자동차용 소음기의 부식특성에 관한 연구

Abstract

자동차의 소음기는 고온의 배기가스가 상온의 소음기 내부를 통과하면서 급격히 온도가 낮아지므로써 소음기 내부에 결로현상에 의해 응축수가 발생되고, 배기가스 중에 유황화합물, 산화물 등의 유해 가스는 응축수에 혼입되면 산성화된 응축수가 되거나 비저항이 낮아진 응축수에 의해 소음기의 부식손상은 더욱 가속되고 있다. 이러한 부식에 의한 손상은 가스가 누설되면서 소음이 높게 되고, 대기환경오염을 유발할 수 있다. 그러나 본 논문은 자동차용 소음기의 부식특성에 대한 연구를 하기 위하여, 전기화학적 분극시험장치에 의해 산성의 pH 변화 및 Cl이온에 따른 비저항 변화에 따른 응축수 환경 중에서 소음기 재료에 따른 부식 특성에 대해 각 응축수 환경조건에 따른 소음기용 기계재료로 주로 사용되고 있는 알루미늄합금재 및 페라이트계 스테인리스강재의 전기화학적 분극저항, 부식전류밀도, 전면부식거동의 내식성을 평가하고, 공식과 틈부식의 국부부식 특성을 구명(究明)함으로써 자동차용 내연기관 소음기의 부식과학, 최적화 설계, 제작 및 보수 유지의 기초자료로 활용하는데 기여하고자 한다.

According to the industrial and economic development, the consumption of fossil energy like coal, petroleum and natural gas rapidly increases, and it accelerates the environmental pollution. Clearly, the car has been one of the notorious pollution sources - noise, exhaust pollutant gas, etc. Among them, a muffler is usually used for reducing noise from the exhaust gas of the engine. Inside a muffler a condensed water can be generated due to high temperature difference from the exhaust gas, and it can cause a corrosion damage on the surface. Especially, the condensed water, which sometimes be acid, makes these damages worse. In this study, therefore, we carry out the corrosion test in the acidified water with the aim to investigate the corrosion behavior around a muffler which is made of Al-alloy and stainless steel. The main results obtained in this study are as follows ; 1) As increasing SOX, both the pH of condensed water and the polarization resistance of muffler are reduced. 2) As the pH of a condensed water is high in the range of pH 2 to pH 6.2, the polarization resistance of stainless steel is higher than that of Al-alloy. 3) As decreasing the pH, the open-circuit potential decreases, whereas the density of corrosive current increases. 4) As the pH of the condensed water is low except for the condensed water of pH 0.5, the corrosion rate of Al-alloy and stainless steel become high. Therefore, the corrosion resistance for the uniform corrosion of a muffler decreases. 5) Under the natural, subacid and strong acid water, the corrosion current density of stainless steel is lower than that of Al-alloy. In the addition the anodic polarization movement of stainless steel can be classified into 3 representative regions. 6) As the positive hysteresis appears in the cyclic polarization curve of Al-alloy, the damaged passive film in the forward scan anodic polarization curve can not be repaired. However, as negative hysteresis occurs in the cyclic polarization curve of stainless steel, the damaged passive film in the forward scan anodic polarization curve can be repaired of itself. 7) As the repassivation potential of Al-alloy is lower than the open-circuit potential, pits grow. However, as the repassivation potential of stainless steel is higher than the open-circuit potential, pits don't.