Porous Oxide Layer Research Articles

Highly corrosion-resistant and photocatalytic hybrid coating on AZ31 Mg alloy via plasma electrolytic oxidation with organic-inorganic integration

This study explores the development of an organic-inorganic hybrid coating to enhance the corrosion resistance and photocatalytic properties of AZ31 Mg alloy modified by plasma electrolytic oxidation (PEO). The PEO process typically generates a porous oxide layer, which can reduce corrosion protection by allowing corrosive agents to penetrate the substrate. To address this limitation, phenopyridine (PHEN) and 2-methylimidazole (2-IMD) were incorporated into the PEO surface to form a robust organic layer on the Mg alloy. Potassium hydroxide (KOH) was used to adjust the pH, improving the interaction and solubility between the organic molecules and the PEO coating. The hybrid coating exhibited unique twig-like surface structures that contributed to forming a multifunctional coating with high corrosion resistance and superior photocatalytic activity. The PEO-PHEN-2IMD sample on the Mg alloy demonstrated exceptional corrosion resistance, with the lowest corrosion current density (Icorr) of 1.92 × 10-¹⁰ A/cm², a high corrosion potential (Ecorr), and the highest top layer resistance (Rtop) of 2.57 × 106 Ω·cm², indicating excellent barrier properties. Additionally, the coating achieved complete (100%) degradation of methylene blue (MB) within 30 min under visible light. Density Functional Theory (DFT) calculations provide deeper insights into the bonding mechanisms and interaction stability between PHEN, 2-IMD, and the PEO layer on the Mg alloy and MB dye. These findings confirmed the enhanced performance of the hybrid coating in both corrosion resistance and photocatalytic applications.

HfB2-SiC ceramics fabricated by reactive melt infiltration and its long-term oxidation behavior at 1650°C

High-density HfB2-SiC ceramics were prepared by reactive melt infiltration (RMI) at a relatively low temperature (1500°C). The ceramics had the open porosity of 0.5 % and the uniform distribution of constituent phases. The long-term oxidation behavior of HfB2-SiC ceramics in air was investigated at 1650°C. After oxidation for 100 h, the weight gain rates of ceramics varied within the range of 1.30–2.26 %. The oxidation products included SiO2, HfO2, and HfSiO4 phases, among which the HfSiO4 compound possessed the higher stability resulting from the reaction between the HfO2 and SiO2 phases. The samples all exhibited the typical feature of layered scale structure after HfB2-SiC oxidation, which was composed of the outermost SiO2 glass layer with the uneven thickness, followed by a relatively porous oxide layer containing white HfO2 and slight HfSiO4, below it was the unoxidized HfB2-SiC. The presence of this multiphase glass layer endowed the samples with excellent oxidation resistance.

Micro-arc oxidation for balancing antibacterial activity and bone formation on titanium surface

Ti surfaces must balance bone formation and antibacterial activity to achieve bone reconstruction and prevent infections. In this study, a Ti surface was electrochemically modified via micro-arc oxidation in an electrolyte containing calcium ions, phosphate ions, and either Ag, or Cu ions to form an antibacterial porous oxide layer on Ti surface. The resultant porous oxide layers consisted of both electrolyte and substrate elements. The Ag- and Cu-incorporated porous oxide layers exhibited antibacterial activity against methicillin-resistant Staphylococcus aureus. Furthermore, animal experiments were performed to reveal the bone-formation on the Ag- and Cu-incorporated porous oxide layers. Both porous oxide layers promoted bone formation more effectively than untreated Ti. Thus, the Ag- and Cu-incorporated porous oxide layers formed on the Ti surface by micro-arc oxidation balanced bone formation and antibacterial activity owing to their structural and compositional features. Our findings will be useful for the surface design of Ti-based medical devices and can provide a simple electrochemical treatment to form porous surfaces to balance antibacterial activity and bone formation on Ti surfaces.

