Mechanisms of Passivity of Nonequilibrium Al‐W Alloys

124

The pitting behavior of sputter-deposited Al binary alloy thin films was studied. Pitting and repassivation potentials were determined in 0.1M NaCl for samples in freshly deposited and air-aged states. Aging for several years in laboratory air increased the pitting potential for some of the alloy systems but had no effect on others. The repassivation potentials, meaningful values for pits in thin films, were found to be very close to the pitting potentials of freshly-deposited films for many alloy systems. Stable pits initiate in these Al binary alloys at potentials just above the value at which they would repassivate, indicating that pit growth considerations control the pitting process. By determining the pit anodic current density just before passivation it is shown that alloying improves pitting resistance through a reduction in the ability of pits to maintain the critical local environment necessary for growth. The influences of alloying on the passive film chemistry and on the tendency of the metal to repassivate (depassivation pH) are secondary in nature.Sputter-deposited supersaturated Al binary alloys have been shown to exhibit remarkable pitting resistance in chloride solutions.

Section: Discussionmentioning

confidence: 76%

On the Pitting Resistance of Sputter‐Deposited Aluminum Alloys

Frankel

Newman²,

Jahnes

et al. 1993

124

“…• C in the equilibrium state, 2 the formation of Al-W alloy films having high corrosion resistances requires non-equilibrium processes, such as sputtering, 1,[3][4][5][6][7][8] ion implantation, 9 and electrodeposition. [10][11][12] Of these methods, electrodeposition has particular advantages in that a uniform film can be formed relatively quickly and continuously, onto substrates having complex shapes, using simple equipment.…”

mentioning

confidence: 99%

Electrodeposition of Al-W Alloy Films in a 1-Ethyl-3-methyl-imidazolium Chloride-AlCl₃Ionic Liquid Containing W₆Cl₁₂

Higashino

Miyake

Fujii

et al. 2017

The electrodeposition of Al-W alloy films in a Lewis acidic 1-ethyl-3-methyl-imidazolium chloride (EMIC)-AlCl 3 ionic liquid using W 6 Cl 12 as the W ion source was investigated. W 6 Cl 12 dissolved in the ionic liquid at a higher concentration than other W ion sources used in previous studies. Potentiostatic electrodeposition was performed in a bath containing W 6 Cl 12 at a concentration of 49 mM. Dense Al-W alloy films containing up to 12 at.% W were electrodeposited at potentials more negative than 0 V vs. Al/Al(III). The deposition current density at >0 V was lower than 0.3 mA cm −2 , while that for Al-W alloy films was higher and reached 38 mA cm −2 at −0.5 V. The deposition of W was induced by the deposition of Al. At lower W concentrations, the Al-W alloy films were composed of a super-saturated solid solution, and in the W content range of 9-12 at.% they comprised an amorphous phase. Potentiodynamic polarization and nano-indentation showed that the Al-W alloy films containing 10-12 at.% W exhibited high pitting corrosion resistance, high hardness, and low Young's modulus. Al metal has high oxidation and corrosion resistance, and its chloride-induced pitting corrosion resistance is enhanced by alloying with transition metals. Consequently, Al-transition metal alloys have attracted much research attention as corrosion-protective materials. Among these alloys, Al-W alloys containing approximately 10 at.% are known to exhibit the greatest resistance to pitting corrosion. 1Since the maximum solubility of W in Al phases is only 0.022 at.% at 640• C in the equilibrium state, 2 the formation of Al-W alloy films having high corrosion resistances requires non-equilibrium processes, such as sputtering, 1,3-8 ion implantation, 9 and electrodeposition. 10-12Of these methods, electrodeposition has particular advantages in that a uniform film can be formed relatively quickly and continuously, onto substrates having complex shapes, using simple equipment. Various Al-based alloy films have been electrodeposited in molten salts such as NaCl-AlCl 3 with the addition of ion sources for the alloy component. Recently, electrodeposition using ionic liquids has become more popular owing to their low melting point and low volatility. A representative ionic liquid used for the electrodeposition of Al alloys is Lewis acidic 1-ethyl-3-methylimidazolium chloride (EMIC)-AlCl 3 (where the AlCl 3 /EMIC molar ratio >1 Al-Mn, 18 and Al-Hf 19 are available in the literature. A previous study revealed that electrodeposition in a EMIC-AlCl 3 bath with the addition of W(IV) chloride (WCl 4 ) yielded Al-W alloys, but the W content was lower than 1 at.%.10 Recently, the electrodeposition of Al-W alloy films from an EMIC-AlCl 3 bath with the addition of the W(III) compound K 3 W 2 Cl 9 was reported.11,12 Although the W content of the Al-W alloys reached nearly 96 at.%, the deposits with high W content had powdery morphologies. When the current density was high (>20 mA cm −2 ), dense Al-W alloy films were obtained, but the W content decreased t...

