Resistance to simulated rain of hydroxyapatite- and calcium oxalate-based coatings for protection of marble against corrosion

- Resistance to dissolution in simulated rain was tested for untreated/treated marble.
- Various treatments based on ammonium phosphate and ammonium oxalate were compared.
- Marble dissolution was reduced by phosphate treatment involving ethanol addition.
- The ethanol-based phosphate treatment outperformed commercial ammonium oxalate.

Dissolution in pure and acid rain is among the main environmental degradation processes of marble. Coatings based on hydroxyapatite (HAP) have been proposed to prevent marble corrosion, providing encouraging results. In this paper, the resistance to dissolution was investigated on marble treated with different HAP-treatment formulations and compared to commercial ammonium oxalate. The ability of the coating to prevent corrosion was evaluated by performing wetting/drying cycles, by a custom-designed apparatus simulating rain. When ethanol is added to the solution used to form HAP, a significant reduction in calcium ions leached from marble is found, which is indicative of reduced corrosion.