Effect of cooling rate on oxidation resistance and powder ignition temperature of AM50 alloy with addition of yttrium

This paper focused on the effect of cooling rate on oxidation resistance and ignition temperature (T_i) of AM50 alloy. Y addition of 0.0 wt%, 0.15 wt%, 0.28 wt%, 0.45 wt% and 1.00 wt%, respectively was added to the AM50 alloy. The result showed that the oxidation resistance was directly affected by the microstructure. Rapid solidification (RS) had a positive effect on improving the oxidation resistance. It is noticeable that no Al₂Y intermetallic compound was found in the microstructure after RS. Elemental Y dissolved in the solid solution increased with increasing Y addition after RS. It is confirmed that Y addition dissolved in the solid solution and phase distribution were key factors for improving the oxidation resistance.     

Effect of yttrium addition on the oxide scale of AM50 magnesium alloy

This paper focused on the effect of yttrium (Y) addition on oxidation resistance of AM50 alloy. The oxide scale on the surface was studied. Elemental Y in solid solution played an important role in improvement of oxidation resistance and formation of oxide scale. Yttrium (0.28 wt%) addition in AM50 alloy could greatly enhance the oxidation resistance. Furthermore, with increasing Y addition, the scale/substrate adherence was increased. However, when Y addition exceeded 0.28 wt%, it was decreased.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Effect of dysprosium addition on the cyclic oxidation behaviour of CoNiCrAlY alloy

The cyclic oxidation behaviour of a Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without the addition of 0.2 wt.% dysprosium was investigated at 800 and 1100 °C in static laboratory air. The Dy-containing alloy showed a faster θ- to α-alumina transformation and significantly less weight gain than Dy-free alloy. Under cyclic oxidation at 1100 °C, Dy addition produced a continuous and protective Al₂O₃ scale. The Dy-free alloy exhibited poor oxidation resistance. Scale spallation led to the development of a complex oxide scale and internal precipitation: (Al,Cr)₂O₃ on the surface, followed by a Al₂O₃ layer, then (Al,N) precipitates alone beneath the external scale.     

Tracking the corrosion of magnesium sand cast AM50 alloy in chloride environments

An experimental procedure for tracking corrosion on lightweight alloys has been developed using a combination of microscopy and corrosion studies using commercial sand cast magnesium AM50 alloys. Corrosion penetration depths were measured and characterized with CLSM and SEM/XEDS, respectively. Corrosion depths on α-grains in the alloys were expressed as a function of their Al content. Al-rich β-phases and eutectic α-phase microstructures were observed to be most corrosion resistant due to an enrichment of Al, identified with TEM, near the oxide/alloy interface. Sand cast alloys were found to be susceptible to major corrosion events in regions with depleted Al content.     

Corrosion behavior of the composite ceramic coating containing zirconium oxides on AM30 magnesium alloy by plasma electrolytic oxidation

The deterioration process of a plasma electrolytic oxidation (PEO) coating containing zirconium oxides on AM30 magnesium alloy in 3.5 wt.% NaCl solution have been investigated. The coating consists of an outer porous layer and an inner dense layer. The content of MgF₂ is high in the pores and an MgO-rich layer is evident in the inner layer. The corrosion resistance of the outer layer gradually decreases in the initial immersion stage (96 h) due to the decomposition of MgO, and the deterioration of the inner layer is delayed by the blocking effect of the outer layer.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

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.     

Improvement of the oxidation resistance of Tribaloy T-800 alloy by the additions of yttrium and aluminium

The microstructures and oxidation behaviour of the modified Tribaloy T-800 alloys by additions of yttrium and yttrium plus aluminium have been studied. At the presence of yttrium alone, the oxidation rate decreased, and the selective oxidation of chromium was promoted, which was related to the refinement of alloy phase size. The addition of yttrium plus aluminium further reduced the oxidation rate. The selective oxidation of chromium and aluminium were both promoted significantly. The benefits were especially pronounced at 1000 °C, with the formation of protective alumina external layer and no internal oxides, which may be detrimental to the alloy mechanical property.     

Corrosion of friction stir welded magnesium alloy AM50

The microstructure of a friction stir welded magnesium alloy AM50 was examined by means of optical light microscopy. The chemical composition, particularly the iron content, and morphology of the oxide film were analyzed and discerned via auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Corrosion behaviour of the welds and base materials were investigated by virtue of neutral salt spray tests and potentiodynamic polarization measurements in conventional cells and in a mini cell. The results demonstrate that minor increases in iron concentration as might be speculated to occur as a consequence of tool/work piece interaction during the welding process on the corrosion resistance of the weld can be ignored. The corrosion morphology was predominantly influenced by the distribution of the Mg₁₇Al₁₂ phase. Here, it was also found that the corrosion resistance of the friction stir weld varied in response to changes in the joint microstructure.     

