EFFECTS OF ACID RAINS, ACID FOG AND ACID MIST
Acid rains, acid fog and acid mist cause quite serious damages to natural and man-made things. These damages may be studied under following categories:
Effects on materials, buildings and man-made objects
The chemical weathering and corrosive processes of materials like coated and uncoated carbon steel, painted steel, galvanized steel, nickel-plated steel, iron, copper, nickel and other metals exposed to rains, fog and mist of acidic pH is speeded in various ways. Ferrous metals are particularly attacked by oxides of sulphur. The iron rusts, its surface becomes flaky and flakes fall off to expose more metal thus resulting in continued corrosion. As a consequence, the ion in the buildings, vehicles, railway stock and tracks, electrical and telecommunication installations etc. suffer badly. In areas having acidic rains, fog and mist, corrosion of zinc products may be ten times faster than in clean areas. Acid rain and mist also damage paint coatings and thus expose the underlying material for further damage. Acidification of surface and groundwater in the affected areas results in corrosion of submerged structures and thus submerged parts of bridges, dams, industrial equipment, water storage tanks and hydro-electric turbines are seriously damaged.
All types of buildings, especially those built of sandstone, limestone and marble are seriously damaged and their rate of decay in affected areas is often 2-3 times higher than in unaffected areas. Both stone and the mortar of buildings is affected by acid rains.
Limestone buildings are worst affected due to reaction of sulphur with calcium carbonate in presence of moisture forming calcium sulphate which is soluble and is washed away with rainwater. Soluble sulphates, nitrates, chlorides and other salts being washed over the surface of stonework crystallize within the stone when the surface water evaporates. The expansion of such salts during crystallization enlarges the cracks leading to crumbling (exfoliation) of stone surface. This, in turn, further exposes the fresh underlying stone to chemical corrosion by acid rains. Cement that has high lime content is seriously affected by acid rains while sandstone which has high silica content, is comparatively less damaged. Main visible effect of acid rains on sandstone is formation of hard, black surface coating on the exposed surface. Granite, light-coloured stones and bricks become darkened and black in acid rains. Brick-built structures are less vulnerable than stone-built structures and for this reason, historical monuments, buildings and sculptors which are mostly made up of stone and marble, are seriously damaged by acid rains. In India, acid rain damage is markedly evident on Taj Mahel of Agra, Red For and Jama Masjid of Delhi.
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