CORROSION SCIENCE AND TECHNOLOGY, Vol.16, No.3(2017), pp.115~127 pissn: 1598-6462 / eissn: 2288-6524 [Research Paper] DOI: https://doi.org/10.14773/cst.2017.16.3.115 국내원전에매설된콜타르코팅배관의음극방식과 FEM 법을이용한방식성능시뮬레이션 장현영 1 김기태 2 임부택 1 김경수 1 김재원 1 박흥배 1 김영식 2, 1 KEPCO E&C, 미래전력기술연구소, 경상북도김천시혁신로 269 2 안동대학교신소재공학부, 청정에너지소재기술연구센터, 경상북도안동시경동로 1375 (2016 년 12 월 21 일접수, 2017 년 6 월 20 일수정, 2017 년 6 월 20 일채택 ) Protection Performance Simulation of Coal Tar-Coated Pipes Buried in a Domestic Nuclear Power Plant Using Cathodic Protection and FEM Method H. Y. Chang 1, K. T. Kim 2, B. T. Lim 1, K. S. Kim 1, J. W. Kim 1, H. B. Park 1, and Y. S. Kim 2, 1Power Engineering Research Institute, KEPCO Engineering & Construction Company, 269, Hyeoksinro, Gimcheon, Gyeongbuk, 39660, Korea 2Materials Research Centre for Energy and Clean Technology, School of Materials Science and Engineering, Andong National University, 1375 Gyeongdongro, Andong 36729, Korea (Received December 21, 2016; Revised June 20, 2017; Accepted June 20, 2017) Coal tar-coated pipes buried in a domestic nuclear power plant have operated under the cathodic protection. This work conducted the simulation of the coating performance of these pipes using a FEM method. The pipes, being ductile cast iron have been suffered under considerably high cathodic protection condition beyond the appropriate condition. However, cathodic potential measured at the site revealed non-protected status. Converting from 3D CAD data of the power plant to appropriate type for a FEM simulation was conducted and cathodic potential under the applied voltage and current was calculated using primary and secondary current distribution and physical conditions. FEM simulation for coal tar-coated pipe without defects revealed over-protection condition if the pipes were well-coated. However, the simulation for coal tar-coated pipes with many defects predict that the coated pipes may be severely degraded. Therefore, for high risk pipes, direct examination and repair or renewal of pipes are strongly recommended. Keywords: nuclear power plant, coal tar-coated buried pipe, FEM, cathodic protection 1. 서론매설배관의경우다양한사용재료및환경의영향으로거의모든종류의부식이발생할수있는데, 매설배관주위의다른토양조건, 피복결함, 제조과정에형성된밀스케일의불균일성등의결과로외부로는균일부식, 틈새부식, 미생물부식, 미주전류부식등이발생할수있고내부에서는유체에의한침식부식, 공식등의부식으로인해누설, 파단, 막힘등의손상이발생한다. 이러한부식손상을막기위한제어방법으로는설계, 피복, 내식성재료선정, 미주전류제어, 음극방식등이있으며일반적으로한가지또는두가지 Corresponding author: yikim@anu.ac.kr 이상적용하고있다. 주로매설배관에서는피복과음극방식을동시에적용하여방식을진행하고있으며원전의경우검사절차서를작성하여관리중에있다. 원자력발전소의경우 EPRI 를중심으로매설배관의경년열화를체계적으로관리하고자 2008 년에매설배관손상관리지침서, EPRI-1016456 이최초로발간되었으며, 미국원전산업계에서는매설배관건전성그룹 (Buried Pipe Integrity Group: BPIG) 을결성하여주기적으로기술회의를통해기술적인문제점과현안을논의하여문제를해결하고있다 [1]. 일반적으로피복방법은그분류방법도다양하고상업적인종류도수많이나와있으나, 근본적으로부식매체를차단한다는 1차적인목적을갖고있으며, 경우에따라서는희생양극성기능등을부여하기도한다. 피복은배관매설시에나
