAlN cathodic arc physical vapor deposited coatings

The corrosion behavior of cathodic arc physical vapor deposited CrAlN and TiAlN coatings were examined in 1 M HCl solution before and after vacuum annealing at 700, 800, 900, and 1000 °C. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods were used to study the corrosion behavior and porosity of the coatings in comparison with the bare steel substrate (304SS). Structural and mechanical characterization of the coatings were also conducted. It is found that with increasing annealing temperature, the mechanical properties of TiAlN increased due to age hardening caused by spinodal decomposition while the hardness of CrAlN decreased as result of relaxation. Similarly, EIS and PDP results revealed that the as‐deposited and annealed coatings offer higher corrosion resistance as compared to the bare 304SS substrate. The coatings susceptibility to corrosion is reduced after annealing as indicated by the increasing nobility of E_corr. Both PDP and EIS tests revealed that CrAlN coating annealed at 1000°C exhibited superior corrosion resistance properties. It is found that the reduced current density for CrAlN coating annealed at 1000°C was due to the reduction in the porosity. Annealed TiAlN coating follows similar behavior until an optimum annealing temperature of 800°C. Beyond this temperature, porosity enlargement and an increase in the number of pores subsequent to structural changes deteriorated the corrosion resistance of TiAlN coating.     

Synthesis and high-temperature corrosion resistance of Ti-Zr-Si-N coatings deposited by means of sputtering

ABSTRACT

We present a study of the synthesis and an evaluation of the corrosion resistance of Ti-Zr-Si-N coatings deposited by means of the co-sputtering technique on 316L steel and Ti6Al4V alloy substrates. The thin films were obtained using Ti₅Si₂ and Zr targets with 99.9% purity for the deposition, and a power of 200 W was applied in the DC source and 170?W in the RF source. The corrosion resistance was evaluated via the cyclic oxidation test with consistent cycles of 1 hour of cooling, 1 hour of heating, and 16 minutes of stabilisation for 300 cycles at a temperature of 600°. The coatings were characterised by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and confocal laser microscopy. The coating was effective up to 150 cycles for the case of 316L steel, and up to 50 cycles for the Ti6AlV alloy.     

The physical and electrochemical properties of electroless deposited nickel–phosphorus black coatings

The manufacture of electroless black nickel surfaces had been obtained, through etching electroless nickel deposits by oxidizing acid solution. The morphology, chemical composition and reflectance of pre-etch and post-etch coatings were compared to examine influence of phosphorus and sulfur content on preparation of black surfaces. The non-metallic elements content of electroless nickel deposits was greatly improved after black treatment. It indicated the etching treatment was a selective dissolving process. The nickel atoms can be preferentially removed during the period. Optimum phosphorus content range to produce low reflectance black surface was proposed, after reflectance measurement of black surfaces. Additionally, the effect of co-deposited phosphorus and sulfur atoms on corrosion resistance of electroless nickel deposits was evaluated by electrochemical impedance spectroscopy (EIS). The fitting results indicated that corrosion resistance of high-phosphorus electroless nickel deposits will be significantly reduced after black treatment. However, that of low-phosphorus deposits will be improved.     

Sodium Diethyl Dithiocarbamate—an effective complexone for Cold Zinc Phosphating

A cold zinc phosphating bath has been formulated and optimized with a view to study the role of sodium diethyl dithiocarbamate (DEDTC) as a complexing additive in such a bath. The resultant coatings were assessed for their physical properties and corrosion resistance. The study concludes that the addition of sodium diethyl dithiocarbamate (DEDTC) increases the coating weight, reduces the processing time and increases the corrosion resistance of the resultant coatings.     

Preparation and characterisation of silica/epoxy hybrid nanocomposite coatings containing boehmite nanoparticles for corrosion protection

Abstract

A hybrid silica/epoxy nanocomposite film containing boehmite nanoparticles has been developed in this work through the sol–gel method to protect AA2024 alloy from corrosion. The hybrid sols were prepared by copolymerisation of 3-glycidoxypropyltrimethoxysilane, tetraethylorthosilicate and aluminium isopropoxide. The films were prepared by dip coating technique. The morphology and the structure of the hybrid sol–gel films were studied by scanning electron microscopy and atomic force microscopy. The corrosion protection properties of the films were investigated by potentiodynamic scanning and electrochemical impedance spectroscopy. The results indicate that the presence of the boehmite nanoparticles in hybrid structure of the silica/epoxy films, highly improved the corrosion protection performance of the coating system.     

