After ten years of satisfactory operation, economizer-tube failures occurred in a large black liquor recovery boiler for a paper mill. The economizer contained 1320 finned tubes. Two fins ran longitudinally for most of the tube length and were attached by fillet welding on one side. The economizer tube leaks occurred at the end of the fin near the bottom of the economizer. A sample from a tube that had not failed showed heavy pitting attack on the inside of the tube, probably due to excess oxygen in the feedwater. Penetrant testing revealed numerous longitudinal cracks on the inside in...