Selective Leaching of Brass: Electrochemical Aspects

Brass, an alloy of copper and zinc, has been used in various industries due to its higher malleability than bronze and zinc as well as its good flow characteristics for casting. In Korean nuclear power plants(NPPs), brass has been mainly used for tubes in some older model heat exchangers and some valve bodies in the fire protection systems. Notably, however, the use of brass may lead to selective leaching, one of corrosion phenomena. Selective leaching is the preferential removal of specific alloying elements from a solid alloy via corrosion processes, leaving behind a weakened spongy or porous residual structure. Materials that are known to be susceptible to selective leaching include gray cast iron and copper alloys that contain greater than 15 percent zinc. Selective leaching proceeds slowly during a long period of time and causes a decrease in strength without changing the overall dimensions of original material, and consequently is difficult to identify. For this reason, as part of the One-Time Inspection Program according to the requirements of the guidelines for continued operation of pressured water reactors (PWRs) in Korea and license renewals in the United States, entitled the 'Generic Aging Lessons Learned (GALL) report,' visual inspection and hardness measurement should be carried out to confirm that selective leaching exerts no effects on the structural integrity of the related components for continued operation of NPPs. In the newly released GALL report, the quantitative acceptance criteria for hardness are given: no more than a 20 percent decrease in hardness for gray cast iron and brass containing more than 15 percent zinc. In the present work, the selective leaching of brass is investigated in terms of electrochemical aspects as part of an ongoing research project to study the changes in metal properties by selective leaching