Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition

A special issue of Coatings (ISSN 2079-6412). This special issue belongs to the section "Surface Characterization, Deposition and Modification".

Deadline for manuscript submissions: 30 May 2024 | Viewed by 9976

Special Issue Editors

Department of Chemical and Materials Engineering, Universidad Complutense de Madrid, Madrid, Spain
Interests: corrosion and protection of light alloys; surface modification; plasma electrolytic oxidation coatings; active protection; biomaterials
Special Issues, Collections and Topics in MDPI journals
Department of Chemical and Materials Engineering, Universidad Complutense de Madrid, Madrid, Spain
Interests: corrosion and protection of light alloys; material characterization; plasma electrolytic oxidation coatings; biomaterials

Special Issue Information

Dear Colleagues,

We would like to invite you to submit your work to a Special Issue on "Plasma Electrolytic Oxidation (PEO) Coatings (Second Volume)". Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), functionalizes surfaces, improving the mechanical, thermal, and corrosion performance of metallic substrates, along with other tailored properties (e.g., biocompatibility, catalysis, antibacterial response, self-lubrication, etc.). The extensive field of applications of this technique ranges from structural components, in particular, in the transport sector, to more advanced fields, such as bioengineering.

The aim of this Special Issue is to present the state of the art of PEO for Al, Mg, Ti, and Zr alloys and steels, through a combination of original research papers, short communications, and review articles from leading research groups around the world.

In particular, the topics of interest include, but are not limited to:

  • Fundamental understanding of the PEO process: mechanistic study and modeling of coating growth;
  • Properties and performance of PEO coatings: corrosion, mechanical, catalytic, and/or electric evaluation;
  • Hybrid PEO coatings;
  • Functionalization of PEO coatings;
  • Active protection based on PEO;
  • Bio-applications of PEO coatings;
  • Advanced PEO processes.

Dr. Marta Mohedano
Dr. Raul Arrabal
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Coatings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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Published Papers (9 papers)

