In Situ Investigation of the Initial Stages of KCl-Induced Corrosion of a Chromia-Forming Steel at 450°C Using an Environmental Scanning Electron Microscope

In the present work, the initial stages of KCl-induced high-temperature corrosion of a chromia-forming alloy (42Fe25Ni22CrWCuNbN), UNS S31035, is investigated by means of in situ environmental scanning electron microscope (ESEM) exposures in an O-2/H2O gaseous environment up to 1 h at 450 degrees C. Prior to the exposures, KCl particles were sprayed on the samples. In order to perform reliable in situ ESEM exposures, temperature calibration of the ESEM hot stage was done. Additionally, the in situ results were validated by reference tube furnace exposures, and the feasibility of the ESEM technique for obtaining dynamic information regarding the chlorine-induced corrosion process of the steel is discussed. Relatively large oxide crusts, with sizes of several micrometers, formed in the vicinity of the KCl particles. In between the KCl particles, a thin double oxide scale (20 nm to 100 nm), consisting of a top layer rich in iron oxide and a bottom layer rich in chromia, developed all over the surface of the alloy. Metal chloride was present beneath the thin base oxide, at the alloy/oxide interface, which shows the corrosive nature of KCl toward UNS S31035 in the studied environment. However, no extensive corrosion occurred at these locations after the short exposure times used in this work (1 h).