XPS and UPS examinations of the formation of passive layers on Ni in 1 M sodium hydroxide and 0.5 M sulphuric acid

Abstract

The passivation of nickel has been studied in 1 M NaOH and 0.5 M H₂SO₄ by XPS. An electrochemical specimen preparation and transfer to an ultra‐high vacuum, without contact with oxygen, permits a detailed examination of the oxide growth with potential and time resolution. Quantitative evaluation of the data yields values for the thickness of the partial layers of the duplex film structure. The Ni/NiO/Ni(OH)₂ sequence for the passive range and Ni/NiO/NiOOH sequence for the transpassive range in alkaline solutions have been confirmed by a detailed peak analysis, including angular‐dependent XPS measurements. These results are closely related to the polarization curves. An interpretation is given on the basis of a simple semiconductor model involving the results of UPS and work function measurements. The data of the acidic and the alkaline electrolytes are related to each other and their differences are explained by the increased solubility for low pH of the outer Ni(OH)₂ and NiOOH layers, respectively. These properties cause thinner films in acidic electrolytes and prevent the formation of a stable NiOOH overlayer for these conditions.