Effect of Austempering Time on Electrochemical and Immersion Corrosion Behaviour of High Carbon, Carbide-Free Nanostructured Bainitic Steel in an Aqueous 3.5% NaCl

Abstract

Abstract: In the present study, carbide-free nanostructured bainite is produced by austempering high carbon and silicon steels at 250 °C. Fine pearlitic structure is also made by patenting the steels of selected compositions at 550 °C. The formation of carbide-free nanostructured bainite and retained austenite in the austempered steels and ferrite and cementite in patented steels were confirmed through X-ray diffraction and TEM studies. The optical, scanning and transmission electron microscopies reveal the presence of nanoscale bainite, filmy and blocky austenite in austempered steels and the presence of lamellar pearlite in patented steels. Increasing austempering time enhances the amount of bainite but decreases the blocky retained austenite. The corrosion behaviour of the newly developed bainitic steels is compared with that of pearlitic steel of the same compositions. Electrochemical and immersion corrosion tests are conducted in an aqueous 3.5% NaCl solution. The corroded surfaces are analyzed with the scanning electron microscope (SEM) and X-ray photoelectron spectrometer (XPS). Corrosion resistance increases with increasing carbon percentage due to the formation of magnetite (Fe3O4), provided the content of other alloying elements remains the same. The presence of Ni decreases corrosion rate significantly, i.e., enhances corrosion resistance, charge transfer resistance (Rct) and polarization resistance (Rp). Reducing retained austenite in nanostructured steel decreases the corrosion rate due to the lesser area of Galvanic couples and the formation of uniform, compact, and non-porous passive layer. The bainitic steel samples demonstrated superior corrosion resistance to that of the pearlitic steel due to lesser cell formation. The corrosion mechanisms of high-carbon bainitic and pearlitic steel are discussed. Graphic Abstract: [Figure not available: see fulltext.]. © 2023, The Author(s) under exclusive licence to The Korean Institute of Metals and Materials.