Alkaline Phosphate Electrolyte Research Articles

In-situ incorporation of polydopamine surface modified bioactive glass nanoparticles into a plasma electrolytic oxidation coating on biodegradable AZ31 Mg alloy: A study of bioactivity and corrosion performance

Magnesium alloys are promising candidates for next-generation biodegradable biomaterials. However, their poor corrosion resistance has limited their medical applications. In this study, to improve the bioactivity performance and control the degradation rate of an AZ31 Mg alloy, anodic oxide coatings containing bioactive glass (BG) were created on the alloy using plasma electrolytic oxidation (PEO). First, BG nanoparticles were synthesized through the sol-gel method and surface-modified with polydopamine (BG/PDA). These nanoparticles were then dispersed in an alkaline phosphate electrolyte and incorporated into the coatings grown using unipolar and bipolar waveforms. According to EDS results, incorporation of BG/PDA nanoparticles was higher when the bipolar waveform was applied. Electrochemical impedance spectroscopy revealed that the coatings significantly enhanced the corrosion resistance of the AZ31 Mg alloy, with the BG/PDA nanoparticle-containing coatings demonstrating the lowest degradation rate during long-term immersion. These coatings achieved the highest faradic resistance of ∼ 12 kΩ.cm² after 28 days of immersion. The incorporation of BG and BG/PDA nanoparticles enhanced the bioactivity performance of the coatings, facilitating hydroxyapatite formation on the coating surface. Based on the results, the newly developed BG-coated alloy shows promising potential for biomedical applications.

Study of the wear resistance of composite coatings modified with h-BN particles on AZ31 magnesium alloy

A low wear resistance is a significant disadvantage of magnesium-based alloys widely used in industry. The results of plasma electrolytic oxidation (PEO) carried out in an aqueous-alkaline phosphate electrolyte with the addition of hexagonal boron nitride (h-BN) powder to obtain coatings with greater wear resistance on the surface of AZ31 magnesium alloy are presented. The PEO method is one of the most promising for surface treatment of magnesium alloys, since oxidation is carried out in alkaline aluminate, silicate or phosphate electrolytes with various functional additives. The addition of nanocrystalline hexagonal h-BN powder in the form of a suspension into the electrolyte volume does not affect the electrical parameters of PEO, and h-BN particles are incorporated into the structure of the formed composite coating, increasing the wear resistance. It is shown that the resulting coatings have a relief typical of PEO with developed morphology and porosity, which change depending on the oxidation time. In this case, the incorporation of h-BN particles into the coating occurs by an inert mechanism, since they do not undergo chemical transformations with the formation of new phases. Composite coatings obtained on the surface of the AZ31 magnesium alloy by the PEO method consist of crystalline phases of MgO and Mg3(PO4)2, regardless of the addition of h-BN particles to the electrolyte. The wear resistance of coatings is 6 – 8 times higher compared to the untreated alloy. The results obtained can be used to produce PEO coatings with increased wear resistance and use them in various sectors of the economy.

Innovative approach to boosting the chemical stability of AZ31 magnesium alloy using polymer-modified hybrid metal oxides

Meeting the demands of complex and advanced applications requires the development of high-performance hybrid materials with unique properties. However, the integration of polymeric frameworks with MgO/WO3 composite layers faces challenges due to the lack of understanding of the formation mechanism and the challenge of determining the impact of self-assembled architecture on anticorrosive properties. In this study, we aimed to enhance the corrosion resistance of the MgO layer produced by plasma electrolysis (PE) of AZ31 Mg alloy by incorporating WO3 with partially phosphorated poly(vinyl alcohol) (PPVA). Two types of porous MgO layers were produced using the PE process with an alkaline-phosphate electrolyte, one with and one without WO3 nanoparticles, which were subsequently immersed in an aqueous solution of PPVA. Incorporating PPVA into the WO3-MgO layer resulted in hybrids being deposited in a fragmented manner, creating a ``laminar reef-like structure'' that sealed most of the structural defects in the layer. The PPVA-sealed WO3-based coating exhibited superior corrosion resistance compared to the other samples. Computational analyses were employed to explore the mechanism underlying the formation of PPVA/WO3 hybrids on the MgO layer. These findings suggest that PPVA-WO3-MgO hybrid coatings can potentially improve corrosion resistance in various fields.

