Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium Yunping Li a,⇑ , Ning Tang a , Phacharaphon Tunthawiroon b , Yuichiro Koizumi a , Akihiko Chiba a a Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan b Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japan article info Article history: Received 18 October 2012 Accepted 29 March 2013 Available online 8 April 2013 Keywords: A. Cobalt B. SEM B. XPS C. Interfaces C. Oxide coatings abstract Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the cor- rosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr 2 O 3 -rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr 2 O 3 , result- ing in a single-layered oxide film dominantly composed of Cr 2 O 3 . The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. 1. Introduction Co-based alloys show superior mechanical properties at ele- vated temperatures [1–4] and high corrosion resistance to liquid Al [5–6]. Hence, they are expected to serve as better candidates than the currently used tool steels in Al die-casting moulds. There- fore, the study of the solid–liquid interfacial reactions between Co- based alloys and liquid Al is of considerable importance. Recent studies by Tang et al. [7,8] indicated that despite its relatively high corrosion resistance to liquid Al, Co–29Cr–6Mo (Co–Cr–Mo) alloy is gradually corroded by liquid Al, resulting in the formation of two intermetallic compound layers between the matrix and liquid Al. To enhance the corrosion resistance of the Co–Cr–Mo alloy to liquid Al, economical and simple methods such as gas nitridation [9] or oxidation [7] on the sample surface have been proposed. Resistance to corrosion of liquid Al can be accomplished by creat- ing a compact and inert film barrier between the sample and liquid Al. Co–Cr–Mo alloys subjected to oxidation by exposure to air at 750 °C for 24 h exhibited substantial improvement in the corrosion resistance to liquid Al. Tang et al. [7] ascribed this improvement to the homogenous coverage of the sample surface by a dense and compact oxide film (approximately 500 nm in thickness) that effectively protected the alloy from coming into direct contact with liquid Al for approximately 2 h. X-ray diffraction (XRD) analysis indicated that the oxide film was possibly composed of (Cr, M) 2 O 3 . However, further research is yet to be carried out to characterise the oxide film in detail. The oxidation behaviour of Co–Cr-based alloys at high temper- atures has been extensively studied for several decades for the application of Co–Cr-based alloys as heat resistant materials [10– 19]. Kofstad and Hed analysed the oxidation behaviour of Co– 10 wt% Cr alloys at 800–1300 °C [10,11] and observed the duplex oxide scales consisting of an outer CoO layer and an inner layer of CoO containing inclusions of Cr 2 O 3 and a Co–Cr spinel (CoCr 2 O 4 ). When the Cr content increased (about 30 wt%), a single layer of Cr 2 O 3 was observed on the flat surface of sample at lower partial pressure of oxygen [12]. The mechanism for formation of the monophase Cr 2 O 3 layer was ascribed to the selective oxidation of Cr under low partial pressure of oxygen [12]. The oxide scales formed on the Co–Cr alloys at temperatures ranging from 800 to 1300 °C have been proven to have a protective effect on the matrix by inhibiting the further contact of the matrix with oxygen. How- ever, these scales are very brittle owing to their large thicknesses that range from a few micrometers to several 100 lm [10–19]. Consequently, these scales invariably spall off when the samples are cooled to room temperature driven by the thermal stress initi- ated between the matrix and the oxide scales [10–12]. Tang et al. [7] showed that the oxide films formed at lower temperatures (i.e., below 750 °C) were effective in protecting the alloy from reac- tion with molten Al, implying that low temperature favoured the formation of compact and dense oxide scales. Al die casting is generally carried out at temperatures slightly higher than the melting point of Al. When Co–Cr–Mo alloys are used in Al die-casting moulds, the oxide films formed on the alloy 0010-938X/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.03.026 ⇑ Corresponding author. Tel.: +81 22 215 2452; fax: +81 22 215 2116. E-mail address: lyping@imr.tohoku.ac.jp (Y. Li). Corrosion Science 73 (2013) 72–79 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci