冷煖房水用STS 304 配管鎔接部의 腐蝕特性및 그 防止에 관한 硏究

Abstract

최근 경제발전에 따라 각종빌딩, 대학등의 학교, 관공서, 병원, 호텔, 백화점 등과 같은 대형건물 및 선박에는 흡수식 냉난방 공조시스템이 거의 필수적으로 사용되고 있으며 이들 시스템은 각종배관에 의해 서로 연결되어 있고 이러한 배관은 인체의 혈관에 해당하는 것으로써 건축물을 건설하고자 할때 가장 중요한 설계요소중의 하나이다. 수도용아연도 강관 (KSD3537)의 부식으로 인해 많은 문제(녹물발생, 누수사고 등)가 발생되어 국가에서는 1994년 4월 이후 건축허가를 받아 시공하는 건축물의 음용수용 배관재료를 스테인리스강관 등 내식성 배관재 사용을 의무화 하였다. 이러한 규정 때문에 최근에는 급수관,급탕관에 스테인리스강관 사용이 증가하고 있으며 흡수식 냉난방 공조시스템의 냉난방수 배관에도 사용이 증가하고 있다. 각종배관은 접합공정상 용접이 불가피하고 특히, 스테인리스강관은 용접시 발생하는 열사이클 때문에 용접열영향부의 Cr탄화물 석출로 Cr결핍영역이 나타나 입계부식이 발생한다. 또한, 국민의 소득증가와 경제의 발전으로 화석에너지 소비 증가로 발생된 유황화합물(SOx), 질소화합물(NOx),CO,HCL 등의 대기오염가스가 대기중의 수분과 혼합됨으로써 황산,질산 및 개미산 등이 되어 전면부식이나 국부부식을 촉진시킬수 있다. 이와 같은 현상으로 인해 용수 및 가스 등이 누설되고 중단된다면 경제적손실과 인명피해도 초래할 수 있으므로 스테인리스강관 용접부위의 부식거동에 관한 연구가 계속되고 있다. 따라서 본 연구에서는 건축물이나 산업시설의 냉난방수, 급수 및 급탕배관 등에 주로 사용되고 있는 STS304강관의 모재와 용접부에 대해 전기화학적분극시험을 실시 전면부식과 국부부식에 대한 부식전위,공식전위 및 부식전류밀도에 미치는 부식특성, 환경조건변화와 용접조건에 따른 부식특성을 고찰하고 또한, EDX분석에 의한 용접부 부식원인을 규명하고 음극방식 실험을 통하여 부식방지특성을 고찰함으로써 부식/방식 과학분야의 발전과 더불어 각종 건축물 배관설비(냉ㆍ난방수배관과 급탕 및 급수배관)공사의 설계와 노후배관 보수 및 배관 수명연장의 기초자료로 활용하는데 기여하고자 한다.

The absorptive refrigerating and heating air-conditioning systems are necessarily used to a large building, ships etc., and these systems are connected by every kinds of pipes. These pipes play an important role as a blood vessel in a human body, it is the most important design element that we construct a building. The anti-corrosive pipes like copper alloy and stainless steel pipe and so on are liable to new construction law legislated in April 1994 to use with the piping materials of building for the drinking water. Specially, austenitic stainless steel is widely applied to various fields of industry, because it is good to corrosion resistance, mechanical properties and weldability. The welding of pipes is generally inevitable. With welding, the mechanical properties and corrosive resistance of welded zone greatly decreases more than base metal due to structural transformation, a change of local compositions and residual stresses by heating cycle on welding. Industrial development and income increase caused rapid increase in using of the fossil energy such as coal, petroleum and natural gas, and resulted in accelerating the environmental pollution. Specially, fresh water like stream and river water was acidified. The corrosion occurred in the heat exchanger and pipes of boiler and gas absorption refrigerating and heating systems is caused by using the refrigerating and heating water mixed air pollution substance. In this study, the electrochemical polarization test used potentiostat/galvanostat corrosion apparatus and cathodic protection test used galvanic anode are carried out to investigate on corrosion characteristics and corrosion protection for welding zone of STS 304 pipes. As the result of experiments, the uniform and the local corrosion behavior for base metal and welded zone of STS 304 pipes were considered as corrosion environment change and welding method. The composition analysis of welded zone of corroded STS 304 pipe is investigated by SEM/EDX, it was compared with that of base metal. Finally, the corrosion protection characteristics for welded zone of STS 304 pipe were investigated by Mg-alloy galvanic anode protection method. The main results obtained are as follows : 1) The corrosion current density of STS 304 pipe manufactured by arc welding is sensitive in order of base metal < weld metal < heat affected zone, and the pitting potential is also low in order of base metal < weld metal < heat affected zone. 2) The nitrogen composition of weld metal and heat affected zone was incredibly similar to base metal, but chromium composition decreased owing to arc welding. 3) Degree of sensitivity of HAZ showed about 0.05 from the electrochemical double loop polarization curves. In HAZ, intergranular corrosion and the pitting were occured. 4) The passive zone of heat affected zone by arc welding was smaller than that of base metal, and the pitting potential was decreased with increasing Cl-ion concentration. 5) With increasing chloride concentration, potential of welded zone of STS 304 pipe approaches ath the anodic potential of Evans polarization diagram and corrosion current density depends on reaction rate of cathode. Therefore, The corrosion reaction of welded zone makes the mechanism of the cathodic control. 6) The decrement of chromium composition for heat affected zone of STS 304 pipe manufactured by argon gas welding has lower than that of arc welding. Therefore, the degrees of the uniform and local corrosion in welding heat affected zone formed by argon gas welding were more restraint than those of arc welding. 7) Cathodic protection potentials of base metal, arc and argon welding zone used Mg-alloy galvanic anode were cathodicaily polarized below -770mV/SCE in tap water and 1000ppm chlorine solution. Therefore, the corrosion occurred in the welded zone of STS 304 pipe could be restrained by Mg-alloy galvanic anode method.