Corrosion Behavior of Ultra-High-Strength Hot-Press-Formed B-Pillar Parts

The corrosion behavior of hot-press-formed (HPF) B-pillar components fabricated from Al-Si-coated boron steel was investigated with an emphasis on the forming-induced crack morphology. The specimens were extracted from the inner and outer surfaces of the top, flat, and radius regions. Microstructural characteristics and coating cracks were examined using optical microscopy, as well as field-emission scanning electron microscopy (FE-SEM) in combination with energy-dispersive spectroscopy (EDS), and corrosion behavior was evaluated using cyclic corrosion immersion and potentiodynamic polarization tests in a 3.5 wt.% NaCl aqueous solution. The Al-Si coating exhibited a multilayered structure composed of alternating Al- and Fe-rich layers. The crack morphology strongly depended on the local stress state: wide macrocracks were mainly formed on the outer surface of the radius region under tensile deformation, whereas the narrow microcracks predominated on the inner surface subjected to compressive deformation. Cyclic corrosion immersion tests showed that the corrosion propagated preferentially along the coating cracks and was more severe on the inner surfaces, where narrow microcracks promoted aggressive crevice corrosion owing to chloride ion accumulation and local acidification. By contrast, wider macrocracks on the outer surface mitigated crevice corrosion by allowing electrolyte exchange. Potentiodynamic polarization tests indicated similar corrosion rates for all regions; however, the outer radius region exhibited a relatively noble corrosion potential owing to oxide film formation on the locally exposed substrate areas. These results demonstrate that the crack morphology induced by curved forming is a key factor governing the corrosion behavior of HPF B-pillar components.