The electrochemical properties of metallic glasses

0013-0655/57 s3 W+000 Pos.,non

EkHrorniarim aen. Vol. 32. No . tL pp . n-26. iw7.

Primed in

. Jour.. 1-tit

Great Britain.

REVIEW ARTICLE THE ELECTROCHEMICAL PROPERTIES OF METALLIC GLASSES M. D. ARCHER, C. C . CORKE and B . H . HARn Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP, U . K .

(Received 20 March 1986) Abstract-Recent work on the corrosion resistance and electrocalalytic properties of glassy metals is reviewed . Their resistance to general corrosion is discussed in terms of the roles played by the composition, both of metal(s) and metalloid(s), and by the structure . The resistance of glassy metals to localized corrosion, in particular pitting, crevice corrosion Bad stress corrosion cracking, is described . The inhomogeneities, both physical and chemical, that have been found in melt-spun alloys are frequently overlooked, but may play an important role in the corrosion behaviour of glasses . Finally, the use of glassy metals as catalysts for hydrogen evolution, methanol oxidation, chlorine evolution, the Fischer-Tropsch reaction and other hydrogenation reactions is critically discussed .

galvanic effects associated with multiphase structures should be absent in glassy metals . In 1974, Naka er al .[11] reported that several Febased melt-spun glassy alloys possessed corrosion resistance superior to that of crystalline stainless steels, particularly in acidic solutions containing chloride . This corrosion resistance arises from the formation of highly protective oxide films by alloying elements such as Cr, Ti and Zr. In this respect, glassy metals resemble conventional stainless steels, except that good passivation ability may be found for lower contents of Cr etc. than are necessary in steels . Since the preliminary Japanese work, there has been increasing interest in the electrochemical characteristics of glassy metals against a background of growing technological awareness of their possible material applications . These applications include the use of glassy metals as electrocatalysts, since their novel compositions may allow good catalytic behaviour allied with good corrosion resistance . Several reviews[12 15] have appeared, most of them dealing with the commercially important melt-spun Fe-based alloys in aqueous environments . Melt-spun glassy ribbons have pronounced physical irregularities and are not always chemically homogeneous. Early accounts of their electrochemical behaviour suggest insufficient awareness of these complications. In general, one might expect unusual electrochemical behaviour in glassy metals on the grounds either of their unusual compositions, or their unusual structure, or because they possess physical or chemical inhomogencitics .

INTRODUCTION Glassy metals are single-phase alloys without longrange crystalline order which can be made by very rapid solidification of certain metal-metalloid or metal-metal melts or, in a few cases, by electrodeposition . In the former and more important group, the metal-metalloid alloys, typical glass-forming compositions are 80%* late transition metal(s) (eg Fe, Co, Ni, Pd, Au) plus 20 % metalloid(s) (B, C, Si, P, Gel. In the latter group, binary metal-metal glasses (eg Cu-Zr, Ni-Nb, Ti-Be, Ca- Mg) are typically - 50 :50 in composition . Although glassy metals have by definition no translational periodicity, many show XRD evidence of pronounced short-range chemical order . Melt spinning is the usual method of fabrication, yielding 20--60 i m thick, 1-20 mm wide ribbons . Planar flow casting yields wider (up to 75 mm) ribbons . Some glassy metals are commercially available from suppliers such as Allied Corporation, New Jersey, U .S .A (Metglas alloys) and Vacuumschmelze GmbH, Hanau, W . Germany (Vitrovac alloys) . Glassy metals show some remarkable properties[l--6] . They are mechanically hard and have extremely high tensile strengths. Some low-temperature annealed ferromagnetic glasses have unusually soft magnetic characteristics compared with crystalline ferromagnetic materials[7, 8] . Selected glassy alloys possess very small temperature coefficients of linear expansion[9] and electrical resistivity[10] . Some glassy metals are chemically very inert and this has sometimes been ascribed to their amorphous nature : they have no crystalline defects or interphases at grain boundaries, both of which are vulnerable to localized attack in conventional alloys . Moreover,

GENERAL CORROSION Figure 1 shows a schematic linear sweep voltammogram (LSV) for an alloy that undergoes an active-passive transition . The corrosion resistance of an alloy in a given solution is the better the more

* Here and elsewhere, glassy metal compositions are given as atomic percentages . 13



14

M. D .

ARCHER

er a!. ivy 10

0

i04 'E

a

a

i Io

i02

I Er

Active potentials

Noble potentials 0

Fig . 1 . Schematic steady state linear sweep voltammogramof a passivable alloy . extensive the passive region, and the lower the current density at the active-passive transition (aait) and in the passive region (ice ). Thus the lower EPo ,, E a, ia,t and ice, and the higher the transpassive or b"reakdown potential E,, the more easily passivated is the material . Non-passivating materials show a current which increases monotonically with increasing potential .

Compositional

effects related

to the metal

Simple Fe-, Ni- and Co-metalloid glasses passivate in alkali, as do the (crystalline) metals themselves . In neutral or acidic media, metal-metalloid glasses tend to show poorer ability to passivate on anodic polarization than the metal itself [ 17-22], although hydrogen evolution may be inhibited, and the corrosion current at the free corrosion potential consequently decreased, by the presence of the metalloid[23] . For example, glassy Fe-13P-7C shows no active-passive transition in its LSV in 1 M H2 SO4 whereas iron does[24] . The replacement of Fe in Fc-13P-7C by either a more noble metal (Cu, Pd, Pt, Rh) or a more readily passivated metal (Cr, Ti, Zr, Mo, V, Ta and RE, though not Mn) markedly decreases the corrosion rate in a range of media[24-29] . Chromium is particularly effective : Fig . 2 shows LSVs of glassy Fe-xCr-13P-7C (x = 1-10) in I M H, S0430] . The active region almost disappears for x > 8 . In the same solution, i m, and ip , of glassy Fe-5Cr-13P-7C are lower than those of crystalline I8Cr-SNi • stainless steel, which contains -17 at .% Cr[31] . In general, smaller Cr contents are required for good passivity in glassy alloys than in stainless steels, and this has often been ascribed to the chemically homogeneous nature of the substrate_ The protective surface film on high-Cr glassy alloys has been shown to consist mainly of hydrated chromium oxyhydroxide, CrO,(OH) 3 -2i nH 2O, where n and x depend on the composition of the underlying alloy and the conditions of formation . The surface films formed on high-Cr stainless steels are

