The role that Al3normalFe intermetallic inclusions play during initiation of pitting corrosion on aluminum alloys was investigated in 0.6 M NaCl. In aerated solutions microscopic observations showed the growth of cavities in the host metal adjacent to inclusions. The rest potential of synthetic Al3normalFe was measured in aerated and deaerated NaCl solutions over a range of bulk pH values between 2 and 12 and was found to act as a cathode. In aerated solutions rotating disk electrode experiments on synthetic Al3normalFe electrodes verified that the cathodic reaction corresponded to the reduction of dissolved oxygen. With scanning pH microelectrodes, measurements were carried out near synthetic Al3normalFe electrodes which were coupled in a galvanic cell with Al-6061 in NaCl solution. In addition, the pH measured over Al-6061 at a distance of 25–30 μm from the surface was observed to fluctuate between pH 4 and 8.5 for the first 2 h of immersion in NaCl solutions. Atomic force microscopy images of Al-6061 immersed in buffered aerated 0.6 M NaCl solution (pH 5.5) did not show any evidence of dissolution of the Al around intermetallics. These measurements support the view that Al3normalFe particles in Al-6061 serve as local cathodes, that a high pH develops around the intermetallic particles and creates cavities in the host metal, and that interactions among a cluster of intermetallic particles causes a large number of slowly dissolving alkaline cavities to evolve into a small number of rapidly dissolving acidic pits. © 1999 The Electrochemical Society. All rights reserved.

The presence of corrosive environments, particularly chloride-containing solutions, usually lowers the fatigue resistance of metals by speeding the process of crack initiation (I). Such cracking phenomena generally initiate at corrosion pits (2) which, for many aluminum alloys, serve as stress concentrators once they reach a critical size of some 40 to 50 ~m (3,4). A growing body of empirical information is being reported on the effect of convective mass transport on early stages of pitting corrosion (5). The present investigation reports on the effect of fluid flow on corrosion fatigue crack initiation. EXPERIMENTALExperiments were performed with AI-2017-T4 aluminum alloy. Important characteristics of this material include its composition (weight percent, 93.5 %AI, 4.4 %Cu, 0.8 %Mn, 0.5 %Mg and 0.8 %Si), the 0.2% offset yield strength (290MPa), the fatigue limit (170MPa), and the pitting or breakdown potential (about -710 mVsc e in 0.6M NaCI solution at 25~Longitudinal tensile specimens, slightly necked with a 0.09 cm minimum cross sectional area at the neck were employed. Each specimen was polished chemically at 95~ for 2 minutes in H3PO4:H2SO4:HNO 3 = 80:15:5mi, degreased in acetone and then rinsed in ethyl alcohol. The specimen was then coated with a thin layer of ductile lacquer ("Lacomit", #2567A, Canning 2& Co., England) except a small area of Imm in the center of the front surface. It was found that the coating prevented corrosion of masked regions during fatigue testing.Corrosion fatigue experiments were *Current Affliliation: E1 Tech Research Corp.Fairport Harbor, Ohio 44077 **Electrochemical Society Actmve Member conducted on specimens immersed in 0.6 M NaCI at pH=6. The specimen was insulated from the grips of the MTS fatigue machine by high strength insulating sleeves of phenolic resin which were fitted to the four-way pin joints of the grips. Tension-tension testing with R = 0.i was done at 3 Hz under load control in the hydraulic close loop machine. The specimen passed through flexible cell walls consisting of neoprene sheets sealed with silicone cement and quick setting epoxy.The Plexiglas corrosion cell was also fitted with a nozzle for directing a jet of flowing electrolyte toward the small exposed region. The electrolyte flow through the jet was laminar, and the distance between the nozzle tip and the specimen front surface was 5mm. Tests were also carried out in stagnant solution as a baseline.