rapidly in these strong alkali solutions. Aluminum artefacts, such as old engines,
would have to be treated in a solution with a pH in the range 4–8. Care must be
exercised in the use of E-pH diagrams as there are some limitations to the use of
these in predicting the corrosion behaviour of metals. As they are based on
thermodynamics, kinetic considerations are neglected. Prior to selecting a pH
range from the E-pH diagram for the metal that one wishes to conserve, it is
prudent to test metal coupons in the solution to ensure that a passive film will
form and that there is no corrosion of the uncorroded metal or its corrosion
products. If this was the case, there will be no artefact left in the solution after a
few weeks of soaking!
3 Field Treatment and Transportation
Metal artefacts immersed in seawater for long periods of time such as over
hundreds of years, may become completely corroded and 100% mineralised. In
certain instances this is the case but in others, the metals or alloys have only
exhibited minimal amounts of corrosion. Even ferrous artefacts recovered from
the same site may experience vastly different corrosion rates.
136 Chapter 6
Figure 1Pourbaix diagram for iron; in an environment with a dissolved iron concentration
of 10^6 gatoms L^1 at a temperature of 25°C
(Modified by Wranglen, 1985.)