Environmentally assisted cracking behaviour of plasma electrolytic oxidation coated AZ31 magnesium alloy

Abstract

A wrought AZ31 magnesium alloy was plasma electrolytic oxidation (PEO) coated in phosphate and silicate based alkaline electrolytes. The effect of these PEO coatings on the stress corrosion cracking (SCC) behaviour of the alloy was investigated by slow strain rate tensile (SSRT) tests in ASTM D1384 solution. The untreated and PEO coated AZ31 magnesium alloy specimens were found to be susceptible to SCC, despite the fact that the PEO coatings offered an excellent general corrosion resistance. The results of the polarisation tests on the untreated AZ31 alloy specimen after prolonged immersion in ASTM D1384 electrolyte suggested the formation of a film on the surface constituted by the corrosion products. The cracking of this film and the evolution/ingress of hydrogen at these defective sites during the SSRT tests in the corrosive environment was believed to be responsible for the SCC of the untreated alloy. Similarly, the cracking of the PEO coatings during the SSRT test, the consequent exposure of the underneath magnesium alloy substrate and the associated electrochemical reactions were attributed as reasons for the SCC of the PEO coated specimens. The transgranular mode of fracture in all the cases avowed that the hydrogen induced cracking was the mechanism of SCC.     

Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al-Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al-Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al-Mg matrix and dendrite precipitates of Mg₂Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg₁₇Al₁₂. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg₂Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.     

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.     

Effect of laser surface melting on friction and wear behavior of AM50 magnesium alloy

In the present study, an attempt has been made to enhance tribological properties of AM50 magnesium alloy by laser surface melting (LSM) with a 2 kW continuous wave CO₂ laser. The microstructure of the laser surface melted zone consists of fine columnar dendrites growing epitaxially from the liquid-solid interface. Microhardness of the melted zone was improved to 55-75 HV as compared to 40 HV of the substrate. The friction and wear behavior of the laser surface melted layer were investigated using a ball-on-flat apparatus under dry sliding condition. It was found that the friction coefficient curve of the laser surface melted layer was similar to that of substrate. They showed a lower initial friction coefficient about 0.18 that after the running-in period increased up to about 0.38. Furthermore, compared with the AM50 substrate, the wear volume of the laser surface melted layer was decreased by 42%, the wear resistance of the laser surface melted layer was improved.     

The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62

Magnesium alloys are increasingly being used as lightweight materials in the automotive, defense, electronics, biomaterial and aerospace industries. However, their inherently poor corrosion and wear resistance have, so far, limited their application. Plasma electrolytic oxidation (PEO) in an environmentally friendly aluminates electrolyte has been used to produce oxide coatings with thicknesses of ~ 80 μm on an AJ62 magnesium alloy. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm–800 nm) was employed to characterize the PEO plasma. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the coated materials, and potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution were used to determine the corrosion behavior. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings and, hence the corrosion resistance. The corrosion resistance of the coated alloy was increased by changing the current mode from unipolar to bipolar, where the strong plasma discharges had been reduced or eliminated.     

Characterization of the surface layers formed on titanium by plasma electrolytic oxidation

This paper is concerned with the surface modification of titanium by the PEO method (plasma electrolytic oxidation) in the solutions which contain Ca, P, Si and Na. The chemical composition of the thus formed surface layers was examined by XPS and EDS. The morphology of the surface was observed by SEM. The phase composition was determined by X-ray diffraction (XRD). The adhesive strength of the oxide layers was evaluated by the scratch-test. The corrosion resistance was determined in a simulated body fluid (SBF) at a temperature of 37 °C by electrochemical methods for various exposure times.The oxide layers obtained were porous and enriched with Ca, P, Si and Na and their properties depended on the electrolyte solution and the parameters of the oxidation process. The results of the electrochemical examinations show that the surface modification by PEO does not worsen the corrosion resistance of titanium after a 13 h exposure in SBF. The electrochemical impedance spectroscopy (EIS) results indicate that the surface layers have a complex structure and that their electric properties undergo changes during long-term exposures in SBF.     

Formation of corrosion resistant plasma electrolytic oxidation coatings on aluminium alloy with addition of sodium tungstate species

Plasma electrolytic oxidation (PEO) coatings were formed in silicate based electrolytes without and with the addition of sodium tungstate on AA?6063 aluminium alloy. Microstructure, composition and corrosion resistance of PEO coatings were investigated by scanning electron microscopy, X-ray diffraction and electrochemical impedance spectroscopy and potentiodynamic polarisation test respectively. The effects of additive sodium tungstate were examined. The results showed that the additive containing PEO coatings were of dense structure with additional phase (WO₃) and of less cracks than the additive free PEO coating. In addition, additive containing coatings were of better corrosion resistance than the additive free PEO coating, which was confirmed by electrochemical impedance spectroscopy and potentiodynamic polarisation tests. Furthermore, long time immersion test revealed that the PEO coated alloy with the addition of 12?g?L^??1 sodium tungstate maintained high impedance over 82?h in 3.5?wt-%NaCl, while the PEO coating without additive was unable to protect the substrate after such long time immersion.     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.     

