Steel Corrosion by Air: South of the Sahara

Steel is enriched iron (74.5-98.4% Fe) with major metal components and trace elements but its impurities affect the strength. Exposure to an electrolytic air/solution in the atmosphere oxidises the iron component into iron oxides to cause corrosion which speeds up after a long period of exposure at critical pH range of 7.85.5. This deterioration may occur uniformly or unevenly when a potential difference is created which may show defects of various textures as pits. Based on the concentration of chlorite, corrosion either commences at the outer or inner rim. Aggressive local corrosion may occur within cracks and crevices. Weathering of rocks could also release into the air and rain, soluble salts which may include sodium chloride. Fine dust which is more severe during the Harmattan over the region south of the Sahara Desert, usually contains sulphide and various salts that could reduce humidity levels. Hence in the Sahel regions, the first rainwater contains also inorganic anions such as NO3- and SO42- while suspended dust in the air from soil may have elevated levels of Cl− which promotes corrosion. The rate of electrochemical reactions, diffusion processes and increase in temperature also speed up rate of corrosion.