Ferritic-martensitic steels: Improvement of the oxidation behavior in steam environments via diffusion coatings

D. Schmidt, M.C. Galetz, M. Schütze

New generation heat-resistant ferritic–martensitic steels are of great interest as superheater materials in fossil fuel power plants. The oxidation resistance of these chromium steels (9 wt.%) in water vapor containing environments above 600 °C still requires improvement. One major problem is the breakdown of the chromia layer by formation of volatile chromium species CrO2(OH)2 promoted by water vapor in the atmosphere. Both, the fire-side as well as the steam side of the superheater tube can be affected due to high contents of water vapor in these atmospheres.

With the goal to operate ferritic–martensitic steels under such conditions, enrichment of the substrate with chromium and manganese was carried out using three different pack cementation processes: pure chromium coating, 2-step chromium and manganese process and simultaneous deposition of chromium and manganese.

All three surface-treated alloys show exceptional stabilities in steam environments, as illustrated by oxidation exposures in argon/50% water vapor at 650 °C for 3000 h. The pure chromium coating on the P91 substrate revealed the best oxidation behavior in steam.

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