Søren Aakjær Jensen

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 (1994–2005). 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 ∼48 000 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.

Field Investigation of Steamside Oxidation of TP347H

The steamside oxide formed on two TP347H superheater tubes was compared. The two specimens investigated were exposed in situ in power plants in Denmark, one specimen was coarsegrained and the other was fine-grained. Parts of both the coarse-grained and fine-grained specimens were turned (machined on the inner side) to give a constant metal thickness so more precise wall thickness measurements could be undertaken. Machined and non-machined areas were investigated using light optical microscopy, and scanning electron microscopy with EDS analysis. The oxide on the fine-grained specimen was also investigated with grazing incidence X-ray diffraction. Results from coarse-grained and fine-grained specimens (machined and non-machined) show that grain size influenced oxide thickness and morphology. The oxides from non-machined specimens had an outer iron rich oxide and an inner iron chromium oxide. However a thinner oxide had grown on the fine-grained steel. The machining of fine-grained and coarse-grained specimens resulted in a thin chromium rich oxide layer. The presence of surface deformation of the inner metal surface was evident on the coarse-grained specimen but not on the fine-grained specimen. However other indications that the fine-grained specimen had been deformed were observed. The influence of temperature, grain size, surface finish and exposure duration is discussed.

Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes

Abstract X20CrMoV121 is a 12%Cr martensitic steel which has been used in power plants in Europe for many decades. Superheater tubes exposed for various durations up to 135,000 hours in power plants in Denmark at steam temperatures varying from 450 to 575°C were investigated. Light optical and scanning electron microscopy was used to investigate steamside oxide morphologies. At all temperatures there is a double layered oxide, however, the inner:outer oxide thickness is not always equal. At the lower steam temperature range of <500°C, there is an internal oxidation zone at the oxidation front indicating that chromium is less mobile at these temperatures. At a higher steam temperature range of 540 – 575°C the inner oxide consists of chromium rich and chromium poor oxide running parallel to the oxidation front indicating that the chromium is more mobile within the steel. Both types of morphology have been observed in the laboratory, however the internal oxidation is observed up to 600°C and the chromium rich oxide striation are observed at 700°C.