Influence of solution pH and anion composition on the electrical resistance of films on nickel

Abstract

Many works have been devoted to investigation of the composition and thickness of anodic films on nickel. According to data obtained in situ* (in an electrolyte) by spectroscopic methods formation of the film occurs with potentials 150 and more millivolts more positive than the reversible potential of formation of Ni(OH) 2 [i, 2]. With lower overvoltages formation of Ni(OH) 2 phase films on the surface of nickel is not observed. Information on the change in properties of the film with an increase in potential in the passive area is contradictory. The majority of authors report a linear increase in film thickness without sharp changes with an increase in potential [3]. According to the other point of view the quantity of nickel oxide on the surface does not depend upon potential [4]. In accordance with the data of [2] the composition of the film changes with an increase in potential with NiO accumulating in a film consisting of Ni(OH) 2 while according to other information the film consists only of Ni(OH) 2 [5] or NiO [6] in the whole area of passivation. In a few works touching upon the question of the influence of electrolyte composition on film properties the identity of the film composition in the neutral and alkaline areas (pH 8.4, ii, 13) is noted and also the absence of a basic difference in the mechanism of film formation in sulfate and borate solutions despite the different thicknesses in these solutions [7]. The influence of the chloride ion on film thickness has been observed only in acid and neutral media but has not been observed in alkaline media [8]. In the majority of works [3, 5] the influence of anion composition of the electrolyte on the mechanism of film formation and composition is not taken into consideration at all.