Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels Kelly Alvarez a , Soong-Keun Hyun a , Hiroaki Tsuchiya b , Shinji Fujimoto b , Hideo Nakajima a, * a The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan b Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan Received 2 January 2007; accepted 6 June 2007 Available online 13 July 2007 Abstract Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pit- ting potential, E b , also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nickel-free stainless steels; A. Lotus-type porous metals; B. Potentiodynamic polarisation; C. Pitting corrosion; C. High temperature nitriding 1. Introduction A large body of continually growing literatures exists on the fabrication methods and properties of high nitrogen nickel-free stainless steels. The austenitic stabiliser property of nitrogen allows the nickel content in steel to be reduced practically to zero, offering additional advantages such as lower production costs, higher strength, higher corrosion resistance and also biocompatibility. The first attempts to produce low nickel content steels was made possible by the addition of manganese (20 wt.%), which enhances the stability of austenite indi- rectly because it raises the solubility of nitrogen quite in contrast to nickel [1]. These steels were produced with pres- surised induction furnace melting [2], pressurised plasma furnace melting [3] or pressurised electroslag remelting (PESR) methods [2]. High manganese Ni-free austenitic stainless steels were not completely satisfactory because manganese has been pointed out as an element that pro- duces a detrimental influence on localised corrosion resis- tance in chloride containing environments [4]. Commercial production of high nitrogen steels is cur- rently accomplished mainly using PESR method. High nitrogen-bearing steel of more than 1 wt.% N has been suc- cessfully fabricated without the addition of manganese using FeCrN powders as a nitrogen source in a furnace capable of producing 20 kg ingots under a maximum N 2 gas pressure of 6 MPa [5]. However, PESR is a very expen- sive fabrication method and requires nitrogen gas pressures above the melt of approximately 3 MPa to obtain 1.0 wt.% N uniformly distributed in the ingot in both axial and radial directions [5]. In austenitic stainless steels, the total solubility of nitro- gen in the solid-state is actually greater than in the liquid [6]. Therefore, solid-state processing is also an attractive way to produce small components made of high nitrogen stainless steels. 0010-938X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.06.004 * Corresponding author. Tel.: +81 6 6879 8436; fax: +81 6 6879 8439. E-mail address: nakajima@sanken.osaka-u.ac.jp (H. Nakajima). www.elsevier.com/locate/corsci Available online at www.sciencedirect.com Corrosion Science 50 (2008) 183–193