05

Potentidt

t V (sce)

Fig. 2. LSVs of glassy Fe Cr 13P 7C alloys in 1 M H 2SO4 at 303K . The numbers refer to the atomic percentage of Cr[30] . similar[32] except that the concentrations of H,O and OH - arc lower[33-36] . Titanium also forms its own passive film and, unlike Cr, does not readily undergo transpassive dissolution to form a soluble species at any pH . However, the comparison of glassy Ni-15Ti-20P and Ni-15Cr-20P shows Cr to be more effective than Ti in improving corrosion resistance[37] . Molybdenum and tungsten additions have also received considerable attention . Like Cr, both are effective in promoting anodic passivation, even in acidic chloride solutions[28] . However, XPS studies of glassy Fe (8 15)Mo--13P-7C[38] and Fe-(12-16)Mo-18C[39] anodically passivated in I M HCI showed only trace amounts of Mo in the hydrated iron oxyhydroxide film . Hashimoto[40] has proposed the role for Mo shown in Fig . 3 . In the active region, a issolution De

4tty j 4 Mo species

Passive

FeUU 40HIr nH,o 1

12

W)

DissoWtion

rate

Mo species

Passive

cr0,(OHI Sz nH,0 VIM,

S (b)

• Stainless steel and solution compositions are given as Po mass percentages-

Fig. 3 . Diagram to show (a) passivation of a glassy Fe-based alloy containing Mo ; (b) repassivation of a glassy Fe-based alloy containing Cr and Mo in acidic media[40] .



The electrochemical properties of metallic glasses non-protective Mo-containing film forms and this acts as a diffusion barrier against further dissolution of the alloy . This assists the accumulation of passivating species (Fe, Cr) at the alloy/corrosion product interface, leading to passive film formation with subsequent dissolution of Mo(VI) . The passivating capability of glassy metals in which both Cr and Mo are present together with P is remarkable[41, 42], although some are prone to hydrogen embrittlement[42, 43] and to transpassive dissolution[44]. Figure 4 illustrates the ability of glassy Fe-xCr-yMo-13P-7C alloys to passivate on anodic polarization even in 12 M HCI[45] . The ability of certain glassy metals to passivate without pitting in highly aggressive media is superior to that of steels of comparable Cr content . However, glassy metals have at least one very small dimension, and this limits their commercial applicability . The production of corrosion-resistant glassy coatings on conventional metallurgical substrates by laser or electron beam rastering offers tremendous possibilities although these methods are still in their infancy[4] .

wr

V

lc r 2

-15

05 os 0 Potentiat/V (see)

1

Is

2

of

Compositional effects related to the metalloid The reactivity of metal-metalloid glassy metals is significantly affected by the metalloid element(s) present . We have already remarked that metalloid addition may improve lie lower) the corrosion current at E, a,, : this may result from the low exchange current density of the H * ,H, couple on metalloids[46] . However, the effect of a given metalloid depends on what else the glassy metal contains and on the solution composition and it is not easy to make generalizations about their role . For example, Naka et aL[47] examined the influence of various metalloids X and Y (B, C, Si and P) on the passivation ability of Fe-(5 or 10)Cr-13X-7Y glasses in 0 .1 M H, SO4 and 3 NaCI . As illustrated in Figs 5a and 5b, P is by far the most effective metalloid, particularly in NaCl . The differences between the other metalloids are slighter and depend on the medium .

Fig . 4 . LSVs of glassy Fe-xCr-yMo-13P-7C in 12 M HCI at 303 K ; x and y are atomic percentages of Cr and Mo, respectively[45] .

U

10'

Fig . 5. 1SV s of glassy Fe-5Cr-20B and Fe-SC r-13B-7X (X = C, Si or P) in (a) 0_05 M H, SO 4 ; ( b) 3 % NaCI[47].

Particular attention has been paid to the role of phosphorus in metal-metalloid glasses because some Fe-based glasses containing both Cr and P have good with stainless corrosion resistance compared steels[30] . For example, the addition of 7 30 % Cr to glassy Fe-, Co- or Ni-20B was necessary to prevent perceptible weight loss in I M HCI but the addition of only 10% Cr to the P-containing alloys Fe-13P-7C, Co-13P-7C and Ni-20P was as effective[48] . Moreover, small (2-5%) additions of phosphorus significantly improve the corrosion resistance of some glassy metal-metal alloys (Ti-(Ni, Cu or Pd) in 1 M HC1 and 1 M HNO 3 [49] ; Cu-4OZr in 0 .5 M H 2 SO4 , I M HCI and 3 .5 °% NaCI[50] ). Although phosphorus in low (typically < 0 .2 %) concentrations improves the corrosion resistance of some steels, it is a base element, readily oxidized in aqueous media to P(V) oxoanions. Thus it would be surprising if phosphorus, present in concentrations up to 20 % in glassy metal-metalloid alloys, were invariably to exert a beneficial effect on the corrosion resistance, and indeed it does not . For example, as shown in Fig . 6[51], Fe-(22-26)P passivates less well in acid than does iron itself. Unsurprisingly, P has been shown to dissolve from glassy Fe-20P under anodic galvanostasis as P(V)[21] (Fig . 7) .



