Influence of second phases on the electrochemical behavior of hot dipped Al–Mg–Si coated steel Chaiyaput Kruehong a , Gamal A. EL-Mahdy a,b, * , Atsushi Nishikata a , Tooru Tsuru a a Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, S8-7, O-okayama, Meguro-ku, Tokyo 152-8552, Japan b Chemistry Department, Faculty of Science, Helwn University, Helwn, Egypt article info Article history: Received 2 December 2009 Accepted 12 April 2010 Available online 18 April 2010 Keywords: A. Aluminum A. Magnesium B. EPMA B. SEM B. Polarization C. Passivity abstract The influence of second phases on the electrochemical behavior of hot dipped Al–Mg–Si coated steel has been investigated using an open circuit potential (OCP) and anodic polarization measurements. The results of OCP monitoring data for Mg 2 Si particles show fast ennoblement of OCP then slowly rise until the last stage of monitoring. The localized corrosion starts from the surrounding matrix of Mg 2 Si particles and propagates in the coating surface. The passivation of the Al–Mg–Si coating surface during anodic polarization can be attributed to the formation of the surface oxide film, which inhibits further dissolu- tion of the coating. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Aluminum alloys have been used as candidate for automotive and aerospace applications due to supreme mechanical properties [1,2]. Many researchers investigated the corrosion performance of Al alloys and reported that the corrosion behavior of Al alloys is associated with inclusion and second phase particles [3–9]. Al–Fe intermetallic compound (Al–Fe IMC) and Mg 2 Si phases are the main precipitated second phases in the hot dipped Al–Mg–Si coated steel. It has been reported that Al–Fe IMC is more active for cathodic reaction than the surrounding of Al matrix and in- duces localized corrosion to the adjacent Al [10]. However, Mg 2 Si particles are found to be more active for anodic reaction than the matrix [11–13]. Eckermann et al. [11] examined the cathodic abil- ity of the second phase particles (Al–Fe IMC and Mg 2 Si particles) and reported that passive film formation on the second phase particles has a minor role on the initial electrochemical dissolu- tion. They reported that Mg 2 Si particles are not considered as the corrosion sites for the Al alloys. Wloka et al. [13] recently re- ported that Al–Fe IMC compounds can accelerate the dissolution rate of Mg 2 Si particle and Al matrix. Yasakau et al. [12] examined the localized area of the intermetallic phases using Kelvin probe force microscopy, in situ atomic force microscopy, scanning elec- tron microscopy coupled with energy dispersive spectroscopy. The study revealed that the corrosion product of Mg 2 Si particles acted as an additional propagation barrier hindering of pits formation. The electrochemical behavior of the gradient polished surface of the hot dipped Al alloy coated steel has been studied [14,15]. The influence of the second phases on the electrochemical behavior of hot dipped Al–Mg–Si coating is still unclear. The aim of this investigation is to apply a new method for coating surface polishing and to study the influence of second phases on the elec- trochemical behavior of the hot dipped Al–Mg–Si coated steel using an open circuit potential (OCP) and anodic polarization measurements. 2. Experimental 2.1. Materials and surface coating polishing A sample of 100 lm thick hot dipped Al–Mg–Si (Al + 8.2 wt% Mg + 4.8 wt% Si) coated steel was used throughout the whole experiments. The outer surface of the sample was carefully pol- ished with a very small angle (h) lower than 0.6° to expand the width of the exposed coating surface as shown in Fig. 1a. This can be done by partially covered the surface with a very thin tape to avoid the entire surface from polishing as shown in Fig. 1b. This step was successively repeated for different parts of the surface in order to polish the surface with a very small angle. 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.04.013 * Corresponding author at: Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, S8-7, O-okayama, Meguro-ku, Tokyo 152-8552, Japan. Tel.: +81 3 5734 3146; fax: +81 3 5734 3135. E-mail address: gamalmah2000@yahoo.com (G.A. EL-Mahdy). Corrosion Science 52 (2010) 2379–2386 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci