On the corrosion of binary magnesium-rare earth alloys

The corrosion properties of high-pressure die cast (HPDC) magnesium-rare earth (RE) based alloys have been studied. Binary additions of La, Ce and Nd to commercially pure Mg were made up to a nominal 6 wt.%. It was found that the intermetallic phases formed in the eutectic were Mg₁₂La, Mg₁₂Ce and Mg₃Nd, respectively. Results indicated that increasing RE alloying additions systematically increased corrosion rates. This was also described in the context of the electrochemical response of Mg-RE intermetallics - which were independently assessed by the electrochemical microcapillary technique.This study is a discrete effort towards revealing the electrochemical effect of carefully controlled binary alloying additions to magnesium in order to elucidate the microstructure-corrosion relationship more generally for HPDC Mg alloys. Such fundamental information is seen to not only be useful in understanding the corrosion of alloys which presently contain RE additions, but may be exploited in the design of magnesium alloys with more predictable corrosion behaviour. There is a special need to understand this relationship - particularly for magnesium that commonly displays poor corrosion resistance.     

Electrochemical behaviour and corrosion of Mg–Y alloys

Corrosion of cast magnesium–yttrium (Y) alloys with systematic Y additions up to a nominal 18 wt.% were studied. Corrosion performance was related to the quantitative alloy microstructure and found to increase significantly with the level of alloying and volume fraction of the Mg–Y intermetallic present. In the alloy microstructures, Mg₂₄Y₅ was principally formed; the electrochemistry of which was characterised using the electrochemical microcell method. Electrochemical testing revealed the fundamental corrosion behaviour of Mg–Y alloys and elucidated the corrosion mechanisms at play.     

Electrochemical investigation of the influence of laser surface melting on the microstructure and corrosion behaviour of ZE41 magnesium alloy – An EIS based study

Surface melting of a magnesium alloy, ZE41 (4%-Zn, 1%-RE) was performed to achieve electrochemical homogeneity at the surface by microstructure refinement. Large secondary precipitates are particularly known to cause severe pitting in magnesium alloys. The corrosion resistance of the laser treated and untreated alloy was investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy. Contrary to the reported behaviour of other magnesium alloys (such as AZ series alloys), laser surface melting did not significantly improve the corrosion resistance of ZE41. This observation is attributed to the absence of beneficial alloying elements such as Al in ZE41 alloy.     

On the degradation mechanism of corrosion protective poly(ether imide) coatings on magnesium AZ31 alloy

In previous publications of the authors, good performance of poly(ether imide) as corrosion protective coatings for magnesium AZ31 alloy was reported. It was suggested that during the sample degradation magnesium hydroxide could react with the imide ring forming magnesium polyamate and polyamic acid, but this could not be experimentally confirmed. In the present letter, we confirm the occurrence of this reaction by infrared and X-ray photoelectron spectroscopy and discuss its influence in the corrosion behavior observed in electrochemical impedance spectroscopy tests.     

The influence of the microstructure on the atmospheric corrosion behaviour of magnesium alloys AZ91D and AM50

Even though magnesium, as a structure metal, is most commonly used in an atmospheric environment, most investigations of magnesium are performed in solution. In the present work the atmospheric corrosion of two commonly used magnesium alloys, AZ91D and AM50, has been investigated from the initial stages up to the most severe forms of corrosion. A detailed investigation of the morphology of a corrosion attack and its development over time shows that the atmospheric corrosion mechanism is similar for the two alloys. Based on these findings a schematic model of the initial atmospheric corrosion attack on AZ91D is presented and discussed.     

The influence of zirconium additions on the corrosion of magnesium

Sixteen custom binary Mg–Zr alloys and four commercial Zr-containing Mg-alloys were used to investigate the role of Zr on the corrosion of Mg. Mg–Zr alloys were manufactured with a range of different Zr concentrations. It was observed that the Mg–Zr alloys with a smaller mean Zr particle size had more Zr dissolved in solid solution. Both the Zr in solid solution and in metallic particle form were observed to have a deleterious effect on the corrosion rate of Mg. However, this deleterious effect is less pronounced to effect in alloys with multiple alloying additions.     

Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy

The electrochemical behaviour of an AZ91 magnesium alloy was investigated in a low aggressive medium by means of local and global electrochemical impedance measurements. The results were compared to those obtained on a pure magnesium sample. It was possible to show the individual contribution of each phase constituting the alloy, and it was observed that the corrosion mechanism of the two phases was the same, controlled by the Mg dissolution. However, local impedance diagrams clearly indicated that the kinetics dissolution of the β-phase was slower than that of the α-phase, which was in good agreement with SEM observations.     

The effect of epoxy coating containing emeraldine base and hydrofluoric acid doped polyaniline on the corrosion protection of AZ91D magnesium alloy

Corrosion protection of epoxy coatings containing emeraldine base polyaniline (PANI) or hydrofluoric acid doped PANI on AZ91D magnesium alloy were studied by EIS and Pull-Off Adhesion Test. The results indicated that the addition of emeraldine base PANI or hydrofluoric acid doped PANI could improve the corrosion resistance of epoxy coating. The epoxy coating containing hydrofluoric acid doped PANI had the best performance of the corrosion protection among three systems under investigation. The corrosion product film was analyzed by XPS indicating that PANI changed the chemical structure of the corrosion film. The protective mechanism imparted by PANI was discussed.     

Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions

This paper studies the corrosion rate of magnesium alloys AZ31 and AZ61 exposed in humid air under continuous condensation conditions. The shape of the gravimetric curves for corrosion progress suggests that the process is controlled by factors related with the corrosion product layer growing on the metallic surface according to gravimetric results there is an initial period in which only a small part of the corroded metal is incorporated in the corrosion product layer, but after longer testing times the proportion of metal that comes to form part of this layer tends to increase very significantly.     

Influence of microstructure on corrosion properties of multilayer Mg–Al intermetallic compound coating

A multilayer Mg–Al intermetallic coating was fabricated on the Mg alloy in molten salts at 400 °C with treatment time range from 2 to 8 h. The coating consists of a single Al₁₂Mg₁₇ intermetallic layer or Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ intermetallic layers. The corrosion resistance of the coating which is obtained at 400 °C for 2 h is the best. When the treatment time is higher than 2 h, some cracks developed in the layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the substrate and the intermetallic layer during the rapid cooling process.     

Comparative study on the corrosion behavior of milled and unmilled magnesium by electrochemical impedance spectroscopy

The corrosion behavior of milled Mg prepared by high-energy ball milling for 10 h has been investigated in alkaline solutions by electrochemical impedance spectroscopy and compared with that of unmilled Mg. X-ray powder diffraction indicates a crystallite size of 34 nm for the milled Mg compared to >100 nm for the unmilled powder. Chemical analyses show no significant iron contamination in milled Mg powder, indicating the absence of tools erosion during the milling procedure. In contrast, significant MgO enrichment in the milled powder is observed (6.5 wt.% after 10 h milling compared to 1.0 wt.% before milling). The oxygen contamination is mainly attributed to the powder oxidation occurring during milling. From XPS analyses, no MgO enrichment is detected on milled Mg electrode surface, confirming that MgO is dispersed homogeneously in the bulk of the material rather than to segregate on its surface. Electrochemical impedance spectroscopy demonstrates clearly the better corrosion resistance of milled Mg compared to unmilled Mg in passive conditions (KOH solution, pH=14) and in more active corrosion conditions (borate solution, pH=8.4). This is illustrated by a nobler corrosion potential and by a significant increase of the interfacial resistance related to the film and charge-transfer reaction. Moreover, the variation of the different electrochemical parameters (corrosion potential, interfacial resistance and capacitance) with immersion time is less accentuated and tends more rapidly to a steady state with milled Mg, suggesting an enhancement of the Mg(OH)₂ formation kinetic. The origin of the distinctive passivation behavior of ball-milled Mg is discussed.     

An investigation of the corrosion mechanisms of WE43 Mg alloy in a modified simulated body fluid solution: The influence of immersion time

The corrosion mechanisms and kinetics of WE43 Mg alloy in a modified simulated body fluid (m-SBF) are investigated by electrochemical, hydrogen evolution and analytical techniques. The changes in the impedance response over time are related to four corrosion stages involving the formation of a partially protective corrosion layer and adsorption of Mg intermediates, formation of an inner passive MgO layer with increasing coverage over time, rupture of the corrosion layer and lateral growth of stable pits. ATR-FTIR, XRD and XPS results show the presence of an amorphous carbonated apatite/Mg(OH)₂ mixed corrosion layer.     

