Abstract
In this study, we investigated by FIB-SEM the microstructure of protective coatings formed by two inorganic treatments (namely, ammonium oxalate and ammonium phosphate solutions), with the aim of identifying the factors limiting the protective efficacy of the coatings and outlining possible improvement strategies. In the case of the oxalate treatment, the resulting layer of whewellite was found to contain vertical channels, which can allow water to reach the marble surface and trigger dissolution. Possible prevention of the formation of these channels by addition of a calcium source to the oxalate solution was attempted, but even micromolar additions led to rapid precipitation in the solution. In the case of the phosphate treatment, the resulting hydroxyapatite and octacalcium phosphate coating was found to be cracked and porous. To prevent cracks and pores, diminishing the ammonium phosphate concentration and adding ethanol to the solution were found to be effective strategies, as cracks were prevented and pores were reduced almost to zero. The resulting protective efficacy of the coating was found to be significantly improved, although still not perfect.
Highlights
Marble sculptures and architectural decorations exposed outdoors are subject to dissolution in rain, because of the slight solubility of calcite in water [1]
We investigated by focused ion beam (FIB)-SEM the microstructure of protective coatings formed by two inorganic treatments, with the aim of identifying the factors limiting the protective efficacy of the coatings and outlining possible improvement strategies
We present an example of the use of a focused ion beam (FIB) microscope to obtain cross sections of protective coatings with minimum alteration of the original microstructure
Summary
Marble sculptures and architectural decorations exposed outdoors are subject to dissolution in rain, because of the slight solubility of calcite in water [1]. Organic protectives have been widely investigated and applied in the past decades In spite of their high efficacy in making marble hydrophobic, preventing dissolution in rain, organic protectives have exhibited severe compatibility and durability issues [2]. Inorganic treatments are aimed at forming a superficial layer of a new mineral, able to prevent dissolution of the substrate. For this purpose, the newly formed layer needs to meet several requirements: (i) it must have reduced solubility, compared to calcite; (ii) it needs to be continuous, to prevent bare areas from triggering dissolution; (iii) it needs to be crack-free and pore-free, to prevent rain from reaching the substrate by permeation through the protective layer. SEM observation of cross sections encapsulated in resin could be one route, but possible alteration of the cross section by resin penetration and polishing needs to be taken into account
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