CHAPTER 17: CORROSION AND DEGRADATION

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

2 OCW Chemistry (Best 3min) Lecture corrosion (10min)

3 KOCW 재료의부식과열화 0f

4 Cost of ( 비용 ) : - 금속에서일어나는비의도적이고파괴적인반응. - 자동차바디패널, 라디에이터, 배기관부품 Cost: - 4 to 5% of the Gross National Product (GNP)* ( 국가의수입 5% 정도부식방지와부식에의해오염 / 손실되는생산물에대한보수나교체 )

5 of Iron Iron 4Fe + 3O 2 + xh 2 O 2Fe 3 O 3 xh 2 O = Oxidization >e - > Reduction Fe Fe e - : Oxidized = loses electron O 2 + 2H 2 O +4e - 4OH - : Reduced = gain electron

6 Galvanic cell Zinc and Copper in Sulfuric (SO 4 2- ) acid Battery Zn Zn e - : Anode (-) is oxidized -0.76V Cu e - Cu : Cathode (+) is reduced +0.34V Zn Cu e - e - e - e - Zn 2+ +SO 4 2- Zinc sulfate Cu 2+ +SO 4 2-

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

Forms of ( 부식형태 ) Uniform Attack Oxidation & reduction reactions

Selective Leaching Preferred corrosion of one element/constituent

more anodic metal 8 Intergranular along grain boundaries, often

Stress corrosion at crack tips when a tensile stress

Combined chemical attack and mechanical wear (e.g.

8 Forms of ( 부식형태 ) Uniform Attack Oxidation & reduction reactions occur uniformly over surfaces. Selective Leaching Preferred corrosion of one element/constituent [e.g., Zn from brass (Cu-Zn)]. more anodic metal 8 Intergranular along grain boundaries, often where precip. particles form. Stress corrosion at crack tips when a tensile stress Erosion-corrosion is present. Combined chemical attack and mechanical wear (e.g., pipe Galvanic Dissimilar metals Cu- Fe are physically joined in the presence of an electrolyte. The corrodes. Forms of corrosion elbows). Pitting Downward propagation of small pits and holes. Crevice Narrow and confined spaces. Rivet holes

Environments( 부식환경 ) Atmosphere( 대기 ) environment

environment have a wide range of composition.

9 Environments( 부식환경 ) 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( 염의양 ), Alalinity( 알칼리도 ), Acidity( 산도 ), Bacteria( 박테리아 )

10 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. Lower the temperature (reduces rates of oxidation and reduction) Apply physical barriers - e.g., films and coatings Metal oxide Metal (e.g., Al, stainless steel)

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

12 Ceramic 세라믹재료 금속 + 비금속화합물 거의모든환경, 특히상온에서부식에대한저항성이큼

13 Polymer Degradation( 열화 ) Polymer degradation: Physiochemical( 물리화학적 ) Swelling( 부풀음 ) and dissolution( 용해 ) when exposed to liquids. Bind Rupture( 결합파괴 ) : Scission( 절단 ) - 분자사슬결합의분열또는파괴에의해품질저하 Weathering( 풍화작용 ) : Oxidation, Water absorption

14 Bind Rupture ( 결합파괴 ) Radiation( 복사선 ) : 복사선이특정원자에전자를제거 공유결합깨짐 Chemical Reaction( 화학반응 ) : 산소오존등과화학적반응을일으켜사슬절단, 인장응력 균열과틈새화학반응가속화 Thermal( 열 ) : 열적열화 고온에서분자사슬들의절단

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page 1end C 0.0.2 ma Cr 14.5~16.5 Co 2.5 max Iron 4~7 Mn 1 max Mo 15 ~ 17 Ni Balance P 0.03 max Si 0.08 max S 0.03 max W 3 ~ 4.5 V 0.35 max 8.89g/cm 3 Multipurpose corrosion resistance of NickelMolybdenumChrome.