*3.2. Corrosion Rate during Tribocorrosion*

The corrosion rate was calculated using the corrosion current (*Icorr*) obtained during the tribocorrosion tests for wrought and AM Inconel 718. The corrosion rates were calculated for both with and without wear conditions as described in the tribocorrosion test

methodology. The observed Tafel plot, along with their respective corrosion rates, is shown in Figure 4. The shift in corrosion potential and corrosion current density due to the effect of corrosion and tribocorrosion can be clearly observed in this figure.

**Figure 4.** Tafel plots and corrosion rate of wrought and AM Inconel 718 for various tribocorrosion test conditions.

It can be observed in Figure 4 that the corrosion potential (*Ecorr*) and corrosion current (*Icorr*) under without wear condition for AM Inconel 718 is −0.029 V and 8.18 μA/cm<sup>2</sup> as compared to −0.103 V and 7.006 μA/cm<sup>2</sup> for wrought Inconel 718, respectively. This indicates that the additively manufacturing Inconel 718 provides an enhanced corrosion resistance potential, which is the point of corrosion onset. This resistance potential for AM Inconel 718 is found to be 71.8% better than that for the wrought counterpart. Even though the corrosion rate of the under the same condition of without wear is found to be slightly more for AM Inconel 718 at 0.082 mm/year as compared to the wrought Inconel 718 at 0.074 mm/year, it can be expected that the onset of corrosion is delayed by 71.8% for AM Inconel 718. Hence, the observed reduction in wear under OCP conditions for AM Inconel 718 as compared to the wrought counterpart (Figures 2b and 3b, respectively), where no corrosion potential was applied. Under the OCP wear condition, the Inconel 718 showed wear volumes of 1.71 mm<sup>3</sup> and 2.28 mm<sup>3</sup> for AM and wrought materials, respectively. The slightly lower corrosion current during the absence of wear for wrought Inconel 718 is found to be what provides it a 9.8% better corrosion resistance without wear but with an early onset of corrosion as compared to the AM Inconel 718.

Considering the wear with corrosion (tribocorrosion) it is observed that the *Ecorr* and *Icorr* for both AM and wrought Inconel 718 shift to have an overall reduced corrosion rate, which confirms that the observed surface degradation is a corrosion accelerated wear. The wear volume is observed to increase in the same conditions (Figures 2a and 3a), but due to the delayed onset of corrosion, the wear in the case of AM Inconel 718 (~1% increase) is minimal as compared to wrought Inconel 718 (55.25% increase). This unique behavior of Inconel 718 can be attributed to the anodic-to-cathodic transition potential (reverse portion of the potentidynamic scan) is more noble than corrosion potential. It can be believed, according to the Silverman's interpretation [24], that any film created on the reference Inconel 718 surface at the corrosion potential might not be very passivating and would allow uniform corrosion to occur at the corrosion potential. The above analysis can be summarized (in a similar way as in Davydov et al. [25]) by stating that using AM

manufacturing significantly increases resistance to the pitting corrosion of Inconel 718, but does not increase its ability for passivation in NaCl solution.
