In vitro corrosion and biocompatibility study of phytic acid modified WE43 magnesium alloy

Phytic acid (PA) conversion coating on WE43 magnesium alloy was prepared by the method of immersion. The influences of phytic acid solution with different pH on the microstructure, properties of the conversion coating and the corrosion resistance were investigated by SEM, FTIR and potentiodynamic polarization method. Furthermore, the biocompatibility of different pH phytic acid solution modified WE43 magnesium alloys was evaluated by MTT and hemolysis test. The results show that PA can enhance the corrosion resistance of WE43 magnesium especially when the pH value of modified solution is 5 and the cytotoxicity of the PA coated WE43 magnesium alloy is much better than that of the bare WE43 magnesium alloy. Moreover, all the hemolysis rates of the PA coated WE43 Mg alloy were lower than 5%, indicating that the modified Mg alloy met the hemolysis standard of biomaterials. Therefore, PA coating is a good candidate to improve the biocompatibility of WE43 magnesium alloy.     

Electrodeposition of Al-Mn alloy on AZ31B magnesium alloy in molten salts

The Al-Mn alloy coatings were electrodeposited on AZ31B Mg alloy in AlCl₃-NaCl-KCl-MnCl₂ molten salts at 170 °C aiming to improve the corrosion resistance. However, in order to prevent AZ31B Mg alloy from corrosion during electrodeposition in molten salts and to ensure excellent adhesion of coatings to the substrate, AZ31B Mg alloy should be pre-plated with a thin zinc layer as intermediate layer. Then the microstructure, composition and phase constituents of the coatings were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction (XRD). It was indicated that, by adjusting the MnCl₂ content in the molten salts from 0.5 wt% to 2 wt%, the Mn content in the alloy coating was increased and the phase constituents were changed from f.c.c Al-Mn solid solution to amorphous phase. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization measurements in 3.5% NaCl solution. It was confirmed that the Al-Mn alloy coatings exhibited good corrosion resistance with a chear passive region and significantly reduced corrosion current density at anodic potentiodynamic polarization. The corrosion resistance of the alloy coatings was also related with the microstructure and Mn content of the coatings.     

Novel strategy in increasing stability and corrosion resistance for super-hydrophobic coating on aluminum alloy surfaces

A novel super-hydrophobic coating was prepared by chemical modification on the anodized aluminum alloy surface. The surface structure was characterized by water contact angle measurement, scanning electron microscopy (SEM), and the composition was measured by X-ray photoelectron spectroscopy (XPS). The corrosion behavior of the super-hydrophobic coating was evaluated by the polarization curve and the electrochemical impedance spectroscopy (EIS). It was found that the static water contact angle on the surface of super-hydrophobic coating was as high as 167.7 ± 1.2°, and the sliding angle was 5°. The super-hydrophobic coating resulted in excellent corrosion resistance property and the super-hydrophobic coating showed a good stability.     

Studies on influence of zinc immersion and fluoride on nickel electroplating on magnesium alloy AZ91D

The effect of zinc immersion and the role of fluoride in nickel plating bath were mainly investigated in nickel electroplating on magnesium alloy AZ91D. The state of zinc immersion, the composition of zinc film and the role of fluoride in nickel plating bath were explored from the curves of open circuit potential (OCP) and potentiodynamic polarization, the images of scanning electron microscopy (SEM) and the patterns of energy dispersive X-ray (EDX). Results show that the optimum zinc film mixing small amount of Mg(OH)₂ and MgF₂ is obtained by zinc immersion for 30-90 s. The corrosion potential of magnesium alloy substrate attached zinc film will be increased in nickel plating bath and the quantity of MgF₂ sandwiched between magnesium alloy substrate and nickel coating will be reduced, which contributed to produce nickel coating with good performance. Fluoride in nickel plating bath serves as an activator of nickel anodic dissolution and corrosion inhibitor of magnesium alloy substrate. 1.0-1.5 mol dm⁻³ of F⁻ is the optimum concentration range for dissolving nickel anode and protecting magnesium alloy substrate from over-corrosion in nickel plating bath. The nickel coating with good adhesion and high corrosion resistance on magnesium alloy AZ91D is obtained by the developed process of nickel electroplating. This nickel layer can be used as the rendering coating for further plating on magnesium alloys.     

