Growth mechanism of industrial Ti/Zr no-rinse sprayed conversion coating applied on automotive aluminium coils: preferential deposition on the intermetallic phases

Growth mechanism of industrial Ti/Zr no-rinse sprayed conversion coating applied on automotive aluminium coils: preferential deposition on the intermetallic phases P. Bouckenoogea, O. Bauerb, K. Eckhardb, H.-J. Brinkmanb, H. Terryna aResearch group of Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium bHydro Aluminium Rolled Products GmbH, Research & Development Bonn, Georg-von-Boeselager-Str. 21, 53117 Bonn, Germany The use of aluminium in the automotive industry keeps growing. Due to outstanding density and formability characteristic, car-body parts are manufactured from aluminium coil sheets. Prior to the sheets processing by the automotive industry, the aluminium coils are conventionally coated with a conversion coating by the aluminium producer. The purpose of this coating is two folded. First, conversion coating is applied to provide enhanced performance of the coil material in adhesive bonding. Secondly, the conversion coating forms a first corrosion protection layer on top of the bare aluminium surface. Due to the increasing importance of adhesive bonding in car-body manufacturing, an in-depth understanding of the growth mechanism of conversion coating is required. Presently, commercially available Ti/Zr-based conversion coatings are applied on the automotive aluminium coils. Several techniques can be used to apply this Ti/Zr-based coating, divided in rinse (dipping) and no-rinse (spraying and roller coater) applications. In industry the no-rinse applications are the preferred coating deposition techniques, due to their short deposition time (seconds) compared to the long immersion time (minutes) in the dip and rinse application. However, published research was only focused on dip applications. This literature revealed the preferential deposition of the coating elements on top of cathodic intermetallic particles of the substrate surface. 1-3 The present study investigates the relevant mechanisms of (ultra-)thin-film growth of a Ti/Zr conversion coatings from a spray-type no-rinse application. All measurements were performed on mill-finished automotive aluminium alloys, coated with a Ti/Zr-based conversion coating applied by an industrial spraying process. In order to analyse the (typically) 10-20 nm thick Ti/Zr conversion coating, FE-AES (Field Emission – Auger Electron Spectroscopy) and FE-SEM-EDS (Field Emission – Scanning Electron Microscope- Energy Dispersive Spectroscopy) techniques were used. The nanometre lateral resolution and the nanometre vs. micrometre depth resolution of the complementary techniques made it possible to investigate the coating distribution near the intermetallic particles on a sub-micrometre scale. Point measurements and line profiles were obtained with FE-AES and FE-SEM-EDS. Both techniques revealed an enriched concentration of the conversion coating elements above and around the cathodic intermetallic particles. FE-AES depth profiles showed that these enrichments, observed from the point measurements and line profiles, are the result of a physical thicker conversion coating at the second phase particles. From FE-SEM-EDS mappings it could be derived that the mentioned increase in coating thickness is not completely homogeneous around and on top of the intermetallic particles. Meaning local increases and decreases within the enriched zones could be observed. The experimental results reveal the preferential deposition of the conversion coating at the cathodic second phase particles. Hence, we conclude that, on a sub-micrometre scale, similar mechanisms are relevant during film growth for both spray/no-rinse and dip/rinse techniques even on an industrial scale, despite their distinctive contrasting production techniques. 1. Lunder, O. et al. Surf. Coatings Technol. 184, 278–290 (2004). 2. Andreatta, F. et al. Surf. Coatings Technol. 201, 7668–7685 (2007). 3. Cerezo, J. et al. Appl. Surf. Sci. 366, 339–347 (2016).