Electrochemical response of MgO/Co3O4 oxide layers produced by plasma electrolytic oxidation and post treatment using cobalt nitrate

This work looked into the influence of the sealing treatment on the structural feature and electrochemical response of AZ31 Mg alloy coated via plasma electrolytic oxidation (PEO). Here, the inorganic layers produced by PEO in an alkaline-phosphate electrolyte were subsequently immersed for different periods in cold (60 °C) and hot (100 °C) aqueous solutions containing either 1 or 3 gr of cobalt nitrate hexahydrate in the presence of hydrogen peroxide as an initiator. The results showed that the sealing treatments in the hot solutions could trigger the hydration reactions of PEO coating which would largely assist the surface incorporation of Co 3 O 4 into the coating. In contrast, the sealing in cold solutions led to less compact coatings, which was attributed to the fact the hydration reactions would be restricted at 60 °C. A nearly fully sealed coating with a porosity of ∼ 0.5 % was successfully formed on the sample immersed in the hot solution containing 1 gr of cobalt nitrate hexahydrate. Thus, the electrochemical stability of this fully sealed coating was superior to the other samples as it had the lowest corrosion current density (4.71 × 10 −10 A.cm −2 ) and the highest outer layer resistance (3.81 × 10 7 Ω.cm 2 ). The composite coatings developed in this study are ideal for applications requiring high electrochemical stability.

Pre-intercalation of phosphate into Ni(OH)2/NiOOH for efficient and stable electrocatalytic oxygen evolution reaction

Layered Ni(OH)2/NiOOH hydroxides are effective oxygen evolution reaction (OER) electrocatalysts, but their activities are still inferior to those of benchmark RuO2 and IrO2. Notably, the long-term stability becomes a significant challenge. Herein, we present a novel volume OER (VOER) strategy to solve the problems via pre-intercalation of phosphate. Phosphate-intercalated Ni-Ni(OH)2/NiOOH electrode was prepared by a two-step electrochemical method. The obtained porous electrode exhibits outstanding OER performance in the alkaline phosphate electrolyte with the overpotentials of 129 and 243 mV at 10 and 100 mA cm−2, respectively, and stability for 1000 h. The exceptional performance is mainly ascribed to effective VOER and elimination of the interlayer-produced protons, because the phosphate-intercalation enhances the interlayer water content, interlayer spacing and proton transport ability, endows the interlayers with proton buffering capacity, and alters the chemical environment of the interlayer Ni-O. The finding provides a strategy to develop highly efficient and stable OER materials and reaction systems for layered transition metal hydroxides.

Fatigue Behavior of an AM50 Die-Casting Alloy Anodized by Plasma Electrolytic Oxidation.

While an anodizing process is essential for magnesium alloys to be used under corrosive environments, it sometimes stimulates a fatigue fracture that initiates at the interface between the coating layer and the substrate. In this study, a plasma electrolyte oxidation (PEO) technique was employed to provide excellent adhesion between the anodizing layer and the AM50 die-cast by applying an extremely high dielectric discharge in an alkaline phosphate electrolyte, and its effect on corrosion and fatigue behaviors was investigated. The stress intensity factor at the fatigue limit was estimated to be 0.28 MPam0.5. The specimen anodized using the PEO technique exhibits enhanced strength and corrosion resistance compared to the unanodized counterpart. Furthermore, it shows a relative fatigue life in spite of the thick anodizing layer because the crack initiates from the interface, not from the pore near the interface.

Catalytic activity of platinized titanium and titanium coated with ruthenium oxide in the processes of electrooxidation of ethyl alcohol in alkaline media

AbstractBACKGROUNDThis study investigates the catalytic activity of platinized titanium and titanium coated with ruthenium oxide in the processes of electrooxidation of ethyl alcohol in alkaline phosphate electrolyte and potassium hydroxide electrolyte in non‐flowing membraneless fuel cells proposed by the authors. In these systems, an interface between the phases formed, which played the role of a membrane. Cyclic voltammetry, pulse chronoamperometry and gas–liquid chromatography were used to study the corrosion resistance and catalytic activity of ethanol oxidation on the selected electrodes. Platinized titanium and titanium coated with ruthenium oxide, involving a thin layer of platinum or ruthenium oxide on a perforated titanium mesh, were investigated.RESULTSIt is found that the oxidation of ethyl alcohol proceeds more actively on the surface of titanium coated with ruthenium oxide in alkaline phosphate electrolyte, with the formation of acetaldehyde as the main reaction product. In potassium hydroxide electrolyte, acetaldehyde and acetic acid are formed on the surface; two and four electrons are released, respectively. This indicates the C2 reaction pathway of ethanol oxidation. In contrast, the oxidation of ethyl alcohol in potassium hydroxide electrolyte proceeds more actively on the platinized titanium with the formation of acetaldehyde.CONCLUSIONSThe results obtained show that titanium electrode coated with ruthenium oxide is promising in phosphate electrolyte, but platinized titanium electrode is promising in potassium hydroxide electrolyte in the proposed non‐flowing membraneless alcohol fuel cells. © 2021 Society of Chemical Industry (SCI).

