Tribological and corrosion behavior of PEO coatings with graphite nanoparticles on AZ91 and AZ80 magnesium alloys

The effect of the addition of graphite nanoparticles into the electrolyte used to produce plasma electrolytic oxidation (PEO) coatings on AZ91 and AZ80 magnesium alloys was studied. The corrosion and wear resistances of the obtained coatings were investigated. A solution that contained both phosphates and silicates was used as electrolyte. Moreover, two different PEO treatment times were studied. The corrosion resistance was analyzed with potentiodynamic polarization and EIS tests; the wear resistance was investigated with a flat on ring tribometer. The results were related to the morphology, microstructure, elemental composition and thickness evaluated with SEM analysis. The presence of the graphite nanoparticles increased the thickness, produced a densification of the coating and sealed the pores on the surface, thus improving both the corrosion and wear resistance. The increase in the corrosion and wear resistances was more evident for AZ91 than for AZ80 due to the higher aluminum content.     

镁合金表面PEF膜与PEO膜腐蚀防护行为的对比研究

目的 对比研究镁合金表面新型等离子体电解氟化(PEF)膜与传统等离子体电解氧化(PEO)膜的腐蚀防护行为.方法 分别在中性和酸性腐蚀介质中,通过开路电位监测和动态电位极化曲线测试表征了膜层的电化学腐蚀行为,通过浸泡实验和盐雾实验表征了膜层的长效腐蚀行为.通过SEM、EDS和XRD等方法表征了膜层的原始微观结构和组成,分析了腐蚀形貌和腐蚀产物.结果 PEF膜与PEO膜均可以为镁合金基材提供有效的腐蚀防护作用.相较于PEO膜,PEF膜在浸泡实验和盐雾实验中,都具有更为优异的腐蚀防护性能,但在动态电位极化测试中,具有更正的自腐蚀电位和更大的自腐蚀电流密度,表明其腐蚀倾向更低,但腐蚀速率更高.结论 总体而言,PEF膜在中性和酸性环境中都具有更好的腐蚀防护性能.PEO膜在中性环境中的腐蚀防护失效机制主要是腐蚀介质的扩散,在酸性环境中的腐蚀防护失效机制主要是膜层化合物的溶解和消耗;PEF膜在中性和酸性环境中的腐蚀防护失效机制都是腐蚀介质的扩散.     

Wear and corrosion resistant coatings on surface of cast A356 aluminum alloy by plasma electrolytic oxidation in moderately concentrated aluminate electrolytes

Plasma electrolytic oxidation of a cast A356 aluminum alloy was carried out in aluminate electrolytes to develop wear and corrosion resistant coatings. Different concentrations of 2, 16 and 24 g/L NaAlO₂ solutions and a silicate electrolyte (for comparison) were employed for the investigation. Wear performance and corrosion resistance of the coatings were evaluated by WC (tungsten carbide) ball-on-flat dry sliding tests and electrochemical methods, respectively. The results show that the coating formed for a short duration of 480 s in 24 g/L NaAlO₂ solution generated the best protection. The coating sustained 30 N load for sliding time of 1800 s, showing very low wear rate of ～4.5×10^?7 mm³/(N·m). A low corrosion current density of ～8.81×10^?9 A/cm² was also recorded. Despite low α-Al₂O₃ content of the coating, the compact and nearly single layer nature of the coating guaranteed the excellent performances.     

Self-similar scaling of discharge events through PEO coatings on aluminium

Plasma electrolytic oxidation (PEO) typically involves hundreds to thousands of discharges per square millimetre per second. In the present work, the characteristics of individual discharges are measured and investigated through an experimental survey of over 3.7 million individual events. The parameters considered include the voltage at which a discharge event commences, the peak current to which it develops, the duration of the discharge, and the current-time profile of individual discharges. It is shown that there is a strong correlation between the voltage at which a discharge event commences and the peak current to which it subsequently develops. There are also strong correlations between these characteristics and the duration of the discharges. These correlations show no significant influence of the coating thickness. The current-time profile is similar for most discharges, scaling according to the initiation voltage, suggesting that all PEO discharge events, whether weak or powerful, all develop according to the same physical mechanisms.     

