In vitro corrosion behavior of TiN layer produced on orthopedic nickel-titanium shape memory alloy by nitrogen plasma immersion ion implantation using different frequencies

In the present work, the NiTi surface was modified by nitrogen plasma immersion ion implantation (PIII) in an effort to improve the corrosion resistance and mitigate nickel release from the materials. The implanted nitrogen depths and thicknesses of the surface TiN barrier layers were varied by changing the pulsing frequencies during PIII. In order to determine the optimal parameters including the pulsing frequencies, electrochemical tests including open circuit potential (OCP) measurements and potentiodynamic polarization tests were conducted on the untreated and N-implanted NiTi in simulated body fluids (SBF). Our results reveal that the nitride layer produced using a frequency of 50 Hz has the best stability under the OCP conditions and the TiN layer produced using 200 Hz has the highest potentiodynamic stability after immersion in SBF for a long time. The observation can be correlated to the temperature during PIII and the thickness of TiN layer. The TiN layer on the NiTi surface favors deposition of Ca-P composites thereby compensating for the instability of the TiN layer produced at a higher frequency.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

Polyoxadiazole-based coating for corrosion protection of magnesium alloy

In this study, polyoxadiazole-based coatings were molecularly designed by attaching two different functional groups, i.e., diphenyl-ether and diphenyl-hexafluoropropane, in the main polymer chain for the purpose of low water permeability and eventually for high corrosion protection of AM50 magnesium alloy. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the coating performance of the two polymers. Electrochemical experiments showed that POD-6FP (poly(4,4′-diphenyl-hexafluoropropane-1,3,4-oxadiazole)) coated alloy exhibited 3-4 orders of magnitude higher corrosion resistance as compared to the POD-DPE (poly (4,4′-diphenyl-ether-1,3,4-oxadiazole)) coated alloy. The high coating performance of the POD-6FP polymer can be attributed to the hydrophobic group attached to the polyoxadiazole chain.     

Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting

An AZ91D magnesium alloy was treated using duplex techniques of laser surface melting (LSM) and plasma electrolytic oxidation (PEO). The microstructure, composition and corrosion behavior of the laser melted surface, PEO coatings, LSM–PEO duplex coatings as well as the as-received specimen were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical corrosion tests, respectively. Especially, the effect of LSM pre-treatment on the microstructure, composition and corrosion resistance of the PEO coatings was investigated. Results showed that the corrosion resistance of AZ91D alloy was marginally improved by LSM due to the refinement of grains, redistribution of β-phase (Mg₁₇Al₁₂) and increase of Al on the surface. Both the PEO and duplex (LSM–PEO) coatings improved significantly the corrosion resistance of the AZ91D alloys, while the duplex (LSM–PEO) coating exhibited better corrosion resistance compared with the PEO coating.     

The effect of interlayer on corrosion resistance of ceramic coating/Mg alloy substrate in simulated physiological environment

Ceramic coatings are deposited on biodegradable magnesium alloys by physical vapor deposition to reduce the electrochemical activity in the simulated physiological environment. Although an interlayer is generally used to reduce the mismatch between the hard coating and soft substrate, the effects of the interlayer on the electrochemical corrosion behavior have seldom been explored. In this work, AlO_xN_y ceramic coatings were deposited on AZ31 magnesium alloys with Al or Ti interlayers. Polarization tests and electrochemical impedance spectroscopy (EIS) were conducted to evaluate the corrosion resistance in the cell culture medium. The AlO_xN_y ceramic coating significantly improved the bio-corrosion resistance of the magnesium alloy, but the Ti interlayer accelerated the corrosion rate. In comparison, although the addition of an Al interlayer led to smaller enhancement in the surface mechanical properties of the AlO_xN_y coating, corrosion could be impeded effectively. Our results indicate that an Al interlayer is preferred over Ti and the corrosion failure mechanism is discussed from the perspective of defects.     

