thick, which did not show up on the ultrasonic survey. New plates were inserted
if the area ofdamage was excessive or a patch weld applied if the area of dam-
age was not excessive. These restoration areas were documented for future
reference.
The interior of the ship was conserved by the electrolytic method as this
had the highest levels of chloride in the rust layers. Sections of the hull were
segregated from each other and ensured that they were all watertight. A section
was flooded with water and the pH adjusted to 9.5 by adding sodium carbonate.
This maintained the steel in the passive region and improved the conductiv-
ity of the electrolyte. The latter reduces the power requirements of the dc sup-
ply used to electrolyse the cell. The anodes were manufactured from 316
stainless steel (austenitic stainless) mesh (Figure 11). By using the mesh,
gases such as oxygen and chlorine, which may be produced on these surfaces,
were able to escape and the higher grade of corrosion-resistant stainless steel
considerably reduced the pitting of the anodes. The chloride content of the
electrolyte was monitored daily at first and then weekly. If the chloride con-
tent became too high, the electrolyte was changed in order to reduce the likeli-
hood of pitting corrosion. The current density used throughout the conservation
programme was approximately 350 mA m^2. Once no more chloride was
observed to be coming out of the rust layers, the electrolysis was terminated.
Each section was left with the sodium carbonate solution in it to keep the
Metals 153
Figure 11Stainless steel anodes used in the electrolytic conservation of M33