The influence of cathodic pulse parameters was evaluated on plasma electrolytic oxidation (PEO) coatings grown on 7075 aluminum alloy in a silicate-based electrolyte containing potassium titanyl oxalate (PTO) using pulsed bipolar waveforms with various cathodic duty cycles and cathodic current densities. The coatings were characterized by SEM, EDS, and XRD. EIS was applied to investigate the electrochemical properties. It was observed that the increase of cathodic duty cycle and cathodic current density from 20% and 6 A/dm2 to 40% and 12 A/dm2 enhances the growth rate of the inner layer from 0.22 to 0.75 mu m/min. Adding PTO into the bath showed a fortifying effect on influence of the cathodic pulse and the mentioned change of cathodic pulse parameters, resulting in an increase of the inner layer growth rate from 0.25 to 1.10 mu m/min. Based on EDS analysis, Si and Ti were incorporated dominantly in the upper parts of the coatings. XRD technique merely detected gamma-Al 2 O 3 , and there were no detectable peaks related to Ti and Si compounds. However, the EIS results confirmed that the incorporation of Ti4+ into alumina changed the electronic properties of the coating. The coatings obtained from the bath containing PTO using the bipolar waveforms with a cathodic duty cycle of 40% and current density values higher than 6 A/dm2 showed highly appropriate electrochemical behavior during 240 d of immersion due to an efficient repairing mechanism. Regarding the effects of studied parameters on the coating properties, the roles of cathodic pulse parameters and PTO in the PEO process were highlighted.

Due to their low density and high specific strength, aluminium alloys provide a great range of application, but they are limited regarding corrosion and wear resistance. Plasma electrolytic oxidation (PEO) is an innovative process to produce oxide ceramic surfaces on aluminium to protect against corrosion and wear. However, due to their brittle nature and an unfavourable residual stress ratio (substrate/coating), PEO coatings have a limited fatigue resistance. The characterisation of the failure behaviour using conventional material testing methods is mostly elaborate and time-consuming. Furthermore, microstructure analysis methods using metallographically prepared specimen sections only provide local two-dimensional information about failure. In contrast, acoustic emission (AE) analysis is an integral method for the examination of the cracking behaviour and fracture mechanisms of materials. However, its application for PEO coatings is a novel approach. In this work, the suitability of the AE analysis for these coatings is evaluated by the characterisation of the fracture and failure behaviour under three-point bending loading. Since the fatigue test is a complex loading case, the focus is first on the static test. Signals were acquired during mechanical testing using an AE sensor and processed by Fourier transform. Two characteristic frequency bands between 150 kHz and 250 kHz as well as 350 kHz and 450 kHz were observed in the generated frequency spectra. From the correlation with the microscopic images, the low-frequency band was assigned to vertical surface cracks, whereas the higher-frequency band was primarily attributed to horizontal interface cracks.

Depassivation processes known by literature detected by electrochemical methods during polarization experiments of Mg substrates in alkaline silicate PEO electrolytes below the ignition voltage were investigated in more detail by SEM images of the generated reaction layers. Remarkable crater structures were discovered on the Mg alloy AZ31 that do not form on the aluminum AlMgSi1 substrate under identical experimental conditions. The formations of these structures were described using the concepts of the high-field model. Through systematic comparative investigations of PEO processes on the two studied substrates by means of in situ process diagnostics, electrochemical subprocesses in the plasma electrolytic oxidation of magnesium were identified by analyzing electrical and photoelectric process signals. A negative influence on the morphology of PEO layers on magnesium was attributed to subprocesses in the context of the depassivation mechanism below the ignition voltage as well as microstructure images of the generated PEO layers.

