The Effect of Chloride Concentration on the Corrosion-Fatigue Crack Behavior of an Age-Hardenable Martensitic Stainless Steel (Custom 465)

Ultra-high strength stainless steels (UHSSS) are desirable for aerospace structural components due to their desirable mechanical properties and corrosion resistance. While these steels are more resilient to corrosive environments than their low-alloy counterparts, concentrated aqueous chloride environments are reported to be highly detrimental to the fatigue endurance limit of these stainless steels when compared to less concentrated environments. This research seeks to better understand this deleterious effect by independently studying the effect of various environments (and related corrosion damage) on the pit-to-crack transition, crack initiation life, and short/long crack growth kinetics. To study this behavior fatigue experiments are carried out in aqueous solutions ranging from distilled water to 0.0006-3.0M NaCl. Two types of experiments are performed. First, dog-bone fatigue specimens are subject to one of four pre-fatigue corrosion conditions to induce pitting damage to induce a pit-to-crack transition under constant maximum load fatigue. Scanning electron microscopy is used to characterize the initiation site and capture load-induced marker bands imparted during fatigue. These markings are used to quantify initiation life and small crack growth behavior. Second, single edge notch specimens are used to quantify the long crack growth kinetics, in which a direct current potential drop technique is used to monitor the crack progression, thus providing real-time control of the ∆K-based loading protocol.

Custom 465 is shown to be highly corrosion resistant, such that localized surface corrosion damage (from several exposure conditions in NaCl-based solutions) is not sufficiently severe to form a crack. Rather, microstructural inclusions and other features dictate crack nucleation location. Short crack growth is shown to be highly dependent on local microstructure and relatively insensitive to the bulk chloride concentration. Long crack growth was shown to be relatively insensitive to the bulk chloride concentration as modeling predictions estimate that crack growth rates taken from various NaCl environments exert a minimal effect on fatigue life. Initiation life, however, exhibits a strong dependence on bulk chloride concentration and is likely the main factor governing similar trends in the overall fatigue life. Such precise knowledge of factors governing fatigue life will improve fatigue behavior modeling by incorporating environmental effects on fracture mechanics and stress/strain life predictions.