The corrosion reaction for low-alloy steel is identical to that of regular steel; the difference can be seen in the effect of the alloys on the formation of the protective layer.
A good explanation of the mechanism of atmospheric corrosion of steel seems to be that of Misawa et al. , where corrosion begins with the anodic dissolution of iron into ferrous ions (Fe 2+ ). These ferrous ions react with moisture (hydrolysis) on the surface of the steel to form FeOH + . The FeOH + in turn reacts with the oxygen (oxidation) in the atmosphere to form -FeOOH, which crystallizes and precipitates out of the system (the rate of crystallization and precipitation is increased if a drying cycle occurs). Moisture mixed with pollutants, such as sulphur dioxide SO2, has a relatively low pH. In contact with this mixture, the crystalline -FeOOH dissolves to form the amorphous FeOx(OH)3-2x, which precipitates again. Finally, the FeOx(OH)3-2x undergoes a solid state transformation (deprotonation with hydroxyl ions from rainwater) to become -FeOOH
Kamimura et al.  examined the ratio of -FeOOH to -FeOOH in weathering steel exposed to industrial and rural environments, and the ratio of -FeOOH to -FeOOH, - FeOOH and Fe2O3 in weathering steel exposed to marine environments. They found that once these ratios achieved a certain value, the corrosion rate remains below 0.01 mm/year, which is considered a slow rate of corrosion. This finding essentially verified the research in [Misawa et al. 1974]. It should be noted that the corrosion process is fostered by accessible oxygen, the creation of an acidic solution layer by means of sulphur oxides (a product of vehicle exhaust, among other things), and a continuous wet-dry cycle
According to Misawa et al. , on a microscopic level, the corrosion layers of regular mild steel and weathering steel vary significantly. The layer on mild steel is uneven, with cracks and fissures that permit penetration of moisture, oxygen, and pollutants. The layer on weathering steel, however, is much more uniform and continuous; it is also found to contain relatively high amounts of the alloying elements particular to this type of steel, such as copper, chromium, and phosphorus. It appears, therefore, that these elements, which are evenly dispersed throughout the steel, enable the formation of a uniform layer of the corrosion product.