16

M . D . ARCHER ei al. 4 Pure iron (pH Q3)

• Fe-(22-26)P(PH 0.31 o Ni-19P(PH 15) . N1-19P4pH 10 * 2826IpH15) • Ni-270 (pH 15)

N

Q2 Is

P

t

Fe ~. J I I I 1 I 1 41 0 I 12 02 0,2 0.4 06 09 Potentiat/V (sce)

-06 -Q4

Fig . 6. LSVs of crystalline Fe and Ni-270, and glassy Fe-(22-26)P, Ni-19P and Fe-40M-14P-614 in acidic sulphate[51]-

30

P nert P+3e

P- Ps" 5.

0

10

20 30 Current density/mA cm

40

berates and silicates in the passive film and so to lower the concentration of the more effective oxide. This hypothesis does not appear to rest on secure evidence. The rate of repassivation of an alloy may be measured chronoamperometrically following in situ abrasion or scratching of an electrode held at constant potential in its passive region. Figure 8 shows repassivation transients for glassy Fe-IOCr-13B-7X in 0 .05 M H2 SO4 [59]. While it is clear that, in this experiment, the 7 P alloy repassivated more quickly than the 7Si or 7C alloys, it is not at all clear that the extrapolation which indicates faster initial dissolution from the 7P alloy is valid . Moreover, the PO ; - content of the passive film on glassy Fe- IOCr-13B-7P reported in the same paper[59] is by no means negligible at 7 % of the total anions, nor is it significantly different from the reported Si0 4 content (8 %) of the passive film on glassy Fe-lOCr-13B-7Si . Diegle and Lineman's current decay transients for two similar glassy alloys and one stainless steel in 0.5 M Ii 2 S04 and in 0 .5M H 2 SO 4 plus 1 M NaCI[60] are not in complete accord with those of the Sendai group . In 0 .5 M H,S04 , they found glassy Fe-36Ni-14Cr-12B-6Si to repassivate faster, not slower, than glassy Fe-36Ni-l4Cr-12P-6B (Fig . 9a). However, in the presence of chloride (Fig . 9b), the B/Si alloy repassivated much more slowly than the P/B alloy. The Sendai group[12, 61] have found repassivation of glassy Fe-l0Cr-13P-7C in acidic chloride to occur much more quickly than that of an 18Cr-8Ni stainless steel. Moffat et al .[62] have found that P decreases the corrosion rate of glassy metals in their active regions, relative to the corrosion of P-flee glasses . Prepassive films on P-containing glassy metals are P-enriched[62, 63] and this may be a precursor to P enrichment in or under passive films . Two studies[64, 65] of the native or thermally formed oxide films on Mctglas 2826A (Fe-36Ni-14Cr-12P-6B) have shown pronounced

Fig . 7. Weight loss of glassy 8OFe-20P on galvanostasis in I M H2SO4-(e) Specimen rinsed with CCI4; (A) further rinsed under ultrasound. The three lines correspond to the calculated weight loss for the phosphorus reactions shown . Fe is assumed to dissolve as Fe"[21] .

P can exert both detrimental and beneficial effects on the corrosion behaviour of Ni[52] . Crystalline Ni is significantly more noble than Fe but, as illustrated in Fig. 6, glassy Ni-19P does not passivate in acid . A nonprotective surface film of nickel phosphate forms on glassy Ni-P in acidic media, and the material is subject to pitting and severe general corrosion[53-55] . Anodic polarization of various glassy Fe-Ni-(Cr)-l' alloys produces similar Ni-rich non-protective prepassivc fihns, yielding to more protective iron or chromium oxyhydroxide films at higher potentials[56, 57] . The role of P in promoting the corrosion resistance of Fe-Cr glasses therefore requires explanation . The Sendaigroup[ 12,44,58] has put forward the view that P serves to enhance active dissolution prior to passive film formation, promoting the enrichment of Cr at the alloy/solution interface and the rapid formation of an unusually uniform hydrated chromium oxyhydroxide film in which very little P is incorporated[33, 44] . By contrast, boron and silicon are said to incorporate as

Fig. 8 . Current decay transients of glassy Fe-lOCr-l311-7X alloys after potentiostatic abrasion in 0.1 M H 2 SO 4. The metalloid X and the controlled potential (us sue) of the sample are shown in the figure[591 .



The electrochemical properties of metallic glasses

to ,

C,

05

I

1 .7

2

2.5

Time/.

11M H2504 • I M Nod

Passive potentials

17

temperatures but even at much lower temperatures, very small crystallites, undetectable by X-ray diffraction, may form . Slowly quenched glasses may also contain crystal embryos or patches of compositional inhomogeneity or heavy superficial oxidation which can greatly affect their mechanical and magnetic properties . Processing parameters and structural relaxation can also have a perceptible influence on the corrosion behaviour of glasses. For example, Kovacs et aL[23] found the corrosion current of Fe-16 .6B in acid to decrease slightly with decreasing wheel speed (ie cooling rate) until a threshold possibly connected with crystallization was reached. Comparison of glassy and crystalline alloys of identical chemical composition would allow one to assess the effect of structural disorder on corrosion behaviour . Most easy glass-forming compositions, particularly in the case of metal-metalloid glasses, are in the region of eutectics in the equilibrium phase diagram, and hence form multiphasic structures when crystallized. However, the glass-forming range of some metal-metal glasses encompasses single compounds in the phase diagram, and selected glasses can therefore be compared with single-phase crystalline analogues of identical composition . It appears from such studies on 60Cu 4OZr[68] and 50Cu-5OZr[69] that there is no significant influence of structure per se on corrosion behaviour alloy composition is the determinant of chemical behaviour . However, if crystallization produces two or more phases, galvanic and other inhomogeneous modes of corrosion become likely . Devitrification then produces a material of corrosion resistance inferior to that of the parent glass : the difference is less marked for metal-metal glasses ([70, 71J and Fig . 10) than for metal-metalloid glasses [72, 73] .

