Development of a novel method for the in-situ dechlorination of immovable iron elements: optimization of Cl\u2212 extraction yield through experimental design

Marco Veneranda,Jose Antonio Carrero,Kepa Castro,Ilaria Costantini,Juan Manuel Madariaga,Aitor Larrañaga,Gorka Arana,Nagore Prieto-Taboada

doi:10.1038/s41598-021-90006-y

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.

Highlights

The conservation of iron objects exposed to marine aerosol is threatened by the formation of akaganeite, a highly unstable Cl-bearing corrosion phase
Akaganeite formation triggers a cyclical mechanism that often ends with the total consumption of the iron c ore[15]
One of the most conventional repair strategies consist in the re-passivation of rebars, followed by the replacement of the contaminated concrete with an alkaline material highly resistant to chloride penetration

Summary

Introduction

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. According to the experimental work here presented, the use of 0.7 M NaOH solutions applied at high temperatures (above 50 °C) is recommended These conditions enhance chloride extraction and iron leaching inhibition, while promoting corrosion stabilization. To prevent the Cl-driven alteration of iron components, preservation treatments are required to (1) extract Cl− ions penetrated within the iron-corrosion layers and (2) shelter dechlorinated elements from further chloride contaminations. One of the most conventional repair strategies consist in the re-passivation of rebars (e.g., by applying surface corrosion inhibitors[27] or by using electrochemical chloride extraction methods28), followed by the replacement of the contaminated concrete with an alkaline material highly resistant to chloride penetration

Methods

Results

Conclusion