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Chapter 17: Corrosion and Degradation

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Chapter 17: Corrosion and Degradation CHAPTER 17: CORROSION AND DEGRADATION ISSUES TO ADDRESS... • Why does corrosion occur ? • What metals are most likely to corrode? • How do temperature and environment affect corrosion rate? • How do we suppress corrosion? Chapter 17 - 1 THE COST OF CORROSION • Corrosion : -- the destructive electrochemical attack of a material. -- Al Capone's ship, Sapona, off the coast of Bimini. • Cost: Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission. -- 4 to 5% of the Gross National Product (GNP)* -- this amounts to just over $400 billion/yr** Leaking battery Chapter 17 - 2 Electrochemical Considerations • Oxidation (anodic reaction) M Mn+ + ne - metal atoms lose or give up electrons • Reduction (Cathodic reaction) Mn+ + ne - M Mn+ + e - M(n-1)- metal ions gain electrons Chapter 17 - 3 CORROSION OF ZINC IN ACID • Two reactions are necessary: 2+ − -- oxidation reaction: Zn → Zn + 2e + − -- reduction reaction: + → 2H 2e H2(gas) H+ Oxidation reaction 2+ Zn Zn H+ + Acid Zinc flow of e - H 2e - H+ solution in the metal H+ H+ Adapted from Fig. 17.1, Callister 7e . H2(gas) (Fig. 17.1 is from M.G. Fontana, H+ Corrosion Engineering , 3rd ed., reduction reaction McGraw-Hill Book Company, 1986.) • Other reduction reactions: -- in an acid solution -- in a neutral or base solution + − − − O + 4H + 4e → 2H O O + 2H O + 4e → 4(OH) 2 2 2 2 Chapter 17 - 4 Not all metallic materials oxidize to form ions with the same degree of ease. Chapter 17 - 5 Galvanic couple: two metals eclectically connected in a liquid electrolyte wherein one metal becomes an anode and corrodes, while the other acts as a cathode. Chapter 17 - 6 The magnitude of such a voltage is the driving force for the electrochemical oxidation-reduction reaction. Chapter 17 - 7 STANDARD HYDROGEN (EMF) TEST electromotive force • Two outcomes: -- Metal sample mass -- Metal sample mass e- e- e- e- H (gas ) + - ne - 2 2e - ne - H 2e H+ n+ n+ M H+ M ions ions H+ Platinum Platinum metal, M metal, metal, M metal, 25°C 25°C 1M M n+ sol’n 1M H + sol’n 1M M n+ sol’n 1M H + sol’n --Metal is the anode (-) --Metal is the cathode (+) o < o > Vmetal 0 (relative to Pt) Vmetal 0 (relative to Pt) Adapted from Fig. 17.2, Callister 7e . Standard Electrode Potential Chapter 17 - 8 STANDARD EMF SERIES • EMF series • Metal with smaller V o o metal metal Vmetal corrodes. Au +1.420 V • Ex: Cd-Ni cell Cu +0.340 Pb - 0.126 - + Sn - 0.136 Ni - 0.250 o Co - 0.277 ∆V = more cathodic Cd - 0.403 0.153V Cd 25°C Ni Fe - 0.440 Cr - 0.744 Zn - 0.763 1.0 M 1.0 M Al - 1.662 Cd 2+ solution Ni 2+ solution Mg - 2.363 more anodic Na - 2.714 Data based on Table 17.1, Adapted from Fig. 17.2, Callister 7e . K - 2.924 Callister 7e . Chapter 17 - 9 CORROSION IN A GRAPEFRUIT Cathode Anode + - Cu H+ Zn H+ Zn 2+ reduction 2e - oxidation + + − → 2H 2e H2(gas) H+ + + + − → + O 2 4H 4e 2H2O H Acid H+ H+ H+ Chapter 17 -10 EFFECT OF SOLUTION CONCENTRATION • Ex: Cd-Ni cell with • Ex: Cd-Ni cell with standard 1 M solutions non-standard solutions o − o = − = o − o − RT X VNi VCd 0.153 VNi VCd VNi VCd ln - + - + nF Y n = #e - per unit oxid/red Cd 25°C Ni Cd T Ni reaction (= 2 here) F = 1.0 M 1.0 M X M Y M Faraday's Cd 2+ solution Ni 2+ solution Cd 2+ solution Ni 2+ solution constant = 96,500 • Reduce VNi - VCd by C/mol. --increasing X --decreasing Y Chapter 17 -11 GALVANIC SERIES • Ranks the reactivity of metals/alloys in seawater Platinum Gold Graphite Titanium Silver (inert) 316 Stainless Steel Based on Table 17.2, Callister Nickel (passive) 7e . (Source of Table 17.2 is more cathodic M.G. Fontana, Corrosion Copper Engineering , 3rd ed., McGraw- Nickel (active) Hill Book Company, 1986.) Tin Lead 316 Stainless Steel Iron/Steel Aluminum Alloys (active) Cadmium Zinc more anodic Magnesium Chapter 17 -12 FORMS OF CORROSION • Stress corrosion Stress & corrosion • Uniform Attack work together • Erosion-corrosion Oxidation & reduction at crack tips. Break down of passivating occur uniformly over layer by erosion (pipe surface. elbows). • Selective Leaching • Pitting Preferred corrosion of Forms Downward propagation one element/constituent of small pits & holes. of Fig. 17.17, Callister 7e . (e.g., Zn from brass (Cu-Zn)). (Fig. 17.17 from M.G. corrosion Fontana, Corrosion • Intergranular Engineering , 3rd ed., Corrosion along McGraw-Hill Book grain boundaries, • Galvanic Company, 1986.) • Crevice Between two often where special Dissimilar metals are pieces of the same metal. phases exist. physically joined. The Rivet holes g.b. more anodic one prec. corrodes.(see Table 17.2) Zn & Mg attacked Fig. 17.15, Callister 7e . (Fig. 17.15 is zones very anodic. courtesy LaQue Center for Corrosion Technology, Inc.) Fig. 17.18, Callister 7e . Chapter 17 -13 CONTROLLING CORROSION Metal oxide • Self-protecting metals! Metal (e.g., Al, -- Metal ions combine with O stainless steel) to form a thin, adhering oxide layer that slows corrosion. • Reduce T (slows kinetics of oxidation and reduction) • Add inhibitors -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O 2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface (e.g., paint it!). • Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected. e.g., zinc-coated nail e.g., Mg Anode Zn 2+ Adapted from Fig. 17.22(a), Adapted Cu wire from Fig. e- Callister 7e . (Fig. 17.22(a) is zinc zinc from M.G. Fontana, Corrosion 17.23, steel Mg Mg 2+ Callister 2e- 2e- Engineering , 3rd ed., McGraw-Hill pipe anode Book Co., 1986.) 7e . steel Earth Chapter 17 -14 Chapter 17 -15 SUMMARY • Corrosion occurs due to: -- the natural tendency of metals to give up electrons. -- electrons are given up by an oxidation reaction. -- these electrons then used in a reduction reaction. • Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals. • The Galvanic Series ranks the reactivity of metals in seawater . • Increasing T speeds up oxidation/reduction reactions . • Corrosion may be controlled by: -- using metals which form -- adding inhibitors a protective oxide layer -- painting -- reducing T -- using cathodic protection. Chapter 17 -16.
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