Towards improved prediction of scale exfoliation from steam tubes

A review of the oxidation behaviour of structural alloys in steam

“…In particular, progressive reduction of heat transfer by thickening oxides can lead to tube overheating and failure, and spallation (or exfoliation) of these oxides can result in blocking of tubes or, if the oxide fragments are transported in the steam, erosion of the steam turbine. 1 Mitigation of such problems requires a mechanistic understanding of not only the rate of growth of these oxides, but also of the evolution the specific scale structures and the influence of factors such as time, temperature, steam conditions and alloy composition. This need exists not only for current steam-generating power plant technology, but also for new and planned power plants that will operate at higher steam temperatures and pressures for increased overall efficiency.…”

“…Laboratory studies have covered a much wider range of alloys, but correlation with plant experience has been lacking, partly because of difficulties in defining the environments experienced in actual boiler operation (such as effects of heat flux, and temperature and pressure cycling). 1 In addition, the experimental problems associated with providing a source of flowing steam for long-term exposures at elevated temperatures and pressures have led to testing in more readilycontrolled environments such as water vapour in a carrier gas. Recent interest in the role played by water vapour (present in environments resulting from combustion of various fuels) in accelerating surface degradation at high temperatures of materials designed to form protective surface oxides in air has led to extensive studies of high-temperature oxidation in mixed gases containing water vapour.…”

“…In contrast, in steam the lower effective oxygen partial pressure reduces significantly the rate of formation of such volatile species to the extent that Cr loss becomes a factor in the oxidation process only at the highest temperatures and pressures envisioned for advanced steam plant. 1 As a result, the aspect of chemical breakdown of initially protective oxide scales that plays a large role in high-temperature oxidation behaviour in air-water vapour mixtures is largely absent in steam; hence there are important differences in the evolution of scale morphologies in the two environments. Since the air/oxygen content of the environments of interest in steam power plants is rigorously controlled to very low levels, this review is focused on high-temperature oxidation in nominally 100% steam, and the mechanistic and practical aspects of high-temperature oxidation in gas mixtures containing water vapour are not further considered.…”

Steam-side scale morphologies associated with scale exfoliation from ferritic steel T22

“…The expectation from analytical considerations is that steam-grown scales on alloy T22 will fail in tension via through-scale cracking perpendicular to the alloy-oxide interface when sufficient stresses are accumulated [4,[11][12][13][14]. As has been observed on these tubes and elsewhere [22], the laminated inner layers on T22 often develop porosity along the interface between the spinel layer in one lamination and the magnetite layer of the next subjacent lamination.…”

“…Renewed efforts in recent years have emphasized modeling of the exfoliation process as a means of accounting for the influences of the many variables involved and, in particular, in understanding the influences of the different scale structures formed on the different alloys [10][11][12][13][14][15][16][17][18][19]. The expectation from analytical considerations is that steam-grown scales on alloy T22 will fail in tension via through-scale cracking perpendicular to the alloy-oxide interface when sufficient stresses are accumulated and the alloy is subjected to the additional thermal stress associated with a boiler shut-down event [4,[11][12][13][14]. In fact, this mode of scale loss involving the full scale thickness does not agree with common observations of loss of relatively thin flakes of scale from this alloy at scale thicknesses less than those associated with attaining the requisite strains.…”