The role of anions in the formation of hydroxide films on aluminum in hot aqueous solutions

Corrosion Science, Vol. 3 I, pp. 243-248, 1990 Printed in Great Britain

0010-938X/90 $3.00+0.00 © 1990 Pergamon Press plc

THE ROLE OF ANIONS IN THE FORMATIONOF HYDROXIDE FILMS ON ALUMINUM IN HOT AQUEOUSSOLUTIONS

H. TAKAHASHI, M. YAMAKI, AND R. FURUICHI

Faculty of Engineering, Hokkaido University, Kita-ku N13 W8, Sapporo 060, Japan

ABSTRACT The formation characteristics of hydroxide films on aluminum were studied at ~9.5 C in neutral solutions of c i t r a t e , s i l i c a t e , and phosphate (10 -4 i0 -L M) as well as in doubly d i s t i l l e d water(DDW), by gravimetry, chemical analysis, TEM, and XPS. In s i l i c a t e and phosphate solutions both the oxidation of aluminum and formation of hydroxide films are retarded with increasing anion concentration, and in citrate solutions the oxidation rate increases but the film formation rate decreases. TEM showed that all the fil~s formed in the test solutions consist of one layer except with DDW and IO-~M s i l i c a t e solution, and the thickness decreased with increasing anion concentration. The film formed in DDW and IO-~M s i l i c a t e solution had a two layered structure. According to XPS, appreciable amounts of s i l i c a t e or phosphate were included in the film formed in all solutions. The role of the anions on hydration of aluminum is discussed in terms of the s t a b i l i t y and dissolution of surface films. KEYWORDS Hydration of aluminum; anion effect; hot water. INTRODUCTION The reaction between aluminum and hot water h~ye been investigated to improve the corrosion orotection of aluminumL - ~} and for use as electrolytic capacitors 6 - 12}. Immersion of aluminum in boiling pure water causes hydroxide films to form on the surface by the following reaction: 2Al + (3 + x)H20

~Al203.xH20+ 3H2

(1)

The hydroxid~ film consists of a fibrous outer layer and a relatively dense inner layer 5,8,1~), and is a poorly c r y s t a l l i z e d ~o~h~t@~,or 'pseudoboehmite' with the structure Al203.xH20(x:2 - 2.7)J,°,°,~u,IL). After a short Induction' parabolic rate law'per~gd'°), the growth kinetics of the hydroxide film follow a I t is known that the reaction characteristics in pure water are changed by small additions of i n h i b i t i v e or aggressive anions1,2,4). Vermilyea et. al. 1,2) examined the anion effect at 10-o M by impedance measurements, and classified anions into four groups depending on their a b i l i t y to i n h i b i t hydration: i)B02-, Mn04-, Cl04-, Cl03-, BrO3-, NO3-, NO2", C l ' , and C03~243

244

H. TAKAHASHI et al.

ions cause no inhibition, ii)103-, S042", Se042", GeO42"A Cr042"~ citrate, oxal te, and M 04L- cause moderate . . inhibition, . . . i!i)Te 4L', Si04~-, WO4L-, As04~-, and I04 ~- cause strong inhlbltlon, ~nd iv) P04~- causes an e~remely strong inhibition. In the regions of 10-~ - 10-~ M, McCune et.al, nj used Rutherford ba~k scattering spectroscopy and classified anions into thr~e types: i)SiOa L- reacts strongly to prevent hydration, ii)NOa- and SOALprevent the ~ormation of hydroxide, films to an extent, iii)P~4~-, MoO4L-, and W04~- reacts to form anlon incorporated hydroxy complexes. In the present investigation, three typical anions, phosphate, silicate, and citrate, with different concentrations, which interact strongly with Al, have been examined to study the hydration characteristics and film structure. EXPERIMENTAL METHOD 1)Specimen: Highly pure aluminum foi1(99.99%) was used as test specimens(S = 20 cm ) after electropolishing in a perchloric acid/acetic acid solution. 2)Test solution: Doubly d i s t i l l e d water(DDW) and 10-~, 10-°, and 10-~ M of s i l i c a t e , phosphate, and citrate solutions(pH = 7.0) were prepared as test solutions. The silicate solutions were prepared by dissolving Na4SiO4 into DDW and by adjusting the solution pH with acetic acid. The phosphate and citrate solutions were prepared by mixing the acid solution with its sodium salt. 3)Boiling : Electropolished specimens(weight: WI) were immersed in the test solutions at 99.5 • for t b = O - 60 min(weight: W2), and then dipped in a chromic acid/phosphoric acid solution(85 ~) for 10 min to strip the hydroxide films(weight: W~). The total amount of oxidized aluminum, Wt(= WI - W3), and amount of hydroxide films, Wf( = W2 - W3) were obtained as functions of t b and the anion concentration, C~,. The amount of dlssolved Al ions, Wd, durlng ~b~ bo111ng was measureB by colorlmetry with an oxinate extraction method~a~, and the film formation efficiency, ~= 1 - Wd/Wt), was calculated. 4)Characterization of films: Surface films formed by boiling were characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy(XPS). In TEM vertical cross sections of boiled specimens were observed with a HITACHI H-7OOH transmission electron microscope at 200KV by an ultra thin sectioning technique~ j . The XPS was carried out by the irradiation with Mg K ray (1253.6 eV) at 10-9 mbar with a VG Scientific ESCALAB5 Mk2. •

