Citation: Herbáth, B.; Kovács, K.; Jakab, M.; Makó, É. Crystal Structure and Properties of Zinc Phosphate Layers on Aluminum and Steel Alloy Surfaces. Crystals 2023, 13, 369. https://doi.org/10.3390/ cryst13030369 Academic Editors: Dah-Shyang Tsai and Christiane Scharf Received: 30 January 2023 Revised: 14 February 2023 Accepted: 19 February 2023 Published: 21 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). crystals Article Crystal Structure and Properties of Zinc Phosphate Layers on Aluminum and Steel Alloy Surfaces Beáta Herbáth 1,2, * , Kristóf Kovács 2 , Miklós Jakab 2 and Éva Makó 2 1 BPW-Hungária Ltd., 9700 Szombathely, Hungary 2 Department of Materials Engineering, Faculty of Engineering, University of Pannonia, 8201 Veszprém, Hungary * Correspondence: herbathbea@gmail.com Abstract: Many studies have been carried out on the phosphating of steel and aluminum alloys used in automotive engineering, but characterization of the properties of the phosphate layers formed by the co-phosphating of these alloys in the presence of different base metals is still lacking. In this study, the crystal structure and properties of the phosphate conversion layers formed on the surface of the aluminum alloys important in vehicle manufacturing (cast and forged AlSi1MgMn, and AA6014 panel) and the CRS SAE 1008/1010 reference steel plate by co-deposition prior to painting were investigated. On a process line set up for the phosphating of typical iron and steel alloys, the phosphate coating was formed using nitrite and nitroguanidine accelerators under identical technological parameters. The microstructure of the formed phosphate layers was examined using scanning electron microscopy (SEM), its phase composition using X-ray diffraction (XRD), and its elemental composition using energy-dispersive X-ray analysis (EDX). The suggested main crystalline phase (Zn 2.3 (Ni 0.1 Mn 0.6 )(PO 4 ) 2 ·4H 2 O) in the surface phosphate layer of both aluminum alloys studied was similar to hopeite, whereas in the steel plate, a minor hopeite phase were identified in addition to the main crystalline phosphophyllite phase (~95%). It can be concluded that, during the combined phosphating treatments, the surfaces of different aluminum and steel alloys behaved similarly to the individual treatments and did not impede the coating reactions of the other metal. To obtain an adequate coating of aluminum and steel alloys, fluoride should always be present in the production line. Comparing the effects of accelerators, we found that the use of nitrite accelerator with the same amount of fluoride resulted in a higher coverage and better quality of the surface protective layer of the aluminum alloys. However, for the steel plate, there was no significant difference between the phosphate coatings prepared with the two different accelerators. Keywords: zinc phosphate; phosphating; aluminum alloy; steel alloy; accelerator 1. Introduction In the automotive industry, particularly in agricultural and road vehicle manufactur- ing, corrosion-resistant multi-layer coatings that encounter metal usually have a phosphat- ing layer at the bottom layer. This adheres tightly to the underlying metal, protecting the base metal from corrosion and improving the correct adhesion of the paints and organic finishes to be applied subsequently [1,2]. For vehicle manufacturing applications, this layer is usually zinc phosphate as it provides the best corrosion protection in outdoor and extreme (salty, wet, marine) conditions [2–5]. Complex agricultural vehicle bodies usually contain a phosphate layer with a varying composition and microstructure on the surface of the raw materials, depending on their physical and chemical properties. This affects the corrosion protection properties of the final coating [6]. Vehicle manufacturers assemble parts of complex design from various base metals, such as aluminum and steel alloys, that undergo different machining stages (weld- ing, machining, sheet metalworking, forging, etc.) and mechanical pre-treatments [1,3]. Crystals 2023, 13, 369. https://doi.org/10.3390/cryst13030369 https://www.mdpi.com/journal/crystals