High temperature sulphide corrosion of manganese and MnNb Alloys

Abstract

Rapid degradation of high temperature alloys in sulphur-containing atmosphere is mainly the result of the poor protective properties of sulphide scales. Manganese and refractory metals are therefore particularly interesting as alloying additions because of their exceptional resistance to sulphide corrosion. To estimate the possible role of these elements in the development of new materials resistant to hot corrosion, a better understanding of their sulphidation mechanism is urgently needed. The present paper is an attempt to fill this gap by systematic studies of the kinetics and mechanism of sulphidation of manganese, niobium and MnNb alloys as a function of temperature (673–1373 K) and sulphur vapour pressure ((10−8) × 10 4Pa)). The sulphidation rate has been studied thermogravimetrically with an accuracy of 10 −6 g and the mechanism of scale growth by marker and radiotracer techniques using the 35S isotope. The morphology of the scale and its grain size as a function of reaction time, temperature and sulphur pressure have been studied by scanning electron microscopy. It has been found that the rate-determining step of α-MnS scale growth on manganese is the outward diffusion of the metal. In the high temperature range (above 1000 K) a coarse-grained scale is formed, and manganese diffuses in the form of Mn 2+ cations and electrons via doubly ionized cation vacancies and holes (volume diffusion). In the low temperature range (below 800 K), however, a fine-grained scale is formed, and manganese diffuses mainly through grain boundaries, but also in the ionized form. In the intermediate temperature range (800–1000 K) the mechanism of scale growth depends on the reaction time. In the early stages of sulphidation a fine-grained scale is formed which grows mainly by outward grain boundary diffusion. As the reaction proceeds, the density of grain boundaries decreases because of grain growth of the scale, and volume diffusion becomes important. Thus, during the initial incubation period of the reaction, negative deviations from the parabolic rate law are observed. The duration of this period increases as the temperature decreases, because of the smaller grain size and a lower recrystallization rate. It has been found that the parabolic rate constant k p of manganese sulphidation is the following function of the temperature and sulphur vapour pressure P S2 for the high and low temperature ranges respectively: k p (h)=7.23×10 -3P S 2 1 6 exp − 120 kJ mol -1 RT k p (l)=5.2×10 -9P S 2 1 6 exp − 34 kJ mol -1 RT Sulphidation of niobium also follows the parabolic law with a rate considerably lower than that for manganese at 1 kPa sulphur vapour pressure: k p=1.6×10 -6 exp − 66 kJ mol -1 RT The very thin scale consists mainly of NbS 2 and is characterized by excellent adhesion to the metal in contrast with manganese sulphidation. Sulphidation of MnNb alloys results in double-layer scale formation. The inner barrier layer consists mainly of NbS 2 and the outer layer of α-MnS. The adherence of this double-layer scale to the base is excellent. The growth of the scale proceeds parabolically with a much greater apparent activation energy than that for manganese and niobium sulphidation: k p=1.4×10 -1 exp − 188 kJ mol -1 RT This effect results from continuous changes in the structure and composition of the scale with temperature.