Al2O3 coatings, which can be produced by plasma electrolytic oxidation (PEO) on aluminum substrates, provide an excellent protection against corrosion and wear. However, due to the brittle nature of the oxide ceramic, the fracture toughness is limited. One approach to enhance the tolerance to fracture is the incorporation of ZrO2 to form zirconia toughened alumina (ZTA). In addition to its use as a bulk material, the application as a coating material enables a broader field of application. In this study, an Al2O3-ZrO2 composite coating was applied on a 6082 aluminum alloy using an aluminate-phosphate-based electrolytic solution containing a Zr-based salt. Polarization measurement as an indicator of the passivability of a given system revealed that Zr-based salt improves the passivation of the aluminum alloy. The coatings’ characteristics were evaluated by SEM, EDS, and XRD. ZrO2 incorporated into alumina as a metastable high-temperature modification led to a thicker coating with new morphologies including lamellar and dendritic structures. Nano-indentation showed that the incorporated Zr increase the average hardness of the compact layer from 16 GPa to 18 GPa. The fracture toughness of the coatings was investigated locally with nano-scratches applied on the compact outer layer of the coatings’ cross-sections. The Zr-containing electrolytic solution resulted in a coating with significantly higher fracture toughness (6.9 MPa∙m1/2) in comparison with the Zr-free electrolytic solution (4.6 MPa∙m1/2). Therefore, it is shown, that the PEO process stabilized a high-temperature allotrope of zirconia at room temperature without the need for rare-earth dopants such as Y2O3. Furthermore, it was demonstrated that the nano-scratch method is a suitable and accurate technique for the investigation of the fracture toughness of coatings with inherent cracks.

Potentiodynamic and potentiostatic polarization tests in the potential range between open circuit potential (OCP) − 0.1 V and OCP + 4 V were carried out in aluminate–phosphate electrolytes with an aluminate concentration of 0.2 mol/L and varying phosphates contents between 0 and 0.1 mol/L. The pH was adjusted between 11.5 and 12.0 due to phosphate and optional KOH addition. A high-strength, dual-phase steel, which is relevant for lightweight construction, served as the substrate material. The layer microstructure was investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy and Raman spectroscopy were used for element and phase analyses. We found that iron hydroxides or oxides are initially formed independently of the electrolyte composition at low potentials. At around 1 V vs. standard hydrogen electrode (SHE), the current density suddenly increases as a result of oxygen evolution, which causes a significant reduction in the pH value. Precipitation leads to the formation of porous layers with thicknesses of 10 µm to 20 µm. In the case of a pure aluminate solution, the layer mainly consists of amorphous alumina. When adding phosphate to the electrolyte, the layer additionally contains the hydrous phosphate evansite. At the highest phosphate content in the electrolyte, the highest P content and the most pronounced crack network were observed.

Al2O3-TiO2 coatings were obtained from a silicate-based electrolyte using pulsed bipolar current by PEO process. Nano-particle titania (NP-TiO2) and potassium titanyl oxalate (PTO) were used as Ti-based additive sources in the PEO electrolytic solution, separately. The coatings were characterized using scanning electron microscope, energy dispersive spectroscopy, and an X-ray diffractometer. The mechanical properties of the coatings were investigated using nanoindentation and ball-on-disk tests. SEM results showed that the PTO developed a more compact inner layer besides the increase of coating thickness. However, NP-TiO2 created an inner layer with less thickness but with higher compactness, without any effect on the outer layer morphology. XRD and Raman spectroscopy analyses showed that the NP-TiO2 had inert incorporation into the alumina. However, TiO2 produced by PTO had reactive incorporation into alumina and made a polymeric titanium oxide structure on the coating with doped rutile and anatase phases. Nanoindentation and tribology analyses approved that the Ti incorporation through PTO provides appropriate mechanical properties owing to the more compact and thicker inner layer. The mechanism of PTO performance in the PEO process was discussed regarding its effect on coating characteristics.

The influence of chemical redissolution in the PEO layer-growth equilibrium on aluminum is evaluated differently in literature. In order to estimate whether and to what extent various alumina modifications could be affected by redissolution processes during PEO, immersion experiments were carried out on PEO coatings in model electrolytes of different alkalinity and silicate concentration. Their composition was determined spectroscopically before and after the experiments. The layers were characterized by XRD before and after the tests and examined at affected and unaffected zones using SEM, EDX, and EBSD. The results show that chemical oxide dissolution can only be observed at the layer/substrate interface and that primarily amorphous alumina is affected. This process is intensified by higher alkalinity and inhibited by silicate additives. The crystalline Al2O3 modifications show no significant attack by the electrolytes used. Transferring these observations to plasma electrolytic oxidation, they allow the conclusion that the electrochemically active zone in the pore ground of discharge channels interacts with the electrolytic and electrical process parameters throughout the entire PEO procedure. Influences of bath alkalinity and silicate content on layer growth rates are to be understood as impact on the passivation processes at the layer/substrate interface rather than chemical redissolution.

