High Temperature Corrosion Resistance of Pt-Based Superalloys in 0 . 2 % SO 2N 2 Gas
Pt-based superalloys are a new generation of materials intended to be used in various high temperature applications. Effluent gases emitted to the environment during combustion operations can lead to degradation of high temperature materials with deleterious effects on mechanical integrity and the corrosion of alloys. This paper focuses on sulphidation of one quaternary, and four ternary, Pt-based superalloys of various chemical compositions. Pt-based superalloys were exposed at 900°C in a reducing environment. The alloys were compared to coated and uncoated Ni-based superalloys (NBSAs). X-Ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize corrosion products that formed. The Pt-based superalloys exhibited higher corrosion resistance than NBSAs as a result of the formation of a protective Al2O3 scale, whereas NBSAs suffered internal sulphidation and formed Cr and Ni sulphide corrosion compounds. The formation of brittle needle-like phases and the transformation of the NiAl phase to Ni3Al, led to early failure of the coated alloy. Pt-based superalloys alloyed with Co showed poor corrosion resistance as a result of depletion of alloying elements.
PRACTICAL GUIDE TO HIGH-TEMPERATURE ALLOYS
The contents of this guide to high temperature alloys cover: the acceleration of corrosion by high temperatures; oxidation and scaling in high temperature processes; materials available for specific high temperature applications, including alloy steels, superalloys and metallic coatings; the comparative costs of the different materials; and special corrosion problems, among which are catastrophic oxidation, sulfidation, hot corrosion, deposits and fuel ash corrosion, carbonaceous environments, and halogen reactions.
Hot Corrosion Behavior of CM 247 LC Alloy in Na2SO4 and NaCl Environments
Hot corrosion studies of CM 247 LC alloy werecarried out in pure sodium sulfate, as well as sodiumchloride and sodium sulfate mixtures of differentconcentrations at various temperatures. A crucible test was employed to study the suitability of CM 247LC as a gas turbine blade material. It was observed thatbare CM 247 LC was severely corroded in just 4 hr, whileit was completely consumed in 70 hr when tested in 90% Na2SO4 +10% NaCl at 900°C. The results show that a chloridecontaining melt is more corrosive than pure sodiumsulfate. The weight loss is linearly related tot1/2 (time) and temperature in the different environments studied. Thecorroded samples were characterized by EPMA, SEM, XRD,and metallographic techniques. The results show that hotcorrosion of CM 247 LC is an electrochemicalphenomenon.