Any corrosion process (electrochemical reaction) must satisfy the mixed-potential theory. The mixed potential theory states that during the corrosion process the total rates of anodic reactions must equal the total rates of cathodic reactions. An example is pitting of stainless steels in aerated acidic solutions containing ferric chlorides (as an illustrative example).
Anodic reactions at interface: Interface Iron flux = J (anodic) = anodic current density (ia) / n F
Fe ----- Fe (II) + 2e occurs at a rate of ia
Cathodic reaction
1- 2H(+) + 2e ----- H2(gas) occurs a at rate of i1 in amperes/square cm
2- O2(gas) +4 H(+) + 4 e ----2 H2O occurs at a rate of i2
3- Fe(III) + e ----- Fe(II) occurs at a rate of i3
The mixed potential theory requires that
Ia = i1+ i2 + i3
1-Non-faradic processes at the electrode surface may result in a disagreement with the mixed potential theory?
2- The are 4 possible reasons or more for the initiation of a pit such as the depletion of oxygen, lowering of pH, buildup by migration of species such as chloride ions, and accumulation of dissolved metal ions. However, one must know the location of the cathodic reactions during pit initiation. Is it in occluded regions or outside the pit?
Cathodic reaction at initial stage of pitting occurs next to the anodic location.
First, not all the 1, 2 and 3 could happen at the same time in any pitting or your example. There should have faradic processes during pitting. There are two hypothesises when you use mixed potential theory: 1. any chemical reaction could be divided to two or more oxidation-reduction processes; 2. there is not any net charge accumulation. So, Non-faradic processes couldn't use mixed potential theory.
It is very diffcult to answer the location of cathode during pitting which are varialble according to experiment condition. When there are multiply oxidation-reduction processes it could have different areas. So this question really need to fix to certain condition.
By pit initiation, the site i.e. pit becomes an anodic reaction site (due to depletion of O2, which is major cathodic branch) and hence the region in the vicinity just outside pit becomes the cathodic site, because the electrons generated by accelerated anodic reaction will be preferably consumed in the viscinity of an anodic site i.e. the pit.
1. Mixed potential theory is strictly only valid as an average over time, since the double layer capacitance will charge or discharge as a result of transient changes in potential.
2. Pit initiation can be observed as a transient drop in potential (see the work of Pistorius on pit nucleation transients for example, together more recent work on metastable pitting and electrochemical noise). This implies that at least some of the charge comes from the double layer capacitance.
3. The balancing increase in the cathodic reaction that cause the potential to rise back to the inital value only happens because of the drop in potential (in effect the cathodic reaction only 'knows' about the potential, it knows nothing about the anodic reaction).
4. How far the drop in potential propagates across the surface of the specimen, and consequently the region of the specimen that supports the enhanced cathodic reaction that follows will depend on the impedance of the cathodic reaction and the solution resistance. For small specimens in conductive solution it is probable that all of the specimen will be affected (which means that specimen size is important in pitting studies).
About the 2nd question, it is important to distinguish between the intitiation and the propagation of pits.
1- The initiation is due to a local rupture of the passive film, and numerous causes are possible, all linked with the interaction of agressive anions (chlorides, fluorides..). It occurs before the propagation of the pit, so in an aerated media. At this early step, there is not yet any depletion of oxygen, nor acidification, since the pit is not yet developped. A local rupture of the passive film is not a pit.
2 - The depletion of oxygen, local lowering of pH, buildup by migration of anionic species occurs inside the pit mainly during the propagation step . As the oxygen and Fe(III) is quickly depleted in the pit, reactions 2 and 3 do not contribute significatively inside the pit. However, as the solution becomes higly acidic in the pit due to Cr hydrolysis (here Cr(III) + 3 H2O Cr(OH)3 + 3 H+) , reaction 1 should contribute.
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