Influence of a chromium ion implantation on the passive behaviour of nickel in artificial sea-water: An EIS and XPS study

The passive behaviour in artificial sea-water of Cr-implanted (4 x 10(16) ions/cm(2), 60 keV) and non-implanted Ni was studied at room temperature by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (SPS). The transfer resistance obtained for the implanted specimen (R-t similar to 5586 k Omega cm(2)) is extremely enhanced compared to the corresponding value of the non-implanted Ni (R-t similar to 94 k Omega cm(2)) indicating a considerable increase in the protective efficiency of the implanted layer. The EIS spectra of ion-implanted Ni exhibit one capacitance loop while in the non-implanted Ni two distinct loops can be observed. These changes in EIS behaviour by the ion-implantation are related to the increase of the superficial layer density resulting in a more stable passive layer. Equivalent circuits were proposed to fit the impedance spectra and corresponding electrochemical parameters deduced. These findings were compared with the results obtained by analysis of the passive film formed on Ni, performed by XPS, and with previous investigations of similar alloys. It appears that the passive film formed on Ni consists of NiO and Ni(OH)(2) whereas the chromium implantation leads to the formation in the passive state of Cr oxides and hydroxides which are responsible of the large increase of the transfer resistance. (C) 2001 Elsevier Science B.V. All rights reserved.