The Importance of Electrochemically Active Surface Area in the Corrosion Behavior of Additively Manufactured 316L Stainless Steel

Surfaces of additively manufactured (AM) stainless steel (SS) by fusing metallic powders are known to be much rougher than those produced by conventionally fabrication methods. The measured surface roughness (Sa or Sq) can range from a few to tens of microns depending on the build angle. Recent studies have shown that the large surface roughness associated with printing conditions makes the AM SS surfaces susceptible to localized corrosion and open-circuit corrosion [1, 2]. Sa or Sq have also long been used to characterize the corrosion behavior of traditionally manufactured metals [3]. However, the open-circuit corrosion rate of the traditional metals has been mainly studied in the submicron-scale surface roughness, and some results have showed that changes in the roughness of hundreds of nanometers or more have little effect on the change in the corrosion rate [4]. In addition, when applied to AM materials, no consistent correlation between the pitting breakdown potential (Eb) and surface roughness has been observed. In this study, the dependence of the corrosion behavior on the electrochemically active surface area of AM 316L SS, defined as the total area of the curvilinear surface that is exposed to the electrolyte, has been explored. The localized corrosion susceptibility was evaluated under full immersion in 3.5 wt% NaCl solution through potentiodynamic polarization for testing articles with different surface roughness. While no correlation to Sa or Sq was displayed, the Eb values showed a clear statistical trend with respect to the electrochemically active surface area: the lager the active surface area, the smaller the Eb. The observed correlation fits well with a previously reported stochastic pitting model on metal surfaces [5]. In addition, by normalizing linear polarization resistance and electrochemical impedance spectra with the electrochemically active surface area, it can be confirmed that the as-printed surface roughness of AM 316L SS did not have a significant effect on the change of the open-circuit corrosion phenomenon under static immersion conditions. Our results suggest that electrochemically active surface area is an appropriate parameter to characterize the corrosion behavior of AM metal surfaces. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. References 1. Melia, M.A., et al., How build angle and post-processing impact roughness and corrosion of additively manufactured 316L stainless steel. Npj Materials Degradation, 2020. 4(1). 2. Ni, C., Y. Shi, and J. Liu, Effects of inclination angle on surface roughness and corrosion properties of selective laser melted 316L stainless steel. Materials Research Express, 2019. 6(3). 3. Walter, R. and M.B. Kannan, Influence of surface roughness on the corrosion behaviour of magnesium alloy. Materials & Design, 2011. 32(4): p. 2350-2354. 4. Li, W. and D.Y. Li, Influence of surface morphology on corrosion and electronic behavior. Acta Materialia, 2006. 54(2): p. 445-452. 5. Shibata, T., 1996 W R Whitney Award lecture: Statistical and stochastic approaches to localized corrosion. Corrosion, 1996. 52(11): p. 813-830.