Effect of H2S on the corrosion behavior of austenitic and martensitic steels in supercritical CO2 at 550 °C and 20 MPa for Brayton cycle system

Corrosion of alloy materials significantly influences the stability and efficiency of the supercritical carbon dioxide (S-CO2) Brayton cycle. In this study, we investigated the corrosion behavior of the austenitic alloy SP2215 and the martensitic alloy X19CrMoNbVN11-19 (X19) in S-CO2 at 550°C and 20 MPa. The effect of impurities, H2S, on corrosion behavior was studied. After 900 hours, SP2215 formed a 1.1 μm thick oxide layer (Cr2O3). X19 formed a loose and porous double oxide layer of 4.1 μm (outer: Fe3O4, inner: FeCr2O4). After H2S doping, SP2215 had an oxide layer of 6.2 μm (FeS, SiO2, and Cr2O3), while X19 was 9.8 μm (outer: Fe3O4, SiO2, FexSy and Fe2O3, inner: FeCr2O4, Fe2SiO4 and Cr2S3). Metal sulfides formed and the corrosion layer thickened. This indicates that H2S not only changes the corrosion mechanism but also aggravates the corrosion. However, the corrosion of CO2 is dominant. SP2215 has better corrosion resistance than X19.

Increasing the Wear and Corrosion Resistance of a CP-Ti Surface by Plasma Electrolytic Borocarburizing and Polishing

The paper examines the possibility of increasing the wear and corrosion resistance of a CP-Ti surface by duplex plasma electrolytic treatment (borocarburizing and polishing). The structure and composition of diffusion layers, their microhardness, surface morphology and roughness, wear resistance during dry friction and corrosion resistance in Ringer’s solution were studied. The formation of a surface-hardened layer up to 200 μm thick with a microhardness of up to 950 HV, including carbides and a solid solution of boron and carbon, is shown. Subsequent polishing makes it possible to reduce surface roughness and remove weak areas of the porous oxide layer, which are formed during high-temperature oxidation in aqueous electrolyte vapor during borocarburizing. Changing the morphology and structural-phase composition of the CP-Ti surface helps reduce weight wear by a factor of three (the mode of frictional interaction changes from microcutting to oxidative wear) and corrosion current density by a factor of four after borocarburizing in a solution of boric acid, glycerin and ammonium chloride at 950 °C for 5 min and subsequent polishing in an ammonium fluoride solution at a voltage of 250 V for 3 min.

Biocompatibility analysis of titanium bone wedges coated by antibacterial ceramic-polymer layer

This paper presents the surface treatment results of titanium, veterinary bone wedges. The functional coating is composed of a porous oxide layer (formed by a plasma electrolytic oxidation process) and a polymer poly(sebacic anhydride) (PSBA) layer loaded with amoxicillin (formed by dip coatings). The coatings were porous and composed of Ca (4.16%-6.54%) and P (7.64%-9.89% determined by scanning electron microscopy with EDX) in the upper part of the implant. The titanium bone wedges were hydrophilic (54° water contact angle) and rough (surface area (Sa):1.16 μm) The surface tension determined using diiodomethane was 68.6 ± 2.0° for the anodized implant and was similar for hybrid coatings: 60.7 ± 2.2°. 12.87 ± 0.91 µg/mL of amoxicillin was released from the implants during the first 30 min after immersion in the phosphate-buffered saline (PBS) solution. This concentration was enough to inhibit the Staphylococcus aureus ATCC 25923, and Staphylococcus epidermidis ATCC12228 growth. The obtained inhibition zones were between 27.3 ± 2.1 mm–30.7 ± 0.6 mm when implant extract after 1 h or 4 h immersion in PBS was collected. Various implant biocompatibility analyses were performed under in vivo conditions, including pyrogen test (3 rabbits), intracutaneous reactivity (3 rabbits, 5 places by side), acute systemic toxicity (20 house mice), and local lymph node assay (LLNA) (20 house mice). The extracts from implants were collected in polar and non-polar solutions, and the tests were conducted according to ISO 10993 standards. The results from the in vivo tests showed, that the implant’s extracts are not toxic (mass body change below 5%), not sensitizing (SI < 1.6), and do not show the pyrogen effect (changes in the temperature 0.15ºC). The biocompatibility tests were performed in a certificated laboratory with a good laboratory practice certificate after all the necessary permissions.

