Corrosion at the Weld Nugget of the Friction-Stir-Welded Medium Strength Steel: Effect of Microstructure

In the present investigation, friction stir welding (FSW) was carried out on C-Mn steel using various welding parameters. The microstructural examination of the weld nugget (WN) region revealed different area fractions of cementite, pearlite, bainite, and martensite within the ferrite matrix. The average hardness in the region of the WN was higher than the base material. The corrosion behavior of the WN and base metal was investigated. The base metal underwent uniform corrosion. Localized pitting corrosion was dominant in the region of the WN. The primary corrosion products were γ-Fe2O3 (maghemite), γ-FeOOH (lepidocrocite), and Fe3O4 (magnetite). The corrosion rate of the WN was significantly higher than that of the base metal. However, the same was considerably lower than the conventionally fusion-welded specimens. Further, the corrosion rate of the WN increased with the increment in the rotational speed of the tool. The change in the corrosion rate at the WN for various specimens was influenced by the matrix grain size, quantity of second phase, area fraction of special boundary, and relative quantity of low- and high-angle grain boundaries.