Metastable pits

In the potential range catliodic to one frequently observes so-called metastable pitting. A number of pit growtli events are initiated, but tire pits immediately repassivate (an oxide film is fonned in tire pit) because the conditions witliin tire pit are such that no stable pit growtli can be maintained. This results in a polarization curve witli strong current oscillations iU [Pg.2728]

Figure 25 Current versus time behavior for Type 302 stainless steel in 1,000 ppm NaCl at (a) a potential between its repassivation and breakdown potentials, and (b) at a potential below its repassivation potential. Note the existence of an incubation time before stable localized corrosion occurs in (a). The small, short-lived current spikes during the first 400 s are due to the formation and repassivation of metastable pits, which can also be observed in (b), although they are of a smaller magnitude.

Due to their separation in time, it is possible to analyze the metastable pit transients in terms of the electrochemical processes occurring. If each transient is considered to be from a single, hemispherical pit, the dissolution current density in that pit can be calculated ... 

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

Figure 42 Cyclic polarization curve for Type 302 stainless steel in 1,000 ppm NaCl. Note the definition of the breakdown and repassivation potentials, the vertex current density, and the appearance of metastable pits.

Figure 50 Electrochemical noise (spontaneous potential and current fluctuations) associated with the metastable pitting of austenitic stainless steel. (From Ref. 57.)...

Figure 4 Metastable pitting rate (k) for 316 stainless steel in 1 M NaCl solution as a function of potential for untreated samples and samples passivated with 20% or 50% nitric acid for 1 hour. (From J. S. Noh, N. J. Laycock, W. Gao, D. B. Wells. Corrosion Sci. 42, 2069 (2000).)...

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

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. 

In addition to the constraints mentioned above, active areas must be sufficiently long-lived and nearly fixed in position for detection by SVET. This is not normally a problem in the study of coatings where the location of active areas are often fixed by existing or emergent coating defects. However, some pitting phenomena, especially metastable pitting, may not be detected well by this technique. 

The goals of this laboratory session are to introduce you to potentiodynamic polarization measurements for the determination of localized susceptibility, dem-onslralc the effect of the presence of non-CL ions on pitting, and present examples of metastable pitting (one type of electrochemical noise ). 

Test 3 Potentiostatic Holds on 302SS—Metastable Pitting... 

In class, we discussed metastable pitting, and you may have seen some indications of it in some of the previous tests. The easiest way to see metastable pits is to perform potentiostatic holds at potentials just below the pitting potential. In order to reduce our background current, as well as provide a baseline, we will first grow the passive film at a low potential before moving it to a potential where metastable pitting can be observed. Finally, we will polarize just above the pitting potential to see the induction time and the similarity between metastable and stable pits. 

Figure 12 Data from the first two poteatiostatic holds in test 3 showing metastable pitting.

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. 

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 ... 

Noise measurements are extensively used in the studies of metastable pits. Pistorius37 discussed several factors that can influence the proper interpretation of electrochemical noise measurements (ENM). These factors can be probe size, sampling rate, and system noise. The current measurements seem to give clearer information on the corroding system than that of the potential28 46... 

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... 

---