Characterization of calcium containing plasma electrolytic oxidation coatings on AM50 magnesium alloy

An attempt was made to produce calcium containing plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy using an alkaline electrolyte. This study was performed in three alkaline electrolytes containing calcium hydroxide and sodium phosphate with three different mass ratios viz., 1:2.5, 1:5 and 1:7.5. All the three coatings produced were found to contain Ca and P in appreciable amounts. The concentration of P was found to be higher in the coatings obtained in the electrolytes with higher concentration of phosphate ions. Even though all the three coatings were found to be constituted with magnesium oxide and magnesium phosphate phases, X-ray diffraction analyses revealed that the phase composition was influenced by the phosphate ion concentration/conductivity of the electrolyte. Further, the PEO coating obtained in the 1:7.5 ratio electrolyte was found to contain di-calcium phosphate (monetite) and calcium peroxide phases, which were absent in the other two coatings. Potentiodynamic polarization studies performed in 0.1 M NaCl solution showed that the coatings obtained from the 1:5 ratio electrolyte possessed a superior corrosion resistance, which is attributed to the combined effect of thickness, compactness and phase/chemical composition of this coating.     

Comparison of coating properties obtained by MAO on magnesium alloys in silicate and phytic acid electrolytes

Anodic coatings were prepared by using microarc oxidation (MAO) on AZ91HP in silicate containing solution (Si-solution) and phytic acid containing solution (P-solution), respectively. The influence of the electrolytes on coating structure, morphology and composition was studied by using X-ray diffraction (XRD), environmental scanning electron microscope (ESEM) and energy dispersive X-ray spectroscopy (EDX). Potentiodynamic polarization test and immersion test were employed to evaluate the corrosion resistance of anodic coatings. Different electrolytes caused the differences in the MAO process and coating properties. The breakdown voltage and the final voltage in P-solution were higher than those in Si-solution. The pore uniformity of anodic coatings obtained in Si-solution (Si-film) was worse than that in P-solution (P-film). XRD analyses indicated that Si-film was amorphous, while P-film consisted of MgO. The corrosion resistance of the sample coated with P-film was better than that with Si-film.     

Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte

In the present work, four processes were carried out to produce ceramic coatings on aluminum substrate in two kinds of electrolytes (silicate and phosphate solution systems) using plasma electrolytic oxidation (PEO) technology. The voltage-time responses were recorded during different PEO processes. SEM/EDX and XRD were adopted to investigate the microstructure, elements distribution and phase composition of the coatings prepared in the two electrolyte systems. It is found that coatings produced in the silicate electrolyte have a more homogeneous morphology than those produced in the phosphate system. EDX analysis shows that silicon element tends to present primarily in the outer region of the coatings while phosphorus distributes uniformly throughout the coating thickness. According to the conventional anodic oxidation mechanism, a model is set up to explain the different characteristics of ceramic coatings fabricated in different electrolytes which is helpful to understand the growth mechanism of PEO coatings.     

Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO₄ into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO₄ in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO₄. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO₄ demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa.     

Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy

Oxide coatings were produced on AM60B magnesium alloy substrate making use of microarc oxidation (MAO) technique. The effect of KF addition in the Na₂SiO₃-KOH electrolytic solution on the microarc oxidation process and the structure, composition, and properties of the oxide coatings was investigated. It was found that the addition of KF into the Na₂SiO₃-KOH electrolytic solution caused increase in the electrolyte conductivity and decrease in the work voltage and final voltage in the MAO process. Subsequently, the pore diameter and surface roughness of the microarc oxidation coating were decreased by the addition of KF, while the coating compactness was increased. At the same time, the phase compositions of the coatings also varied after the addition of KF in the electrolytic solution, owing to the participation of KF in the reaction and its incorporation into the oxide coating. Moreover, the coating formed in the electrolytic solution with KF had a higher surface hardness and better wear-resistance than that formed in the solution without KF, which was attributed to the changes in the spark discharge characteristics and the compositions and structures of the oxide coatings after the addition of KF.     

