High Temperature Air and Steam Oxidation and Fireside Corrosion Behavior of 304HCu Stainless Steel: Dichotomous Role of Grain Boundary Engineering

The 304HCu stainless steel is a candidate material for superheater and reheater tubes in advanced ultra-supercritical power plants due to its excellent creep and oxidation resistance. However, these operating conditions involve exposure to steam at high pressure and temperature on the steam-side and hot coal-ash products on the fireside. In this study, the role of grain boundary character distribution (GBCD) on oxidation and fireside corrosion behavior of 304HCu steel is investigated. The GBCD was modified through grain boundary engineering (GBE) via optimized strain-annealing treatment on the as-received (AR) specimen. The air oxidation, steam oxidation (pressure ~ 243 bar) and fireside corrosion studies were conducted at 973 K for up to 1000 h, in custom-designed setups precisely simulating the operating conditions. Following GBE, the grain size (excluding twins) and coincident site lattice boundary (Σ ≤ 29) fraction increased from 21 ± 1 to 60 ± 12 μm and from 62 ± 4 to 74 ± 3%, respectively, resulting in disruption of the random high angle grain boundary networks through the introduction of twins. Evaluation of oxidation behavior revealed that the GBE specimens have lower oxidation resistance (i.e., higher weight gain and oxide scale thickness) in both air and steam, while the same specimen displayed improved fireside corrosion resistance (lower percolation depth) as compared to the AR specimen. From a detailed analysis of the oxidation/fireside corrosion products and cross-sectional microstructures of the oxide layers, the above responses could be correlated with the GBCD and grain size, and the possible mechanisms operative during the air/steam oxidation and fireside corrosion are also presented.