Importance of the effect of dechlorination treatments on cultural heritage
Metallic cultural heritage exposed to marine subaquatic conditions, for example due to a shipwreck, face an aggressive corrosion environment because of the ions that composed the sea water and the own live organisms that can gather over the surface of samples. In despite of the great amount of ions and the hardest conditions faced by metallic cultural heritage remains, there is an ion that takes the first place because it exerts a great effect over the morphological, structural and corrosion processes of the metallic samples: the chloride ion.
Chloride ion is a small size ion with great electronegativity that can diffuse through the corrosion layers and the microstructure of the metallic sample. Once the sample is under the sea, redox process begins on the surface and the corrosion mechanism implies the decrease of the pH related to the formation of HCl. Furthermore, in the case of iron, for example a grey-iron cannon ball, the corrosion products formed can evolve along time, developing cracks that continuously exposed the metallic surface or reactivates the localized corrosion phenomena. Due to inherent conditions of ion chlorides and corrosion mechanism the degradation of metallic samples continues until all the metallic core of the sample disappears in spite of the formation of an external protective layer.
On a traditional way, electrochemical chloride extraction treatment can be performed by means of the polarization of the sample in order to promote the migration of the chloride ions outside of the microstructure and corrosion layers. This procedure allows removing great amount of the chloride ions on the external layers. However, the surface exhibits a modification due to the reduction of corrosion products to more stable species. The removal of chloride ions enhances the protective barrier effect and reduces the corrosion rate of the sample. However whether storage or display conditions are inadequate the chloride corrosion mechanism can reactivate.
Study Case – Artillery in the Gulf of Mexico
The Gulf of Mexico has been scenery for the travel and commerce since the XV century between the New and Old World. In order to offer security and protect ships and cities the zone has been fortified, specifically in the east of Mexico, the San Juan de Úlua fortress has been edified. Some of the artillery storage inside the fortress were sank on the sea and later recovered. The samples were surface analysed and a conservation treatment was applied. As a conservation treatment, chloride extraction and the application of a barrier layer was employed. In order to determine the best conditions for the application of the treatment, chloride extraction was carried out on a replica of historical metals for almost 300 hours.
Once the conservation procedure have finished, the characterization of the historical sample were performed determining that conservation treatment exhibits a positive behaviour on the corrosion rate and protective effect show by the sample in spite of the time taken on the chloride removal treatment.
As a conclusion manner, electrochemical chloride extraction is an important task to perform in the conservation treatments in spite of the time taken due to this process allow to enhance the corrosion resistance of the samples furthermore, the stabilisation and reduction of the corrosion products increase the barrier effect provided and preserves the surface because avoids the loss of integrity. However, each conservation treatment will be discussed and its suitability discussed by the conservator in the best case assessed by a scientific group and analysis results.
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