to achieve in most circumstances in museums or storage rooms. Finally, cor-
rosion reactions involving chlorides can produce acids such as hydrochloric
acid. These reactions may take place in crevices or pits on the metal surface and
the pH can drop from 8.1 (seawater) down to 2.5 at the base of a pit. Metals will
invariably corrode faster in acids than in neutral electrolytes, and hence corro-
sion will be accelerated at the base of these pits. From the above, it is apparent
that if the artefact is to survive long after it has been excavated, the removal of
the deleterious chloride is essential. This is one of the main tasks in the con-
servation of metals as once these have been removed, the metal is said to be
stabilised.
Dissolution of the chlorides from the corrosion products is an essential part of
the conservation process. It is essential that the artefact is immersed in an
electrolyte that will not corrode the metal any further, while this dissolution is
taking place. Corrosion scientists have developed redox potential – pH diagrams
from thermodynamics in order to predict the most stable form of the metal.
These diagrams are divided into three zones. Where metal ions are the most
stable phase, this is classed as a zone of corrosion. If the metal itself is the most
stable species, this is said to be the zone of immunity. The third zone is where
solid metal compounds such as oxides, hydroxides, etc, are the most stable and
may form a protective layer over the metal surface. This zone is termed passiv-
ity and the metal will not corrode as long as this film forms a protective barrier.
The thickness of this passive layer may only be approximately 10 nm thick but
as long as it covers the entire metal surface, it will prevent further corrosion.
A typical E-pH diagram for iron is illustrated with a diagram in Figure 1. The
zones where Feand Feare stable are the zones of corrosion, while the
zone of immunity is where the metal, Fe, is the stable phase. The zones where
the oxides Fe 2 O 3 and Fe 3 O 4 are stable are termed passivity and the iron will not
corrode. By measuring the pH and the steady potential of the metal against a
reference electrode, it is possible to determine the zone where the metal is situ-
ated in any given environment. Iron in seawater (pH 8) and freshwater (pH
7) will give a point on the E-pH diagram where Fe is most stable i.e.
iron will corrode. As one is only too well aware, steels and cast irons readily
corrode in these environments. If the pH was raised to above 9 by the addi-
tion of alkali such as sodium hydroxide to freshwater, the point on the diagram
would be in the zone of passivity where Fe 2 O 3 is the most stable phase. The
iron will remain passive and not corrode as long as the pH is maintained at this
value. Using alkalis such as sodium hydroxide or sodium carbonate in solu-
tions, above pH 9, is one of the most accepted methods for the removal of
chloride ions from ferrous artifacts without corroding any remaining metal.
Inspection of the relevant E-pH diagram for the metal/water system will indicate
the range of pH which may be used for the soaking of chlorides from the arte-
fact. A pH of 9 or above would be a disaster for aluminum as it would corrode
Metals 135