Relation between microstructure and adhesion of hot dip galvanized zinc coatings on dual phase steel G.M. Song a,⇑ , T. Vystavel b , N. van der Pers a , J.Th.M. De Hosson b , W.G. Sloof a a Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands b Department of Applied Physics, Zernike Institute for Advanced Materials and Materials Innovation Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received 10 October 2011; received in revised form 11 January 2012; accepted 1 February 2012 Available online 6 March 2012 Abstract The microstructure of hot dip galvanized zinc coatings on dual phase steel was investigated by electron microscopy and the coating adhesion characterized by tensile testing. The zinc coating consists of a zinc layer and columnar f-FeZn 13 particles on top of a thin inhi- bition layer adjacent to the steel substrate. The inhibition layer is a thin compact and continuous layer that consists of g-Fe 2 Al 5–x Zn x fine and coarse particles. The coarse faceted particles are on top and fine faceted particles are at the bottom. The steel surface is covered with small fraction manganese oxides, which may impair adhesion of the zinc coating. The adhesion at various interfaces that exist in zinc- coated steel was quantitatively estimated using a so-called “macroscopic atom” model. In addition, the adhesion at the interfaces in zinc- coated steel was qualitatively assessed by examining the fracture and delamination behavior upon tensile testing. In accordance with this model, fracture along zinc grain boundaries preceded fracture along the zinc layer/inhibition layer and f-FeZn 13 particle/inhibition layer interfaces. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zinc coatings; Microstructure; Interface cracking; Work of adhesion; Coating adhesion 1. Introduction A higher demand for safety together with the urge to reduce weight to increase fuel efficiency are driving the rapid replacement of zinc-coated low carbon steel sheets in the automotive industry with zinc-coated high strength steel sheets [1–5]. As one kind of high strength steel dual phase (DP) steels have a combination of high tensile strength, a high work hardening rate in the early stages of plastic deformation and good ductility, conferred by a complex microstructure consisting of hard martensite islands embedded in a soft ferrite matrix [4,5]. Besides these features, properties such as continuous yielding behavior, uniform plastic deformation, good formability and high dynamic energy absorption during high strain rate defor- mation of DP steels enable their application to improve vehicle crash performance without an increase in weight [6]. However, the alloying elements in high strength steels, such as Mn, Si, P, Al in DP steels, tend to segregate to the steel surface upon annealing at a temperature in an inter- critical range between A C1 and A C3 and can be selectively oxidized prior to zinc deposition [7–12]. Thus the wettabil- ity of the surface areas covered by oxides becomes poor in a melted zinc bath, even leading to bare spot defects [11]. Consequently, adhesion of the zinc coating to the steel sub- strate as well as protection against corrosion is diminished [11–13]. This adhesion is crucial when zinc-coated high strength steel sheets must be deformed into the desired shape, e.g. for automotive applications [14,15]. Although various zinc coatings on steel and their deposi- tion routes have been extensively investigated, understand- ing the relationship between the coating/steel interface 1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2012.02.003 ⇑ Corresponding author. E-mail addresses: g.song@tudelft.nl, songguim@yahoo.com (G.M. Song). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 60 (2012) 2973–2981