Initial microzonal corrosion mechanism of inclusions associated with the precipitated (Ti, Nb)N phase of Sb-containing weathering steel.

• There are the combined inclusions and precipitates within the steel sample, and the morphologies of the combinations are different. • The precipitated (Ti, Nb)N phases have the highest Volta potential, followed by that of the matrix. The inclusions have the lowest Volta potential. • When precipitated phases completely encapsulated inclusions, the rates of pit initiation and propagation are low. • When inclusions are in contact with the matrix, pitting initiation occurs very quickly and is accompanied by the generation of corrosion products. In this work, the effects of combined inclusions and precipitates on the initiation and propagation of pits in weathering steel were studied. The SKPFM results showed that the precipitated (Ti, Nb)N phases had the highest Volta potential, followed by that of the matrix. The inclusions had the lowest potential. The corrosion initiation and propagation processes were observed, in situ, at the site of the combined inclusion and precipitate; the matrix around the precipitated phase dissolved slowly. When inclusions were in contact with the matrix, they rapidly dissolved, forming pits. Subsequently, the surrounding matrix dissolved rapidly, causing the pits to expand. [ABSTRACT FROM AUTHOR]