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Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967dissolved in the solution (2H 2 O+O 2d +4e - 4OH - ) accompanied by the reduction ofoxyhydroxide andOxide layers (3FeO(OH)+e - Fe 3 O 4 +H 2 O+OH - and Fe 3 O 4 +4H 2 O+8e - 3Fe+8OH - ) areobviously favoured and this is reflected by the pH increase. Further investigation are requiredto find a reason for the decrease of pH when corroded nails are polarized.3.3 Stabilisation of the porthole and its frameworkSince similar stabilisation treatments were applied on the two artefacts only theporthole will be described in the following.The treatment started by a mechanical cleaning of the corrosion layers on the greycast iron elements. The objective was to remove any physical barrier to the extraction ofchloride species. It was performed both with a scalpel and a hand held rotary drill with a softabrasive wheel. Powdery oxyhydroxides layers could be removed but attention was paid notto damage the remaining, but loose, white paint covering the graphitized surface of the greycast iron elements. No consolidation of the paint was performed at that stage.Due to the extensive corrosion of the cast iron elements the electrical contact wasmade on the copper based rim. The wire was attached on a pre-polished area with “Araldite”rapid set epoxy adhesive. Stainless steel grids were placed in and outside the porthole.Through a bridge a good connection was made between the inner and the outside anode. Theobject together with the anode were placed in a plastic tank. To limit the volume of electrolyteto the minimum (to be able to better follow up the extraction of chlorides) spare containerswere placed in the tank. The porthole was connected to the negative (-) terminal of the powersupply and the stainless steel anode to the positive (+) terminal. The 1% (w/v) NaNO 3solution was prepared with deionised water and to avoid its evaporation and pollution aplastic sheet covered the tank. The pH of the electrolyte at the beginning of the stabilisationtreatment was 6.2 (the device is shown in Figure 8).a/ monitoring of the electrolytic parameters© Published by the National Museum of Australia www.nma.gov.au437
Proceedings of Metal 2004 National Museum of Australia Canberra ACT 4–8 October 2004ABN 70 592 297 967b/ covering of the tank with a polyethene sheet. Note the presence of spare containers in thetank to limit to the minimum the volume of the electrolyteFigure 8: General device for the cathodic polarization of the porthole.Based on the results obtained with “corroded nails”, an anodic potential value of+1300 mV /Ag-AgCl was applied at the anode (Figure 9). The cathodic potential measured on theporthole was less negative than with the iron nail experiments:-300mV /Ag-AgCl . The penetration of the solution within the graphitic corrosion layer expelledair bubbles that were trapped on the metal surface. These were not hydrogen bubbles since theapplied cathodic potential was too low. The pH decreased very fast, in one hour from 6.2 to4.8. No increase of pH could be obtained by increasing the anodic potential(to +1400 mV /Ag-AgCl ). The contrary happened: the pH became even lower; it decreased from4.8 to 4.1. So the anodic potential was left at +1300 mV /Ag-AgCl . The next morning the pH was4.0 and the cathodic potential was –371 mV /Ag-AgCl . In order to speed up the chlorideextraction, the anodic potential was once again increased to +1400 mV /Ag-AgCl but both thecathodic potential and the pH decreased, the latter from 4.0 to 3.6.© Published by the National Museum of Australia www.nma.gov.au438
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