The surface area of the test sample depicted in figure 2 was calculated wrong (without the contribution of the lateral area). That’s why the current densities in Table 3, Figure 4 and Figure 5 were wrong also. We adjusted the mentioned article elements as well as the corresponding statement within the abstract: As a result of this, it was possible to obtain alumina layers of 80 micrometers in thickness, with a high corundum content of approximately 50 to 90%, after 37 minutes of treatment time, at a current density below 10 (instead of 25) A/dm2 on C8C-steel.

The plasma electrolytic oxidation is an innovative method for the surface treatment of titanium and its alloys. This review provides an overview of the historical development of the process and summarizes the current state of the art. The chemical as well as the electro- and plasma-chemical basics of the layer forming mechanisms, which comprises the substrate/electrolyte interface before discharge initiation and the different types and stages of plasma electrolytic discharge phenomena are explained within the context of titanium-based materials. How these phenomena can be influenced by the use of suitable electrolytes and controlled by the electrical regime is described. Subsequently, the microstructures and composition of the layers are described in detail, and the properties for specific applications are then discussed. The resistance of a PEO coating to corrosive environments, tribological factors, and alternating mechanical stress is viewed critically, and the extensive functional properties such as physiological compatibility, photocatalytic activity, and decorative properties are revealed. Finally, examples of various practical applications in the medical engineering, aviation, automotive, and environmental technology fields, as well as other branches of industry, are presented.

The plasma electrolytic oxidation (PEO) is an innovative method for providing light metals and their alloys with protective ceramic surfaces. However, for iron-based materials, the process requires very high current densities and results in the formation of coatings which consist of less stable iron compounds. Therefore, it was the aim of this study to design a PEO procedure on low-carbon steel at moderate current densities, which allows for the formation of ceramic coatings whose chemical composition is dominated by the electrolyte constituents. The electrolyte used was based on aluminate and preselected by systematic electrochemical passivation experiments. The PEO treatment was monitored by electrical and optical process diagnostics. As a result of this, it was possible to obtain alumina layers of 80 micrometers in thickness, with a high corundum content of approximately 50 to 90%, after 37 minutes of treatment time, at a current density below 25 A/dm2 on C8C-steel. However, the coating’s microstructure was inhomogeneous and showed poor substrate bonding. Based on the results of the experimental work, explanatory approaches were provided and a course of action is suggested for counteracting these problems.

Plasma electrolytic oxidation (PEO) is a method to obtain protective coatings on metallic light-weight construction materials. Here, the workpiece receives a strong anodic polarisation in a suitable aqueous electrolyte, which leads to the formation of a passive layer and a gaseous shell. Afterwards, plasma electrolytic discharges appear on the substrate surface and convert it into a ceramic layer. The properties of the passive layer are influenced by the selected substrate/electrolyte combination and are essential for the PEO process-initiation and characteristics. In this work, a new method for the systematic investigation of the substrate/electrolyte interactions during the pre-discharge stage is presented. The procedure is carried out by a polarisation experiment and allows for a quantitative characterisation of the passivation behavior, based on a small electrolyte volume. The method is used to investigate a literature-known electrical conduction mechanism on passive films formed on magnesium, by cross-comparison between different Mg and Al materials. In addition, the influence of phosphate, glycerol, and fluoride on the passivation behaviour of the Mg alloy AZ31 in an alkaline environment is considered and quantified. The results provide an explanatory approach for the positive influence of toxic fluorides within the electrolyte on the morphology of PEO layers on magnesium.

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is an innovative method in producing oxide-ceramic coatings on metals, such as aluminum, titanium, magnesium, zirconium, etc. The process is characterized by discharges, which develop in a strong electric field, in a system consisting of the substrate, the oxide layer, a gas envelope, and the electrolyte. The electric breakdown in this system establishes a plasma state, in which, under anodic polarization, the substrate material is locally converted to a compound consisting of the substrate material itself (including alloying elements) and oxygen in addition to the electrolyte components. The review presents the process kinetics according to the existing models of the discharge phenomena, as well as the influence of the process parameters on the process, and thus, on the resulting coating properties, e.g., morphology and composition.

