Lab-Scale Studies on the Activation Energy Regarding the Surface Alteration of Super-Heater Materials in Contact with KCl at Elevated Temperatures
The aim of this work is to study the activation energy of chromium oxide alteration on the surface of genuine heat exchanger materials (high-alloy stainless steel Sanicro 28 and low-alloy ferritic steel 10CrMo) in the presence of potassium chloride. The activation energy is defined as the sum of electronic and thermal free energies, and can be calculated from data obtained using chronoamperometric techniques, varying temperature and set potential. These results have been combined with surface specific information obtained by using X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM-EDX). With these techniques, the reaction current under various conditions (Eapp, T, t) has been measured and the depth profile together with the chemical composition of the formed oxides has been determined.
At the very early stage of the oxidation reaction (t≈1 min) substantial amounts of both Cr6+ and Cr3+ could be detected by both XPS and CA (as a continuing oxidation current). An example of a set of CA-measurements is presented in Figure 1. Therefore, it seems apparent that oxidation of trivalent chromium to hexavalent chromium takes place despite temperatures well below the melting point of the used salt. For this solid-solid interaction, assuming a linearly changing reaction quotient (Q) under each condition, the activation energy can be calculated. However, due to overlapping reactions, the calculated activation energy reflects the sum reaction rather than one individual reaction, albeit the reaction with the highest activation energy will dictate the calculated value. Based on these results, it is apparent that the changes in the Cr6+/Cr3+-ratio and oxide layer thickness can be determined and activation energy for the sum reaction can be calculated in selected points at the very early stage of the reaction. This information can potentially increase the understanding of the overall reaction mechanism of KCl-induced corrosion at elevated temperatures.