Mechanism of Sulphation by Atmospheric SO 2 of the Limestones and Marbles of the Ancient Monuments and Statues: II. Hypothesis concerning the rate determining step in the process of sulphation,and its experimental confirmation

Abstract

This work is a continuation of that published in Part I (BCJ, 1981, 16, 63) under the same title.

In this paper a new hypothesis is presented to explain the mechanism of the sulphation process taking place on marbles and limestones in humid, SO₂-containing atmospheres. It is suggested that the reaction rate, in the initial, rectilinear, stage of the process (up to∼300 Å thickness of gypsum film) is determined by the rate of desorption of CO_3(aq) ²⁻(CO₂ + ½O₂ + 2e)from the CaCO₃ surface, and its diffusion through pores in the gypsum film towards the SO₂-containing environment.

In the second, parabolic, stage, at thicknesses > 300 Å, the gypsum is no longer porous and growth is now controlled by solid state diffusion of Ca²⁺ through the gypsum to the environment. The system can be conceived as a galvanic cell with the CaCO₃ as the negative pole, (SO₂ + air + H₂O) the positive pole, and the gypsum itself as the electrolyte. This parallels the Wagner mechanism for film growth on metals and, following the Wagner theory, expressions are given for the rate constant of the growth of the gypsum film in the parabolic stage, and the driving potential of the galvanic cell involved, both expressions in terms of the physical characteristics of the various participating species.The various steps in the hypothesis have been verified experimentally.