Characterization of the Oxide Scale Formed on T12 Water Wall Tube After Long-term Service in Supercritical Power Plant

Abstract

Oxide scale control is one of the critical maintenance issues in fossil fuel power plant. Hence, the water treatment of the feed water has been changed from all-volatile treatment (AVT) to oxygenated treatment (OT) by added some amount of oxygen in the system to form a stable oxide and minimize the scale growth rate. In this work, the oxide scales formed on T12 water wall tube after one cycle of chemical cleaning treatment under OT condition in a supercritical coal power plant were characterized by using various methods. It was found that the oxide scale formed on the inner surface of the tube had a multi-layer structure with a porous outer layer consisting of Fe 3 O 4 and Cr-rich α-Fe 2 O 3 oxides and a dense inner layer consisting of Cr-rich spinel oxide. A Cr-and Mn-enriched outermost layer was also observed. Cavities, pits, and exfoliation were observed in the oxide scales. The cavities and grain boundaries behaved as the short path of the diffusion of oxygen into the metal. Intergranular oxidation was observed between the inner layer of oxide scale and matrix. The distribution, morphology, and chemical compositional of these oxides were assessed and discussed from a point of view of the suitability of current chemical cleaning solution and procedure which has been used for AVT descaling.

Mechanistic understanding for the formation of Cr-rich haematite layer on the T12 ferritic steel in supercritical water

“…This is an evidence for that the Cr can diffuse outward through the oxide scale and then result in the formation Cr-rich haematite layer. It was also found that the oxide scale formed on the inner surface of the T12 water wall tube after long-term service in supercritical fossil power plant contains Cr-rich α-Fe 2 O 3 oxides [10]. However, the oxygen content in the actual operation condition is about 100 ppb.…”

“…Many reports focused on the corrosion behaviour of F-M steels in SCW at the temperature range of 400-650 °C showed that the oxidation process followed parabolic oxidation kinetics and the oxide scales consisted of multi-layer structures with Cr-rich inner layer and Fe-rich outer layer [4 9]. In our previous work, the Cr-rich haematite layer was observed in the oxide scale formed on the T12 water wall tube after long-term service in supercritical fossil power plant with temperature of 430 °C for 164171 h [10]. The Cr-rich haematite layer was also observed in our lab tests in simulated SCW environment with DO = 3 ppm [11].…”