The oxidation-sulfidation of Ni-Nb alloys at 600–800 °C in H 2-H 2S-CO 2 mixtures

Abstract

The corrosion of two Ni-Nb alloys containing 15 and 30 wt% Nb has been studied at 600–800 °C in H 2-H 2S-CO 2 mixtures providing a sulfur pressure of 10 −8 atm and oxygen pressures ranging between 10 −24 atm and 10 −20 atm. The corrosion kinetics were approximately linear under all conditions, except for Ni-30Nb at 600 °C, when nearly parabolic kinetics were observed. At 600 °C the scales presented an outer layer of Ni sulfide, an innermost layer of mixed Ni-Nb double sulfide and Ni metal as well as a complex intermediate layer containing a mixture of sulfides of both Nb and Ni with niobium oxide. At 700 and 800 °C the scales formed on both alloys contained a complex mixture of Ni sulfide, double sulfide and pure Ni metal but no oxide, very likely due to the formation of a liquid Ni-S solution which allowed a very fast transport of reactants through the scales. The Nb addition was able to reduce the corrosion rate at 600 °C but was practically inefficient at the higher temperatures.