镁合金微等离子体电解氧化陶瓷层生长及膜层微观形态和耐蚀性能的研究

通过对镁合金微等离子体电解氧化（Plasma Electrolytic Oxidation，PE0）沉积陶瓷层的生长过程、微观形貌及相组成的分析，研究探讨了镁合金微等离子体电解氧化沉积陶瓷层的生长过程与生长机理，并采用盐雾腐蚀试验对镁合金微等离子体氧化沉积陶瓷层耐蚀性进行了研究对比。实验结果表明，镁合金微等离子体电解氧化得到的陶瓷层分为3部分：外层为疏松层，表面有很多孔洞；中间层为紧密层，结构紧密；内层为过渡层，为陶瓷层与基体镁合金相互渗透的衔接部位，是经典的冶金结合；在微等离子体电解氧化处理过程中，由于在高压高温的等离子体环境下，促进了氧负离子和镁离子借助于放电通道向膜层深处的渗透和迁移，形成表面的盲孔，提高了防护膜的致密性和与镁合金基体结合的坚韧度。     

AC PEO of aluminium with porous alumina precursor films

The effect of the composition, morphology and thickness of pre-formed porous anodic films on the subsequent formation of coatings on aluminium by plasma electrolytic oxidation is investigated. The pre-formed films, of a wide range of thickness, are shown to be incorporated into the coatings. Further, an increased thickness of the film is revealed as beneficial to the establishment of a microarc regime that is favourable for the formation of the coatings. The anodic alumina undergoes melting under the microdischarges, which changes the cellular film morphology and reduces the concentration of anion contaminants.     

The effect of substrate composition on the electrochemical and mechanical properties of PEO coatings on Mg alloys

Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl₂O₄) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20 ± 2 °C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings.     

Corrosion pathways in aluminium alloys

The corrosion pathways in AA2024-T3, AA5083-O and AA6082-T6 alloys have been investigated. The objective of the investigation is to further the understanding of the complex localised corrosion mechanism in aluminium alloys. The investigation was carried out by examining the corroded surfaces of the alloys after potentiodynamic polarization tests in a 3.5% NaCl solution with the aid of a scanning electron microscope, and by analysing the flow of anolyte solution using the scanning vibrating electrode technique. The results revealed that the overall corrosion pathways in the alloys are distinctively different and are influenced by the flow of anolyte solution. Also revealed, was the fact that corrosion propagates in two ways (particularly in the AA5083-O and AA6082-T6 alloys): an overall pathway in the corrosion front (filiform-like pathway in the AA5083 alloy and organized linear pathways in AA6082 alloy); and the crystallographic channelling along the 〈100〉 directions. These are dependent on the grain distinct features of the AA5083-O and AA6082-T6 alloys and are not influenced by the presence of coarse second phase particles in these alloys, compared with the AA2024-T3 alloy, where the corrosion pathways are more dependent on the presence of second phase particles and grain boundary character.     

Characterization of AC PEO coatings on magnesium alloys

Optical emission spectroscopy, fast video imaging and coating characterization are employed to investigate AC plasma electrolytic oxidation (PEO) of magnesium alloys. The findings revealed initiation and gradual increase in the number of discharges after 2-4 ms of each anodic pulse once a critical voltage was reached. No discharges were observed during the cathodic half-cycles. The lifetimes of discharges were in the range of 0.05-4 ms. A transition in the voltage-time response, accompanied by a change in the acoustic and optical emission characteristics of discharges, was associated with the development of an intermediate coating layer with an average hardness of 270-450 HV_0.05. The coatings grew at a rate in the range 4.0-7.5 µm min^− 1, depending on the substrate composition. Regardless of the substrate, the coatings consisted of MgO and Mg₂SiO₄, with incorporation of alloying element species. Electrolyte species were mainly present in a more porous layer at the coating surface, constituting 20-40% of the coating thickness. A thin barrier layer consisting of polycrystalline MgO was located next to the alloy. The corrosion rate of the magnesium alloys determined using potentiodynamic polarization in 3.5 wt.% NaCl was reduced by 2-4 orders of magnitude by the PEO treatment.     

