The Effect of Pre-Oxidation on High-Temperature Corrosion Resistance of Superheater Steels

The possibility to raise steam temperatures in biomass- and waste-fired boilers has drawn increased attention, when suitable alternatives for the use of fossil fuels in power production are considered. Currently in straw-fired boilers, for example, maximum steam temperatures are commonly between 450°C and 485°C, depending on the composition of the superheater steel, as compared to 560-600°C in coal-fired boilers. Therefore it is obvious, that increased steam temperatures are required for biomass- and waste-fired boilers in order for them to converge the power production efficiency of coal-fired boilers. In fact, every 10°C rise in the steam temperature results in an approximate increase of 2% in power production efficiency. Unfortunately, higher steam temperatures can easily lead to higher corrosion rates of the superheaters due to aggressive species such as potassium chloride (KCl), which can be found in the deposits at the superheater surfaces. From the material point of view, corrosion can be reduced to a certain extent by modifying the chemical composition of superheater steels to enhance their resistance to corrosion. From the chemical point of view, the challenge can be approached by studying the corrosion reactions and mechanisms. Instead of preventing corrosion with expensive high-alloyed materials, possibilities such as oxide layer manipulation/passivation should be studied more thoroughly to find alternative and more cost-efficient ways to design materials with improved capability to resist corrosion. The aim of this work is to study, whether certain pre-treatment conditions could enhance the high-temperature corrosion resistance of steels used in power plants combusting renewable fuels. More precisely, the effect of such variables as pre-oxidation temperature, time and atmosphere will be addressed.

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.