Development of a novel method for the in-situ dechlorination of immovable iron elements: optimization of Cl− extraction yield through experimental design

Abstract

The conservation of iron objects exposed to marine aerosol is threatened by the formation of akaganeite, a highly unstable Cl-bearing corrosion phase. As akaganeite formation is responsible of the exfoliation of the rust layer, chlorides trigger a cyclic alteration phenomenon that often ends with the total consumption of the iron core. To prevent this degradation process, movable iron elements (e.g. archaeometallurgical artefacts) are generally immersed in alkaline dechlorination baths. Aiming to transfer this successful method to the treatment of immovable iron objects, we propose the in-situ application of alkaline solutions through the use of highly absorbent wraps. As first step of this novel research line, the present work defines the best desalination solution to be used and optimizes its extraction yield. After literature review, a screening experimental design was performed to understand the single and synergic effects of common additives used for NaOH baths. Once the most effective variables were selected, an optimization design was carried out to determine the optimal conditions to be set during treatment. According to the experimental work here presented, the use of 0.7 M NaOH solutions applied at high temperatures (above 50 °C) is recommended. Indeed, these conditions enhance chloride extraction and iron leaching inhibition, while promoting corrosion stabilization.

Tratamiento de conservación a gran escala de piezas de artillería pesada de hierro fundido y procedencia subacuática. Procedimientos metodológicos

“…Este resultado es especialmente interesante ya que revela la efectividad de la misma en condiciones de temperatura ambiental media-alta, validando así su potencial aplicación a pie de obra en piezas de gran envergadura de origen marino. En un trabajo reciente se ha planteado como novedad la aplicación de un procedimiento similar al nuestro para hierros inamovibles consistente en la aplicación de NaOH mediante envolturas de material absorbente (Veneranda 2021), utilizado normalmente para la extracción de iones cloruro en piedra (Pozo A., J. S. et al 2016), demostrando la originalidad de la técnica aplicada en su momento en nuestras piezas.…”