Pit Propagation

Electrochemical, chemical, and physical processes associated with the anodic current determine the conditions leading either to local repassivation or to pit propagation. Since the particular set of processes determining repassivation or propagation is specific to each metal/envi-ronment combination, a generally applicable mechanism of propaga- 

Anodic Current and Cation Concentration in Occluded Regions. 

Anion Migration into Occluded Regions. Anions in the external environment, particularly chloride ions, will migrate into the occluded region as a consequence of the potential difference between the solution at the metal/environment interface in the pit and the solution at the external surface or, equivalently, in response to the increase in positive charge resulting from the increased cation concentration in the bottom of the pit. Chloride ions are known to stabilize the hydrolysis reactions and actually further lower the pH (Ref 19). If the increase in metal-ion concentration associated with the anodic current density at the pit inter- 

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. 

An Analysis of Pitting Corrosion in Terms of IR Potential Changes in Occluded Regions and Relationship to Polarization Curves (Ref 20) 

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Polarization techniques have also been used to determine mechanisms by which microorganisms induce localized corrosion in the forms of pitting or crevice corrosion. In most cases itpit was determined in the presence and absence of bacteria, itpit provides data as to the tendency for pitting, but not the rate for pit propagation. Salvarezza et ah " and De Mele... 

Figure 43 Cyclic polarization behavior of 430 stainless steel in 1 M NaCl, demonstrating the striking effect of pit propagation on 7ipr,t. (From Ref. 43.)...

M Na2Cr2Ov to aerated 1.0 M NaCl solution has virtually no effect on the pit polarization curve compared to a dichromate-free solution. Additions of 0.5 M Na2Cr2Ov increase the repassivation potential by about 90 mV, while a 2.0 M Na2Cr2Ov addition nearly stops pit propagation. The concentrations of dichromate required to inhibit thin film pit growth are consistent with those proposed by Kaesche (23) but are much greater than those required to slow metastable pitting in bulk A1 samples. 

The admittance response at 1 kHz has also been interpreted in terms of the behavior at residual defects in anodic films. This interpretation is based on electron optical characterization, which shows that anodic films contain a distribution of preexisting defects associated with substrate inclusions and mechanical flaws (96,102). In aggressive environments, pits nucleate from these defects and propagate into the metal substrate. In this model, pits are distinct from anodic film flaws, and both can contribute to the measured admittance. Measurements of anodic films exposed to chloride solutions showed that the dissipation factor increased with time, but the capacitance remained nearly constant. Under these conditions, pit propagation at a flaw led to a pit area increase, which increased the resistive component of the admittance, resulting in an increased dissipation factor, but no increase in the capacitance. Measurements at 100 kHz were reflective of the electric double layer and not the components of the oxide film. 

The results of this study indicate that pit initiation takes place in areas where the oxide film is broken or damaged under a stagnant environment in the presence of sufficient moisture and oxygen. It is possible that pit propagation could occur without oxygen, and that it is accelerated by the copper redeposit reaction. In such a case, preventing pit initiation becomes very important. 

Furthermore, the secondary mechanism then causes the pit propagation to become autocatalytic where one of the products formed in the reaction serves to accelerate it. Figure 7.104 describes the following mechanism. Pitting occurs in three stages initiation, propagation and termination. 

The major question is whether the influence of molybdenum on pitting is related to the structure and composition of the passive film, resulting in restricted pit initiation, or that molybdenum quickly stops or severely retards pit propagation. With respect to pit initiation, the mo-... 

There is also evidence that the beneficial effect of molybdenum is to interfere with pit propagation. If the mechanism is active at the initiation of localized breakdown of the passive film, then, effectively, pitting will not occur. Based on the low solubility of molybdenum chloride, Mo03, and polymolybdates in acid solutions, one mechanism proposes that molybdenum enhances the formation of salt films of these species within the pit. This can decrease the IR potential drop to the pit... 

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