The influence of chromium on early high temperature corrosion of ferritic alloys under SO₂ atmosphere

Abstract

Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650°C to observe the influence of chromium during the corrosion under SO₂. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron-sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt-% Cr shows hematite and mixed Fe–Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterisation of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.

Keywords:

• Sulphidation
• early stages
• high temperature corrosion

Acknowledgement

The authors thank Artur Göbel for help with the experiments, Nicole Wollschläger for the FIB cut, Romeo Saliwan Neumann for SEM images, Christoph Berthold and Tobias Kiemle from the Eberhard-Karls-Universität Tübingen for their assistance with μXRD² measurements, and Wencke Schulz for valuable discussions.