Improvement of corrosion properties of microarc oxidation coating on magnesium alloy by optimizing current density parameters

The microstructure, composition and corrosion performance of oxide coatings formed on AM60B alloy using microarc oxidation techniques at different waveforms of applied current densities were investigated within this study. It is found that the use of optimizing current density waveforms, i.e. decaying freely current density in the later stage and stepped decreasing current density, significantly improved the microstructure of oxide coatings compared with the constant current density mode, which are connected with changes in behaviors of spark discharges on the surface in oxidation process. The optimal waveform of current density is showed to be decaying freely current density in the later stage, which results in sealing the originally formed large micropores. The optimisation of the microstructure results in a significant improvement of the corrosion resistance of oxide coating.     

Preparation and properties of titanium oxide film on NiTi alloy by micro-arc oxidation

Titanium oxide ceramic coatings were prepared by micro-arc oxidation (MAO) in galvanostatic regime on biomedical NiTi alloy in H₃PO₄ electrolyte using DC power supply. The surface of the coating exhibited a typical MAO porous and rough structure. The XPS analysis indicated that the coatings were mainly consisted of O, Ti, P, and a little amount of Ni, and the concentration of Ni was greatly reduced compared to that of the NiTi substrate. The TF-XRD analysis revealed that MAO coating was composed of amorphous titanium oxide. The coatings were tightly adhesive to the substrates with the bonding strength more than 45 MPa, which was suitable for medical applications. The curves of potentiodynamic porlarization indicated that the corrosion resistance of NiTi alloy was significantly improved due to titanium oxide formation on NiTi alloy by MAO.     

Electrodeposition of high corrosion resistance Cu/Ni-P coating on AZ91D magnesium alloy

High corrosion resistance Cu/Ni-P coatings were electrodeposited on AZ91D magnesium alloy via suitable pretreatments, such as one-step acid pickling-activation, once zinc immersion and environment-friendly electroplated copper as the protective under-layer, which made Ni-P deposit on AZ91D Mg alloy in acid plating baths successfully. The pH value and current density for Ni-P electrodeposition were optimized to obtain high corrosion resistance. With increasing the phosphorous content of the Ni-P coatings, the deposits were found to gradually transform to amorphous structure and the corrosion resistance increased synchronously. The anticorrosion ability of AZ91D Mg alloy was greatly improved by the amorphous Ni-P deposits, which was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS). The corrosion current density (I_corr) of the coated Mg alloy substrate is about two orders of magnitude less than that of the uncoated.     

Effect of electrodeposition modes on surface characteristics and corrosion properties of fluorine-doped hydroxyapatite coatings on Mg-Zn-Ca alloy

The microstructure, morphology and composition highly determine the corrosion resistance and bioactivity of coating. In traditional cathodic electrodeposition process, because of the unfavorable effects of the polarization of concentration difference and H₂ evolution, fluorine-doped hydroxyapatite coating was loose and porous. This coating could not ensure the long-term stability of the Mg alloy implants. In order to improve the corrosion resistance and bioactivity of coating, pulse electrodeposition and H₂O₂ were introduced into the electrodeposition to deposit fluorine-doped hydroxyapatite coating. As a comparative study, microstructure, corrosion resistance properties and bioactivity of traditional cathodic electrodeposition coating and pulse electrodeposition coating were investigated, respectively. The results revealed that nano fluorine-doped hydroxyapatite coating could be prepared by pulse electrodeposition, and the coating was dense and uniform. The potentiodynamic polarization experiment indicated that the dense and uniform coating could effectively protect Mg alloy substrate from corrosion. Immersion testing was performed in simulated body fluid. It was found that pulse electrodeposition coating could more effectively induce the precipitation of Mg²⁺, Ca²⁺ and PO₄³⁻ in comparison with traditional cathodic electrodeposition coating, because the nano phase had comparatively high specific surface area. Thus magnesium alloy coated with fluorine-doped nano-hydroxyapatite coating may be a promising candidate as biodegradable bone implants, and was worthwhile to further investigate the in vivo degradation behavior.     