I M HCL

Fig . 9 . Current decay transients of Metglas 2826A (glassy Fe-36Ni-14Cr-12W 6Si), Si B (glassy Fe-36Ni-14Cr12B- 6Si) and stainless steel T316 (crystalline FeNi-l8Cr2Mn-L4Mo-2Si) held at constant potential in the passive in H 2 SO4 ; (b) 0 .5MF1 2 SO 4 region (a) 0.5M + 1 M NaCI[60] .

10 6

variation in film composition with depth : the outermost part of the film is actually P-depleted but there is a P-enriched layer under the main film, in a layer associated with Cr depletion . Thus while it is clear that there exists synergism between Cr and P in promoting the passivity of glassy metals, its cause is still unclear .

Structural effects Several important physical properties (eg density, resistivity, coercivity, Curie temperature) of metallic glasses are affected by the cooling rate employed during their formation[66, 67] . Glasses formed at high quench rates have slightly lower densities, and contain more free volume and quenched-in stress, than those formed more slowly . Low-temperature anneals, which permit some topological relaxation without causing crystallization, also change some physical properties . Glasses crystallize rapidly near their glass transition es 32,1-s

to , to I 2

I I I 1 0 -1

Fbtentiot/V (sca ) Fig. 10 . LSVs of glassy Fe-9Ni-l 8Cr-I I W in 1 M HCl after various heat treatments: (a) as-quenched material; (6) 673 K, 24 h ; (c) 773 K, 24 h (d) 873 K, 1 h ; (e) 973 K, 1 h[71].



18

M . D . ARCHER et al. INHOMOGENEITIES AND LOCAL CORROSION

Many glassy metals arc susceptible to localized forms of electrochemical corrosion such as pitting, crevicing, hydrogen embrittlement and stress corrosion cracking. Such local attack would be unlikely to occur if glassy metals were perfectly homogeneous . However, glassy metals are seldom physically homogeneous and they are, moreover, sometimes chemically inhomogeneous . Both types of inhomogeneity may serve as initiating sites for local attack .

eoo

a E

400 Inhomogeneity Melt spinning in inert gas, the most commonly used technique for the fabrication of glassy metals, produces physical inhomogeneities known as gas entrainment furrows on the wheel side of the ribbon . These are created by the trapping of bubbles of inert gas between the incipient ribbon and the rotating wheel . They can cause sealing and crevicing problems in the fabrication of electrodes[20, 62] . The top side is normally gently undulant, smoother and shinier than the wheel side, but it may have been more heavily oxidized during formation[74], particularly at the edges[75] . Chemical inhomogeneities or composition gradients or fluctuations have been detected in asformed amorphous metals fabricated both by meltspinning[76] and by sputtering[70, 77] . Moreover, crystallites of diameter less than .-10 nm are undetectable by X-ray diffraction but their presence in asquenched (100-x)(Co, Fe, B)-xCr for x > 4 has been inferred from the development of magnetic anisotropy[77] . It is thus not surprising that several workers have found electrochemical differences between the wheel side and top side o f the same ribbon[ 18,23,78-81] and effects arising from quench rate[23] . For example, Fig . 11[78] shows the marked difference between the potentiodynamic anodic polarization curves for asformed wheel-side (Fe-rich) and top-side (Cr-rich) glassy Fe-35Ni-15Cr-14P-6B in 3 % NaCl . As can be seen from Fig . 11, a light mechanical polish usually removes these superficial effects . Glassy metals are thermodynamically metastable with respect to bulk crystallization . They owe their room-temperature metastability to kinetic barriers to atomic translation . Their structures and compositions are therefore liable to evolve with any heat treatment that imparts significant mobility to any of their ingredients. Thus even mild heat treatment at temperatures far below the crystallization temperature may greatly affect their physical properties[82, 83] . Rapidly quenched ribbons are extremely strong and have good bending elasticity. They can deform plastically with formation of shear bands on bending without being fractured . However, on low-temperature annealing, some glassy metals suffer a severe loss in bending ductility although they are not perceptibly crystallized . Heat treatment is likely to cause accumulation of the small and relatively mobile metalloid on the free surface if the surface energy is thereby decreased[84, 85]. The consequent embrittlement may be regarded as analogous to the heat-induced embrittle-

o

10-

Current

density /A

i0-s

Cm 2

Fig. 11 . LS V s of glassy Fe-35M-14P-61l--l 5Cr in 3 % NaCl solution (- -) wheel side of the as-quenched material ; ( ) top side of the as-quenched material ; (---) mechanically polished top side[78] . ment of steels by solute segregation at grain boundaries. Phosphorus and sulphur segregate at the surface of certain Fe-based glasses at temperatures as low as 400K, whereas the smaller boron and carbon do not, possibly because they are in more stable positions in the bulk or because they are not surfactants[84] . The thermal embrittlement of several P-containing glasses is linked with the thermal diffusion and superficial segregation of phosphorus . Discrete P-rich islands or clusters of diameter < 6 nm have been found along the fracture surfaces of low-temperature annealed Metglas 2826 (Fe-4ONi-14P-6B)[76, 86] . These clusters may be the precursors to crystallization to Fe, P (among other phases) at higher temperatures . The decrease in toughness of glassy Fe-36Ni-14P-6B on annealing above 353 K has been correlated with the accumulation of P on the ribbon surface, probably accompanied by internal P clustering[86] . The results of Massiani et aL[78] on anneal-induced changes in the corrosion resistance of glassy Fe-35Ni-15Cr-14P-6B provide a startling cautionary talc . These workers observed deterioration in the corrosion resistance of material annealed at low temperatures in argon . This they ascribed to the pronounced surface segregation of sulphur, apparently present in the Allied Corporation material as an adventitious impurity . Ribbons subjected to identical thermal treatment in flowing hydrogen showed no superficial sulphur, presumably because of its removal as H 2 S, and no decrease in corrosion resistance . Pitting Some glassy metals, particularly those Fe Cr glasses which are resistant to general corrosion, possess



