Corrosion behavior of binary and ternary aluminum-lithium alloys

Janet Marie Sater, Purdue University

Abstract

An attractive feature of lithium-containing aluminum alloys is decreased density with a corresponding increase in strength and elastic modulus compared to conventional structural aluminum alloys. Due to the reactive nature of lithium, however, these alloys may be subject to severe corrosive attack. In order to investigate more fully the relationship between microstructure and corrosion behavior a series of six binary alloys, rolled, (1.2 to 3.3wt.% Li). a ternary Al-2.8Li-0.14Zr(wt.%) alloy, rolled, and a ternary Al-2.6Li-0.09Zr (wt.%) alloy, extruded at two temperatures and two ratios, all under various aging conditions have been tested in NaCl solutions. Corrosion tests included corrosion potential, anodic potentiodynamic and weight loss (corrosion rate) measurements. Results show that potential for the binary alloys becomes more anodic with increased Li content, but aging time and temperature have little effect. Zero current potential and breakdown potential become more anodic up to the peak-aged condition for higher solute alloys, then increase. Passive current density increases with Li content, and, for higher solute alloys, it increases up to the peak-aged condition, then decreases. Corrosion rate is virtually constant for all alloys and aging conditions, and attack is generally in the form of pits. The rolled Al-2.8Li-0.14Zr(wt.%) alloy exhibits a corrosion potential shift to an extremely anodic value only for long aging times at higher aging temperatures. Zero current potential becomes more anodic and passive current density increases with aging time. Corrosion rates are constant, increasing only for more severe aging conditions. For the ternary extrusions potential is more anodic for lower aging and extrusion temperatures and follows a trend similar to that for zero current potential and breakdown potential vs. aging time for the binary alloys. A larger subgrain size, or small Z, improves corrosion potential behavior. Both zero current and breakdown potential become more anodic only when extremely overaged and are little affected by extrusion conditions. Passive current density increases with higher extrusion temperatures and/or ratios up to about 48 hours aging time (200$\sp\circ$C) then decreases. Corrosion rate is virtually constant for all the extrusions, and attack is usually in the form of pits. Due to the low volume fraction of $\delta$ in all of the alloys investigated long-term corrosion behavior is controlled by the matrix.

Degree

Ph.D.

Advisors

Sanders, Purdue University.

Subject Area

Materials science

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