Heat treatment effect on the corrosion behaviour of black passivated ZnNi alloys

Electrochemical corrosion behaviour of ZnNi and black passivated ZnNi alloys in aerated and partially deaerated 5% NaCl solution was studied using dc and ac techniques. The heat treatment (24 h, 120 °C) effect on black CCC properties was analyzed. No changes in the R_p values were observed, but differences in the potentiodynamic curves and in EIS diagrams suggested a change in the corrosion mechanism. Comparative studies by AFM and electrochemical techniques showed that these changes in the corrosion behaviour could be explained by taking into account the different effect of heat treatment on the inner and outer layers of the CCC.     

Investigation of the corrosion behaviour of laser-TIG hybrid welded Mg alloys

The paper presents the corrosion behaviour of the laser-tungsten inert gas welded Mg alloy. The effects of microstructure variations of Mg alloy joint on the corrosion behaviour and reliabilities of joint are investigated. The results demonstrate that the effects of some weld defects and precipitated phases on the corrosion behaviour of weld joint are very little, and corrosion resistance of joint is predominantly influenced by grain refinement or interactions of grain refinement and continued net-shaped β phases. Moreover, the corrosion resistance of weld joints and welding mode (butt and lap joint) keep a close relation, which must not be ignored.     

Some particularities of the corrosion behaviour of Mg-Zn-Mn-Si-Ca alloys in alkaline chloride solutions

The influence of silicon and calcium on corrosion behaviour of Mg-6Zn-Mn alloys was investigated in alkaline solutions. The corrosion behaviour of the alloys with varied silicon and calcium content mainly depends on volume fractions and morphology of the intermetallics. In Cl-ion-free solutions silicon increased the corrosion resistance of alloys causing the growth of the surface passive film mostly composed of hydroxide products. Calcium had caused trouble to the formation of the stable protective film through the formation of Ca-Mg-Si intermetallics. In Cl-ion containing solutions intermetallic Ca-Mg-Si phase was responsible for extreme initial corrosion of Mg-6Zn-Mn-Si-Ca alloys.     

Effect of alternating voltage treatment on the corrosion resistance of pure magnesium

Pure magnesium was treated by alternating voltage (AV) treatment technique. The optimal AV-treatment parameters for greatly improving corrosion resistance were determined by the orthogonal experiments. Polarization curves, electrochemical impedance spectroscopy (EIS), and scanning electrochemical microscopy (SECM) were performed to understand the effect of AV-treatment on the corrosion resistance of pure magnesium. AFM, contact angle measurement and XPS were employed to further investigate the influence of AV-treatment on the properties of the surface film formed on pure magnesium after AV-treatment. The results showed that a uniform and stable film was formed and the corrosion resistance of pure magnesium was greatly improved after AV-treatment. This was caused by the noticeable change of chemical structure and semi conducting properties of surface film after AV-treatment.     

Enhancement of corrosion resistance of Mg-9 wt.% Al-1 wt.% Zn alloy by a calcite (CaCO3) conversion hard coating

A calcite (CaCO₃) coating on Mg alloy, formed by chemical conversion treatment, was investigated. Aqueous with Ca²⁺ concentration of ∼220 ppm was employed in the chemical conversion treatment. Cross-sectional microstructures of the coated sample after 2 h of treatment revealed a two-layer coating structure. The corrosion current density (I_corr) of the coated sample was approximately two orders of magnitude lower than that of the untreated sample. Electrochemical impedance spectroscopy (EIS) and an appropriate equivalent circuit suggested that each of the layers of the two-layer coating effectively protects Mg alloy against corrosion.     

Effect of excimer laser surface melting on the microstructure and corrosion performance of the die cast AZ91D magnesium alloy

Excimer laser surface melting (LSM) of the die cast AZ91D alloy has been investigated in terms of microstructure and corrosion behaviour. Excimer LSM of the alloy resulted in a highly homogeneous and refined melted microstructure, which improved the corrosion resistance of the alloy. The latter was associated with the large dissolution of intermetallic phases and the enrichment of aluminium within the melted layer. An increased number of laser pulses resulted in thicker melted layers, but also in enhanced porosity and the formation of micro-cracks at the overlapping area. Both factors diminished the corrosion resistance of the laser-treated alloy.