Field test of superheater corrosion in a CFB waste boiler: Part II - Scale formation characteristics

This study concerns the scales formed on the steels T22, Alloy 310, Alloy 28 and the nickel-based Alloy 65 in a superheater test coil at 460-540\ub0C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe 2 O 3 and Fe 3 O 4 on the T22 steel, NiFe 2 O 3 and Cr 2 O 3 on Alloy 310 and Alloy 28, and Cr 2 O 3 and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine-containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65. \ua9 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim