Corrosion resistance of micro-arc oxidation coating on laser-treated magnesium alloy

Improvement on corrosion resistance of micro-arc oxidized AZ91D magnesium alloy by a pore-sealing coating

The automotive sector is particularly interested in magnesium alloys, which can decrease the weight of the vehicle leading to improved fuel efficiency and decreased emissions. However, poor corrosion resistance, especially in solutions containing chlorides, is a major limitation for its widespread exploitation in exposed automotive applications. Plasma electrolytic oxidation (PEO) coatings have been shown as a promising technology to improve the corrosion resistance of magnesium alloys 1. The properties of PEO synthesized coatings have been shown to significantly dependent on the process parameters employed to produce them2,3. Understanding the influence of process parameters on the overall corrosion resistance of AZ31 is essential if PEO technology is to be used in an industrial application. The present investigation examines the effect of processing time, current density, and electrolyte temperature on structural morphology of PEO coatings made on AZ31B linked to their corrosion resistance. PEO coatings were produced using a sodium silicate basic electrolyte using current densities ranging between 10 mA/cm2 and 20 mA/cm2. The temperature of the electrolyte was varied between 10-40ºC, while the processing time was varied between 15 and 30 minutes. The overall corrosion rate of PEO-coated samples was evaluated using mass loss testing and electrochemical impedance spectroscopy (EIS), while the composition and morphology of the PEO coatings were analyzed using a combination of x-ray diffraction (XRD), electron microscopy, and white light profilometry. The phase composition of the synthesized PEO coatings was analysed using XRD, see Figure 1. Spectra indicated that the PEO coating comprised two main phases, namely magnesium oxide (MgO) and forsterite (Mg2SiO4). The ratio between magnesium oxide and magnesium silicate was estimated via the reference intensity ratio (RIR) analysis. The mass ratio (MgO/Mg2SiO4) for PEO coatings made on AZ31B decreased from 0.63 (10 mA/cm2) to 0.11 (20 mA/cm2). The observed increase in the weight fraction of forsterite when higher current densities were is related to polymerization of silicate ions during the deposition process. It has been previously reported that the extremely high energy generated by the plasma discharges promote polymerization of the silicate4. Increasingly favorable polymerization resulted in greater incorporation of silicates into the coating, ultimately leading to higher weight fraction of forsterite and lower weight fraction of magnesium oxide. The corrosion rates of the two coated specimens (10 mA/cm2 and 20 mA/cm2) in addition to the bare metal AZ31 substrate were measured by 5-day mass loss testing in a 0.086M NaCl solution, see Figure 2. Both PEO coatings exhibited significantly improved corrosion resistance properties compared to as-received AZ31. The corrosion rates of coatings produced using an applied current density of 10 mA/cm2 were significantly lower than those of coating produced using a current density of 20 mA/cm2. The current research effort is focusing on providing explanations for observed differences in corrosion resistance properties of PEO produced when applying different current density values. The effect of changes in processing time and electrolyte temperature is also under investigation.

