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26 pages, 50563 KiB

Open AccessArticle

Systematic Investigation of Silicon Content Effects on the PEO Coatings’ Properties on Al–Si Binary Alloys in Silicate-Based and Tungstate-Containing Electrolytes

by Masoud Moshrefifar, Hadi Ebrahimifar and Amin Hakimizad

Coatings 2022, 12(10), 1438; https://doi.org/10.3390/coatings12101438 - 30 Sep 2022

Cited by 7 | Viewed by 1586

Abstract

The present study evaluates the effect of a substrate’s silicon content on the coatings’ morphology, structure, and properties in an electrolyte containing sodium tungstate on Al–xSi alloys. The PEO-coated samples demonstrated the structure of the pancake and crater with irregular micro-cracks and micro-pores. [...] Read more.

The present study evaluates the effect of a substrate’s silicon content on the coatings’ morphology, structure, and properties in an electrolyte containing sodium tungstate on Al–xSi alloys. The PEO-coated samples demonstrated the structure of the pancake and crater with irregular micro-cracks and micro-pores. The incorporation of the element W in the coatings decreased the surface roughness and porosity due to increasing the electrolyte conductivity and, hence, decreasing the breakdown voltage and intensity of micro-discharges. The friction coefficient of all the coatings stayed constant during the wear test due to the contact of the ball with the inner layer with less porosity and higher micro-hardness. The track depth to the thickness ratio of the coatings was found to be 0.60–0.75 for those without additive coatings and 0.55–0.65 for those with additive coatings. As a result, the higher wear rates, volume losses, track widths, and depths in the specimens with a lower silicon content of the substrate were found due to the greater thickness of the coating and the outer sublayer for both specimens with and without Na₂WO₄. The coatings formed using the W-containing electrolyte showed a higher micro-hardness and a lower wear rate in comparison with the coatings produced in the silicate-based electrolyte. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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21 pages, 8956 KiB

Open AccessArticle

Silicate and Hydroxide Concentration Influencing the Properties of Composite Al₂O₃-TiO₂ PEO Coatings on AA7075 Alloy

by Mehri Hashemzadeh, Keyvan Raeissi, Fakhreddin Ashrafizadeh, Amin Hakimizad, Monica Santamaria and Thomas Lampke

Coatings 2022, 12(1), 33; https://doi.org/10.3390/coatings12010033 - 28 Dec 2021

Cited by 3 | Viewed by 1560

Abstract

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al₂O₃-TiO₂ coatings produced through plasma electrolytic oxidation in a solution containing 3 g L⁻¹ potassium titanyl oxalate, (PTO), using a [...] Read more.

This work evaluates the effect of sodium meta-silicate pentahydrate (SMS) and potassium hydroxide concentrations on properties of Al₂O₃-TiO₂ coatings produced through plasma electrolytic oxidation in a solution containing 3 g L⁻¹ potassium titanyl oxalate, (PTO), using a unipolar waveform with constant current density. The surface and cross-section characteristics of PEO coatings including morphology, elemental distribution, and phase composition were evaluated using FESEM, EDS, and XRD techniques. Voltage-time response indicated the concentration of SMS and KOH had a significant effect on the duration of each stage of the PEO process. More cracks and pores were formed at the higher concentrated solutions that resulted in the incorporation of solution components especially Si into the coating inner parts. Ti is distributed throughout the coatings, but it had a dominant distribution in the Si-rich areas. The coating prepared in the electrolyte containing no silicate consisted of non-stoichiometric γ-Al₂O₃ and/or amorphous Al₂O₃ phase. Adding silicate into the coating electrolyte resulted in the appearance of α-Al₂O₃ besides the dominant phase of γ-Al₂O₃. The corrosion behaviour of the coatings was investigated using the EIS technique. It was found that the coating prepared in the presence of 3 g L⁻¹ SMS and 2 g L⁻¹ KOH, possessed the highest barrier resistance (~10 MΩ cm²), owing to a more compact outer layer, thicker inner layer along with appropriate dielectric property because this layer lacks the Si element. It was discovered that the incorporation of Ti⁴⁺ and especially Si⁴⁺ in the coating makes the dielectric loss in the coating. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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12 pages, 7146 KiB

Open AccessArticle

Characterization of Dark-Colored Nanoporous Anodic Films on Zinc

by Ryoya Masuda, Damian Kowalski, Sho Kitano, Yoshitaka Aoki, Taisuke Nozawa and Hiroki Habazaki

Coatings 2020, 10(11), 1014; https://doi.org/10.3390/coatings10111014 - 22 Oct 2020

Cited by 6 | Viewed by 2826

Abstract

In this study, zinc is anodized at different voltages in 0.1 mol·dm⁻³ KOH electrolyte to form nanoporous anodic films. Dark-colored anodic films are formed at anodizing voltages ≤6 V, whereas colorless anodic films are developed at voltages ≥7 V. The anodic films [...] Read more.