A comparative study on the oxidation behaviour of NbC-Ni, WC-Co, and high entropy carbide-Ni based cermets

The oxidation behaviour of NbC-13.5(wt%)Ni, NbC-5.7Mo2C-13.3Ni and WC-12Co was compared to that of equimolar quinary (Ti0.2V0.2Nb0.2Ta0.2W0.2)C-10Ni and non-equimolar senary (Ti0.2V0.2Nb0.2Ta0.2Mo0.1W0.1)C-10Ni high entropy carbide (HEC)-Ni cermets. Isothermal oxidation experiments were performed at 700 °C, whereas non-isothermal calorimetric experiments were conducted to determine the onset temperature of oxidation. HEC-based cermets exhibited superior oxidation resistance and a unique oxide layer morphology, which was attributed to the formation of a chemically complex oxide scale mainly consisting of (Ti,V,Nb,Ta,Ni)Ox and Ni(Mo,W)O4. The NbC-based cermets showed the least oxidation resistance and formed a porous oxide layer of Nb2O5 and (Nb,Ni)Ox, whereas WC-Co showed an intermediate oxidation behaviour with respect to the HEC-Ni based cermets. All composites followed a linear oxidation rate law, and the senary HEC cermet demonstrated the slowest oxidation kinetics.

Formation of Through-Hole Porous Anodic Aluminum Oxide Layer Locally with 3D Printed Solution Flow Type Microdroplet Cell

In this study, a 3D-prinited solution-flow type microdroplet cell (SF-MDC) is employed as a new technique for the fabrication of porous anodic aluminum oxide (AAO) layer using oxalic acid electrolyte on aluminum. The surface morphology of the porous AAO film was characterized by a scanning electron microscope. The aim of this study was to fabricate a through-hole porous alumina layer in a single step anodizing process and to investigate the influence of anodized voltages and scanning speeds on the thickness and pore structure of alumina layer. The results showed that the pore diameter and interpore distance were directly proportional to the anodizing voltage. The thicknesses of formed AAO films were found to be 35.5, 50.7, and 81.6 μm at scanning speeds of 10, 5, and 2.5 μms−1, respectively. Through-hole porous AAO was successfully fabricated at room temperature without chemical etching. The SF-MDC fabrication technique is proposed as an environmentally attractive and suitable process for the fabrication of porous AAO layers.

Oxidation behavior and corrosion resistance of CoCrFeNi high entropy alloy compared with FeCoNi medium entropy alloy

The entropy-based alloy with excellent oxidation and corrosion resistance is of great significance for practical applications in high-temperature environments. Thus, the difference in anti-oxidation property and corrosion behavior of FeCoNi MEA and CoCrFeNi HEA in a simulated marine environment was studied via a series of measurements. The results reveal that the HEA possesses superior oxidation and corrosion resistance in comparison with the MEA, which can be demonstrated by comparing the oxidation rate and thickness of oxide layer, corrosion current density (icorr) as well as corrosion morphology after polarization. The microstructure and composition of the oxide layer are the primary reasons resulting in the difference of oxidation and corrosion behaviors of the two alloys. The oxide layer with a compact, even and intact structure is produced on the HEA surface, which hinders the diffusion of detrimental ions and oxygen, whereas the loose, porous and cracked oxide layer consisting of inner Fe3O4 and outer CoO exhibits poor protection against corrosion and oxidation of the MEA. For the HEA, the preferential combination of Cr and O elements forms the protective oxide of Cr2O3, effectively enhancing corrosion resistance and inhibiting oxidation. However, the low protective CoO and relatively active Fe3O4 grow on the MEA surface, which cannot provide a good barrier to restrain oxidation and corrosion.

