the range where the steel remains passive. Reference electrodes were positioned
throughout the submarine to monitor the potentials of both the steel and brass in
order to ensure that the steel remained just in the passive region and the brass did
not corrode with release of toxic copper ions.
The tank was filled up with the required amount of sodium carbonate solution
and regular chloride analysis was carried out, first at weekly intervals and
subsequently at approximately monthly intervals. Solution changes were not
carried out as frequently as desired as the cost of chemicals to fill a 820,000 dm^3
was rather excessive. After 3 years, no more chlorides were detected in the
electrolyte. The next stage of conservation was the construction of a viewing
gallery where the relative humidity could be maintained below 35% at all
times even with large numbers of visitors passing through. This ensured that
no moisture films formed on the metal surfaces of the submarine. The sub-
marine was coated with an industrial wax. This was relatively hard wearing,
but did allow simple visual detection of any subsequent sign of corrosion of the
ferrous and copper based components of the submarine.
4.3 Electrolytic Conservation
In this method, the artefact is immersed in a tank of electrolyte in which the
metal will not corrode, i.e.it remains passive. An electrolytic cell is formed with
the artefact being the cathode with an inert anode. A small dc current is passed
between the two electrodes and the corrosion products on the surface undergo a
reduction process to a different compound. In the case of iron, the following
takes place on the cathode surface.
3FeOOHeFe 3 O 4 OH–H 2 O (14)For one mole of Fe, the volume of FeOOH (red rust) is 21.3 cm^3 and the
volume of Fe 3 O 4 (magnetite) is 14.9 cm^3. This involves a 30% reduction in
volume, which allows the electrolyte to penetrate more easily through the cor-
rosion products and reach the deeply-buried chloride ions. In addition, the
negatively-charged chloride ion migrates due to the influence of the electric
field, from the cathode (artefact) toward the anode. This speeds up the rate of
chloride ion removal and decreases the time for conservation.
The anodes that have been used include stainless steels, mild steel, lead and
platinised titanium, while typical electrolytes for ferrous materials have been
0.5 M sodium hydroxide, 0.2 M sodium carbonate, 0.5 M sodium sesquicar-
bonate and tap water. For bronze cannons recovered from the Mary Rose, both
sodium hydroxide and sodium carbonate electrolytes were employed while
pewter artefacts (plates) from the same ship were treated in similar electrolytes
or in a 0.5% solution of EDTA as a sodium salt in alkaline solution.
150 Chapter 6