to\vard higher current densities and more cathodic potentials. T h e critical and passive currents increase \vith temperature slo~vlyu p to about 50" and 70" C.. respectively. but at much higher rate a t higher temperatures. Figure 7 sho\vs the temperature dependence of critical and passive current densities. respectively. for carbon steel in 54.57c H 2 S 0 4 . From a practical standpoinr. an increase of I , with temperature is undesirable. since this indicates a higher corrosion rate (with .4,P,), T h e variation of the passive potential range with increasing temperature. shown in Figure 8. appears to decrease \\-ith increasing temperature Lvithin the temperature range studied. Transpassivity
For carbon steels the increase in current observed a t -1.4 volts cs. S.C.E. and more cathodic potentials (Figure 1) is due to the oxygen evolution reaction. which is the primary reaction taking place in this potential region. T h e potential a t which oxygen evolution begins becomes more cathodic \vith increasing acid concentration and results in a larger passive range, as shobvn in Figures 2 and 6. Chromium-containing alloys such ai Alloy 20 exhibit transpassivity Lvhich is the potential range of a f t w tenths of a volt before the beginning of oxygen evolution where passivity breaks down and metal dissolution becomes severe. Transp,assivity becomes more pronounced with alloys containing 18%, chromium or more. T h e mentioned element becomes oxidized to its sexivalent soluble state in this region of potential and dissolution becomes appreciable (,3. 5. 7. 7 0 ) Conclusions
Sulfuric acid concentration has a n appreciable effect on the electrochemical behavior of carbon steel in the passivation region. T h e dependence of the critical as well as the passive current on per cent HzS04 shows that anodic protection is feasible for carbon steel in concentrations higher than 6070 H2S04 and lo\\er than 20y0 H 2 S 0 , . At medium acid concentrations, the
current requirements and corrosion rates become enormously large and use of anodic protection appeai-s to be impractical. T h e potential range within the passive region is sufficiently large for practical applications. Both the critical and the pashive currents increase appreciably with temperature, particularly above 60" to 70" C., indicating no promise for the use of anodic protection with carbon steels in high temperature and medium concentration sulfuric acid services. Nevertheless. there is some promise that anodic protection could be used with carbon steel and sulfuric acid concentration (>60% and