Formation mechanism of high-temperature oxidation film on WE43 magnesium alloy and its effect on corrosion performance

The oxidation behavior of WE43 magnesium alloy in dry air at three temperatures (225, 440 and 525 °C) and the corresponding corrosion performance of samples attached to oxide film in 3.5 wt% NaCl solution was investigated. The results show that the oxide films formed at all three temperatures are a complex MgO·RE2O3·ZrO film with different compositions. The film formed at 225 °C is flat and dense, whose components are 3.2MgO·1.8RE2O3·1ZrO. The oxidation ridges begin to form, and then gradually grow into nodular oxides and form a loose and porous oxide layer as the temperature increases to 525 °C. The oxide films formed at all three temperatures improve the corrosion resistance of the alloy due to the MgO·RE2O3·ZrO, with the protective properties of oxide films following the order of 225 °C > 440 °C > 525 °C, because the dense MgO·RE2O3·ZrO film formed at 225 °C can provide better protection to the substrate than the loose oxide film formed at higher temperatures.

Enhanced photocatalytic reduction of p-nitrophenol by polyvinylpyrrolidone-modified MOF/porous MgO composite heterostructures

Catalysts derived from Metal-Organic Frameworks (MOF) present a compelling blend of cost-effectiveness and superior performance in reduction reactions. In this study, polymer-decorated MOF-Cobalt (MOF-Co) was deposited onto porous magnesium oxide (MgO) layers, acquired through surface modification of AZ31 Mg alloy via plasma-assisted oxidation, with the aim of fabricating novel catalysts for reduction reactions. Herein, this synthesis involved the utilization of Cobalt-benzene dicarboxylic acid (Co-BDC), where cobalt ions served as metal nodes and 1,4-benzene dicarboxylic acid (H2BDC) functioned as an organic linker, with and without the presence of polyvinylpyrrolidone (PVP) as an active site enhancer. The morphology of the prepared catalysts was affected by several factors such as pH, hydrothermal treatment duration, PVP content, and the presence of a MgO layer. The optimal catalyst, designated as metal-organic frameworks-cobalt at 80 °C temperature and 1 h of hydrothermal treatment at pH 4 (MOF-Co 1.4), was synthesized on the MgO layer using a solution containing 1.7 g of PVP, H2BDC, and Co(NO₃)₂.6H₂O. The paper-like morphology of the MOF-Co 1.4 catalyst facilitated exceptional performance, efficiently degrading p-nitrophenol (p-NP) under visible light irradiation with an impressive 99.74 % efficiency within just 5 min of exposure, while also demonstrating stability over five successive cycles. ·O2− species was found to drive the reduction reaction to p-aminophenol, with harmless compounds as byproducts, while GC-MS analysis identified intermediates in the reduction of p-NP. Density functional theory (DFT) calculations suggested that H2BDC and PVP jointly provided multi-active sites, enabling effective contact with reactants and rapid electron transfer, thus playing a synergistic role in the catalytic reduction process. This study pioneers a novel method for designing efficient bulk catalysts, achieving high efficiency and stability in pollutant degradation with fast, low-energy fabrication.

Mechanistic Insights into Plasma Oxidation of Ag Nanofilms: Experimental and Theoretical Studies.

Plasma oxidation of metals has been studied extensively to fabricate nanoporous oxides with the merits of room temperature treatment and facile control of the oxidation rate. Plasma oxidation of Ag, motivated by studies on atomic oxygen corrosion of Ag, is one of the most studied systems. However, several important questions remain unaddressed and even overlooked traditionally: the critical role played by atomic O in promoting oxidation, evolution of microstructures during plasma exposure, and a sound framework for quantitative oxidation kinetic analyses. In this paper, the O2 plasma oxidation behavior of Ag films deposited on Si substrates was systematically studied both experimentally and theoretically. The effects of plasma pressure and power on the microstructural evolution and oxidation kinetics of Ag films of various thicknesses were investigated using comprehensive characterization, as well as numerical analysis of plasma chemistry for deriving atomic O concentration. The findings here provide a full picture and deep mechanistic insights into the morphology and microstructure evolution of Ag films and the growth of dense or porous Ag2O and AgO oxide layers by plasma oxidation, revealing the intricate interplay between atomic O, vacancy creation, Ag ion diffusion, Kirkendall effect, formation of pores, and interfacial void coalescence. The methodology developed here can be easily transferred to help understand the plasma oxidation behavior of other metals.

Self-Ordered Anodic Porous Oxide Layers as a High Performance Electrocatalyst for Water Oxidation

Self-Ordered Anodic Porous Oxide Layers as a High Performance Electrocatalyst for Water Oxidation

The Effect of Sulfuric Acid Anodization on the Electrochemical Properties of Aluminum Alloy AlSi10Mg Prepared by Selective Laser Melting

Aluminum alloy, AlSi10Mg, prepared by selective laser melt (SLM) fabrication was anodized in 9.8% sulfuric acid (Type II) at 15 V for a total of 23 min. Experiments were performed to study the potentiostatic anodization process and its effects on the oxide coating morphology, thickness, and electrochemical properties of the alloy. Prior to anodization, the alloy microstructure is composed of aluminum cells encapsulated in a silicon network. Anodizing the abraded and polished AlSi10Mg surface produced a porous oxide layer with a thickness of 5 μm. The oxide coating weight was 698 ± 29 mg/ft2. The oxide coating forms in the aluminum cells that are isolated from one another by the silicon eutectic phase. In electrochemical tests, the anodic and cathodic potentiodynamic polarization currents were suppressed by factors of 15× and 215×, respectively, as compared to the unanodized controls. The data indicate the anodic oxide coating suppresses the cathodic more than the anodic reaction rate. Linear polarization resistance (R p) values increased by 279× after anodization. The corrosion current density values (j corr) decreased by 133× after anodization. Taken together, the electrochemical data indicate the anodic oxide coating (unsealed) increases the corrosion resistance of the SLM alloy by two orders of magnitude.

Optimization of electrolyte composition for enhanced photocatalytic performance of the ceramic coating produced on brass by plasma electrolytic oxidation

PEO coatings are formed by subjecting a metal substrate to an electrolytic plasma discharge, resulting in the formation of a porous and rough oxide layer. Herein, the significant role of the chemical composition of the electrolyte system and additives in the formation of an oxide coating on brass alloy in the PEO process was investigated. It was observed that the coating created with 8 g/l aluminate, 1 g/l NaOH, and 1 g/l KF electrolyte had higher homogeneity than other coatings. This is because a composite oxide coating of Cu2O, ZnO, and AlCuO2 was formed. Methylene Blue (MB), a widely used dye in various industries, is known to be persistent and environmentally harmful. The photocatalytic activity of the PEO coating on brass for the removal of MB was investigated under visible light exposure. According to the results, the presence of the sample produced in an electrolyte containing 8 g/l Aluminate, 1 g/l NaOH, and 1 g/l KF resulted in a degradation rate of approximately 65 % for MB photodegradation after 6 h of irradiation. The PEO-coated brass shows promising photocatalytic efficiency compared to our research group's PEO-coated TiO2 coatings, indicating its potential as a photocatalyst. The results highlight that PEO coating on brass could be a potential candidate for photocatalytic and photoelectrochemical applications.

Effect of synergistic surface treatment on the bonding strength and durability performance of aluminum-lithium alloy adhesive joints

The effect of synergetic surface treatments on the adhesion performance and strength durability of aluminum-lithium alloy bonded joints was investigated. The synergistic modes combine the advantages of different surface treatments, rough morphologies were firstly generated and then porous oxide layer structures were produced on the rough surfaces, so more porous structures and suitable rough characteristics were generated on the substrate to enhance the bonding performance of adhesive joint. The surface characteristics, surface wettability, fracture morphology, shear strength and strength durability of samples subjected to different surface treatments were compared and analyzed. The experimental results indicated that the synergistic treatments had a positive effect on the strength performance and environmental durability of bonded joints, with the highest surface free energy of 80.6 mJ/m2 and shear strength of 40.2 MPa, and also had the best environmental durability with a strength reduction of 8.2 % during the hot-humid exposure.