A corrosion resistant cerium oxide based coating on aluminum alloy 2024 prepared by brush plating

Cerium oxide based coatings were prepared on AA2024 Al alloy by brush plating. The characteristic of this technology is that hydrogen peroxide, which usually causes the plating solution to be unstable, is not necessary in the plating electrolyte. The coating showed laminated structures and good adhesive strength with the substrate. X-ray diffraction and X-ray photoelectron spectroscopy analysis showed that the coatings were composed of Ce(III) and Ce(IV) oxides. The brush plated coatings on Al alloys improved corrosion resistance. The influence of plating parameters on structure and corrosion resistance of the cerium oxide based coating was studied.     

Enhanced in vitro biocompatibility of ultrafine-grained biomedical NiTi alloy with microporous surface

Bulk ultrafine-grained Ni_50.8Ti_49.2 alloy (UFG-NiTi) was successfully fabricated by equal-channel angular pressing (ECAP) technique in the present study, and to further improve its surface biocompatibility, surface modification techniques including sandblasting, acid etching and alkali treatment were employed to produce either irregularly roughened surface or microporous surface or hierarchical porous surface with bioactivity. The effect of the above surface treatments on the surface roughness, wettability, corrosion behavior, ion release, apatite forming ability and cytocompatibility of UFG-NiTi alloy were systematically investigated with the coarse-grained NiTi alloy as control. The pitting corrosion potential (E_pit) was increased from 393 mV (SCE) to 704 mV (SCE) with sandblasting and further increased to 1539 mV (SCE) with following acid etching in HF/HNO₃ solution. All the above surface treatment increased the apatite forming ability of UFG-NiTi in varying degrees when soaked them in simulated body fluid (SBF). Meanwhile, both sandblasting and acid etching could promote the cytocompatibility for osteoblasts: sandblasting enhanced cell attachment and acid etching increased cell proliferation. The different corrosion behavior, apatite forming ability and cellular response of UFG-NiTi after different surface modifications are attributed to the topography and wettability of the resulting surface oxide layer.     

Microstructure and corrosion resistance behavior of ceramic coatings on biomedical NiTi alloy prepared by micro-arc oxidation

In this paper, ceramic coatings were prepared on biomedical NiTi alloys by micro-arc oxidation (MAO) in constant voltage mode. The current density-time response was recorded during the MAO process. The microstructure, element distribution and phase composition of the coatings prepared at different MAO treatment times were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), thin-film X-ray diffraction (TF-XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion behavior of the coatings in 0.9% NaCl solution was evaluated by the potentiodynamic polarization test. It is found that the coatings become more compact with increasing the MAO treatment time, and the growth rate of coating decreases. The results of TF-XRD, EDS and XPS indicate that the coatings are composed of a large amount of γ-Al₂O₃ and a little α-Al₂O₃, TiO₂ and Ni₂O₃. The Ni content of the coatings is about 3 at.%, which is greatly lower than that of NiTi substrate. The bonding strength of coating-substrate is higher than 40 MPa for all the samples in this study. The corrosion resistance of the coatings is about two orders of magnitude higher than that of the uncoated NiTi alloy.     

Mild corrosion behavior on sulphurized steel surface during friction

After treatment by low temperature ion sulphuration, the solid lubrication sulphuration layers (FeS films) were produced on the AISI 1045 and stainless steel. A mass of corrosion peeling pits occurred on the worn scars of 1045 steel sulphuration layer after wear test, whereas none of them on the stainless steel one. AFM was used to observe the morphology of sulphuration layer, SEM equipped EDS was utilized to analyze the morphologies and compositions of worn scars. XPS and XRD were employed to detect the valence states of sulphuration layer and its worn scars, as well as the phase structures. The results showed that during friction, under the frictional heat, the sulfate radical with mild corrosion was produced, so that the 1045 steel without any anti-corrosion was corroded in some certain, meanwhile the stainless steel was not corroded depending on its excellent corrosion resistance.     