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM Table 1 Inspection of buried and underground piping and tanks [8] Current AMP XI.M41 LR-ISG-2015-01 Condition Years 30-40 Years 40-50 Years 50-60 Years 30-40 Years 40-50 Years 50-60 Conditions: C 1 1 1 1 1 1 D 2 2 2 2 2 2 E 7 10 12 3 3 3 F 15 20 25 6 6 6 C : CP operational and meeting operational and effectiveness goals in AMP XI.M41 D: CP demonstrated to be not required E: CP operational but does not meet operational and effectiveness goals in AMP XI.M41; however, coatings and backfill meet preventive action recommendations of AMP XI.M41, operating experience does not reveal leaks, significant coating degradation, or metal loss, and the soil is not corrosive. F: Condition C, D, or E not met, and either: plant-specific operating experience has revealed leaks, significant coating 사용시간이증가함에따라서열화발생에따른손상부위가발생하며부식성환경에금속재료가직접노출되는것을의미한다. 현재비굴착식매설배관의피복손상의심부를탐측하는간접검사기술로는 Pearson method, Electromagnetic Current Attenuation Survey, Close Interval Potential Survey, ACVG(Alternating Current Voltage Gradient), DCVG(Direct Current Voltage Gradient), APEC(Area Potential Earth Current) 등이있다 [2-4]. 미국에서는인허가갱신을신청하는원전의신청서류를검토하기위한기술적근거문서로서 NUREG-1801 (GALL, Generic Aging Lessons Learned Report) 을활용한다. 본지침서에서매설배관관리와관련된내용은 Ⅺ.M41, Buried and Underground Piping and Tanks Inspection 에기술되어있다. NUREG-1801 은미국 NRC가계속운전을신청하는발전사업자를대상으로인허가갱신 (License Renewal) 심사를위한지침서로써안전관련및위험물질배관 / 탱크를대상으로한다. 상기경년열화관리프로그램지침 XI.M41, Buried and Underground Piping and Tanks Inspection 의부속검사지침서인 LR-ISG-2011-03 의개정판 LR-ISG-2015-01 에서는음극보호시스템의유효성을기준으로다소완화된검사대상수를제시하고있음을최근에발표한바있으며 Table 1에나타내었다 [5-8]. 또한음극방식성능기준에도변화가있는데, 기존의일률적으로적용되던황산동기준전극 (CSE, Cu/CuSO 4 Electrode) 대비 -850 mv 또는포화감홍기준전극 (SCE, Saturated Calomel Electrode) 대비 -800 mv는세분화되어다음과같은기준이마련되었다. 10k 100 kω cm토양비저항의경우 ; -750 mv(cse) 또는 -700 mv(sce) 100 kω cm초과토양비저항의경우 ; -650 mv(cse) 또는 -600 mv(sce) 전기저항부식속도센서로재료의부식속도가 1 mpy 이하임을입증해야함국내에서는원자력안전법시행규칙제20 조및제21조의경년열화에관한사항, 요구되는안전여유도유지를위하여평가대상구조물, 계통및기기의경년열화관리계획이확립되어있어야함 을기술기준으로매설및지하배관의경년열화관리계획을수립및이행하여매설배관의건전성을입증하고있다 [9]. 한편원전기기의부식열화기구로서선택적침출 (Selective Leaching) 혹은선택적부식 (Selective Corrosion) 의대상재료는회주철, 황동및청동등일부구리계합금에국한되었으나, 최근 USNRC 의 GALL-SLR 에서는구상흑연주철 (Ductile Cast Iron) 에대해서도이러한경년열화현상에민감한재료로포함할것을권고하고있다 [10]. 