Wear and corrosion behavior of clay containing coating on AM 50 magnesium alloy produced by aluminate-based plasma electrolytic oxidation

This study aims to examine the effect of clay micro particles addition on the microstructure, wear and corrosion behavior of PEO coatings on AM 50 magnesium alloy. PEO coatings were prepared using an aluminate-based electrolyte with and without the presence of 5 g/L clay particles. The structure and composition of the coatings were evaluated using SEM, EDS and XRD. The wear investigations were conducted using a ball-on-disk tribometer at 2, 5 and 10 N loads. The corrosion behavior of the coatings was examined using polarization and EIS tests in 0.5 wt.%NaCl. The results revealed that the addition of clay particles deteriorated the wear resistance of the coatings under the loads of 5 and 10 N. The SEM examinations of the worn surfaces indicated that a combination of adhesive and abrasive wear mechanisms was activated for the coating with clay particles. The poor wear performance of the clay-incorporated coating was related to its lower adhesion strength and higher roughness. The potentiodynamic polarization examinations revealed that the addition of clay particles slightly decreased the corrosion rate of the coatings. Corrosion resistance of the clay-containing coating was attributed to its compactness, as indicated by the results of EIS tests.     

铝合金阳极氧化膜电解着色工艺概况

介绍了几种传统的阳极氧化电解着色技术原理及典型工艺,在此基础上归纳了传统电解着色的技术特点并介绍了该领域的最新研究动向,并详细介绍了最新研究方向之一"多彩电解着色"的工艺原理和主要方法。最后,预测电解着色技术在未来相当长的时间内仍然是铝合金着色的主要工艺技术,同时也指明了多彩着色技术在未来是一个重要的发展方向。     

铝电解惰性阳极研究现状

惰性阳极是铝工业新技术的一个重要课题,本文对近年来铝电解阳极材料多已集中在金属、金属陶瓷、氧化物陶瓷和氧化铈涂层几个方面的现状及研究进展分别加以介绍。指出铝电解惰性阳极的研制一开始就遇到的问题,如氧化,腐蚀,产出铝受惰性阳极材料的污染,阳极寿命,导电性和成本问题仍未完全解决。     

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024‐T3 with the aim of replacing chromate‐based technologies. Cerium diphenyl phosphate (Ce(dpp)₃) and mischmetal diphenyl phosphate (Mm(dpp)₃) were added to epoxy coatings applied to AA2024‐T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp)₃ was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp)₃ appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.     

Effect of NaOH on plasma electrolytic oxidation of A356 aluminium alloy in moderately concentrated aluminate electrolyte

Plasma electrolytic oxidation(PEO) of cast A356 aluminum alloy was carried out in 32 g/L NaAlO₂ with the addition of different concentrations of NaOH. The stability of the aluminate solution is greatly enhanced by increasing the concentration of NaOH. However, corresponding changes in the PEO behaviour occur due to the increment of NaOH concentration. Thicker precursor coatings are required for the PEO treatment in a more concentrated NaOH electrolyte. The results show that the optimal NaOH concentration is 5 g/L, which improves the stability of storage electrolyte to about 35 days, and leads to dense coatings with high wear performance (wear rate: 4.1×10⁻⁷ mm³·N⁻¹·m⁻¹).     