Effect of porosity sealing treatments on the corrosion resistance of high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous metallic coatings

High-velocity oxy-fuel-sprayed FeCrMoMnWBCSi amorphous metallic coatings were sealed with sodium orthosilicate (Na₃SiO₄), aluminium phosphate (AlPO₄), and cerium salt sealants. The microstructure of the sealed coatings was characterised by scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction. Corrosion behaviour was examined using electrochemical methods of potentiodynamic polarisation, cyclic polarisation, electrochemical impedance spectroscopy, and Mott-Schottky tests. The results indicated that the uniform corrosion resistance of the three sealed coatings was enhanced greatly, and the passive current densities were decreased by one order of magnitude after the sealing treatments. The AlPO₄ sealant can penetrate the coatings by no less than 50 μm and enhance their hardness, which exhibited a more uniform corrosion resistance, fairly good pitting corrosion resistance, and can be applied in long-term corrosive and/or abrasive environments. The cerium salt-sealed coating showed better pitting corrosion resistance but inferior corrosion resistance in the local regions of micro-cracks, which was practically used for temporary corrosion protection. The Na₃SiO₄-sealed coating showed better uniform corrosion resistance and inferior pitting corrosion resistance, which can be applied in short-term corrosion environments. The stability of the passive film affected the corrosion behaviour of the sealed coatings. The AlPO₄-sealed coating performed better as a protective passive film during the long-term immersion test for a lower defect concentration and a more protective passive film.     

Electrochemical frequency modulation and inductively coupled plasma atomic emission spectroscopy methods for monitoring corrosion rates and inhibition of low alloy steel corrosion in HCl solutions and a test for validity of the Tafel extrapolation method

The inhibition effect of glycine (Gly) towards the corrosion of low alloy steel ASTM A213 grade T22 boiler steel was studied in aerated stagnant 0.50 M HCl solutions in the temperature range 20-60 °C using potentiodynamic polarization (Tafel polarization and linear polarization) and impedance techniques, complemented with scanning electron microscope (SEM) and energy dispersive X-ray (EDX). Electrochemical frequency modulation (EFM), a non-destructive corrosion measurement technique that can directly give values of corrosion current without prior knowledge of Tafel constants, is also presented here. Experimental corrosion rates determined by the Tafel extrapolation method are compared with corrosion rates obtained by electrochemical, namely EFM technique, and chemical (i.e., non-electrochemical) method for steel in HCl. The chemical method of confirmation of the corrosion rates involved determination of the dissolved cation, using ICP-AES (inductively coupled plasma atomic emission spectrometry) method of analysis. Corrosion rates (in mm y⁻¹) obtained from the electrochemical (Tafel extrapolation and EFM) and the chemical method, ICP, are in a good agreement. Polarization studies have shown that Gly is a good “green”, mixed-type inhibitor with cathodic predominance. The inhibition process was attributed to the formation of an adsorbed film on the metal surface that protects the metal against corrosive agents. Scanning electron microscopy (SEM) and energy dispersion X-ray (EDX) examinations of the electrode surface confirmed the existence of such an adsorbed film. The inhibition efficiency increases with increase in Gly concentration, while it decreases with solution temperature. Temkin isotherm is successfully applied to describe the adsorption process. Thermodynamic functions for the adsorption process were determined.     

Structure stability and corrosion resistance of nano-TiO2 coatings on aluminum in seawater by a vacuum dip-coating method

A stable nano-TiO₂ coating was prepared by vacuum dip-coating TiO₂ sol-gel onto the anodized aluminum surface. The effect of vacuum dip-coating method and anodization pretreatment on compactness and stability of the coatings was proved. The structure and composition of the coatings were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicate that the surface of coatings is uniform and compact, with high load capacity. The particles of TiO₂ anatase is filled into the Al₂O₃ nano-pores formed by anodization. The electrochemical measurements show that the nano-TiO₂ coatings significantly decrease the corrosion currents densities (i_corr), as simultaneously increased the values of polarization resistance (R_t) of aluminum. It indicates the nano-TiO₂ coatings exhibit excellent anticorrosion properties in sterile seawater at the room temperature.     

