Electrochemical and Microstructural Study of 18th Century Cannon Ball

 The main degradation phenomena that affect the metallic cultural heritage are the corrosion reactions with its environment: the burial soil, the outdoor atmosphere or the indoor environment of the museum, display cases or storage. The best way to avoid, or at least delay, such degradation is through preventive conservation. ⁽¹⁾

 The conservation and restoration of archaeological metals is one of the most complex conservation issues. Therefore, conservators usually employ the most advanced techniques, with the purpose of obtaining positive results for the preservation of the objects. ^(2-3)

 Nowadays, electrochemical techniques are provided with control devices, such as potentiostatic control and could be considered again as a very valuable option, in combination with traditional and vanguard techniques, to recover the archaeological metallic heritage.⁽⁴⁾

 The objective of this work is to show how electrochemical measurements and  microstructural characterization can be used as an efficient tool to contribute to the diagnosis of the conservation state of metallic artefacts. In the case of the cannon ball, In San Juan de Ulua Fortress, stationary electrochemical measurements are able to characterise the allowable electrochemical effect of thick corrosion layer on the corrosion rate of metal but are not sufficient to give a complete diagnosis.

 The electrochemical behavior of a cannon ball has been studied in two media: 0.01 M sodium borate and 1M sodium chloride. Techniques used include potential measurements as function of time, linear polarization resitance, potentiodynamic polarization curves and electrochemical impedance spectroscopy.

 These techniques were carried out on cannon ball fragment, in order to relate microstructural aspects with functionality, manufacturing technology of the piece. Metallographic characterization was performed by optical microscopy and scanning electron microscopy, and chemical analysis by energy dispersive X-ray spectrometry. The results allowed determining that the piece was of lamellar grey iron. ^(5-7)

 The obtained data suggest a relationship between alloy composition and electrochemical properties of the material.