Mitigation of Active Corrosion Fatigue and Corrosion Pit Initiated Fatigue

Corrosion related fatigue failures of aluminum structural alloys adversely impact the structural integrity of aging aircraft. A chemically active environment, susceptible material and alternating tensile stresses are all required for failure. Conventional mitigation strategies isolate the corrosive environment with coatings, change the alloy or heat treatment, or reduce the applied stress by redesign, all expensive solutions. This paper describes an alternate approach, employing surface enhancement by low plasticity burnishing (LPB) to introduce a deep, stable layer of compressive residual stress to eliminate the tensile stresses necessary for failure without altering environment, material or design. Either 100 hr salt fog pitting or active corrosion in 3.5 % NaCl during fatigue reduced the endurance limit of the baseline machined 7075-T6 surface from 205 MPa (30 ksi) to 103 MPa (15 ksi). LPB prior to corrosive exposure increased the endurance limit for 100 hr pitted samples to 310 MPa (45 ksi), and for active corrosion fatigue to 275 MPa (40ksi). The results support the use of LPB processing of fatigue critical aircraft structural components as a cost effective alternative to alloy substitution or component redesign