CRITICAL CORROSION PIT DEPTH FOR FATIGUE CRACK INITIATION IN 2024-T3 AND 7075-T6 ALUMINUM ALLOYS A study of the effects of corrosion pitting of was accomplished on aluminum alloys (2024-T3 and 7075-T6 aluminum alloys in 1.6 mm (0.063 inch) sheet thickness) common in aging aircraft in order to determine the critical pit depth required for fatigue crack initiation. The corrosion pitting was accomplished using a 3.5% NaCl solution with hydrogen peroxide added as an accelerant; altering the degrees of concentration and exposure times in order to vary the magnitude of corrosion. The pitting specimens were fatigue cycled in one-way bending until failure and the cycles to failure recorded. The specimens were examined with a metallurgical microscope, using the focus method of ASTM G46 to determine pitting depth. The presence of corrosion significantly reduced the fatigue life of the alloys tested as compared to the fatigue life of pristine (un-corroded) specimens. The average pitting depth required to initiate fatigue cracks was determined to be 2-4 μm (0.00008 – 0.00016 inches), which was much less in severity than anticipated. This pitting depth corresponded with typical grain sizes at the surface and cracks began to initiate once pitting had penetrated about 50% of the grain thickness. Basically any visible pitting appreciably reduced the fatigue life of the specimens tested; revealing pitting need not be significant to impact structural performance. This indicates the need to detect and address even minute pitting damage a requisite in ensuring continued structural integrity. Additionally, ratchet marks were noted on the fracture surface of the failed specimens, indicating multiple fatigue initiation sites; making it a useful indicator of fatigue crack nucleation.