Heat-insulation film with gold layer deposited on antireflection subwavelength-structured surface

The optical properties of gold layer deposited on antireflection subwavelength-structured surface are first exposed. The experimental results of the reflectances and transmittances for several different thicknesses of gold-deposited layers on the subwavelength structures are carried out. The nanostructured surface with spatial period and a diameter of about 230 nm and height of about 150 nm on polyethylene terphthalate (PET) film is fabricated by micro-replication process of UV imprinting. Comparing these with the bare gold-deposited layer and bare nanostructure, the results show that the optical films with the suitable gold layer deposited on antireflection subwavelength-structured surface has high transmittance and low glare in the visible spectral range and high reflectance in the infrared range. That is to say, when the antireflection subwavelength-structured surface is coated with a gold layer of several tens of nm thickness, it will have some unique optical characters.     

Surface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloy

NiTi alloys are attractive materials that are used for medicine, however, Ni-release may cause allergic reactions in an organism. The Ni-release rate is strongly affected by the surface state of the NiTi alloy that is mainly determined by its processing route. In this study, a NiTi shape memory alloy (50.9 at.% Ni) was heat-treated by several regimes simulating the shape setting procedure, the last step in the manufacture of implants. Heating temperatures were between 500 and 550 °C and durations from 5 to 10 min. Heat treatments were performed in air at normal and low pressure and in a salt bath. The purpose of the treatments was to obtain and compare different surface states of the Ni-Ti alloy. The surface state and chemistry of heat-treated samples were investigated by electron microscopy, X-ray photoelectron spectroscopy and Raman spectrometry. The amount of nickel released into a model physiological solution of pH 2 and into concentrated HCl was taken as a measure of the corrosion rate. It was found that the heat treatments produced surface TiO₂ layers measuring 15-50 nm in thickness that were depleted in nickel. The sample covered by the 15-nm thick oxide that was treated at 500 °C/5 min in a low pressure air showed the best corrosion performance in terms of Ni-release. As the oxide thickness increased, due to either temperature or oxygen activity change, Ni-release into the physiological solution accelerated. This finding is discussed in relation to the internal structure of the oxide layers.     

Effect of current density on the microstructure and corrosion behaviour of plasma electrolytic oxidation treated AM50 magnesium alloy

Plasma electrolytic oxidation (PEO) of an AM50 magnesium alloy was accomplished in a silicate-based electrolyte using a DC power source. Coatings were produced at three current densities, i.e. 15 mA cm⁻², 75 mA cm⁻², and 150 mA cm⁻² and were characterised for thickness, roughness, microstructural morphology, phase composition, and corrosion resistance. Even though the 15 min treated coatings produced at higher current density levels were thicker, they showed poor corrosion resistance when compared to that of the coatings obtained at 15 mA cm⁻². Short-term treatments (2 min and 5 min) at 150 mA cm⁻² yielded coatings of thickness and corrosion resistance comparable to that of the low current density coatings. The superior corrosion resistance of the low thickness coatings is attributed to the better pore morphology and compactness of the layer.     

Electroless deposition of Ni-W-P coating on AZ91D magnesium alloy

Ternary Ni-W-P alloy coating was deposited directly on AZ91D magnesium alloy by using an alkaline-citrate-based baths. Nickel sulfate and sodium tungstate were used as metal ion sources, respectively, and sodium hypophosphite was used as a reducing agent. The pH value of the electroless bath was tailored for magnesium alloy. The coating was characterized for its structure, morphology, microhardness and the corrosion properties. SEM observation showed the presence of dense and coarse nodules in the ternary coating. EDS analysis showed that the content of tungsten in the Ni-W-P alloy was 4.5 wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution revealed that the ternary Ni-W-P coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy.     

