Salt fog corrosion behavior in a powder-processed icosahedral-phase-strengthened aluminum alloy

Highlights

• Pitting corrosion resistance has been evaluated for an Al-Cr-Mn-Co-Zr alloy.

• Pit densities and depths are far lower than for other high-strength Al alloys.

• Corrosion proceeds by selective oxidation of the Al matrix around the other phases.

Abstract

The pitting corrosion resistance has been evaluated for a powder-processed Al-Cr-Mn-Co-Zr alloy which contains ≈35% by volume of an icosahedral quasi-crystalline phase and a little Al₉Co₂ in an Al matrix. ASTM standard salt fog exposure tests show that the alloy exhibits far lower corrosion pit densities and depths than commercial high-strength aerospace Al alloys under the same conditions. Electron microscopy data show that the salt fog exposure leads to the selective oxidation of the face-centered cubic Al matrix around the other phases, and to the development of a porous outer oxide scale.

Introduction

Aluminum alloys exhibit good oxidation and corrosion resistance over a wide variety of conditions due to the formation of passivating oxide scales. These alloys are, however, susceptible to localized pitting corrosion in environments that contain significant concentrations of chloride ions [1], [2], [3], [4], [5]. The pitting process corresponds to the local breakdown of the passivating film, leading to the dissolution of the underlying metal. The nucleation of localized pits is a complex process that can involve: Cl⁻ ion penetration into the porous Al oxide, adsorption and displacement leading to oxide thinning, and then breakdown of the oxides [1], [3], [5]. Thereafter, the rate at which the pits grow is dictated by some combination of: the ion concentrations in the pit, the overall alloy composition, and the local alloy microstructure. The corrosion potentials of Al alloys are mainly determined by the concentrations of solid solution alloying elements in the FCC matrix, but any precipitates or dispersoids present may lead to local galvanic cells that could accelerate pit growth [2], [6], [7].

Over the last few years there have been extensive efforts to develop Al alloys and metal-matrix composites with strengths and thermal stabilities that exceed those of conventional Al alloy systems. In our work, we have explored the use of powder metallurgy routes for this purpose. Much of this work has involved Al – rare earth – transition metal alloys that are marginal glass formers, enabling these materials to be processed via metastable vitreous intermediates. This allows the solubility limits of FCC Al to be bypassed [8], [9], [10], [11], [12], [13]; in this manner, Al alloys with very high volume fractions of strengthening intermetallic phases were obtained. Recently we have extended this approach to the formation of icosahedral-phase (I-phase) strengthened alloys [14]. Nanocomposite FCC Al/I-phase powders were produced by gas atomization from an Al-Cr-Mn-Co-Zr alloy and consolidated into bulk material. It was shown that the alloy exhibited an excellent combination of room temperature mechanical properties (elastic modulus, tensile yield strength/ductility, and high-cycle fatigue life) and promising values of modulus and yield strength at elevated temperatures. Thus, it was proposed that such materials could form the basis of the first practical quasicrystal-strengthened aluminum alloys for structural applications.

Here we present a preliminary evaluation of the pitting corrosion behavior of this new alloy. ASTM standard salt fog testing [15] was performed on samples of the alloy and the behavior was compared with that for other high-strength aerospace aluminum alloys in standard microstructural conditions. It is shown that the I-phase-strengthened alloy is far less susceptible to pitting than any of the other alloys, and a combination of electron microscopy techniques is used to reveal the microstructural features associated with this remarkable behavior.