Despite more than 15 years of operational reliability, a 317L (UNS S31703) stainless steel steam surface condenser at a power plant in the Northeastern United States was revealed to have a significant waterside pitting corrosion problem. The pitting was identified during Eddy Current testing and verified by removing sample tubes. The severe pitting in the tubes was primarily focused at the seam weld region. Possible causative factors for the pitting attack were identified. Metallography and electrochemical testing confirmed non-optimal corrosion resistance along the original laser seam welds, as compared to the base metal. Modifications to the cooling water chemistry were instigated to reduce the severity of the pitting attack in an effort to extend the service life of the condenser bundle. Re-tubing options with alternative stainless steel alloys are also discussed.
The power plant is a natural gas-fired, combined cycle plant with three combustion turbines and a single steam turbine. A large stainless steel surface condenser is used to condense steam off of the turbine, and provide high purity steam condensate return to the boiler system. The steam condenser was put into service approximately 15 years ago. This plant takes makeup water for its open recirculating cooling tower water system from a river location that is inland from an ocean coastal area. When the plant was built, the tubing material selected for the main steam condenser was 317L (UNS S31703). This was likely done to assure additional corrosion resistance due to the expected higher chloride content of the cooling water. The higher chloride content in the cooling water can develop during tidal incursion of seawater into the river, and/or drought or seasonal variation, which can impact the relative salinity of the make-up water to the cooling tower system.
This is a medium dispatch power generation plant, operating approximately 16 hours per day. The cooling water flow circulation and water treatment have been maintained at all times, even when the plant is not generating power. There has never been a report of a condenser leak, but it has been known for years that there is some pitting in the condenser tubes. However, the extent and location of the pitting damage were not thoroughly investigated until recently.
