Abstract
The effect of Cr(VI) pollution on the corrosion rate of corrugated iron roof samples collected from tanning industry areas was investigated through simulated laboratory exposure and spectrophotometric detection of Cr(III) deposit as a product of the reaction. The total level of Cr detected in the samples ranged from 113.892 ± 0.17 ppm to 53.05 ± 0.243 ppm and showed increasing trend as sampling sites get closer to the tannery and in the direction of tannery effluent stream. The laboratory exposure of a newly manufactured material to a simulated condition showed a relatively faster corrosion rate in the presence of Cr(VI) with concomitant deposition of Cr(III) under pH control. A significant (P = 0.05) increase in the corrosion rate was also recorded when exposing scratched or stress cracked samples. A coupled redox process where Cr(VI) is reduced to a stable, immobile, and insoluble Cr(III) accompanying corrosion of the iron is proposed as a possible mechanism leading to the elevated deposition of the latter on the materials. In conclusion, the increased deposits of Cr detected in the corrugated iron roof samples collected from tanning industry zones suggested possible atmospheric Cr pollution as a factor to the accelerated corrosion of the materials.
Highlights
Accelerated corrosion of corrugated iron roof is a subject of global concern because of its importance to the service life of the material and its aesthetic appearance [1, 2]
Atmospheric corrosion is the result of a redox reaction between the metal component of the material and its atmospheric environment that occurs in the presence of a conducting thin aqueous adlayer [3]
Chromium is naturally present in corrugated iron roof as component of the steel alloy
Summary
Accelerated corrosion of corrugated iron roof (galvanizedsteel) is a subject of global concern because of its importance to the service life of the material and its aesthetic appearance [1, 2]. The ability of zinc to galvanically protect iron is relatively effective in neutral environment but very sensitive to any change of atmospheric acidity [2] Air pollutants such as sulfur dioxide, hydrogen sulphide, oxides of nitrogen, and chlorides and weathering factors such as temperature, moisture, rainfall, solar radiation, and wind velocity have been recognized as conventional atmospheric parameters that may contribute to the corrosion [1,2,3]. The presence of metallic pollutants such as Cr that can galvanically couple with the iron of the roofing material through the adlayer can accelerate the degradation process [4,5,6,7]. Reduction of Cr(VI) to Cr(III) by reaction with zerovalent iron (Fe0) and subsequent precipitation of Cr(III) oxyhydroxides can occur through the reaction sequence [4]: CrO24−(aq) + Fe0(s) + 8H+(aq) → Fe3(a+q) + Cr3(a+q) + 4H2O(l)
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