Influence of different layer microstructures induced by different gas compositions on corrosion behavior of plasma nitrided stainless steel Saeid Amiri, Masoud Moradshahi ⁎ Laboratory of Applied Plasma, Centerof Nuclear Fusion Research, Atomic Energy Organization of Iran (AEOI), North Karegar Ave. Tehran, Iran Received 6 November 2006; accepted in revised form 1 February 2007 Available online 13 February 2007 Abstract AISI 316 austenitic stainless steels have been plasma nitrided using a dc glow discharge unit in order to investigate the influence of gas composition on microstructure and corrosion behavior of treated samples. Corrosion properties of untreated and plasma nitrided 316 steels have been evaluated using anodic polarization tests in 1 N H 2 SO 4 solution. Qualitative evaluation was carried out using surface analyses such as SEM, EDX, XRD and GDS before and after the corrosion tests. The results showed that the untreated sample suffered localized pitting corrosion under the testing conditions. Nitriding in different gas compositions resulted in different surface structures that affect the corrosion behavior of the modified layer. Increasing of H 2 to gas mixture results in improving nitriding efficiency but only in the case of sample nitrided at 723 K and N 2 /H 2 with ratio of 50/50 a precipitate-free single phase nitrided layer of nitrogen expanded austenite (γ N phase) was produced, which considerably improved the corrosion properties of the austenite stainless steel. Nitriding at these conditions due to formation of a surface layer without defects and higher thickness allows obtaining a significant improvement of corrosion resistance even at higher treatment time and temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma nitriding; Corrosion; Anodic polarization; Pitting; Expanded austenite 1. Introduction It is well known that the 300 series austenitic stainless steels (ss) are prone to pitting corrosion especially in the presence of halide ions and acidic environments [1,2]. This has limited their use in a wide range of engineering applications particularly in offshore installations, chemical tankers and chemical industries. In fact, corrosion depends strongly on the microstructure and composition of the material at the near-surface region. In this regard, surface modification by means of lasers [3], magnetron sputtering [4], low energy ion implantation [5] and plasma nitriding [6–9] has become a promising alternative for corrosion resistance improvement of stainless steels. Nitriding is one of the widely used surface engineering technologies to improve the surface hardness and wear resistance of various engineering materials such as low alloy steels and tool steels. In recent years, nitriding of austenitic ss using the low temperature nitriding technology has been exten- sively investigated and rapidly gained industrial applications [10]. It has been established that plasma nitriding techniques are effective in improving both surface hardness and corrosion resistance of austenitic ss, only when they are performed at temperatures lower than about 723 K [8,11–16]. In fact, by using such low temperatures the modified surface layer consists essentially of a metastable phase, known as supersaturated or expanded austenite γ N [9,11–14,17] or S-phase[6–8,16,18], which has proved to have high hardness and very good corrosion resistance. So, the main critical parameter in development of a precipitate-free s-phase is recognized to be the process temperature. Moreover, it has been observed that for a specific temperature there exists a critical time beyond which nitride formation occurs. For example for treatment tempera- tures of 703 K and 673 K, this time was longer than 30 h and 60 h respectively [19,20]. Nevertheless, nitrides were observed to form at the temperature of 683 K or lower for treatment times of a few hours [21–23]. These observations suggest that even if treatment temperature and time play a fundamental role in the nitride formation, the influence of the other conditions should Surface & Coatings Technology 201 (2007) 7375 – 7381 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +98 2188004015; fax: +98 2188023547. E-mail address: mmoradshahi@aeoi.org.ir (M. Moradshahi). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.02.006