Passive Film Structure of Supersaturated Al‐Mo Alloys

Abstract

Reflection-extended x-ray absorption fine-structure spectroscopy has been used to probe the local atomic structure of the passive film of supersaturated A1-Mo alloys polarized in KC1. These alloys (with 7-11 atom percent Mo) exhibit resistance to localized attack with an increase in the pitting potential of -600 mV relative to pure aluminum. Measurements show that the structure of the A1-Mo passive films resembles that of a-A12OjA1OOH, whereas an oxide film grown on pure A1 in tartaric acid, which does not possess enhanced passivity, is more like ~-A12OJA1OOH. Complementary x-ray photoelectron spectroscopy measurements indicate the A1-Mo passive film composition to be near that of A1OOH and the tartaric-acid film to be A1203. Because corundum (a-A1203) and diaspore (a-A1OOH), which contain only octahedrally coordinated A1 atoms, are very stable and inert while ~/-A1203, which contains both octahedral and tetrahedral sites, is reactive, the change in structure of the passive film suggests that improved passivity may be correlated with a reduction in the density of tetrahedrally coordinated atoms. This structural change likely results from the incorporation of oxidized Mo into the passive film; only as this Mo is hydrated with a presumed change in local structure does the alloy pit.Aluminum and conventional aluminum alloys are susceptible to localized attack in chloride-containing environments. Although passivity-promoting alloying species, such as Cr and Mo, are commonly added to iron to form corrosion-resistant stainless steels, this approach has traditionally not been applied to aluminum because these alloying additions have very low solubilities in aluminum. As a result, precipitates form that can degrade the alloy's resistance to pitting attack. Recently, improved passivity has been observed in non-equilibrium, supersaturated aluminum alloys formed by sputter deposition (1-8), ion implantation (9, 10), melt spinning (11), or physical vapor deposition (12). For co-sputter-deposited alloys containing 6-8 atom percent (a/o) Mo, Cr, Ta, or W, pitting potentials range from -200 mV (SCE) to 860 mV (SCE) in 0.1N KC1, compared with a pitting potential for pure aluminum and its conventional alloys of --600 mV (SCE). Similar qualitative improvements can also be seen in salt fog tests.The passivity of A1-Mo, A1-Cr, and A1-Ta alloys is achieved through an oxidized solute species that impedes the progress of chloride and, in some cases, oxygen anions, through the passive film. Increased stability of the passive film leading to improved general and localized corrosion resistance may be governed by the oxide structure. For example, corundum (a-A1203) consists of only octahedrally coordinated A1 atoms and is very stable; in contrast, cubic ~-A1203, or "activated alumina," consists of both octahedrally and tetrahedrally coordinated A1 atoms and is reactive and comparatively unstable (13-15). E1-Mashri et al.have proposed to extend this correlation between bulk structure and stability to electrochemically formed amorphous alumi...

Electrochemical Behavior and Surface Chemistry of Nonequilibrium Aluminum Alloys: Passivity Mechanism and Fabrication Methods

“…Several different mechanisms have been proposed to explain the passivity of stainless aluminum alloys, including electrostatic repulsion of Cl" by oxidized solute atoms [1][2][3], formation of an oxidized solute barrier layer [3,4], blockage of Cl" transport through the passive film [22], stabilization of the passive film oxide structure [6], replacement of oxidized Al in the passive film by oxidized solute to form a more stable oxide [28,29], reduction of the critical pH for pit propagation [17][18][19][20], and reduction in C1-adsorption due to pHpzc changes [11][12][13][14][15]. Additionally, Macdonald and coworkers [34,35] studying Mo-and W-containing stainless steels have proposed a solutevacancy interaction model (SVIM).…”

“…Several alloys have shown significant improvements in passivity in chloridecontaining environments. For our sputter-deposited thin films, AI-W, Al-Ta, and AI-Mo alloys have shown the best performance with breakdown potentials Eb well above 0 V (SCE) and passive regions exceeding 1000 mV in 0.1 M Cl-solutions [4][5][6][7][8][9].…”

“…In the last several years, supersaturated aluminum alloys with Mo, Cr, Ta, W, Zr, Nb, Zn, V, Cu, Ti, and Si have been produced by several groups using rapid solidification or other nonequilibrium methods , such as sputter deposition [1][2][3][4][5][6][7][8][9][10][16][17][18][19][20][21][22][23][28][29][30][31][32][33], ion implantation [11][12][13][14][15]24,25], melt spinning [27], and vapor deposition [26,33]. Several alloys have shown significant improvements in passivity in chloridecontaining environments.…”

Development of Multifunctional Nanocomposite Coatings for Wind Turbine Blades

“…Experimental results show that such a coating protects against lightning failure and damage [2], However, a metallic coating suffers corrosion failure because of expose to chlorine in the marine environment. The surfaces suffer from pitting corrosion attack when an aluminum coating is exposed to an environment that riches chloride ion (Cl") [4][5].…”

A study of Al-Mo powder processing as a possible way to corrosion resistent aluminum-alloys

“…However, among the alloying elements contained in technical alloys, only Cu shifts the pitting potential up to about 150 mV to the noble direction, but this is not enough to obtain a substantial improvement of the corrosion resistance. In the nineties researchers produced supersaturated Al alloys, mostly alloyed with transition elements such as Mo, W, Ta, Zr, Nb, Cr, Ni [1][2][3][4] . Also rare earth elements have been added more recently 5 .…”