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소재의 부식 및 방식(Corrosion & Corrosion Control of Metallic Materials)
이 성 민
한국가스공사
연구개발원 배관연구센터
[email protected] www.cordoc.pe.kr
Materials & Corrosion Control
외부부식 내부부식 응력부식
기계적손상 지반이동
피로파괴 외부부식 내부부식 응력부식
기계적손상 지반침하/이동 타공사
매설배관의 위해 인자
Materials & Corrosion Control
부식(Corrosion)이란?
환경(environment) 속에 노출된 (금속)재료가 화학적으로또는 전기화학적으로 퇴화(degradation)되는 현상
Materials & Corrosion Control
부식현상의 응용 (I)A wise man can learn from everyone, a fool can learn from nobody.
구리지붕의 멋진 파티나(patina)
내화지지물 (Refractory support): Steel casing of refractory bricks
음극방식의 희생양극 (Sacrificial anodes)
은도금막대에서 살균작용의 은 이온(silver ions )의 방출 (외과용)
치과 치료시 아말감 (amalgam): 충진제 및 치아소재
칼라 anodizing, Al, Mg, Ti
PCB (Printed Circuit Boards) 제작
금속학적 에칭 (Etching)
해양용 방오 도료 (Antifouling coatings): poisonous copper ions.
Materials & Corrosion Control
부식현상의 응용 (II)A wise man can learn from everyone, a fool can learn from nobody.
내후성강 Weathering steel: steel develops the rust which acts as a protective coating.
콘크리트 철근의 녹 Rusting of concrete rebars: Rebars grip concrete better if their surface is slightly rusted before the concrete is procured. Actually this is one of the way of improving the ultimate stress of pre-stressed concrete.
하이드라진 부식억제제 Addition of Hydrazine to boiler water: It helps to form adherent and protective magnetite on exposed surface.
원자력 발전 Nuclear application: One can run water through the primary system before inserting fuel and initiating operation. It passivates the piping and remove any corrosion products formed before they are subjected to neutron field.
Materials & Corrosion Control
부식현상의 열역학과 속도론
GB
GA
A → B
∆G
∆G*A
B
• 반응의 자발성∆G = GB - GA
• 반응의 속도γA->B = k exp {-∆G*/RT}
Materials & Corrosion Control
부식발생의 필요조건
Anode: oxidation reaction
M → M2+ + 2e-
Cathode: reduction reaction
O2 +2H2O +4e- → 4OH-
2H+ + 2e- → H2
Electronic Conductor: metal
Ionic Conductor: electrolyte
Closed Circuit Loop Thus ProtectionIs What?
Materials & Corrosion Control
전기화학적 반응과정
염산용액 내의 아연 Zinc in HCl solution
Zn + 2HCl → ZnCl2 + H2
- Overall: Zn + 2H+ + 2Cl- → Zn2+ + 2Cl- + H2- Oxidation: Zn → Zn2+ + 2e-
- Reduction: 2H+ + 2e- → H2
Anode: the place where oxidation reaction occursCathode: the place where reduction reaction occurs
Corrosion is an electrochemical process in nature (Wollaston, circa 1815)
Materials & Corrosion Control
금속용해(산화)시 환원반응
Hydrogen evolution: 2H+ + 2e- → H2
Reduction of oxygen (acidic): O2 +4H+ +4e- → 2H2O
Reduction of oxygen (neutral to alkaline):O2 +2H2O +4e- → 4OH-
Reduction of metallic ion: M3+ + e- → M2+
Deposition of metallic ion: M+ + e- → M
Materials & Corrosion Control
부식의 유형 Forms of Corrosion
Uniform Corrosion (general corrosion)Localized Corrosion
Stress-insensitive corrosionPitting corrosion, galvanic corrosion, crevice corrosion, intergranular corrosion, exfoliation, dealloying
Stress-induced corrosionErosion corrosion, corrosive wear or fretting, environmentally-induced crackings (SCC, CF, HAC)
Hydrogen blistering
Materials & Corrosion Control
예측적이며, 설계시 두께로 반영하는 경향이 있음
공장의 입지조건 선정 관련 인자 Geographic location Plant preferably be sited upwind from stacks.
