Gaseous corrosion resistance of Fe–Al based alloys containing Cr additions: Part II. Scale morphology

Abstract

Iron–aluminum based weld overlay claddings are currently being considered as corrosion resistant coatings for corrosion protection at 500 °C in aggressive sulfidizing and oxidizing atmospheres. These alloys rely on the formation of a thin passive Al 2O 3 layer for their corrosion protection. Once the alloy can no longer maintain the protective oxide scale, passive layer breakdown can occur, where non-protective nodules form at random locations along the surface of the sample. These nodules can continue to grow together and eventually overgrow the protective oxide rendering the sample no longer corrosion resistant. In this current study, 10 iron–aluminum based alloys containing chromium and titanium additions were exposed to three high-temperature corrosive environments at 500 °C for 100 h. The passive layer breakdown phenomenon was observed in all three environments on several non-protective alloys. The extent of nodule growth was measured and used to identify corrosion resistant alloy compositions. It was found that aluminum and chromium additions help to form and maintain a protective oxide layer and critical alloying contents were required to prevent passive layer breakdown during 100 h of exposure. Critical alloying values necessary to prevent nodule formation were compared to critical alloying levels required to prevent rapid corrosion kinetics. While corrosion kinetics are helpful in determining protective alloy compositions, the corrosion scale morphology must be characterized to ensure that the alloy is completely protective during the exposure time.