University College - Chemistry Chem 112A Lecture 30: Sacrificial anodes Passivation and Electrolysis
4 Apr 2017
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Zn/fe -> zn would have to be replaced regularly to prevent fe oxidation as zn is consumed, which can be inconvenient or expensive in some situations. Mg + fe2+ -> mg2+ + fe (spontaneous) Mg is working as a sacrificial anode even though it isn"t directly coating the pipe. Passivation creation of a protective oxide layer. Al3+ + 3e- -> al e red = -1. 66 v. O2 + 2h2o + 4e- -> 4oh- e red = 0. 40 v. 4al + 3o2 + 6h2o -> al3+ + 12oh- Not brittle i. e. a protective coating that prevents oxidation of the metal below. Anode: 6h2o -> o2 + 4h3o+ + 4e- Cathode: 2e- + 2h2o -> h2 + 2oh- Anode: ered = e red rt/nef*ln[a(h2o)/a(o2)1/2 x a(h3o+)2] = 0. 82 v @ ph = 7, 1atm, 298k. = -0. 414 v @ ph = 7, 1atm, 298k. Ecell = ered, c ered, a = -0. 414 0. 82 = -1. 23 v.