The electrochemical properties of metallic glasses outstanding resistance to pitting corrosion- For example, glassy Fe-25Cr-xMo-20B does not pit on anodic polarization in 6 M HCI at room temperature when x > 5[44] . It is generally held that glasses are relatively immune to pitting corrosion by comparison with steels of similar composition in similar media because their passive films are highly uniform and less porous than those formed on steels[58] . When pits do form, they are smooth-walled and usually roughly hemispherical . Preferred sites for pitting in a uniform environment include the bottom and lip of gas entrainment furrows on the ribbon wheel side and any point at which a defect is introduced into the passive film . Table 1 summarizes literature information on pitting and shows that many glassy metals are indeed extremely resistant to pitting corrosion . The severity of the tests to which these materials were subjected differs so the yes/no answers should be treated with caution . For example, by using a slower scan rate and a nitrogen rather than air atmosphere, Hanham[89] found pitting in two glassy alloys which previous workers[30] had found to be unpittable. Pitting corrosion has been found in Fe-Ni-B-Si alloys with c 12 % Si during anodic polarization in a borate buffer solution of pH 8 .4[20] . This is attributed to there being insufficient Si atoms in the surface to form a protective Si-rich passive film, allowing localized corrosion to occur in the areas of low Si concentration . Amorphous ion-plated Fe-Cr films have been found to pit in neutral 1 M NaCl although only above - LO V (sce)[91] . Pitting has also been observed in glassy Fe-25Cr-10Zr in 1 M HCI at > 0.55 V (sce)[92]. Crystallization, by producing chemical heterogeneity, lays the material open to pitting by galvanic However, Fe--40Ni-14P-6B and means. Fe-35Ni-15Cr-14P-6B have been found to be resistant to pitting in various media even after crystallization, although no results for an acidic chloridecontaining medium were quoted[73] . In a study of sputtered xCr-B films in 1 M HCI, annealed microwere crystalline films with a Cr content of x > 60 found to be susceptible to pitting on anodic polarization, while the as-sputtered amorphous films were resistant[95] .

Crevice corrosion As has already been mentioned in connection with inhomogeneities, some workers[20, 62] have reported problems with crevice corrosion when manufacturing electrodes of glassy metal ribbons . However, little systematic work has been performed on the resistance of glassy metals to crevice corrosion . Hanham[89], in his work on Fe-Ni-B alloys of differing B contents in 2 M H 2 SO4, reported a dissolution rate so high that it was impossible to obtain the complete anodic polarization curve . The samples crevice-corroded right through the ribbon at the interface of the electrode with the epoxy coating . In the same study, no crevice corrosion was found for Fe-10Cr-13P-7C or Fe-IOCr-lONi-13P-7C alloys ptibility to using the same electrode desi . So crevice corrosion was therefore attributed to the absence of Cr and P .

19

A comparative study has been made of the susceptibility to crevice corrosion of Fe-N1-Cr-P-B alloys with different Cr contents[80, 96, 97] . This series of alloys is of interest since they show exceedingly good resistance to general corrosion but also display certain characteristics which are used as pointers to possible susceptibility to crevice corrosion in conventional crystalline compounds : they passivate at sufficiently anodic potentials and the rate of the active dissolution increases with decreasing pH. In the earliest study[80], the localized corrosion resistance of Fe-3514i-15Cr-14P-6B was investigated by anodic polarization both potentiostatically and potentiodynamically in a variety of chloride-containing media . No pitting was found at any pH or C1 - concentration . However, the alloy was susceptible to crevice corrosion even in dilute NaCl solutions. Cold-rolling the alloy markedly enhanced the corrosion rate : this was attributed to crevice corrosion at the numerous cracks introduced by this deformation . Crevice corrosion in regions of oxygen depletion thus presents a possible Achilles' heel in the otherwise remarkable corrosion resistance of this alloy . Only 2 % Cr was necessary to confer significant resistance to pitting and crevice corrosion on Fe-30Ni-xCr-15P-6B[96, 97] . The alloys could be forced to crevice-corrode at pH = 1 but only at very anodic (transpassive) potentials ; hence they are considerably more resistant to crevice corrosion than crystalline alloys containing equivalent amounts of Cr . Cold-rolling of the alloys greatly increased the initial values of both i_, and i., but the active current rapidly declined . This was attributed to the onset of crevice corrosion which ceases as soon as the crevices widen and passivate . The increased susceptibility to crevice corrosion of the cold-rolled alloy was again attributed to the presence of microscopically invisible microcracks which can act as initiating sites. The main conclusion drawn was that the corrosion resistance of glassy alloys extends to resistance to the propagation, as well as to the initiation, of localized corrosion .