Effect of potassium fluoride on structure and corrosion resistance of plasma electrolytic oxidation films formed on AZ31 magnesium alloy

Plasma electrolytic oxidation films on AZ31 magnesium alloy were prepared in silicate–KOH–glycol (base electrolyte) electrolyte with the addition of different KF concentration. The effect of KF on the characteristic of discharge in electrolytes was studied. The compositions, structures and morphologies of the oxide films formed in different KF concentration were determined by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion behavior of the oxide films was evaluated in 3.5 wt.% NaCl solution using potentiodynamic polarization tests, electrochemical impedance spectroscopy and potentiodynamic scanning tests. The films were mainly composed of Mg, Al, O, C, Si as well as a trace of Na and K. Major phases were MgSiO₃ and forsterite Mg₂SiO₄. The appearance of KF in the base electrolyte resulted in the decrease of the equilibrium current density and the appearance of MgF₂ in the films. Corrosion resistance depends on the amount of MgF₂ composition in the films. The films formed in the electrolyte containing 8.0 g/L KF exhibit the highest uniform corrosion resistance and the lowest pitting-corrosion tendency in NaCl solution.     

Microstructure and corrosion behavior of FeCrNiMnMox high-entropy alloys fabricated by the laser surface remelting

In this study, an Nd:YAG laser was used to carry out laser surface remelting treatment on FeCrNiMnMo_x (x = 0, 0.5, 1) alloys. A study was conducted on the potential impact of Mo on the microstructure and corrosion resistance of the laser-remelted layer. According to the research results, FeCrNiMnMo_x alloys were more effective in refining the dendrites, compared with the matrix, whereas the FeCrNiMn alloys' remelted layer exhibited an almost single face-centered cubic (FCC) structure. In comparison, FeCrNiMnMo_0.5 and FeCrNiMnMo₁ alloys' remelted layer displayed the FCC and σ phase. In addition, the dendrite crystals' microstructure can be clearly refined by Mo alloying. Mo is effective in improving the corrosion resistance of the FeCrNiMnMo_x alloys' remelted layer in 3.5% NaCl solution. The pitting resistance of Mo-containing-remelted layers is significantly higher, compared with Mo-free alloy's remelted layer, and the FeCrNiMnMo_0.5-remelted layer shows the most satisfactory corrosion resistance. As revealed by X-ray photoelectron spectroscopy analyses, the addition of molybdenum promotes the generation of Cr₂O₃ and enhances the corrosion resistance of the remelted layer.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

Assessment of duplex coating combining plasma electrolytic oxidation and polymer layer on AZ31 magnesium alloy

Mechanical and corrosion tests were performed on a polymer-coated AZ31 magnesium alloy pre-treated by plasma electrolytic oxidation (PEO). Results were compared with a fluorotitanate–zirconate conversion coating pre-treatment. Mechanical performance was assessed by standardized adhesion (ISO 2409:2007), impact (ISO 6272‐1:2004) and impact + adhesion (ISO 6272/ASTMD2794) pass/fail tests. Corrosion behavior was monitored using electrochemical impedance spectroscopy (EIS), ac/dc/ac measurements and continuous exposure to salt fog per ASTM B117 and cyclic exposure per VDA 621‐415 (VDA). The PEO + polymer coating revealed lower impact resistance but better corrosion resistance than the Ti/Zr + polymer coating. The ac/dc/ac procedure demonstrated to be faster than EIS measurements for evaluation of the corrosion performance of studied coatings.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

Effect of laser surface treatment on corrosion and wear resistance of ACM720 Mg alloy

A creep resistant Mg alloy ACM720 was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. This treatment was found to be beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarization resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarization resistance values of laser surface treated alloy were twice as high as that for the untreated alloy. The improved corrosion resistance was attributed to the absence of the second phase Al₂Ca at the grain boundary, microstructural refinement and extended solid solubility, particularly of Al, in α-Mg matrix owing to rapid solidification. The laser treatment also increased surface hardness two times and reduced the wear rate considerably due to grain refinement and solid solution strengthening.     

