Reviews ChemElectroChem doi.org/10.1002/celc.202100216 Zinc Anodizing: Structural Diversity of Anodic Zinc Oxide Controlled by the Type of Electrolyte Katja Engelkemeier,*[a, c] Aijia Sun,[a, c] Dietrich Voswinkel,[b, c] Olexandr Grydin,[b, c] Mirko Schaper,[b, c] and Wolfgang Bremser[a, b] ChemElectroChem 2021, 8, 1–15 1 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH These are not the final page numbers! �� Wiley VCH Dienstag, 18.05.2021 2199 / 204431 [S. 1/15] 1 Reviews ChemElectroChem doi.org/10.1002/celc.202100216 Anodic zinc oxide (AZO) layers are attracting interdisciplinary The article gives an overview of the different possibilities of research interest. Chemists, physicists and materials scientists anodic treatment, whereby the voltage and the current type are are increasingly devoting attention to fundamental and the main distinguishing criteria. Presented is the electrolytic application-related research on these layers. Research work oxidation (anodizing) and the electrolytic plasma oxidation focuses on the application as semiconductor, corrosion protec- (EPO). The electrolytic etching is also a process of anodic tor, adhesion promoter, abrasion protector, or antibacterial treatment. However, it does not produce AZO layers, but rather surfaces. The structure and crystallinity essentially determine a degradation of the zinc layer. The review article shows the the properties of the AZO coatings. The type and concentration parameters used so far (electrolyte, current type, current of the electrolyte, the applied current density or voltage as well density, voltage) and points out the influence on the formation as the duration time enable layer structures of structural variety. of AZO structures in dependency to the used electrolyte. The European parliament voted in favor of a general ban on 1. Importance of Zinc Anodization for the use of the carcinogenic chrome (Cr) VI compounds.[12,13] Corrosion Protection and Further Applications Alternative surface treatment methods are required, thus anod- izing is becoming more attractive as a method of surface Zinc is essential as an protector layer primarily for steel treatment.[14] materials and is used for car bodies,[1] fence elements, or joining In the 1960s, the international lead zinc research organ- elements[2] like screws, nuts, and nails. However, zinc coatings ization inc. (ILZRO, USA) was engaged in zinc anodizing and tend to stain, tarnishes quickly and is susceptible to atmos- galvanized coatings. The treatment process was commercially pheric corrosion.[3] Under atmospheric conditions, zinc materials introduced by the Allied Kelite Product Division (AKPD) under exhibit a certain corrosion resistance in comparison to steel and the trade name “Iridizing”. Granted patents are those of A. G. form a smooth, compact, and weather-resistant protective layer. White[15] (1961) and M. M. Wright[16] (1967). They describe their The layer consists of zinc oxide (ZnO), zinc hydroxide, and process as “anodic treatment under alternating current (AC)” carbonate (Zn(CO3)·Zn (OH)2), known as white rust. However, instead of direct current. The specified parameters for the the native protective layer, less than 20 nm thickness, is not anodization treatment: Voltages between 50 V and 250 V, sufficient for long-term corrosion protection. Furthermore, the current densities between 37.6 mAcmÀ 2 (35 AftÀ 2) and natural oxide layer is not scratch-resistant.[4] 48.4 mAcmÀ 2 and temperatures between 65.6 °C and 82.2 °C. The international lead and zinc study group (ILZSG) has The process of anodic treatment takes place at high voltages statistically captured, the demand for zinc increased up to 4 and is a process of plasma electrolytic oxidation (PEO).[17] percent between 2013 and 2018.[5] Even the increase in global The process generates corrosion protective coatings with a zinc prices did not affect the demand. The worldwide export of thickness of up to 300 μm. However, commercial acceptance is galvanized steel doubled between 2001 and 2017.[6–8] marginal. Reasons are high costs associated with the process in Produced were up to 247,000 tons of zinc, primarily in terms of equipment, the energy-inefficient process design, and demand by automotive and construction companies.[9] Galvan- the high-risk potential due to excessive clamping voltages of ized steels are furthermore attractive components in hybrid the AC power supply. Furthermore, the application of the materials. However, the smooth zinc surface does not provide process recommends sodium hydroxide and potassium any possibilities for mechanical interlocking. Surface treatments hydroxide based electrolytes (pH 5 to pH 11.4), phosphate ions must be applied to adapt the zinc surface to the requirements and chromate ions (Cr(VI), Cr(III).[18] of the hybrid material.[10] It is known that specific structures of The zinc anodizing process MIL-A-81801[19] (1971), devel- AZO increase the adhesive strength between the steel and the oped by the ILZRO and financed by the US military, also works fiber composite component.