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Scale Growth Mechanisms The prominent effects of RE additions on scale growth processes for Cr20 3-forming alloys are illustrated in Fig. 6.22, which compares the oxidation rates for IDNiCr (Ni-20%, Cr-2vol%, Th02) to Ni-30% Cr and Co-35% Cr alloys at l200°C [50]. The platinum markers were found to be situated at the alloy-scale interface for the binary alloys without any active element/oxide additions, and at the scale-gas interface for the thoria dispersed alloy. This clearly suggests that outward chromium diffusion is the important process for the binary alloys, whereas inward oxygen diffusion is important for the thoria dispersed alloy. The

Figure 6.22 Comparison of the oxidation ofNi-30%Cr, Co-35%Cr, and TDNiCr (Ni20%Cr-2voi%Th02) at 1200°C [50].