Corrosion of calcareous stones in humid air containing SO 2 and NO 2

Samples of limestone, marble and travertine were exposed to mixtures of sulphur dioxide and nitrogen dioxide gases in humid air. A strong synergistic effect on corrosion was found when the gases were combined. Sulphur dioxide alone causes the formation of calcium sulphite while in the presence of nitrogen dioxide as well, the sulphate (gypsum) is produced the mexhanism of deposition and the catalytic oxidation of suuphur dioxide are discussed.