Comportamento corrosivo em uma solução aquosa de solo e subproduto de termelétrica em liga de aço

Abstract

The corrosion caused by the soil in buried pipes in a thermal power plant in Candiota, RS, Brazil, involves costs for replacing degraded materials. During the burning of coal in this location, combustion gases are emitted which, when passing through the desulfurization system, together with hydrated lime, generate a desulfurization waste product. In this study, the corrosive behavior of carbon steel alloy in the soil mixed with fly ash from coal burning and desulfurization by-product was evaluated. The soil is mainly composed of silt, clay and sand. Standard aqueous soil solutions were prepared using 200 g / L of sieved soil and other solutions containing sieved soil (100 g / L), ash (100 g / L and 50 g / L) and desulfurization by-product (100 g / L and 50 g / L). Three other solutions were also prepared by adding 3%wt of NaCl to the first preparations containing soil and desulfurization by-product to have severe corrosion conditions. The corrosion of carbon steel alloy was evaluated by chronopotentiometry, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The potentiodynamic polarization curves showed a transpassive region with current densities of approximately 10 μA / cm² in the solution with 100 g / L of desulfurization by-product. The curves in the standard solutions, however, did not exhibit the same behavior. These results suggest that the addition of the desulfurization by-product to the solution probably induced a transpassive layer, thus reducing the corrosion rate of the steel alloy in the soil. The EIS confirmed a considerably higher polarization resistance of 470 kΩ for the transpassive layer than 0.7 kΩ for the film formed in the standard solution after ten days. Raman spectroscopy indicated that the transpassive layer was composed of lepidocrocite with a roughness of 16.05 nm measured by Atomic Force Microscopy.