High corrosion resistance of electroless composite plating coatings on AZ91D magnesium alloys

The process of electroless plating Ni-P on AZ91D magnesium alloys was improved. The Ni-P-ZrO₂ composite coatings and multilayer coatings were investigated based on the new electroless plating process. The coatings surface and cross-section morphologies were observed with scanning electron microscopy (SEM). The chemical compositions were analyzed by EDXS. The corrosion behaviors were evaluated by immersion, salt spray and electrochemical tests. The experimental results indicated that the Ni-P-ZrO₂ composite coatings suffered attack in NaCl solution but displayed passivation characteristics in NaOH and Na₂SO₄ solutions. The corrosion resistance of Ni-P-ZrO₂ coatings was superior to Ni-P coatings due to the effect of ZrO₂ nano-particle. The multilayer coatings consisting of Ni-P-ZrO₂/electroplating nickel/Ni-P (from substrate to surface) can protect magnesium alloys from corroding more than 1000 h for the salt spray test.     

Long-term corrosion resistance and fast mineralization behavior of micro-nano hydroxyapatite coated magnesium alloy in vitro

To improve the long-term corrosion resistance of biodegradable AZ31 magnesium alloy, the micro-nano structural hydroxyapatite (HA) coating was fabricated on AZ31 substrate by hydrothermal treatment. The compact and high crystallinity HA coating prepared at 120 °C had excellent electrochemical properties. Moreover, the cell viability experiment revealed that the micro-nano structure coating was conducive to the viability and proliferation of MC3T3-E1 osteoblasts. The immersion experiment in simulated body fluid (SBF) solution showed that the micro-nano structural HA coatings could quickly induce the production of HA mineralization, and then the mineralization evolved into a compact mineralized layer on the surface of coated sample, which provided a long-term protection for the specimen. Even after 147 days of immersion, the coated samples remained the relatively complete macroscopic shape, the corrosion rates were lower than 0.500 mm/y and the pH values of the SBF solution maintained in the range of 7.10–7.80, suggesting when these coated AZ31 magnesium alloys were used as degradable biomaterial implants, they could provide a long-term mechanical support during the healing of damaged bones.     

Effect of ball milling on the corrosion resistance of magnesium in aqueous media

The influence of the high-energy ball milling on the corrosion behavior of magnesium in aqueous media has been investigated through electrochemical experiments complemented by morphological, structural, chemical and surface analyses. The milling duration was varied from 0 to 40 h. Polarization curves show that the milling procedure improves the magnesium corrosion resistance in passive conditions (KOH solution) and in more active corrosion conditions (borate solution). This is illustrated by the corrosion potential which becomes nobler with milling. The variation of the polarization resistance and related corrosion current with milling time is also an indication of the improvement of the Mg corrosion resistance due to the milling. Moreover, the passive current is significantly lower for milled Mg. The Mg crystallite size is reduced from >100 to 34 nm after 10 h of milling and does not decrease significantly with further milling. The iron contamination of the Mg powder due to the erosion of the milling tools is very low (0.09 wt.% after 40 h of milling). In contrast, a significant oxygen contamination occurs during milling (2.6 wt.% after 40 h of milling). XPS and AES data indicate MgO enrichment in the bulk of the milled Mg without significant MgO increase at the powder surface. The corrosion improvement was attributed to the increase through the milling process of the density of surface defects and grain boundaries susceptible to increase the number of nucleation sites for Mg hydroxylation in aqueous media, leading to the rapid formation of a dense and protective Mg(OH)₂ layer.     

Preparation and corrosion resistance of a self-sealing hydroxyapatite- MgO coating on magnesium alloy by microarc oxidation

An ideal self-sealing hydroxyapatite (HA)-MgO coating was designed on an AZ31 Mg alloy by one-step microarc oxidation (MAO) with the addition of HA nanoparticles into a base electrolyte. The formation mechanism of the self-sealing HA-MAO coating was discussed. The effect of the nano-HA addition on the corrosion resistance of the MAO coating was evaluated by corrosion tests in Hank's solution. The results show that HA nanoparticles inertly incorporated into the MAO coating during the process of coating growth. HA and MgO were the main constituents of the HA-MAO coating. The HA nanoparticles were absent in the inner barrier layer but concentrated in the outer porous layer. In addition, HA nanoparticles accumulated much more inside coating defects than in the other zones, which resulted in the nearly ideal sealing of micropores on the coating surface. By forming a denser and more stable outer layer, the incorporation of HA nanoparticles greatly enhanced the anti-corrosion properties of the MAO coating.     

Preparation and corrosion resistance studies of zirconia coating on fluorinated AZ91D magnesium alloy

Novel anti-corrosion zirconia coating was prepared via the sol–gel method for AZ91D magnesium alloy using zirconium nitrate hydrate as a precursor modified with acetylacetone (AcAc). Magnesium alloy substrates were first fluorinated in 20% HF aqueous solution at room temperature for 20 h, then, the zirconia coating was deposited on the fluorinated sample by dip coating. Basing on the sol–gel process, a chelate complex from the reaction of zirconium coordinating AcAc was formed which was supported by UV–vis spectrum analysis. The result showed that the absorption peak could be seen for the sol at 308 nm, which was red-shifted by 36 nm from that of methanol form of AcAc (272 nm). Moreover, Fourier transform infrared (FT-IR) spectrum analysis was performed to examine the structural differences between the gel and AcAc. The results indicated that the chelate complex with a bidentante structure was formed through the interaction chemically between zirconium nitrate and AcAc. The surface morphology of the zirconia coating was characterized by scanning electron microscope (SEM), an uniform coating can be obtained on the fluorinated sample. The corrosion resistance of the substrate, the fluorinated with and without the zirconia coating in the 3.5 wt.% NaCl solution was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests, respectively. The results demonstrated that the zirconia coating could greatly improve the corrosion resistance of the AZ91D magnesium alloy. Furthermore, the effect of the different heat-treatment temperatures for the zirconia coating on corrosion resistance was also discussed.     

Corrosion behaviors of electroless plating Ni-P coatings deposited on magnesium alloys in artificial sweat solution

Magnesium alloys are the optimum shell materials for electronic products. These electronic products inevitably contact with the hands of users, and then are corroded by the sweat solution mainly consisting of 0.1% urea, 0.5% NaCl and 0.5% lactic acid. Electroless plating Ni-P coatings can provide protection to the magnesium alloys shell. The corrosion behaviors of Ni-P coatings deposited on AZ91D magnesium alloys in artificial sweat solution were investigated by electrochemical tests and SEM observations. The results indicated that urea acted as corrosion inhibitor. The inhibiting effect of urea was reduced in NaCl solution or lactic acid solution. NaCl and lactic acid were the main corrosive mediums, and their synergistic effect can significantly accelerate the corrosion of Ni-P coatings.     

镁及其合金的腐蚀与阳极化处理

综述了镁及其合金的腐蚀行为和阳极化处理工艺，较详细介绍了镁的电化学特性，合金元素和溶液介质对其腐蚀行为的影响，重点介绍了镁合金的阳极氧化膜的结构和组成及微弧氧化方面的研究进展。     

Localized electrochemical study of corrosion inhibition in microdefects on coated AZ31 magnesium alloy

In the present work corrosion inhibition in microdefects of protective coatings on magnesium alloy was studied by SVET (scanning vibrating electrode technique) and SIET (scanning ion-selective electrode technique) in 0.05 M NaCl. Mg²⁺- and pH-selective microelectrodes were developed to be used by SIET. The microelectrodes were characterized from the standpoint of properties important for corrosion applications, aiming at the reliable functioning during measurements. The combination of SVET and SIET demonstrated to be a useful approach to investigate the inhibition of corrosion processes in microdefects on coated AZ31. In this paper the corrosion inhibiting properties of 1,2,4,-triazole, F⁻ and Ce³⁺ on AZ31 alloy were analyzed. According to the results, 1,2,4-triazole in concentration of 0.01 M showed the highest inhibition efficiency among the studied inhibitors and was able to prevent the increase of pH in the corroding defects, by keeping the corrosion activity on a very low level during the tested immersion period.     

