Search

In this study, plasma electrolytic oxidation (PEO) process was applied to AA1060 aluminum alloy in an alkaline‐based electrolyte with different concentrations of EDTA‐2Na. The investigation of coatings was concentrated on sparks, phase, structure, morphology, and corrosion resistance. The whole process presented a "soft spark" regime which proved by the reduction of light and acoustic emission with increase of current. The results of SEM and EDS showed that more discharge channels were found under surface and the proportion of silicon in the inner layer increased for the sample treated in solution with 5 g/L EDTA‐2Na, which was attributed to D type discharge causing some light emission invisible on sample surface. The polarization test presented a better corrosion resistance with the increase of EDTA‐2Na. However, EIS measurement showed a bigger corrosion resistance for outer layer of sample prepared with the addition concentration up to 3 g/L that meant outer layer played a more critical role in terms of corrosion resistance. [ABSTRACT FROM AUTHOR]

Ceramic coatings were prepared on 6061 Al alloy in a mixed electrolyte with/without MgO powders at different treatment durations. The results of energy dispersive spectroscopy (EDS) and X‐ray diffraction (XRD) showed that MgO powder was incorporated into the coatings, and Mg species gradually aggregated into coating inside as prolonging the oxidation time. Scanning electron microscopy (SEM) showed that MgO additive had a certain effect on the microstructures and coating thickness. The corrosion behavior tests evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution suggested that at the same treatment time, the addition of MgO powders can improve the corrosion resistance of the coating, and the Mg‐rich layer can affect the corrosion resistance of the coating. The tests of mechanical properties showed that the addition of MgO powders improved the stability and hardness of the coating. [ABSTRACT FROM AUTHOR]

A plasma electrolytic oxidation (PEO) coating was fabricated on AA1060 alloy in an alkaline sodium hexametaphosphate ((NaPO 3) 6) electrolyte. The growth characteristic of the PEO coating was investigated by means of a substrate-detachment technique, Scanning electron microscope (SEM) equipped with energy dispersive spectrometry (EDS), X-ray photoelectron spectrometer (XPS), X-ray diffractometer (XRD) and Mott-Schottky analysis. The results showed that an ordered and porous anodic aluminum oxide (AAO) film was formed at the anodic oxidation stage by the influence of (NaPO 3) 6 in the electrolyte, and the microstructure of the AAO film induced the plasma discharge events to initiate inside the AAO film. The Mott-Schottky analysis showed that the PEO coated samples were all represented n -type semiconducting behavior. The microstructures of detached coatings showed that the coating/substrate interfaces formed at the anodic oxidation stage and sparking stage consist of numerous hemispherical cap structures, but with different sizes. It was also revealed that the inward growth of the hemispherical cap structures is dependent on the diffusion of oxygen from the electrolyte or plasma gases to substrate, and P atoms mainly exist in the amorphous phase of the PEO coating. • The interfaces of the PEO coatings were analyzed by detaching the substrate. • The plasma initiation was observed at the inside of the anodized film. • The cap structure formation mechanism was proposed. • An Al-based PEO coating growth pattern in an alkaline (NaPO 3) 6 electrolyte was built. [ABSTRACT FROM AUTHOR]

Various plasma electrolytic oxidation (PEO) ceramic coatings were fabricated on aluminum in the electrolytes with different concentrations of Na 2 SiO 3 ·9H 2 O. The morphology of fractured cross-section and coating/substrate (C/S) interface, phase compositions, and corrosion behavior of these ceramic coatings were characterized employing field emission scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The results showed that adding Na 2 SiO 3 ·9H 2 O in the electrolyte could reduce the decrease amplitude of the voltage-time curve, and it can affect the types of the anodic film before the beginning of plasma discharges. A novel phenomenon was recorded as PEO treatment was carried out in the electrolyte with 1.85 g l−1 Na 2 SiO 3 ·9H 2 O that the substrate was first covered by a barrier-type anodic film, and then a porous-type anodic film appeared after the barrier-type anodic film was broken down. The anodic film continued to grow for a long duration in the PEO process after being broken down. Characteristics of PEO ceramic coatings with a similar thickness acquired in various electrolytes were also evaluated. XRD results indicated that these ceramic coatings of similar thickness were mainly composed of α-Al 2 O 3 and γ-Al 2 O 3 , and the α-Al 2 O 3 proportion decreased with increasing Na 2 SiO 3 ·9H 2 O concentration. The electrochemical data showed that the corrosion resistance of these PEO ceramic coatings of similar thickness decreased by adding Na 2 SiO 3 ·9H 2 O in the electrolyte. [ABSTRACT FROM AUTHOR]

In the current investigation, plasma electrolytic oxidation (PEO) ceramic coatings on Al are galvanostatically synthesized at various processing stages in an alkaline silicate system. The resultant coatings are systematically surveyed in terms of the following respects: the applied voltage and surface sparking evolution over the studied course of PEO are recorded by the signal acquisition system and the real time imaging, respectively; the phase constitution, the surface morphology, polished cross section of the coated specimens, the fractured cross section and coating/substrate interface of the detached coatings are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) assisted with energy-dispersive X-ray spectrometer (EDS), respectively. In particular, the coatings detached by an in-house developed electrochemical way disclose the coating microstructure in a new angle, which allows the cross section and the coating/substrate interface to come clearly into focus. The microstructure evolution of the oxide ceramic coatings and the correspondences among the characteristic microstructures of the free surface, the fractured cross section and the coating/substrate interface are presented and discussed. Further, based on the present PEO coating microstructure, a model of the coating growth that evolves over time is proposed. [ABSTRACT FROM AUTHOR]

Ceramic coatings were performed on aluminum through plasma electrolytic oxidation (PEO) in a Na 5 P 3 O 10 solution with various concentrations of NaOH. It was revealed that the concentration of NaOH affected the types of anodic films and the activity of plasma discharges. Emphatic investigation was performed on the coatings fabricated in the electrolyte with addition of 1 g L−1 NaOH, focusing on the coating microstructure evolution and coating corrosion resistance. Some coatings were detached from the substrate, showing that the oxide particles on the anodic film C/S interface underwent profile variations from hemisphere to polygon, and its interpore distance decreased over time. The formations of pores of anodic films and plasma discharge of the PEO coatings were both easy to occur around the existing pores and the existing discharge channels. Fractured cross-section showed that a pancake-like structure corresponded to a discharge channel, a cavity, and a barrier layer protrusion whose area was similar to that of the corresponding pancake-like structure. The data of potentiodynamic polarization measurements showed that the sample treated with PEO for 10 min possessed the lowest i corr value of 4.97E-8 A cm−2, the most positive E pit value of −0.63 V, and the highest R p value of 2.66E5 Ω cm2. EIS data of 10 min sample exhibited the highest R 2 value of 1.61E6 Ω cm2, demonstrating the best corrosion resistance among these samples. • NaOH concentrations affect the anodic film type and the activity of plasma discharge. • Particles on porous anodic film undergo profile changes from hemisphere to polygon. • The pore formation in the anodic film is easy to occur around the existing pores. • PEO coating acquired at 10 min has the best corrosion resistance. [ABSTRACT FROM AUTHOR]