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Pitting is a form of localized corrosion that initiates by breakdown of passivity at the tube surface. An electrolytic cell is formed between active metal (anode) and passive metal (cathode). Dissolved metal ions in the cell react with halide species such as chlorine to form metal hydroxides and acid. This reaction lowers the overall pH of the cell and accelerates further corrosion.
Pitting produces perforations of the tube wall through localized corrosion on the internal surface. Craterlike corrosion sites develop from galvanic activity and may have a red or rusty appearance due to the production of Fe203 (hematite) corrosion products.
Pitting is caused by exposure of tube metal to water with a high concentration of oxygen or pH excursions. In economizer tubing, the cause of pitting is likely to be high levels of oxygen in the feed water entering the economizers during boiler startups and low load operating periods. In Reheater tubing, the cause of pitting is likely to be the collection or pooling of condensate in bends and low points in horizontal sections during start up/shut down.
The root cause of pitting can be verified by monitoring the level of oxygen entering the economizer, especially during startups and low load operating periods. The cause of the high oxygen concentration may be a malfunction of the deaerator, inadequate injection of oxygen scavenger chemicals, or excessive air in-leakage in low pressure heaters, condensate piping, and the main condenser. During boiler outages, inadequate draining and standby protection methods can be verified by tube sampling to reveal the formation of any localized corrosion cells.
Corrective actions involve control of the oxygen level in the economizer feed water and protection of the tubing during non-operating periods. When oxygen levels exceed 10 ppb in the feed water, steps must be taken to immediately locate and isolate the source.
Ultrasonic (UT) tube wall thickness measurements are used to detect wall thinning that results from localized corrosion. Access to the tube surface for a complete wall thickness survey is limited in some cases. External surface preparation is necessary since the UT transducer must traverse this surface in search of wall penetrations.
Pitting can occur anywhere in the boiler including economizers, Superheaters, Reheaters, and the non-heated portion of water wall tubes. Locations where high levels of oxygen can be present are most likely to experience pitting. Inlet tubing in economizers and low points in Reheater tubing where condensate can collect during outages are locations which have frequent pitting problems. Horizontal Reheater tubing is most susceptible due to the relatively thin wall thickness of the tube and the direct exposure of the internal surface to the atmosphere during unit outages.