Oxides on anode

Phosphates, molybdates, and (at high pH) silicates act as anodic inhibitors much as do alkalis, except that the iron oxides/hydroxides formed on anodic sites then contain some PO43-, M0O42-, or Si044- ( basic iron phosphates, etc.). These inhibitors require the presence of 02 to produce basic iron(III) phosphate, molybdate, or silicate films, whereas oxidizing anions such as chromates and nitrites oxidize Fe2+ (aq) rapidly to insoluble iron(III) oxides on anodic sites. Dianodic inhibitors combine complementary inhibition mechanisms for example, sodium triphosphate may be used with sodium chromate, or sodium molybdate with NaN02. 

It is easy to do this if the electrode is a noble metal, he., does not itself easily oxidize on anodic polarization. One evolves H2 cathodically until the entire surface is covered with H atoms (0H =1) and then reverses the direction of the current, making it anodic and dissolving the adsorbed H atoms (MH —> M + H + e). It is easy, by use of a cathode ray oscillograph, to record the i,-t relation during dissolution at a constant potential. Apart from an initial charging potential, one obtains the coulombs used in... 

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