Electrochemical deposition of amorphous aluminum oxides on lead pipes to prevent lead leaching into the drinking water

Over 5000 public drinking water systems in the US are out of compliance with the Lead and Copper Rule. Lead leaching from lead pipes is limited by the solubility of a naturally occurring scale. Changes in water quality may cause this scale to become more soluble, releasing lead into the drinking water. We propose a novel electrochemical method to prevent lead leaching from lead pipes. In this method, an aluminum wire and an alkaline phosphate electrolyte are inserted into the pipes. The pipes are then anodized for 2 h by using an external power supply, resulting in the electrodeposition of an insoluble aluminum oxide layer on top of the preexisting scale. This technology was tested on lead pipes from the EBMUD water distribution systems located in Berkeley, CA, using recirculating synthetic and actual tap water for 120 days. The untreated pipes leached an average of 23 ppb and 38 ppb of lead respectively, when using free chlorine and monochloramine as disinfection residuals. In contrast, the treated pipes leached 3 ppb and 5 ppb respectively. These results suggest that the proposed treatment has the potential to prevent lead leaching regardless of the disinfection residual and thus should be further explored in a field trial.

High temperature oxidation of Zr[sbnd]1Nb alloy with plasma electrolytic oxidation coating in 900–1200 °C steam environment

A continuous ceramic coating of 8 μm thick on Zr1Nb alloy was fabricated by plasma electrolytic oxidation (PEO) in alkaline phosphate electrolyte. The high-temperature steam oxidation behavior of bare and PEO coated Zr1Nb alloys was evaluated using a thermogravimetric analyzer (TGA) in 900–1200 °C water vapor environment. Their morphologies, compositions, phase constituents before and after the steam oxidation were characterized by scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD). It was found that the mass gain of PEO coated Zr alloy was about 38%, 56%, 93% and 100% of bare Zr alloy after 3600 s steam oxidation at 900 °C, 1000 °C, 1100 °C and 1200 °C, respectively. The steam oxidation resistance of Zr1Nb alloy was obviously improved after PEO treatment below 1000 °C, but was hardly enhanced above 1100 °C. The mass gain rate for Zr1Nb alloy samples in high-temperature steam is related to the oxygen diffusion coefficients in β-Zr, α-Zr(O) and ZrO2 phases. After steam oxidation at 900–1200 °C, the oxide layers for bare and PEO coated Zr1Nb alloys contain monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phases below 1100 °C, but the T-ZrO2 phase disappears at 1200 °C.

Study on Preparation, Microstructure and Properties of Micro-Arc Oxidation Ceramic Coating on AZ91 Magnesium Alloy in Phosphate Electrolyte

The micro-arc oxidation behavior of AZ91 magnesium alloy in alkaline phosphate electrolyte was studied. The effect of phosphate on the microstructure and properties of micro-arc oxide film was investigated and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectrum (EDS) and salt spray test. The results show that phosphate concentration has an important effect on the surface morphology, phase structure, thickness and corrosion resistance of micro-arc oxidation film. With the increase of phosphate concentration from 5g/L to 30g/L, the film quality decreases, the surface roughness increases, the thickness decreases and the corrosion resistance decreases. So in phosphate electrolysis the lower phosphate concentration in the liquid system is helpful to the formation of better film quality.

An interesting anodic oxidation behavior of plasma electrolytic oxidation coatings fabricated on aluminum in alkaline phosphate electrolyte

Plasma electrolytic oxidation (PEO) of Al was carried out in dilute alkaline sodium tripolyphosphate (Na5P3O10) electrolyte. Surface and cross-section morphologies, elemental and phase compositions were characterized by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometer (XRD). The results revealed that a severe thinning of the anodic film resulted in the voltage decrease at the initial stage of the PEO process. Porous-type anodic film with uniform pore diameter were fabricated at the initial stage of PEO process. Subsequently, detachments and cracks took place on the anodic films, and served as weak regions occurring dielectric breakdown preferentially due to the concentration of electric current at these sites. Two origins of PEO coatings, the transformation of anodic films and the normal generation of PEO coatings through the plasma discharges, were proposed. The results also indicated that P element mainly participated in the anodic oxidation and existed as amorphous phase in the inner side areas of anodic films except the barrier layer. Based on the analyses of the microstructure evolution and the chemical composition, a schematic of coating growth mechanism was proposed in the end.