구상흑연주철은우리나라원전의소화수배관계통의옥외소화전관로에적용중이다. 본연구진은매설배관의전기방식상태에대한전위분포시뮬레이션연구 [11] 및시뮬레이션을통한간접적인결함탐지기술 [12] 등에대하여최근보고한바있다 ; 3D FEM 모델링법을이용하여매설배관에대한전기방식상태및코팅건전성을예측할수있는가능성이있음을확인하였다. 상기와같은매설배관음극방식기준의변화및경년열화평가대상의변화에따른국내원전의매설배관적합성을평가하기위해서본연구에서는국내원전특정호기의실제 CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD (c) Fig. 1 3D layout of domestic nuclear power plant; Area 1, Area 2, (c) Area 3. 매설배관을대상으로 3D FEM 모델링을수행하여현재적용중인음극방식설비및운전조건의적절성을평가하였으며, 실제측정방식전위와모델링과의비교를통해매설된구상흑연주철배관의표면코팅에건전성등을평가하였다. 2. 연구방법 2.1 매설배관에대한방식상태현장측정시험국내원전의매설배관의방식상태점검을위해임의의원전을선정하여 #1~#3 구역의매설배관에대한전위분 CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM (c) Fig. 2 3D FEM model domain extracted from 3D CAD data base; Area 1, Area 2, (c) Area 3. 포를측정하였다. 측정방법으로는매설배관에서인출한전선과토양표면에접촉시킨황산동기준전극사이에볼트메타를연결시켜기준전극에대한배관의 On 전위를측정하였다. 2.2 국내원전매설배관의 3D 모델화본해석에사용된모델은현재운전중인국내원전특정호기에현장적용연구를위해특별히제작된정류기와모니터링시스템그리고기준전극이적용된현장의배관망이며, 이중 일부영역을 3등분하여 3D CAD 데이터를모델기하조건으로적용하였다. 3D 배관망데이터는 Bentley 사의 Microstation 으로작성되어관리중에있으며 Microstation 의확장자파일인 dgn 파일을 AutoCAD 파일인 dwg 로변환시켜 COMSOL Multipysics 로불러들여모델화하였다. Fig. 1에는모델링에적용된실제원전 3D CAD 도면으로서위상 (level) 별로층상화 (layered) 되어있고, 각층상내에포함된배관과건물및구조물과같은여러요소들이함께배치되어있다. 기존정류기및전극들에더하여현장 CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD 적용연구용정류기및전극들이영향을줄수있는부위를선별하여대상구역을선정하였고, 이구역을 3개로나누어모델링을진행하였다. 2.3 1차전류분포를통한음극방식시뮬레이션음극방식유한요소모델링은 COMSOL Multiphysics 를이용하였으며, 전기화학기구로 1차전류분포 (Primary Current Distribution Physics) 를적용하여거시적인전위분포를해석하였고지배방정식및경계조건은 Table 2 와 Table 3과같다 [13]. 1차전류분포기구에서는토양내화학종이나화학반응및전극계면반응에관계없이외부에 서인가된방식전류가모두배관의결함부로흘러들어간다는가정하에진행하는계산이다. 배관을제외한모든구조물은절연이라가정하였으며 Fig. 1에서모든구조물을제외하고 Fig. 2와같이토양에매설된배관과양극을배치하여모델기하조건을구성하였다. 대상의하부도메인에서매설배관실체 ( 양각 ) 들은모두삭제하고음각 (intaglio) 화하여모델링할대상인토양및배관과의계면만남기게된다 [14]. 음각화된모델은부위별로적절한크기의요소 (mesh) 로나누었다. 방식전위는포화감홍전극 (Saturated Calomel Electrode: SCE) 으로나타내었다. Table 2 Govern equations for 3D simulation using a primary current distribution Primary Current Distribution Electrolyte-Electrode Boundary Electrolyte Potential Govern equation Table 2 Govern equations for 3D simulation using a secondary current distribution Governed equation Secondary Current Distribution i -s f, Ñ i = 0 Electrolyte-Electrode Boundary Electrolyte Potential Key i l = Current density in liquid i 0 = Exchange current density i s = Current density in solid α a = Anodic transfer coefficient i loc = Local current density α c = Cathodic transfer coefficient Q l = Total charge in liquid(soil) η = Overpotential Q s = Total charge in solid φ l = Potential in liquid σ l = Conductivity of liquid(soil) φ s = Potential on solid = Conductivity of solid σ s Table 3 Parameter for simulation runs l = l l l n il = itotal itotal = åi m f = f loc, m l l, bnd Parameter Value Description σ 0.02 S/m Soil conductiviry E eq_cs -0.72 V Equilibrium potential of carbon steel (vs. SCE) I 0_CS 9.31e -4 A/m 2 Exchange current density of carbon steel α a 0.5 Butler-Volmer coefficient (+) α c 0.5 Butler-Volmer coefficient (-) i app 1.2 A Applied total current CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM Fig. 3 Configuration and mesh for 3D modelling; Configuration of buried pipe and anode, mesh formation. 2.4 2차전류분포를통한음극방식시뮬레이션모델링결과와현장측정데이터를부합시킬수있도록매설배관코팅제의상태를변화시켜가며전해질전위값을계산하였다. 이계산을위해서는코팅제의상태에따라배관표면의분극상황등전기화학적상태가변하기때문에기구학적지배방정식으로서 2차전류분포 (Secondary Current Distribution Physics) 를적용하였다. 2차전류분포기구는재료계면에서의활성화분극을고려하고, 전하의이동과분극과의관계또한수식에의해정의된다. 전극과전해질에서전류의전도를묘사하기위해전하보존법칙과결합된 Ohm s law를적용한다. 시간에따른계의변화를묘사할필요가없으므로정적 (Stationary) 해석을수행하였으며, 계산에적용된지배방정식은 Table 2 와 Table 3에정리하였다 [13]. 이계산에서도앞서사용했던실제원전 3D CAD 도면을사용하였으며, 계산의복잡성과이에따른계산시간을줄이 기위해모델은 1 구역중 2 개의배관및 1 개의양극그리고 Fig. 4 Cathodic polarization curve of ductile cast iron in aerated 0.01% NaCl solution at room temperature (scan rate; 0.33 mv/s). CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD Table 4 Cathodic protection status of domestic nuclear power plant Areas Applied voltage, V Applied current, A V vs. SCE Maximum value Minimum value Mean value Area 1 12 1.2-0.61-1.83-0.90 Area 2 12 1.2-0.49-0.69-0.55 Area 3 6.9 0.12-0.50-1.13-0.70 (c) Fig. 5 Simulation result for area 1 using a primary current distribution; current density and its vector, electrolyte potential of defect #1, (c) electrolyte potential of defect #2. CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM (c) Fig. 6 Simulation result for area 2 using a primary current distribution; current density and its vector, electrolyte potential of defect #1, (c) electrolyte potential of defect #2. 이들을둘러싼토양으로국한하여모델링하였다. Fig. 3 에계산에사용하기위해 3D CAD 도면에서추출 하여모델화된도메인및메쉬를나타내었다. 코팅제가손 상된배관결함부 ( 부식부 ) 의부식속도는금속표면전기화 CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD 학반응에의해결정되는데, 이러한현상을모사하기위한경계조건으로몇가지방정식을적용할수도있으나, 본연구에서는보다실제환경에가까운조건을입력하기위해토양모사용액 ( 호기성 0.01% NaCl) 에서구상흑연주철의음극분극시험을수행하였고, Fig. 4와같이그데이터를함수화하여경계조건으로적용하였다. 방식전위는포화감홍전극 (Saturated Calomel Electrode: SCE) 으로나타내었다. 3. 연구결과및고찰 3.1 현장측정전위데이터상기모델링결과와의비교를위해 1구역 ~ 3구역에해당하는매설배관에대하여황산동구리전극대비전위값 (On potential) 을측정하였고, 운전중인정류기상의인가전압, 인가전류값을 Table 4에정리하였다. Table 4에서나타난바와같이측정된전위값이상기모델링의전해질전위와매우큰차이를나타내고있다. 