Investigation on the corrosion and oxidation resistance of Ni-Al2O3 nano-composite coatings prepared by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique from a Watt's type electrolyte containing nano-Al₂O₃ particles. The corrosion resistance and high temperature oxidation resistance of resulting composite coatings were investigated. It was found that the incorporation of nano-Al₂O₃ particles in Ni matrix refined the Ni crystal and changed the preferential orientation of composite coatings. Meanwhile, the corrosion and oxidation resistance were improved after the incorporation of nano-Al₂O₃ particles into Ni matrix. The nano-Al₂O₃ content in deposits plays an important role for improving the corrosion and oxidation protection. The corrosion and oxidation resistance of Ni-Al₂O₃ nano-composite coatings produced via SCD technique are superior to that of CEP technique. Compared to pure Ni and Ni-Al₂O₃ composite coatings fabricated using CEP technique, the Ni-7.58 wt.% Al₂O₃ composite coating obtained by SCD technique exhibits better corrosion resistance and enhanced high temperature oxidation resistance. Moreover, the mechanism of corrosion and high temperature oxidation resistance of Ni-Al₂O₃ nano-composite coatings are discussed.     

Synthesis and corrosion protection study of poly(o-ethylaniline) coatings on copper

Poly(o-ethylaniline) coatings were synthesized on copper (Cu) by electrochemical polymerization of o-ethylaniline in an aqueous salicylate solution by using cyclic voltammetry. The characterization of these coatings was carried out by cyclic voltammetry, UV-visible absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results of these characterizations indicate that the aqueous salicylate solution is a suitable medium for the electrochemical polymerization of o-ethylaniline to generate strongly adherent and smooth poly(o-ethylaniline) coatings on Cu substrates. The performance of poly(o-ethylaniline) as protective coating against corrosion of Cu in aqueous 3% NaCl was assessed by the potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). The results of these studies demonstrate that the poly(o-ethylaniline) coating has ability to protect the Cu against corrosion. The corrosion potential was about 0.078 V versus SCE more positive in aqueous 3% NaCl for the poly(o-ethylaniline) coated Cu (∼ 15 μm thick) than that of uncoated Cu and reduces the corrosion rate of Cu almost by a factor of 70.     

Mechanical properties and corrosion resistance of nanocrystalline ZrNxOy coatings on AISI 304 stainless steel by ion plating

Transition metal oxynitrides have become emerging decorative coating materials due to their adjustable coloration and high hardness and corrosion resistance. This research studied the effect of oxygen content on the coloration, mechanical properties and corrosion resistance of ZrN_xO_y thin films deposited on AISI 304 stainless steel using hollow cathode discharge ion plating (HCD-IP). The Zr/N/O ratios of the ZrN_xO_y films were determined using X-ray photoelectron spectroscopy (XPS). The color of the ZrN_xO_y thin film changed from golden yellow to blue and then slate blue with increasing oxygen content. X-ray diffraction (XRD) patterns revealed that phase separation of ZrN and m-ZrO₂ occurred as the oxygen content reached 31.2 at.%. ZrN(O) (ZrN with dissolving oxygen) is dominant at oxygen content less than 18.1 at.%, while m-ZrO₂ phase was prevailed at oxygen content above 40.3 at.%. Phase separation lowered the hardness of the ZrN_xO_y films as the fraction of ZrO₂ was less than 40%. The residual stresses in ZrN phase was higher than that in ZrO₂, and the residual stress decreased for the specimen containing 30 to 37% ZrO₂. For the samples containing more than 44% ZrO₂, the average residual stress was close to that in ZrO₂ phase. The corrosion resistance was evaluated by salt spray test and potentiodynamic scan in two solutions: 0.5MH₂SO₄ + 0.05 M KSCN and 5% NaCl solutions. The results showed consistent trend in the two solutions. From the results of potentiodynamic scan, corrosion resistance increased as the packing density of the film increased, whereas the film thickness was not a crucial factor on corrosion current; moreover, the electrical conductivity of the film may be one of the significant factors in corrosion resistance. Results of salt spray tests suggested that the corrosion of ZrN_xO_y in NaCl may play an important role in corrosion resistance.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

Effect of electrodeposition conditions on the composition, microstructure, and corrosion resistance of Zn-Ni alloy coatings

The formation, composition, and structure of electrodeposited zinc-nickel alloys were investigated. It has been shown that both anomalous and normal codeposition of zinc and nickel can be realized by changing the bath composition and deposition conditions, with the nickel content in the resultant deposit being varied in a wide range (from 2 to 90 at.%). It has been also shown that the ammonical diphosphate electrolyte allows deposition of Zn-Ni coatings with a homogeneous phase structure (Ni₅Zn₂₁ and Ni₃Zn₂₂ intermetallides, a solid solution of Zn in Ni, or a solid solution of Ni in Ni₅Zn₂₁), whereas the weak acid chloride electrolyte produces two-phase coatings consisting of Ni₅Zn₂₁ with the admixture of polycrystalline Zn or Ni. The Zn-Ni coating with a nickel content of 19 at.% consisting of Ni₅Zn₂₁ intermetallic phase exhibits the highest corrosion resistance.     