With the progress in materials science and production technology and the establishment of light-weight design in many fields of the industry, the application of light metals no longer requires only mechanical strength, but also a significant protection of the material against wear and corrosion. Hard and wear-resistant oxide coatings on aluminium are produced by plasma electrolytic oxidation (PEO). During PEO, a conversion of the aluminium substrate to a ceramic oxide takes place. While the role of strength-giving alloying elements like Cu, Mg/Si, Zn, and Zn/Cu on the PEO process has selectively been subject of investigation in the past, the significance of the alloy composition for the service properties of the coatings is still unknown. Therefore, the performance of PEO coatings produced on the widely used commercial high-strength alloys AlCu4Mg1 (EN AW-2024), AlMgSi1 (EN AW-6082), and AlZn5.5MgCu (EN AW-7075) is examined with regard to their behaviour in the rubber-wheel test according to ASTM G65 and the current density-potential behaviour of the substrates with undamaged and worn coatings in dilute NaCl solution. To give a reference to the unalloyed material the testings were carried out also on Al 99.5 (EN AW-1050) which was treated in an adjusted PEO process. Although differences in the conversion of intermetallic phases during PEO and the phase composition of the coatings on the various substrates are determined, the service properties are hardly depending on the alloying elements of the investigated aluminium materials. The wear rates in the rubber-wheel test are low for all the alloyed samples. The current density-potential curves show a decrease of the corrosion current density by approximately one order of magnitude compared to the bare substrate. Eventually, previous wear of the coatings does not deteriorate the corrosion behaviour. PEO layers on technically pure aluminum can resist the testing regimes if they are prepared in an electrolyte with an elevated silicate content and without additional hydroxide ions, during a longer process time.

Magnesium and its alloys are increasingly in use as lightweight construction materials. However, their inappropriate corrosion and wear resistance often prevent their direct practical use. The plasma electrolytic oxidation (PEO) is a promising, environmentally friendly method to improve the surface characteristics of magnesium materials by the formation of oxide coatings. These PEO layers contain components of the applied electrolyte and can be shifted in their composition by increasing the concentration of the electrolyte constituents. Therefore, in contrast to the use of conventional low concentrated electrolytes, the process results in more stable protective coatings, in which electrolyte species are the dominating constitutes. In the present work, the influence of the composition of highly concentrated alkaline silicate electrolytes with additives of phosphate and glycerol on the quality of PEO layers on the magnesium alloy AZ31 was examined. The effect of ultrasound coupled into the electrolyte bath was also considered. The process was monitored by recording the electrical process variables with a transient recorder and by observation of the discharge phenomena on the sample surface with a camera. The study was conducted on the basis of a design of experiments. The effects of the process parameter variation are considered with regard to the coatings thickness, hardness and corrosion resistance. Information about the statistical significance of the effects of the parameters on the considered properties is obtained by an analysis of variance (ANOVA).

Plasma electrolytic oxidation (PEO), also called plasma anodizing or micro arc oxidation (MAO), is an appropriate method to improve surface characteristics of valve metals. Micro arc discharges, which appear at the working piece in an aqueous electrolyte under strong anodic polarization, are typical for this process. The discharge initiation is discussed controversially. Contradicting theories, which are mainly based on investigations of tantalum, exist concerning the meaning of electrolytic anions, which are initially incorporated in the anodic film. Dual step polarization and PEO experiments were carried out on aluminum, magnesium, and titanium, which are in practical use as lightweight construction materials. For Al 99.5, AZ31, and TiAl6V4 it could be shown that the incorporation of electrolytic anions in the anodic film is not required for the ignition process. Also, the influence of electrolyte conductivity on the ignition voltage was investigated by polarization experiments with silicate, aluminate, and hydroxide solutions using the example of AZ31. It has been demonstrated that not the electrical electrolyte conductivity, but its specific electron injection ability at the electrolyte/substrate interface defines the ignition voltage. This injection ability depends on the kind and concentration of the electrolytic anions.

Plasma electrolytic oxidation (PEO) is a common means for the surface modification of light metals. However, PEO of magnesium substrates in dilute electrolytes generally leads to the formation of coatings consisting of unfavorable MgO magnesium oxide. By incorporation of electrolyte components, the phase constitution of the oxide coatings can be modified. Coatings consisting exclusively of MgAl2O4 magnesium-aluminum spinel are produced by PEO in an electrolyte containing hydroxide, aluminate, and phosphate anions. The hardness of the coatings is 3.5 GPa on Martens scale on average. Compared to the bare substrate, the coatings reduce the corrosion current density in dilute sodium chloride solution by approx. one order of magnitude and slightly shift the corrosion potential toward more noble values.