The entry of chloride ions into passive films on nickel studied by spectroscopic (ESCA) and nuclear ( 36Cl radiotracer) methods

Abstract

The incorporation of chloride ions in passive films formed on nickel in Cl −-containing acid solution and the entry of Cl − into passive films formed in Cl −-free solution and subsequently exposed to Cl − have been investigated quantitatively by electrochemical measurements, ESCA (Electron Spectroscopy for Chemical Analysis) and in situ radiotracer ( 36Cl) methods. At potentials in the active region, chloride ions are adsorbed on the nickel surface and form a thin hydroxy-chloride layer. The amount of Cl − increases with potential (beyond the equivalent full monolayer coverage) up to the active/passive potential where a marked drop of the Cl − surface concentration is observed. In the passive region, chloride ions are incorporated in the pass7ive film. When the film is formed in Cl −-containing solution at potentials below the pitting potential, Cl − ions are found in the outer hydroxide layer and at the metal/oxide interface. When the film is formed without Cl − in the solution, and then exposed to Cl −, chloride ions are found in the outer hydroxide layer with the same concentration as the one found after passivation in Cl −-containing solution, whereas no significant amount of Cl − is found in the inner oxide. The concentration of Cl − in the hydroxide layer is governed by a chemical reaction. It increases with increasing Cl − concentration in the electrolyte but does not vary with applied potential. In contrast with the equilibrium Cl − concentration found in the hydroxide layer, the concentration of Cl − in the oxide near the oxide/metal interface is not in equilibrium. An irreversible trapping of Cl − at the metal/oxide interface is caused by the growth of the passive film on a Cl −-covered surface. The concentrations of Cl − in the passive film have been measured. The Cl − concentration in the hydroxide layer is ∼ 10 × 10 −3 mol cm −3 (i.e. Cl −/(Cl − + OH −) ∼ 10%) for Cl − = 5 × 10 −3 M in the solution. At potentials above the pitting potential, measurements have been performed during the incubation period in order to investigate the mechanisms of pit initiation. The hydroxide layer acts as a reservoir for Cl − which enter the passive film. A critical concentration of Cl − in the oxide layer of ∼ 10 × 10 −3 mol cm −3 has been measured in the stage immediately preceding pitting. A similar critical concentration in the film has been measured for different Cl − concentrations in the solution. The passive film was found to have a bilayer structure both in the absence and in the presence of Cl − in the stage preceding pitting. The passive film does not become thinner in the presence of Cl −; on the contrary a growth of the film is observed when Cl − is added to the electrolyte. Cl − must be adsorbed in order to penetrate into the film but the surface coverage of adsorbed Cl − always remains very low, as Cl − starts to enter the passive film at low surface coverage. The view of pit initiation emerging from the above results is the following. Marked modifications of the passive film are caused by the presence of Cl −. Cl − is incorporated or enters the hydroxide layer by a chemical reaction probably involving place exchange of OH − and Cl −. Above a certain electrochemical potential, Cl − enters the inner oxide layer. A critical concentration of Cl − in the film is reached, of the order of 10 × 10 −3 mol cm −3 for nickel. Local differences around this average value are likely to exist, and a local over-concentration may cause the onset of pitting. It is suggested that the establishment of the observed critical concentration of Cl − in the film (and the presence of associated vacancies) is a prerequisite for the existence of local over-concentrations which cause localized breakdown of the film.