Influence of the β phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the β phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of α + β with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating.     

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.     

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.     

Electroless Ni-Sn-P coating on AZ91D magnesium alloy and its corrosion resistance

An electroless Ni-Sn-P coating was deposited on AZ91D magnesium alloy in an alkaline-citrate-based bath where nickel sulphate and sodium stannate were used as metal ion sources and sodium hypophosphite was used as a reducing agent. The phase structure of the coating was amorphous. SEM and attached EDS observation revealed the presence of dense and uniform nodules in the ternary coating and the content of tin was 2.48wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution proved that the ternary Ni-Sn-P coating exhibited better corrosion resistance than the Ni-P coating in protecting the magnesium alloy substrate.     

Preparation of MgO coatings on magnesium alloys for corrosion protection

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

Evaluation of corrosion resistance of magnesium alloys in radiator coolants

Abstract

Coolant corrosion is a major drawback for the use of magnesium alloys in engine and cooling system, but the coolant is not normally intended to prevent corrosion of magnesium alloys. This research assessed the corrosion performance of two magnesium alloys, AZ91D and AM50A, in two newly formulated radiator coolants using immersion test, potentiodynamic polarisation test, and corroded surface analysis. Two coolants were named as Irgacool Plus L and Irgacool Plus S. C7, C8-organic acids and polycarboxylic acid were the main inhibitor species in Irgacool Plus L while Irgacool Plus S was formulated with C7, C8-organic acids and sebacic acid inhibitors. Corrosion rates of magnesium alloys decreased twice in Irgacool Plus L compared with Irgacool Plus S. AZ91D alloy had better corrosion resistance than AM50A alloy in both radiator coolants. Both alloys suffered corrosion due to microgalvanic coupling between cathodic β-Mg₁₇Al₁₂ intermetallic and anodic α-Mg matrix, and the presence of Al₈Mn₅ and Al₁₁Mn₄ intermetallics in AM50A led to further microgalvanic corrosion. A continuous network of β-Mg₁₇Al₁₂ phase and higher Al content α-Mg matrix accounted for better corrosion resistance of AZ91D alloy.     

Chemical conversion coating on AZ31B magnesium alloy and its corrosion tendency

The morphology change of the magnesium matrix after pre-treatment and the mor-phology as well as the phase composition of chemical conversion coating formed by phosphate were studied using scanning electron microscope and X-ray diffraction. The corrosion resistance of the coating was studied by salt spray and damp test, and the corrosion tendency during salt immersion test was analyzed. The results show that the phase composition before and after pre-treatment is almost change- less, and the deep microflaw appears between α and β phases during acidic pickling. The phosphate conversion coating is mainly composed of Mg, MgO, and some amor-phous phase, and it can provide a good protection for the AZ31B alloy. Results from corrosive morphology indicate that the growth and the corrosion resistance of the phosphate conversion coating are related to the forming process of the AZ31B matrix.     

Influence of magnesium nitrate on the corrosion performance of sol-gel coated AA2024-T3 aluminium alloy

Traditional anticorrosion technology has relied heavily on using reducible metal species, predominantly hexavalent chromium (Cr(VI)), for protecting reactive metal alloys such as aluminium which is extensively used in the aerospace sector. However, the impending changes in the use of Cr(VI) in Europe and the United States have forced aerospace manufacturers to examine alternative materials for protecting aluminium. One of the most promising alternatives being investigated are organosilane based sol-gels containing anticorrosion additives. In this work the anticorrosion properties of magnesium (II) nitrate (Mg(NO₃)₂) as an inhibitor were investigated at different concentrations (0.1%-1.0 wt.%) in a methyltrimethoxysilane (MTEOS) sol-gel on the aluminium alloy AA 2024-T3 and compared to Alodine^TM 1200 (the established Cr(VI) pre-treatment). Electrochemical evaluation of the coating system by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) measurements correlated strongly with results obtained from Neutral Salt Spray (NSS) exposure data. The surface morphology of the coating was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS). The results indicated that the optimum performance was achieved with a Mg (NO₃)₂ level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg²⁺ rich sol-gel are due to the pore blocking mechanism of insoluble magnesium precipitates formed during the hydrolysis process.     

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.     

Influence of anodising current on the corrosion resistance of anodised AZ91D magnesium alloy

The thickness, chemical composition and microstructure of anodised coatings formed on magnesium alloy AZ91D at various anodising current densities were measured. It was found that all these parameters could be affected by anodising current density, and hence the coatings formed at different anodising current densities had different corrosion resistances. This suggests that the corrosion performance of an anodised coating could be improved if a properly designed current waveform is used for anodising. In addition, based on the experimental results, some physical, chemical and electrochemical reactions involved in the anodising process were proposed to explain the anodising behaviour in this paper.     

