Corrosion(부식) and Degradation Associate Professor Su-Jin Kim School of Mechanical Engineering Gyeongsang National University

OCW Corrosion Chemistry (Best 3min) https://youtu.be/T4pSuflO9fk Lecture corrosion (10min) https://www.youtube.com/watch?v=Il-abRhrzFY

KOCW 재료의 부식과 열화 http://www.kocw.net/home/cview.do?lid=cfe6f21ec766380f

Cost of Corrosion (비용) • Corrosion: • Cost: - 금속에서 일어나는 비의도적이고 파괴적인 반응. - 자동차 바디패널, 라디에이터, 배기관부품 • Cost: - 4 to 5% of the Gross National Product (GNP)* (국가의 수입 5%정도 부식방지와 부식에 의해 오염/손실되는 생산물에 대한 보수나 교체)

Corrosion of Iron Iron 4Fe + 3O2 + xH2O  2Fe3O3 · xH2O Corrosion = Oxidization >e-> Reduction Fe  Fe2+ + 2e- : Oxidized = loses electron O2 + 2H2O +4e-  4OH- : Reduced = gain electron

Galvanic cell Zinc and Copper in Sulfuric (SO42-) acid  Battery Zn  Zn2+ + 2e- : Anode (-) is oxidized -0.76V Cu2+ + 2e-  Cu : Cathode (+) is reduced +0.34V e- Zn2++SO42- Zinc sulfate Cu2++SO42- Cu Zn

Galvanic Series (+) noble, resistant to corrosion Platinum Gold Graphite Titanium Silver Copper Tin Lead Steel Iron Aluminum Zinc Magnesium (-) easy to corrode

Forms of Corrosion(부식 형태) • Stress corrosion Corrosion at crack tips when a tensile stress is present. • Uniform Attack Oxidation & reduction reactions occur uniformly over surfaces. • Erosion-corrosion Combined chemical attack and mechanical wear (e.g., pipe elbows). Forms of corrosion • Selective Leaching Preferred corrosion of one element/constituent [e.g., Zn from brass (Cu-Zn)]. • Pitting Downward propagation of small pits and holes. • Intergranular Corrosion along grain boundaries, often where precip. particles form. • Crevice Narrow and confined spaces. Rivet holes • Galvanic Dissimilar metals Cu-Fe are physically joined in the presence of an electrolyte. The more anodic metal corrodes.

Corrosion Environments(부식 환경) Atmosphere(대기) environment containing moisture(습기), sodium(염화나트륨) and acid rain(산성비) Water(물) environment: Sea water(바닷물, 염화나트륨)  Pitting(피팅), Crevice corrosion(틈새부식) Soil(토양) environment have a wide range of composition. Moister(습기), Oxygen(산소), Salt content(염의 양), Alalinity(알칼리도), Acidity(산도), Bacteria(박테리아)

Corrosion Prevention (i) • Materials Selection - Use metals that are relatively unreactive in the corrosion environment - e.g., Ni in basic solutions - Use metals that passivate - These metals form a thin, adhering oxide layer(산화막) that slows corrosion. Metal (e.g., Al, stainless steel) Metal oxide • Lower the temperature (reduces rates of oxidation and reduction) • Apply physical barriers - e.g., films and coatings

Corrosion Prevention (ii) Inhibitors(억제제): substances added to solution that decrease its reactivity - Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it with an inhibitor). - Slow oxidation reaction by attaching species to the surface. Using a sacrificial anode steel pipe Mg anode Cu wire e - Earth 2+ • Cathodic protection (음극보호) - Attach a sacrificial anodic(회생양극) material to the one to be protected. zinc Zn 2e e.g., zinc-coated nail Galvanized Steel e.g., Mg Anode

Ceramic Corrosion 세라믹 재료  금속 + 비금속 화합물 세라믹 재료  금속 + 비금속 화합물 거의 모든 환경, 특히 상온에서 부식에 대한 저항성이 큼

Polymer Degradation(열화) Polymer degradation: Physiochemical(물리화학적) Swelling(부풀음) and dissolution(용해) when exposed to liquids. Bind Rupture(결합파괴) : Scission(절단) - 분자 사슬 결합의 분열 또는 파괴에 의해 품질 저하 Weathering(풍화작용) : Oxidation, Water absorption

Bind Rupture (결합 파괴) Radiation(복사선) : 복사선이 특정 원자에 전자를 제거공유결합 깨짐 Chemical Reaction(화학반응) : 산소 오존 등과 화학적 반응을 일으켜 사슬 절단, 인장응력 균열과 틈새 화학 반응 가속화 Thermal(열) : 열적 열화고온에서 분자 사슬들의 절단