Critical Deposit Loading Thresholds for Under Deposit Corrosion in Steam Generators

Significant industrial facilities typically operate a steam system with multiple steam generators (or boilers) that are potentially vulnerable to under deposit corrosion (UDC). This mechanism begins with the deposition of a porous layer of magnetite particles on the waterside heat transfer surfaces. Beneath this layer, wick-boiling causes the concentration of contaminants (such as chlorides) until the concentration exceeds some critical threshold that results in rapid corrosion. Industry practice to mitigate this risk includes monitoring the extent of deposition and then chemically cleaning the equipment before the deposits reach the critical level. One significant factor in this mechanism is the heat flux, which drives both the magnetite deposition and the wick-boiling processes. This paper investigates the relationship between heat flux and the critical deposit thickness for the initiation of chloride-driven UDC. Ex-service samples from industrial steam generators have been examined, an electrochemical method has been developed for in situ measurement of the deposit loading, and a wick-boiling model has been used to estimate the local conditions beneath the porous deposits during the progress of UDC. For 1.0 Cr 0.5 Mo steel coils from a steam generator operating at approximately 75 bar(a) under an all volatile treatment regime, the threshold for the onset of rapid UDC was determined to be at a heat flux of 350 kW/m2, deposit loading of approximately 50 mg/cm2, and deposit thickness of approximately 95 µm, corresponding to a local chloride concentration beneath the deposits of approximately 80 ppm and a local at-temperature pH below ∼2.7.