Oxidation resistance and corrosion behavior of hot-dip aluminized coatings on commercial-purity titanium

Aluminizing is an effective method to protect alloys from oxidation and corrosion. In this article, the microstructure, morphology, phase composition of the aluminized layers and the oxide films were investigated by SEM, EDS and X-ray diffraction. The high temperature oxidation resistance and electrochemical behavior of hot dip aluminizing coatings on commercial-purity titanium had been studied by cyclic oxidation test and potentiodynamic polarization technique. The results show that the reaction between the titanium and the molten aluminum leads to form an aluminum coating which almost has the composition of the aluminum bath. After diffusion annealing at 950 °C for 6 h, the aluminum coating transformed into a composite layer, which was composed of an inner layer and an outer layer. The inner layer was identified as Ti₃Al or Ti₂Al phase, and the outer layer was TiAl₃ and Al₂O₃ phase. The cyclic oxidation treatment at 1000 °C for 51 h shows that the oxidation resistance of the diffused titanium is 13 times more than the bare titanium. And the formation of TiAl₃, θ-Al₂O₃ and compact α-Al₂O₃ at the outer layer was thought to account for the improvement of the oxidation resistance at high temperature. However, the corrosion resistance of the aluminized titanium and the diffused titanium were reduced in 3.5 wt.% NaCl solution. The corrosion resistance of the aluminized titanium was only one third of bare titanium. Moreover, the corrosion resistance of the diffused titanium was far less than bare titanium.     

316L表面激光熔覆复合等离子体电解氧化制备多孔陶瓷涂层及其生物应用（英文）

为了提高316L不锈钢的生物活性,采用激光熔覆(LC)技术在316L不锈钢表面制备钛层,然后利用等离子体电解氧化(PEO)技术在钛层上形成多孔陶瓷涂层。采用三维表面轮廓仪、SEM、EDS、XRD和XPS等测试方法对涂层试样的形貌、微观结构和组成进行表征。通过动电位极化曲线和模拟体液(SBF)浸泡试验,分别对涂层的耐腐蚀性和生物活性进行评价。结果表明,多孔陶瓷涂层主要由锐钛矿和金红石组成,并检测到高结晶HA。陶瓷涂层的主要元素为Ca、P、Ti和O。在模拟体液中,LC+PEO复合生物涂层比316L基质具有更优异的耐腐蚀性,并且复合涂层能有效提高316L不锈钢的生物活性。     

Porous TiO2 film prepared by micro-arc oxidation and its electrochemical behaviors in Hank's solution

Porous titanium oxide film was prepared by micro-arc oxidation (MAO) method on the surface of titanium alloy in electrolyte containing Ca and P. Surface characterizations of the film were carried out using X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) before and after immersion in Hank's solution. Electrochemical behaviors and corrosion resistance were studied by electrochemical techniques. The film was mainly composed of titania, α-tricalcium phosphate (α-TCP) and amorphous Ca-P compounds. α-TCP and amorphous compounds could transformed into hydroxyapatite (HA) when immersed in Hank's solution. MAO film showed higher corrosion potential and lower corrosion current than the titanium alloy and its chemical stability was slightly changed after formation of HA. Fitted electrochemical impedance spectroscopy (EIS) data indicated that after immersion for 2 weeks the MAO film kept good corrosion resistance. Porous TiO₂ film on titanium alloy by MAO method showed good chemical stability in Hank's solution and the transformation of Ca-P compounds into HA indicated that MAO was an effective method for preparing titanium alloys as bioactive artificial bone substitute even when Ca and P in the tissue environment were not abundant.     

Effects of duty ratio on properties of micro-arc film on Ti–3Zr–2Sn–3Mo–25Nb

Porous titanium oxide layers, which are important features for improving the biological activity of Ti implants with bone tissues, have been obtained through the technique of micro-arc oxidation (MAO). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to evaluate the micromorphology and crystalline structure of these oxide films, and the chemical compositions were measured by electron dispersive X-ray spectroscopy (EDS). TiO₂ layers presented the crystalline phases of rutile and anatase. During the micro-arc oxidation treatment, Ca and P ions were incorporated into the oxide layer, and incorporation of Ca and P with the Ca/P content (%) of around 1.38 is similar to that in the human body. Nb₂O₅ was also identified in the treatment samples. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves. In the electrochemical corrosion tests, the treated samples presented lower values of corrosion current density than untreated Ti, indicating a better corrosion resistance. Diffusion phenomena were present in the process of corrosion.     

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.     

Preparation and cytocompatibility of Si-incorporated nanostructured TiO2 coating

Silicon (Si)-incorporated TiO₂ coating with porous and nanostructured surface on titanium was prepared by plasma electrolytic oxidation (PEO). The microstructure, phase composition and elemental distribution of the coating were characterized using scanning electron microscopy, X-ray diffraction and energy-dispersive X-ray spectrometry. MG63 cells were cultured on the surface of the coating to investigate its cytocompatibility. Potentiodynamic polarization tests were used to measure its corrosion resistance. The results showed that Si was successfully incorporated and distributed homogeneously in the coating. The Si-incorporated coating showed a denser and finer coating structure and higher MG63 cell proliferation rate and vitality than Si-free coating. The corrosion resistance of the Si-incorporated coating was also significantly improved. In conclusion, it is an interesting and promising way to regulate the functions of the coatings by introducing desired elements, such as Si, by PEO technique.     

