Sulfidation- and oxidation-resistant alloys prepared by sputter deposition

In order to tailor novel alloys resistant to high temperature corrosion in multicomponent sulfidizing-oxidizing environments, amorphous or nanocrystalline Al-refractory metal alloys with and without silicon and Cr-refractory metal alloys have been prepared by sputter deposition. The sulfidation and oxidation behavior of the alloys has been studied as a function of temperature in sulfur vapor pressure of 10(3) Pa and in oxygen of 2 x 10(4) Pa, respectively. The sulfidation of these alloys generally follows a parabolic rate law, being thus diffusion controlled. The sulfidation rates of Al-refractory metal and Cr-refractory metal alloys are several orders of magnitude lower than those of conventional high temperature alloys and comparable to or even lower than those of the corresponding refractory metals. The sulfide scales formed on these alloys consist of two layers, comprising an outer Al2S3 or Cr2S3 layer and an inner refractory metal disulfide layer. The formation of the inner layer is attributed to the excellent sulfidation resistance of these alloys. The oxidation resistance of Al-refractory metal alloys is not sufficiently high, but the addition of silicon improves remarkably their oxidation resistance by synergistic effect of aluminum and silicon. Although the Cr-Mo alloys possess poor oxidation resistance, due to the formation of volatile molybdenum oxide, the oxidation resistance of the Cr-Nb and Cr-Ta alloys is as high as that of typical chromia-forming alloys. (C) 1999 Elsevier Science S.A. All rights reserved.