Removal of marine crust from iron objects

Iron based objects recovered from marine environments are usually covered with a thick crust. Its mechanical removal can damage the remaining metal. Therefore an electrolytic removal process using an alkaline electrolyte  (KOH, 1% by weight) is recommended. The objective is to favour the decomposition of the electrolyte to produce hydrogen. The cathodic linear voltammetry below shows that when reduction Fe2+ + 2e- ⇒ Fe occurs, hydrogen bubbling is simultaneously initiated. Therefore the operator works at the corresponding potential (about -1000mV/standard hydrogen electrode – SHE (0V)) which favours the reduction of iron corrosion products and also a slight bubbling of hydrogen. The bubbling initiates the “mechanical” removal of the crust. The counter-electrode is a stainless steel 316 mesh (Degrigny 2010).

                                 a                                                             b

Cast iron cannon covered with a thick marine crust (a) and cathodic followed by anodic linear voltammetries on an oxidized iron coupon in alkaline electrolyte (KOH, 1% by weight) (b). The potentials are given versus the mercury sulphate reference electrode (640mV/SHE).

A relatively short period is needed for the development of hydrogen bubbling over the whole surface of the residual metal; as visualised by the shape of the artefact in the surface of the electrolyte (left figure below). Several days are usually sufficient, but the electrolytic process is continued for one month. At the end of this period the crust can be removed mechanically with a hammer and chisel (right picture below). This is the first step of the conservation treatment which is followed by an electrolytic stabilization at -800mV/SHE: enabling the reduction of chlorinated iron (Fe3+) compounds (see the cathodic linear voltammetry above).

Removal of tarnish from silver and gilt silver objects

The cathodic voltammetric plots (below) carried out on the gilt silver Charlemagne’ cup (13th century) from the Treasury of Saint-Maurice Abbey, Valais (CH), show that the tarnishing is due to handling (AgCl – 0-200mV/Ag-AgCl) and poor exhibition atmospheres (Ag2S - -800-1000mV/Ag-AgCl). The electrolytic tarnish removal requires two steps: the first consists of the reduction of silver corrosion products to silver while the second step transports the reduced silver into solution. The anodic voltammetric plots below indicate that this latter step has to be performed between 200 and 250mV/Ag-AgCl. The electrolyte used (citrate buffered NaNO3 1% by weight, pH=4.8) must be changed between each step. The counter-electrode is a platinum mesh (Degrigny et al. 1996).

                                    a                                                                       b

Cathodic (a) and anodic (b) linear voltammetries on two medallions on Charlemagne’s cup in citrate buffered NaNO3 electrolyte. The potentials are given versus the Ag-AgCl reference electrode (210mV/SHE).

The pictures below show the change in the surface appearance of the top part of Charlemagne’s cup during the reduction and oxidation steps.

                          a                                             b                                                c

Upper part of Charlemagne’s cup during electrolytic cleaning: reduction in progress (a), surface appearance at the end of the reduction step (b), surface appearance after the dissolution of reduced silver by anodic polarisation (c).

References:

Degrigny, C., Wéry, M., Vescoli, V. et Blengino, M., Altération et nettoyage de pièces en argent doré, Studies in Conservation, 41 (1996) 170-178.

Degrigny, C., Use of electrochemical techniques for the conservation of metal artefacts: a review, Journal of Solid State Electrochemistry, 14, 3 (2010) 353-361.