The effect of cathodic polarization on the corrosion fatigue behavior of a precipitation hardened aluminum alloy |
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Authors: | E F Smith D J Duquette |
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Affiliation: | (1) Olin Metals Research Labs., 91 Shelton Avenue, 06504 New Haven, CT;(2) Materials Engineering Department, Rensselaer Polytechnic Institute, 12181 Troy, NY |
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Abstract: | Fatigue experiments were conducted on polycrystalline and monocrystalline samples of a high purity Al, 5.5 wt pct Zn, 2.5
wt pct Mg, 1.5 wt pct Cu alloy in the peak-hardened heat treatment condition. These experiments were conducted in dry laboratory
air and in 0.5N NaCl solutions at the corrosion potential and at applied potentials cathodic to the corrosion potential. It has been shown
that saline solutions severely reduce the fatigue resistance of the alloy, resulting in considerable amounts of intergranular
crack initiation and propagation under freely corroding conditions for polycrystalline samples. Applied cathodic potentials
resulted in still larger decreases in fatigue resistance and, for poly crystals, increases in the degree of transgranular
crack initiation and propagation. Increasing amounts of intergranular cracking were observed when applied cyclic stresses
were reduced (longer test times). The characteristics of cracking, combined with results obtained on tensile tests of deformed
and hydrogen charged samples, suggest that environmental cracking of these alloys is associated with a form of hydrogen embrittlement
of the process zones of growing cracks. Further, it is suggested that stress corrosion cracking and corrosion fatigue of these
alloys occurs by essentially the same mechanism, but that the often observed transgranular cracking under cyclic loading conditions
occurs due to enhanced hydrogen transport and/or concentrations associated with mobile dislocations at growing crack tips. |
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