Pitting scans

Other electrochemical techniques covered include measurements of the corrosion potential, the redox potential, the polarization resistance, the electrochemical impedance, electrochemical noise, and polarization curves, including pitting scans. A critical review of the literature concerned with the application of electrochemical techniques in the study of MIC is available [1164]. 

Figure 5 Nonmetallic inclusions (before pitting). Scanning electron microscopy, ><3000. (a) Steel A MnS nucleated on an aluminum oxide, (b) Steel B Ti nitride surrounded by sulfur compounds.

Comparative electrochemical Cyclic Polarization ("pitting") scans in aerated 3.5% NaCl, for aluminum alloy samples. 

The pitting scans or single-cycle pitting curves at a rate (v) of 10 mV/min were recorded after equilibration at open circuit for 10 min (t ). When the forward current reached about SxlO Acm , the potential scan was reversed until complete repassivation had occurred. 

Pitting scans are illustrated in Fig. 5.12 for bare, SiPy- and Si-treated AA6082 and AA5083 in naturally aerated 0.6 M NaCl solution. Curves for specimens coated by multiple deposition of the SiPy and Si films are included in the plot. 

The pitting scans for Si-treated AA6082 and AA5083 are presented in the right part of Fig. 5.12a,b. Single immersion treatment provides some barrier protection as indicated by the lower passive currents with respect to the bare alloys. However, the multiple approach is better than the former with lower currents and wider passive regions. These differences are attributed to defects hidden by layer-on-layer deposition. The hysteresis on the reverse scan has the same features as those discussed above, irrespective of the film thickness. 

The notably different pitting scan behaviour of AA2024 treated with SiPy and Si is illustrated in Fig. 5.13. No passive regions are obtained for any treatment condition. The corrosion potential of the SiPy-coated alloy is shifted towards the noble direction by ca. 100-150 mV with respect to the response of bare metal (Fig. 5.13, left), while the multiple Si-treated substrate retards pit propagation to some extent (Fig. 5.13, right). Upon reversing the scan, the hysteresis closely follows that of the bare alloy but with a less pronounced potential step ( ptp). 

Typical view of a semi-developed pit (scanning electron microscopy xlOOO). A thin metallic film is still present and covers part of the pit. Secondary pits are visible all around the main hole. The pit may either go on or repassivate if the thin "top" film breaks down. 

Potentiodynamic pitting scans for 25Cr duplex stainless steel in 20% NaCl + 0.03 M H2S, pH 3.4 (acetate-buffered) at 80°C, with different hold times just above the pitting potential, showing the paradoxical raising of the repassivation potential (Er) with increasing pit (crack) depth, due to depletion of HjS at the dissolving surface. (Courtesy of Mat, S. B.)... 

Pitting and Crevice Corrosion The general literature for pre-dic ting pitting tendency with the slow scan reviews the use of the reverse scan if a hysteresis loop develops that comes back to the repassivation potential below the FCP (E ) the alloy will pit at... 

Close examination of the weld under a low-power stereoscopic microscope revealed small openings (Fig. 15.6). Probing these sites with a pin revealed a large pit that had been covered by a thin skin of weld metal. These sites contained fibrous, metallic remnants (Fig. 15.7). Examination under a scanning electron microscope further revealed the fibrous character of the material (Fig. 15.2) and also the convoluted shapes of the individual fibers (Fig. 15.21). Energy-dispersive spectrographic analysis of this material revealed the compositions in Table 15.1. 

Optimal Salinities Because the EACN values ascribed to oil systems are usually derived from salinity scanning, it was considered appropriate to evaluate the salinity tolerance of ICI NP6, and to determine the EACN value for toluene so that a comparison could be made between the two experimental techniques employed in this study. Inspection of Table III reveals an EACN value for toluene of -10.3 which can be compared to the value of -16 determined from PIT data. Such large negative values... 

Figure 3. Scanning electron microscope picture of laser pits in different materials (left,) feldspar (right) pyroxene. The difference in the amounts melted is caused by the different absorbencies of the minerals.

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