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Research

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21 pages, 6130 KiB  
Article
Degradation Rate Control Issues of PEO-Coated Wrought Mg0.5Zn0.2Ca Alloy
by Lara Moreno, Marta Mohedano, Raul Arrabal and Endzhe Matykina
Coatings 2024, 14(3), 309; https://doi.org/10.3390/coatings14030309 - 02 Mar 2024
Viewed by 535
Abstract
Bioactive plasma electrolytic oxidation (PEO) coatings were developed on a wrought Mg0.5Zn0.2Ca alloy using a transparent electrolyte for easy maintenance and waste disposal, compared to a conventional suspension-based solution. Treatment times of 300, 600, and 900 s were evaluated for their effects on [...] Read more.
Bioactive plasma electrolytic oxidation (PEO) coatings were developed on a wrought Mg0.5Zn0.2Ca alloy using a transparent electrolyte for easy maintenance and waste disposal, compared to a conventional suspension-based solution. Treatment times of 300, 600, and 900 s were evaluated for their effects on coating morphology, composition, and corrosion resistance. A short-time electrochemical impedance spectroscopy (EIS) screening was utilized to identify coatings with optimal corrosion protection. To assess the degradation rate and corrosion mechanisms, hydrogen evolution was monitored under pH-controlled quasi-in vivo conditions over extended immersion periods. Coating thickness increased by only 3% from 300 to 900 s of treatment (13 and 18 µm, respectively), with pore bands formed near the barrier layer at 900 s. The short-term EIS screening revealed that the coatings produced at 600 and 900 s were less protective and consistent than those at 300 s due to the presence of pore bands, which increased permeability. Hydrogen evolution measurements during 5 days of immersion at pH 7.4 indicated a tenfold higher degradation rate of the PEO-coated alloy compared to the bare substrate. Therefore, none of the PEO coatings provided effective corrosion protection after 24 h of immersion, which is attributed to crack formation at the PEO/corrosion products interface. This highlights the importance of crevices in the corrosion of Mg-Zn-Ca alloys. The presence of ZnO exacerbates the corrosion of magnesium in crevice areas. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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23 pages, 7194 KiB  
Article
Functionalization of Plasma Electrolytic Oxidation/Sol–Gel Coatings on AZ31 with Organic Corrosion Inhibitors
by Borja Pillado, Endzhe Matykina, Marie-Georges Olivier, Marta Mohedano and Raúl Arrabal
Coatings 2024, 14(1), 84; https://doi.org/10.3390/coatings14010084 - 07 Jan 2024
Viewed by 857
Abstract
In this investigation, the sol–gel method is employed along with a corrosion inhibitor to seal a plasma electrolytic oxidation (PEO) coating, aiming to improve the long-term corrosion resistance of the AZ31 Mg alloy. Following an initial screening of corrosion inhibitors, 8-hydroxyquinoline (8HQ) is [...] Read more.
In this investigation, the sol–gel method is employed along with a corrosion inhibitor to seal a plasma electrolytic oxidation (PEO) coating, aiming to improve the long-term corrosion resistance of the AZ31 Mg alloy. Following an initial screening of corrosion inhibitors, 8-hydroxyquinoline (8HQ) is incorporated into the hybrid PEO/sol–gel system using two methods: (i) post-treatment of the PEO layer through immersion in an inhibitor-containing solution; (ii) loading the inhibitor into the sol–gel precursor. The characterization includes scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis), and water drop contact angle measurements. The rheological properties of the inhibitor-loaded sol–gel precursors are assessed by measuring flow curves. The corrosion processes are evaluated in a saline solution through electrochemical impedance spectroscopy (EIS) and immersion tests with unscratched and scratched specimens, respectively. The results demonstrate the successful incorporation of the inhibitor for both loading strategies. Regardless of the loading approach, systems containing 8HQ exhibit the most favourable long-term corrosion resistance. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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28 pages, 6731 KiB  
Article
Influence of Cathodic Polarization on Plasma Electrolytic Oxidation of Magnesium and AZ31 and AZ91 Magnesium Alloys
by Zhengzhou Le, Zihua Liu, Xiaorui He, Yulin Cheng, Panfeng Hu and Yingliang Cheng
Coatings 2023, 13(10), 1736; https://doi.org/10.3390/coatings13101736 - 06 Oct 2023
Viewed by 802
Abstract
In this study, the influence of cathodic polarization on the plasma electrolytic oxidation (PEO) behaviors of pure magnesium and AZ31 and AZ91 magnesium alloys with varied Al alloying contents was systematically examined in a dilute alkaline silicate electrolyte by adjusting the cathodic-to-anodic current [...] Read more.
In this study, the influence of cathodic polarization on the plasma electrolytic oxidation (PEO) behaviors of pure magnesium and AZ31 and AZ91 magnesium alloys with varied Al alloying contents was systematically examined in a dilute alkaline silicate electrolyte by adjusting the cathodic-to-anodic current density ratio (R = jc/ja) from 0 to 3.2. The results show that moderate cathodic polarization (R = 0.6) led to the thickest coatings on the Mg and Mg alloys, and the coatings grew in an outward-and-inward mode compared with the inward growth at R = 0. Excessive cathodic polarization (high R ratios) differently influenced the PEO behaviors of the magnesium alloys. For the pure magnesium and AZ31 alloy, the coatings blistered or peeled off when the R ≥ 0.9. However, the tolerance to cathodic polarization was significantly improved for the AZ91 Mg alloy. The coatings were undamaged even with the highest R ratio of 3.2, and their compactness was further improved as the R ratio increased to 0.9 and 1.2. An increase in cathodic polarization led to a reduction in the anodic potential and spark softening but did not result in an improvement in the coating quality. Optical emission spectroscopy identified two spectral lines at 559.79 and 570.11 nm, which are assigned to the Mg species but not found in databases or the literature. The corrosion and wear resistance of the PEO coatings were also investigated. The coating formed on the AZ91 magnesium alloy at R = 1.2 displayed the narrowest wear track due to its high compactness. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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16 pages, 5640 KiB  
