• Ali-Löytty, Harri

Abstract

Ferritic stainless steels (FeCr-based alloys with low Ni content) have gained recent interest due to their excellent corrosion resistance, mechanical strength, and competitive price, which make them an attractive choice for many energy applications, for example, energy conversion and exhaust systems. The corrosion resistance results from the Cr-rich protective oxide layer that forms spontaneously under oxidizing conditions. At elevated temperatures, oxidation resistance can be enhanced by controlled surface treatments or by microalloying with elements that affect the oxide layer formation through segregation and interfacial oxidation. Today, further alloy development is required concerning the high-temperature oxidation resistance due to the increased operation temperatures. Furthermore, new alloy materials that form an electrically conductive non-volatile oxide layer under high-temperature conditions are required for the solid-oxide fuel cell interconnect applications.In this thesis, the segregation and oxidation phenomena on non-stabilized and Ti–Nb stabilized ferritic stainless steel alloys were investigated at 50–800 °C by photoemission spectroscopy, inelastic electron energy-loss background analysis, and electrochemical impedance spectroscopy. Firstly, the influence of controlled surface treatments on the initial stages of oxidation was investigated. The surface enrichment of Cr was induced by H2O preadsorption at low temperatures and by thermally induced cosegregation with N at high temperatures. The Cr-enriched surface was found beneficial against further oxidation by O2, but the effect was the most pronounced at low temperatures where the thermal diffusion of ions is not fast enough to support the oxidation.Secondly, microalloying with Nb was shown to improve the electrical properties of ferritic stainless steel alloys at 650 °C. The role of excess Nb was attributed to its high segregation rate and formation of conductive oxides at the oxide–metal interface. Furthermore, the Nb alloying induced the formation of (FeNbSi)-type Laves intermetallic phase in the alloy, which resulted in the non-uniform distribution of electrically resistive SiO2 at the interface. Therefore, the results presented in this thesis can be applied to design ferritic stainless steel alloys and surface treatments that facilitate the formation of the protective oxide layer with the optimum composition under various demanding application conditions, particularly, in the solid-oxide fuel cells.

Topics

• impedance spectroscopy
• surface
• stainless steel
• corrosion
• phase
• strength
• intermetallic