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Electrochemical techniques have been used to monitor the environmental stability of archaeological iron artifacts, to evaluate the effects of environmental pollutants on the stability of the artifacts, and to establish criteria for evaluating this stability.
The electrochemical techniques have been developed to the stage where all the desired measurements can be carried out. Measurements can be made without any electrical connection to the artifact, although this does not allow measurement of absolute corrosion potentials. At this stage it is felt that most required monitoring information can be obtained by linear polarization resistance (LPR) measurements in conjunction with potential mapping or corrosion potential measurements. The impedance studies have provided, and will continue to provide, an essential validation of data interpretation methods.
The study of the effect of pollutants on the stability of passive hydroxide treated precorroded surfaces and artifacts has shown that the critical factor is local pH, with a sufficient quantity of acidic pollutant reducing the pH at the metal surface leading to a loss of passivity.
The correlation of electrochemical responses with visual assessment after exposure to high or cyclic humidity was initially confused by low impedances and resistances associated with stable artifacts. However it is felt that a combination of polarization resistance with electrode capacitance and corrosion potential does enable correlation with stability. The criteria for stability are being refined, with a wide range of treatment methods. Together with the studies of the effects of the environment on artifacts this will lead to development of proposed standards for the assessment of conservation procedures.
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