Article
Comparison of Tribological Characteristics of AA2024 Coated by Plasma Electrolytic Oxidation (PEO) Sealed by Different sol–gel Layers
by Hafiza Ayesha Khalid, Sajjad Akbarzadeh, Yoann Paint, Véronique Vitry and Marie-Georges Olivier
Coatings 2023, 13(5), 871; https://doi.org/10.3390/coatings13050871 - 05 May 2023
Cited by 1 | Viewed by 1084
Abstract
The application of sol–gel on plasma electrolytic oxidation (PEO) coatings can increase wear resistance by sealing the surface defects such as pores and cracks in the outer layer of the PEO layer and strengthen the coating. Four different sol–gel formulations based on precursors—(3-glycidyloxypropyl)trimethoxysilane [...] Read more.
The application of sol–gel on plasma electrolytic oxidation (PEO) coatings can increase wear resistance by sealing the surface defects such as pores and cracks in the outer layer of the PEO layer and strengthen the coating. Four different sol–gel formulations based on precursors—(3-glycidyloxypropyl)trimethoxysilane (GPTMS), methyltriethoxysilane (MTES), methacryloxypropyltrimethoxysilane (MAPTMS), (3-aminopropyl)triethoxysilane (APTES), and zirconium(IV) propoxide (ZTP) along with tetraethoxysilane (TEOS)—were used to seal PEO pores, and the samples were tested tribologically. A sliding reciprocating tribometer was used to carry out a wear test with an alumina ball as the counter body in two different conditions: (a) 2.5 N load for 20 min, and (b) 3 N load for 40 min. The coefficient of friction and wear rate as volume loss per unit sliding length were obtained for all sol–gel-sealed specimens and unsealed PEO-coated and bare AA2024 substrate. 3D mechanical profilometer surface scans were used to compare the depth of wear traces. The elemental color mapping using SEM and EDS revealed that silicon remains present in the wear tracks of PEO coatings sealed with sol–gel layers containing GPTMS (PSG) and ZTP (PSG-ZT). GPTMS (PSG) was able to fill the pores of the PEO layer efficiently due to its cross-linked network. Moreover, sol–gel containing ZTP (PSG-ZT) was deposited as a thick layer on top of the PEO layer which provided good lubrication and resistance to wear. However, other sol–gel formulations (PSG-MT and PSG-AP) were worn out during tests at a higher load (3 N). The most stable friction coefficient (COF) and specific wear rates were observed with sol–gels with GPTMS and ZTP. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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23 pages, 5810 KiB  
Article
Photoluminescent Coatings on Zinc Alloy Prepared by Plasma Electrolytic Oxidation in Aluminate Electrolyte
by Hanna Maltanava, Stevan Stojadinovic, Rastko Vasilic, Sergey Karpushenkov, Nikita Belko, Michael Samtsov and Sergey Poznyak
Coatings 2023, 13(5), 848; https://doi.org/10.3390/coatings13050848 - 28 Apr 2023
Cited by 1 | Viewed by 1080
Abstract
Thick ZnO/ZnAl2O4 coatings were synthesized on zinc alloy Z1 substrates through plasma electrolytic oxidation (PEO) for different anodization times. The prepared coatings were characterized by scanning SEM, XRD, diffuse reflectance and photoluminescence spectroscopy in order to establish the relationship between [...] Read more.
Thick ZnO/ZnAl2O4 coatings were synthesized on zinc alloy Z1 substrates through plasma electrolytic oxidation (PEO) for different anodization times. The prepared coatings were characterized by scanning SEM, XRD, diffuse reflectance and photoluminescence spectroscopy in order to establish the relationship between their structural and optical properties and PEO processing parameters. Under different PEO processing conditions (anodization time—1–10 min and applied voltage—370 and 450 V) ceramic coatings with a mean thickness of 2–12 μm were prepared. XRD analysis explored the coating structure composed of zinc oxide (wurtzite) and zinc aluminate spinel. The content of ZnAl2O4 in the coatings grows with increasing the applied voltage and anodization time. Photoluminescence (PL) measurements showed that the PEO coatings have several bands in the visible and near-infrared regions associated with their composite structure. The PL spectra significantly depend on the PEO processing parameters due to varying ZnO and ZnAl2O4 content in the coatings. The insight in the relationship between the ZnAl2O4 structure and the photoluminescent properties of ZnO/ZnAl2O4 coatings has been provided using the combination of XRD and luminescence spectroscopy. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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19 pages, 5690 KiB  
Article
Passivation and pH-Induced Precipitation during Anodic Polarization of Steel in Aluminate Electrolytes as a Precondition for Plasma Electrolytic Oxidation
by Roy Morgenstern, Claudia Albero Rojas, Frank Simchen, Vanessa Meinhold, Thomas Mehner and Thomas Lampke
Coatings 2023, 13(3), 656; https://doi.org/10.3390/coatings13030656 - 20 Mar 2023
Viewed by 1313
Abstract
Potentiodynamic and potentiostatic polarization tests in the potential range between open circuit potential (OCP) − 0.1 V and OCP + 4 V were carried out in aluminate–phosphate electrolytes with an aluminate concentration of 0.2 mol/L and varying phosphates contents between 0 and 0.1 [...] Read more.
