Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

Abstract

The corrosion behavior and microstructure of AA7055 alloy with a high Cu content, and a Cu-free Al-Zn-Mg alloy in various aging conditions were studied. Polarization measurements and ASTM B117 salt-spray exposure were used to assess the corrosion characteristics. Transmission electron microscope and atom probe tomography were used to evaluate the microstructure and the composition of precipitates. The matrix composition of AA7055 changed with aging because of precipitation. Thus, the galvanic relationship between Al 7 Cu 2 Fe particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. The susceptibility to pitting corrosion damage of AA7055 was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. However, aging had only a small influence on pitting corrosion damage for the Cu-free Al-Zn-Mg alloy. Furthermore, despite the more active breakdown potential of the Cu-free Al-Zn-Mg alloy, this alloy exhibited less severe pitting corrosion damage than AA7055, in particular for the over-aged samples. This was attributed to the weaker galvanic coupling between Al 3 Fe particles and the surrounding matrix. Although Cu in the intermetallic particles is detrimental to pitting resistance, the galvanic effect can be reduced by increasing the matrix Cu content. High-strength precipitation-hardenable aluminum alloys are commonly used for structural components of aircraft due to their high strength-to-density ratio.1 However, they are also very susceptible to pitting corrosion, in particular for Navy aircraft subjected to extended periods of salt water spray and/or salt fog.2 These corrosion pits can act as initiation sites for corrosion fatigue cracks and reduce the fatigue life, which is a significant damage mechanism in aging aircraft structures. 2,3From a microstructural viewpoint, initiation of localized pitting corrosion is often associated with coarse cathodic intermetallic particles (IMPs), such as Al 7 Cu 2 Fe, Al 3 Fe, Al 2 Cu, and Al 2 CuMg, which are common in 2xxx and 7xxx series Al alloys.4-9 A number of studies have been performed on electrochemical characteristics of these particles. 4,8,9 In general, they act as local cathodes and sustain oxygen reduction at a high rate to drive the pitting corrosion attack at the particle-matrix interface. In addition, a few studies have related the precipitate size to metastable and stable pitting corrosion in Al-Cu-Mg and Al-Zn-Mg-Cu alloys.10-13 Ralston et al. 11,12 suggested that there was a critical S-phase precipitate size range from ∼3 -8 nm to trigger pitting events. Aged samples with smaller precipitate size had pitting resistance similar to as-quenched samples. The susceptibility to pitting corrosion was greatly enhanced after prolonged aging to create precipitates larger than this critical size range. Gupta et al. 13 reported that a critical precipitate size existed for a transition in electrochemical behavior of 7xxx series Al alloys. For Cu-rich AA7150, increased ...

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