Degradation and structure evolution in corrosive LiOH solution of microarc oxidation coated Zircaloy-4 alloy in silicate and phosphate electrolytes

To improve the corrosion resistance of zirconium alloys, two separate coatings with distinct structure were prepared on Zircaloy-4 alloy by microarc oxidation in alkaline phosphate and silicate electrolytes, respectively. The coating formed in silicate solution has more dense and homogeneous structure than the other, thus it shows better corrosion resistance. According to the electrochemical impedance spectroscopy of the two samples immersed in LiOH solution for different time periods, the coating formed in phosphate solution is able to protect the substrate alloy from corrosion for 30days, while the coating formed in silicate can provide sufficient protection to the substrate up to 50days.

Degradation behavior of PEO coating on AM50 magnesium alloy produced from electrolytes with clay particle addition

Amorphous plasma electrolytic oxidation (PEO) coating with sealed discharge channels can be formed in alkaline phosphate electrolyte containing montmorillonite clay particles. The effect of various concentrations of phosphate and hydroxide ions in the clay-containing electrolyte on the microstructure of the coatings was studied in the present work and correlated with the corrosion behavior. The clay particles were reactively incorporated into the coating. Single amorphous phase appears in PEO coatings produced from electrolytes containing higher concentration of phosphate. These amorphous coatings are degrading within a relatively short period in 0.5wt% NaCl solution. Electrolytes containing higher concentration of KOH tend to produce mixed PEO coatings composed of crystalline and amorphous phases. These layers demonstrate higher corrosion resistance and degradation stability. Thus, the degradation rate of PEO coatings is governed mostly by the stability of their phase composition, which might be controlled by varying electrolyte composition.

Structure and corrosion behavior of oxide layer with Zr compounds on AZ31 Mg alloy processed by two-step plasma electrolytic oxidation

The formation of the oxide layer with Zr compounds on AZ31 Mg alloy processed by two-step plasma electrolytic oxidation (PEO) has been investigated. After the PEO process first in an alkaline phosphate electrolyte and second in an acid electrolyte containing K2ZrF6, the microstructure, chemical composition, and phase composition of the oxide layers were analyzed via SEM, EDS, and XRD, respectively. The electrochemical reaction and the high temperature caused by the plasma discharges in the electrolyte were the main factors leading to the fabrication of an oxide layer containing Zr compounds on AZ31 Mg alloy. The micro-pores were filled with ZrO2 formed during the PEO process. The results of a potentiodynamic polarization test clearly indicated that the corrosion properties of AZ31 Mg alloy were significantly improved by the two-step PEO process.

Effective regime of microarc oxidation (MAO) on AZ 91 magnesium alloy

8 1 At the present time, the relative volume of produc� tion and use of magnesium alloys is continuously growing due to their lower density compared to other structural materials (1). Nevertheless the use of parts made of these Mg alloys require anticorrosive coatings on their surfaces. To obtain such protective coatings one of the most promising method consists of Microarc Oxidation (MAO) (2, 3) with a subsequent sealing or varnishing and painting of the surface since microarc coating (MAC) mostly serves as a high� porous layer for further finishing treatment on magne� sium alloys products. It was established (4-6) that the addition of fluo� rinecontaining chemical components in basic elec� trolytes leads to the improvement of the mechanical and tribological properties of microarc oxidation coat� ings formed on the surface of magnesium alloys, increases the efficiency of the MAO process and con� sequently reduces the power consumption of the pro� cess. In this work (3) it is shown that within alkaline phosphate electrolytes, the growth rate of microarc oxidation coating on the surface of AZ91 magnesium alloy is significantly higher (not less than 1.4 times) for a process conducted in the anodic regime than in the anodiccathodic regime. Thus, at the stage of sparking discharges, growth coverage is slowed down with the advent of microarc discharges on a surface of the working electrode. It is obvious that the performance of the process of microarc oxidation of magnesium alloys increases sig� nificantly if it is conducted by passing a direct current between the electrodes in basic electrolytes with the introduction of fluorides. Therefore, the main objec� tive of this work is the investigation of the process to obtain MAO precoats on the surface of magnesium 1 The article was translated by the authors. alloys. This work also aims at establishing technologi� cal parameters that allow us to conduct the process with higher performances. Therefore the optimal con� centration of ammonium fluoride in the base alkaline phosphate electrolyte, the current density and the shape of the current waveform in galvanostatic mode are investigated

Preparation Al2O3/ZrO2 composite coating in an alkaline phosphate electrolyte containing K2ZrF6 on aluminum alloy by microarc oxidation

