Environmental, Processing, and Fabrication Factors Affecting Pitting Corrosion of SS316L

The maintenance and repair of the storage canister for spent nuclear fuel are of importance to safely maintaining the national strategic fuel supply chain. End-of-life storage is an integral component of the infrastructure supporting the long-term viability of the existing U.S. reactor fleet. Stainless steels (SSs) are widely used as a structural material for nuclear power plants. Specifically, in the dry storage of spent nuclear fuels within the concrete cask, SS canisters are used as an important barrier to separate spent fuel (radioactive material) and the environment. Due to the formation of a protective passive film, austenitic SSs have a wide spectrum of resistance to atmospheric corrosion. However, such passive film is susceptible to breakdown in the presence of chloride ions, resulting in pitting corrosion. It is known that RH and temperature fluctuations tend to influence corrosion under natural exposure conditions. Nevertheless, most work performed up to date primarily includes immersion testing. In addition, the fabrication method of the canisters includes welding and if it is not heat treated correctly, it can introduce a sensitized microstructure. In this research the initial stage of atmospheric corrosion under seawater and MgCl2 solutions droplets on austenitic SS 316L was evaluated. The current research aims to understand the effect of environmental, fabrication (specifically additive anufacturing), and processing parameters such as droplet size, relative humidity (RH), sensitization, temperature, and chloride droplet solution on the atmospheric pitting corrosion of SS 316L. For this purpose, atmospheric pitting resistance testing, electrochemical techniques (under atmospheric and immersion conditions), microstructure characterization, and thermodynamic modeling were performed using potentiodynamic polarization, double loop electrochemical potentiokinetic reactivation, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), ThermoCalc and JmatPro. The results indicate that increasing temperature, droplet size and decreasing in RH tends to decrease pitting resistance (individual effects). Moreover, exposure to MgCl2 droplets led to higher pitting corrosion susceptibility than seawater droplets. The environmental effect was also observed on pit morphology (faceted, shallow, equiaxial, columnar pitting). Alloy fabrication methods have a large effect on pitting. The pitting resistance of a emergent material, in this case carbon nanotube reinforced SS 316L and 3D printed using selective laser melting (SLM) technique, showed enhanced pitting resistance (2 % carbon nanotube composition). Sensitization has a deleterious effect on pitting corrosion. The results from this work will provide knowledge, not currently existent in the literature for SS 316L (conventional and SLM) to inform the safe conditions to operate nuclear waste storages and predict their life-cycle.