The effect of solid diffusion surface alloying on properties of ZM5 magnesium alloy

In this research, solid diffusion process was used to form a diffusion alloying layer on the surface of ZM5 magnesium alloy to improve corrosion and wear resistance. It is shown that the solid diffusion layer was mainly composed of Mg–Al–Zn intermetallic compounds and Mg–Al–Zn solid solution transition zone that had more Zn and Al elements than untreated ZM5 magnesium substrate. The continued immersion test in 3% NaCl solution displayed that the diffusion-treated specimen had better corrosion resistance compared to the untreated ZM5 specimen. The polarization test indicated that the Mg–Al–Zn intermetallic compounds of the diffusion alloying layer were an effective corrosion barrier to decrease the corrosion rate for ZM5 magnesium alloy when exposed to 3% NaCl solutions. In addition, the microhardness values of the Mg–Al–Zn intermetallic compounds were much higher than those of the substrate and this would greatly contribute to the enhancement of wear resistance.     

Effect of electroless-Ni-plating on corrosion fatigue behavior of magnesium alloy

Fatigue tests were performed on electroless nickel-plated magnesium alloy specimens in laboratory air and 3% sodium chloride solution. In laboratory air, the effect of surface treatments (plating, blasting and polishing) on the fatigue lives of specimens was found to be minimal. However, in 3% sodium chloride solution, the electroless Ni-plated specimens were found to have shorter fatigue lives than those of the polished and blasted specimens. In order to study the fatigue mechanisms, successive observations of the specimen surfaces were conducted during the fatigue process in both laboratory air and sodium chloride solution. Observations of the fracture surfaces were also conducted to clarify the fatigue mechanism.     

Anticorrosive magnesium phosphate coating on AZ31 magnesium alloy

A novel anticorrosive film with a thickness of approximately 50 μm was successfully coated on an AZ31 magnesium alloy by chemical and low-heat treatments (50 °C). The film was a single-phase system of newberyite (MgHPO₄•3H₂O) having an orthorhombic crystal structure. The corrosion current density of the newberyite film coated on the AZ31 magnesium alloy decreased by more than two orders of magnitude as compared to that of the AZ31 magnesium alloy. The static water contact angle of the newberyite film was less than 10°. The average value of the scratch critical load for the newberyite coating was estimated to be approximately 15 mN.     

Influence of electrolyte on corrosion properties of plasma electrolytic conversion coated magnesium alloys

The synthesis of oxides in a low-temperature electrolytic plasma allows to cover surfaces of magnesium and its alloys with multifunctional protective oxide-ceramic coatings. The corrosion properties of these layers are strongly dependent on their porosity. In order to minimize the porosity and to optimize the corrosion properties of the layers, the electrolyte concentration and composition (addition of CrO₃ as corrosion inhibitor) were varied, and the influences on layer structure, composition, and properties with a main focus on corrosion behaviour were studied.The corrosion properties of various layers thus generated were studied in 5% NaCl solution by measuring electrochemical polarization curves and by electrochemical impedance spectroscopy (EIS) at pH 3 and 6. Using XRD, LM, SEM and EDX to evaluate the composition and microstructure of the modified surfaces, the corrosion results were related to the microstructure and composition of the specific layer. The better results were obtained for layers produced at higher electrolyte concentration, whereas the addition of CrO₃ had no significant beneficial effect.     

Modification of biomedical NiTi shape memory alloy by TiC/Ti films using PIIID

TiC/Ti coatings were deposited on the surface of Ti-50.6 at.% Ni alloy by plasma immersion ion implantation and deposition (PIIID) technique. The microstructure, mechanical properties and hemocompatibility of the samples were investigated by means of XRD, AFM, nanoindentation, and scratch and platelet adhesion tests. The result of XRD analysis shows that the crystalline TiC coating has a preferential orientation of (111) in the normal direction. The surface presents a very smooth and dense microstructure with 1.517 nm root mean square roughness (RMS). The average hardness and modulus values of the TiC coating are much higher than those of the NiTi substrate. In the initial stage of scratching, some obvious transversal cracks and worm-like cracks spreading into the film from both side of the scratch track were observed. At higher normal loads for the scratching test, the film delaminated from the substrate at the margins of the substrate. Platelet adhesion tests demonstrate that the hemocompatibility of the coated sample is improved.     

