loss was due to the turbulence in the surrounding seawater while the ship was
underway.
Metallurgical analysis revealed that the steel plates were normalized plain
carbon steel with a carbon content of 0.06–0.08% and a hardness of 172 VPN.
Chloride analysis of the rust films gave an average value of 0.33% which may
appear to be relatively low but could be explained by the continual washing of
the metal surface by rain water which assisted in the prevention of build up of
high chloride concentrations in the rust films. Samples taken from the interior
of the ship gave considerably higher values for chloride content in the corrosion
products. The chloride levels were 8–10 times those found on the exterior, with
the magazine compartment giving values of 3.93%. This highlights the benefits
gained by the washing of metal surfaces by rainwater. The magazine compart-
ment contained a primitive refrigeration system, which worked by means of a
brine tank. Leakage of brine from this could have contributed to the high chlor-
ide levels discovered in the rust layers recovered from this area.
Once in dry dock, the outside of the ship was pressure-washed with mains
water to remove marine growths attached to the steel plates as well as the loosely
adherent corrosion products. This was repeated several times to assist in the
removal of chloride ions from the rust layers. Approximately 11 tons of debris
were removed from the external structure of the ship by this process. Several
sections of the steel plates were found to have very thin areas less than 1 mm
152 Chapter 6
Figure 10M.33 in dry dock in Portsmouth dockyard