Stress corrosion cracking and hydrogen embrittlement Glassy metals have received some attention as potential hydrogen storage materials owing to the capacity of some alloys, such as Pd-Zr and Ni-Zr, to dissolve large quantities of hydrogen while exhibiting less severe embrittlement in the process than crystalline alloys[98] . Much work has been performed on the susceptibility to, and causes of, hydrogen embrittlement in glassy alloys, especially Fe and Ni-based alloys, both in solution and in moist atmospheres . It is not appropriate here to review the work that has been done on gas-phase hydrogen storage and embrittlement : only work of relevance to the understanding of stress corrosion cracking (SCC) will be considered. Two recent reviews[99, 100] cover the absorption of hydrogen in glassy metals . Stress corrosion cracking in crystalline alloys has been widely studied but the underlying causes are not yet fully understood . The cracking, caused by the combination of an applied stress and a corrosive environment, proceeds along grain boundaries or preferred crystal planes in crystalline alloys . Since

20

M . D . ARCHER et al. Table 1 . Pitting of glassy metal alloys Alloy

Solution

Anodic sweep or free corrosion

Pitting

Reference

Fe-Cr-13P-7C C'> 7% Fe--Cr-13P7C and Fe-Ni-Cr-P-C

I M HCI 3 % NaCl 6 % FeCI, 0.01-1 M HCI 1 M NaCI 1 M H, SO4

fc as

No No

11

till fe as as

No No No No

30

Fe-l0Cr-13P-7C Fe-10(7r-tONi-13P-7C

2 M "2 S04 2 M H 2 SO4

as as

Yes Yes

89

Fe-N4-13P-7C M=Cr>4% M=Mo>4% M = W > 6

I M HCI 1 M HCI I M HCI

as as as

No No No

Fe-Co-13P-7C Fe-2(Ti, Nb, V, W or Mo)-3Cr-13P-7C Fe 2OCr-13P-7C Fe-C(Cr, P, Mot Fe-Cr-Mo + C or B or (P+C) or (B+Si) Fe-12Mo 18C Fe-16Mo 18C FeB-C+Cr > 8% Fe-B-Si +Cr > 8

3 M NaCl

as

No

24

1 M HCI 1 M HC1 I M HCl

as as as

No No No

58 87 87

6M HCI 1 M HCI 1 M HCI 3 % NaCI 3 % NaCI

as as as as as

No No No No No

44,45 39 39 27 27

Fe--4ONi-20B Fe-39M-10B-xSi x < 12% x>12% Fe-2SCr-xMo-20B x>5% Fe-25Cr-IOZr Fc-Cr film Cr-25B Ti-25B Ni-P + Cr, W, Mo

Borate buffer, pH = 8 .4

as

Yes

20

6 M "Cl

as as as

Yes No No

20 20 44

1 M HCI 1 M NaCI I M HCI 1 M HCI 0.5 M H 2 SO, 0.3 M H, PO 4 1 M HCI 1 M HCI IMNaCl 10% FeCI,

as as as as as as as as as fc

Yes Yes No No No No Yes No No No

92 91 90 90 54 54 54 58 58 58

1 M HCI

as

No

94

1 M 1 M 0 .1 I M

as as as as

No No No No

94 73 73 73

as as

No Yes

71 71 71

as

No

as as

No Yes

Ni Cr 15P 5B Cr >7% Ni-Cr- 15P-5B Cr=5,7,9% Co-13P-7B and other elements Co-Cr-20B and other elements Fe-4ONi-14P 7B Fe-35Ni-15Cr-14P-7B Fe-4ONi-15Cr-16P-4B Fc-19Ni-18Cr-11 W after Ihat773K 1 h at 873K Fe-5Ni-9Cr-54W after DSC thermal scan to 1098 K xCr-B, x > 60% Amorphous Microcrystalline

28

HCI H 2 SO4 M NaCl NaCI

I M NaCI pH-7

HCI, pH = 0.4

these do not exist in glassy alloys, it is of considerable interest whether and in what manner SCC occurs . There is certainly no reason for believing that metallic glasses need be immune to SCC because they lack grain boundaries . As already described, metallic glasses display considerable physical non-uniformity, espec-

71 95 95 95

ially on the wheel side, and are in some cases also highly chemically inhomogeneous . These inhomogeneities may well act as SCC initiation sites. Glassy metal SCC, like that of crystalline alloys, is closely linked with the process of hydrogen embrittlement . Kawashima and co-workers[I0l, 102] have



The electrochemical properties of metallic glasses studied SCC of two FeNi-Cr-P-C alloys in solutions of H 2 SO4 with and without added chloride . Under similar conditions, austenitic stainless steels suffer SCC at room temperature. The fracture stress of the ribbon was lowered by the presence of the solution and also by cathodic polarization, which causes hydrogen embrittlement with or without added Cl . In the passive region, SCC was found only for acidic solutions with high Cl - concentrations, while in the transpassive region, general corrosion occurred and the fracture stress approached that found in air . The cracking, both in the cathodic and in the passive region, was attributed to hydrogen embrittlement . More recently, SCC of Fe-10Cr-13P-7C has been studied by much the same means in 5 M HCI and 5 M H 2 SO 4 , and hydrogen embrittlement was again concluded to play a major part in SCC[163] . The nucleation of stress corrosion cracks on glassy Ni-29Fe-14P-oB-2A1 at the free corrosion potential in 3 .5 M NaCI has been studied[ 104] . The cracks were observed to nucleate at an embrittled surface layer . Cracking of an embrittled surface layer and pitting appeared to compete with one another as SCC initiation mechanisms. SCC of two Fe-Ni-B alloys, one containing phosphorus[10S] and one not[106], has been studied in 0.39 M FeCI„ I M HCI and polythionicacid by bend tests similar to those used by other workers[101-103] . Both alloys were found to be vulnerable to SCC in FeCI, at their free corrosion potentials (0 .219 V us she for 4OFe-4ONi-20B and 0 .150 V vs she for 40Fe-40Ni-14P-6B) . In polythionic acid, hydrogen embrittlement appeared to have a greater effect and the cracks formed were larger . In 1 M

21

HCI, cracking was induced only by anodic polarization. Strikingly regular and elaborate cracks (Fig . 12) were formed on bent Fe- 4ONi-14P-6B in FcCI3 at the free corrosion potential . It is apparent that, in the absence of grain boundaries and significant internal stress, cracks should propagate from pits normal to the net stress at the crack tip in a rather reproducible way. It is also clear that hydrogen embrittlement, the presence of chloride ions and the composition of the alloy all play a part in SCC .

