The Impact of Humidity on High Temperature Corrosion of Ferritic-Austenitic Model Alloys in the Presence of KCL

Abstract

This study investigates the impact of humidity on the corrosion behavior of an Fe–18Cr–12Ni alloy in environments containing KCl, as a deposit, and either laboratory air or ${\text{SO}}_2$ at 560°C for exposure times up to 330 h. Corrosion characteristics were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD) with subsequent Rietveld analysis to quantify the obtained reaction products. Samples exposed to lab air and KCl showed half the weight gain in humid air than in dry atmospheres ($0.80\frac{\text{mg}}{{\text{cm}}^2}\pm 0.06\frac{\text{mg}}{\text{cm}{}^2}$ at 30.00% $\pm$ 0.07% relative humidity (r.H.) vs. $1.63\frac{\text{mg}}{{\text{cm}}^2}\pm 0.01\frac{\text{mg}}{\text{cm}{}^2}$ at 1.58% $\pm$ 0.09% r.H.). The surface porosity, as determined by image analysis of SEM surface images, was lower for the samples exposed to humid air compared to dry air (5.5% $\pm$ 2.1% vs. 11.0% $\pm$ 2.2%, respectively). In a SO₂ containing environment humidity decreased the scale thickness for deposit induced corrosion significantly by one order of magnitude, reflected by a low amount of oxide phases such as ${\text{Cr}}_2{\text{O}}_3,{\text{Fe}}_2{O}_3$, and ${\text{Fe}}_3{\text{O}}_4$. This study shows that humidity present in a hot gas containing other corrosive species, such as SO₂, can be beneficial for KCl deposit induced corrosion of stainless steels.