Analysis of corrosion scale structure of pre-oxidized stainless steel 316 in molten lead bismuth eutectic and the relation to impedance spectroscopy response

Stainless steel 316 is a candidate structural material for heavy liquid metal spallation neutron sources and fast reactor systems. Impedance spectroscopy (IS) provides a means of real-time measurements of the stability of protective oxide scales in these systems. The IS technique has been shown to yield quantitative results at lower temperatures, but does not yet yield quantitative results at temperatures of interest for engineering applications. To understand this, pre-oxidized stainless steel 316 samples were immersed in oxygen-saturated stagnant lead bismuth eutectic at 550 °C for 1335 h. During the exposure, real-time impedance spectroscopy was taken from different samples. Negligible impedance was observed from one sample at the end of 1335 h, indicating that the oxide scale was non-protective. A variety of microanalysis techniques revealed that oxide layers formed on the sample surface suffered corrosion attack from the LBE. A duplex oxide structure composed of outer iron oxide and inner iron chromium oxide was found on some sample surface area. The major phases of the duplex oxide scales were Fe₃O₄ and FeCr₂O₄. The high conductivity of this duplex scale explains the corrosion rate as well as the lack of IS response.