In this study, zinc is anodized at different voltages in 0.1 mol·dm⁻³ KOH electrolyte to form nanoporous anodic films. Dark-colored anodic films are formed at anodizing voltages ≤6 V, whereas colorless anodic films are developed at voltages ≥7 V. The anodic films formed at all voltages consist of crystalline ZnO, which was identified by X-ray diffraction and Raman spectroscopy. The Raman spectra of the dark-colored anodic films show the enhanced intensity of the LO phonon mode due to electric-field-induced Raman scattering, which may be associated with the presence of metallic Zn nanoparticles in the anodic films. Scanning electron micrographs and transmission electron micrographs of the cross-section of the dark-colored anodized zinc reveal the formation of two-layer porous anodic films with a highly rough metal/film interface. In contrast, nanoporous anodic films of uniform thickness with a relatively flat metal/film interface are formed for the colorless anodized zinc. The transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) analysis suggested the presence of zinc nanoparticles in the dark-colored anodic films. The non-uniform anodizing and the formation of metal-nanoparticle-dispersed porous anodic films cause the formation of dark-colored anodic films on zinc. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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19 pages, 5958 KiB

Open AccessEditor’s ChoiceArticle

The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy

by Amirhossein Toulabifard, Maryam Rahmati, Keyvan Raeissi, Amin Hakimizad and Monica Santamaria

Coatings 2020, 10(10), 937; https://doi.org/10.3390/coatings10100937 - 30 Sep 2020

Cited by 40 | Viewed by 3644

Abstract

Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that [...] Read more.

Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that all coatings consist of MgO and MgF₂, while specific products such as Mg₂SiO₄, MgSiO₃, Mg₂P₂O₇, and MgAl₂O₄ were also present in specimens based on the selected solution. Surface morphology of the obtained coatings was strongly affected by the electrolyte composition. Aluminate-containing coating showed volcano-like, nodular particles and craters distributed over the surface. Phosphate-containing coating presented a sintering-crater structure, with non-uniform distributions of micro-pores and micro-cracks. Silicate-containing coating exhibited a scaffold surface involving a network of numerous micro-pores and oxide granules. The aluminate-treated sample offered the highest corrosion resistance and the minimum wear rate (5 × 10⁻⁵ mm³ N⁻¹ m⁻¹), owing to its compact structure containing solely 1.75% relative porosity, which is the lowest value in comparison with other samples. The silicate-treated sample was degraded faster in long-term corrosion and wear tests due to its porous structure, and with more delay in the phosphate-containing coating due to its larger thickness (30 µm). Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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23 pages, 6982 KiB

Open AccessArticle

Immobilized Nano-TiO₂ Photocatalysts for the Degradation of Three Organic Dyes in Single and Multi-Dye Solutions

by Umberto Bellè, Filippo Pelizzari, Andrea Lucotti, Chiara Castiglioni, Marco Ormellese, MariaPia Pedeferri and Maria Vittoria Diamanti

Coatings 2020, 10(10), 919; https://doi.org/10.3390/coatings10100919 - 25 Sep 2020

Cited by 9 | Viewed by 2639

Abstract

Heterogeneous photocatalysis with titanium dioxide (TiO₂) is considered one of the most promising Advanced Oxidation Processes (AOPs). In order to solve issues related to catalyst recovery and possible agglomeration, which are typical of catalysts in nanoparticle form, self-organized nanotubular TiO₂ [...] Read more.

Heterogeneous photocatalysis with titanium dioxide (TiO₂) is considered one of the most promising Advanced Oxidation Processes (AOPs). In order to solve issues related to catalyst recovery and possible agglomeration, which are typical of catalysts in nanoparticle form, self-organized nanotubular TiO₂ films directly immobilized on a metal substrate can be produced through anodization. In this study, a nanotubular anodic oxide was tested in the degradation of three organic dyes, namely Direct Red 80, Methylene Blue, and Rhodamine B, in single, binary, and ternary mixtures, to simulate industrial effluents with the co-presence of multiple dyes. To better understand the dyes’ behavior and possible interaction effects, spectrophotometry was used to analyze the degradation of each dye in the mixture. The zero-crossing first-order derivative approach and double divisor ratio spectra derivative method were used for the analysis of binary and ternary mixtures, respectively, to overcome quantification problems due to spectra overlapping. The photocatalytic system demonstrated good efficiency, supporting the use of nanotubular TiO₂ as a photocatalyst for dye mixtures. Moreover, the interaction among dyes can actually affect, both positively and negatively, photodegradation kinetics, posing an issue in understanding the actual efficiency of the purification process as a function of the effluent composition. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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14 pages, 4457 KiB

Open AccessArticle

Investigation of Electrochemical Oxidation Behaviors and Mechanism of Single-Crystal Silicon (100) Wafer under Potentiostatic Mode

by Weijia Guo, Senthil Kumar Anantharajan, Kui Liu and Hui Deng

Coatings 2020, 10(6), 586; https://doi.org/10.3390/coatings10060586 - 24 Jun 2020

Cited by 3 | Viewed by 3242

Abstract

Electrochemical oxidation (ECO) has been used widely to oxidize single crystal Si wafers. Aiming at optimizing the ECO assisted machining methods, the oxidation behaviors of single- crystal silicon (100) wafer under potentiostatic mode are experimentally investigated. It is shown that the Si wafer [...] Read more.

Electrochemical oxidation (ECO) has been used widely to oxidize single crystal Si wafers. Aiming at optimizing the ECO assisted machining methods, the oxidation behaviors of single- crystal silicon (100) wafer under potentiostatic mode are experimentally investigated. It is shown that the Si wafer can be electrochemically oxidized and the oxidized film thickness reaches to 239.6 nanometers in 20 min. The hardness of the oxidized surface is reduced by more than 50 percent of the original surface. The results indicate that the oxide thickness and the hardness can be controlled by changing the voltage. Based on the experimental findings, a hypothesis on the ECO mechanism under potentiostatic mode was proposed to explain the fluctuations of current density under specific applied voltage. The occurrence of the multiple peaks in the current density curve during the oxidation process is due to the formation of discharge channels, which was initiated from the defects at the interface between the oxide bottom and the substrate. This breaks the electrical isolation and leads to the discontinuous growth of the electrochemical oxide layer. The present work contributes to the fundamental understanding of the ECO behaviors for the single-crystal Si (100) wafer under potentiostatic mode. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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Review

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32 pages, 6627 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Passive Layers and Corrosion Resistance of Biomedical Ti-6Al-4V and β-Ti Alloys

by Patrizia Bocchetta, Liang-Yu Chen, Juliana Dias Corpa Tardelli, Andréa Cândido dos Reis, Facundo Almeraya-Calderón and Paola Leo

Coatings 2021, 11(5), 487; https://doi.org/10.3390/coatings11050487 - 21 Apr 2021

Cited by 115 | Viewed by 10347

Abstract

The high specific strength, good corrosion resistance, and great biocompatibility make titanium and its alloys the ideal materials for biomedical metallic implants. Ti-6Al-4V alloy is the most employed in practical biomedical applications because of the excellent combination of strength, fracture toughness, and corrosion [...] Read more.

The high specific strength, good corrosion resistance, and great biocompatibility make titanium and its alloys the ideal materials for biomedical metallic implants. Ti-6Al-4V alloy is the most employed in practical biomedical applications because of the excellent combination of strength, fracture toughness, and corrosion resistance. However, recent studies have demonstrated some limits in biocompatibility due to the presence of toxic Al and V. Consequently, scientific literature has reported novel biomedical β-Ti alloys containing biocompatible β-stabilizers (such as Mo, Ta, and Zr) studying the possibility to obtain similar performances to the Ti-6Al-4V alloys. The aim of this review is to highlight the corrosion resistance of the passive layers on biomedical Ti-6Al-4V and β-type Ti alloys in the human body environment by reviewing relevant literature research contributions. The discussion is focused on all those factors that influence the performance of the passive layer at the surface of the alloy subjected to electrochemical corrosion, among which the alloy composition, the method selected to grow the oxide coating, and the physicochemical conditions of the body fluid are the most significant. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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30 pages, 5551 KiB

Open AccessReview

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

by Mariana Paz Martínez-Viademonte, Shoshan T. Abrahami, Theodor Hack, Malte Burchardt and Herman Terryn

Coatings 2020, 10(11), 1106; https://doi.org/10.3390/coatings10111106 - 18 Nov 2020

Cited by 110 | Viewed by 18067

Abstract

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists [...] Read more.

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives. Full article

(This article belongs to the Special Issue Anodizing Processes for the Production of Advanced Functional Coatings)

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