Pitting Susceptibility of Concrete Reinforcing Steel Bars Having Manganese Sulfide Inclusions

Long-term wetting-drying treatment of specimens with reinforcing bars having manganese sulfide (MnS) inclusions in their microstructure showed a strong pitting tendency under sufficient chloride concentration. On the other hand, reinforcing bars free from MnS inclusions tested under identical conditions did not exhibit such pitting. The mass loss for both types of reinforcing bars computed after 3 years of exposure were comparable. However, the mortars embedded with manganese sulfide inclusion (MSI) reinforcing bars exhibited cracking after 3 years of wet/dry treatments. No such cracks were found for inclusion-free (IF) reinforcing bars. Scanning electron microscopy, space and time-dependent electro-chemical alternate current (AC) impedance, and potential studies were performed to examine the kinetics and mechanism of these reinforcing bars. The accelerating effect of chloride on pitting of metals and alloys with MnS inclusions has been explained in this paper considering the theory of acid regeneration cycle, increased solution conductivity, and the availability of high charge density on these ion forming compounds, which destabilized the passive layer. Raman spectroscopy and X-ray diffraction studies indicated the formation of ferric chloride in the rust layers of severely pitted MSI reinforcing bars. It is suggested that this ferric chloride, in the presence of oxygen and moisture, regenerated hydrochloric acid and lepidocrocite, resulting in autocatalytic chain reactions on the surface of MSI reinforcing bars.