Nanocrystalline Al[sub 87]Ni[sub 8.7]Y[sub 4.3] and Al[sub 90]Fe[sub 5]Gd[sub 5] Alloys that Retain the Localized Corrosion Resistance of the Amorphous State

Abstract

The localized corrosion properties of Al 87 Ni 8.7 Y 4.3 and Al 90 Fe 5 Gd 5 , were investigated in the amorphous, high strength partially nanocrystalline, and fully crystalline states. The critical pitting potentials and pit growth behaviors of amorphous and nanocrystalline alloys were improved compared to high purity, polycrystalline Al in 0.6 M NaCl solution. Transition metal (TM) and rare earth additions (RE) were retained in solid solution in amorphous alloys and the remaining amorphous matrix of partially nanocrystalline alloys. Substantial TM and RE solute rejection occurred from nano-and microcrystals. Notably, the improved resistance to pitting was retained in the partially nanocrystalline condition, but was completely lost in the fully crystalline alloys. The opportunity exists to create partially nanocrystalline Al-TM-RE alloys of great strength with corrosion resistance equal to their amorphous counterparts. © 1999 The Electrochemical Society. S1099-0062(99)02-016-7. All rights reserved.Manuscript received February 2, 1999. Available electronically March 18, 1999. It is well established in the literature that glassy alloys and alloys with a low degree of crystallinity offer superior resistance to corrosion and vastly improved mechanical properties over their conventional counterparts. 1,2 Amorphous alloys often have corrosion resistance one to two orders of magnitude greater than polycrystalline alloys with the same alloy composition. 1,2 One theory links the improved resistance of amorphous alloys to their ability to promote amorphous oxide formation. In high temperature gas, vitreous or amorphous oxides offer improved oxidation resistance due to the absence of oxide grain boundaries which provide a rapid diffusion path for concentration gradient driven ion movement. 3,4 In the case of metal passivity in aqueous solutions, ion transport is mostly driven by the electric field across the oxide film. The lack of oxide grain boundaries may lower ion migration rates, rendering the passive film more protective. [3][4][5] The rapid transport and accumulation of cation vacancies at the oxide/metal interface is one theory accounting for oxide breakdown. 6 Moreover, vitreous oxides on amorphous alloys perform well due to enhanced bond flexibility since the vitreous or amorphous material can rearrange to accommodate lattice mismatch and strain between the oxide and the metal. 5,7 As a result of this flexibility, almost all surface atoms can bond with oxygen or OH -without requiring an optimal epitaxial relationship between an ordered metal substrate and oxide. 5 Intolerable changes in oxide/metal misfit strain with halide incorporation is another theory accounting for the rupture of protective oxide films on metals. 8 The improved corrosion resistance associated with the addition of 18% Cr to Fe is attributed, in part, to a change in the protective oxide structure from a well oriented spinel structure at 0-12% Cr to a noncrystalline structure at 18% Cr. 5 This disordering has been shown by low energy electr...

Composition Profiles around Solute-Lean, Spherical Nanocrystalline Precipitates in an Amorphous Matrix: Implications for Corrosion Resistance

“…An approximately uniform matrix composition profile would also be expected for interface-controlled growth. Figure 8 shows the dependence of the precipitate volume fraction, the distance separating the precipitates, [38] bcc Fe 10 to 20 polarization in 1-M NaCl no effect Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 [39] bcc Fe(Si)* 10 to 20 polarization in 1-, 3-, and 5-M H 2 SO 4 no effect Fe 75.5 Cr 8 P 16.5 [29] bcc Fe not reported immersion in 9-M H 2 SO 4 preferential attack of bcc Fe Fe 70 Cr 10 P 13 C 7 [40] bcc Fe(Cr) 50 polarization in 1-M HCl increases i pass Fe 73.5-x Si 13.5 B 9 Nb 3 Cu 1 Cr x [14] bcc Fe(Si) 10 to 20 polarization in 0.02-M NaCl decreases i pass Zr 67 Cr 33 , Zr 60 Cr 40 [12] hcp Zr 10 to 22 polarization in 6-M HCl increases E pit Nb 60 Cr 40 [41] bcc Nb 22 polarization in 12-M HCl preferential attack of bcc Nb Ni 36 Fe 32 Cr 14 P 12 B 6 [40] fcc Ni(Fe,Cr) 50 polarization in 1-M HCl increases i pass Ni 55 Cr 15 Mo 10 P 16 B 4 [18] fcc Ni 20 polarization in 6-M HCl increases i corr Ni 65 Cr 10 Ta 5 P 16 B 4 [18] fcc Ni 2 polarization in 6-M HCl higher i corr Al 87 Ni 8.7 Y 4.3 [10,30,35] fcc Al 25 polarization in 0.6-M NaCl, in-situ scanning probe microscopy in acidified NaCl increases E pit ; preferential attack of Al in acid Al 90 Ni 6 La 4 , Al 89 Ni 6 La 5 [42] fcc Al not reported polarization in 0.1-, 0.01-, 0.001-M NaCl no effect on E pit or i pass Al 88 Ni 6 La 6 [43] fcc Al not reported polarization in 0.01-M NaCl, decreases i pass Al 90 Fe 5 Gd 5 [10,11,32] fcc Al 6 to 15 polarization in 0.6-M NaCl, artificial pit growth in 0.6-M NaCl increases E pit ; no effect on pit growth kinetics Al 85 Fe 7 Gd 8 [11,32] fcc Al 100 polarization in 0.6-M NaCl increases E pit Al 87 Ni 7 Gd 6 [11,32] fcc Al 20 to 40 polarization in 0.6-M NaCl, artificial pit growth in 0.6-M NaCl no effect * The symbol bcc Fe(Si) means that the nanocrystal is bcc iron with silicon in solid solution. and the amorphous matrix composition as a function of the precipitate radius, assuming an alloy composition of 0.1, a close-packed distribution, and a precipitate number density of 10 23 /m 3 .…”

“…The bulk pitting and repassivation potential of several Al-TM-RE alloys are not diminished by partial devitrification. [10,11,32,35] Figure 10(a) shows the calculated amorphous matrix composition for Al 90 Co 3 Ce 7 after partial devitrification to form a given volume fraction of fcc Al precipitates. The corrosion resistance of the amorphous matrix for Figure 6(a).…”

“…The corrosion resistance of the amorphous matrix for Figure 6(a). [6] (b) Repassivation potential is calculated assuming the alloy composition is Al 90 Zr 10 . The E rp -vs-composition data are shown in Figure 6(b).…”

“…In fact, the localized corrosion resistance of some amorphous-nanocrystalline alloys that have undergone primary crystallization is comparable or superior to that of the fully amorphous state. [10][11][12][13][14] One argument is that the overall pitting resistance of the partially nanocrystalline states may be enhanced, or at least not diminished, because solute buildup in the amorphous matrix inhibits the growth of corroding sites that readily initiate at surface nanocrystals. Enhanced pitting resistance is possible, because solutes in solid solution improve the corrosion resistance of the alloy (such as solutes in Al).…”

Amorphous layer formation in Al86.0Co7.6Ce6.4 glass-forming alloy by large-area electron beam irradiation

“…Recently, several researchers have investigated the corrosion behaviour of several Al-TM-RE amorphous alloys including Al-Fe-Gd, Al-NiGd, Al-Ni-Y and Al-Co-Ce [4][5][6]. The critical pitting potential and pit growth behaviour of this family of amorphous alloys were shown to be improved compared to high purity, polycrystalline Al.…”