16 M. Stem, unpublished data, Corrosion Laboratory, M.I.T.17 G. Gemmell found that the corrosion rate for mild steel (0.06 per cent C) in deaerated 4 per cent NaCl of pH 5.6 was zero within the experimental variations of such measurements, whereas at pH 3.8 the rate averaged 24 mg/dm2/day (mdd), in agreement with Stern's value for iron. This means that the rate at pH 5.6 must be less than perhaps the order of 0.5 mdd. Pryor and Cohen, J. Electrochem. Soc., 100, 206 (1953), report a value of 0.3 mdd for the corrosion rate of mild steel in deaerated water. . 21 Based on atomic percentages and atomic radii of Cd and Hg, the fraction of surface composed of Cd atoms for 10-13 per cent Cd amalgam is equal to about 0.2. However, in measurements of anodic polarization, this fraction remains constant up to fairly high current densities because the potential for formation of Hg2++ is not approached so long as the surface concentration of Cd is appreciable. Therefore, measured values of b are for an anodic area similar to that which applies to the corrosion process, and, hence, the apparent value of A in eq. (2) under these conditions is equal to unity. Data by Wagner and Traud, op. cit., for zinc amalgam confirm this conclusion. For iron, on the other hand, the anodic surface becomes larger, the higher the value of current density externally applied for anodic polarization, and, hence, A is usually less than unity.

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