ELECTROCATALYTIC PROPERTIES The excellent corrosion resistance of some metallic glasses combined with the possibility of fabricating new metastable compositions has led to interest in their possible use as catalysts and electrocatalysts. In some cases, metallic glasses have shown greater activity and longer catalyst life than their crystalline counterparts.

Hydrogen evolution Naka et al.[107] have studied hydrogen evolution on Fe Zr, Ni-Zr, Co-Zr, Ni-Nb and Cu-Ti amorphous alloys in I M H 2 SO4 by galvanostatic polarization . Figure 13 shows that, for all alloys except those containing Nb and Cu, the hydrogen overpotential was similar to that expected of a simple mixture of the pure crystalline components . In the case of the Nbcontaining alloys, the hydrogen overpolential was

Fig. 12 . Scanning electron micrograph of stress corrosion crack formed in glassy Fe-40Ni-14P-6B by 50 min immersion in 0 .39 M FeCl3 at the free corrosion potential[105] .



22

M. D . ARCHER

et al.

200 > E G IW

C 0. 200 > E

C IOo

0

Fig. 13. Hydrogen overpotential p of various glassy alloys measured galvanostaticallyat 10 2 A m - ' in 0.5 M H 2 SO, as a function of alloy composition[107]. increased in the amorphous state . The Cu-containing alloys, on the other hand, showed . a lowered overpotential, that for the Cu-30Ti alloy being exceptionally low. This was attributed to the transfer of electrons from Cu to Ti in this alloy, creating a d-electron deficiency in the Cu atoms which results in increased electrocatalytic activity. Other workers[108] have investigated hydrogen evolution on glassy Cu-5OTi, Cu 65Ti and Cu 67Zr, both in the as-quenched state, and after surface activation by immersion in 1 M HF for a few minutes at room temperature . XPS studies showed Cu enrichment at the surface after treatment with HF caused by the leaching out of Zr or Ti . This also produces a superficial porous Raney-type Cu layer . The activity of the as-quenched alloys in 1 M NaOH was too low to be measured, but surface activation resulted in substantially improved activity . Figure 14 shows cathodic Tafel plots for the hydrogen evolution reaction obtained for the surface-treated alloys in comparison with that for crystalline Cu . The Tafel-extrapolated exchange current density, corrected for roughness and taken as a measure of the electrocatalytic activity, was higher for the amorphous alloys than for crystalline Cu . The Cu-Zr alloy appeared to be five to ten times more active than the Cu-Ti alloys. In 0.5 M H,SO 4 , surface activation is destroyed and the electrocatalytic activity of the HFpretreated alloys is very low. It is possible that the porous copper layer is oxidized in air and it is this oxide layer that has the high catalytic activity in NaOH rather than Cu itself. The dissolution of this oxide layer in acid would then explain the lack of activity in that medium .

Methanol oxidation A great deal of interest has focussed on the development of small-scale transportable fuel cells which employ organic fuels . Methanol has many advantages as a fuel : it is cheaper and easier to store and handle

-r

-B tog

, /A cm 2 ,

-5

-4

t rue

Fig. 14 . Cathodic polarization behaviour of HI-treated glassy Cu-Ti and Cu-Zr in 1 M NaOH at 303 K on (a) the apparent unit area basis; (b) the true unit area basis : ( ) pure copper in I M NaOH with an assumed roughness factor of three[126]; (o)Cu50Ti5a ; (9) Cu,,Ti,, ; (A .A) Cu, Zr 67 [lOg] . than hydrogen . It is also readily available and has a fairly high electrochemical reactivity at moderate temperatures . Much effort has therefore been put into the development of an electrode with a high elect trocatalytic activity for the oxidation of methanol and its derivatives. Fe, Co, Ni, Zr and Pd-based metallic glasses have been studied [108-111] both in the asquenched and the surface-activated state as electrodes for methanol oxidation in 1 M KOH . The surfaces were activated by the electrodeposition of Zn onto the mechanically polished glassy metal surface, followed by heat treatment at 473-573 K for 30 min and subsequent removal of the zinc by 6MKOH . This treatment produces a thick porous layer on the surface of the glassy metal . In all cases, the as-quenched alloys showed very low or no activity for alkaline methanol oxidation . However, the performance of the Pd-based alloys was improved by surface treatment to a level comparable with that of platinized platinum . This was attributed to an increase in the porosity of the surface . The activity of the Pd-based alloys was also improved by alloying with P, Ni, Pt, Rh and Ru . Excellent activity was found for surface-activated I'd-5Ni-19P alloy heat treated at 573K: this exceeded that of platinized platinum, and Fig . 15 shows the stability also to be good in comparison . The activity for methanol oxidation of two Cucontaining glassy alloys, Cu-65Ti and Cu 62Zr, has also been studied[112] both in the as-spun state and after surface activation by HF treatment as above . Once again, the activity was negligible without pretreatment but, by comparison with that of crystalline Cu, a much larger anodic electro-oxidation current was found for the HF-treated electrodes of both alloys,



The electrochemical properties of metallic glasses

23

Table 2. Current densities of various electrodes at 1 .15 V (see )Current density (Am -2) 4 M NaCl I M Na 2 SO 4

Electrode -350

-450 0

2

4

6

Time/h

s

ID

Fig . 15. Potential of surface-activated glassy Pd-Ni-19P alloys during galvanostatic oxidation of methanol at 50mAcm -2 in 2MCH3OH+IMKOH at 303K. Comparison is made with platinized platinum (Pt/Pt) and surface-activated crystalline palladium[1101 . especially those with large roughness factors . This activity was also stable over several days .

