Microstructural Analysis of the Brass Seat of a Valve from the 1907 Steam Tug 'Hercules'

摘要

The steam tug Hercules was an ocean-going and bay tug for 55 years before being retired. It is now being restored by the National Park Service (as of 1993 printing of this article). A broken steam valve was obtained for microstructural examination. The body was gray cast iron, and the stem and seat were brass. The examination centered on corrosion of the brass components. The seat and shaft were a brass, with a hardness of 64 and 79 DPH, respectively. A nut held the shaft onto the seat and was α-β brass with a hardness of 197 DPH. Welded on the end of the shaft was a ring of hard (DPH 294) α-β brass, which seated against the nut. The brass seat and stem showed little corrosion. However, the α-β brass nut and welded tip show extensive dezincification. This process of removal of Zn and the retention of Cu began in the high Zn β phase, but eventually both phases were attacked. The depth of penetration was consistent with dezincification rates reported in the literature for such brasses in salt water if the valve had been in service about 55 years.