It is essential for materials used at high-temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high-temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low-Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni-base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550 8C in the case of low-alloy steels, 750 8C in the case of an austenitic steel (TP347) and up to 1000 8C in the case of the Ni-base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low-Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short-circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr 2 O 3 with some FeCr 2 O 4 at localized sites, while for specimens with larger grain size a non-protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni-base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high-angle grain boundaries.

The global pressure for recycling and ecological energy production increases steadily in combination with the demand of cost-effective application of materials. However, some severe corrosion problems, associated with the high internal/intergranular corrosion rates in boiler components need to be avoid. Some commercial boiler materials contain a Cr content of 0.55 (wt. (%)) - 2.25 (wt. (%)). This Cr concentration in the alloys is not sufficient for the formation of a complete dense Cr2O3 scale. Hence, high oxidation kinetics may result. In this study, pure Fe and the steel 2.25Cr1Mo were oxidized in laboratory air at 550 °C using a thermobalance system. The surface as well as the cross section of oxidized specimens were analysed using scanning electron microscopy in order to quantify several factors (e.g. surface finishing, cold working and grain size) on the overall oxidation kinetics. For alloys with low Cr content, a decreasing in the grain size leads to an acceleration of the oxidation rate by facilitating the oxygen diffusion along alloy grain boundaries leading to an inward oxide layer formation. The study of effects of surface finish and cold working yielded results revealing that the oxidation process is complex and comparison of results from different laboratories is difficult and should be done

Oxidation of two low-Cr (Cr content 1.5 wt% and 2.25 wt%) and three high-Cr (Cr content 9 wt%, 12 wt% and 18 wt%) boiler steels was investigated at temperatures between 550°C and 830°C in laboratory air. Thermogravimetry (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied for evaluation of the oxidation kinetics, oxides phase identification and scale structure analysis. Particular attention was paid to the phenomenon of inward oxidation and its relationship with the Cr content of the steels under investigation. The results showed that the inward oxidation of the Cr steels is governed by grain boundary diffusion. Two different Cr-depending tendencies concerning the effect of alloy grain size on the inward oxidation were observed. For low-Cr steels (less than 2.25 wt% Cr), an increase in the grain size improved the oxidation resistance, while steels with high Cr content (18 wt% Cr) can form a thin and protective chromia scale on the surface more easily at finer grain size. In the latter case an increase in grain size deteriorates the oxidation resistance by the formation of a thicker scale composed of both an outer and an inner Fe-oxide-based layer.