Surface Chemistry of Sputter‐Deposited Al‐Mo and Al‐Cr Alloys Polarized in 0.1N   KCl

Abstract

A study was performed to investigate the changes that take place in the chemistry of the passive film that forms on A1 alloys containing either Mo or Cr, as a function of solute concentration and applied potential in 0.1N KC1. The results show that the mechanisms by which the solute protects the Al alloy differ. Molybdenum forms a film containing MoO4 -2 that impedes the ingress and movement of the C1-anion in the film. In contrast, Cr forms a CrOOH barrier layer that inhibits the oxidation of the A1 substrate and restricts the C1-anions from reaching the metal/film interface. The results show that pitting of A1-Mo alloys occurs when sufficient MoO4 2 is replaced by hydrated Mo § compounds, whereas pitting of the A1-Cr alloys occurs when Cr +3 is oxidized to the more soluble Cr § state.Aluminum and its alloys are known for their poor resistance to localized attack and, in particular, for pitting in aqueous chloride-containing environments. The formation of pits is caused by the breakdown of the passive film in the presence of chlorides due to its interaction and chemical reaction with the chloride anion. This results in the formation of a more soluble, and therefore less protective, film which eventually leads to the rapid localized dissolution of A1 metal and the formation of a pit.Elements like Mo or Cr promote passivity in A1 if they can be retained in solid solution without the formation of precipitates which can serve as active microgalvanic cells. Retention of these additions in solid solution can be accomplished by several nonequilibrium alloying techniques (1-4). Since the corrosion behavior of A1 is dependent upon the protective properties of the passive film, the presence of these elements in the film improves the protective nature of the film by decreasing its susceptibility to pitting attack.Changes in the passive film can be studied by monitoring the products of the reactions. X-ray photoelectron spectroscopy (XPS) is an ex situ surface-sensitive technique that has been widely used to study the chemistry of passive films (4)(5)(6)(7)(8)(9)(10)(11)(12). This technique provides a quantitative analysis of the passive film and yields information regarding the oxidation state of the elemental species present, thereby providing insight into the nature of the chemical changes that occur during passivation and film breakdown in the presence of chlorides.The purpose of this paper is to study the changes occurring in the passive film on sputter-deposited A1 alloys containing either Mo and Cr as a function of applied potential from the open-circuit potential (Eoc) to the breakdown, or pitting, potential (Eb). The changes in the surface chemistry, are used to study the mechanism by which these alloying additions affect the passive behavior of A1 in aqueous chloride environments.

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).…”

“…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.…”

Microstructure and metal-insulator transition in single crystalline KMo4O6

“…Also, some samples display superconducting behavior with surprisingly critical temperature (T C ) as high as 10 K 8 . Additionally, molybdates have attracted great attention due to the existence of many valences of the Mo ions (from 2+ to 6+) 9 . Recently, the study of the Mo valences in the Li 0.9 Mo 6 O 17 compound unambiguously showed an interesting connection with the formation of 1D channels in this compound 10 .…”

X-ray Photoelectron Spectroscopic Study on the Anodic Passivity of Sputter-deposited W-Nb Alloys in 12 M HCl Solution

“…Anderson et al studied the corrosion behavior of the sputter-deposited amorphous Ni-Nb coatings on commercial 316L stainless steel in different aggressive media and they found that the amorphous Ni-Nb coatings have good corrosion resistance [10]. It has been reported that the alloying of niobium with copper [11], aluminum [12], chromium [13], molybdenum [14] and magnesium [15] enhances the protective quality of the passive films in aggressive media. The sputter-deposited Cu-Nb [11], Al-Nb [12] and Mg-Nb [15] alloys were spontaneously passivated and showed high corrosion resistance in aggressive media even though these alloys did not show higher corrosion resistance than those of alloy−constituting elements.…”

“…It has been reported that the alloying of niobium with copper [11], aluminum [12], chromium [13], molybdenum [14] and magnesium [15] enhances the protective quality of the passive films in aggressive media. The sputter-deposited Cu-Nb [11], Al-Nb [12] and Mg-Nb [15] alloys were spontaneously passivated and showed high corrosion resistance in aggressive media even though these alloys did not show higher corrosion resistance than those of alloy−constituting elements. However, the most interesting fact has been reported, during last two decades, that the corrosion rates of the sputter-deposited Cr-Nb [13,16,17] and Mo-Nb [14,17] alloys were lower than those of alloy constituting elements due to the spontaneous passivation in aggressive HCl solutions.…”