The objectives are: validation of procedures for monitoring the stability of treated and untreated corroded archaeological and historical iron artifacts; utilization of the procedures to evaluate the effects of environment on the stability of artifacts; and the establishment of criteria and standards for the assessment of current and proposed conservation treatments.
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.
Much work has been performed on various treatments and stabilization methods for archaeological and historical iron artifacts. However, this work has not led to a consensus about a preferred treatment. One of the major reasons for this is that there are no standard criteria for the assessment of the stability of artifacts. The common assessment method is the high humidity chamber test (over periods of only a few weeks) which provides only a visual observation and does not aim to quantify degradation rates. There is thus a need to develop and validate quantitative tests for the stability of artifacts. Although this project is concerned predominantly with iron, the techniques proposed will also be applicable to other metals.
Several experimental methods are utilized to provide complementary information about degradation processes and degradation rates. These include:
the standard chamber test, extended cyclic humidity tests and chamber tests in typically polluted atmospheres;
chemical methods, especially oxygen consumption studies as a direct measurement of the corrosion rate;
and electrochemical methods similar to those used in industrial corrosion monitoring. These include polarographic methods with direct and alternating current as well as electrochemical noise measurements.
Of these, the latter are most clearly appropriate to rapid, in situ monitoring, and the aim is to develop one or more of the electrochemical methods for this purpose. The other tests provide an essential comparison at this stage of development.
The work will result in the description of standard test methods and quantitative criteria for stability. This will make it possible to compare and evaluate different conservation treatments and the influence of various environmental factors on stability.
Funding SchemeCSC - Cost-sharing contracts
M60 1QD Manchester