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 corrosion of the underlying substrate. -- Galvanized iron is coated with a protective layer of ____. zinc galvanized electrical conduit

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

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

Fe 2+ /Fe  E o red = –0.44 V Cr 3+ /Cr  E o red = –0.74 V For Cr to plate out on the Fe pipe, the equation is: 2 Cr Fe  3 Fe Cr and E o (for the galvanic cell) would be: = –0.74 V – –0.44 V = –0.30 V Cathode – Anode 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. = “Cr 3+ /Cr”– “Fe 2+ /Fe” Consider plating chromium onto an iron pipe: