Effects of mill scale and varying exposure conditions on corrosion behavior of A572 structural steel

Today, many steel structures are multi-decades old and commonly suffer from corrosion that induces progressive loss of the base metal in various forms including localized and distributed losses. In addition, Old steel structures were constructed using as-delivered steel plates without sand-blasting of the mill scale. While the mill scale provides a protective layer in general, it adds a layer of complexity to the electro-chemistry of the corrosion process. In this study, the evolution of corrosion of A572 structural steel is investigated under varying aqueous environmental conditions. Dog-bone samples were cut from steel plates without removing their mill-scale and were conditioned in aerated water baths containing deionized water with 0%, 1%, 2%, and 3% chloride concentrations for up to 16 months. For each concentration, 3 baths were prepared and stored at 26 °C, 40 °C, and 50 °C. An additional experimental program (with a three-month duration) was performed using coupon samples with and without mill scale at room temperature with 0% and 1% chloride contents to further investigate the effect of mill scale on corrosion initiation. The results can be subdivided into two main categories: (1) The effects of exposure conditions and mill scale presence on corrosion morphology and progress rate; (2) The effects of corrosion morphology on mechanical strength and ductility. Temperature had an obvious activation effect on the corrosion rate supporting the conclusion that the corrosion process is driven by a nucleation and growth mechanism. The higher the temperature the higher the corrosion rate and the more uniform the corroded surface is with more distributed corrosion initiation sites and smaller early pit dimensions. Chloride concentration showed an expected saturation trend between 1% and 2% even with mill scale presence which is explained by the effect of excess chloride presence on the reduction of dissolved oxygen content. However, the chloride-free samples showed evident pitting corrosion. After further analysis, it was concluded that the solution was not contaminated and the only source for this observed corrosion initiation is the presence of the mill scale that contains, cracks, crevices, and oxides expected to have dissolved in water within the crevices and initiated the pitting corrosion. Additionally, the mill scale was shown to induce nonuniform corrosion at all chloride concentrations increasing the roughness of the corroded metal surface. On the mechanical properties side, the reduction in tensile strength showed a direct linear correlation with the weight loss with very limited dependence on corrosion morphology. However, the reduction in ductility was very sensitive to the corrosion morphology showing much more reduction for rough corrosion surfaces as compared to smoother ones.