Electrodeposition of Hydroxyapatite Coatings for Marble Protection: Preliminary Results

Enrico Sassoni,Giulia Masi,Maria Bignozzi,Elisa Franzoni

doi:10.3390/coatings9030207

Abstract

Surface coatings made of hydroxyapatite (HAP) have been proposed to protect marble artworks from dissolution in rain, originated by the aqueous solubility of calcite. However, HAP coatings formed by wet chemistry exhibit incomplete coverage of marble surface, which results in limited protective efficacy. In this study, electrodeposition was explored as a new route to possibly form continuous coatings over the marble surface, leaving no bare areas. Electrodeposition was performed by placing marble samples in poultices containing the electrolyte (an aqueous solution with calcium and phosphate precursors) and the electrodes. The influence of several parameters was investigated, namely the role of the working electrode (cathode or anode), the distance between the marble sample and the working electrode, the deposition conditions (potentiostatic or galvanostatic), the electrolyte composition and concentration, the applied voltage, and time. The coating morphology and composition were assessed by SEM/EDS and FT-IR. The protective ability of the most promising formulations was then evaluated, in all cases comparing electrodeposition with traditional wet synthesis methods. The results of the study suggest that electrodeposition is able to accelerate and improve formation of HAP coatings over the marble surface, even though the obtained protective efficacy is not complete yet.

Highlights

When exposed outdoors, marble artworks suffer from dissolution in rain because of the aqueous solubility of calcite
A possible strategy is forming a surface coating with reduced solubility, compared to calcite. This has been attempted by several routes, e.g., by treating marble with aqueous solutions of ammonium oxalate, ammonium hydrogen tartrate, and ammonium phosphate to transform calcite into calcium oxalate, calcium tartrate, or calcium phosphates, respectively [2]
The specimens were subjected to the electrodeposition tests described in the following without preliminary polishing, because a very flat electrodeposition tests described in the following without preliminary polishing, because a very flat surface would not be representative of marble condition in the field

Summary

Introduction

Marble artworks suffer from dissolution in rain because of the aqueous solubility of calcite (the mineral constituting marble, having solubility product Ksp = ~5 × 10−9 at 25 ◦ C [1] and dissolution rate Rdiss = ~10−10 mol cm−2 ·s−1 [2]). A possible strategy is forming a surface coating with reduced solubility, compared to calcite. This has been attempted by several routes, e.g., by treating marble with aqueous solutions of ammonium oxalate, ammonium hydrogen tartrate, and ammonium phosphate to transform calcite into calcium oxalate, calcium tartrate, or calcium phosphates, respectively [2]. HAP can be formed in a few hours in ambient conditions (necessary to treat monuments in the field), by reacting marble with an aqueous solution of a phosphate

Methods

Discussion

Conclusion