Fireside corrosion and steamside oxidation of 9–12%Cr martensitic steels exposed for long term testing

Abstract

To obtain long term corrosion and steam oxidation data for the 9-12%Cr ferritic steels, test tube sections have been exposed in Amager 3 and Avedøre 1 coal fired power plants in Denmark (formerly run by ENERGI E2). Thus direct comparisons can be made for T91 and T92 for the 9%Cr steels and X20CrMoV121 and HCM12 for the 12%Cr steels. The test tubes were welded in as part of the existing final superheaters in actual plants and exposure has been conducted over a ten year period (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005). Compared to the older steel types, T92 and HCM12 utilise tungsten to improve their creep strength. From Avedøre I testing, T91 and T92 can be compared for exposure times up to y48 000 h exposure. From Amager 3 testing, X20, HCM12 and T92 were tested; T92 has been exposed for up to 31 000 h and X20 and HCM12 have had 84 500 h exposure. Tube sections were removed for various exposure durations such that steamside oxidation and fireside corrosion could be investigated with respect to exposure time. The fireside corrosion rate was assessed by oxide thickness and in some cases residual metal thickness. The growth of steamside oxide was assessed by inner oxide thickness. The microstructure and chromium content of the corroded layers has been investigated using light optical and scanning electron microscopy. The fireside corrosion rate for the T92 and HCM12 steels are comparable to those of T91, however X20CrMoV121 has a higher fireside corrosion rate after the longest exposure time. For steamside oxidation, it was HCM12 that revealed high oxidation rates after the longest exposure time.

Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes

“…Here the only difference is variation in heat flux around the tube resulting in different surface metal temperatures. This was also observed for HCM12 [3]. The increased temperature due to a thicker oxide, results in a higher localised temperature at the oxidemetal interface thus increasing the oxidation rate.…”

“…Wright [1] has cited that other workers observed iron oxides in the inner oxide for 2.5%Cr steel. The formation or iron rich oxide within the inner oxide has been also observed for T92 but after shorter exposure times of 50,000 h [3].…”

“…Cr rich oxide was observed as striations within the oxide parallel to the oxide -metal interface. This has been observed in laboratory tests for 12%Cr steels [7] and power plant exposure for 9 and 12%Cr steel [3,11]. The explanation is described in detail by [4,7]: first chromium rich spinel is preferentially formed at the metaloxide interface leading to chromium depletion of the underlying metal.…”

“…Spallation of the oxide can also cause serious problems with blockage of tubes and is especially relevant for austenitic steels [2] but could also apply to ferritic steels exposed for long times. It was previously observed that oxidation rates for the 12%Cr HCM12 and the corresponding oxide microstructure changed when exposures were over 40,000 h [3]. It was discussed in this paper whether the increased tungsten in this alloy is a contributory factor to the increased oxidation rate which produced a thicker inner oxide.…”

“…For X20CrMoV121 it has been shown that the outer magnetite and haematite grows by Fe ion diffusion outwards and the inner oxide grows from diffusion of oxygen bearing species inwards [4], such as transport of OH À species into the oxides [3]. This paper compares steamside oxides formed at two temperature ranges from plants with laboratory studies.…”