선박용 Al-황동세관의 SCC에 미치는 부식환경의 영향
Effect of corrosion environment on the SCC of Al-brass tube for vessel
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Al-황동의 SCC특성에 미치는 부식환경의 영향에 대하여, 3.5% NaCl 수용액에 포함된 NH4OH 농도 변화에 따른 Al-황동의 분극특성, 응력부식균열 파면거동 및 탈아연현상에 대하여 연구한 결과 다음과 같은 결론을 얻었다. 1 수용액의 NH4OH 함유량이 증가할수록 개로전위로부터 재부동태전위로의 상승폭은 적게 나타남으로써 공식 및 간극부식은 더 활성화 될 것으로 판단된다. 2. 3.5% NaCl 수용액 중에서 황동의 Cu2O 보호피막이 형성되지만, 3.5%NaCl 수용액중에 NH4OH 함유량이 증가할수록 SCC는 활성화된다. 3. 수용액에 NH4OH가 함유됨에 따라 응력집중부에 국부적인 선택적 양극용해에 의한 탈아연현상이 일어나는 것으로 판단되며, 이러한 탈아연범위는 수용액의 NH4OH 함유량이 증가할수록 확산되는 양상이다.
Abstract
Al-brass is usually used as the tube material of vessel‘s heat exchanger for seawater cooling system because it has high thermal conductivity and good mechanical properties and high corrosion resistance due to cuprous oxide (Cu2O) layer against seawater. However, Al-brass tubes of heat exchanger for vessel at the actual environment is reported that local corrosion such as stress corrosion cracking occurred by synergism effect between mechanical factor and corrosion environment. In this paper, the effect of corrosion environment on the stress corrosion cracking of Al-brass in various NH4OH of 3.5% NaCl solution, under flow by constant displacement tester. Based on the test results, the behavior of polarization, stress corrosion crack propagation and dezincification phenomenon of Al-brass are investigated. The main results are as follows : (1) Increasing range of potential from open circuit potential to repassivation gets lower, as the contain rate of NH4OH gets higher. (2) As contain rate of NH4OH gets higher, SCC of Al-brass is become activation but the protection film(Cu2O) of Al-brass is created in 3.5% NaCl solution. (3) According as content of NH4OH increases in 3.5% NaCl solution, the dezincifiction area is spread. It is concluded that dezincification occurred by localized preferential anodic dissolution at stress focusing region.