Atmospheric Corrosion of Metallic Coated and Pre-painted Steel Roofing Sheets as Influenced by the Environment in Diani Beach, Mariakani, and Mlolongo Sites in Kenya.

Abstract

Atmospheric corrosion of roofing sheets contributes to devastating economic losses to the construction industry. Due to the diverse nature of atmospheric environmental conditions that roofing sheets and metallic structures are exposed to, there is a need to study the atmospheric corrosive behavior of metallic coated and pre-painted steel roofing sheets in different atmospheric environments. Roofing sheets were obtained from Mabati Rolling Mills and the other four selected manufacturers, cleaned, and cut into 130*190 mm sizes. The sheets were weighed and mounted on the rack and exposed to the atmosphere for two years at Diani beach-severe marine, Mariakani-marine, and Mlolongo-urban industrial environments. Atmospheric pollutants sulphur dioxide, nitrogen dioxide, and chloride were collected by active sampling and analyzed by spectrophotometric method and ion chromatography for chloride. The fading and glossiness of pre-painted sheets upon exposure were assessed at each of the sites using spectroguide: (BYK Gardner 82638 GmbH). The levels of nitrogen dioxide were highest at Mlolongo (146.12±7.30 μg/m3) in the dry season, followed by Mariakani (116.14±14.35 μg/m3) and Diani (112.39±10.59 μg/m3). The levels of sulphur dioxide were higher in the dry season compared to the wet season with concentrations of 859.50±105.40 μg/m3 at Mlolongo, 695.67±53.72 μg/m3 at Mariakani, and 438.71±45.06 μg/m3 at Diani. Chloride levels were highest in the wet season at Diani (3.48±12.12 μg/m3), followed by Mariakani (2.13±0.19 μg/m3) and Mlolongo (1.22±0.10 μg/m3). Relative humidity was highest at Diani (81 %), followed by Mariakani and Mlolongo, 78 % and 70 %, respectively. Mlolongo registered the highest mean rainfall of 114 mm, followed by Mariakani at 111mm, and Diani at 93 mm. Ultraviolet radiation was highest at Diani with 12 mW/cm2, followed by Mariakani (6 mW/cm2) and lowest at Mlolongo (5 mW/cm2). The average temperature for Diani and Mariakani was 27 oC, while Mlolongo recorded 21 oC. Mlolongo performed better in terms of fade resistance for both single and double-coat pre-painted sheets. Single coat performed poorly at 0.56±0.00, 3.90±0.06 and 1.51±0.00 Hunter units per year (Hupy) for charcoal, tile red, and sky blue, except for dark green (1.29±0.01) while glossiness order followed charcoal 47.50±12.30 %, tile red 52.90±3.75 % and sky blue 79.60±2.63 %. The data suggest that charcoal roofing sheets are superior for both marine and urban-industrial environments. Blue and green roofing sheets from MR performed better at Diani with the fade of 0.77±0.12 and 0.86±0.00 Hupy, while gloss retention was 63.51±12.85 % and 77.45±0.00 % respectively. Blue sheets performed better than green suggesting the superiority of the product in all environments. The corrosion rates of the Al-Zn coated roofing sheets for the coating masses Al-Zn 85 and Al-Zn 70 varied from 0.089-0.091 and 0.066-0.070 mmpy respectively, indicating that these roofing sheets were suitable for use in both marine and urban- industrial environments. The corrosion rate for the roofing sheets was in the order of Diani ˃ Mlolongo ˃ Mariakani. Diani beach had the highest corrosion rate values of 0.332±0.09 for Al-Zn 150, 0.089±0.00 for Al-Zn 85, 0.070±0.01 for Al-Zn 70 and 0.072±0.00 mmpy for Al-Zn 60. High chloride ions, sulphur dioxide and nitrogen dioxide levels contributed to the differences in corrosion rate observed at the sites. The corrosion product of Al-Zn coated steel was composed of α-FeOOH, γ- Fe2O3, β-FeOOH and α-Fe2O3. Chloride ions and sulphur dioxide served as a catalyst in promoting the corrosion of Al-Zn-coated steel sheets in the marine environment, leading to γ-FeOOH, the final product of corrosion formation. The Fourier transform infrared radiation spectral analysis of exposed and controlled pre-painted roofing sheets showed that ultraviolet radiation caused the degradation of paint components. The findings of this study are significant in improving the quality of roofing sheets to withstand diverse corrosive atmospheric environments.