Pitting corrosion potential drops

Here, JC represents the electrolyte conductivity in the corrosion pit, L(t) is the depth of the pit that changes with time, and A0 corresponds to the potential difference between the pit base and the pit opening. For relatively deep corrosion pits, the potential drop in the electrolyte outside the pit is negligible compared to that within the pit because of the much larger cross section for current flow. In this case, A

potential difference between anode and cathode. According to Faraday s law, the growth rate of the pit is proportional to the anodic current density ... 

PITTING AND CREVICE CORROSION arise fix)m the creation of a localized aggressive enviion-ment that breaks down the normally corrosion-resistant passivated surface of the metal. This localized environment normally contains halide anions (e.g., chlorides) and is generally created because of differential aeration, which creates corrosion potential drops between most of the surface and occluded regions (e.g., pits or crevices) that concentrate the halide at discrete locations. 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

If the flaw in the passive film is smaller in cross section and greater in depth, then with reference to Fig. 7.6, the resulting increase in resistance can lead to an (IR)2 potential drop that decreases the potential in the bottom of the flaw and/or pit to E2. Then passivity cannot be maintained, and the corrosion current density increases to i2 in the active range. The local corrosion rate is much higher, and a stable pit is initiated at the much higher current density. When the pH of the bulk envi-... 

In the opposite case, hv > g, photons produce electron-hole pairs. Accumulation of holes at the oxide surface increases the local potential drop which may cause a fast photocorrosion. Ion migration is enhanced in the thin film, corrosion is enhanced, and altogether a fast dissolution of metal takes place by a photoelectro-chemical process in the passive film. An example is given for Ti [160]. This technique can be used for microstructuring of Ti- or Al surfaces [104]. On the other hand, anodic metal ion dissolution competes with the opposite anodic film forming ITR of oxygen ions. Therefore, in dependence on the special conditions, laser induced oxide growth may overcome pit formation [160]. 

Also in crevice and pitting corrosion, the ohnrdc potential drop may be responsible for the stability of local attack on passive surfaces. 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

In the second phase of pit growth corrosion products of dissolved metal ions and chloride anions are deposited in the hole. This film further stabilizes the potential drop and provides the conditions for an electrochemical polishing of the surface in the hole. This finally creates the larger semi-spherical forms of the pits. 

Figure 1-48. Potential drop A within large corrosion pits shifting the local potential from the applied value E to negative to the passivation potential Ef.

Figure 4 Double logarithmic plot of the increase of the geometric current density with time for electropolished iron during pitting corrosion, 1 h prepassivated at 1.18 V and potential drop to 0.78 V (sohd hne), At = time between potential change and chloride addition, 1 h piepassivated at 0.78 V (dashed line), phthalate buffer, pH 4.9, 0.1 M SO, , 0.01 M CH. (From Ref 6.)...

XPS measurements of passivated Fe and Ni electrodes that have been exposed to aggressive anions (Ni and Fe to F Fe to Cl , Br, and I ) but have not already formed corrosion pits support this mechanism. The quantitative evaluation of the data clearly shows a decrease of the oxide thickness with time of exposure [22,48], Not only F but also the other halides cause thinning of the passive layer (Fig. 8) [48], The catalytically enhanced transfer of cations from the oxide to the electrolyte leads to a new stationary state of the passive layer. Its smaller thickness yields an increased electrical field strength for the same potentiostatically fixed potential drop, which in turn causes faster migration of the cations through the layer to compensate for the faster passive corrosion reaction (1) at the oxide-electrolyte interface (Fig. 2a). Statistical local changes... 

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