True active surface area as a key indicator of corrosion behavior in additively manufactured 316L stainless steel

Laser powder bed fusion (LPBF) additively manufactured (AM) 316L stainless steels (SS) possess much more complex surfaces than their wrought counterparts which affects the corrosion behavior. Surface roughness, a typical metric for assessing corrosion of conventionally manufactured metals, is often ineffective as an independent parameter in characterizing corrosion of the AM metals for their higher surface roughness ranging from 5 to 50 µm. This study experimentally shows that the true active surface area (ATA) is a proper parameter for quick assessment of localized corrosion response of AM 316L SS. Through the potentiodynamic polarization testing on surrogates under full immersion in 0.6 M NaCl solution, the pitting corrosion susceptibility was evaluated. While no consistent correlation to surface roughness was displayed, the pitting breakdown potential (Ep) showed a clear statistical trend at $1/$$\mathrm{A}$ $_\mathrm{TA}^{0.5}$. In addition, normalization of the polarization resistance with the ATA reveals the corresponding surface roughness did not significantly affect the change in open-circuit corrosion phenomenon. This correlation fits well with a previously reported stochastic pitting model on metal surfaces. The results suggest that the importance of ATA as a predictor for predisposition to corrosion in AM 316L SS extends far beyond what has been established for wrought materials.