Transmission electron microscopy study of the passive films on nickel

Abstract

Nickel has a high resistance against certain forms of chemical attack by the formation of a corrosion resisting passive oxide film: The structure of this passive film has been the subject of controversy due to the thinness of the film [1]. Previous studies of the passive films formed on nickel in borate buffer solution (pH 8.4) were carried out by the author using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) [2]. The results showed that the passive film formed at low anodic potentials consisted of NiO and Ni(OH)2. The transmission electron microscopy plays an important role in the understanding of the nature of the passive films. Foley etal. [3] have used the this technique to determine the structure of the passive films on iron. This letter presents some initial observations on the structure of the passive film on nickel by using the electron diffraction method. High purity polycrystalline nickel plates obtained from Inco were cold-rolled to foils with thickness of approximately 0.15/am. The foils were annealed at 850 ° C for 3 h in a vacuum-sealed quartz tube and then subsequently cooled in air. Disc specimens of 3 mm in diameter were punched out from these annealed foils. The foils were finally thinned by jet polishing until perforation occurred, followed by a rinse in reagent-grade methanol and distilled water. The passivation treatments were conducted in borate buffer solution prepared from an equal volume mixture of 0.15 M H3BO3 and 0.0375 M Na2B407 (pH 8.4). The electrochemical cell contained about 20ml of this solution and was deaerated with argon before and during the experiment. The reference electrode was an ~-Pd/H bead and platinum counter electrode. All potentials quoted in this letter are referred to the reversible hydrogen electrode (RHE). The electrode potential was controlled by a PAR model 173 potentiostat combined with a PAR model 175 universal programmer. The disc specimen was introduced into the electrochemical cell and held at a cathodic potential of -0 .180V for 10rain to remove any airformed oxide films. After the cathodic reduction, the electrode potential was stepped to the desired value for 2 h to form the passive film. When the passivation treatment was completed, the disc specimen was removed from the cell, rinsed with distilled water and transferred immediately to JEOL JEM-200CX scanning transmission electron microscope. Electron diffraction was carried out at an operating voltage of 200 kV. Fig. 1 shows the electron diffraction pattern of the passive film formed on nickel at 0.65 V. One can see that the diffraction pattern exhibits the diffuse halo structure. This indicates that the passive film formed on nickel at this potential has an amorphous structure. This result is in agreement with the previous studies which indicate that the passive film formed at 0.65 V is predominantly hydroxyl [2]. It is well recognized that hydrogen in the passive film can lead to the formation of