Pyrrole-based silane primer for corrosion protection of commercial Al alloys. Part II. Corrosion performance in neutral NaCl solution

A chromate-free, direct-to-metal treatment using pyrrole-based silane (PySi) was developed for protection against corrosion of as-received commercial Al alloys, following the typical procedure for silane deposition. The protection performance of composite PPySi films, containing polysiloxane linkages and polypyrrole units, was evaluated in near neutral NaCl solution by simple corrosion tests such as single-cycle anodic polarization, corrosion potential monitoring and long-term immersion experiments. Control coatings of polymethylsiloxane (PMeSi) and electrochemically synthesized polypyrrole (Ppy) were also studied. The superior performance of PPySi with respect to PMeSi and Ppy was attributed to highly crosslinked, well-packed and adherent composite films of thickness of the order of microns, manifesting both barrier action and active protection. The use of pyrrole-based silane for corrosion protection of Al alloys constitutes a promising approach for effective replacement of chromium-based treatments in practical applications. Further investigation from the fundamental point of view is deserved.     

Improvement of corrosion resistance of magnesium metal by rare earth elements

Mg metal containing rare earth metals (REs) can be electrowon directly by molten salt electrolysis. The clarification of the optimum RE content in Mg is necessary to fix the electrolytic conditions in the direct electrowinning of Mg with RE. From this point of view, effect of RE addition in Mg metal on its corrosion property was studied in detail in this study. The specimen was prepared by adding La, Nd, or Ce in melted Mg metal, and its corrosion resistance was examined by an immersion test in 3 mass%-NaCl solution at room temperature. The corrosion resistance of Mg was improved greatly by adding a small amount of RE, whereas the excess addition of RE deteriorated the corrosion resistance. The optimum RE content was about 0.5 mass%. In this study, the corrosion property of Mg with an artificial surface oxide layer was also studied to clarify the effect of surface oxide. The corrosion resistance of Mg was particularly strengthened by conversion coating in a solution including La(NO₃)₃, Nd(NO₃)₃, or Ce(NO₃)₃, with Mg(NO₃)₂. This result suggests that the surface oxide film consisting of both Mg and RE gives ideal corrosion resistance to Mg metal. Mg metal with conversion coating including RE should also be of use as a corrosion-resistant material.     

High corrosion protection of a polyaniline/organophilic montmorillonite coating for magnesium alloys

Epoxy coatings containing polyaniline (PANI) and polyaniline/organophilic montmorillonite (PANI/OMMT) powders were prepared on the surface of AZ91D magnesium alloy. The corrosion performance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS) and open-circuit potential analysis in 3.5% NaCl. The results indicate that the PANI/OMMT coating retained its high corrosion protection for AZ91D magnesium alloy after 6000 h of immersion. The protective mechanism conferred by the PANI/OMMT coating was also discussed. The effects of oxygen on the protective mechanism of PANI were evaluated by EIS measurements in a 3.5% deaerated NaCl solution.     

The corrosion behaviour of rare-earth containing magnesium alloys in borate buffer solution

In this work, the corrosion behaviour of magnesium alloys ZK31, EZ33 and WE54 was studied in sodium borate buffer solution at pH 9.2.The electrochemical processes were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The composition and morphology of the alloys and corrosion products formed were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).The experimental findings highlighted the differences in the corrosion mechanisms of the different alloys tested. The results showed that the presence of rare-earth elements (RE) only increases the corrosion resistance when present in solid solution, as is the case of the WE54 alloy. At pH 9.2, an amorphous yttrium oxide/hydroxide thick film was formed, which possesses greater stability when compared to magnesium oxide/hydroxide. The role of RE in the corrosion mechanism was discussed.     