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EXPERIMENTAL RESULTS Hydration kinetics Fig. i sho~s the time-variation in (a) Wt, (b)Wf, and (c)Wd during immersion in 10" M silicate(semi-solid circle~, phosphate(square), and citrate (open circle) solutions as well as DDW(triangle). Both the vertical and horizontal scales are plotted in log units. In DDW, log Wt vs. log t b and log Wf vs. log t b curves are straight lines with a slope of i/2, following a parabolic growth rate, as reported by Alwitt 8). DDW shows a small In the silicate and phosphate solutions, dissolution rate of Al ~+ Ions. " the oxidation and film formation are much slower than in DDW, and log Wt vs. t curve shows a leveling off after long immersion. The dissolution rate o~ Al 3+ ions in the silicate solution is much lower than that in DDW, but in the phosphate solution i t is higher than in DDW. Different from silicate and phosphate solutions, the citrate solution shows high rates of oxidation and dissolution during immersion. The log Wt vs. log t b and log Wd vs • log t b curves show straight lines with the slope one, and Wf remains small and constant. This shows that the rates of oxidation and dissolution are equal here. Fig. 2 shows the change in the film formation efficiency,

Hydroxide films on aluminum

245

n, with boiling time, t b, obtained in I0-2M solutions of s i l i c a t e , phosphate, and citrate as well as DDW. The values of n remain nearly constant during immersion in all the solutions, and decreases in the order of DDW(= ca. 1.0) > s i l i c a t e > phosphate > c i t r a t e ( = ca. 0 ) . i

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Fig. 1 Time-variation in (a)Wt , (b)Wf, and (c)Wd during immersion at 99.5 °C in 10-: M of silicate(semi-solid c i r c l e ) , phosphate(square), and citrate(open circle) as well as d i s t i l l e d water(triangle). Fig. 2 Changes in the film formation efficiency, n, with boiling time, t b. The effect of anion concentration Fig. 3 shows the relationship between Wt, Wf, and Wd and the anion concentration, Can, obtained for t b = 60 min inwnersion in silicate(semi-solid c i r c l e ) , phosphate(square), and citrate(open circle) solutions at Can = 10"~ - 10-~ M. Addition of small amounts of s i l i c a t e to DDW causes no change in Wt and ~f, and with increasing concentration Wt and Wf decrease steeply. At 10- M, Wt and Wf in phosphate solutions are smaller than those in DDW, and they decrease further with increasing concentration. By adding citrate ions at 10-~ M to DDW, Wt decreases appreciably, But incre~ses rapidly with increasing citrate 1on concentration from 10-~ M to i0 -L M. The dissolution characteristics in phosphate and s i l i c a t e solutions show a maximum Wd ~tween 10-4 and 10-3 M. In citrate solutions the dissolution rate of A1° ions increases with increasing concentration. This behavior reflects the change in n with anion concentration(Fig. 4). In citrate solutions n decreases with increasing concentration, and reaches almost zero at 10-2 M; phosphate and s i l i c a t e solutions show a minimum n at i0 -° M.

246

H. TAKAHASHI

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Fig. 3 Effect of anion concentration, Can, on (a)Wt, (b)Wf, and (c)Wd, obtained for 60 min boiling Tn silicate(semi-solid circle), phosphate(square), and citrate(open circle) solutions. Fig. 4 Changes in the film formation efficiency, n, with anion concentration, Can.

