AES characterization of oxide grains formed on ductile cast irons in exhaust environments

Abstract

Cast alloys (SiMo and Ni-resist) were oxidized at 850• C for 50 h in dry air and synthetic diesel exhaust gases. High-resolution AES analysis and SEM studies on sections of the inner oxides showed that, for both alloys, thicker silicon-containing oxides layers were formed at metal/oxide interfaces in diesel than in air. For SiMo, fayalite appeared to precipitate within this layer in diesel gases, thus lowering its protectiveness. Further out, silica may reprecipitate. Where the austenitic Ni-resist failed to maintain a protective interfacial oxide, a highly irregular oxide/metal interface led to metallic Ni-rich particles becoming trapped in the oxide.

High Temperature Corrosion of Cast Alloys in Exhaust Environments I-Ductile Cast Irons

“…The inner oxide developed in Diesel at 850°C has been described in a previous publication [22]. Already after 1 h, the outer Fe-oxide covered the entire surface of the sample.…”

“…3b. The inner oxide developed after 50 h at 850°C in Diesel was described in a previous publication [22]. It was composed of a rather pure bright Fe oxide surrounded by speckled strings which were proposed to consist of nano-sized particles of silica and magnetite (Fig.…”

“…Fayalite was observed in Diesel inside the interfacial silica layer [22] and in larger amounts in Dry Gasoline but may have retransformed into silica and Fe oxide further out in the oxide. This can possibly be seen in Fig.…”

“…Fresh metal fragments got entrapped into the oxide by the growing oxide and depleted in Fe and Si. The very irregular metal-oxide interface and the slow diffusion processes in austenite were proposed to have promoted local failure of the protective oxide film [22]. The inner oxide then consisted of Ni particles trapped in a rather pure or Si-rich Fe oxide with a thin and weak interfacial Si(-Cr) film.…”

Field test of superheater corrosion in a CFB waste boiler: Part II – Scale formation characteristics

“…Thermodynamic data [4] shows that molybdenum can be selectively oxidised while nickel still is in metallic form. Selective oxidation of austenitic alloys leaving metallic nickel inclusions in internal oxides has been observed previously [18]. The second particular feature was that no cracks perpendicular to the tube were observed through the outer oxide layers on Alloy 28.…”