The Influence Of Surface Quality Of Grade 91 Tubing On Long-Term Oxidation In Steam

Oxide scale formation in the inner bore of steam tubing has been identified as a key metric for determining operational parameters and life expectancy of modern boiler systems. Grade 91 tubing is commonly used for the construction of key components within boiler systems designed for power generation operating in the temperature range of 500 to 650 degrees C. Standard laboratory test procedures involve grinding the surface of test coupons to homogenise their surface structure and improve experimental consistency, however, data presented here shows a discrepancy between laboratory and industrial practices that has long term implications on scale growth kinetics and morphological development. Microstructural analysis of both virgin and ex-service tubing reveals the presence of a pre-existing oxide structure that is incorporated into the inwardly growing scale and is implicated in the formation of multiple laminar void networks. These void networks influence thermal diffusivity across the scale and may function as regions of spallation initiation and propagation.