Continuous and cyclic oxidation of T91 ferritic steel under steam

Abstract

The oxidation behaviour of T91 ferritic steel in steam has been studied under isothermal and non-isothermal conditions within a temperature range between 575 and 700 °C. Isothermal treatments resulted in parabolic oxidation kinetics. Three clearly defined oxide layers constituted the oxide scales. The innermost layer was a (Fe,Cr)₃O₄. The intermediate layer was porous magnetite (Fe₃O₄) followed by a compact thinner layer of hematite (Fe₂O₃). Under non-isothermal conditions the oxide scales were irregular and evidently cracked. An increase of the oxidation temperature produces an acceleration of the oxidation process, causing an increase of the oxide scale thickness that depends on the temperature increase and the exposure time.

Introduction

Ferritic steels, containing chromium and molybdenum are well known for their excellent mechanical properties combining high temperature strength and creep resistance with high thermal fatigue life, as well as with good thermal conductivity, weldability, and resistance to corrosion and graphitisation. Because of these characteristics this type of steels have attracted special interest for application in industrial processes related to carbochemistry, oil refining, carbon gasification and energy generation in thermal power plants, where components like, heat exchangers, boilers and pipes operate at high temperatures and pressures for long periods of time [1], [2]. Amongst these the modified T91 steel, containing 9Cr–1Mo with small additions of V and Nb, favourably compares to the austenitic grades, for instance the AISI 316 or 304 types, because of its better mechanical properties that allows it to support higher stresses at operating temperatures up to 600 °C [3]. Also, because of its higher rupture stress, as compared for instance to the $\text{2}\text{1}\text{4}$Cr–1Mo ferritic steel, a reduced wall thickness may be used resulting in important weight reductions and savings in the welding process [2], [3], [4], [5], [6].

Additionally, during high temperature exposure the interaction between a metal or an alloy and the surrounding gases and combustion products leads to corrosion, which is one of the main causes of failure for materials and structures [7], [8], [9]. One corrosion mechanism of this type is the oxidation of materials operating in oxygen rich environments, which causes chemical attack by reaction with the oxygen contained in the surrounding media. The kinetics of the oxidation process, as well as the characteristics of the oxidation products are critically dependent on the oxygen partial pressure of the aggressive environment [9], [10], [11], [18]. A particular case is found in thermal power generating plants where metallic materials in the heater and reheater zones are subjected to temperatures between 500 and 600 °C in contact with steam.

Given the importance of oxidation on the design, optimisation, safety and the performance of power generating plants during their lifetime, extensive research has been devoted to study the oxidation behaviour of chromium containing steels in contact with steam [12], [13], [14], [15], [16], [17], [20]. In these works, it is commonly reported that as a result of the oxidation process under isothermal conditions a protective Cr-containing oxide is developed on the surface of the steel causing a decrease of the oxidation rate with time. There also seems to be wide agreement that the oxidation resistance of these alloys is appreciably reduced in steam as compared to that in air. Additionally, several reports confirm that the oxide scale is constituted by a layered structure with compositional and microstructural variations from the substrate to the outer interface [12], [13], [14], [15], [16], [20]. On the other hand, depending on the oxidation temperature and the chemical composition of the steel, both, the mechanisms of formation and the microstructural characteristics of the oxide scale, along with the degree of protection it provides, are reportedly different.

The present report is intended as a contribution to the knowledge of the oxidation behaviour of the T91 ferritic steel in a steam saturated atmosphere under cyclic and isothermal conditions. Especially designed experiments along with the microstructural characterisation of the oxide scales formed, were carried out in an attempt to understand more fully the mechanisms of formation of the oxide scale.