[11] with voltages of up to 200 V. Layers of up to 80 μm in thickness can be realized through the addition of chromate and phosphate compounds. The AZO layers have a salt water [a] K. Engelkemeier, A. Sun, Prof. W. Bremser resistance of above thousand hours and have an increased Paderborn University [20–22] Department of Chemistry, ‘Coatings, Materials & Polymers’ abrasion resistance compared to the base metal. 33098 Paderborn, Germany Anodizing is an alternative option to increase the corrosion E-mail: [email protected] resistance and wear properties of zinc and galvanized layers, as it [b] D. Voswinkel, Dr. O. Grydin, Prof. M. Schaper, Prof. W. Bremser [18] Paderborn University can generate protective oxide layers. Researchers are increasingly Department of Chemistry, ‘Chair of Material Science’ turning to basic research on zinc anodizing, with a focus on the 33098 Paderborn, Germany semiconductor functionalities. Zinc oxide is a wide bandgap II–IV [c] K. Engelkemeier, A. Sun, D. Voswinkel, Dr. O. Grydin, Prof. M. Schaper Institute for Lightweight Design with Hybrid Systems (ILH) semiconductor material (3.37 eV at 300 K). It is used in application [23–25] [26,27] Paderborn University areas such as microelectronics, optoelectronics, sensor 33098 Paderborn, Germany technology[28–30] and photocatalysis.[31,32] Zinc oxide surfaces also © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This have antibacterial properties that make them interesting for is an open access article under the terms of the Creative Commons [33,34] Attribution License, which permits use, distribution and reproduction in any biotechnical and biomedical applications. medium, provided the original work is properly cited. ChemElectroChem 2021, 8, 1–15 www.chemelectrochem.org 2 © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH These are not the final page numbers! �� Wiley VCH Dienstag, 18.05.2021 2199 / 204431 [S. 2/15] 1 Reviews ChemElectroChem doi.org/10.1002/celc.202100216 In an earlier review article, Zaraska et al. discussed the oxidation (PEO),[15,45–48] and electrolytic etching.[49] The Electro- anodic formation of zinc oxide nanostructures.[35] They dis- lytic oxidation is known as anodizing and PEO is commonly cussed the formation of different structures and showed the referred to as micro-arc oxidation (DC-current) or iridizing (AC- impact of specific applied parameters and electrolytes. The current). In all cases, the workpiece forms the anode. Figure 1 amount of literature on anodizing has increased in recent years shows a short overview of the three methods. Influenced is the and allows correlations between the structure formation and anodic formation of zinc oxide by the type of method, type of the anodizing parameters depending on the electrolyte. This electrolyte, concentration and pH value of the electrolyte, the article focuses on the anodizing process in different electrolytes applied current density or potential, the type of current (direct and shows the relationship between the AZO formation in current or alternating current), the current mode (pulsed or various electrolytes depending on other anodizing parameters. constant), the duration time, additives and its topography. Anodizing of zinc products leads to further possibilities for application such as wear protection for tools,[36] sensor technology,[37] automotive and aircraft construction,[38–40] bio- 2.1. Electrolytic Oxidation (Anodizing) medical and medical technology.[41–43] The anodic treatment is typically applied below the dielectric breakdown. Figure 2 shows the oxidation process at the 2. Types of Zinc Anodizing interphase between the electrolyte and the zinc anode. The affinity of zinc to oxygen is exploited to increase the thickness Tree types of anodic treatment. The methods of anodic treat- of the oxide layer. The anodic formation of the oxide layer is ment include electrolytic oxidation,[44] plasma electrolytic less established for zinc than for aluminum. Zinc tends to have Katja Engelkemeier studied chemistry at the Dr. Olexandr Grydin gained a Ph.D. degree at Paderborn University from 2009 to 2014 and the National Metallurgical Academy of Ukraine is completing her Ph.D. this year at the Chair (Ukraine) in 2004. He habilitated to a Dr. tech. of Material Science (LWK) in the Faculty of sc. at the same university in 2014. Since 2014, Mechanical Engineering. Since 2019, she has he is working as a senior engineer at the been employed as a junior research group Department of Materials Science of the Pader- leader in the Faculty of Natural Sciences in the born University (Germany). He has expertise in working group Coatings, Materials and Poly- twin-roll casting of light metals, in particular mers at the Paderborn University and is high-strength aluminum alloys, and metallic actively involved as a specialist group repre- clads such as aluminum and steel, thermome- sentative at the Institute for Lightweight chanical treatment of metallic materials, sur- Construction with Hybrid Systems (ILH). She is face treatment of metals for advanced bond- currently focusing on the development of ing properties.