The important precaution to be considered in this method is the evolu-
tion of hydrogen gas on the surface of the artefact, which is the cathode in the
cell. For this reason the current densities must be kept to low values. Typi-
cal current densities that have been used range from 300 to 2000 mA m^2
depending on the metal/alloy being protected. Careful monitoring of the
potential of the cathode by use of reference electrodes must be employed in
order to maintain the potential of the artefact below the hydrogen evolution
potential. This can be obtained from the relevant E-pH diagram. If hydrogen
gas is evolved, it can cause layers of corrosion products to spall off the sur-
face and significantly alter the shape of the artefact. For situations involving
artefacts with high-chloride levels in the rust layers, there is the strong possi-
bility that chlorine gas may be liberated at the anode. This can be a problem
in enclosed spaces, particularly if this gas reaches high levels of concentra-
tion. Despite these reservations, this is quite an effective process for the con-
servation of metals and the conservation time can vary from 2 to 33 months
depending upon the original chloride content buried within the corrosion
products.
The only surviving First World War ship belonging to the Royal Navy was
brought into Portsmouth harbour in 1990 and then into dry dock beside HMS
Victoryin 1997 for the purpose of conservation. At the beginning of the First
World War, the Admiralty embarked on a very extensive ship-building pro-
gramme. These included vessels called Monitors, which were to provide shore
bombardment. In order to be able to sail close to the shore, the 6000 tons
Monitors had shallow drafts of 1.8–3 m (6–10 ft) and two revolving 15 in gun
turrets. For use in very shallow waters and river estuaries, smaller vessels of only
600 tons with a draft of only 1.2 m (4 ft) and two 6 in revolving guns placed
fore and aft were ordered in March 1915. One of these was M.33 which was
built in 2 months and launched in May 1915. She had a top speed of 9.6 knots but
had to be towed for most of her first journey to the Aegean and the Dardenelles
as her engines were only designed for 3 days continuous use. M.33 remained in
active service for the remainder of the war and then served in campaigns in
Russia during 1919. Later in 1925, she was converted to a minesweeper and
re-named Minervaand finally converted to a staff office and workshop during
the Second World War.
HMS Minervawas finally “paid off ” in 1984 and eventually brought by
Hampshire County Council and berthed in No 1 Basin in Portsmouth Naval
Base in 1990 and transferred into dry dock in 1997. (Figure 10) An ultrasonic
survey was conducted over strategic parts of the ship in order to determine the
extent of corrosion of the steel plate. The original thickness of the metal was
12.5 mm and the results showed that the average metal remaining over the
ship was 8 mm. Near the stern of the ship around the steering gear and propellers,
only 3 mm of the original metal plate was left. The reason for the high-metal
Metals 151