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.     

Oxidation behavior of molten magnesium in atmospheres containing SO2

The microchemistry and morphology of the oxide layer formed on molten magnesium in atmospheres containing SO₂ were examined. Based on the results and the thermodynamic and kinetic calculations of oxide-growth process, a schematic oxidation mechanism is presented. The results showed that the oxide scales with network structure were generally composed of MgO, MgS, and MgSO₄ with different layers, depending on the SO₂ content, the time and the temperature. The formation of MgSO₄ was important for the formation of the protective oxide scales. The growth of the oxide scales followed the parabolic law at 973 K and was controlled by diffusion.     

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.     

Film formation in the second step of micro-arc oxidation on magnesium alloys

Magnesium alloys were anodized firstly in a neutral solution and then in an alkaline solution by micro-arc oxidation (MAO). The second step of MAO was a competition between old film dissolution and new film formation. The curves of voltage and mass gain with time showed that in the initial stage, the old film was dissolved. Surface morphologies and EDX analyses indicated that a new film was formed after 1 min. The transition from the process dominated by old film dissolution to that by new film formation took place when the new film had completely covered the old film.     

Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys

In a neutral solution, coating formation by micro-arc oxidation (MAO) on AZ91HP magnesium alloy was studied. The process involved the substrate dissolution and coating development. During the first 5 s, the sample mass decreased, indicating substrate dissolution dominating the process. After 5 s, the sample mass began to increase and the coating development began to predominate the process. The coating was firstly nucleated on α phase and sparks initially appeared mainly on the edges of the sample. With treating proceeded, the coating was becoming uniform; meanwhile, microscopic pores of anodic coatings increased in size and decreased in number.     

Oxidation behaviour of molten magnesium and AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The oxidation rates, the morphologies and compositions of the surface films on molten magnesium and AZ91D alloy in covering gas of 1,1,1,2-tetrafluoroethane/air at 760 °C were investigated using thermogravimetric techniques, SEM, XRD and XPS. The oxidation rates of the two melts followed parabolic law in the atmosphere of high concentrate 1,1,1,2-tetrafluoroethane, and obeyed linear law in the atmosphere of low concentrate 1,1,1,2-tetrafluoroethane. But the oxidation rates of molten magnesium and AZ91D alloy presented a certain difference. The oxidation products on its surface films were composed of MgF₂ which played an import role to prevent the molten oxidation and other compounds.     

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.     

Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment

A layer containing dicalcium phosphate dihydrate (DCPD) and β-Ca₃(PO₄)₂ was prepared on magnesium alloy by hydrothermal treatment of micro-arc oxide (MAO) layer. The biocorrosion resistance of the oxide layers before and after hydrothermal treatment was analyzed by anodic polarization and electrochemical impedance spectroscopy (EIS) in Hank’s solution. The prepared MAO layers consisted mainly of MgO and MgAl₂O₄, and Ca and P inside the oxide layers existed with amorphous phase. Hydrothermal treatments not only made the amorphous Ca and P change into DCPD and β-Ca₃(PO₄)₂ crystals, but also improved the biocorrosion resistance of magnesium alloys, especially the pitting corrosion resistance.     

ToF-SIMS depth profile of the surface film on pure magnesium formed by immersion in pure water and the identification of magnesium hydride

Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH₂ is a result of the Mg corrosion mechanism.     

Electrolytic MgO/ZrO2 duplex-layer coating on AZ91D magnesium alloy for corrosion resistance

By a two-step fabrication process of electrolytic deposition and annealing treatment, an MgO/ZrO₂ duplex-layer coating has been prepared on AZ91D magnesium alloy as a protective film against corrosion. Owing to the chemical bonding formed after the condensation of precursory hydroxides, the adhesion strength, thickness and compactness of MgO coating on the substrate are significantly enhanced by the intermediate ZrO₂ layer which prevents the formation of corrosion product Mg₂(OH)₃Cl·4H₂O. As a result, the MgO/ZrO₂ duplex-layer coated specimen reveals relatively high corrosion resistance and superior stability in 3.5 wt% NaCl solution with respect to the MgO single-layer coated specimen.     

Evaluation of the anti-corrosive effect of acid pickling and sol-gel coating on magnesium AZ31 alloy

The effect of different acid pre-treatment procedures on the corrosion of magnesium AZ31 alloy was compared by measuring the amount of hydrogen gas formed when the surface was in contact with aqueous 5% sodium chloride solution. A 4-7 μm thick sol-gel coating prepared by phosphoric acid catalyzed sol-gel processing of a methyltriethoxysilane/tetraethoxysilane mixture was applied to the differently pre-treated magnesium surfaces. The corrosion rate of the alloy decreased by a factor of up to 60 by combination of acid pickling and sol-gel coating. The addition of triethylphosphate or 1,2,4-triazole as corrosion inhibitors led to further improvements. Composition and texture of the films was investigated by scanning electron microscopy and energy dispersive X-ray analysis.     

