Characterisation of the electrochemical behaviour of cerium implanted stainless steels C.M. Abreu, M.J. Cristo ´bal, X.R. No ´ voa *, G. Pena, M.C. Pe ´rez Universidade de Vigo, E.T.S.E.I., Lagoas-Marcosende, 9 E-36200 Vigo, Spain Received 22 June 2001; received in revised form 11 September 2001 Abstract Passive layers formed on a Ce implanted stainless steel in alkaline media are studied to clarify its beneficial effect on stainless steel corrosion resistance. An important decrease in iron and chromium activity peaks is detected by electrochemical techniques. X-ray photoelectron spectroscopy (XPS) analysis shows passive film thickness decreasing together with changes in chemical composition. Cerium implantation modifies the conductivity properties of the passive film, as it is inferred from EIS measurements. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Austenitic stainless steel; Ce-implantation; Cyclic voltammetry; XPS; EIS 1. Introduction The beneficial effect of Ce on the oxidation resistance of stainless steels and Cr alloys when exposed to high- temperature environments has been widely studied [1  / 4]. Several mechanisms have been proposed, but most of them attribute the decrease in oxidation rate to a change in the transport process for scale growth, due to the segregation of the Ce 3 and Ce 4 along the oxide grain boundaries [4]. Moreover, the addition of cerium either by immersion in boiling cerium nitrate solutions [5  /7] or by ion implantation [8] has been found to be an effective way of improving localised corrosion resistance of stainless steels. The reviewed literature denotes this effect as a result of cathodic reactions inhibition most likely due to a blocking effect of Ce compounds. In the chemical treatment with cerium nitrate, the precipitation of insoluble Ce(OH) 3 at cathodic sites takes place [5], while in the Ce-implanted steels this cathodic inhibition is attributed to the formation of a stable cerium oxide [8]. Some of these investigations pointed out a reduction in the passivity current during anodic polarisation for steels after Ce treatment [6,8], but the mechanism involved in this process is not clarified. The work reported in this paper gives an insight to the nature of the passive film generated by cyclic polarisa- tion on a Ce implanted stainless steel, in order to explain the electrochemical behaviour of the oxide layer. The obtained results are compared with unimplanted steel. 2. Experimental Specimens of about 1.1 /1.1 cm were cut from a 1.5 mm thick sheet of AISI 304L stainless steel, and polished to 1 mm diamond paste. The chemical composi- tion is given in Table 1. The steel samples were ion- implanted with Ce  at room temperature, using a 150 keV beam, to a dose of 1 /10 17 ions cm 2 . The cerium source was CeCl 3  /CeO 2 . The comparative study of unimplanted and Ce implanted stainless steel by cyclic voltammetry was performed in a conventional three electrode cell, where the working electrode was the selected steel (0.5 cm 2 exposed surface). A Pt mesh was used as large counter electrode and a Hg/HgO 0.1 M KOH as the reference one. The experiments were carried out at 30 8C in NaOH 0.1 M deaerated solutions (N 2 bubbling). * Corresponding author. Tel.: 34-986-812-213; fax: 34-986-812- 201. E-mail address: rnovoa@uvigo.es (X.R. No ´ voa). Electrochimica Acta 47 (2002) 2215  /2222 www.elsevier.com/locate/electacta 0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(02)00058-0