The Effect of Pre-Oxidation on High-Temperature Corrosion Resistance of Superheater Steels
Dual samples of three genuine superheater steels; a low alloy ferritic 10CrMo9-10 steel, a Nb-stabilized austenitic AISI347 steel, and a high alloy austenitic Sanicro 28 steel, were first pre-oxidized under dry conditions at various temperatures (200°C, 500°C, and 700°C) for either 5 or 24 hours. After the pre-oxidation one sample piece of each steel was studied with X-ray Photoelectron Spectroscope (XPS) and with a scanning electron microscope coupled to an X-ray detector (SEM-EDX), whereas the other sample piece was exposed to KCl at 550°C for 168 hours. After the salt exposure the samples were cast into epoxy, cut for cross-sectional samples and analyzed with SEM-EDX. The XPS measurements provided information about the chemical composition and the thickness of the thin oxide layers formed during pre-oxidation, whereas the morphology of the surface and chemical composition of small surface features were studied with SEM (Fig. 1, left). Average oxide layer thickness, structure, and chemical composition were measured and analyzed from the cross-sectional samples with SEM (Fig. 1, right).
Figure 1. SEM image with chromium oxide nuclei formed on the steel surface after the pre-oxidation (left) and an SEM image of the cross section of the same steel after exposed to KCl (right).
According to the preliminary results, pre-treatment conditions affect the average oxide layer thickness, its structure and chemical composition. Most importantly, the resistance towards KCl-induced high-temperature corrosion was improved with pre-oxidation carried out under certain conditions. The final results are expected to provide more tools to material designers when developing novel materials with improved abilities to withstand material degradation in hostile environments.