792 High Temperature Corrosion Resistance

Oxidation It is well known that elements having greater thermodynamic affinity for oxygen tend for form passive barriers in alloy systems, thus providing the required resistance. Chromium, aluminum, and silicon are the three major elements that account for these passive barriers. The usefulness of protective chromia Cr2O3 is limited to around 950°C due to the formation of volatile chromium oxide (CrO3). The higher thermodynamic stability of the alumina sublayer, at even very low partial pressures of oxygen, improves the alloy 602CA oxidation resistance in cyclic tests. Rare-earth elements further reduce the cracking, fissuring, and spalling of the protective oxide.

Table 7.15 presents the laboratory test data on cyclic oxidation testing (24-h cycles—1.5-h heat up, 16 h hold at temperature, and furnace cool down, for test temperatures up to 1100°C, and cooling in air for temperatures higher than 1100°C) for periods up to 1200 h. As is evident, alloy 602CA gave superior performance when compared to many other iron, nickel, and cobalt base alloys. Metallographic examination of alloy 602CA showed a continuous alumina sublayer without any selective internal oxidation by comparison to alloy 601 (Fig 7.5). Further tests conducted on alloy 602CA and alloy 601 for 3150 h

TABLE 7.15 Cyclic Oxidation Data—1200 h, 24-h Cycles

Alloy

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