Atomic-scale dissolution corrosion mechanism of additively-manufactured 316L steels in liquid lead-bismuth eutectic

Abstract

In this paper, the dissolution corrosion mechanism of 316L steels fabricated by laser-powder-bed-fusion (LPBF) with and without subsequent hot-isostatic-pressing (HIP) has been studied under multilength scales after exposure to static lead-bismuth eutectic (LBE) with a reactor-relevant oxygen concentration of ∼5×10⁻⁷wt.% dissolved, at 500°C for up to 4000h. The results show that both steels are subjected to dissolution corrosion with LBE preferentially attacking defective areas. The average dissolution depths of “LPBF 316L” steel are much larger than those of “LPBF+HIP 316L” steel, suggesting that the nonequilibrium defects generated by LPBF significantly exacerbate dissolution corrosion, whereas dissolution resistance recovers following HIP treatment. The dissolution zones invariably undergo austenite-to-ferrite phase transformation. Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) are the predominant orientation relationships (ORs) of α and γ phases, especially in “LPBF+HIP 316L” steel. While Pitsch OR is also identified, it constitutes only a small fraction. Atomic-resolution characterization reveals that at the penetration tips along mechanical nanotwin boundaries, a one-nanometer-thick, Bi- and Ni-rich, coherent interfacial phase forms at the interface between steel matrix and a nanosized amorphous oxide scale, mediating the leaching of Ni. Theoretical computations confirm that Bi is energetically more favorable than Pb in segregating on austenitic steel surfaces. The interfacial segregation of Bi facilitates the outer diffusion of Ni due to the negative enthalpy of mixing of the Ni-Bi system. Fe and Cr are also extracted through oxidation/decomposition processes. Consequently, the dissolution process involves simultaneous removal of all three steel elements, rather than just Ni. An atomic-scale dissolution mechanism scheme is proposed.