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

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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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