Figure 7 Effect of pH on the Corrosion Rate of Iron in Water

Figure 6 Representation of Cathodic Depolarization by Oxygen

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Corrosion DOE-HDBK-1015/1-93 GENERAL CORROSION Rev. 0 CH-02 Page 15 Figure 7 Effect of pH on the Corrosion Rate of Iron in Water Figure 8 Effect of pH on the Relative Attack Rate of Iron in Water First, consider the exposure of iron to aerated water at room temperature (aerated water will contain dissolved oxygen). The corrosion rate for iron as a function of pH is illustrated in Figure 7. In the range of pH 4 to pH 10, the corrosion rate of iron is relatively independent of the pH of the solution. In this pH range, the corrosion rate is governed largely by the rate at which oxygen reacts with absorbed atomic hydrogen, thereby depolarizing the surface and allowing the reduction reaction to continue. For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it is formed rather than depositing on the metal surface to form a film. In the absence of the protective oxide film, the metal surface is in direct contact with the acid solution, and the corrosion reaction proceeds at a greater rate than it does at higher pH values. It is also observed that hydrogen is produced in acid solutions below a pH of 4, indicating that the corrosion rate no longer depends entirely on depolarization by oxygen, but on a combination of the two factors (hydrogen evolution and depolarization). For pH values above about pH 10, the corrosion rate is observed to fall as pH is increased. This is believed to be due to an increase in the rate of the reaction of oxygen with Fe(OH) (hydrated FeO) in the oxide layer to form the more protective Fe O (note 2 2 3 that this effect is not observed in deaerated water at high temperatures). A plot of the relative corrosion rate for iron at various pH values in 590?F, oxygen-free water is presented as Figure 8. The curve illustrates that the corrosion rate of iron in high temperature water is lower in the pH range of 7 to 12 than it is at either lower or higher pH values (at very high pH values, greater than pH 13.0, the oxide film becomes increasingly more soluble because of increased formation of soluble FeO at 2 high temperatures, and corrosion rates increase). As a result of the data plotted in Figure 8 and other similar measurements, it is general practice to maintain high temperature water in the alkaline condition (but below very high pH values) to minimize the corrosion of iron and the steels exposed to the high temperature water.

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