Microstructure, bonding strength and thermal shock resistance of ceramic coatings on steels prepared by plasma electrolytic oxidation

Ceramic coatings were successfully prepared on steel by plasma electrolytic oxidation (PEO) in aluminate electrolyte and silicate electrolyte, respectively. The microstructure of the coatings including surface morphology, phase and element composition were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The bonding strength between the ceramic coating and the substrate was tested using different methods including tensile tests and shearing tests. The thermal shock resistance of the coatings was also evaluated. The results indicated that coatings obtained in both electrolytes were porous and coarse. The average diameters of the pores were below 10 μm. PEO coatings obtained in aluminate electrolyte were composed of Fe₃O₄ and FeAl₂O₄, while those obtained in silicate electrolyte were in a noncrystal state. PEO coatings obtained in aluminate electrolyte showed similar change trend of tensile strength and shearing strength with increasing treating time, namely, a relatively high values with middle time treating and low value with short and long time treating. The best coating was the samples treated with 30 min, whose tensile strength was 20.6 MPa and shearing strength was 16 MPa. The tensile strength and shearing strength of coatings obtained in silicate electrolyte were not strongly influenced by the treating time, the values of which were range in 14 ± 2 MPa and 11 ± 2 MPa, respectively. Coatings obtained in both electrolytes showed the best thermal shock resistance with middle time treating. Coatings obtained in silicate electrolyte show a little better thermal shock resistance than those obtained in aluminate electrolyte.     

The study of zirconium alloy coatings produced by microarc oxidation using Rutherford and nuclear backscattering spectrometry

The effect of the electrolyte composition during microarc oxidation of the E110 zirconium alloy on the corrosion resistance of coatings is investigated using proton Rutherford backscattering spectrometry, nuclear backscattering spectrometry, and other methods.     

Formation of titania composite coatings on carbon steel by plasma electrolytic oxidation

Titania composite coatings were prepared on carbon steel by plasma electrolytic oxidation in silicate electrolyte and aluminate electrolyte with titania powers doping in the electrolytes. The microstructure of the coatings was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Investigation results revealed that the coating obtained in silicate electrolyte was composed of anatase-TiO₂, rutile-TiO₂ crystal phases and some Fe, Si, P elements; coating obtained in aluminate electrolyte consisted of anatase-TiO₂, Al₂TiO₅ and some Fe, P elements. Coatings obtained in two types of electrolytes show porous and rough surface. With increasing the concentration of titania powers in the electrolytes, the coating surface first became more compact and less porous and then became more porous and coarse. The bond strength and thickness were not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa and for 66 μm for coatings obtained in aluminate electrolyte, and 21 MPa and 35 μm for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration of titania powers in the electrolytes. All coated samples showed better corrosion resistance than the substrate in 3.5 wt% NaCl solution. The best coatings were also obtained with middle concentration of titania powers doping in both electrolytes whose corrosion current density was decreased by 2 orders of magnitude compared with the substrate.     

Preparation of ceramic coating on Ti substrate by plasma electrolytic oxidation in different electrolytes and evaluation of its corrosion resistance: Part II

The aim of this work is to discuss the growth characteristics and corrosion behavior of the prepared ceramic coatings on titanium by plasma electrolytic oxidation (PEO) technique in different electrolytes. PEO process was carried out on titanium under constant voltage regime using a pulse power supply. Three kinds of electrolytes, phosphate, silicate and borate based solutions, were used to evaluate the influence of electrolyte composition on the structure, surface morphology, phase composition and corrosion behavior of prepared ceramic oxide films (titania). The phase composition of the coatings was investigated by X-ray diffraction. Scanning electron microscopy was employed to evaluate the growth and surface morphology of coatings. Elements of coatings were investigated with energy dispersive spectrometer. Corrosion behavior of the coatings was also examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The spark voltage of oxide films had a significant effect on the surface morphology, size and homogeneity of micro-pores, thickness and corrosion properties of coatings.     

Influence of K2TiF6 in electrolyte on characteristics of the microarc oxidation coating on aluminum alloy

Black and gray microarc oxidation (MAO) coatings were prepared in a phosphate electrolyte with and without K₂TiF₆ on 2A70 aluminum alloy, respectively. Voltage–time curves were recorded during the MAO process. The effects of K₂TiF₆ on the morphology, composition, abrasive resistance and corrosion resistance of MAO coatings were investigated. The results showed that the MAO coating produced in the electrolyte with K₂TiF₆ was thicker, and more uniform than that produced in the electrolyte without K₂TiF₆. Ti was detected in the surface of the MAO coating formed in the electrolyte with K₂TiF₆. The results of abrasive resistance and corrosion resistance showed that the MAO coating formed in the electrolyte with K₂TiF₆ exhibited better abrasive resistance and corrosion resistance.     

