Eng Mat Chapter 4.ppt - ISSUES TO ADDRESS. How does...

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How does corrosion occur?Which metals are most likely to corrode?What environmental parameters affect corrosion rate?How do we prevent or control corrosion?ISSUES TO ADDRESS...
3Corrosion:-- the destructive electrochemical attack of a material.-- Ex:Al Capone'sship, Sapona,off the coastof Bimini.Cost:-- 4 to 5% of the Gross National Product (GNP)*-- in the U.S. this amounts to just over $400 billion/yr*** H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.**Economic Report of the President (1998).Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.
4Two reactions are necessary:-- oxidation reaction:-- reduction reaction:ZnZn22e2H2eH2(gas)Other reduction reactions in solutions with dissolved oxygen:-- acidic solution-- neutral or basic solutionO24H4e2H2OO22H2O4e4(OH)Adapted from Fig. 17.1, Callister & Rethwisch 8e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)ZincOxidation reactionZnZn2+2e-Acid solutionreduction reactionH+H+H2(gas)H+H+H+H+H+flow of e-in the metalEx: consider the corrosion of zinc in an acid solution
5Two outcomes:0ometalV(relative to Pt)Standard Electrode PotentialAdapted from Fig. 17.2, Callister & Rethwisch 8e. -- Corrosion-- Metal is the anode (-)Platinummetal, MMn+ionsne-H2(gas)25ºC1M Mn+sol’n 1M H+sol’n 2e-e-e-H+H+-- Electrodeposition-- Metal is the cathode (+)Mn+ionsne-e-e-25ºC1M Mn+sol’n1M H+sol’nPlatinummetal, MH+H+2e-0ometalV(relative to Pt)H2(gas)
6metaloMetal with smallerVcorrodes.EMF seriesAuCuPbSnNiCoCdFeCrZnAlMgNaK+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.363- 2.714- 2.924metalVmetaloData based on Table 17.1, Callister 8e.more anodicmore cathodicV = 0.153VoAdapted from Fig. 17.2, Callister & Rethwisch 8e. -1.0 M Ni2+solution1.0 M Cd2+solution+25ºCNiCdCdoNioEx: Cd-Ni cellV< VCd corrodes
7Zn2+2e-oxidation reactionAcidH+H+H+H+H+H+H+-+Zn (anode)Cu (cathode)O24H4e2H2O2H2eH2(gas)reduction reactionsZnZn2+2e
8Ex: Cd-Ni cell withstandard 1 M solutionsVNioVCdo0.153 V-Ni1.0 M Ni2+solution1.0 M Cd2+solution+Cd25ºCEx: Cd-Ni cell withnon-standard solutionsYXlnnFRTVVVVoCdoNiCdNin= #e-per unitoxid/redreaction(= 2 here)F= Faraday'sconstant= 96,500C/mol.Reduce VNi- VCdby-- increasing X-- decreasing Y-- increasing T-+NiYM Ni2+solutionXM Cd2+solutionCdT
9Ranking of the reactivity of metals/alloys in seawaterBased on Table 17.2, Callister & Rethwisch 8e. (Source of Table 17.2 is M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)PlatinumGoldGraphiteTitanium

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