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41 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 rich chromium phase, likely chromium car- bide (Cr 3 C 2 ). This indicated a direct reac- tion between titanium oxide(s) and the substrate or corrosion scale on the surface. Intergranular precipitation with Al 2 O 3 was also observed in the substrate. Below the scale layers, voids or intergranular attack were not visible in either case. Coated Alloys in 90% CO- 2.5% H 2 -7.5% Ar at 1,100 °C Pre-test examination of aluminide dif- fusion-coated samples verified that the coatings were coherent, nonporous, and covered the entire outer surface with no v i si b l e c ra c k i n g . T h e m i c r o st r u c tu re showed a rough outer layer, a middle layer with voids at its base, and an inner layer with filamentary penetration into the sub- strate, including dif fusion of aluminum and formation of a metallurgical bond between the coating and substrate. The observed attack on the coated substrates depended on local coating behavior. For UNS N07214 and N06025, there was little or no attack where the coating was present. Where significant regions of the coating had been damaged, the attack was similar to that on uncoated coupons of the same materials (Figure 4). The coating micro- structure also had changed, with the for- mation of a mixed aluminide layer with voids remaining in the coating. For UNS N06601, only a small f law was present in the coating, which led to massive grain boundary precipitation radiating outward from thi s region . For al l three c o at ed alloys, the attack was deeper than observed on uncoated substrates, but only in regions where the coating was damaged. Overall Alloy Performance Difficulties were encountered in accu- rately measuring penetration depth per- pendicular to the surface, as the original substrate surface could not be clearly dis- cerned. Where this was the case, measure- ments were taken from the visible remain- ing surface. All alloys and penetration depths are summarized in Figure 5. Based on the results, UNS N07214 was deemed unsuitable for this service. UNS N06230 experienced low penetration with little intergranular attack, although a Cr- depleted layer formed below the scale surface. UNS N06625 had moderately promising performance—low penetration combined with void formation and no intergranular attack. The surface did not display a signifi- FIGURE 4 Light micrograph of beta-aluminide coated UNS N06601 sample cross section after 336 h at 1,100 °C in flowing 90% CO-2.5% H 2 -7.5% Ar while covered with TiO 2 powder. Grain boundary precipitation is visible for several hundred µm below this region. FIGURE 5 Maximum penetration depths for all alloys tested for 336 h at 1,100 °C in flowing 90% CO-2.5% H 2 -7.5% Ar while covered with TiO 2 powder. Some penetration depths were outside the scale of this graph; these are noted. cant melted/corroded appearance. This may be due to the presence of niobium, which acts as a carbide stabilizer and may lead to preferential formation of niobium carbides over chromium carbides. These two materials appeared to be the most promising. UNS N06600 had the lowest penetration into the substrate, and voids and carbides had formed below the substrate surface. Since the original surface could not be defi- nitely discerned, it was regarded as promis- ing with caution. High-Temperature Material Selection for a Powder Processing Environment