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Estudo da eficiência de técnicas electroquímicas na remoção de cloretos em amostras de uma bala de canhão proveniente de um naufrágio do séc. XVIII
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Resumo(s)
Desde finais do séc. XIX são várias as teorias propostas para explicar a corrosão de artefactos arqueológicos em metal, assim como têm vindo a ser desenvolvidos tratamentos e metodologias para a conservação destes artefactos. No âmbito da conservação de artefactos em ferro provenientes de contextos arqueológicos subaquáticos, nomeadamente marítimos, a remoção dos iões cloreto constitui a principal intervenção, no intuito de cessar o processo de corrosão do ferro e permitir a sua estabilização. Com este estudo pretendeu-se aplicar alguns procedimentos de conservação reportados na literatura de modo a avaliar a sua eficiência ao nível da remoção de cloretos. Os métodos de remoção de cloretos utilizados neste trabalho foram: imersão em soluções alcalinas, redução electrolítica, redução galvânica e método do sulfito alcalino. Para este fim foi recuperada uma bala de canhão em ferro fundido, de meados do século XVIII, pertencente ao contexto arqueológico de um navio almirante francês de nome Ócéan, que naufragou junto à costa sul de Portugal, em 1759. A partir desta bala foram seccionadas várias amostras que foram utilizadas como eléctrodos de trabalho, e que permitiram o estudo dos métodos acima enunciados. Antes da aplicação dos diferentes métodos, as amostras foram caracterizadas por voltametria cíclica, microscopia electrónica de varrimento com microanálise por raios-X e difracção de raios-X. A análise e quantificação dos cloretos extraídos em cada técnica foi efectuada por cromatografia de permuta iónica (IEC), o que permitiu efectuar uma análise global comparativa relativamente à sua eficiência para a remoção de cloretos, tendo-se concluído que, nas condições experimentais utilizadas, o método mais eficiente foi a redução electrolítica.
Since the late nineteenth century, several theories have been proposed to explain the corrosion of archaeological metallic artifacts. Different treatments and methodologies for the conservation of those artifacts have been developed. Considering the conservation of iron artifacts from underwater archaeological sites, namely, sea sites, the removal of chloride has been one of the main interventions, aiming to stop the corrosion process in order to stabilize the artifact. This study has been addressed to the application of some procedures reported in the literature for the conservation of archaeological artifacts aiming to evaluate its efficiency in the removal of chlorides. The methods of chloride removal tested in this study were immersion in alkaline solutions, electrolytic reduction, galvanic reduction and the alkaline sulfite method. Several samples were prepared from a cast iron cannonball, dated from the middle of the XVIII century, belonging to the archaeological remains of a French flagship, Océan, which sank near the southern Portuguese coast, in 1759. The samples were used in this work as working electrodes and have allowed testing the efficiency of the electrochemical methods listed above. Before the application of the different methods, samples from the original object were characterized by cyclic voltammetry, scanning electron microscopy coupled to X-ray microanalysis and X-ray diffraction. The quantification of chlorides extracted by each technique was performed by ion exchange chromatography. Data has allowed comparing the efficiency of the various methods for the removal of chlorides. It was concluded that, under the established experimental conditions, the more efficient method was the electrolytic reduction.
Since the late nineteenth century, several theories have been proposed to explain the corrosion of archaeological metallic artifacts. Different treatments and methodologies for the conservation of those artifacts have been developed. Considering the conservation of iron artifacts from underwater archaeological sites, namely, sea sites, the removal of chloride has been one of the main interventions, aiming to stop the corrosion process in order to stabilize the artifact. This study has been addressed to the application of some procedures reported in the literature for the conservation of archaeological artifacts aiming to evaluate its efficiency in the removal of chlorides. The methods of chloride removal tested in this study were immersion in alkaline solutions, electrolytic reduction, galvanic reduction and the alkaline sulfite method. Several samples were prepared from a cast iron cannonball, dated from the middle of the XVIII century, belonging to the archaeological remains of a French flagship, Océan, which sank near the southern Portuguese coast, in 1759. The samples were used in this work as working electrodes and have allowed testing the efficiency of the electrochemical methods listed above. Before the application of the different methods, samples from the original object were characterized by cyclic voltammetry, scanning electron microscopy coupled to X-ray microanalysis and X-ray diffraction. The quantification of chlorides extracted by each technique was performed by ion exchange chromatography. Data has allowed comparing the efficiency of the various methods for the removal of chlorides. It was concluded that, under the established experimental conditions, the more efficient method was the electrolytic reduction.
Descrição
Tese de mestrado em Química, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2010
Palavras-chave
Arqueologia Conservação Ferro Tratamentos electroquímicos Remoção de cloretos Teses de mestrado - 2010