이러한이유는크게 3가지로유추해볼수있는데, 첫째는매설배관표면을덮고있는코팅제가열화되어절연성이크게저하되었거나코팅제에무수한결함이발생한경우이고, 둘째는방식전류대부분이 Fig. 7 Simulation result for area 3 using a primary current distribution; current density and its vector, electrolyte potential of defect #1, (c) electrolyte potential of defect #2. CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM 주변의특정구조물이나기기로흘러들어가는경우, 셋째는방식시스템과양극들의결선이단절되어있는경우이다. 두번째의경우는이러한많은전류가지속적으로흘러들어가면특정구조물이나기기가급속히부식되어손상을주게되므로, 아직까지이와같은현상이보고된바없어가능성이적은것으로판단할수있다. 또한결선단절의경우는굴착외에정확히확인하기는어려우나정류기상에각각의양극별로차이는크지만, 각채널별양극전류값이읽히는것으로보아다량의양극에결선이단절된것으로판단되지는않는다. 따라서, 현재로서는매설배관상피복제 ( 코팅제 ) 가열화되었거나박리등에의해다량의결함부가존재하고있는것으로판단되어이에대한확인평가를수행하였다. 3.2 1차전류분포시뮬레이션해석결과인가전압중심의모델링의해석을위해 1차전류분포를이용하여 3가지구역에대해서모델링을진행했다. Fig. 5 는 1 구역에대한음극방식모델링결과이다. 현장의방식조건과동일하게인가전압 12 V 및인가전류 1.2 A를입력변수로적용하였다. Fig. 3의 는결함부의전류밀도및전류밀도벡터를, 는결함부 -1의전위, (c) 는결함부 -2의전위를보여주고있다. 인가전압과인가전류는대상의실제원전에서운전중인값을적용하였으나, 이정도방식전압과전류에서는결함부의전해질전위가인가전압 (-12 V) 이하로형성되어심한과방식이진행중임을알수있다. 적절한방식기준은포 Fig. 8 Effect of the number of coating defect on the cathodic potential (applied current: 1.2A, total defect area: 0.9546ft 2 ); defect number; 8, defect number; 16. CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD 화감홍전극기준으로 -0.75 V ~ -1.2 V 내외이다. Fig. 6은 2 구역에대한음극방식모델링결과로서 전류밀도및전류밀도벡터, 결함부 -1의전해질전위, (c) 결함부-2 의전해질전위를도시하고있다. 인가전압및인가전류는현장과동일한 12 V/1.2 A로서 1구역과마찬가지로전해질전위가방식기준을현격히초과하고있어과방식이발생하는것으로해석되었다. 3구역에대한음극방식모델링결과는 Fig. 7과같다. 1구역및 2구역과마찬가지로매설배관표면의코팅층이이상 적인상태로절연이잘되고있다면결함부는상당한과방식현상으로수소취성및코팅박리등의현상이발생될것으로예측할수있다. 3.3 2차전류분포시뮬레이션해석결과 Fig. 8은대상모델매설배관의코팅결함증가에따른전해질전위분포변화로서 는 0.9546ft 2 면적의결함수가 8개, 는같은면적결함수를 16개도입하여계산을수행한결과이다. 배관상의코팅결함수가증가함에따라방식전 Fig. 9 Effect of the number of coating defect on the cathodic potential vector (applied current: 1.2A, total defect area: 0.9546ft 2 ); defect number; 8, defect number; 16. CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
H. Y. CHANG, K. T. KIM, B. T. LIM, K. S. KIM, J. W. KIM, H. B. PARK, AND Y. S. KIM 위는토양전반에걸쳐 + 방향으로상승하는데, 이것은결함수가증가할수록소모되는전류의양이많아지기때문이다. 배관상의코팅결함수증가에따른방식전류벡터를 Fig. 9에나타내었다. 그림에서와같이결함수가증가할수록결함을보호하기위해결함방향으로진행하는전류벡터의크기와굵기가증가하고그수도증가함을알수있다. 한편, 배관표면에서전위분포를더욱세밀하게관찰하기위해 1D Line Plot을사용하여배관1 과배관2 의표면전위 를도시하였다. Fig. 10은배관 1의코팅결함증가에따른표면전위분포로결함수가 8개인 의경우양극과가까운부분전위는 -3.6 V(SCE) 이며, 거리가가장먼결함은약 -2.6 V(SCE) 의방식전위를보이고있다. 배관 2의코팅결함증가에따른표면전위분포를 Fig. 11 에나타내었으며, 결함수가 8개인 의경우양극과가까운부분전위는 -2.3 V(SCE) 이고, 거리가가장먼결함은약 -1.2 V(SCE) 의방식전위를보이고있다. 이것은앞서 Fig. 10 11 1D line plot of potential distribution by increasing defects of pipe #1 #2 (applied current: 1.2A, total defect area: 0.9546ft 2 ); defect number; 8, defect number; 16. CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017
