A relationship between indigenous impurity elements and protective oxide scale adherence characteristics

Abstract

A new and a radically different mechanism to account for the beneficial effects that small additions of elements such as yttrium have on the adherence of oxide scales is proposed. It has been long:known and has been reaffirmed here that indigenous impurities such as sulfur, known to be present at tramp levels (<100 ppm) within nickel and nickel-based alloys, can segregate to metal surfaces. However, here it has been disclosed that such sulfur segregation can also markedly affect the adherence of the protective oxide scale. In the absence of elements like yttrium, such segregation effects weaken the bond between the protective scale and the substrate metal. The role of the yttrium is to interact with such indigenous sulfur to form a refractory sulfide. This interaction lessens the amount of sulfur available to segregate to and concentrate at the critical scale-metal interface. Results reported here have focused on sulfur because sulfur has been long known to be a common tramp impurity in nickel and nickel-based alloys. However other elements, e.g., phosphorus, chlorine, etc., could very probably produce similar effects. The results of experiments involving Auger spectroscopy, optical, scanning electron microscopy, scanning electron microprobe, and scanning transmission electron microscopy techniques in conjunction with isothermal and cyclic oxidation testing which have led us to propose this mechanism are presented. © 1986 The Metallurgical of Society of AIME.