Abstract
This paper presents findings of an investigation into the cause of contamination in alloy 59 (UNS N06059) weld qualification. Qualification specimens were welded and tested as part of the welding procedure development supporting the on-site fabrication of a large flue gas desulfurization (FGD) absorber and associated storage tanks. The specimens were fabricated by welding together carbon steel plates clad with alloy 59, or welding clad plates to solid alloy 59 with UNS N06059 filler metal. During the course of welding procedure development, various welding processes and techniques were evaluated for optimum as-welded corrosion resistance of the weldment. The evaluation included corrosion testing in the Green Death solution, which is a modified ASTM G 28 B test (the sulfuric acid content is reduced from 23 % to 11.5 % whilst keeping other constituents the same). The results of these Green Death corrosion tests included several pitting failures, which prompted a metallurgical examination to determine the cause of attack. During this investigation, iron was identified on or near the weldment surfaces of specimens not yet exposed to corrosion testing, despite substantive welding procedure development specifically designed to limit dilution from the carbon steel backing material. Such iron contamination was subsequently identified on all-alloy qualification specimens as well. This investigation determined that iron contamination was introduced during weld interpass cleaning with stainless steel power brushes. This paper describes the occurrence and solutions to this problem.
