Aluminum-magnesium alloys, designated as AA5xxx by the Aluminum Association, are used readily in naval applications because of their low density, moderate strength, formability, weldability, and superior corrosion resistance. In order to impact sufficient strength, Mg additions are typically higher than the solubility limit of ~3.5%. At room temperature a Mg supersaturated solid solution is kinetically stable in these high Mg Al alloys; but it has been well established that exposure to elevated temperatures, even those typically experienced in-service, for extended periods of time leads to precipitation of a Mg rich β phase on grain boundaries. The literature establishes that the precipitation of β phase on grain boundaries leads to a severe degradation in corrosion and stress corrosion cracking resistance. In fact, intergranular corrosion issues have been reported on more than 200 commercial vessels containing AA5083-H321. Because marine structures are exposed to corrosive seawater environments while simultaneously experiencing cyclic (fatigue) stresses from 1) ship machinery vibrations, 2) diurnal thermal variations leading to thermal expansion and contraction, 3) wind and wave action (hog/sag motion), and 4) combat/operational loading events; understanding the impact of β phase on corrosion fatigue (CF), the simultaneous interaction of corrosion and fatigue, resistance is of interest. A combination of high resolution fracture mechanics based studies and corrosion experimentation is utilized to understand and quantify the effect of the formation of β phase along grain boundaries on CF of two high Mg AA5xxx alloys. Specifically, results will be discussed showing that the resistance to CF is severely degraded under low frequency loading, and the deleterious effect of grain boundary β phase is magnified as fatigue loading frequency decreases. Crack growth rates for microstructures heavily decorated with β phase are accelerated by ~3 orders of magnitude compared to the as-received microstructure when loading frequencies typical of wave motion are investigated. Additionally, comparison of stress corrosion cracking (SCC) resistance via K1SCC, the threshold below which SCC does not occur, with CF results shows that severe degradation in CF resistance is triggered by loading conditions which promote SCC. Should time allow results on precipitation hardened Al alloys aimed at examining the ability to use Bayesian Network modeling to predict corrosion to corrosion fatigue crack transition will also be discussed.