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.     

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.     

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.     

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.     

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.     

Effect of plasma electrolytic oxidation coating on the stress corrosion cracking behaviour of wrought AZ61 magnesium alloy

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁷ s⁻¹, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.     

Investigation of the plasma electrolytic oxidation of Ti6Al4V under single-pulse power supply

Plasma electrolytic oxidation (PEO) was studied using electrochemical impedance spectroscopy (EIS) and transient waveform analysis. First, EIS measurements on coatings prepared under constant current density were conducted in the working electrolyte. The EIS analysis showed that the coating structure changed with the termination voltage. Second, the pulse waveform of the power supply was recorded during the PEO process. It showed that a charging process occurred across the breakdown coating at the beginning of the pulse. The present study analysed this charging process in detail.     

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.     

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.     

Corrosion behaviour of AZ31B magnesium alloy in NaCl solutions saturated with CO2

Corrosion behaviour of AZ31B magnesium alloy in different concentrations of NaCl solution saturated with CO₂ was studied by electrochemical techniques, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive X-ray. The corrosion rate increases with increasing NaCl concentration both in the presence and absence of CO₂. The corrosion rate in NaCl solution saturated with CO₂ is bigger than that in single NaCl solution. The inhibitive effect of CO₂ is also observed with immersion time increased in NaCl solution saturated with CO₂, showing that CO₂ reduces the average corrosion rate due to the formation of insoluble products.     

Influence of cerium addition on the resistance to oxidation of AM50 alloy prepared by rapid solidification

The influence of various cerium (Ce) additions and starting temperature on the resistance to oxidation of AM50 alloy prepared by rapid solidification (RS, RS-AM50 alloy) has been investigated. The Ce addition has two opposite effects on the oxide scale. To the RS-alloy, the beneficial effect outweighs the detrimental one. However, when the alloy was prepared by slow solidification at the normal cooling rate (SS, SS-AM50 alloy), the detrimental effect was dominant. The improvement of oxide scale is closely related to the adherence of metal/oxide and tightness of oxide scale of the alloy. In addition, the reaction rate at the surface increased with starting temperature, which finally results in the formation of a protective oxide scale.     

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.     

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.     

Characterization of the plasma electrolytic oxidation of aluminium in sodium tungstate

Morphology, composition and microhardness of oxide coatings formed during DC plasma electrolytic oxidation (PEO) of aluminium in sodium tungstate were investigated. Oxide coatings were characterized by optical emission spectroscopy, AFM, SEM-EDS and XRD. Oxide coatings morphology is strongly dependent on process duration, while microhardness decreases with extended PEO time. The microdischarges characteristics were studied as well and it is shown that size of microdischarges becomes larger, while the surface density of microdischarge sites becomes lower, with increasing PEO time. Optical emission spectrum of microdischarges has several intensive band peaks caused by electronic transition in Al, W, Na, O, H atoms.     

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.     

Electrochemical study of growth behaviour of plasma electrolytic oxidation coating on Ti6Al4V: Effects of the additive

The growth behaviour of plasma electrolytic oxidation (PEO) coating on Ti6Al4V was studied by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, focusing on effects of the electrolyte additive - calcium hypophosphite. The EIS analysis of the outer layer of the PEO coating provided insight into the structure of the coating, which was confirmed by SEM results. The EIS analysis of the inner layer of the PEO coating provided information matching well with the results of the potentiodynamic polarization test.     

Corrosion behaviour of a magnesium matrix composite with a silicate plasma electrolytic oxidation coating

The corrosion behaviour of AZ92 magnesium alloy reinforced with various volume fractions of silicon carbide particles (SiCp) and treated by alternating current (AC) plasma electrolytic oxidation (PEO) was investigated in humid and saline environments. For untreated composites, corrosion attack started around the Al-Mn inclusions and gradually developed into general corrosion without significant galvanic coupling between the matrix and the SiCp. PEO coatings consisted mainly of MgO and Mg₂SiO₄, and revealed increased hardness, reduced thickness and slightly higher corrosion resistance with increasing proportion of reinforcement. Pit formation and hydration of the outer layer were the main mechanisms of corrosion of PEO-treated specimens.     

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.     

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.     

Effect of sintering temperature on corrosion properties of sol–gel based Al2O3 coatings on pre-treated AZ91D magnesium alloy

A novel anti-corrosion sol–gel based Al₂O₃ coating was developed on the AZ91D magnesium alloy. The morphology, microstructure and composition of the coatings were investigated by scanning electron microscope coupled with energy dispersive spectroscopy, Fourier transform infrared spectrum analysis, X-ray diffraction, thermo-gravimetric and differential thermal analysis. The corrosion resistance of the coatings in 3.5 NaCl wt.% solution was studied using electrochemical measurements. The results demonstrated that a homogeneous Al₂O₃ coating could be obtained and the sol–gel coated samples sintered at 380 °C had the best corrosion resistance properties as compared to the specimens sintered at 120 and 280 °C.