Copper content in boiler feed water
Inexpensive determination using anodic stripping voltammetry (ASV)
Copper alloys are used in condensers and heat exchangers in the water-steam cycle because of their superior heat transfer properties. However, these materials are susceptible to corrosion from ammonia and dissolved oxygen. This can lead not only to stress-induced corrosion cracking but also to the loss of megawatts as copper deposits form on high-pressure turbine blades. Monitoring the copper levels in feed water helps to control corrosion before it negatively affects power plant efficiency. Anodic stripping voltammetry (ASV) is a sensitive and inexpensive method for determining copper in feed water.
For copper alloys in the boiler system, the primary causes of corrosion are the pH value (P-alkalinity) and dissolved oxygen. A deaerator and water treatment chemicals, such as sodium sulfite or hydrazine as oxygen scavengers, can be used to control the dissolved oxygen content in boiler water and condensate. However, hydrazine can decompose into ammonia, which is corrosive to copper. Because dissolved copper redeposits as metallic copper on steel surfaces, it causes galvanic corrosion of iron. Due to the negative effects of copper corrosion, the American Society of Mechanical Engineering (ASME) recommends a copper concentration of 0.01 to 0.05 ppm (mg/L) in boiler feed water depending on the drum pressure in its water quality guidelines. The limits set by EN 12953-10 are in a similar range of <0.03 to <0.05 ppm of copper in feed water and less than 0.1 ppm copper in make-up water.
Voltammetry is a sensitive method for the determination of metals such as copper or iron. Anodic stripping voltammetry (ASV) according to DIN 38406-16 allows the determination of copper concentrations down to 0.001 ppm. This method uses a mercury drop electrode. Alternatively, a gold microwire electrode can be used, which allows to reach a detection limit of about 0.0005 ppm copper. No sample preparation of the collected boiler water is required prior to the analysis for both methods. Compared to Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma (ICP), ASV is a viable, less sophisticated alternative for the determination of copper in boiler feed water with only a moderate investment in hardware and low running costs. To learn more about this method, download our free application documents about the analysis of copper in water samples.