Experimental Observations on the Anodic Polarization of Iron

A representative anodic polarization curve for iron in a buffered environment of pH = 7 is shown in Fig. 5.4. The solid curve is representative of experimental observations the dashed curve is an extrapolation of the Tafel region to the equilibrium half-cell potential of -620 mV (SHE) and aFg2- = 10 6. This extrapolation allows estimation of an exchange current density of 0.03 mA/m2. The essentially steady minimum current density of the passive state is ip = 1 mA/m2. 

Research on the polarization of iron in a buffered solution of pH = 8.4 and higher has been interpreted to show that a series of electrochemical reactions occur as the polarization potential increases (Ref 4). Reactions 5.1 to 5.5, identified below by letter, are considered to be the dominant reactions in the potential ranges identified by the corresponding letters along the polarization curve in Fig. 5.4  

The onset of passivity is associated with reaction C, which results in a layer having the sequence of phases shown in Fig. 5.5(a). 

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

The above series of reactions indicates that pH should be a major variable affecting the position of the active-passive polarization curve of 

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A separate chapter, Chapter 5, is used to introduce the corrosion behavior of active/passive type metals. This allows emphasis on the more complex anodic polarization behavior of these metals and the associated problems in interpreting their corrosion behavior. The chapter is introduced by discussing experimental observations on the anodic polarization of iron as a function of pH and how these observations can be related qualitatively to the iron-water Pourbaix diagram. Pedagogically, it would be desirable to analyze the corrosion behaviors of active/passive metals by relating their anodic polarization curves to curves for cathodic reactions as was done in Chapter 4 for nonpassive alloys. Because of the extreme sensitivity of an experimental curve to the environment, a reasonably complete curve usually can only be inferred. To do so requires understanding of the forms of experimental curves that can be derived from individual anodic and cathodic polar-... 

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