Introduction Magnesium alloys can be successfully joined by friction stir spot welding (FSSW) to achieve high mechanical integrity of the weld. Previous work done on AZ31 FSSW joints has shown that the welding process reduced their corrosion resistance, and resulted in a localized attack around the weldment1-2. The welding process resulted in the dissolution of noble β-Mg17Al12 and Al-Mn intermetallics3, eliminating harmful microgalvanic couples between the particles and the α-Mg matrix in the stir zone. The localization of the corrosion attack resulted from the newly formed noble stir zone coupled with the active base metal. If FSSW technology is to be used in an industrial application, the challenges associated with the corrosion behaviour of the weld will need to be addressed. Plasma electrolytic oxidation (PEO) coatings have been shown as a promising technology to improve the corrosion resistance of friction stir welds in AZ314. It is important to understand the effect of process parameters on the microstructure of the coating in order to optimize their properties. The present work explores possible post welding treatments that minimize the negative effect of welding on the corrosion resistance of magnesium alloys. The paper assesses the feasibility of these treatments through mass loss testing supplemented with scanning reference electrode technique (SRET), microcapillary polarization (MCP), and scanning electron microscopy. Experimental Methods All samples were spot welded using a flat shoulder tool from 1.6 mm thick AZ31B magnesium sheets. PEO coatings were produced using a sodium silicate basic electrolyte with a current density of 10 mA/cm2and oxidation time between 15 and 30 minutes. SRET experimental procedures are described elsewhere2. The reference electrode chosen for this investigation was a 10 µm diameter platinum wire encapsulated in a glass tube. The measurements were performed in 0.086 M NaCl electrolyte solution. Microcapillary polarization was performed in 0.1M NaClO4 electrolyte, using a capillary with an inner diameter of 40 μm. Further details can be found elsewhere1. Mass loss testing was conducted by submerging samples in NaCl solutions of varying concentrations for a duration between 120 and 480 hours. The samples were imaged and analyzed to produce their respective corrosion rates. Results and Discussions PEO coating of FSSW joints yielded uniform coating across the weld and base metal for all coating parameters studied. Increases in processing time and polyethylene glycol (PEG) concentration in the bath electrolyte resulted in a smoother and thicker coating formed on the surface of the weld, as seen from the micrographs in Figure 1. The coating thickness increased from 6.6 µm for the coating processed for 15 minutes to 12.3 µm for the coating processed for 30 minutes. The roughness of the coatings was measured by white light profilometry. The roughness was reduced from 6.3 µm to 5.8 µm with increased processing time and PEG concentration in the electrolyte. Figure 1shows that application of PEO coating to FSSW joints increased the corrosion resistance by at least a factor of two for all processing parameters investigated. The results indicate that the corrosion resistance of the joint is proportional to the coating thickness, and inversely proportional to its roughness. Conclusions and Future Work The results suggest that PEO coating can be successfully applied to FSSW joints made in AZ31 to improve their corrosion resistance. Process parameters were shown to affect the microstructure of the coating and its subsequent corrosion resistance. Further work will examine the breakdown mechanism of PEO coatings through a successive series of potential maps using SRET. Other post welding treatments to improve the corrosion resistance of the joint will be evaluated as well. References Y. Savguira, T.H. North, S.J. Thorpe, Mater. Corros. 65 (2014): p.1055.Y. Savguira, W. H. Liu, D.J. Miklas, T.H. North, S.J. Thorpe, Corrosion 70 (2014): p.858.A. D. Sudholz, N.T. Kirkland, R.G. Buchheit, N. Birbilis, Electrochem. Solid-State Lett.14, 2 (2011): p. C5.T. Chen, W. Xue, Y. Li, X. Liu, J. Du, Mater. Chem. Phys. 144, 3 (2014): p. 462. Figure 1

Enhanced corrosion resistance of micro-arc oxidation coated magnesium alloy by superhydrophobic Mg−Al layered double hydroxide coating

Micro-arc oxidation (MAO) technology, as an efficient and widely used technology, can fabricate a ceramic film on magnesium (Mg) alloys in situ to significantly improve corrosion resistance. The study aims to investigate the effect of voltage modes on the preparation and performance of MAO coating on Mg alloys. The scanning electron microscope, X-ray diffractometer, potential dynamic polarization, and electrochemical impedance spectroscopy measurements were applied for characterizing the microstructure, composition, and corrosion resistance of the MAO film, respectively. The results revealed that employed 300/350 V step-up mode, the MAO film showed a more uniform distribution of surface micro-pores, fewer micro-cracks and denser inner layer, lower corrosion current density (1.871 × 10−3 mA·cm−2), and higher electrochemical impedance value, compared to other voltage treated parameters. Therefore, it could be concluded that the step-up voltage parameters are conducive to the optimization of microstructure, improvement of corrosion resistance of MAO coating, and reduction of the energy consumption during processing.

Growth process and corrosion resistance of micro-arc oxidation coating on Mg-Zn-Gd magnesium alloys

Based on environmentally friendly and green manufacturing of magnesium alloys, micro-arc oxidation coatings were prepared in optimized metasilicate solutions on AZ63B magnesium alloys surfaces. Effect of nickel additive on working voltage, surface morphology, thickness, hardness, composition and corrosion resistance of micro-arc oxidation coating of magnesium alloys was studied. The surface morphology, composition, microstructure and corrosion resistance of conversion coatings were investigated by using scanning electron microscopy (SEM), energy dispersion spectrometry (EDS), X-ray diffraction (XRD) and electrochemical tests respectively. The results show that brown coating was prepared in electrolyte with the addition of nickel acetate, which can accelerate growth of micro-arc oxide film layer and make the smaller hole on the surface. The micro-arc oxide coating mainly consists of Mg2SiO4, MgSiO3, MgAl2O4, Ni2SiO4. Potentiodynamic measurements and electrochemical impedance spectroscopy (EIS) testify that the corrosion resistant of micro-arc oxidation film adding nickel acetate electrolyte increases significantly.

The article presents the promising field of micro-arc oxidation as a surface modification method with the purpose to improve the corrosion resistance of titanium, aluminum, aluminum alloys and composites. Micro-arc oxidation is an electrochemical process that forms a thick, hard and highly adhesive ceramic oxide layer on the metal surfaces, thereby providing excellent protective properties. This method of hardening the metal surface was used in production in the manufacture of parts from AA5086 and VT6 alloys for industrial fans operating in chemically hazardous facilities. When using micro-arc oxidation, the strength and wear resistance of the internal parts of the fan to aggressive medium was increased. By that increased the trouble-free operation time and reduced the likelihood of destruction of the internal parts of the fan, which subsequently made it safer for employees to operate the fans in close proximity to the work area. This work examines the corrosion resistance of aluminum alloys and composites, since the indicated materials are also widely used in various branches of industry. It also considers the role of alloying elements in influencing corrosion behavior, as well as the influence of microstructure and processing methods on corrosion resistance. In addition, the discussion extends to the corrosion behavior and protective measures for aluminum matrix composites, including the inclusion of reinforcing phases such as ceramic particles, fibers or nanoparticles. Throughout the article, the corrosion resistance of micro-arc oxidation treated surfaces, anodized aluminum layers, aluminum alloys and composites are critically assessed, analyzing the factors influencing their protective characteristics, such as the morphology, composition and thickness of the oxide layers, as well as environmental factors.

Growth characteristics and corrosion resistance of micro-arc oxidation coating on Al–Mg composite plate

PurposeUniform nanostructured TiO2 thin film has been applied as an over coat on micro‐arc oxidized substrate, using the sol‐gel method. The anticorrosion performance of the coating have been evaluated using electrochemical techniques. Owing to increasing application of light alloys in industry, the purpose of this paper is to report effort to increase the corrosion and wear resistance properties of these alloys by applying a TiO2 nanostructured coating using the sol‐gel method on the micro‐arc oxidation (MAO) surface. This approach will decrease the time for the MAO process, especially for achieving good mechanical properties, and will minimize energy consumption as well as achieving better results from the obtained coatings.Design/methodology/approachSol‐gel coatings were deposited (on titanium substrates) by spin coating techniques. The morphologies and nanostructures of thin films were analyzed using scanning electron microscope, atomic force microscopy and grazing incidence X‐ray diffraction (XRD). The anticorrosion performance of the coating has been evaluated by using electrochemical techniques. Tafel polarization measurements provide an explanation for the increased resistance of nanostructured TiO2 coated specimen against corrosion. Effective sol‐gel coating parameters were optimized with respect to this enhancement. Electrochemical impedance spectroscopy measurements showed the role of barrier layer on corrosion resistance of MAO and nanostructured TiO2 coating.FindingsThe results showed that icorr is decreased from 0.258 to 0.169 (μA/cm2). An optimized TiO2 nanostructured coating with thickness of 74 nm will shift the open circuit potential (OCP) about 165 mV and will improve the corrosion prevention properties of coated samples. Corrosion resistance by these duplex coatings can be improved by a factor of more than three times, compared to that of the uncoated substrate. Increasing the coating thickness to more than 74 nm will decrease the physical and corrosion properties of coated samples. It can be concluded that samples with the optimized coating showed higher values of charge transfer resistance, due to the presence of a newly formed layer that accounted for the greater corrosion protection.Practical implicationsThe results obtained in this research into nanostructured coating can be used wherever good corrosion and wear resistances are required.Originality/valueThe speed of treatment by this technique makes this method very suitable for industrial surface treatment of different components.

Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges

For the purpose of detecting the influence of grain structure of a Mg matrix on the microstructure and corrosion resistance of micro-arc oxidation (MAO) coating, prior to MAO processing, sliding friction treatment (SFT) was adopted to generate a fine-grained (FG) layer on coarse-grained (CG) pure Mg surface. It showed that the FG layer had superior corrosion resistance, as compared to the CG matrix, owing to the grain refinement; furthermore, it successfully survived after MAO treatment. Thus, an excellent FG-MAO coating was gained by combining SFT and MAO. The surface morphology and element composition of FG-MAO and CG-MAO samples did not show significant changes. However, the FG layer favorably facilitated the formation of an excellent MAO coating, which possessed a superior bonding property and greater thickness. Consequently, the modified FG-MAO sample possessed enhanced corrosion resistance, since a lower hydrogen evolution rate, a larger impedance modulus and a lower corrosion current were observed on the FG-MAO sample.