Microstructural and corrosion resistance characterisation of NiTi shape memory alloy modified at low-temperature plasma with carbon coatings produced via RFCVD and IBAD methods

ABSTRACT

NiTi shape memory alloy, both before and after low-temperature oxynitriding under glow discharge conditions combined with the production of a nano-sized outer carbon coating, was investigated. The study compares two types of carbon coatings: produced via the RFCVD (radio frequency chemical vapour deposition) and IBAD (ion beam-assisted deposition) methods. The latter was also enriched by randomly distributed silver nanoparticles. The study showed slight changes in roughness parameters and increased corrosion resistance after surface treatment processes. The largest impedance module and corrosion potential and the smallest density of corrosion current were achieved for samples with a carbon coating produced via the RFCVD process. The presented surface modifications using hybrid processes can extend the use of NiTi alloy in medicine due to the increased biocompatibility.     

Annealing temperature effect on the corrosion parameters of autocatalytically produced Ni-P and Ni-P-Al2O3 coatings in artificial seawater

Ni-P and Ni-P-Al₂O₃ amorphous alloy coatings with 9.3 and 8.3 wt.% P respectively were obtained by autocatalytic deposition at 90 °C on carbon steel substrates. The effect of annealing temperature (100, 200, 300, 400 and 500 °C) upon the corrosion parameters of the coatings in artificial seawater with pH 5.0 and 8.1 at room temperature was evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. It was found that deposits annealed at 400 and 500 °C presented an increase of the charge transfer resistance and negligible changes on samples annealed at lower temperature. Polarisation tests showed a charge transfer controlled anodic kinetics on both Ni-P and Ni-P-Al₂O₃ deposits and diffusion controlled cathodic reaction in artificial seawater at pH 5.0 and 8.1. The coatings did not present passive behaviour in the electrolytes and impedance measurements showed a single time constant for all cases with the lowest double layer capacitance (C_dl) for samples annealed at 400 and 500 °C. The best corrosion parameters were observed on Ni-P and Ni-P-Al₂O₃ coatings annealed at temperatures higher than 400 °C, which is the temperature where crystallisation of this kind of coatings takes place.     

Influence of cold gas spray parameters on the corrosion resistance of Al-Al2O3 coatings sprayed on carbon steel

ABSTRACT

This work describes the influence of standoff distance (SoD), and gas temperature on the morphology and corrosion resistance of Al-10%Al₂O₃ coatings deposited by cold gas spray (CGS) on carbon steel. The results showed that the standoff distance had little effect on the thickness and microstructure of the coating. However, a 100 °C decrease of the spraying temperature reduced the coating thickness by 300?µm. The use of electrochemical analyses and SEM images showed that all the coatings studied were able to protect the substrate during at least 1300?h of immersion, due to the dense microstructure obtained by CGS.     

Improvement in the corrosion resistance of TiB2/A356 composite by molten-salt electrodeposition and anodization

This study reported a novel treatment to improve the corrosion resistance of TiB₂/A356 composites. The method was employed in combination of the molten-salt electrodeposition and subsequent electrochemical anodization technique. By means of molten-salt electrodeposition, the Al coatings were deposited on the surface of TiB₂/A356 composites. It was found that the morphology of the Al coatings is closely related to the current density. Thus, under the suitable condition, a dense and uniform Al coating can be obtained, with the crystal size in the range of 0.5–2 μm and the coating thickness of ~ 9 μm. This continuous Al layer can eliminate the adverse corrosion contribution of TiB₂ particles in Al matrix. The following step of anodization was designed to convert the Al film to an anodized Al oxide film for further corrosion protection. The electrochemical corrosion behavior was evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy. These results showed the corrosion resistance was greatly enhanced in TiB₂/A356 composites with an anodized Al-coating than that of the anodized composites. It is evident that the new treatment of metal electrodeposition in molten salts and following anodization is an effective method of anti-corrosion in composites.