Electrochemical extraction of chlorides from corrosion products of iron archaeological objects

摘要

Chloride removal from the archaeological findings is important for their corrosion protection in indoor atmosphere of common relative humidity (30 -50%). A prototype of an electrolyzer was used for the purpose of desalination of small objects with a metallic core. Desalination proceeds in potable water with stainless steel cathode and MMO titanium or platinum anodes. We tested changes of chloride concentration in a rust layer under the conditions when (1) the metallic core of an object was connected to the cathode during electrolysis, (2) only corrosion products (not the metallic core) were in contact with the cathode, (3) rust or corroded specimens were placed in the electric field between the anode and cathode (the objects thus worked partially as bipolar electrode). The testing material was a salinised rust, artificially salinised rusted forged nails and porous iron specimens and rusted iron reinforcement of concrete. The effect of cathodic polarization on the extent of chlorides removal from a model crevice in a steel specimen was also tested. For all experimental alternatives the drop of chloride content in the rust was compared to a reference experiment with the identical setup, however, with no direct current flow. The chloride content decrease in the rust by diffusion was dominant for most of the experiments and contribution of the migration was negligible. Only in the case of a compact metallic core galvanically connected to the cathode, a greater amount of chlorides was demonstrably removed than it was by a pure diffusion. As for real archaeological findings there is a problem with how to remove chlorides not only from the superficial rust layer but also from cracks and cavities filled with corrosion products in the basic metallic material. Cathodic polarization thus directly constrains the diffusion flow induced by the chloride concentration gradient. The cathodic polarization has a negative effect also if the metallic core of an object is not in a galvanic contact with the cathode but it is only exposed to a electric field between the cathode and anode.