Results of failure examination of chromized reheater tubes from a 2400 psig (16.5 MPa) coal-fired boiler operating typically at 1000°F (538°C) are presented. The failed tubing was in service for only 8 years. Key events in the service life of the tubes including cleaning with an alkaline detergent following one year-storage prior to installation are reviewed. The tubes were examined using scanning electron microscopy/X-ray energy dispersive analysis, stereomicroscopy, and optical metallography techniques. The findings show that reheater tubes failed due to corrosion fatigue cracking initiated on the internal chromized layers which were exposed to sodium and sulfate ions. In addition, chromized layers were found to be thicker than the layers typically formed during the chromizing of the steamside surfaces of superheater and reheater tubing.
Keywords: chromized reheater tubes, ferritic steel, Alloy T22, corrosion fatigue cracking.