In a previous study (1) of the kinetics of anodic oxide film growth on iron in a neutral solution, it was found that in the steady growth region the rate of thickening of the oxide varies with both film thickness and electrode potential in accordance with the equation i = k' exp (BE --QT/B} [1] where E is the potentiaI, QT the film thickness, and k', /~, and B are constants. Accordingly, under galvanostatic condition the potential is a linear function of the film thickness E = E' Jr" KQT [2] and at constant potentialwhich can be integrated to the equation of logarithmic lawIncrease of the film thickness, QT, during potentiostatic oxidation can be measured electrochemically by integrating the current-time curve obtained. However, since after a certain period of time the anodic current falls to less than the limit of experimental error, it becomes difficult for extended periods of time to obtain an accurate measure of the amount of charge passed during potentiostatic oxidation. The present note reports another convenient method for estimating the thickness of the oxide film formed by prolonged oxidation.The experimental procedure was almost the same as that described in previous papers (1-3); the specimen was an electropolished carbonyl iron sheet of 5 x 1 cm, and the solution was a deaerated aqueous mixture of 0.15N boric acid and 0.15N sodium borate (pH = 8.42). Before the measurement, the air-formed or passive oxide film on the specimen was completely removed by cathodic reduction, and the solution was renewed at least three times.The specimen was anodically oxidized at a constant potential of 0.Or (SCE scale) at 25 ~ _ 0.1~ for eight different periods of time ranging from 0.5 to 50 hr, which was immediately followed by the galvanostatic oxidation at a current density of 10 ~a/cm 2. Results of the galvanostatic oxidation subsequent to the potentiostatic oxidation for the various periods of time are shown in Fig. 1. It can be seen in agreement with Eq.[2] that the potential rises linearly with increase of AQT in the steady growth region, and that the E --AQT curve in the straight line range shifts in a parallel fashion toward the more noble potential region with increased time of the preceding potentiostatic oxidation and hence with increasing the initial thickness of the oxide film sQr E = E' + K (~QT -b sQr) [5]Accordingly, sQT, the thickness of the oxide film grown during the preceding potentiostatic oxidation, can be estimated from Eq.[5], where K and E" are constants at constant current density. From Eq.[5] + 1,0 +O.S +O.E +0.7 +O.E +0.5 +0.4 +0.3 0 50h 4 ~_. or2 /01 /00,5 hr. /" i = I0 Fo/cm= J , .'o 2;0 3.0 AQT ( mC/cm = ) Fig. 1. Change of potential with coulomb passed during galvanostatic oxidation at 10 /~a/cm 2 subsequent to potentiostatic oxidation at O.Ov for eight different hours. 5.2 1.0 5.0 0.8 4.8 _ 06 .~ 0 0.4 ~ E ~ 4.4 ~ 0.2 0 <3 q , 4.2 ~ 0.0 <3 4.2 -0.2 J i 38 o:s , 2 ; ,o 2b ,~ ,oo t ( hr, scale in log t" ) Fig. 2. Change of film thickness in coulomb with time of potentiostafic oxidati...