Potentiodynamic and potentiostatic polarization tests in the potential range between open circuit potential (OCP) − 0.1 V and OCP + 4 V were carried out in aluminate–phosphate electrolytes with an aluminate concentration of 0.2 mol/L and varying phosphates contents between 0 and 0.1 mol/L. The pH was adjusted between 11.5 and 12.0 due to phosphate and optional KOH addition. A high-strength, dual-phase steel, which is relevant for lightweight construction, served as the substrate material. The layer microstructure was investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy and Raman spectroscopy were used for element and phase analyses. We found that iron hydroxides or oxides are initially formed independently of the electrolyte composition at low potentials. At around 1 V vs. standard hydrogen electrode (SHE), the current density suddenly increases as a result of oxygen evolution, which causes a significant reduction in the pH value. Precipitation leads to the formation of porous layers with thicknesses of 10 µm to 20 µm. In the case of a pure aluminate solution, the layer mainly consists of amorphous alumina. When adding phosphate to the electrolyte, the layer additionally contains the hydrous phosphate evansite. At the highest phosphate content in the electrolyte, the highest P content and the most pronounced crack network were observed. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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12 pages, 2854 KiB  
Article
Chromium Luminescence in Plasma Electrolytic Oxidation Coatings on Aluminum Surface
by Aleksejs Zolotarjovs, Rudolfs Piksens, Krisjanis Smits, Virginija Vitola, Gatis Tunens, Ernests Einbergs, Arturs Zarins and Gunta Kizane
Coatings 2022, 12(11), 1733; https://doi.org/10.3390/coatings12111733 - 13 Nov 2022
Viewed by 1211
Abstract
With plasma electrolytic oxidation (PEO), one can easily obtain thick (tens of microns), mechanically resilient and chemically stable oxide coating on aluminum and other valve metal alloys. The study of luminescent PEO coatings is a relatively new subfield of the already well-established coating [...] Read more.
With plasma electrolytic oxidation (PEO), one can easily obtain thick (tens of microns), mechanically resilient and chemically stable oxide coating on aluminum and other valve metal alloys. The study of luminescent PEO coatings is a relatively new subfield of the already well-established coating preparation methods. In recent years, many new luminescence-based approaches have been developed, one of which is the detection of ionizing radiation of carbon-doped PEO alumina coating. This study presents an improved approach by doping the alumina coating with chromium using citric acid as an additive in the electrolyte. Trivalent chromium ions replacing aluminum in the crystalline lattice of the coating exhibit characteristic sharp lines in the luminescence spectrum. The effectiveness of different DC voltages, process times and citric acid concentrations in electrolyte were examined. The use of citric acid in the electrolyte also provides the conditions required for the formation of an energy trap in the bandgap of the material, thus opening up the possibility for the coating to be used as an ionizing radiation detector by measuring its thermoluminescence. Chromium atoms are incorporated in the coating from the Al6082 aluminum alloy itself and are not added in the electrolyte, therefore making the process much more reliable, repeatable, and environmentally friendly. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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16 pages, 18049 KiB  
Article
Dissolution Behavior of Different Alumina Phases within Plasma Electrolytic Oxidation Coatings
by Frank Simchen, Roy Morgenstern, Steffen Clauß, Thomas Mehner and Thomas Lampke
Coatings 2022, 12(8), 1205; https://doi.org/10.3390/coatings12081205 - 17 Aug 2022
Cited by 3 | Viewed by 1215
Abstract
The influence of chemical redissolution in the PEO layer-growth equilibrium on aluminum is evaluated differently in literature. In order to estimate whether and to what extent various alumina modifications could be affected by redissolution processes during PEO, immersion experiments were carried out on [...] Read more.
The influence of chemical redissolution in the PEO layer-growth equilibrium on aluminum is evaluated differently in literature. In order to estimate whether and to what extent various alumina modifications could be affected by redissolution processes during PEO, immersion experiments were carried out on PEO coatings in model electrolytes of different alkalinity and silicate concentration. Their composition was determined spectroscopically before and after the experiments. The layers were characterized by XRD before and after the tests and examined at affected and unaffected zones using SEM, EDX, and EBSD. The results show that chemical oxide dissolution can only be observed at the layer/substrate interface and that primarily amorphous alumina is affected. This process is intensified by higher alkalinity and inhibited by silicate additives. The crystalline Al2O3 modifications show no significant attack by the electrolytes used. Transferring these observations to plasma electrolytic oxidation, they allow the conclusion that the electrochemically active zone in the pore ground of discharge channels interacts with the electrolytic and electrical process parameters throughout the entire PEO procedure. Influences of bath alkalinity and silicate content on layer growth rates are to be understood as impact on the passivation processes at the layer/substrate interface rather than chemical redissolution. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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Review

