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The use of dissimilar metal construction in power plant condensers is a common practice. It is not unusual to encounter condensers fabricated with stainless steel or titanium tubes, copper alloy tubesheets, and cast iron or carbon steel waterboxes. Furthermore, condensers initially furnished with copper alloy tubes and copper alloy tubesheets are frequently being retubed with stainless steel or titanium tubes. In several instances, the use of dissimilar alloys in power plant condensers has led to severe galvanic corrosion, particularly of the tubesheet.
The primary factors affecting galvanic corrosion in a condenser include the electrochemical characteristics (that is, potential and polarization) of the different alloys, the effective area relationships of the different alloys, and the specific characteristics of the cooling water (for example, conductivity, temperature, and so forth). A number of studies have been conducted to develop a better understanding of the relative influence and the interrelationship of these various factors. These studies, conducted under simulated condenser flow conditions, have shown that (1) the length of tube involved in a tubesheet/tube galvanic cell can extend beyond 6 m down the tube, depending on the tube alloy and water salinity; (2) the use of either titanium or stainless steel tubes with a copper alloy tubesheet can result in significant galvanic corrosion of the tubesheet; (3) galvanically coupling a Muntz metal tubesheet to either stainless steel or titanium can cause severe dezincification of the tubesheet; (4) the intensity of galvanic attack will tend to diminish as the salinity of the cooling water decreases; (5) the intensity of the galvanic attack will tend to diminish as the cooling water temperature decreases; (6) certain copper alloy tubesheets may be galvanically compatible with either stainless steel or titanium tubes if the cooling water salinity or temperature is sufficiently low; (7) as a tubesheet alloy coupled to either stainless steel or titanium tubes, aluminum bronze (Alloy D) is significantly less susceptible to galvanic attack than Muntz metal; (8) cathodic protection properly implemented can effectively mitigate galvanic attack of copper alloy tubesheets caused by more noble alloy tubes, and (9) the cathodic protection current required to mitigate galvanic corrosion will be greater in higher salinity, higher temperature cooling water.
steam condensers, galvanic corrosion, simulated testing, copper alloy tubesheets, stainless steel tubes, titanium tubes, cathodic protection
President, Ocean City Research Corporation, Ocean City, NJ