Current Understanding of Steam Oxidation - Power Plant and Laboratory Experience

Abstract

This review discusses key papers presented at an EPRI sponsored Workshop on “Scale Growth and Exfoliation in Steam Plant” that was held at the National Physical Laboratory (NPL) in September 2003 [1]. Additionally, some more recent developments on modelling both scale growth and exfoliation are described. Scale exfoliation in the steam circuit of power plant boilers leads to tube blockages and, further downstream in the power plant, to erosion of the steam turbine blading; and this can have serious consequences for plant performance. Factors controlling this behaviour are reviewed. These include the thermochemistry of oxide formation as a function of operating conditions, scale microstructure and scale growth rates. It is well known that the oxidation rate of steels in steam is about an order of magnitude greater than that in air or oxygen, but the mechanism responsible for this increased rate is still unclear. Various hypotheses, which consider transport of volatile species through cracks and pores, diffusion of OH - or protons and direct access of steam to the metal oxide interface, are proposed to account for the increased rates of reaction in steam compared with air. Modelling exfoliation of thick oxide scales is considered in a number of ways. The basis of the original model by Armitt et al [2] has been extended and further developed. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach is to consider the energetics of steady state through-thickness cracking that involves the fracture energy for through-thickness cracking and avoids the difficult issue of needing to know the defect size that initiates through-thickness cracking. Additionally, the need to incorporate kinetics of scale growth into the developing exfoliation models is briefly discussed.