Film structure Fig. 5 shows electron microgra~hs of vertical sections of specimens immersed for t b = 60 min ~n (a)lO- M silicate, (b)lO-4 M phosphate, (c)i0 "4 M ~itrate, and (d)lO"~ M silicate solutions. Hydroxide film formed in 10-~ M silicate solution consists of a fibrous outer layer and a dense inner layer with a total thickness of about 700 nm. The mor~glogy and thickness of~ this film are very close to those formed in DDW J. Films formed in 10-~ M phosphate and citrate solutions consist of one l~yer, and the thicknesses of these films are about 120 and 200 nm. In 10" M s i l i cate solution, a thin film with 50 - 80 nm thickness is formed(F~g. 5-d), and strongly protects the metal substrate from hydration. In 10-: M phosphate and citrate solutions, a similar thin film was observed on a rough surface, which may be caused by preferential dissolution. XPS measurements showed that appreciable amounts of phosphate and silicate ions are included in the film, and that the anion content in the film increases with increasing Can. Films formed in citrate solutions showed C ls peak from the carboxyllc a c i d group, which was higher for films formed at higher CanHence, the chemical composition of films formed in these solutions can he expressed as AlOa(OH)b(An)c (An: anion), and the value of c increases with Can.

Hydroxide films on aluminum

247

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Electron micrographs of v e r t i c a l sections of specimens boiled f o r t b = 60 min in (a)~O -4 M s i l i c a t e , (b)lO -4 M phosphate, (c)lO -4 M c l t r a t e , and (d)10 -~ M s i l i c a t e s o l u t i o n s . DISCUSSION

The r o l e of anions in the hydration of aluminum in hot aqueous s o l u t i o n s containing anions is discussed below. In c i t r a t e , phosphate, and s i l i c a t e s o l u t i o n s f i l m formation proceeds by the f o l l o w i n g equation: A1 + (a +b)H20 + cAn x- .. >AlOa(OH)b(AnX-)c + (a + I/2)H 2

(2)

The i n h i b i t i o n of hydration with anions depends on the s t a b i l i t y and the p r o t e c t i v e capacity of the surface f i l m against hydration. At the low conc e n t r a t i o n of 10 -~ M, the a b i l i t y of f i l m s decreases in the orde T ~( phosphate > c i t r a t e > s i l i c a t e , in agreement with Vermilyea e t . a l . ~ , ~ ) . An-

H. TAKAHASHI e t al.

248

other role of anions is the a b i l i t y to dissolve the film. The addition of anions to pure water increases the solubility of Al ~+ ions by the formation of complexes. dAl3+ + eAnx-

> (AldAne)3d - xe

(3)

Anions which form a stable complexes with A13+ ions may be able to attack the surface film to dissolve, provided that the charge of complexes~= 3d xe)is not zero. The maximum Wd, or the minimum n , at about 10-~ M in phosphate and silicate solutions can be explained by the following: In low Can regions, phosphate or silicate ions are adsorbed on the surface of hydroxide films to cause the dissolution of the film by forming cationic complexes(3d xe > 0 in eq.(3)). In this region the dissolution rate increases with Can. At higher ~a the anions adsorbed on the surface form zero charged complexes(3d -xe 8 in eq.(3)), and are incorporated in the film to inhibit the dissolution. This ks reflected in a significant protective a b i l i t y of films formed in 10-: M silicate solution. I t may be assumed that the addition of more anions would enhance the dissolution by the formation 9f anionic complexes(3d - xe < 0 in eq.(3)). This was ascertained in 10-~ M phosphate solution, where the value of n was 0.53. The extremely high dissolution a b i l i t y of citrate anions may be caused by the formation of stable anionic complexes even in low concentration regions. In conclusion, anions retard hydration of aluminum by forming stable anionincorporated surface films and accelerate the dissolution of the film by forming ionic complexes. This behavior changes drastically with the kind of anion and with concentration.

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W. Vedder and D. A. Vermilyea: Trans. D. A. Vermilyea and W. Vedder: ibid., T. Abe, S. Tsuda, and T. Tominaga: J. 700 (1973) R. C. McCune, S. S. Shilts, and S. M.

Faraday Soc_oc_~_~64, 561 (1968) 66, 2644 (1970) Metal Fini.shin 9 Soc. Jpn., 24, Furguson: Corros. Sci.~ 22, 1049

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12) 13) 14)

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