Fig. 2

Schematic illustrating the expected crack tip conditions (red dashed line) for an Al alloy. Potential is vs. the SHE electrode. The black solid line represents the reversible hydrogen evolution reaction The numbers represent experimental results reported by Cooper and Kelly¹⁶ for an SCC crack of 7050 in the near T6 temper when immersed in 0.5 M Na₂CrO₄ + 0.05 M NaCl. Numbers with a * are for bulk conditions and without are for crack tip conditions.  The (1) is for measurements taken when applying the open circuit potential and (2) is when an ~ 200 mV anodic polarization is applied. Note that the slope in the crack tip curve at basic pH is truly schematic as no research was found in the literature presenting crack E and pH measurements for a similar Al alloy under cathodic polarization. The slope in the cathodic polarization region of the curve is estimated based on pH levels measured by Nguyen et al.⁴⁹