Effect of Eu2O3 on sintering densification and corrosion resistance of magnesium aluminate spinel

The effect of the doping amount of Eu₂O₃ on the densification behaviour of magnesium aluminate spinel (MAS) and its corrosion resistance to aluminium electrolyte were studied. The relative density, phase composition, micro morphology and hardness of the sintered samples were characterised by Archimedes’ drainage method, X-ray diffractometer, scanning electron microscope and automatic micro Vickers hardness tester. Results showed that the doping of Eu₂O₃ was conducive to the densification of MgAl₂O₄. When the content of Eu₂O₃ was 3 wt.%, the relative density of MAS was the largest (99.32%), the microstructure was more compact and the hardness was the largest (2293.4 kgf/mm²). The MAS sample with 3 wt.% Eu₂O₃ had the best corrosion resistance to aluminium electrolyte, and the corrosion depth was 80.99 μm. It was speculated that the electrolyte may penetrate into the sample through the micropores, and the fluoride salt chemically reacted with MgAl₂O₄ to form Al₂O₃, NaF and MgF₂.     

Electrochemical noise analysis of the corrosion of AZ91D magnesium alloy in alkaline chloride solution

The corrosion behavior of AZ91D magnesium alloy in alkaline chloride solution was investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and power spectral density (PSD), had been employed to analyze the EN data. It was revealed that there exist three different stages of corrosion for AZ91D magnesium alloy in alkaline chloride solution, including an anodic dissolution process companying with the growth, absorption and desorption of hydrogen bubbles, a development of pitting corrosion, an inhibition process by protective MgH₂ film. The results demonstrated that energy distribution plot (EDP) was a powerful tool to provide useful information about the dominant process for the different corrosion stages.     

A calcium phosphate coating improving corrosion resistance of the biodegradable magnesium alloy with graphene oxide modifying the deposition

Biodegradable magnesium alloy is an ideal material for medical implant applications, but its application is limited by its rapid degradation. Therefore, it becomes the main goal to improve corrosion resistance. In this study, a calcium phosphate dihydrate/graphene oxide composite coating was designed on the AZ60 alloy for medical applications. A calcium phosphate dihydrate coating was first prepared by biomimetic deposition on the alkali pretreated magnesium alloy, and graphene oxide was dispersed in the solution to modify the deposition. The results showed that graphene oxide could not only alter the loose striped calcium phosphate coating to the compact flaked composite coating, but also enhance the corrosion resistance with a reduced corrosion current density by 2 orders of magnitude, an increased impedance by 3 orders of magnitude and a corrosion rate down to 7/20. The in vitro biocompatibility of the composite coating was also demonstrated by a series cell experiments, with a cell viability of 120%. The composite coating provides a feasible method to enhance the corrosion resistance and biocompatibility of magnesium alloys.     

Inhibition of magnesium localised corrosion in chloride containing electrolyte

An in situ Scanning Vibrating Electrode Technique (SVET) is used to investigate the inhibition of localised corrosion occurring on unpolarised magnesium (Mg) samples immersed in 5% (w/v) aqueous sodium chloride electrolyte. In uninhibited electrolyte, localised corrosion features taking the form of dark, radially expanding discs are shown to consist of a strongly cathodic interior, surrounded by a ring of anodic activity. A range of potential anionic inhibitors, selected on the basis of their ability to form insoluble precipitates with aqueous Mg²⁺ ions, are systematically investigated and compared with chromate additions. Of the anions evaluated only phosphate additions markedly decrease the extent of localised corrosion observed and provide a comparable level of inhibition to chromate. The onset of profound corrosion inhibition is achieved at threshold concentrations of 10⁻² and 2 × 10⁻⁴ mol dm⁻³ for chromate and phosphate additions respectively. Phosphate inhibition efficiency increases markedly when the pH of the electrolyte is decreased. Consequently at phosphate concentrations of ≥10⁻³ mol dm⁻³, in the pH 4-7 range, Mg is able to withstand over 4 h immersion with little evidence of breakdown. From the empirical evidence presented, it is proposed that phosphate principally acts to inhibit hydrogen evolution at focal cathodic sites, where elevated pH produces sufficient PO₄³⁻ speciation to form a surface precipitate by combining with free Mg²⁺.     

