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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967Figure4: Polarization of a “bare” iron nail at –1000 mV/ Ag-AgCl in 1% (w/v) NaNO 3 solution.A drop of HNO 3 3.5%(v/v) is added after eight hours. No modification of the solution couldbe performed afterwards (night)Values indicated in Figure 4 are average values (around 1300 mV/ Ag-AgCl at theanode). The pH increases slowly but constantly during the first three hours of the experiment.During two short periods while applying a more negative cathodic potential of –1100 mV /Ag-AgCl (after 215 minutes) and –1050 mV /Ag-AgCl (after 445 minutes) the pH increased faster thanduring polarization at –1000 mV /Ag-Ag/Cl . A decrease in the cathodic potential has a directinfluence on the anodic potential (immediate increase). Because the pH had increased to 9.0after eight hours a drop of HNO 3 (3.5% (v/v)) was added to recover a neutral pH of 7.0.During the night when the system cannot be modified and drops cannot be added the pHincreased again and the next morning a pH of 9.5 was measured.© Published by the National Museum of Australia www.nma.gov.au433
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967Figure 5: Polarization of a “slightly polished” iron nail at –1000 mV/ Ag-AgCl in 1% (w/v)NaNO 3 solutionWhen moving from the “bare” nails to the “slightly polished” ones, similar resultswere obtained although the increase of pH was much faster (Figure 5). Drops of HNO 3 (evenmore concentrated than before) had to be added three times in eight hours to recover a neutralpH. Although the cathodic potential is the same as in the previous experiment the anodicpotential was a bit higher, around +1400 mV /Ag-AgCl . The application of a lower cathodicpotential (around –800-900 mV /Ag-AgCl ) tended to slow down the increase of pH as indicatedon Figure 6 but the extraction of chlorides is then less effective. Another way to monitor thetreatment was by applying a constant anodic potential. This approach seems to be much morereliable practically, since the stainless steel is clean of any corrosion product and the surfaceis reproducible from one experiment to another.© Published by the National Museum of Australia www.nma.gov.au434
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