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34 pages, 17972 KiB  
Review
Plasma Electrolytic Oxidation (PEO) as a Promising Technology for the Development of High-Performance Coatings on Cast Al-Si Alloys: A Review
by Patricia Fernández-López, Sofia A. Alves, Jose T. San-Jose, Eva Gutierrez-Berasategui and Raquel Bayón
Coatings 2024, 14(2), 217; https://doi.org/10.3390/coatings14020217 - 09 Feb 2024
Cited by 1 | Viewed by 1062
Abstract
Cast Al-Si alloys, recognized for their excellent mechanical properties, constitute one of the most widely employed non-ferrous substrates in several sectors, and are particularly relevant in the transport industry. Nevertheless, these alloys also display inherent limitations that significantly restrict their use in several [...] Read more.
Cast Al-Si alloys, recognized for their excellent mechanical properties, constitute one of the most widely employed non-ferrous substrates in several sectors, and are particularly relevant in the transport industry. Nevertheless, these alloys also display inherent limitations that significantly restrict their use in several applications. Among these limitations, their low hardness, low wear resistance, or limited anti-corrosion properties, which are often not enough when the component is subjected to more severe environments, are particularly relevant. In this context, surface modification and the development of coatings are essential for the application of cast Al-Si alloys. This review focuses on the development of coatings to overcome the complexities associated with improving the performance of cast Al-Si alloys. Against this background, plasma electrolytic oxidation (PEO), an advanced electrochemical treatment that has revolutionized the surface modification of several metallic alloys in recent years, emerges as a promising approach. Despite the growing recognition of PEO technology, the achievement of high-performance coatings on cast Al-Si is still a challenge nowadays, for which reason this review aims to provide an overview of the PEO treatment applied to these alloys. In particular, the impact of the electrolyte chemical composition on the properties of the coatings obtained on different alloys exposed to harsh environments has been analyzed and discussed. By addressing the existing gaps and challenges, this paper contributes to a better understanding of the intricacies associated with the development of robust PEO coatings on cast Al-Si alloys. Full article
(This article belongs to the Special Issue Plasma Electrolytic Oxidation (PEO) Coatings, 2nd Edition)
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