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.     

Substitution of hexavalent chromate conversion treatment with a plasma electrolytic oxidation process to improve the corrosion properties of ion vapour deposited AlMg coatings

In this paper plasma electrolytic oxidation (PEO) is examined as a potentially beneficial post-deposition treatment for Al-5 wt.%Mg coatings deposited onto Ti alloy substrates using Ion Vapour Deposition (IVD), with a view to replacing toxic hexavalent chromate conversion treatments and at the same time enhancing the barrier protection properties of such IVD coatings. The aqueous corrosion behaviour of PEO layers formed on IVD coatings was evaluated by means of potentiodynamic polarisation scans, open circuit potential measurements and electrochemical impedance spectroscopy. Normally, IVD aluminium-based coatings require a post-coat shot peening treatment to densify the coating structure; however it was found that PEO layers could be formed successfully on as-deposited IVD coatings deposited onto Ti alloy substrates, providing a cost effective process to improve corrosion behaviour. PEO treatment provides unique improvements in the corrosion resistance of IVD coatings; the PEO layer possesses effective anticorrosion properties in aqueous environments containing Cl⁻ ions. A more positive corrosion potential, lower corrosion current and increased polarisation resistance were recorded for PEO layers, compared to traditional chromate conversion treatments.     

Investigation of Plasma Electrolytic Oxidation (PEO) coatings on a Zr-2.5Nb alloy using high temperature/pressure autoclave and tribological tests

A Plasma Electrolytic Oxidation (PEO) process was used to produce thin oxide coatings on a Zr-2.5 wt% Nb alloy. Effects of current density on surface morphologies and wear properties of PEO coatings were investigated and compared to the uncoated substrate and a commercially used black oxide coating. Corrosion properties at ambient and high temperature/pressure conditions were studied using potentiodynamic polarization tests and autoclave tests, respectively. Up to 30-day autoclave experiments were carried out in an aqueous condition of 300 °C and 10 MPa in 0.05 M LiOH solutions. It was found that most of the micro-pores which were produced during the PEO treatment were closed after the autoclave experiments. PEO coatings had larger weight gains in the first 10 exposure days than the black oxide coating. However, after 10 days, the corrosion rate of black oxide coating accelerated and exhibited a similar weight gain to PEO coatings after 30 days. PEO coatings prepared at low current densities had lower weight gains. Although the black oxide coating exhibited a good corrosion resistance, it had a much lower wear resistance than the PEO coatings. Compared with the uncoated substrate, all PEO coatings had a higher corrosion resistance, lower weight gain during autoclave tests and better wear resistance.     

纯Ti表面微弧氧化-碱热处理仿生陶瓷膜的制备及耐蚀性

采用微弧氧化-碱热处理在纯Ti表面制备了含有羟基磷灰石(HA)的仿生陶瓷膜。利用SEM,XRD和电化学工作站等手段研究了膜层的形貌、物相及其耐蚀性。结果表明:在乙酸钙-磷酸二氢钙电解液体系中微弧氧化(MAO),纯Ti表面形成一层含Ca和P的TiO2多孔陶瓷膜。经水热处理后,膜层表面的孔洞变小、致密性增加,膜层中还出现了鳞状、层片状以及针棒状的HA。在Hank's模拟体液中,MAO膜和微弧氧化-碱热处理(MAOAH)膜均表现出较好的耐蚀性。MAO膜经模拟体液腐蚀后,形成了缺钙型HA(Ca8.86(PO4)6(H2O2)2)和CaTiO3;而模拟体液中的阴离子与MAOAH膜层的氧化物作用使膜层孔洞直径和深度增加。     

A study of the corrosion behavior of gamma titanium aluminide in 3.5 wt% NaCl solution and seawater

Gamma titanium aluminide (γ-TiAl) was evaluated for corrosion resistance in 3.5% NaCl and seawater using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Polarization plots indicate very low corrosion rates although in both electrolytes the control samples lost passivity at low polarization potentials. Surface modification treatments were employed by oxidizing the samples in air at 500 °C and 800 °C with the purpose of improving corrosion resistance. While the surface treatments rendered γ-TiAl passive in the polarization experiments in both electrolytes, EIS tests showed that the oxide formed at 800 °C was not protective. EEC models are proposed to explain the EIS results for the three surface conditions tested.     

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.     

Morphology and Properties of Coatings Obtained by Plasma-Electrolytic Oxidation of Titanium Alloys in Pyrophosphate Electrolytes

By means of scanning electron microscopy, atomic-force microscopy, X-ray fluorescence analysis, and X-ray spectral microanalysis it was shown that, under plasma-electrolytic oxidation (PEO) of titanium alloys in pyrophosphate electrolytes, well-adhered oxide coatings with microglobular morphology result. It was demonstrated that the chemical and phase composition of the coatings, as well as the surface topography and grain size, can be controlled by changing the concentration of a pyrophosphate electrolyte and a PEO current density. It was established that the resulting oxide layer is highly resistant to abrasive wear and enhances the titanium corrosion resistance in model media (Ringer’s solution) substantially, which suggests that the coatings are promising for use in biological applications.     

