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Pyrrhotite requires both iron and sulfur to form. [6] Iron is the fourth most abundant element in the Earth's continental crust (average abundance of 5.63 % or 56,300 mg/kg in the crust), [21] and so the majority of rocks have sufficient iron abundance to form pyrrhotite. [6]
Oxidation of pyrite in clay formations in contact with concrete – this produces sulfuric acid which reacts with concrete. Bacterial activity in sewers – anaerobic sulfate reduction at work in the organic-rich sludges accumulated under water in the conduits produces hydrogen sulfide gas (H 2 S).
Iron pyrite was heaped up and allowed to weather (an example of an early form of heap leaching). The acidic runoff from the heap was then boiled with iron to produce iron sulfate. In the 15th century, new methods of such leaching began to replace the burning of sulfur as a source of sulfuric acid. By the 19th century, it had become the dominant ...
Example of flat piece of concrete having dislodged with corroded rebar underneath, Welland River bridge across Queen Elizabeth Way in Niagara Falls, Ontario. The expansion of the corrosion products (iron oxides) of carbon steel reinforcement structures may induce internal mechanical stress (tensile stress) that cause the formation of cracks and disrupt the concrete structure.
These soils contain iron sulfide minerals (predominantly as the mineral pyrite) and/or their oxidation products. In an undisturbed state below the water table, acid sulfate soils are benign. However, if the soils are drained, excavated or otherwise exposed to air, the sulfides react with oxygen to form sulfuric acid. [1]
Anaerobic corrosion is evident as layers of metal sulfides and hydrogen sulfide smell. On cast iron, a graphitic corrosion selective leaching may be the result, with iron being consumed by the bacteria, leaving graphite matrix with low mechanical strength in place. Various corrosion inhibitors can be used to combat microbial corrosion.
The hydrogen sulfide gas is biochemically oxidized in the presence of moisture to form sulfuric acid. The effect of sulfuric acid on concrete and steel surfaces exposed to severe wastewater environments can be devastating. [1] In the USA alone, corrosion causes sewer asset losses estimated at $14 billion per year. [2]
Sulfate-reducing bacteria oxidize the molecular hydrogen to produce hydrogen sulfide ions (HS −) and water: 4 H 2 + SO 4 2− → HS − + 3 H 2 O + OH −. The iron ions partly precipitate to form iron (II) sulfide. Another reaction occurs between iron and water producing iron hydroxide. Fe 2+ + HS − → FeS + H + 3 Fe 2+ + 6 H 2 O → 3 ...