Atmospheric corrosion of field-exposed magnesium alloy AZ91D

The magnesium alloy AZ91D was exposed in three different types of atmospheric environment, viz. urban, rural and marine exposure sites. Corrosion rates, corrosion products formed, and the influence of the microstructure on the corrosion behaviour of the alloy were investigated. The corrosion rate of AZ91D exposed in the marine environment was 4.2 μm/year, and in the rural and urban environments 2.2 and 1.8 μm/year, respectively. The main corrosion product found was magnesium carbonate hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), which was formed at all three exposure sites. The corrosion attack started in the -phase in larger grains at the boundary between the -phase and the eutectic -/β-phase. Microgalvanic elements were formed with the eutectic -/β-Mg phase as cathodic site and the -Mg grains as anodes. The Al–Mn particles played a minor roll in the initiation process, even though these particles are the most noble in the microstructure and thus the driving force for a corrosion attack around these particles could be expected to be high. A close resemblance was observed between the corrosion mechanisms operating under the field-exposure conditions described here and the mechanisms operating under the previously reported laboratory conditions.     

Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys

The corrosion protection afforded by a cerium conversion coating, formed by immersion in a solution containing rare earth salt and hydrogen peroxide, on pure magnesium and two magnesium alloys, AZ91 and AM50, has been studied. The effect of HCl pre-treatments on the morphology and on the corrosion resistance of the cerium conversion layer was investigated. A thicker and more homogeneous distribution of the conversion coating was obtained when the sample surface was pre-treated with acid. Higher amounts of cerium on the surface of the pre-treated samples were detected. The cerium conversion coating increased the corrosion resistance of the alloys because it ennobled the corrosion potential and decreased both the anodic and cathodic current. The acid pre-treatment further increased the corrosion resistance of the coated alloys. After five days of immersion in chloride environment the untreated samples showed localized corrosion while the chemical conversion coated samples appeared unaffected.     

Electrochemical behavior of the Ti6Al4V alloy implanted by nitrogen PIII

Plasma surface treatments have been used very often to enhance the surface properties of metallic materials. In this work, Ti6Al4V titanium alloy was treated by nitrogen plasma immersion ion implantation (NPIII) in order to obtain improvements in its surface properties, such as corrosion resistance evaluated here. The microstructure and corrosion behavior of the implanted and unimplanted samples were evaluated, using, XRD, GDOES and potentiodynamic polarization and impedance electrochemical spectroscopy tests in 0.6 M NaCl solution. It was verified that the NPIII created resistant layers to corrosive attacks. In corrosion tests by polarization, the implanted samples showed corrosion current density reduction of about 10 times compared to the Ti6Al4V alloy without treatment. Besides that, it was also observed a reduction of the passive current density of one order of the magnitude. In all the studied cases, the polarization curves were shifted to more positive values of potentials, indicating a lower tendency of these PIII treated surfaces to corrosion. The implantation process produced a thin TiN surface layer followed by Ti₂N and then a layer with nitrogen in solid solution, all detected by GDOES combined with X-ray diffraction. These layers promoted an excellent polarization resistance of the Ti6Al4V surfaces on impedance spectroscopy tests also. This better performance in these tests can be correlated with the formation of continuous nitride layer, which could retard chloride ions ingress into the substrate.     

Electrochemical studies of stainless steel implanted with nitrogen and oxygen by plasma immersion ion implantation

Plasma Immersion Ion Implantation (PIII) of stainless steel with nitrogen at temperatures lower than 400 °C has been reported to increase the hardness of the material by several times. However, expectations that the corrosion resistance will remain unaffected after implantation were not found to be so. In the present study the influence of post-oxygen implantation on the corrosion resistance of nitrogen implanted stainless steel is presented. Stainless steel samples were subjected to oxygen, nitrogen and post-oxygen ion implantation at different temperatures. GIXRD and microRaman studies of the implanted samples showed that oxygen implantation leads to the formation of an oxide layer consisting of corundum and spinel structures. The corrosion properties of the implanted samples were studied by potentiodynamic polarization and electrochemical impedance techniques in 3.5% NaCl solution. After nitrogen implantation the corrosion current increased and the corrosion potential shifted to the less noble side to − 0.486 V as compared to − 0.284 V for the substrate. Oxygen implantation at 400 °C shifted the corrosion potential to the nobler side to − 0.2 V with decrease of corrosion current. For post-oxygen ion implantation at temperatures lower than 400 °C, the corrosion current was higher than the substrate and the corrosion potential was also on the less noble side. However, post-oxygen ion implantation at 400 °C after nitrogen ion implantation resulted in improved corrosion resistance as the corrosion potential shifted to nobler side and the corrosion current was lower than that of substrate.     

