Pit growth

Harb and Alkire reported comparisons of experimental measurements and finite-element simulations of pit growth on stainless steel and nickel in the presence of flow. The simulation domain 

---

Szkiarska-Smialowska, Z., The Kinetics of Pit Growth on Nickel in Solutions with Different Cl /S04" Ratios , Corros. Sci., 12, 527 (1972)... 

Owing to the hydrolysis reaction, pit development is an autocatalytic process and often there is an induction period before pit growth attains... 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

At the area between the breakdown potential Eb and the critical pitting potential pit local film breakdown occurs, which leads to the creation of pit nuclei. However, these nuclei are immediately repassivated. Consequently, in this potential region it is concluded that breakdown and repair are continuously repeated without creating pit growth. 

Figure 39. Current-time variation in nickel pitting dissolution in NaCl solution.89,91 1, double-layer charging current 2, fluctuation-diffusion current 3, minimum dissolution current 4, pit-growth current (Reprinted from M. Asanuma andR. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film, J. Chem. Phys. 106, 9938, 1997, Fig. 2. Copyright 1997, American Institute of Physics.)...

In their works,51"54 the self-similar fractal dimension dF>ss of the two-dimensional distribution of the pits was determined by the analysis of the digitized SEM images using the perimeter-area method. The value of dF>ss increased with increasing solution temperature,51 and it was inversely proportional to the pit shape parameter and the pit growth rate parameter.53 Keeping in mind that dr>ss is inversely proportional to the increment of the pit area density, these results can be accounted for in terms of the fact that the increment of the pit area density is more decelerated with rising solution temperature. 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

Figure 20 Evans diagrams showing limitations to pit growth (a) diffusion limitation at cathode, (b) salt film formation at anode, and (c) IR limitations between anode and cathode.

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... 

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. 

Hunkeler and Bohni used this approach to show that pit growth in A1 foils occurred under ohmic control (24). It was also shown that nitrate and chromate inhibitors, added to the electrolyte after pit initiation, inhibited pit growth kinetics though the effect due to chromate additions was small. Several other inhibitors added to solution increased pit growth kinetics, since their primary influence was in decreasing the solution resistance. 

Two main points should be remembered from this lab. First, the breakdown potential is not necessarily the best measurement of pitting resistance. This is because pitting can occur at potentials below EM, as was demonstrated by metastable pitting in test 4. Ebi corresponds to the potential for stable pit growth and propagation only. Pits can nucleate, however, at any potential above the repassivation potential. Secondly, the effects that additional anions have on the pitting behavior is concentration dependent and not mass dependent. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

The mechanism of pitting corrosion involves two steps pit initiation and pit growth. Mechanism of pit... 

---