Comparison of the Oxidation Behaviour of Wrought and Additively Manufactured Alloy 625 in a High Temperature CO2 Environment

In pursuit of cleaner and more sustainable power generation, gas turbine power cycles using supercritical carbon dioxide (sCO ₂ ) as a working fluid have emerged as an option to meet emissions targets. The Allam cycle achieves emission reduction through high-pressure sCO ₂ in an oxy-combusted, highly recuperated cycle, operating at 300 bar and up to 800°C. Alloy 625, known for its high corrosion resistance, is used for high-temperature components, including heat exchangers, which can be manufactured traditionally or via additive manufacturing (AM) techniques, such as laser powder bed fusion (LPBF).This study investigates Alloy 625’s microstructural influence on resistance to simulated Allam cycle conditions; comparing as-built and solution heat-treated LPBF AM samples with conventionally manufactured wrought. Isothermal oxidation tests were conducted at 800 ^° C for 1000h in a CO ₂ rich environment (CO ₂  + 2.7mol% H ₂ O + 1.43mol% N ₂  + 0.17mol% O ₂  + 300ppm SO ₂ ). Wrought Alloy 625 demonstrated the lowest oxidation rate, attributed to its homogenized microstructure and fine grain size. Solution heat-treated AM samples exhibited a continuous oxide layer due to grain boundary changes, enhancing oxide scale formation. The K _p values of as-built, solution heat-treated and wrought Alloy 625 were 4.8 x 10 ^− 5 , 5.3×10 ^− 5 , and 4.0×10 ^− 5 mg ² /cm ⁴ respectively.Grain morphology and heat treatment influenced oxidation. As-built LPBF samples formed ridges on the oxide scale and with subsurface voids, while wrought samples displayed uniform oxide layers without subsurface voids. These findings highlight how manufacturing techniques and post-processing affect Alloy 625’s high-temperature performance, crucial for turbine casing and heat exchangers in a baseline Allam cycle environment.