Electrochemical impedance spectroscopic investigation of the role of alkaline pre-treatment in corrosion resistance of a silane coating on magnesium alloy, ZE41

The protective performance of the coatings of bis-1,2-(triethoxysilyl) ethane (BTSE) on ZE41 magnesium alloy with different surface pre-treatments were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M sodium chloride solution. Electrical equivalent circuits were developed based upon hypothetical corrosion mechanisms and simulated to correspond to the experimental data. The morphology and cross section of the alloy subjected to different pre-treatments and coatings were characterized using scanning electron microscope. A specific alkaline pre-treatment of the substrate prior to the coating has been found to improve the corrosion resistance of the alloy.     

Corrosion study of biodegradable magnesium based 1393 bioactive glass in simulated body fluid

The present work deals with study of corrosion behavior of Magnesium based biodegradable alloy added with 1393 bioactive glass in simulated body fluid, which can be used as an orthopedic implant or bone fixation. Various efforts had been done for development of magnesium based alloy but the main problem observed was high corrosion rate. Our work consists of developing a Magnesium based alloy with addition of Zinc, Aluminum and Calcium along with 15% 1393 bio-active glass. The alloying elements have their own advantages associated with them. Further the alloy has been studied for the corrosion behavior after 1 days, 3days, 5 days, 7 days and 14 days. Samples have been tested for their corrosion behavior in simulated body fluid and their corrosion rate has been observed by weight loss method and electro potentio-dynamic tests. Atomic Absorption Spectrophotometry (AAS) of the samples has been done in order to get more precise results. Young's Modulus of the alloy has been found to be in the range of 33.80 GPa–46.72 GPa, which is near to the human bone. Also the surface morphology and phase analysis of the samples has been done by Scanning Electron Microscopy and X-Ray Diffraction Technique. Initial corrosion rate of the sample is fast but after 7 days the decrease in corrosion behavior has been observed. SEM images of the samples have confirmed the formation of hydroxyapatite layer and the X-Ray Diffraction of corrosion products have shown the formation of complexes of Magnesium on the surface.     

Improved corrosion resistance of AZ91D magnesium alloy coated by novel cold-sprayed Zn-HA/Zn double-layer coatings

In order to improve the corrosion resistance and bioactivity of biodegradable Mg alloy substrate, novel Zn-HA/Zn double-layer coatings with different HA/Zn ratios in weight were deposited on AZ91D substrates by cold spraying. Phase compositions and microstructures of as-sprayed coatings and coatings after corrosion tests were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Electrochemical corrosion behaviors of both Zn-HA/Zn double-layer coatings were investigated in Hanks’ simulated body fluid using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that both the pure Zn coating and HA/Zn composite coatings presented the similar phase compositions with their primary powders in addition to Zn oxidizations. Zn powders were plastically deformed and partially oxidized due to its low melting point, while HA powders were mainly crashed into fragments and hill-like splats. Both Zn under layer and HA/Zn upper layer were well bonded and presented dense structures, differences in HA/Zn upper layers were related to the HA/Zn ratios. Potentiodynamic polarization and EIS measurements illustrated that the cold-sprayed Zn-HA/Zn double-layer coatings not only improve the corrosion resistance of Mg alloy substrates, but also enhance its bioactivity due to the HA existed in composite upper layer.     

Corrosion protection of AZ91D magnesium alloy with sol–gel coating containing 2-methyl piperidine

Sol–gel coatings were prepared using 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TMOS) as precursors, diethylentriamine as curing agent. Inhibition effect of 2-methyl piperidine on AZ91D in 0.005%, 0.05% and 0.5% (wt.%) NaCl solution is investigated. Potentiodynamic polarization tests revealed that 2-methyl piperidine significantly decreased the anodic activity of AZ91D especially at high concentration. Corrosion behaviors of sol–gel coatings incorporated with 2-methyl piperidine on AZ91D in the Harrison's solution were analyzed using electrochemical impedance spectroscopy (EIS). EIS results showed the corrosion resistance of sol–gel coatings and sol–gel sealed phosphate conversion coating on AZ91D were significantly improved through addition of 2-methyl piperidine. Fourier transform infrared spectra (FT-IR) confirmed that the 2-methyl piperidine was compatible with sol–gel matrix.