Correlation between texture and corrosion properties of magnesium coatings produced by PVD

Physical vapour deposition with energetic ions is an established technology for creating functional surfaces where changing morphologies are observed with increasing energy deposition. In this presentation, magnetron sputtering (MS) is compared with ion beam sputtering (IBS) and vacuum arc deposition (VAD) for corrosion resistant Mg coatings. With increasing average energy flux along the three methods, a transition from a columnar growth regime towards a layer-by-layer growth at increased energies was observed, while a basal texture with the c-axis normal to the surface was found in all cases. However, the full width at half maximum (FWHM) of the corresponding Mg(002) rocking curves showed a pronounced minimum of 3° for IBS deposited films, apparently caused by the reflected high energy primary Ar⁺ ions. For pure Mg films, no larger differences in the corrosion potential and the corrosion rates were measured.     

Electroless Ni-P coating of different magnesium alloys

Coating of AZ31B, AE 42 and ZRE1 wrought magnesium alloys was carried out using electroless Ni plating technique in a solution of nickel sulphate, sodium hypophosphite, ammonium hydrogen fluoride and glycine with a zinc immersion pre-treatment.The results of SEM/EDX investigations and X-ray diffraction indicate that the coat exhibit a typical surface morphology with compact nodules with good adherence to the substrate. The coat was composed of amorphous structure, which transformed to a mixture of crystalline Ni and Ni₃P precipitates after heat treatment at 673°K for 1 h. The phosphorous (P) content increased gradually from the substrate towards the surface reaching a maximum of 10 wt.% to 18 wt.% on the surface depending on the substrate alloy and the thickness of deposit. The hardness of the coat was found to increase with the P content and also after heat treatment. The electrochemical corrosion test in NaCl solution indicated a great improvement in the corrosion resistance of the Mg substrates and that a noble behaviour of Ni-P was obtained regardless of the heat treatment process. The forming ability test indicates that hot rolling of the coated substrate does not succeed to keep a continuous coat due to cracking of the coat in both as-coated and heat treated specimens.     

Control of biodegradation of biocompatable magnesium alloys

By utilising its rapid corrosion reaction and controlling its degradation process through Zn and Mn alloying, purification and anodization, chemically active magnesium can be developed into a biodegradable biocompatible implant material with a specific biodegradation process and tolerable hydrogen evolution rate, which may replace current problematic biodegradable polymers in applications.     

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.     

New engine coolant for corrosion protection of magnesium alloys

The global trend toward decreasing of atmospheric pollution, by saving fuel consumption in vehicles, has led to extensive interest of using lightweight metals such as magnesium alloys, in engine and cooling system components. The modern coolant is not intended to prevent corrosion of magnesium alloy in the engine cooling systems. We have developed a new coolant that aims to protect Mg alloy parts together with all other commonly used metals. Several inhibitor formulations were tested, according to glassware test (ASTM D1384) and heat transfer conditions (ASTM D4340). Mg alloys EZ33 and WE43 were added to the standard sets of metals and the corrosivity of different types of formulations was determined by weight loss measurements. The new anticorrosive coolant showed high performance in all tested metals including magnesium alloys and it satisfied the requirements (ASTM D3306). Cyclic potentiodynamic polarization curves have been used to study electrochemical corrosion behavior of the magnesium alloys EZ33 and WE43 in aqueous solution containing the inhibitors and ethylene glycol (33 vol%‐EG prepared with corrosive water according to ASTM D1384) and compared to a reference coolant with no inhibitor. It was found that a passive film was created upon the Mg alloys, which exhibited high corrosion resistance against pitting.     

Influence of interlayers on corrosion resistance of diamond-like carbon coating on magnesium alloy

Diamond-like carbon coating (DLC) was deposited on AZ31 magnesium alloy by ion beam deposition technique in this study. A columnar Cr layer with a (110) preferred texture and a columnar CrN layer with a (111) preferred texture were applied as interlayers in the DLC coating/AZ31 substrate systems. The addition of these interlayers improved the adhesion between coating and substrate effectively, but did not enhance the corrosion resistance of the DLC/AZ31 systems due to the formation of galvanic cell between substrate and interlayer in the region of through-thickness defects in 3.5 wt.% NaCl solution. In addition, the effect of bias voltage on the corrosion resistance of CrN/Cr coatings on magnesium alloys was investigated. Although the application of bias voltage induced the coating denser, it was still difficult for CrN/Cr coating to reduce the corrosion current density of AZ31 due to the large difference between coating and substrate in galvanic series.     

