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Corrosion

galvanized electrical conduit. Corrosion. -- spontaneous redox reactions in which a metal reacts with some substance in its environment to form an unwanted compound . -- For some metals (e.g., Al and Mg)… . a protective oxide coating (Al 2 O 3 , MgO) prevents further

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Corrosion

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  1. galvanized electrical conduit Corrosion -- spontaneous redox reactions in which a metal reacts with some substance in its environment to form an unwanted compound -- For some metals (e.g., Al and Mg)… a protective oxide coating (Al2O3, MgO) prevents further corrosion of the underlying substrate. -- Galvanized iron is coated with a protective layer of ____. zinc

  2. Mg is used in the cathodic protection of underground Fe pipe. The Mg has to be replaced every so often. Fe e– Mg Offshore oil rigs are often cathodically protected. Mg2+ -- cathodic protection: protecting a metal by making it the cathode in an electrochemical cell sacrificial anode  oxidized metal is called the ______________ EX.

  3. Ag-plated brass trumpet Cr-plated Fe pipe Cu-plated Pb shot Ni-plated steel rotor using an outside source of electrical energy to cause nonspontaneous redox reactions to “go” Electrolysis: -- Electrolysis occurs in electrolytic cells, which consist of two electrodes in a molten salt or a solution. • reduction at cathode; oxidation at anode

  4. Consider plating chromium onto an iron pipe: Fe2+/Fe  Eored = –0.44 V Cr3+/Cr  Eored = –0.74 V For Cr to plate out on the Fe pipe, the equation is: 2 Cr3+ + 3 Fe  3 Fe2+ + 2 Cr and Eo (for the galvanic cell) would be: Cathode – Anode = “Cr3+/Cr”– “Fe2+/Fe” = –0.74 V – –0.44 V = –0.30 V The rxn is nonspontaneous in the forward direction. A galvanic cell would run opposite the way we want. We need to put an external “oomph” into the rxn to make it go…which is the definition of an electrolytic cell.

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