Section 4: Composite artefacts (PDF 20858kb) - National Museum of ...

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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967Figure 9: Monitoring of the electrolytic parameters during the cathodic polarization of theporthole in neutral NaNO 3 solution.. Drops of NaOH were constantly added during days 2, 3,4 and 5 to maintain a constant neutral pHThe solution had turned yellow due to the corrosion of the stainless steel, and ironspecies formed deposits at the bottom of the tank. Some also formed on the artefact but couldbe easily brushed away. Three millilitres of diluted NaOH solution (10%) was added toincrease the pH from 3.6 to 5.9. Stirring is very important while adding NaOH to obtain anequal pH for the whole electrolyte. The process of adding NaOH was repeated every twohours during the day. The volume of NaOH that was needed to get a higher pH was differenteach time. Before we left the object during the night we decreased the anodic potential to+1300 mV /Ag-AgCl .The process of increasing the anodic potential to a more positive value during theday, decreasing the anodic potential to a less positive value during the night and controllingthe pH by adding a few millilitres of NaOH was repeated during the next five days. Since theincrease of the anodic potential did not increase the pH it was decided to decrease the anodicpotential to see how the pH would react. The first two nights an anodic potential of+1300 mV /Ag-AgCl was applied. The third and the fourth night an anodic potential of+1200 mV /Ag-AgCl. During the fifth night an anodic potential of +1100 mV /Ag-AgCl was appliedand a stable pH was finally obtained.Adding a few millilitres of NaOH every two hours is time consuming and thereforenot a realistic way to treat an artefact in a conservation laboratory. Also it is unknown whetherthis rapid fluctuation of pH (between 4 and 8) can damage the object as well. Therefore it wasdecided to continue the treatment at an anodic potential of +1100 mV /Ag-AgCl for which thecathodic potential measured on the porthole was –500 mV /Ag-AgCl .© Published by the National Museum of Australia www.nma.gov.au439
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967a/b/Figure 10: Monitoring of the extraction of chlorides versus time (days) (a/) and the squareroom of time (b/) for the porthole, under cathodic polarization in neutral NaNO 3 solution© Published by the National Museum of Australia www.nma.gov.au440
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