- Electrolyte – to complete the electrical circuit.
- Electrical contact between anode and cathode – to allow the transfer of
electrons from the anode to the cathode. - Cathode reactant – to use the electrons formed at the anode. Dissolved
oxygen is the most common one in archaeological situations.
If any of these components are absent, the metal will not corrode. For exam-
ple, sachets of silica gel are put into display cabinets to prevent corrosion by
stopping an electrolyte from condensing on the metal surfaces. Museums,
nowadays, use more refined methods of humidity control for cabinets.
This also explains the remarkable state of preservation of numerous artefacts
that have remained undisturbed for many hundreds of years prior to excavation.
A good example of this is the wrought iron strip ring guns recovered from the
Mary Rose. These had lain deeply buried in silt on the floor of the Solent for 440
years and were excavated in very good condition. The reason for this is that the
silt prevented oxygen from reaching the metal surface: no oxygen, no cathode
reactant and no corrosion. In other instances, the artefact has been covered
with thick layers of concretion, which have prevented, once more, oxygen
reaching the metal surface. On removal of the concretion, corrosion will
re-commence as oxygen can now reach the metal surface and act as cathode
reactant (Equation (10)). This highlights the importance of treating the artefact
immediately it has been excavated. Even if the concretion is not removed on
excavation, oxygen can reach the cathode sites as this layer is very brittle and
readily cracks to allow air (oxygen) to the metal surface.
The metal ions formed at the anode (Equation (9)) will react with the
hydroxyl ion formed at the cathode (Equation (10)), or any other ions present in
the electrolyte to form metal compounds. The ferrous ion formed in Equation
(9) will form a compound FeO · OH, which is red rust. While in the presence
of low levels of oxygen in the electrolyte, black magnetite (Fe 3 O 4 ) will be the
main corrosion product. On excavating ferrous artefacts, black corrosion prod-
ucts are often observed but these change to red rust on exposure to the air
after several hours. Seawater contains 3 wt% sodium chloride and ferrous
chloride (FeCl 2 ) may form as well as ferrous carbonate (FeCO 3 ) due to the
hardness of salts found in this natural environment. Over 35 different iron com-
pounds have been identified on ferrous artefacts recovered from soil, fresh-
and sea-water sites. Copper and its alloys produce aesthetically-pleasing
corrosionproducts ranging from red, purple and black through to green and
blue. The famous green patina on copper is a basic copper carbonate either
malachite (Cu 2 (OH) 2 CO 3 ) or azurite (Cu 3 (OH) 2 (CO 3 ) 2 ).
Once the corrosion products have formed on the metal surface, the subse-
quent rate of corrosion will depend on whether these compounds can block the
arrival of oxygen to the cathodic sites or the dissolution of metal from the anode
Metals 133