PROTECTION PERFORMANCE SIMULATION OF COAL TAR-COATED PIPES BURIED IN A DOMESTIC NUCLEAR POWER PLANT USING CATHODIC PROTECTION AND FEM METHOD 예측한바와같이매설배관의코팅결함이증가함에따라소모되는전류의양이많아져방식전위가 + 방향으로상승하는현상이다. 따라서상기 Table 4의실제원전매설배관구역의전위측정과비교하여보면, 현재매설된배관표면의코팅에사용연수증가에따라다량의결함이존재하거나코팅 / 배관계면부가박리내지탈리되어있어토양혹은토양내수분과접촉함으로써다량의방식전류가소모되고있을것으로추정할수있다. 또한, 코팅제에문제가없다면적절량을 10배이상초과하는높은방식전위로부터발생되는과잉방식전류는주변의다른기기혹은구조물로유입되어부식을야기하고고장을초래할수있기때문에매설배관주변에배치된주요기기및구조물에대한주의깊은관찰이필요하다. 4. 결론국내가동원전 1개호기에설치한시험용정류기 / 모니터링 / 제어시스템및전극류의 3D CAD 도면을이용하여매설배관의 3D FEM 전기화학해석을수행한결과와배관의방식전위실측을통해얻은데이터를비교하여다음과같은결론을얻었다. 1) 현재대상호기에적용되어운전중인음극방식시스템에서는적정방식조건보다매우높은조건으로방식을행하고있는것으로나타났다. 이러한상태를매설배관코팅제의경년열화에기인된것으로가정하고 FEM법으로 3D 시뮬레이션을행한결과, 손상된코팅층이다수존재하는것으로예측되었다. 2) 매설배관상의코팅제가건전하다면잉여방식전류는주변의기기및구조물에부정적영향을줄수있으므로지속적인주의와관찰이필요하며, 매설배관에대한모델링, 간접검사및위험도평가를통해고위험배관에대해서는직접검사를수행하고보수 / 교체를시행해야할것으로판단된다. 감사의글 This work was supported by the Nuclear Power Core Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131520000100). References 1. EPRI 1016456, Recommendations for an Effective Program to Control the Degradation of Buried and Underground Piping and Tanks, Revision 1 (2010). 2. KISTEK, Development of corrosion environment survey and analysis method for buried pipe (II) (1999). 3. KOGAS, Protection technique handbook, p. 18 (2003). 4. EPRI 1022962, Plant Engineering: Evaluation of Indirect Assessment Techniques for Coating Flaw Detection, (2011). 5. NUREG-1801, Generic Aging Lessons Learned (GALL) Report, Revision 2, USNRC, Dec. (2010). 6. License Renewal Interim Staff Guidance, Changes to Buried and Underground Piping and Tank Recommendations, LR-ISG-2015-01, USNRC (2015). 7. B. Allik, U.S. NRC Update on Buried and Underground Piping and Tanks, BPIG-CPUG Meeting, EPRI, July (2016). 8. W. C. Holston, Changes to Buried and Underground Piping and Tank Recommendations, BPIG Meeting, EPRI, July (2015). 9. Act No. 13616, Nuclear Safety Act, Article 20 and 21, Periodic Safety review (2017). 10. ML16041A090, Selective Leaching of Ductile Iron for GALL-SLR(Generic Aging Lessons Learned-Subsequent License Renewal), USNRC (2015). 11. K. T. Kim, H. W. Kim, Y. S. Kim, H. Y. Chang, B. T. Lim, H. B. Park, Corros. Sci. Tech., 14, 12 (2015). 12. H. Y. Chang, H. B. Park, K. T. Kim, Y. S. Kim, Y. Y. Jang, KPVP, 11, 61 (2015). 13. COMSOL, Guidebook of COMSOL TM Multiphysics (Chemistry-Electrochemistry), ALTSOFT (2014). 14. M. Tabatabaian, COMSOL TM for Engineers, Mercury Learning and Information (2014). CORROSION SCIENCE AND TECHNOLOGY Vol.16, No.3, 2017