“…The overpotential deposition results in stable non-equilibrium alloy phase with uniform composition and structure. Furthermore, the non-equilibrium alloys exhibit similar chloride-induced pitting potentials as those prepared by physical non-equilibrium alloying methods such as rapid solidification, [16][17][18][19] sputter deposition, [20][21][22][23][24][25][26] and ion implantation. 27 Therefore, the electroplating of aluminum alloys from the Lewis acidic chloroaluminate IL is a possible low-cost alternative to conventional non-equilibrium methods for producing thin films of aluminum alloys.…”

mentioning

confidence: 99%

“…Among the non-equilibrium binary alloys, it is well-known that Al-W prepared by the physical non-equilibrium alloying methods described above show the most anodic chloride-induced pitting corrosion potential. [20][21][22][23][24] When the W content of these alloys exceeds ca. 10 atomic percent (a/o), the passive region of the Al-W alloy extends to nearly +2,600 mV against pure aluminum.…”

mentioning

confidence: 99%

Electrodeposition of Al-W-Mn Ternary Alloys from the Lewis Acidic Aluminum Chloride−1-Ethyl-3-methylimidazolium Chloride Ionic Liquid

Tsuda

Ikeda

Imanishi

et al. 2015

The electrodeposition of non-equilibrium ternary Al-W-Mn alloys was examined in the Lewis acidic 66.7-33.3 m/o aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl 3 -[C 2 mim]Cl) ionic liquid (IL). K 3 [W 2 Cl 9 ] and MnCl 2 were added to provide sources of W and Mn. The Al-W-Mn alloys were deposited on Cu wire substrates rotated at a fixed rate of 1000 rpm by using a dc galvanostatic method. The W content in the ternary Al-W-Mn alloys decreased with an increase in the deposition current density and was independent of the K 3 [W 2 Cl 9 ] and MnCl 2 concentrations in the plating solution. However, both the current density and the metal salt concentrations affected the Mn content of the ternary alloys. X-ray diffraction and composition analysis of the resulting Al-W-Mn deposits revealed the presence of an amorphous non-equilibrium alloy phase without chloride contamination. The chloride-induced pitting potential of the Al-W-Mn ternary alloys was found to be superior to that of the related binary alloys, Al-W and Al-Mn, indicating that the presence of two transition metal solutes has a beneficial additive effect on the corrosion resistance of Al. The many useful features and novel physical properties of ionic liquids (ILs), which are also called room-temperature or ambient-temperature molten salts, make them fertile media for future technologies.1-3 Thermodynamically, aluminum metal cannot be electrodeposited from aqueous solutions containing Al(III), because hydrogen is produced before the Al(III) can be reduced. However, Al can be deposited from aprotic, nonaqueous solutions, e.g., organic solvents containing aluminum salts and from chloroaluminate ILs. 4 However, all processes involving the electrodeposition of Al from organic solvents are based on volatile, flammable solvents and pyrophoric aluminum salts, greatly complicating large-scale aluminum plating operations. As a result, one of the fastest emerging technologies based on ILs is the electrodeposition of aluminum and its alloys from chloroaluminate ionic liquids. Although reactive with moisture, these ILs are virtually non-flammable and have negligible vapor pressure. Among the chloroaluminates, AlCl 3 -1-ethyl-3-methylimidazolium chloride (AlCl 3 -[C 2 mim]Cl) IL is under development for industrial-scale aluminum plating and for lab-scale aluminum alloy electrodeposition. mim]Cl IL. The processes leading to these alloys can be divided into underpotential and overpotential deposition. 5 The former alloys are deposited thermodynamically; i.e., their composition as a function of potential can be calculated from the Nernst equation.5 Al-Cu and Al-Ag alloys form by this underpotential mechanism. However, they tend to suffer from significant chloride contamination. The overpotential deposition results in stable non-equilibrium alloy phase with uniform composition and structure. Furthermore, the non-equilibrium alloys exhibit similar chloride-induced pitting potentials as those prepared by physical non-equilibrium alloying methods such as rapid * Elect...