阴极电流密度对6061铝合金在含Na2WO4电解液中微弧氧化膜结构和耐磨性能的影响

目的 研究恒流模式下阴极电流密度对6061铝合金在含Na2WO4的电解液中制备的微弧氧化膜厚度、形貌、相组成及耐磨性能的影响。方法 固定阳极电流密度为5.0 A/dm²,阴极电流密度分别为0、1.25、2.5、3.75、5.0 A/dm²,对6061铝合金进行微弧氧化40 min。用涡流测厚仪测量了氧化膜的厚度,用扫描电镜观察了微弧氧化膜的表面形貌和截面形貌,用能谱分析仪分析了氧化膜的表面成分,用X射线衍射分析仪分析了微弧氧化膜的相组成,用往复式摩擦磨损试验机测试了氧化膜的耐磨性能。结果 随着阴极电流密度的增加,氧化膜内的W含量逐渐减少,氧化膜颜色逐渐变浅,氧化膜厚度逐渐增加。微弧氧化膜的主要组成相为α-Al2O3和γ-Al2O3。当阴极电流密度从0 A/dm²增加到3.75 A/dm²时,氧化膜内孔洞的数量和尺寸逐渐减少,孔洞到氧化膜/基体界面的距离逐渐增加,氧化膜的耐磨性能逐渐提升。当阴极电流密度为3.75 A/dm²时,氧化膜的磨损率最低,仅为1.07×10^‒4 mm³/(N.m)。但阴极电流密度增加到5.0 A/dm²时,氧化膜表层出现孔洞和剥落,耐磨性能下降。结论 阴极电流的加入有助于增加6061铝合金微弧氧化膜的厚度,提高氧化膜的致密性和耐磨性能,但过高的阴极电流会导致氧化膜表层出现孔洞,降低耐磨性能。     

Spectroscopic characterization of plasma during electrolytic oxidation (PEO) of aluminium

We present results of an optical emission spectroscopic study of Plasma during Electrolytic Oxidation (PEO) of aluminium in citric acid and in sodium-tungstate water solutions. The line shape analysis of the first two hydrogen Balmer lines indicates presence of two PEO processes characterized by relatively low electron number densities N_e ≈ 0.8 × 10¹⁵ cm^− 3 and at 2.5 × 10¹⁶ cm^− 3. Apart from these two N_e values, N_e ≈ 6.0 × 10¹⁶ cm^− 3 is determined from the width of the Al II 704.2 nm line. Three considerably different N_e values imply presence of three types of discharge during PEO of aluminium. The electron temperature T_e is determined from relative line intensities of the O II (T_e ≈ 38,300 K) and W I (T_e ≈ 3300 K) lines. The use of two databases of transition probabilities for O II lines introduces systematic difference of reported T_e results.     

Recent advances in the deformation processing of titanium alloys

Titanium (Ti) alloys are special-purpose materials used for several critical applications in aerospace as well as non-aerospace industries, and extensive deformation processing is necessary to shape-form these materials, which poses many challenges due to the microstructural complexities. Some of the recent developments in the deformation processing of Ti alloys and usefulness of integrating the material behavior information with simulation schemes while designing and optimizing manufacturing process schedules are discussed in this paper. Discussions are primarily focused on the most important alloy, Ti-6Al-4V and on developing a clear understanding on the influence of key parameters (e.g., oxygen content, starting microstructure, temperature, and strain rate) on the deformation behavior during hot working. These studies are very useful not only for obtaining controlled microstructures but also to design complex multi-step processing sequences to produce defect-free components. Strain-induced porosity (SIP) has been a serious problem during titanium alloy processing, and improved scientific understanding helps in seeking elegant solutions to avoid SIP. A novel high-speed processing technique for microstructural conversion in titanium has been described, which provides several benefits over the conventional slow-speed practices. The hot working behavior of some of the affordable α+β and β titanium alloys being developed recently—namely, Ti-5.5Al-1Fe, Ti-10V-2Fe-3Al, Ti-6.8Mo-4.5Fe-1.5Al, and Ti-10V-4.5Fe-1.5Al—has been analyzed, and the usefulness of the processing maps in optimizing the process parameters and design of hot working schedules in these alloys is demonstrated. Titanium alloys modified with small additions of boron are emerging as potential candidates for replacing structural components requiring high specific strength and stiffness. Efforts to understand the microstructural mechanisms during deformation processing of Ti-B alloys and the issues associated with their processing are discussed.     

Recent advances in the thermohydrogen processing of titanium alloys

In this article, the status of the methods and applications of the thermohydrogen processing of Titanium alloys is reviewed. Increased understanding of the mechanisms by which such processing is enhanced and the microstructure refined should lead to industrial applications.