쉬운 배수 조건을 갖춤 easy drainage산성증기응축 (acid-vapor condensation) 의 회피
Neutralization with ammonia or amminesMinimize collection of condensed vapor
대기 및 중부식성 용액의 탄소강 대책:Organic coating, metallic (Al, Zn) coating, cathodic protectionGalvanized steel, weathering steel
균일 부식 Uniform Corrosion
Materials & Corrosion Control
Very harmful due to its acceleration of corrosion.It usually occurs by the action of chloride ions.
Corrosion rate of tip of corrosion fissure > that of side wall.Breakdown of Passivity
Passive Film (Oxide Film): active-passive metalsFe, Ni, Cr, Co and their alloysStainless steels
Mechanism:Oxide film theory: increase in permeability of Cl- in oxide filmAdsorption theory: increase of hydration of Cl- adsorbed on metal surface
Removal of Chloride/Use of Resistant Materials
공식 Pitting Corrosion
Materials & Corrosion Control
Taken place at small gapsMost of metals are susceptible to this type of attack
Metals owe their resistance to protective film.Stainless steel, titanium, aluminum are more susceptible.
MechanismDeoxygenated due to initial corrosion in the creviceCathodic process moves outside the crevice, separating anode and cathode (Galvanic coupling effect due to oxygen concentration)Build up of chlorides and depressed pH in crevice solution may cause the breakdown of passive film.
Avoid small gaps: welding rather than riveting and bolting
틈부식 Crevice Corrosion
Materials & Corrosion Control
• Insulate anode from cathode if possible.• Area ratio is important: avoid small anode to large cathode.• Understand “noble metal coating” concept.
갈바닉부식 Galvanic Corrosion
Initial Coupling
Later after Coupling
Later after Coupling
Materials & Corrosion Control
부식발생의 필요조건
Anode: oxidation reaction
M → M2+ + 2e-
Cathode: reduction reaction
O2 +2H2O +4e- → 4OH-
2H+ + 2e- → H2
Electronic Conductor: metal
Ionic Conductor: electrolyte
Closed Circuit Loop Thus ProtectionIs What?
Materials & Corrosion Control
부식제어 방법
내식성 소재의 선택
구조적인 설계
환경조절
음극방식: SA, IC
피복: 유기라이닝, 금속피복
부식억제제: 무기 및 유기 /음극 및 양극
Materials & Corrosion Control
부식과정 Corrosion Reaction
Cathode Anode
Mz+O2 OH-, O2-
H2H+
ze-
ELECTROLYTE
METAL
cationic current flowelectronic current flow
(CORROSION)
M → Mz+ + ze-
Materials & Corrosion Control
코팅의 적용 개념
격리 장벽 (separation)
유기물 코팅(고분자 코팅)
유전 장벽 (insulation)
산화피막 (유전체 피막)
희생 양극 (sacrificial)
※부식억제제 중 일부는 코팅의 역할
Materials & Corrosion Control
• Insulate anode from cathode if possible.• Area ratio is important: avoid small anode to large cathode.• Understand “noble metal coating” concept.
갈바닉부식 Galvanic Corrosion
Initial Coupling
Later after Coupling
Later after Coupling
Materials & Corrosion Control
전류의 흐름과 전자의 흐름
electron flow(negative ch)
(positive ch)current flow
직류부식
Materials & Corrosion Control
비교 항목 희 생 양 극 법 외 부 전 원 법
시공의 편리성 쉽다 어렵다(외부전원 및 양극부지 확보 필요)
유지보수성 쉽다 주기적인 유지, 보수가 필요함
출력/방식범위 방식전류가 작고 방식범위가 좁다
방식전류가 크고 방식범위가 넓다
토양의 영향 비저항이 큰 경우 비효율적임
비저항이 큰 경우에도 적용 가능
단위전류당비용 높다(양극 소모시 교체해야 함)
낮다(전기비용)
피방식물 상태 피복이 불량할 경우 비효율적임
피복이 불량할 경우에도적용가능
타시설물 관계 비교적 간섭을 유발하지않는다
간섭을 유발하는 경우가많다
기 타 기전력에 제한이 있다 과방식의 우려가 있다
전기방식법의 비교