Mechanism of metal dusting corrosion by pitting of a chromia-forming alloy at atmospheric pressure and low gas velocity

摘要

FeNiCr samples (800HT) were exposed at 570 degrees C, 1 bar to a 47.25CO-47.25H(2)-5.5H(2)O atmosphere (a(c) = 33) flowing at 18 mu m/s. Pitting corrosion was observed. Pits showed a flattened morphology and a constant pit diameter growth rate. Corrosion rings appeared successively at the surface during pit growth. A four step mechanism is proposed which includes internal oxidation of carbides, graphitisation and localised enhanced graphitisation. Gas velocity and thermal cycling play key roles in pit morphology. Thermal cycling induces circular cracks. Low gas velocity induces the gas to evolve in crevices, due to local oxygen consumption. (C) 2016 Elsevier Ltd. All rights reserved.