X‐Ray Photoelectron Spectroscopic Examinations of Electrochemically Formed Passive Layers on Ni‐Cr Alloys

Abstract

The structure and composition of passive films electrochemically formed on Ni-Cr alloys in 1 M NaOH and 0.5 M H7S04 has been studied with x-ray photoelectron spectroscopy. The passivation potential was varied within the passive range up to the transpassive range and the time of passivation from 10 ms to 1000 s. With angular-resolved x-ray photoelectron spectroscopy, the passive film was found to have a two-layer structure with an inner oxide part formed directly at the metal surface and an outer hydroxide part in contact with the electrolyte. The metal surface underneath is enriched in Ni compared to the metal bulk composition. Except for potentials approaching the transpassive range, the hydroxide layer forms the dominant part of the passive film. Above 0.3 V in NaOH and 1.2 V in H2S04, Cr(VI) ions are found within the hydroxide layer. The maximum thickness of the passive layer is about 6 nm in NaOH and 3.5 nm in H2S04. The growth and the pronounced changes of the composition of the passive layers reflect the electrochemical behavior of the pure metal components. The results are in agreement with the thermodynamic data.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 35.8.191.249 Downloaded on 2015-03-29 to IP

Utilization of Special Potential Scan Programs for Cyclic Voltammetric Development of Different Nickel Oxide-Hydroxide Species on Ni Based Electrodes

“…The presence of chromium in the electrode was only noticeable on the i/E st found by other authors in X-rays photoelectron spectroscopy (XPS) examinations of the electrochemically formed passive layers on Ni-Cr alloys [16]. They found that the XPS analysis of the passive layer formed at a potential range from -1.0 V to 0.0 V showed that Ni(II) and Cr(III) oxides and hydroxides were developed.…”

“…They suggested a two-layer model for the passive layers formed at Ni-Cr (80:20) alloy electrodes, an inner oxide layer in contact with the metal and an outer hydroxide layer in contact with the electrolyte. The XPS data showed that Ni(OH) 2 and Cr(OH) 3 are present in the outer hydroxide layer, Ni(III) and some soluble Cr(VI) are only observed in the transpassive potential range [16]. Only the redox couple Ni(II)/Ni(III) was observed on the i/E voltammetric trace at a potential range from 0.0 V to 0.6 V.…”

“…Jabs et al [16] reported that the formation of Ni(III) species at electrode potentials abo the negative going potential scan. This new wave was identified as due to the reduction of γ-NiOOH [18].…”

“…It is well known that the alloying of some metals can markedly lower their corrosion rate. As regards nickel, it has been suggested that its corrosion resistance is considerably improved by alloying it with chromium [14][15][16]. A two-layer structure, with an inner oxide part formed directly at the metal surface and an outer hydroxide part in contact with the electrolyte, was proposed for Ni-Cr alloys in 1 M NaOH solutions as deduced from X-Ray photoelectron spectroscopic examinations of the electrochemically formed passive layers.…”

“…Some soluble Cr(VI) was found within the hydroxide layer. However, due to the low solubility of Ni(OH) 2 in alkaline solutions, it has been suggested that Cr(VI) may be trapped within the hydroxide layer [16]. Above the passivation potential, Ni(III) species also accumulated within the passive layer, behaving as dopants and causing a shift of the Fermi level to lower energy.…”