Oxidation and spallation of austenitic stainless steels for ultra-supercritical plant application

Rosser, Jim (2021). Oxidation and spallation of austenitic stainless steels for ultra-supercritical plant application. University of Birmingham. Ph.D.

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Abstract

A comprehensive understanding of the steam-side oxidation and spallation properties of austenitic stainless steels is required to enable the use of these materials in ultra-supercritical plant in order to raise efficiency.

Isothermal oxidation of austenitic steels Super 304H, 347H FG and HR3C was performed in laboratory air and deoxygenated flowing steam at atmospheric pressure for oxidation periods of up to 3000 hours and 1300 hours respectively. The oxides on two inner tube surface finishes were characterised – A pickled surface finish, as would be installed in plant (Super 304H, 347H FG, HR3C), and a shot peened surface finish (Super 304H).

Oxidation in flowing deoxygenated steam was found to increase the oxidation rate of austenitic stainless steels compared with oxidation in laboratory air. Pickled Super 304H and 347H FG “18-8” austenitic steels exhibited larger mass gain and oxide thicknesses than 25Cr HR3C in all atmospheres. The inner surface of shot peened Super 304H exhibited dramatically thinner oxides in both air and steam.

The pickled austenitic stainless steels formed a duplex oxide featuring an outer iron rich oxide and inner spinel oxide containing the alloy constituent elements. The outer iron oxide grown in laboratory air was identified as haematite and that in deoxygenated steam, magnetite. At 650 and 700 °C in steam, a protective Cr-Mn spinel formed at the base of the spinel oxide. The parabolic rate constant of this oxide was found to be an order of magnitude larger than that of pure chromia, and hence less protective. The oxidation kinetics of the spinel and outer oxide layers at 650 and 700 °C were found to follow two regimes throughout oxidation. Initially growth of these layers follows an accelerated parabolic regime, until the complete formation of a protective Cr-Mn spinel at the base of the inner spinel oxide. Following this, the growth rate is dramatically reduced. This is caused by reduced cation diffusion through the Cr-Mn layer.

Spallation of the outer oxide of a number of samples occurred following oxidation in ineffectively deoxygenated steam. The presence of 20% or more haematite in the outer oxide was a prerequisite for spallation. The spallation interface was often within the magnetite layer and, occasionally, at the spinel/outer oxide interface.

The study was extended to include oxidation tests in flowing air saturated steam. These were performed to promote spallation of the inner surface oxide on 347H FG. The strain energies within spalled oxide were calculated and compared to those found in the literature. A value of γ’ = 13 Jm-2 was calculated for the effective fracture energy for the outer oxide on 347H FG, in good agreement with literature values for type-316 austenitic steel. Analysis of the oxide scales revealed that oxide buckling was the most common mode of failure, and that spallation of these oxides satisfies the critical strain energy criterion. Spallation maps for haematite and magnetite growth on 347H FG were constructed, and data was fit, and corresponded to the area at which unstable buckling or wedging was expected, in agreement with the experimental data.

An additional feature of oxidation in air saturated steam was the formation of oxide blisters on the inner surface of 347H FG. These blisters are proposed to nucleate and grow during oxidation, and rupture on cooling.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):