I have encountered a problem with corrosion of my 5/8", SB111 admirlaty brass heat exchanger tubes and need some insight into possible causes. I have two failure reports both identifying pitting/plug dealloying (dezincification). I have found the following contributors to dezinc... 1. under-deposit 2. stagnant flow 3. chlorides in the water 4. Microbiolical issues 5. slightly acid or alkaline water and low pH 6. water with high oxygen Am I missing any? I have run most of these to ground and none of the condition occur. Although, part of the problem is that these causes are not quantified (pH of ?, high oxygen?) so it is possible I am experiencing some of the above. It was also suggested that I may have a galvanic reaction occurring. The tubesheet and HX heads are 316ss and the shell is carbon. The pitting I am seeing occurs at varous points along the length of the tube. Would a galvanic reaction only occur at the joint between the tube and the tubesheet or would you experience metal loss at various points down the tube? The failure reports indentify pitting as a failure. What cause pitting in the tubes?

les..... I have encountered similar problems with powerplant HXs with SB-111 tubes. After evaluating several alternatives, fixes, water chemistry changes, it was decided to replace the entire HX with one constructed using Type 439 ferritic stainless steel tubes. We had a vendor use a commercial HX design program to "re-design" based on the existing duty. Because SB-111 tubes have such a high HT coefficient compared to the SS tubes, we modified the design slightly (smaller tube diameters, slightly longer bundles) and installed the new replacements. They worked very well and no follow-up inspections were necessary. The 439 tubes are described as: "Allegheny Ludlum Stainless Steel Type 439, a titanium stabilized, 18 percent chromium alloy, also known as ASTM XM-8 and by the UNS designation S43035, is a ferritic stainless steel designed to resist corrosion in a variety of oxidizing environments from fresh water to boiling acids......Excellent resistance to stress corrosion cracking, good weldability, high thermal conductivity and low thermal expansion characteristics make Type 439 stainless steel and ideal consideration for many applications. For shell and tube heat exchangers where carbon steel shells are used with Type 439 tubes the close match of thermal expansion coefficients may possibly eliminate the need for an expansion joint in the heat exchanger. " http://www.matweb.com/search/SpecificMaterialText.asp?bassnum=NL4394 While I did not mean to make this post a commercial recommendation, we did go on to replace several HXs with the new tubes and had excellent luck after that. -MJC