Influence of C3H8O3 in the electrolyte on characteristics and corrosion resistance of the microarc oxidation coatings formed on AZ91D magnesium alloy surface

Ceramic coatings were fabricated on AZ91D Mg-alloy substrate by microarc oxidation in Na₂SiO₃-NaOH-Na₂EDTA electrolytes with and without C₃H₈O₃ addition. The effects of different concentrations of C₃H₈O₃ contained in the electrolyte on coatings thickness were investigated. The surface morphologies, RMS roughness, phase compositions and corrosion resistance property of the ceramic coatings were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and electrochemical corrosion test respectively. It is found that the addition of C₃H₈O₃ into silicate electrolyte leads to increase of the unit-area adsorptive capacity of the negative ions at anode-electrolyte interface and thus improves the compactness and corrosion resistance of the MAO coating. The coating thickness decreases gradually with the increase of concentrations of C₃H₈O₃ in the electrolyte. The oxide coating formed in base electrolyte containing 4 mL/L C₃H₈O₃ exhibits the best surface appearance, the lowest surface RMS roughness (174 nm) and highest corrosion resistance. In addition, both ceramic coatings treated in base electrolyte with and without C₃H₈O₃ are mainly composed of periclase MgO and forsterite Mg₂SiO₄ phase, but no diffraction peak of Mg phase is found in the patterns.     

The effects of anodic and cathodic processes on the characteristics of ceramic coatings formed on titanium alloy through the MAO coating technology

This paper reports on the investigation into the effects of the different anodic j_a and cathodic j_c current densities on the variations of the anodic and cathodic processes and the associated changes in the characteristics of the coatings synthesized on Ti-6Al-4V alloy substrate by microarc oxidation technique. The coated samples were subjected to coating thickness and cross-section fracture observation. Phase and elemental composition at different depth of the coatings were evaluated through X-ray diffraction and energy dispersive spectrum analyses. The experimental results indicate that the increase of j_a leads to the increasing slope of anodic voltage U₊ versus oxidation time plots, the larger coating thickness and the more coarse surface morphology of MAO coatings; while the aggrandizement of the cathodic process results in the lower growth rate and more uniform structure of coatings. The changes of the elements distribution from the interface towards the coating surface with variation of j_a and j_c are affected by the Ti anodic dissolution and the electromigration of electriferous particles, such as Al(OH)₄⁻, in electrolytes.     

Interface adhesion properties of functional coatings on titanium alloy formed by microarc oxidation method

Three functional coatings (namely Al-C, Si-P-Al and P-F-Al coating) were fabricated by microarc oxidation method on Ti6Al4V alloy in different aqueous solutions. The microstructure, phase and chemical composition of coatings were investigated using scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy. The interface adhesion failure mode of the coating is revealed by shear, tensile and thermal shock methods. The coatings exhibit high adhesion strength by the quantitative shearing test, registering as 110, 70, and 40 MPa for Al-C, Si-P-Al and P-F-Al coating, respectively. The tensile test of the coated samples shows that microarc oxidation treatment does not significantly deteriorate mechanical properties of substrate titanium alloy. The observations of the coating failure after subjected to the identical tensile elongation of 3.0% are well in agreement with those results of the shear test. The thermal cycle test indicates that all the coatings have good anti-thermal shocking properties.     

Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti-6Al-4V alloy

The aim of this work is to discuss the growth characteristics of the ceramic coatings on Ti alloy by plasma electrolytic oxidation (PEO) technique. Ceramic coatings were prepared on Ti alloy by plasma electrolytic oxidation in different electrolyte solutions under different pulse modes. The composition and the structure of the coatings were investigated by X-ray diffraction and scanning electron microscopy (SEM), respectively. The amount of the dissolved titanium into the electrolytes during PEO process was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The structure and the composition of the coatings were related to the mode of the spark discharge during PEO process. (a) Under the pulsed single-polar mode: In Na₃PO₄ solution, the spark discharge was mainly due to the breakdown of the oxide film, and the coatings prepared were porous and mainly structured by the Ti from the substrate. In K₄ZrF₆-H₃PO₄ and NaAlO₂-Na₃PO₄ solutions, the main mode of the spark discharge was the breakdown of the oxide film at the initial stage, and then changed into the breakdown of the vapor envelope, and the coatings were rough and thick, and mainly structured by the elements from the electrolyte. (b) Under the pulsed bi-polar mode in NaAlO₂-Na₃PO₄ solution, the spark discharge may be mainly due to the breakdown of the oxide film, the coatings prepared were dense in inner layer and loose in outer layer, and structured by the elements from both the substrate and the electrolyte. Besides, the ICP-AES analyses showed that the amount of the dissolved titanium in the electrolyte during PEO process was more under the breakdown of the oxide film than under the breakdown of the vapor envelope, which was consistent with the changes of the structure of the coatings. Cathode pulse in the pulsed bi-polar mode increased the amount of the dissolved titanium in the electrolyte, compared with the pulsed single-polar one.     

Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium （TA2） substrates in a Na2SiO3NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope （SEM） and x-ray diffraction （XRD） are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile （TiO2）. With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.     

Microstructure and corrosion behavior of coated AZ91 alloy by microarc oxidation for biomedical application

Magnesium and its alloy currently are considered as the potential biodegradable implant materials, while the accelerated corrosion rate in intro environment leads to implant failure by losing the mechanical integrity before complete restoration. Dense oxide coatings formed in alkaline silicate electrolyte with and without titania sol addition were fabricated on magnesium alloy using microarc oxidation process. The microstructure, composition and degradation behavior in simulated body fluid (SBF) of the coated specimens were evaluated. It reveals that a small amount of TiO₂ is introduced into the as-deposited coating mainly composed of MgO and Mg₂SiO₄ by the addition of titania sol into based alkaline silicate electrolytic bath. With increasing concentration of titania sol from 0 to 10 vol.%, the coating thickness decreases from 22 to 18 μm. Electrochemical tests show that the E_corr of Mg substrate positively shifted about 300500 mV and i_corr lowers more than 100 times after microarc oxidation. However, the TiO₂ modified coatings formed in electrolyte containing 5 and 10 vol.% titania sol indicate an increasing worse corrosion resistance compared with that of the unmodified coating, which is possibly attributed to the increasing amorphous components caused by TiO₂ involvement. The long term immersing test in SBF is consistent with the electrochemical test, with the coated Mg alloy obviously slowing down the biodegradation rate, meanwhile accompanied by the increasing damage trends in the coatings modified by 5 and 10 vol.% titania sol.     

Growth process and wear resistance for ceramic coatings formed on Al-Cu-Mg alloy by micro-arc oxidation

The growth process, distribution of chemical elements, phase constitutions and relative wear resistance of the ceramic coatings formed on Al-Cu-Mg alloy by ac micro-arc oxidation are investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscope (EDX), X-ray diffraction (XRD) and reciprocating friction and wear tests. The results indicate that there are three stages with the formation of the ceramic coatings: (1) the formation of ceramic particles, (2) sintering growth in sawtooth structure, (3) the increase of thickness by remolting and sintering. The ceramic coatings are made up of a mixture of α-alumina, γ-alumina and amorphous alumina, whose relative contents varied with the position in the ceramic coatings, respectively. The chemical elements altered in ceramic coatings produced in different electrolytes and varied along the depth in ceramic coatings obtained in phosphate electrolyte. Meanwhile, the results of friction and wear tests against Gr15 after 16 h indicate that the weight loss of ceramic coatings became almost unchanged.     

Iron-containing coatings obtained by microplasma method on aluminum with usage of homogeneous electrolytes

The microplasma synthesis method of multicomponent oxide coatings including iron oxide on an aluminum alloy surface has been studied. The homogeneous electrolytes containing iron complexes with Edta⁴⁻ under pH 9-10 have been used. The dynamics reducing the pH of the electrolyte during formation process have been studied. The content of iron in the coatings has been measured spectrophotometricaly. Morphlogy and phase composition of the coatings have been studied by the scanning electron microscopy and X-ray powder diffraction.     

Mitigation of the deleterious effect of silicon species on the conductivity of ceria electrolytes

The deleterious effect of silicon species on the conductivity of ceria electrolytes is well documented in the literature. A simple process has been developed at Praxair that alleviates the negative effect of high concentrations of silicon species on the conductivity of ceria electrolytes. The impact of this process on the conductivity of commercially available doped ceria electrolyte powders is demonstrated and a likely mechanism for the reduction of the resistivity is proposed. The influence of this process on other properties of the ceria electrolyte is discussed.     

Characteristics of sealed plasma electrolytic oxidation coatings with electrochemical impedance spectroscopy

<正>Plasma electrolytic oxidation(PEO) coatings are prepared on aluminium with graphite powders added into the electrolyte.The scanning electron microscopy(SEM) coupled with an energy dispersive x-ray analysis system(EDX) is used to characterize the surface and the cross-section morphologies of the coatings.The electrochemical impedance spectroscopy(EIS) is used not only to evaluate the corrosion resistance but also to analyse the structure of the coating. Results show that graphite powders are embedded in the PEO coating.The corrosion resistances of both the inner barrier and the outer porous layer are greatly improved,and the EIS could give some valuable detailed information about the coating structure.