Magnesium alloys are actively researched for biodegradable implants in order to avoid an implant-removal surgery after the healing process. However, the high degradation rate of Mg leads to hydrogen evolution and may increase the pH of the environment, causing an inflammatory response. In the present work, bioactive plasma electrolytic oxidation (PEO) coatings on three MgxZnyCa alloys (two cast alloys and one extruded; x = 1, 3 or 0.5; y = 1 or 0.3), manufactured by Helmholtz-Zentrum Hereon, Geesthacht, were generated in order to enhance the corrosion resistance, bioactivity and cytocompatibility of the alloys. AC PEO process was carried out in two environmentally friendly alkaline electrolytes containing Ca, P and Si as bioactive elements. The electrolytes were a true solution and a particle suspension. F-free electrolyte design was employed to ensure cytocompatibility of the coatings with different types of cells. The materials were characterized by SEM, EDS, XRD and optical profilometry. The corrosion behavior was evaluated by EIS and hydrogen evolution measurements during 5 days of immersion in 0.9% NaCl and α-MEM solutions, a complete one and an inorganic part only, at 37 °C. PEO coatings (7–13 µm-thick, Sa = 1.85–4.19 µm) were constituted by MgO, Ca3(PO4)2, Ca5(PO4)3(OH) and Mg2SiO4 phases. Both PEO coatings decreased the degradation rate of Mg alloys; corrosion resistance of coated samples in inorganic α-MEM increased by more than an order of magnitude (|Z|10 mHz, Ω·cm2): MgZnCa = 746, MgZnCa/PEO = 8544…28,277). All materials exhibited considerably greater corrosion rates in 0.9% NaCl than in α-MEM, where phosphate-based additives acted as corrosion inhibitors. Corrosion rates were slightly greater in complete α-MEM than in inorganic α-MEM due to the presence of complexing aminoacids. The developed coatings are considered suitable candidates for the subsequent development of hybrid hierarchical ceramic/biodegradable polymer systems.

The fast corrosion rate of magnesium (Mg) alloys is the main problem associated with the use of such biocompatible alloys for bone fixation applications. The corrosion resistance of Mg alloys can be improved by different post-fabrication processes such as heat treatment and coating. We have heat-treated a biocompatible Mg-1.2Zn-0.5Ca (wt.%) alloy at optimized heat treatment parameters to achieve the highest mechanical strength and corrosion resistance. Afterwards, the heat-treated alloy was coated with a ceramic layer using micro arc oxidation (MAO) process to further enhance the corrosion resistance. The microstructure of the prepared samples was investigated using optical microscopy and scanning electron microscopy (SEM). The corrosion characteristics were determined by conducting in vitro electrochemical and immersion corrosion tests. The results showed that the heat treatment process successfully improved the mechanical and corrosion properties of the Mg-1.2Zn-0.5Mn (wt.%) alloy. Both the in vitro electrochemical and immersion corrosion tests showed that the MAO-coated samples have a significantly higher corrosion resistance which results in a significantly lower corrosion rate. This study indicated that the biocompatible coating produced by MAO process may be suitable for providing heat-treated Mg-Zn-Ca-based alloys with protection from corrosion towards synthesizing bone fixation materials in clinical application.

Lightweight magnesium alloys offer excellent benefits over Al alloys due to their high specific strength and damping properties, but they are more prone to galvanic corrosion. Plasma electrolytic oxidation (PEO) coatings reinforced by nanoparticles have been shown to improve corrosion resistance and possess better mechanical properties. A lot of research has been published that focuses on the effect of nanoparticle concentration in the PEO electrolyte solution, and the type of nanoparticle, on the properties obtained. The aim of paper is to study the effect of processing time on the nanoparticle-reinforced PEO coating on AZ31 magnesium alloy. TiN and SiC nanoparticles were produced using plasma chemical synthesis and added to KOH-based electrolyte to develop PEO coatings. The concentration of nanoparticles was kept constant at 0.5 g/L and the treatment time was varied as follows: 1, 2, 3, 5, and 10 min. The coatings were tested for their microstructure, phase, chemical makeup, nano-mechanical properties, and corrosion resistance. Nanoparticles were found to be clustered in the coating and spread unevenly but led to a decrease in the size and number of pores on the PEO coating surface. The corrosion resistance and nano-mechanical properties of the coating improved with treatment time. The hardness and contact modulus of coatings with TiN particles were 26.7 and 25.2% greater than those with SiC particles. Addition of TiN nanoparticles resulted in improved corrosion resistance of the PEO coatings when the processing time was 5 or 10 min. The lowest corrosion rate of 6.3 × 10−5 mm/yr was obtained for TiN-added PEO coating processed for 10 min.