Segregation and adsorption of sulphur on Ni 50Fe 50(100) alloy: Studies of the surface composition and structure and of the effects on the passivation

Abstract

The surface composition of Ni 50Fe 50(100) alloy was studied and the segregation and adsorption of sulphur were investigated by AES, LEED and radiotracer ( 35S) techniques. Ion etching produces a surface composition identical to the matrix composition (Ni:Fe=1:1). In the temperature range 300–600°C iron segregates on the surface and nitrogen was detected. Heating to higher temperatures (>600°C) also causes the segregation of iron as well as the segregation of sulphur. The durface reaches a stable composition that does not depend on further changes of the temperature in the range 25–800°C. It consists of an almost complete monolayer of iron segregated on the alloy. The segregation of sulphur leads to the formation of a c(2×2) structure. A sulphur coverage of 45 ng cm -2, consistent with the c(2×2) structure, was measured by the radiotracer method after chemisorption in gaseous H 2S/H 2 mixtures at 550° and 200 Torr. This sulphur monolayer is stable in a range of p H 2S / p H 2 =7.4 × 10 -5-6×10 -4. Above this pressure, a preferential sulphidation of iron is observed. The effects of sulphur on the anodic dissolution and passivation of the alloy in acid solution were studied. Adsorbed sulphur promotes the dissolution and delays the passivation. When the alloy is doped with sulphur, bulk sulphur accumulates on the surface during the dissolution of Ni and Fe. This anodic segregation leads to the formation of an adsorbed layer of sulphur, followed by the growth of a sulphide which blocks the formation of the protective oxide film.