The Role of Sulfur in the Growth and Adhesion of Oxide Scales

Abstract

Experiments are reported in which indigenous sulfur, present at low levels within a substrate alloy, is sufficient to produce oxide scale exfoliation effects. The amount of sulfur commonly found at levels less than approximately 50 ppm by weight in normally prepared alloys is sufficient to produce exfoliation. The reduction of such sulfur either through the use of high purity, i.e. low sulfur starting materials or the use of active element additions, is sufficient to produce adherent scales. By reacting with indigenous sulfur to form refractory sulfides, active element, e.g. yttrium, additions effectively reduce the amount of sulfur available to segregate to and weaken the crucial scale-metal interfacial bond.

Pores are frequently observed on substrate surfaces when oxide scales exfoliate. These pores have commonly been attributed to Kirkendall effects. By reacting with and annihilating vacancies resulting from Kirkendall effects at the scale-metal interface, active element oxides at the scale-metal interface have been suggested to promote scale adherence. Studies conducted here have shown that without active element additions small pores few in number are associated with exfoliation effects. Despite adherent scales, similar pores occur at the scale-metal interface. Hence the relationship between porosity at the scale-metal interface and protective scale adherence is unclear.

In a series of experiments involving implanted species that would be little soluble at elevated temperatures, exfoliating scales were produced despite the presence of yttrium in a Ni – 20 wt% Cr – 12 wt% Al alloy. Pores in exfoliated regions of such implanted specimens had all the characteristics normally associated with scale exfoliation effects in unimplanted materials. Hence it is suggested that the porosity commonly observed at surfaces of conventionally produced alloy substrates is rather due to the expulsion of contaminant levels, i.e. low ppm levels, of little soluble elements, e.g. argon, present at low levels.

Experimental results which have led to these conclusions are presented.