Al2O3/ZrO2 composite coating was prepared on 2A70 aluminum alloy by microarc oxidation (MAO) in a phosphate electrolyte containing K2ZrF6. The effects of K2ZrF6 in the electrolyte on the coating thickness, surface morphology, elemental composition and abrasive resistance were investigated. The results indicated that the MAO coating thickness increase due to the addition of K2ZrF6 in the electrolyte. In the electrolyte containing 6.0g/L K2ZrF6, the coating was the most uniform and compact. The MAO coating formed in the electrolyte without K2ZrF6 was mainly composed of γ-Al2O3 and a little α-Al2O3. Zirconium in the form of t-ZrO2 was detected in the MAO coatings formed in the electrolyte containing K2ZrF6, and the Zr content increases with increasing concentration of K2ZrF6. A γ-Al2O3 and t-ZrO2 nanocomposite coating was formed in the electrolyte containing 6.0g/L K2ZrF6, and which exhibited the highest abrasive resistance.

Influence of titania sol in the electrolyte on characteristics of the microarc oxidation coating formed on 2A70 aluminum alloy

The effects of a titania sol in an alkaline phosphate electrolyte on the thickness, surface morphology, composition and abrasive resistance of a microarc oxidation (MAO) coating on 2A70 aluminum alloy were investigated. The voltage–time curve was used to characterize MAO process. The coating thickness increased with increasing the concentration of the titania sol in the electrolyte, accompanied with the increase in surface nonuniformity and size of micropores in the coating surface. The MAO coating formed in the titania sol free electrolyte was mainly composed of γ-Al 2O 3 and a little α-Al 2O 3. Titanium in the form of Al 2TiO 5, Ti 2O 3 and TiO was detected in the MAO coatings obtained in the electrolyte containing titania sol, and the content increases with increasing the concentration of titania sol. The MAO coating formed in the electrolyte containing 6.0 vol.% titania sol exhibited the highest microhardness and abrasion resistance.

Development of decorative and corrosion resistant plasma electrolytic oxidation coatings on AM50 magnesium alloy

Plasma electrolytic oxidation of AM50 magnesium alloy was performed in alkaline phosphate electrolyte with and without the addition of titania sol. The coatings produced in the phosphate electrolyte were constituted with MgO and Mg3(PO4)2. The coatings obtained in the phosphate electrolyte with the addition of titania sol were blue in colour and contained additionally the TiO2 and Mg2TiO4 phases. The phosphate PEO coating provided a two order of magnitude improvement in corrosion resistance to the AM50 magnesium alloy as was shown by the potentiodynamic polarisation measurements. With differences in the physical appearance, microstructural morphology and phase composition, the coatings produced in titania sol containing electrolytes offered an improved corrosion resistance compared to the mere phosphate PEO coating.

Effect of pulse frequency on the microstructure, phase composition and corrosion performance of a phosphate-based plasma electrolytic oxidation coated AM50 magnesium alloy

An AM50 magnesium alloy was plasma electrolytic oxidation treated using a pulsed DC power supply at three different pulse frequencies viz., 10 Hz, 100 Hz and 1000 Hz with a constant pulse ratio for 15 min in an alkaline phosphate electrolyte. The resultant coatings were characterized by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy for their phase composition and microstructural features. The 10 Hz condition yielded relatively thick and rough coatings, which was attributed to the higher energy input per individual pulse during the PEO processing. The phase composition was also found to be influenced by the processing frequency. Electrochemical impedance spectroscopy studies performed in 0.1 M NaCl solutions revealed that the coatings produced at 10 Hz condition had a better corrosion resistance, which was attributed to the higher thickness, more compact microstructural features and a relatively stable phase composition.

Anodizing under sparking of AZ31B magnesium alloy in Na3PO4 electrolyte

The microstructures and corrosion resistance of the anodic oxide films formed on AZ31B magnesium alloy by anodizing under continuous sparking in alkaline phosphate electrolyte (Na3PO4) were investigated. Basically, voltage-current characteristic of AZ31B in Na3PO4 solution was the same to that obtained in other sodium salt electrolytes such as Na2SiO3 and NaAlO2. However, the anodic films formed in Na3PO4 solution exhibited the higher corrosion resistance than that of the films formed in other electrolytes because of the incorporation of substantial amount of magnesium phosphate. The microstructures (e.g., diameter and number of pores) of the anodic films formed under sparking accompanied by current or voltage oscillation and gas evolution were strongly affected by the degree of sparking such as size, number, and appearance frequency, not by the formation voltage itself. X-Ray diffraction analysis revealed the incorporation of crystalline Mg3(PO4)2 into the anodic film formed under the sparking condition of dielectric breakdown.