Corrosion behaviour of Al/Al 3 Ti and Al/Al 3 Zr functionally graded materials produced by centrifugal solid-particle method: Influence of the intermetallics volume fraction S.C. Ferreira a , L.A. Rocha a,⇑ , E. Ariza a , P.D. Sequeira a , Yoshimi Watanabe b , J.C.S. Fernandes c a Centre for Mechanical and Materials Technologies, Department of Mechanical Engineering, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal b Department of Engineering Physics, Electronics and Mechanics, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-Chu, Showa-Ku, 466-8555 Nagoya, Japan c ICEMS/DEQB, Instituto Superior Técnico, TULisbon, 1049-001 Lisboa, Portugal article info Article history: Received 29 March 2010 Accepted 7 February 2011 Available online 12 February 2011 Keywords: A. Aluminium A. Intermetallics B. EIS B. SEM C. Pitting corrosion abstract Intermetallic particles, Al 3 Ti and Al 3 Zr were formed in Al–5mass%Ti and Al–5mass%Zr alloys, respectively, by centrifugal casting, in order to create functionally graded materials (FGMs). At present, no information is available on the influence of the amount of intermetallics on the electrochemical properties of these alloys. In this paper, the corrosion resistance of Al/Al 3 Ti and Al/Al 3 Zr FGMs was investigated by open-circuit measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Results sug- gests that the corrosion resistance of the FGMs is affected by galvanic effects between the intermetallic particles and the metallic matrix. Lower centrifugal forces resulted in an improvement of the electro- chemical properties. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The corrosion behaviour of aluminium alloys has been the subject of a large number of publications, since the naturally formed oxide film can give excellent protection against corrosion, in addition to the advantages resulting from aluminium’s low den- sity [1]. Many commercial aluminium alloys are developed with a wide range of mechanical properties stimulated by careful alloying additions and heat-treatments [2]. According to the Al–Ti and Al–Zr phase diagrams, the intermetallic compounds of Al 3 Ti and Al 3 Zr are formed through crystallization as primary crystals [3]. Both compounds have similar crystal structures i.e., D0 22 in Al 3 Ti and D0 23 in Al 3 Zr with the same tetragonal space samples of I4/mmm and show high melting temperature, low density, good oxidation resistance and good thermal stability [4]. The corrosion resistance of the aluminium alloys is usually dependent on the metal heterogeneities and/or on the medium or exposure conditions. Heterogeneities in the microstructure can cause formation of cathodic and anodic zones that promote differ- ent forms of localised corrosion [2,5–8]. The production of local galvanic cells leads to the dissolution of the less noble areas [9]. In the case of the aluminium alloys, the heterogeneities are frequently intermetallic compounds distributed throughout the metallic matrix that can be either anodic or cathodic relatively to the matrix. In the case of the Al 3 Ti and Al 3 Zr intermetallic compounds, they show a more cathodic potential in comparison with the pure aluminium (pure Al [99,99] = 849 mV SCE ; Al 3 - Ti = 799 mV SCE and Al 3 Zr = 801 mV SCE in 0.6 M NaCl solution) [2]. Consequently, depending on the environmental conditions, they are able of inducing localised corrosion in the metallic matrix surrounding the intermetallic compounds. Additionally, the pres- ence of intermetallic compounds in the aluminium matrix can dif- ficult the growth or promote failures of the protective oxide layer formed in air or in aqueous solutions on alloys that are likely to be passivated [2,5]. However the influence of the volume fraction of intermetallic compounds on the corrosion behaviour of these materials is still unknown. Functionally graded materials (FGMs) are considered as engi- neered materials systems produced to achieve optimum perfor- mance in an intended application. FGMs are produced using techniques that introduce spatial differential distribution of the reinforcing along the component. As end result a gradient of mechanical properties from the surface up to the bulk of the com- ponents is obtained. The Al/ceramics and Al/intermetallic compound particles FGMs can be fabricated by a centrifugal method [10–19]. The composi- tion gradient is achieved primarily by the difference in the centrif- ugal force produced by the difference in density between the molten metal and particles. The fabrication of the intermetallic compounds dispersed FGMs made by the centrifugal method can be classified into two categories based on the liquidus temperature 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.02.010 ⇑ Corresponding author. Tel.: +351 253 510231; fax: +351 253 516007. E-mail address: lrocha@dem.uminho.pt (L.A. Rocha). Corrosion Science 53 (2011) 2058–2065 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci