The adverse effects of nitrates on stone monuments

The adverse effects of nitrates on Pentelic marble and other calcareous stones have been investigated in the field and the laboratory. The concentrations of photochemically formed nitrates in the atmosphere of central Athens were measured over a period of 1 year and were found to be as high as in other polluted cities. The surface density of nitrates on the Parthenon at the end of the dry season was found to be 6.9 μg cm−2. The extent of corrosion of Pentelic marble by the dry deposition of nitrates and sulphates on the Acropolis hill were compared with the aid of a specially designed reactor. It was found that they were of the same order of magnitude. A modified version of this reactor was used to compare the corrosive action of the nitrates and sulphates with that of NOx and SO2. Finally, a nitric acid generator was used in the laboratory to investigate the corrosive action of this acid on Pentelic marble. The variables studied were the surface area of stone, concentration of nitric acid, flow rate of the corrosive mixture and relative humidity. It was found that nitric acid, under almost all conditions, is a very corrosive agent for marble stones.