Corrosion Fatigue Crack Growth Rates of Sensitized AA5456-H116: Contribution of Stress Corrosion Cracking

The interplay between stress corrosion cracking (SCC) and corrosion fatigue was investigated for AA5456-H116 to determine the source of the inverse relationship between fatigue loading (f) and fatigue crack growth rates (da/dN). Sensitization in 5xxx series aluminum alloys refers to the precipitation of β on α-aluminum grain boundaries, which can occur in high-Mg 5xxx alloys after sufficient exposure to temperatures as low as 40°C. Recent research has established that da/dN in sensitized 5xxx series aluminum alloys can be inversely dependent on f. The severity of the inverse relationship between da/dN and f has been attributed to sensitization level, alloy composition, and temper. In this work, the effect that K_max has on corrosion fatigue da/dN was quantified using fracture mechanics-based experiments conducted at different maximum stress intensity factors (K_max) but the same ΔK in 3.5 wt% NaCl. Results suggest that the severity of the inverse relationship between da/dN and f (i.e., the slope of the da/dN vs. f trend) is governed by the magnitude of K_max in relation to the threshold stress intensity factor for SCC (K_ISCC). In addition, results show that an inverse f-dependence, albeit with a lower slope, can exist even when K_max is less than K_ISCC; in other words, absent SCC. This low-slope corrosion fatigue behavior may have been driven by an enhanced rate of hydrogen embrittlement facilitated by the more aggressive local crack tip environment present at progressively lower loading frequencies during corrosion of highly sensitized microstructures.