Characterization of AC PEO coatings on magnesium alloys

Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys. The findings revealed initiation and gradual increase in the number of discharges after 2-4 ms of each anodic pulse once a critical voltage was reached. No discharges were observed during the cathodic half-cycles. The lifetimes of discharges were in the range of 0.05-4 ms. A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270-450 HV_0.05. The coatings grew at a rate in the range 4.0-7.5 µm min^− 1, depending on the substrate composition. Regardless of the substrate, the coatings consisted of MgO and Mg₂SiO₄, with incorporation of alloying element species. Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20-40% of the coating thickness. A thin barrier layer consisting of polycrystalline MgO was located next to the alloy. The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 3.5 wt.% NaCl was reduced by 2-4 orders of magnitude by the PEO treatment.     

2-Hydroxy-4-methoxy-acetophenone as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy

The inhibition behavior of 2-hydroxy-4-methoxy-acetophenone (paeonol) as an environment-friendly corrosion inhibitor for AZ91D magnesium alloy was investigated in 0.05 wt.% NaCl solution by means of polarization curve, AC impedance, weight loss measurement, scanning electron microscopy, Fourier transformation infrared spectroscopy, ultraviolet analysis, and computer molecular simulation. The results show that paeonol can inhibit the corrosion of AZ91D. The maximum inhibition efficiency is achieved when paeonol concentration is 50 ppm by weight in this study. It is proposed that paeonol chelates with Mg to form a paeonol-Mg complex mixing with the original Mg(OH)₂ film on the surface to inhibit the anodic dissolution of AZ91D.     

Characterization and corrosion studies of fluoride conversion coating on degradable Mg implants

Fluoride conversion coating was synthesized on magnesium (Mg) by immersion treatment in hydrofluoric acid (HF) at room temperature, with the aim of improving the corrosion resistance of Mg in applications as degradable implant material. After an immersion period of 24 h in 48% HF, the samples carried a bronze color, and the conversion coating was dense and free of cracks. Field-emission scanning-electron microscopy (FE-SEM) of the cross-section revealed a coating thickness of about 1.5 μm. Atomic-force microscopy (AFM) recorded an average surface roughness of ∼ 21 nm for the coated sample, similar to that of the untreated one (∼ 17 nm). The coating was mainly composed of magnesium fluoride (MgF₂) as identified by thin-film X-ray diffractometry (TF-XRD), consistent with compositional analysis using X-ray photoelectron spectroscopy (XPS). The MgF₂ was in the form of crystallites of a few nm. A small amount of oxygen was present inside the coating, suggesting that some F⁻ ions are replaced by hydroxyl (OH⁻) ions in the MgF₂ structure, or that a small amount of Mg(OH)₂ was present. The corrosion resistance of untreated and conversion coated Mg in Hanks' solution was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization tests, and immersion tests. EIS results showed a polarization resistance of 0.18 kΩ cm² for the untreated Mg and 5.2 kΩ cm² for the coated sample, giving an improvement of about 30 times. Polarization tests also recorded a reduction in corrosion current density from 400 μA/cm² to 10 μA/cm², showing an improvement of about 40 times. The galvanic effect between untreated and fluoride-coated Mg samples was small. Immersion tests in Hanks' solution also resulted in a much milder and more uniform corrosion damage on the fluoride-coated samples. The results of the present study showed that fluoride coating by conversion treatment is a simple and promising way of enhancing the corrosion resistance of Mg in Hanks' solution, or that it may be employed as a pretreatment step for subsequent coating.     

The in vivo and in vitro corrosion of high-purity magnesium and magnesium alloys WZ21 and AZ91

Corrosion was studied in vitro in Nor’s solution (CO₂ – bicarbonate buffered Hank’s solution) at 37 °C, and in vivo implanted in the lower back muscle of rats. Nor’s solution is a good model for HP Mg and WZ21, because (i) the pH is maintained by the same buffer as in blood and (ii) concentrations of corrosive chloride ions, and other inorganic constituents, are similar to those in blood. The higher in vitro corrosion rate of AZ91 was caused by micro-galvanic from second phases. The lower in vivo corrosion rate of AZ91 was tentatively attributed to suppression of micro-galvanic corrosion by tissue encapsulation.