This study investigates the steam oxidation behavior of Alloy 699 XA, a material containing 30 wt.% chromium and 2 wt.% aluminum that forms protective oxide scales in low-oxygen conditions. The research compares four variants of the alloy: conventional bulk material, a laser powder bed fusion (LPBF) additively manufactured version, and two modified compositions. The modified versions include MAC-UN-699-G, optimized for gamma-prime precipitation, and MAC-ISIN-699, which underwent in-situ internal nitridation during powder atomization. All variants were subjected to steam oxidation testing at 750°C and 950°C for up to 5000 hours, with interim analyses conducted at 2000 hours. The post-exposure analysis employed X-ray diffraction (XRD) to identify phase development and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) to examine surface morphology, cross-sectional microstructure, and chemical composition. This study addresses a significant knowledge gap regarding the steam oxidation behavior of 699 XA alloy, particularly in its additively manufactured state.

Long-term performance of high temperature alloys is critically linked to the oxidation behavior in power generation applications in wet air and steam. As power generation systems move towards higher efficiency operation, nextgeneration fossil, nuclear and concentrating solar power plants are considering supercritical CO 2 cycle above 700°C. Wrought solid solution strengthened and precipitations strengthened alloys are leading candidates for both steam and Supercritical CO 2 power cycles. This study evaluates the cyclic oxidation behavior of HAYNES 230, 282, and 625 alloys in wet air, flowing laboratory air, steam and in 1 and 300 bar Supercritical CO 2 at ~750°C for duration of 1000 -10,000h. Test samples were thermally cycled for various times at temperature followed by cooling to room temperature. Alloy performances were assessed by analyzing the weight change behavior and extent of attack. The results clearly demonstrated the effects of alloy composition and environment on the long-term cyclic oxidation resistance. The extents of attack varied from alloy to alloy but none of the alloys underwent catastrophic corrosion and no significant internal carburization was observed in supercritical CO 2 . The performance of these alloys indicates that these materials are compatible not only in oxidizing environments, but also in Supercritical CO 2 environments for extended service operation.

As part of the Boiler Materials for Ultrasupercritical Coal Power Plants program, sponsored by the United States (U.S.) Department of Energy (DOE) and the Ohio Coal Development Office (OCDO), the steamside oxidation and oxide exfoliation behavior of candidate alloys have been thoroughly evaluated in steam at temperatures between 620°C and 800°C (1148°F and 1472°F) for times up to 10,000 hours. The results from this test program indicate that the oxidation rates and oxide morphologies associated with steamside oxidation are a strong function of the crystallographic lattice structure and the chromium content of the material. Oxide exfoliation correlates to oxide thickness. The time required to reach the critical oxide thickness for exfoliation can be estimated based on oxidation kinetic relationships. For austenitic stainless steels, shot peening is effective in reducing steamside oxidation/exfoliation, but the efficacy of this technique is limited by the operating temperature. Nickel-based alloys exhibit very low oxidation/exfoliation rates, but have a propensity to form aluminum/titanium oxides along near surface grain boundaries.

Because of the problems experienced with steam-side oxide scale exfoliation in commercial power plants, there has been increased interest in understanding the steam oxidation resistance of 300- series stainless steels such as 347H and 304H. Model alloys were used in an attempt to understand the effect of varying Ni (9-12%) and Cr (16-20%) on steam oxidation resistance at 650°C. However, the model alloys generally showed superior oxidation resistance than commercial alloys of similar composition. Several surface engineering solutions also were investigated. The commercially favored solution is shot peening. Laboratory steam testing at 650°C found that annealing temperatures of ≥850°C eliminated the benefit of shot peening and a correlation was observed with starting hardness in the peened region. This effect of annealing has implications for the fabrication of shot peened tubing. Another route to improving oxidation resistance is the use of oxidation resistant diffusion coatings, which can be deposited inexpensively by a vapor slurry process. Uniform coatings were deposited on short tube sections and annealed at 1065°C to retain good 650°C creep properties. The coating was thicker than has been investigated in laboratory processes resulting in increased brittleness when the coating was assessed using 4-point bending.

Using oxygen, rather than air, in coal-fired boilers has been studied for several years as a strategy to reduce NOx and concentrate CO 2 for capture. In combination with flue gas recirculation, higher levels of CO 2 are expected but increased H 2 O and SO 2 levels also may occur. In order to understand the role of substrate composition on corrosion, a combination of commercial and model alloys were investigated with synthetic coal ash and gas compositions simulating air- and oxyfiring environments. Exposure temperatures ranged from 600°-800°C to cover current operating temperatures up to advanced ultrasupercritical conditions. Using 500h exposures, no consistent negative effect was found for switching to the oxy-firing environment with the same synthetic ash. For model Fe-Cr alloys, 30%Cr was needed to form a thin protective reaction product across this temperature range. Among the commercial stainless steels, 310-type stainless steel showed low reaction rates with the maximum attack at 650°C. At higher temperatures, the depth of attack on Fe-base type 310 stainless steel was less than for Ni-base alloy 740. Initially, this difference was attributed to the Al and Ti additions in alloy 740. However, cast and hot rolled model Ni-18Cr and -22Cr alloys with various Al and Ti additions showed decreased metal loss with increasing Al and Ti additions in the oxy-firing environment at 700° and 800°C. As expected, metal loss was very sensitive to Cr content. A second set of model alloys also examined the effect of Co and Mo.