High temperature oxidation of AZ91D magnesium alloy granule during in-situ melting

Oxidation behaviour of AZ91D was investigated by heating the alloy granules in a ceramic mould between 650 and 800 °C, for 30 and 60 min. The granules failed to melt in unprotected environment even when the temperature was increased to 800 °C. Raising the temperature increased the oxides thickness linearly, however, oxidation enhanced beyond 750 °C with severe mould–metal reaction and selective oxidation. Heating duration showed more pronounced effect on oxide formation compared to heating temperature. MgO was found to be the dominant compound in oxidation products. Aluminium participated during severe oxidation or combustion to some extent whilst no zinc oxide was detected.     

Formation of self-repairing anodized film on ACM522 magnesium alloy by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) on a die-casting ACM522 Mg alloy was conducted in aqueous silicate and phosphate solutions. The corrosion behavior of the anodized ACM522 Mg alloy was investigated in detail. During the investigation, the self-repairing behavior of the anodized films was evaluated by a salt spray test for 168 h, and the mechanism of this self-repairing was discussed in terms of thermodynamic equilibrium constants. Furthermore, the effects of additives to the phosphate solution on the self-repairing behavior were examined, and the addition of Na₂B₄O₇ was found to effectively reduce cracks on a self-repaired film.     

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.     

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.     

Interactive effect of cerium and aluminum on the ignition point and the oxidation resistance of magnesium alloy

This paper focused on the interactive effect of cerium (Ce) addition and aluminum (Al) content in magnesium alloy on ignition point and oxidation resistance. Ce content played an important role in improving the oxidation resistance of Mg alloy. Ignition point ascended with increasing Ce content. 0.25 wt% Ce content in Mg alloys could greatly improve tightness of the oxide film of Mg alloys. However, when Ce content in the alloy exceeded its solid solubility, ignition point descended. Furthermore, Al content in the alloy also influenced the ignition point. The higher the Al content was, the lower the ignition point.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

Effect of second phases on the corrosion behaviour of wrought Mg-Zn-Y-Zr alloy

The corrosion behaviour of wrought Mg-Zn-Y-Zr alloy was investigated by corrosion morphology observation and electrochemical measurement. The results indicate that the corrosion process can be divided into three stages, corresponding to three types of corrosion features. At the initial stage, corrosion occurred surrounding the second phases, which was driven by galvanic couple effect; at the middle stage, filiform corrosion was found in the central regions of α Mg matrix; at the final stage, pitting corrosion presented around the second phases. The second phases have a great effect on the corrosion process of Mg-Zn-Y-Zr alloy.     

Improvement of corrosion resistance of AZ91D magnesium alloy by holmium addition

The corrosion behaviour of two Mg–9Al–Ho alloys (Mg–9Al–0.24Ho and Mg–9Al–0.44Ho) was evaluated by general corrosion measurements and electrochemical methods in 3.5% NaCl solution saturated with Mg(OH)₂. The experimental results were compared with that of Mg–9Al alloy without Ho addition. Various corrosion rate tests showed that the addition of Ho obviously enhanced corrosion resistance of Mg–9Al alloy. The microstructure of the three magnesium alloys and the morphology of their corrosion product film were examined by Electron Probe Microanalysis (EPMA) and Energy Dispersion Spectroscopy (EDS). The alloys with Ho addition showed a microstructure characterized by α phase solid solution, which was surrounded by some β phase and grain-like Ho-containing phase. The improvement of corrosion resistance of the Mg–9Al–Ho alloys could be explained by the fact that the deposited Ho-containing phases were less cathodic. Moreover, the corrosion product films on the Ho-containing alloy surface demonstrated their ability to restrain further corrosion.     

Microwave-assisted synthesis of lanthanum conversion coating on Mg–Li alloy and its corrosion resistance

Lanthanum-based conversion coating on Mg–Li alloy has been prepared by a microwave-assisted method. X-ray diffractions (XRD) indicate that the intermetallic compounds of lanthanum are formed on Mg–Li alloy surface. Scanning electron microscopy (SEM) images show that the coating has different morphologies and special structures. The corrosion resistance was assessed by means of potentiodynamic polarization curves and electrochemical impedance spectra (EIS). The results indicate that this coating significantly reduces the corrosion rate of Mg–Li alloy in NaCl solution. A comparing experiment indicates that the coating prepared by microwave-assisted process has superior corrosion resistance to the coating obtained at room temperature.     

Effect of pre-oxidization on the cyclic coking and carburizing resistance of HP40 alloy: With and without yttrium modification

The pre-oxidation results demonstrate that rare earth elements play an important role on the scale adherence of alloy after the oxidation at elevated temperature. The better scale adherence of the yttrium modified alloy permits a superior cyclic coking and carburizing resistance. This is because the integrity chromia layer, which prevents fast ion diffusion, can retard the outer catalytic coking and the inner carburizing of pre-oxidized alloy. As for the alloy under the cast condition, severe coking and carburization has been detected, which was also observed on the pre-oxidized samples where the scale suffered from severe spalling.