The ABH-973-48 sample shows minor pitting corrosion and some melt holes of varying sizes and shapes resulting from cracks in the as-built sample (Fig. 16c). The average pit size of the as-built sample (~7.8 μm) is three times larger than that of the ABH-973-48 sample (~2.5 μm), while the average pit size of the as-cast sample (~0.4 μm) is smaller than that of the ABH-973-48 sample. The extent to which corrosion pitting dominates depends on the constituents of the passive film.
EDS mapping analysis shows that the as-cast and ABH-973-48 samples have only small amounts of element segregation (Fig. 3a and Fig. 5a). Aggressive Cl- ions tend to attack regions with lower Cr first, forming and propagating corrosion pits on the surface. Therefore, higher Cr content (point 9) compared to matrix (point 10) can effectively protect against aggressive Cl-, forming a main composition of passive film (Cr2O3).
More severe element segregation is observed in Fig. 5, where the B2 phase constitutes Al-rich precipitates. EDS results for points 11 and 12 show that corrosion leads to formation of Al2O3 and TiO2, which are main parts of passivation film in ABH-973-48 sample. Also, Al and Ti content decreased significantly in corroded area, indicating selective dissolution of Al and Ti oxides/hydroxides in passivation film (region R20), leaving only Cr-rich oxides/hydroxides in passive film (Fig. 16c). Passive film on stainless steel mainly consists of Cr oxide, and selective dissolution of Fe during passivation enriches Cr in passive film [74].
Results also showed that selective corrosion of Al-rich phase is caused by galvanic corrosion in annealed sample. The B2 phase has a lower corrosion potential than the FCC matrix, leading to preferential corrosion of the B2 phase and slowing down the corrosion attack on the FCC matrix. Consequently, the annealed sample exhibited an enhanced degree of corrosion resistance.