Glassy alloys 5lPd-30Rh-19P 41Pd-40Pt-19P 41Pd-401r-19P 4lPd-401r-19P 46Pd-301r-5Ti-19P 41Pd-301r-lOTi-19P 46Pd-30Ir-5Rh-19P 4lPd-301r-lORh-19P 46Pd-301r-5Pt-19P 4lPd-3olr-l0Pt-19P 4lPd-301r-l0Ru-19P Crystalline materials Graphite RuO2 /Ti Rh Pt If

Pd Ru

220 400 2000 2000 3000 1500 700 260 860 500 2200

7 6 5 5 2 5 4 8 17 11 28

41 1800 26 340 340 1300

20 340 6 1 150 16 1400

-From Ref 114.

Chlorine evolution Anodes consisting of a mixture of Ru0 1 and Ti supported by metallic Ti, often called dimensionally stable anodes (DSA), are widely used for chlorine evolution in the chloralkali industry . One of the problems with the DSA is that the overpotential for oxygen evolution is too low, so the chlorine produced contains an undesirable amount of oxygen . Pd-P glassy alloys containing different amounts of Ru, Rh, it and Ti have been studied as possible anodes[113-116] by potentiostatic polarization in 4 M NaCI and I M Na t SO 4 under typical industrial conditions (pH 4, 353K) . Table 2 gives some results . The corrosion resistance in the chlorine evolution region of the glassy alloys containing a second metallic element in sufficient concentration was better than that of their crystalline counterparts . The activities of the glassy alloys were generally higher than those of their crystalline counterparts and also than those of all the Pt group metals except Pd, which dissolves under this treatment. In addition, the overpotential for oxygen evolution was also high . Various alloy compositions were tested : those with the most favourable characteristics were Pd-301r-5Ti-19P, Pd-301r-IORu-19P and Pd-301r-lRh-19P . These all showed a greater activity for chlorine evolution, a higher overpotential for oxygen evolution and good stability as compared with a conventional DSA.

stable and stable crystalline analogues, the yield of C,, and Cs hydrocarbons being especially good in the case of glassy Fe-60Ni-20P[119, 120] . The kinetic study shown in Figs 16 and 17 revealed first order kinetics in P a with little dependence on P co either in the glassy or in the crystalline material . Since no surface area differences could be detected by BET analysis, this suggests that the active sites on the glassy and crystalline catalysts are similar, but that they are more numerous on the glassy surface . alloys Studies of the metal-metal glassy Fe-IOZr[121], Ni-37Zr[122] and Pd-65Zr[123] as possible Fischer-Tropsch catalysts have also been

10

a 0 6 'o

a 4 0

4

2

Fischer-Tropsch reactions Fe and Ni-based glassy alloys with additions of P and B have been investigated as possible catalysts for the Fischer-Tropsch hydrogenation of and carbon monoxide, Fe-40Ni-16P-4B[117] Fe-6ONi-20P[118] both show good activity . Without: exception, fifteen glassy Fe-Ni based glassy alloys with P and B additions performed better than both meta-

2

0

2

04

0.2 Pa'

.6 0

(atm)

Fig. 16 . Rate of hydrogenation of CO vs pressure of hydrogen using glassy and crystalline catalysts[119] .



M . D . ARCHER et al.

24

P,, ' =

e eA

is negligible without some form of surface activation . Schloegl[ 128], in a recent paper, expresses some doubt as to whether these results can truly be ascribed to the presence of a glassy phase . He contends that the surfaces of glassy and crystalline structures have been insufficiently characterized, and that the reported differences could be due to different surface areas or different superficial chemical composition . If the active species on the surface is an oxide, the catalyst may not be glassy, but merely formed from the glassy phase . Thus although metallic glasses show promise as catalysts and electrocatalysts, and rapid quenching has been found to be a useful method of forming catalysts for certain reactions, caution is necessary in attributing increase in activity to the influence of a glassy phase .

Fee.,P,e r Nie0Pzo FepoNiooP2A A A At 230°C

00

v0

I I 002 004

Po (atm)

Fig . 17 . Rate of hydrogenation of CO us pressure of CO using glassy catalysts[119] .

reported. The catalytic activity of the latter was orders of magnitude higher than that of conventional Raney nickel and supported Pd catalysts : the catalytically active species was reported to be a complex and unknown oxide of Pd and Zr[123] . Addition of Si reduced the activity, as the active oxide species was not formed. Other workers[124] have studied CO adsorption on glassy Ni-36Zr to obtain further information on the mechanism of the reaction. The differences between the glassy and crystalline material were small and appeared only below 273 K .

Other hydrogenation reactions Glassy alloys have also been studied as catalysts for other hydrogenation reactions . Glassy Ni-P and Ni-B have been studied as olefin hydrogenation catalysts[125] . Once again, the as-quenched alloys showed no activity but surface activation with dilute HNO 3 , oxygen at 375-523 K and hydrogen at 570 K produced high catalytic activity in comparison with crystalline Ni-P and Ni-B compounds . However, according to a later report on the hydrogenation of 1,3-butadiene over amorphous Ni-xB (x > 15 %), some x-dependent catalytic activity is discernible without prior surface treatment[ 1261 . Glassy Pd-Si and Pd-Ge have been compared with their crystalline counterparts and also with crystalline Pd as catalysts for the dissociation of hydrogen and deuterium, and for hydrogen and deuterium exchange in ciscyclodecane[127] . In all cases, higher activity and better selectivity were reported for the glassy phase as compared with the crystalline phase. It is clear that, in many cases, better catalytic behaviour has been found for metallic glasses as compared with conventional crystalline catalysts. However, although there is good evidence that some glassy alloy surfaces make good catalysts, especially for chlorine evolution, without surface treatment, there are many cases in which the activity of the glassy alloy

Acknowledgements-We thank Professor B . D . Lichler and Dr A. L . Greer for their constructive comments on an earlier draft of this paper.

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