The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62

Magnesium alloys are increasingly being used as lightweight materials in the automotive, defense, electronics, biomaterial and aerospace industries. However, their inherently poor corrosion and wear resistance have, so far, limited their application. Plasma electrolytic oxidation (PEO) in an environmentally friendly aluminates electrolyte has been used to produce oxide coatings with thicknesses of ~ 80 μm on an AJ62 magnesium alloy. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm–800 nm) was employed to characterize the PEO plasma. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the coated materials, and potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution were used to determine the corrosion behavior. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings and, hence the corrosion resistance. The corrosion resistance of the coated alloy was increased by changing the current mode from unipolar to bipolar, where the strong plasma discharges had been reduced or eliminated.     

Stearic acid coating on magnesium for enhancing corrosion resistance in Hanks' solution

Magnesium is potentially a desirable metallic material for degradable orthopedic implants if its corrosion rate can be reduced so that the implants can maintain sufficient mechanical integrity in the initial period of bone fracture healing. To achieve this aim, an organic coating was prepared on magnesium samples by a two-step process: (i) hydrothermal treatment to form a thick hydroxide layer, followed by (ii) immersion coating using stearic acid (SA, CH₃(CH₂)₁₆COOH, an endogenous long-chain saturated fatty acid which is the main component of fat). The hydroxide layer formed in the first step provided sites to anchor the SA coating in the subsequent step, with the formation of magnesium stearate, which would enhance adhesion of the coating to the substrate. The corrosion behavior of the coated samples in Hanks' solution (a simulated body fluid) was studied by electrochemical methods. The corrosion resistance of the coated samples was much higher than that of bare magnesium, especially in the initial period, due to the barrier effect provided by the coating. Degradation of the coating occurred due to cracking in long-term immersion, with the formation of magnesium hydroxide and apatite.     

A composite coating for corrosion protection of AM60B magnesium alloy

Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO₂, ZrO₂ and Al₂O₃). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and cross-section morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/silane composite coatings was evaluated in 0.6 M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO₂ and in Al₂O₃ rich solutions. For this reason specimens anodized in ZrO₂ and Al₂O₃ containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO₂ or Al₂O₃), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time.     

Effect of magnesium hydride on the corrosion behavior of an AZ91 magnesium alloy in sodium chloride solution

The effect of magnesium hydride on the corrosion behavior of an as-cast AZ91 alloy in 3.5 wt.% NaCl solution was investigated using gas collection method and potentiostatic test. The Pourbaix diagram of Mg–H₂O system was built using thermodynamic calculation. It was possible that magnesium hydride could form in the whole pH range in theory. The experimental results showed that at cathodic region, magnesium hydride formed on surface, which was the controlling process for the corrosion behavior of AZ91 alloy; at anodic region and free corrosion potential, magnesium hydride model and partially protective film model, monovalent magnesium ion model and particle undermining model were responsible for the corrosion process of AZ91 alloy.     

Microstructure,corrosion behavior,and surface mechanical properties of Fe oxide coatings on biomedical ZK60 Mg alloy

The Mg‐5.5Zn‐0.6Zr (in wt%, ZK60) alloy has been surface modified by dual Fe&O ion implantation and deposition (II&D) under different O₂ fluxes from 0 to 40 sccm. The microstructure is investigated by glancing angle X‐ray diffraction, atomic force microscope, and scanning electron microscopy. The results show that the modified layers, with a gradient microstructure of outer deposition region and inner implantation region, are composed of α‐Fe + Fe&Mg mixture, FeO/Fe‐rich oxide + Fe&Mg mixture, and α‐Fe₂O₃/Fe‐rich oxide + Fe&Mg mixture for 0, 10, and 40 sccm O₂ fluxes, respectively. The electrochemical and immersion tests in 37°C Hank's solution indicate an improvement in corrosion behavior under 0 and 10 sccm O₂ fluxes, but a deterioration in corrosion resistance under 40 sccm O₂ flux. In addition, the nanoindentation tests suggest that the dual Fe&O II&D simultaneously enhances the surface hardness and elastic modulus due to the formation of Fe and its oxide coatings.     

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.