Preparation and properties of super-hydrophobic coating on magnesium alloy

The super-hydrophobic coating was successfully fabricated on the surface of magnesium alloy AZ31 by chemical etching and surface modification. The surface morphologies, compositions, wettability and corrosion resistance of the coating were investigated with SEM, XPS, contact angle measurement and electrochemical method, respectively. It shows that the rough and porous micro-nano-structure was presented on the surface of magnesium alloy, and the contact angle could reach up to 157.3 ± 0.5° with sliding angle smaller than 10°. The super-hydrophobic coating showed a long service life. The results of electrochemical measurements showed that anticorrosion property of magnesium alloy was improved. The super-hydrophobic phenomenon of the prepared surface was analyzed with Cassie theory, and it finds that only about 10% of the water surface is contacted with the metal substrate and the rest 90% is contacted with the air cushion.     

Electroless Ni-P/Ni-B duplex coatings for improving the hardness and the corrosion resistance of AZ91D magnesium alloy

The Ni-P/Ni-B duplex coatings were deposited on AZ91D magnesium alloy by electroless plating process and their structure, morphology, microhardness and corrosion resistance were evaluated. The duplex coatings were prepared using dual baths (acidic hypophosphite- and alkaline borohydride-reduced electroless nickel baths) with Ni-P as the inner layer. The coatings were amorphous in as-plated condition and crystallized and produced nickel borides upon heat-treatment. SEM observations showed that the duplex interface on the magnesium alloy was uniform and the compatibility between the layers was good. The Ni-P/Ni-B coatings microhardness and corrosion resistance of having Ni-B coating as the outer layer was higher than Ni-P coatings. The Ni-P/Ni-B duplex coatings on AZ91D magnesium alloy with high hardness and good corrosion resistance properties would expand their scope of applications.     

Improvement of corrosion resistance of NiTi sputtered thin films by anodization

Anodization of sputtered NiTi thin films has been studied in 1 M acetic acid at 23 °C for different voltages from 2 to 10 V. The morphology and cross-sectional structures of the untreated and anodized surfaces were investigated by field emission scanning electron microscopy (FE-SEM). The results show that increasing anodization voltage leads to film surface roughening and unevenness. It can be seen that the thickness of the anodized layer formed on the NiTi surface is in the nanometer range. The corrosion resistance of anodized thin films was studied by potentiodynamic scan (PDS) and impedance spectroscopy (EIS) techniques in Hank's solution at 310 K (37 °C). It was shown that the corrosion resistance of the anodized film surface improved with increasing voltage to 6 V. Anodization of austenitic sputtered NiTi thin films has also been studied, in the same anodizing conditions, at 4 V. Comparison of anodized sputtered NiTi thin films with anodized austenitic shape memory films illustrate that the former are more corrosion resistant than the latter after 1 h immersion in Hank's solution, which is attributed to the higher grain boundary density to quickly form a stable and protective passive ?lm.     

Preparation of anti-corrosion films by microarc oxidation on an Al-Si alloy

Thick ceramic films over 140 μm were prepared on Al-7% Si alloy by ac microarc oxidation in a silicate electrolyte. The film growth kinetics was determined by an eddy current technique and film growth features in different stages were discussed. The microstructure and composition profiles for different thick films were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. Their phase components were determined by X-ray diffraction. The electrochemical corrosion behaviors of bare and coated alloys were evaluated using potentiodynamic polarization curves, and their corrosion morphologies were observed. In the initial stage of oxidation, the growth rate is slow with 0.48 μm/min due to the effect of Si element though the current density is rather high up to 33 A/dm². After the current density has decreased to a stable value of 11 A/dm², the film mainly grows towards the interior of alloy. The film with a three-layer structure consists of mullite, γ-Al₂O₃, α-Al₂O₃ and amorphous phases. By microarc discharge treatment, the corrosion current of the Al-Si alloy in NaCl solution was significantly reduced. However, a thicker film has to be fabricated in order to obtain high corrosion-resistant film of the Al-Si alloy. Microarc oxidation is an effective method to form an anti-corrosion protective film on Si-containing aluminum alloys.