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.     

用于制备纯钛表面新型PEO 陶瓷膜最佳电解液配方研究

    为筛选出最佳的含硅电解液配方,配制了4种不同的含钙、磷、硅的电解液;对比研究了不同溶液中,等离子火花放电现象以及所制备PEO陶瓷膜的厚度、微观形貌、元素及相组成.结果表明,在4种电解液中,溶液D中制备的氧化膜生长速度较快,且膜层表观均匀光滑;该氧化膜表面孔隙率较高且微孔大小均匀一致;在膜层D中均具有较高的生物活性元素钙、磷、硅.因此,溶液D成为制备含硅生物活性PEO陶瓷膜的最佳电解液配方.     

Mixed Oxide Films Formed on Titanium Alloy by Plasma Electrolytic Oxidation

The structure and the properties of oxide films formed on titanium in the diphosphate based electrolytes by plasma electrolytic oxidation in the spark-discharge regime at application of inter-electrode voltage 100 to 130 V have been studied. A possibility to obtain oxide layers containing alloying elements by the modification of the composition of electrolytes has been stated. It was found that the chemical and phase composition as well as the topography, the microstructure and the grain size of the formed layers depend on the electrolyte composition, applied current density and inter-electrode voltage. The effect of the chemical composition of the formed mixed oxide films on the corrosion resistance and catalytic activity has been discussed.     

Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting

An AZ91D magnesium alloy was treated using duplex techniques of laser surface melting (LSM) and plasma electrolytic oxidation (PEO). The microstructure, composition and corrosion behavior of the laser melted surface, PEO coatings, LSM–PEO duplex coatings as well as the as-received specimen were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electrochemical corrosion tests, respectively. Especially, the effect of LSM pre-treatment on the microstructure, composition and corrosion resistance of the PEO coatings was investigated. Results showed that the corrosion resistance of AZ91D alloy was marginally improved by LSM due to the refinement of grains, redistribution of β-phase (Mg₁₇Al₁₂) and increase of Al on the surface. Both the PEO and duplex (LSM–PEO) coatings improved significantly the corrosion resistance of the AZ91D alloys, while the duplex (LSM–PEO) coating exhibited better corrosion resistance compared with the PEO coating.     

Influence of alloying elements on the corrosion resistance of rolled zinc sheet

The influence of alloying elements on the corrosion behaviour of rolled zinc sheet in aqueous media has been investigated by means of electrochemical techniques. All the changes in corrosion behaviour seen in this study could be attributed to modification of the formation or the stability of the passivating oxide film on the zinc surface. A low concentration of copper (0.6 wt.%) inhibited the formation of the passivating film and reduced the stability of the film. Conversely, a low concentration of chromium (0.5 wt.%) accelerated the passivation process and raised the stability of the film. The passivation and corrosion behaviour shown by a commercially produced ternary alloy containing copper and titanium additions was almost the same as the behaviour shown by a model binary alloy containing only copper. All the results obtained in this study were consistent with the hypothesis that alloying elements alter the electron-conducting and/or ion-conducting properties of the passivating oxide film.     

钛在H2O2溶液中的腐蚀特性研究

通过分光光度计测定腐蚀后溶液中钛离子浓度的方法，研究了钛在H2O2溶液中的腐蚀特性，以及不同pH值条件下，K^＋，Ca^2＋，Na^＋，Mg^2＋，Cl^-，HPO4^2-，H2PO4^-等添加离子对钛腐蚀的影响。结果表明：Ca^2＋对钛腐蚀具有最强的抑制性，而HPO4^2-则具有最强的促进作用。分析认为：Ca^2＋通过吸附到钛表面形成CaTiO3或者CaO2来抑制钛的腐蚀：而HPO4^2-则是吸附到钛表面来增加表面氧化膜的缺陷，从而促进钛的腐蚀。     

Thin films of titanium and tin oxides and semiconductor structures on their basis obtained by pyrolytic pulverization: Preparation,characterization, and corrosion properties

Peculiarities of obtaining of tin oxide and titanium oxide layers and semiconductor structures on their basis are described. The X-ray diffraction data show that the SnO₂ and TiO₂ layers possess the tetragonal crystal structure (anatase modification for TiO₂). The results of analysis of the elemental composition and impedance investigations of the fabricated structures in model chloride-sulfate solutions demonstrate that the oxide/SiO₂/Si structures are obtained if Si substrates are used. In the case of InP substrates, the oxide layer at the interface is not detected and the corresponding structure is oxide/InP. The results of investigations of corrosion show that a substantial shift of the corrosion potential to the anode region is observed in the case of deposition of SnO₂ and TiO₂ oxide layers on Si and InP crystals and fabrication of corresponding semiconductor structures. This demonstrates the possibility of the use of these materials in photoelectrochemical applications.