MAIN AGENTS OF ALTERATION IN COVERINGS

The main agents of alteration in coverings refers to acidulous substances conventionally handled in domestic environments, but particularly due to active acid rains over external coverings. Amongst potentially harmful domestic substances for the stones, related to the chemical attack and staining, it is cited: citric fruit (specially lemon), vinegar, cleaning products (acid and alkaline), soft drinks, isotonic drinks, gasoline, kerosene, alcoholic colored beverages (most specifically red wine), oily liquids, oils and greases in general.

The term “acid rain” means strictly the humid precipitation of acid constituents. These constituents, derived from biological and anthropogenic activities, are dispersed as gases and particulate composites, becoming dissolved into clouds and raindrops, forming acidulous solutions. More widely, there happens as much humid deposition (acid rain), as dry deposition of gaseous and particulate acid pollutants. In this case the expression “acid deposition” can be used , as the sum of the processes of humid and dry deposition of acid composites. For a more practical approach, it appears that both processes are considered important for the acidification of the physical environment.

Surveys done by CETESB - Company of Technology of Ambient Sanitation (Companhia de Tecnologia de Saneamento Ambiental) - and CETEC - Foundation Technological Center of Minas Gerais (Fundação Centro Tecnológico de Minas Gerais), have made evident the occurrence of acid rain in the Southeastern region of Brazil. The studies of CETEC, in Belo Horizonte, Betim and Contagem (Minas Gerais), had also demonstrated that acidity increases along the rainy period, because of the exhaustion of calcium and other alkaline particles available in the atmosphere for neutralization. The negative impact of acid rains are disclosed in aquatic environments, over ground and vegetation, causing corrosion of metallic structures and painted surfaces, as well as spoiling building materials, paper, leather, fabrics and the above cited carbonate stones.

Among the reactive and acid natured gases present in the atmosphere, the carbonic gas is distinguished (CO2): dissolved in water, it turns into carbonic acid (H2CO3). Within normal concentration and pressure of Co2 in the atmosphere (340 ppm and 1 atm), the rain water pH reaches 5,65, value below which is hence defined the acid rain.
Rains with pH lower than 5,65, are due whether to higher concentrations of H2CO3, or to the contamination by sulfuric acid (H2SO4) and nitric acid (HNO3). Both sulfuric and nitric acids are most of all generated by anthropogenic sources, through the watery dissolution of SO2 and NO2 in the atmosphere.

The chloride acid (HCl), also discriminated as an agent of environmental degradation, happens mainly over the seashores, through the atmospheric dispersion of NaCl (marine salt), acting as much through acid rains, as through marine aerosol.