Since most high-temperature materials are optimised with regard to strength and corrosion resistance at elevated temperatures, their resistance to electrochemical low-temperature corrosion may be less satisfactory. Components made of high-temperature material should therefore be designed and operated so that acid condensates are not formed, or at least so that any such condensates are drained away.
As 4878 is a titanium-stabilised grade, it will probably show the best resistance to aqueous intergranular corrosion.
The resistance of a material to high-temperature corrosion is in many cases dependent on its ability to form a protective oxide layer. In a reducing atmosphere, when such a layer cannot be created (or maintained), the corrosion resistance of the material will be determined by the alloy content of the material. Below, a number of high-temperature corrosion types are treated. However, industrial environments often contain a mixture of several aggressive compounds, so the choice of material will, as a rule, have to be a compromise. All these austenitic steels have a greater thermal expansion and a lower thermal conductivity than ferritic stainless steels. This will result in greater thermal stresses when the temperature changes rapidly – thermo-shock – which must be taken into account during design and operation.
When a material is exposed to an oxidising environment at elevated temperatures, a more or less protective oxide layer will be formed on its surface. Even if oxidation is seldom the primary cause of high-temperature corrosion failures, the oxidation behaviour is important, because the properties of the oxide layer will determine the resistance to attack by other aggressive elements in the environment.The oxide growth rate increases with increasing temperature until therate of oxidation becomes unacceptably high or until the oxide layer begins to crack and spall off, i.e. the scaling temperature is reached. The alloying elements that are most beneficial for oxidation resistance are chromium, silicon, and aluminium. A positive effect has also been achieved with small additions of so-called (re)active elements, e.g. ttrium, hafnium, rare earth metals (REM, e.g. Ce and La). These affect the oxide growth so that the formed layer will be thinner, tougher, and more adherent and thus more protective.
Various sulphur compounds are often present in flue gases and other process gases. As a rule, they have a very detrimental effect on the useful life of the exposed components. Sulphides can nucleate and grow due to kinetic effects even under conditions where only oxides would form from a thermodynamic point of view. In existing oxide layers, Fig. 2. Long-term oxidation at 1100°C. The specimens were cooled down to room temperature once a week for weighing => 165 h cycles. attacks can occur in pores and cracks. It is therefore essential that the material is able to form a thin, tough, and adherent oxide layer. This requires a high chromium content and preferably
also additions of silicon, aluminium, and/or reactive elements.Sulphur attacks Various sulphur compounds are often present in flue gases
and other process gases. As a rule, they have a very detrimental effect on the useful life of the exposed components. Sulphides can nucleate and grow due to kinetic effects even under conditions where only oxides would form from a thermodynamic point of view. In existing oxide layers, Fig. 2. Long-term oxidation at 1100°C. The specimens were cooled down to room temperature once a week for weighing => 165 h cycles. Attacks can occur in pores and cracks. It is therefore essentialthat the material is able to form a thin, tough, and adherent oxide layer. This requires a high chromium content and preferably also additions of silicon, aluminium, and/or reactive elements.