Effect of preoxidation and Ti or Hf additions on the corrosion behavior of Fe-23Cr-5Al alloys in Ar-10SO2 at 1200 K

An Fe-23Cr-5Al alloy and those containing 0.21% Ti or 0.12% Hf were cyclically corroded in flowing Ar-10SO₂ gas under atmospheric pressure and in a temperature range varying from room temperature to 1200 K. The corrosion kinetics were assessed by gravimetry, while morphological examinations were carried out using x-ray diffractometry, scanning electron microscopy, and microanalysis. Similar corrosion tests and examinations of the corrosion products were performed for the specimens preoxidized at 1200 K for 20 ksec in pure oxygen under atmospheric pressure. The main corrosion product under all the experimental conditions was -alumina, growing mainly outward in the form of thin needles. Many voids formed beneath the adherent scales. The degree of outward growth decreased by the additives. Partial spallation and formation of scales repeated on the base alloy, resulting in continuous mass losses. The addition of Ti prevented scale spallation, resulting in gradual mass gains; whereas the addition of Hf resulted in poorly adherent scales, under which grooving at alloy grain boundaries and void formation inside the grains occurred. Preoxidation resulted in the formation of adherent scales on all the alloys; however, with increasing corrosion cycles, the outward growth of the oxide became significant. Penetration of sulfur to the substrate under the adherent scale took place.