Beyond the sulfur effect

The addition of reactive elements (REs) is known to increase the adherence of Al(2)O(3) scales and the underlying alloys. A widely accepted mechanism is that the REs getter the sulfur impurity in the alloy, thus preventing it from segregating to the scale-alloy interface to weaken the interfacial bonding. This paper provides evidence showing that not all S-free interfaces ave strong. Although eliminating the sulfur in the alloy can greatly improve scale adhesion, the presence of RE clearly has additional beneficial effects that make the scale-alloy interface even stronger. Fe(3)Al-base alloys, with or without Zr additions or a desulfurization H(2)-anneal were oxidized at 1000 degrees C in O(2). The amount of sulfur at the Al(2)O(3)-alloy interface was studied after the scales were removed by scratching with a diamond stylus in ultra-high vacuum using Auger spectroscopy. The interface composition was related to the spallation resistance of the scale.