Atomic-Scale Insights into Flow-Accelerated Corrosion of Carbon Steel

The role of flow velocity on the formation and dissolution of oxides on SA106Gr.B carbon steel was investigated at both microscopic and atomic scales. In static water, a compact oxide layer with highly faceted magnetite particles was formed. Atomic-scale transmission electron microscopy images of such a layer revealed highly ordered and parallel lattice fringes, indicating that the oxide had very high crystallinity and minimal lattice defects. In contrast, turbulent water prompted the creation of a porous oxide layer consisting of amorphous magnetite particles. Here, numerous mismatched lattice fringes were observed, indicating a prevalence of point defects within the oxide structure. These differences in oxide properties are attributed to hydrodynamic shear stress induced by turbulent flow. These findings provide atomic-level insights into how carbon steel corrosion accelerates in fast-flowing water.

Hierarchically porous surface of HA-sandblasted Ti implant screw using the plasma electrolytic oxidation: Physical characterization and biological responses.

The surface topological features of bioimplants are among the key indicators for bone tissue replacement because they directly affect cell morphology, adhesion, proliferation, and differentiation. In this study, we investigated the physical, electrochemical, and biological responses of sandblasted titanium (SB-Ti) surfaces with pore geometries fabricated using a plasma electrolytic oxidation (PEO) process. The PEO treatment was conducted at an applied voltage of 280V in a solution bath consisting of 0.15mol L-1 calcium acetate monohydrate and 0.02mol L-1 calcium glycerophosphate for 3 min. The surface chemistry, wettability, mechanical properties and corrosion behavior of PEO-treated sandblasted Ti implants using hydroxyapatite particles (PEO-SB-Ti) were improved with the distribution of calcium phosphorous porous oxide layers, and showed a homogeneous and hierarchically porous surface with clusters of nanopores in a bath containing calcium acetate monohydrate and calcium glycerophosphate. To demonstrate the efficacy of PEO-SB-Ti, we investigated whether the implant affects biological responses. The proposed PEO-SB-Ti were evaluated with the aim of obtaining a multifunctional bone replacement model that could efficiently induce osteogenic differentiation as well as antibacterial activities. These physical and biological responses suggest that the PEO-SB-Ti may have a great potential for use an artificial bone replacement compared to that of the controls.

Corrosion of Old Overhead Lines: Insights into Aluminum-Steel Galvanic Couple Using Electrochemical Techniques

In order to explain the corrosion inside 20th century power lines, the galvanic corrosion of steel and aluminum from 1949 was examined. In this investigation, 0.1 M Na2SO4 + 1 mM NaCl was used as moderately corrosive medium. Various electrochemical methods were applied to determine the corrosion potential and the different reactions involved in the corrosion process. The thickness of the oxide layer that regulates the corrosion of aluminum was obtained from impedance measurements and it was shown that the diffusion of oxygen through a porous oxide layer is the governing stage for the reaction occurring at the steel electrode. Finally, the corrosion of steel regulates the corrosion of both metals when they are in electrical contact.

Influence of the crystallographic orientations of Ti–6Al–4V substrate on the morphology and optical properties of oxide thin films obtained by anodizing

The origin of color inhomogeneity in anodized titanium at the microscopic scale is investigated. Oxide layers were produced by galvanostatic anodizing in sulfuric acid on commercial Ti–6Al–4V substrates, with and without HF/HNO3 etching prior to anodizing. The crystallographic orientations of the Ti–6Al–4V substrates were determined using Electron Backscatter Diffraction (EBSD) before anodizing. The chemical composition of the oxide thin films was characterized using X-ray Photoelectron Spectroscopy (XPS). The oxide thin films were observed optically using a digital microscope. The internal morphology of the oxide layers was observed through a Scanning Transmission Electron Microscope (STEM) on Focused Ion Beam (FIB) lamella preparations, taken at the boundary of two differently colored areas on anodized Ti–6Al–4V samples. By characterizing both the substrate and the oxide layer, a correlation was established between the crystallographic orientations of the underlying Ti–6Al–4V substrate and the morphology of the oxide layer. It is shown that crystallographic orientations of the α phase of the substrate close to the basal plane (0001) lead to a porous oxide layer, resulting in a blue color. For other α grains and the β grains of the substrate, the oxide layer is dense and exhibits a yellow color. The development of an optical model to simulate color, accounting for the porosity of the oxide layer, confirms the correlation between porosity and color.