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39 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 TABLE 1. CHEMICAL COMPOSITIONS OF PARENT MATERIALS AS DETERMINED BY INDUCTIVELY COUPLED PLASMA, OPTICAL EMISSION SPECTROMETRY (A) Element % (m/m) Material C Si Mn S Cr Fe Al Co Cu Ni Ti Mo Nb W Other UNS N06600 0.038 0.48 0.41 <0.003 15.79 9.99 0.072 0.04 0.02 72.6 N/A N/A N/A N/A N/A UNS N06601 0.053 0.20 0.69 <0.003 22.57 13.33 1.21 0.04 0.02 60.9 N/A N/A N/A N/A N/A UNS N06025 0.20 0.10 0.06 <0.003 24.78 9.01 2.10 0.04 <0.01 62.7 0.13 N/A N/A N/A Zr 0.05 Y 0.08 UNS N06625 0.027 0.21 0.13 <0.003 21.34 4.00 0.08 0.13 N/A 60.2 0.25 9.23 3.58 N/A P 0.008 Ta 0.001 UNS N06693 0.036 0.10 0.19 <0.003 28.71 4.26 2.89 0.04 <0.01 61.2 0.40 N/A 0.52 N/A Ta 0.001 UNS N06002 0.089 0.43 0.53 0.003 21.95 17.56 N/A 1.64 N/A 46.9 N/A 9.27 N/A 1.18 B 0.0021 UNS N07214 0.053 0.07 0.18 N/A 15.74 2.28 N/A 0.01 N/A 75.4 N/A N/A N/A N/A Zr 0.03 Y <0.01 B 0.0059 UNS N06230 0.063 0.39 0.49 N/A 21.03 1.41 0.27 0.19 N/A 59.6 N/A 1.21 N/A 15.74 B 0.004 La <0.01 (A) All materials were within standard manufacturer specifications. Some elements were not analyzed; these are noted. anhydrous toluene (at 20 °C) to prevent oxidation. Alumina crucibles were ultrasonically degreased in acetone, then heated in air to 1,100 °C for 24 h. UNS N06601 coupons from each set were air dried, loaded into the crucibles, and placed into the test cell. The polished coupon set was placed in the alumina crucible then loaded into the test cell, still under anhydrous toluene. The cell was purged with flowing nitro- gen for 2 h at 1 L·min –1 and heated to 150 °C for 2 h to remove all toluene (nickel alloys will corrode at <0.05 mm/y from boiling toluene 1 ). The cell was then heated to 1,100 °C at 10 °C/min and a test gas (10% carbon monoxide [CO]-1% carbon dioxide [CO 2 ]-89% argon (Ar) flowed over the sam- ples at 0.15 L·min –1 for 336 h to simulate service conditions. Following a further ni- trogen purge (2 h at 1 L·min –1 ), the samples were extracted, ultrasonically cleaned in acetone, and then weighed. The samples were cross-sectioned and examined by light microscopy (in the etched and un- etched conditions), scanning electron mi- croscopy (SE M), and energ y-dispersive x-ray analysis (EDXA). Initial inspection indicated that the behavior of the UNS N06601 samples did not match behavior experienced by compo- nents in the proprietary industrial cell. By combining the above with updated data from the cell, the test conditions were mod- ified to better simulate service. Coupons of Alloy 600 (UNS N06600), UNS N06601, Alloy 602C A (UN S N06025), Al loy 625 (UN S N06625), Alloy 693 (UNS N06693), Hastelloy X † ( U N S N 0 6 0 0 2 ) , Hay n e s 2 1 4 † ( U N S N07214), and Haynes 230 † (UNS N06230) (some with a 50-µm thick beta-aluminide diffusion coating) were prepared and their compositions (Table 1) were determined by inductively coupled plasma, optical emis- sion spectrometry. UNS N06601 was tested in the four surface conditions previously described, and all other alloys were tested in the as-received state. The coupons were ultrasonically de - greased in acetone and loaded into alumina crucibles prepared as described earlier, then covered with fine titanium oxide (TiO 2 ) powder in the form of rutile to fur- ther simulate the micro environment expe- rienced in service. Toluene was not used. The test cell was purged with nitrogen (2 h at 1 L·min –1 ) and then heated to 600 °C. A test gas with higher carbon activity (90% CO-2.5% hydrogen [H 2 ]-7.5% Ar) was intro- duced and the test cell then heated to 1,100 °C. The test gas was f lowed over the sam- ples at 0.15 L·min –1 for 336 h at 1,100 °C. Four coupons were tested at a time. The test cell was then purged with nitrogen and the samples extracted and cross-sectioned for microstructural analysis by light mi- croscopy, SEM, and EDXA. This included measurement of penetration depth (i.e., the deepest point at which the microstructure had been affected by the environment). Mass change results are not presented for these trials due to varying amounts of par- tially reduced/react ed Ti oxides t ena- ciously adhering to the surface, as shown in Figure 1. The powder was analyzed by x-ray diffraction. † Trade name.