Pit nucleation

Szummer, A., Szklarska-Smialowska, Z. and Janik-Czachor, M., Electron Microprobe Study of the Corrosion Pits Nucleation of Fe-16Cr Single Crystals , Corros. Sci., 8, 827... 

Values of r satisfying Equation 3 (corresponding to the minimum and maximum points in Ag) will yield steady state solutions where a pit radius should remain constant, while the rest of the crystal grows or dissolves depending on the chemical affinity (Equation 2). If the term t b2g /2Tt2Y2 > 1, there are no real solutions to Equation 3 and there is no steady state value of r, which indicates that a small pit nucleated at a dislocation core should spontaneously open up to form a macroscopic etch pit. The critical concentration at which this occurs (setting the above term equal to one) is ... 

For C crit t 161"6 ls a double root to the maximization equation, and there is an inflection point in the AG function (curve D on Figure 1). Since there is no activation barrier to opening up the etch pit, any pit nucleated at a dislocation should open up into a macroscopic etch pit. Similarly, for C < Ccr t, there are no real solutions and no maxima and minima in the A G function, and nucleated pits open up into etch pits. At 300°C, the calculated Ccr t for quartz equals 0.6CQ. 

A very accurate measurement of Ccrjt would allow back-calculation of the surface energy for a given crystal. Because Ccrjt is dependent on the square of Y, such a measurement could be a very sensitive method of measuring interfacial energy at dislocation outcrops. The calculated interfacial energy from our experiments is 280+ 90 mJm- for the rhombohedral face of quartz at 300°C. Parks (10) estimated 25°C value of 360 + 30 mJm is well within the experimental error of our measurement. The best way to determine the value of Ccrjt would be to measure etch pit nucleation rate on... 

On a clean surface of an Fci7Cr alloy in a 0.5M NaCl electrolyte, corrosion is accelerated as pitting corrosion when a potential pulse of 1 s duration extending from the passive region and above the pitting potential is applied. Gugler et al. ° showed by in situ AFM that in this case the pitting corrosion was initiated close to an inclusion on the surface (Fig. 8). Such a surface defect may act as a center for pit nucleation, as was... 

This elevation was attributed to the effect of chromate on metastable pitting. Figure 6b shows plots or current density versus time for high purity A1 wire loop electrodes potentiostatically polarized to -0.500 Wxe in the same solutions as those shown in Fig. 6a. As the chromate concentration is increased from 0 to 25 pM, and then from 25 to 50 pM, the metastable pit nucleation rate (events per unit time) diminishes, as does the magnitude of individual events (event peak current). The presence of chromate appears to decrease the metastable pit growth... 

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. 

From an engineering perspective, the repassivation potential is a more important parameter than the potential for pit nucleation. We want to know the potential below which pits will not grow. This is analogous in theory to measuring KiC or Kiscc in mechanical and SCC testing. One way to test this is to produce a completely bare surface that is dissolving rapidly, and determine at what potential it can repassivate. An easy way to do this is what may be termed an electrochemical scratch. ... 

There are two distinct processes before stable pit formation occurs pit nucleation and growth of the metastable pit and the pit precursors or metastable pits cannot grow until a pitting potential is reached.28 There are many examples of pitting in practice as follows ... 

Fig. 15. The pit nucleation potential ( np) of a clean surface of alloy 600 in deaerated NaCl solutions as a function of chloride concentration and temperature, and the open-circuit potential (Eop) of Alloy 600 in water as a function of dissolved oxygen concentration and temperature [35]. Reproduced with permission. Copyright 1985 by the American Nuclear Society, La Grange Park, Illinois.

Fig. 33. Cartoon outlining various stages of pit nucleation according to the point defect model. Reproduced from J. Electrochem, Sec. 139, 3434 (1992) by permission of the Electrochemical Society.

A number of studies have explored the effects of fluid flow on passivity breakdown (pit nucleation) in aqueous solutions at ambient and elevated temperatures [33, 66-... 

In summary, the existing work suggests that fluid has little effect upon the critical potential for passivity breakdown on aluminium, iron. Type 304SS, and Type 304LSS at ambient temperature, as well as on solution-annealed Type 304SS, sensitized Type 304SS, and mill-annealed Alloy 600 in chloride-containing solutions at elevated temperatures. However, fluid flow may affect the location of pit nucleation (in some... 

Janik-Czachor, M. An assessment of the processing leading to pit nucleation on iron. J. Electrochem. Soc., 128 513C, 1981. 

The nucleation rate of new pits increases with the oxide film damage s and the concentration of aggressive species c. In contrast to that, the ohmic potential drop in the vicinity of an active pit inhibits pit nucleation. The influence of these three variables is combined in an auxiliary variable M ... 

Equations (8.13) and (8.15) reveal the autocatalytic nature of the model, which was before hidden in the stochastic part of the full model (eq. (8.1) and (8.2)) Aggressive ions and a high oxide film damage have an activating effect on the pit nucleation rate. In particular, the presence of active pits increases the nucleation rate in a diffusion-limited area around the active site. Thus, the model contains an autocatalytic component. Further details about the model can be found in [13]. 

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