Chapter_13_11

Chapter_13_11 - 1 Chapter 13 Corrosion 13.1 Introduction...

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Unformatted text preview: 1 Chapter 13 Corrosion 13.1 Introduction 13.2 Electrochemical corrosion 13.3 Galvanic Cells Chapter 6 ME 201: Materials Science, Z. Hao 1 Introduction Corrosion: Deterioration of a metal resulting from chemical attack by its environment. Rate of corrosion: temperature and concentration of Rate of corrosion: temperature and concentration of reactants and products. Driving force: lowering energy through corroding Elemental iron > Fe 2 O 3 (iron oxide) Metals: free electrons setup electrochemical cells within their structure Chapter 6 ME 201: Materials Science, Z. Hao 2 their structure. Ceramics and polymers: suffer corrosion by direct chemical attack . 2 Electrochemical Corrosion Oxidation-Reduction Reactions What kinds of reactions are involved in corrosion? Standard Electrode Half-Cell Potentials Different metals have different tendencies to corrode, so how do we measure their corrosion tendencies? Chapter 6 ME 201: Materials Science, Z. Hao 3 Oxidation Reduction Reactions A metal (Zn) placed in HCl undergoes corrosion. Chemical reaction: Zn + 2HCl ZnCl 2 + H 2 Simplified ionic form: Zn + 2H + Zn 2+ + H 2 Zn Zn 2+ + 2e- 2H + + 2e- H 2 (oxidation half cell reaction) (Reduction half cell reaction) Chapter 6 ME 201: Materials Science, Z. Hao 4 Oxidation(Anodic) reaction: Metals form ions at local anode. Reduction (cathodic) reaction: Metal is reduced in valence charge at local cathode. Oxidation and reduction takes place at the same rate. 3 Standard Electrode Half-Cell Potential (1) Different metals have different tendencies to form ions. How to compare Oxidation/Reduction half cell potentials are compared with standard hydrogen ion half cell potential. Chapter 6 ME 201: Materials Science, Z. Hao 5 How to measure Voltage of metal (Zn) is directly measured against hydrogen half cell electrode (25 C) 1M Zn 2+ 1M H + Zn + 2H + Zn 2+ + H 2 Standard Electrode Half-Cell Potential (2) Two types of metals based on their potentials versus hydrogen Potential Anodic to hydrogen More tendency to corrode Anodic to hydrogen More tendency to corrode M M n+ + ne- (Metal oxidized to ions) 2H + + 2e- H 2 Examples: Fe (-0.44), Na (-2.74) Cathodic to hydrogen Less tendency to corrode M n+ M + ne (Metal ions reduced to atoms) Chapter 6 ME 201: Materials Science, Z. Hao 6 M n+ M + ne- (Metal ions reduced to atoms) H 2 2H + + 2e- Examples: Au (1.498), Cu (0.33), Pt (1.2) Table 13.1 on Page 723 4...
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Chapter_13_11 - 1 Chapter 13 Corrosion 13.1 Introduction...

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