Corrosion behavior of laser melted AZ91HP magnesium alloy

AZ91HP magnesium alloy was melted by CO₂ laser. Compared with as‐received Mg alloy, the grain of the melted layer was refined significantly and the content of Al was increased. The corrosion resistance of the melted layer was improved because of the grain refinement, the redistribution of β‐Mg₁₇Al₁₂ and the increasing of the Al content. As compared to the non‐overlapping zone, the overlapping zone of the melted layer was liable to be corroded.     

微弧氧化处理对铝合金耐霉菌腐蚀特性的影响

在2A12铝合金表面制备了微弧氧化膜层,按照国家军用标准霉菌测试方法对微弧氧化膜层的耐腐蚀性能进行了测试,通过扫描电镜(SEM)、X射线衍射分析(XRD)对铝合金基体及微弧氧化膜层霉菌腐蚀前后的微观结构、相组成进行了表征。结果表明,未经过微弧氧化处理的铝合金表面有少量的霉菌生长,表面产生了一定数量的点蚀坑,长霉等级为1级。经过微弧氧化处理试样表面未发现霉菌生长,长霉等级为0级。微弧氧化处理可以有效提高铝合金表面耐霉菌腐蚀性能,扩大其应用范围。     

Environmental friendly plasma electrolytic oxidation of AM60 magnesium alloy and its corrosion resistance

Plasma electrolytic oxidation of Mg-based AM60 alloys was investigated using 50 Hz AC anodizing technique in an alkaline borate solution, which contained a new kind of organic. The anodic film is relatively smooth with some micro pores and cracks, while the anodic film consists of MgO, MgAl2O4 and MgSiO3. The electrochemical behavior of anodic film was studied by electrochemical impedance spectroscopy and potentiodynamic polarization. Polarization results indicate the PEO treatment can decrease corrosion current by 3-4 magnitude compared with blank AM60 alloy. The anodic film presents a good level of corrosion protection for AM60 magnesium alloy, over 272 h of the salt spray test based on ASTM B 117. The effect of micro-structure and composition on corrosion protection efficiency was also investigated.     

A new nanoparticle penetrant used for plasma electrolytic oxidation film coated on AZ31 Mg alloy in service environment

A new penetrant of improved self-assembled nanophase particle (SANP) solution was used to protect AZ31 Mg alloy with plasma electrolytic oxidation (PEO) film in the service environment. The improved SANP technique was carried out by increasing water concentration, prolonging hydrolytic time, reducing solution acidity and adding phytic acid (PAH) inhibitor. Properties of the improved SANP solution and the PEO + SANP film were evaluated by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and potentiodynamic polarization. Service performance of the PEO + fluorocarbon (FC) paint treated by the improved SANP technique was estimated by thermal shock method (TSM) and neutral salt spray test (NSST). Experiment results indicated that permeability of the improved SANP solution was increased obviously due to the formation of smaller nanoparticles. Uniformity and corrosion protection of the PEO + improved SANP film were further enhanced attributed to the restriction in the formation of bigger inorganic particles and the stability of the PAH inhibitor, respectively. For the composite film of PEO film + improved SANP + FC, it still had strong adhesion and good corrosion protection after the service environment tests of TSM and NSST due to the chemical combinations of SANP/PEO and SANP/FC.     

Influence of potassium pyrophosphate in electrolyte on coated layer of AZ91 Mg alloy formed by plasma electrolytic oxidation

The effect of potassium pyrophosphate in the electrolyte on plasma electrolytic oxidation (PEO) process for AZ91 Mg alloy was investigated. The morphologies and chemical compositions of the coating layer on the AZ91 Mg alloy were evaluated and corrosion resistance was also estimated by potentiodynamic polarization analysis. The coating layer on AZ91 Mg alloy coated from the Bath 2 containing 0.03 mol/L of potassium pyrophosphate for 360 s exhibited considerably dense structure and contained 11%–18% (mass fraction) of phosphorous. The higher content of phosphorous of coating layer coated from Bath 2 could be detected at the bottom of oxide layer, which strongly implied that the phosphorous ion might be concentrated at the barrier layer. Corrosion potential of coating layer of AZ91 Mg alloy increased and corrosion current density decreased with increasing the concentration of potassium pyrophosphate. The polarization resistance (R_p) of coating layer of AZ91 Mg alloy coated from Bath 2 was 4.65×10⁷ Ω/cm², which was higher than that (R_p=3.56×10⁴ Ω/cm²) of the sample coated from electrolyte without potassium pyrophosphate. The coating layer coated from Bath 2 containing 0.03 mol/L potassium pyrophosphate exhibited the best corrosion resistance.