Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/792600
40 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 COATINGS & LININGS Results and Discussion UNS N06601 in 10% CO-1% CO 2 - 89% Ar at 1,100 °C When tested under 10% CO-1% CO 2 - 89% Ar, UNS N06601 showed a mixture of carburization and oxidation, with black particles visible along grain boundaries at depths of up to 150 µm (Figure 2). Mass gains varied from 3.6 to 4.9 mg·cm –2 depend- ing on surface finish. EDXA confirmed the presence of complex corrosion products rich in Cr and O, presumably chromium oxide (Cr 2 O 3 ) on the UNS N06601 surface. The chromium and carbon-rich regions were nearer the surface, alongside detached pieces of substrate. Intergranular attack was present below, with dark particles at the grain boundaries primarily consisting of Al and O, presumably aluminum oxide (Al 2 O 3 ). This indicates minimal oxygen diffusion along the substrate into the grain boundar- ies. Under a more oxidizing environment, the formation of Cr 2 O 3 would be expected to dominate 2 (p[O 2 ] for formation of Cr 2 O 3 is ~10 –19 atm at 1,100 °C vs. ~10 –31 atm for Al 2 O 3 ). No post-testing microstructural dif- ference was observed between the four dif- ferent surface finishes, but in all cases, the surfaces and cross sections appeared less corroded and had a different appearance than the materials in the proprietary indus- trial cell. This gas mixture, therefore, was judged to be an unsuitable simulant for this service. Uncoated Alloys in 90% CO- 2.5% H 2 -7.5% Ar at 1,100 °C Tests with multiple engineering alloys with TiO 2 powder in 90% CO-2.5% H 2 -7.5% Ar yielded the reduction of TiO 2 to a differ- ent titanium oxide (Ti 3 O 5 ) as confirmed by x-ray diffraction. This likely released oxi- dizing species into the local environment. Different amounts of oxide powder adhered to alloy surfaces even after multiple ultra- sonic cleaning cycles and m echanical removal. Metallic f lecks were observed in the powder, indicating reasonably strong adherence to the surface, with pieces of the substrat e removed as th e powder was detached. For UNS N06601, more powder adh ered to th e samples with sur faces ground to 600 grit finishes rather than the heat-treated, polished, or as-received sur- faces, which is likely due to mechanical keying and/or greater surface area. No cor- relation between alloy composition and oxide adherence was observed. All alloys were significantly corroded by this modified environment, but the corro- sion mechanism and corrosion product microstructure varied substantially. For most alloys, corrosion was general and did not vary across different sides of the same coupon. Exceptions are noted below. Over- all, the corrosion behavior of the samples was similar enough to that experienced by the material in the proprietary industrial cell—including a partially "melted" appear- ance on the surface of UNS N06601 and sev- eral other alloys—to consider the combina- tion of 90% CO-2.5% H 2 -7.5% Ar and TiO 2 powder to be a good simulant of service and thus suitable for material selection and further testing. The surface finish of UNS N06601 did not significantly affect the extent or mech- ani sm of c or ro sion , w ith p en etration depths ranging from 91 µm (heat-treated) to 105 µm (as-received), likely the same within the range of experimental error. Fig- ure 3 shows an example. Below the affected region, the composition was unchanged, although Al and O (indicating Al 2 O 3 ) were present along the grain boundaries. The metallic region was depleted in chromium and the surface oxide layer correspond- ingly was enriched with both chromium and titanium. Local aluminum enrichment was observed in some regions of the scale. UNS N06025 exhibited similar inter- granular corrosion to UNS N06601, with Al and O (also presumably Al 2 O 3 ) at the grain boundaries, though with a thicker attacked layer at the surface. UNS N06600, N06625, and N06002 were corroded mainly by void formation beneath a coherent scale layer comprised of a mixture of titanium-chro- mium oxides. Limited intergranular attack was observed in UNS N06600, while UNS N06693 and N07214 both formed multi- phase surface layers. For UNS N06693, the layer was primarily chromium-depleted metallic fragments surrounded by carbon- rich oxide corrosion product, whereas the concentrations of titanium were much hi g h er f or UN S N 0 7 2 1 4 . UN S N 0 6 2 3 0 formed a coherent layer of Cr 2 O 3 , mixed titanium-chromium oxide, and a carbon- FIGURE 1 A UNS N06601 sample (tested in the as-received surface condition) after 336 h at 1,100 °C in flowing 90% CO-2.5% H 2 -7.5% Ar while covered with TiO 2 powder. A millimeter scale is included. FIGURE 2 Light micrograph of a UNS N06601 sample cross section (tested in the polished surface condition) after 336 h at 1,100 °C in flowing 10% CO-1% CO 2 -89% Ar. FIGURE 3 SEM of a UNS N06601 sample cross section (tested in the polished surface condition) after 336 h at 1,100 °C in flowing 90% CO-2.5% H 2 -7.5% Ar while covered with TiO 2 powder.