In-situ study of the formation process of stannate conversion coatings on AZ91D magnesium alloy using electrochemical noise

The formation process of stannate conversion coating (CC) on AZ91D alloy was in-situ investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and spectral noise resistance (R_sn), had been employed to analyze the EN data. It was revealed that there exist two distinguishing stages of stannate CC formation process on AZ91D alloy, including an incubation stage companying with the nucleation and nuclei dissolution process, a periodical growth stage involving hemispherical particles growth and coating dissolution process. Furthermore, the results demonstrated that EN was a powerful tool to investigate rapid electrochemical process, such as CC formation process.     

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.     

Plasma electrolytic oxidation of pre-anodized aluminium

The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al₂O₃, with increased concentrations of silicon, sodium and potassium in an outer region of the coating.     

Characterization and corrosion studies of ceria thin film based on fluorinated AZ91D magnesium alloy

The CeO₂ thin film was prepared via sol-gel method on fluorinated AZ91D magnesium alloy surfaces. The surface morphology, composition and the corrosion resistance of the film were investigated in details using scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. It was found that small amount of MgO and MgF₂ were encapsulated in CeO₂ thin film. The electrochemical measurement results demonstrated that the CeO₂ thin film on fluorinated AZ91D magnesium alloy could improve the corrosion resistance approximately by two orders of magnitude compared with that of the bare substrate.     

Anodic films formed on magnesium in organic, silicate-containing electrolytes

Anodic films were prepared on magnesium in electrolyte solutions consisting of 3.0 M KOH and various concentrations of sodium silicate and aqueous ethylene-glycol solutions. The anti-corrosion properties of the films formed in the ethylene-glycol electrolyte solutions ranging from 10 to 40 wt% far exceeded that of the anodic film produced using the HAE method. The film formed under optimal conditions had excellent anti-corrosion properties, which were more than 10-fold greater than those of the HAE anodic films. The anodic film consisted of two layers—a heterogeneous porous layer and a barrier layer; moreover, both carbon and silicate were detected in the anodic film.     

Formation and breakdown of surface films on magnesium and its alloys in aqueous solutions

The influences of surface films formed by open-circuit exposure to neutral solutions on the corrosion and electrochemical behaviour of pure Mg and Mg alloys have been examined by in situ ellipsometric analysis and electrochemical measurements. Surface films mainly composed of Mg(OH)₂ grew rapidly during open-circuit exposure to 0.1 M NaCl and 0.1 M Na₂SO₄ solutions. These films had protective ability to passivate Mg in the solutions. However, they suffered local breakdown under anodic polarisation. The passive current density decreased and the breakdown potential increased with increasing immersion time and film thickness. Influences of purity and alloying elements on the passivity and its breakdown of Mg have been discussed.     

The protective surface film formed on molten ZK60 magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The composition, microstructure and growth kinetics of the surface film formed on molten ZK60 magnesium alloy in 1,1,1,2-tetrafluoroethane/air mixture were investigated by using SEM, EDS, XRD, XPS and TG. The results show that the film formed in 0.01% 1,1,1,2-tetrafluoroethane/air was non-protective, and its phase composition changed with melt temperature. At 760 °C, the film was mainly composed of MgO and some MgF₂ and ZrO₂, whereas at 660 °C it consisted mainly of MgO and Mg₃N₂. The film formed in air containing 0.1% 1,1,1,2-tetrafluoroethane or higher was comprised of MgF₂ and C with small amounts of MgO, and it was protective.     

Control of polymorphism in Al2O3 scale formed by oxidation of alumina-forming alloys

The selective oxidation of specific components in alumina-forming alloy such as CoNiCrAlY under precisely regulated oxygen partial pressures (P_O2) can be used to control polymorphism in Al₂O₃ scale formed on the alloy. Dense, smooth α-Al₂O₃ scale was formed rapidly by treatment at 1323 K under a thermodynamically determined P_O2, where both aluminum and chromium in the alloy were oxidized and elements such as cobalt and nickel were not oxidized. By contrast, under a higher P_O2 all the components in the alloy were oxidized, the transformation was obviously retarded, and (Co,Ni)(Al,Cr)₂O₄ was produced.     

Effect of alternately changing the dissolved Ni ion concentration on the oxidation of 304 stainless steel in oxygenated high temperature water

Characteristics of oxide films formed on 304 stainless steel under alternately changing Ni²⁺ concentrations in oxygenated high temperature water were examined. Oxides preformed under low Ni²⁺ concentration evolve from hematite to spinel after subsequent immersion under high Ni²⁺ concentration. Meanwhile, Ni content in the surface layer of oxide rises up while Fe content drops. Oxide films preformed under high Ni²⁺ concentration show little change in phase composition after subsequent immersion under low Ni²⁺ concentration. Fe contents in surface layer rise up while the change of Ni contents depends on the original phase composition and whether residual Ni²⁺ is present.