FIGURE 7.5 Internal oxidation attack on alloy 601 after cyclic testing at 1200°C in air for 1000 h (alloy 602CA—no internal attack).

at a lower temperature of 2100°F (1148°C) again showed excessive internal oxidation with alloy 601. In contrast alloy 602CA had no internal attack but only a thin surface oxide scale. This is especially beneficial in applications that utilize thin sheets such as in radiant tubes. No internal oxidation means the entire wall thickness is sound metal and the alloy retains most of its original properties. The higher thermodynamic stability and more than 5 orders of magnitude lower dissociation pressure of alumina are the primary reasons for formation of the protective alumina layers. Another series of cyclic oxidation test at 2100°F (1148°C) for 3000 h (cycle time of 160 h) measured the weight loss as well as total penetration in mils by metallographic examination. These results are shown in Table 7.16.

TABLE 7.16 Weight Gain and Internal Penetration of Various Alloys after 3000 h Exposure in Air at 2100°F (1148°C)


Weight Gain (mg/cm2)

Max Internal Penetration (mils)

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