Corrosion Science, Vol. 35, Nos 1-4, pp. 411-417, 1993 0010--938X/93 $6.00 + 0.00 Printed in Great Britain. © 1993 Pergamon Press Ltd

T E S T I N G T H E F I L M - I N D U C E D C L E A V A G E M O D E L O F S T R E S S - C O R R O S I O N C R A C K I N G

M. SAITO, G. S. SMITH and R. C. NEWMAN

UMIST, Corrosion and Protection Centre, P.O. Box 88, Manchester M60 1QD, U.K.

Abs t rac t - -The effects of coarsening of porosity within dealloyed layers have been investigated from the viewpoint of the film-induced cleavage model of SCC. Tensile tests and pore coarsening measurements of Ag-20 a t % A u foils were carried out in 1 M HCIO 4 at room temperature. Upon ageing at potentials above 550 mV(SCE), the ability of a brittle crack to penetrate into the substrate was destroyed quickly due to rapid pore coarsening. At low potentials brittle behaviour persisted for up to 2h . Chloride ions accelerated the coarsening and pyridine retarded it. The observed time dependence of the coarsening indicates that it occurs by the surface diffusion of Au atoms and is affected by the specific adsorption of ions or molecules.

I N T R O D U C T I O N

THE FILM-INDUCED cleavage model, which has been developed mainly by Sieradzki and Newman, 1-3 is based on the idea that a crack, originating in a surface layer, can obtain a high enough velocity to penetrate into the underlying substrate even if the substrate is a face-centred cubic (fcc) metal.

The film-induced cleavage model has been fractographically and kinetically investigated, and is probably the only mechanism of SCC which can generate discontinuous, cleavage like fracture surfaces, such as those seen in the transgranular SCC of a-brass in cuprous ammonia solutions. 4 Recent work shows that intergranu- lar cracking can also penetrate into the substrate in Ag-Au 5'6 and C u - A u 7 alloys. Recently the main focus has been placed on the study of thin foils, because a thin foil is useful in isolating a single brittle event and evaluating the kinetics of crack propagation. Newman et al. showed that less than 100 nm of surface dealloyed layer gave rise to several/~m of cleavage fracture in an ammonia environment by using 10 ~tm a-brass foils, s Cleavage also occurred at liquid nitrogen temperature. Kelly et al. 6 have investigated the effects of pore coarsening of dealloyed layers on film- induced intergranular fracture of A g - A u foils in 1 M H C 1 0 4. Devine and others have recently confirmed the brittle behaviour (at dealloying potentials) using Cu-Au foils in 0.6 M NaC1 solution. 7"9 Ricker et al. showed evidence for film-induced fracture in Rh-electroplated Ni. 10

In this paper the effects of pore coarsening on the film-induced intergranular fracture of Ag-20 at%Au alloy in 1 M HCIO4 are described in detail.

E X P E R I M E N T A L M E T H O D

The material used for this investigation was a Ag-20 a t % A u alloy foil with a nominal thickness of 10/~m, rolled by Goodfellow Metals (Cambridge, U.K.) .

For the mechanical tests, tensile specimens of dimensions 3.5 × 18 mm were prepared and rounded notches (r = 2.6 mm) cut on both edges. Each specimen was then annealed for 1 h in air at 900°C and rinsed with ethanol before all but 0.04 cm 2 (2 × 2 mm) was insulatcd with lacquer. Tensile tests were

411

412 M. S A I T O , G. S. SMITH and R. C. NEWMAN

FIG. 1.

L)

v >

v

o

<

1000

500

Symbol Fracture mode • Brittle • Mostly brittle

- • Brittle/Ductile Mostly ductile

o Ductile Dealloying

¢

\

Ductile Brittle

\

\ • • • O

I I I I I

0 30 60 90 120 150

Aging time (rain)

The effects of ageing potential and ageing time on the fracture mode of Ag- 20 a t%Au alloy after dealloying (1050 mV x 10 s) in 1 M HCIO4.

1 . 0 - -

0 . 9 - o 0 . 8 - -

0 . 7 - - O'

0 . 6 -

0 . 5 - a-.

¢.9 0 . 4 - -

o N

0 . 3 - -

O Z

0 . 2 -

(rain)

5 10 203060 120 240

I I I I I I I

% )x

Symbol Potential ----o---- 0V m o - - - 550mV

- -.-&- - 700mV

0.1 ~ ~ ~ l t t . I ~ n , l , . . , l . , , I , , , , I , . , I , . ,

10 100 1000 10,000 100,000

Aging t ime (s)

FIG. 2. The effects of ageing potential on the charge passed during a 50 mV step in 1 M HC104 at room temperature (measurement of pore coarsening: Q / Q o ~ t-°3 -= pore

size - t°3).

i !

Aging at 0 V x 5 min

Aging at 700 mV x 30 min

Fro. 3. SEM micrographs of the fracture surface after different ageing t reatments in 1 M HCIO 4 containing (a) no additions; (b) HCI: 1 x 10 3 M; (C) HCI: l x 10 2 M; (d) HCI: 1 x

10-2M + pyridine: 1 x 10 2M; (e) no additions; (f) pyridine: 1 x 10 2M.

413

Film-induced cleavage model 415

performed in 1 M perchloric acid (HC104) solution* at room temperature after dealloying for 10 s at 1050 mV(SCE) (q = 1.8 C cm -2) and ageing for up to 240 min at various potentials. The stress was introduced in a 'single shot' manner (strain rate >25 _+ 5 s-1) by a tensile rig. This procedure is described in detail elsewhere. 5

To investigate the effect of adding surface-active ions or molecules (chloride and pyridine) on the fracture mode, tensile tests were conducted after ageing for 5 rain at 0 V(SCE) in 1 M HCIO4 containing (1) no additions, (2) HCI: 1 x 10 -3 M, (3) HCI: 1 x 10 - 2 M, (4) HCI: 1 x 10 .2 M and pyridine: 1 x 10 .2 M; after ageing for 30 rain at 700 mV(SCE) in 1 M HC104 containing (5) pyridine: 1 x 10 -2 M.

For the measurements of pore coarsening, 5 x 5 mm specimens were cut, annealed for 1 h at 900°C and covered with epoxy resin leaving about 0.1 cm 2 (3.5 x 3 ram) exposed. They were then dealloyed at 1050 mV and aged at lower potentials. Potential steps (50 mV in amplitude) from 0 V(SCE) were applied periodically during ageing at 0 V, 550 and 700 mV(SCE) in 1 M HC104 at room temperature so that the change in pore size could be estimated from the capacitative (double layer) charge passed during each potential step. To avoid cathodic deposition of silver during the measurement, test solutions were completely refreshed immediately before the potential was dropped from the dealloying potential to the ageing potential or from the high ageing potentials to 0 V(SCE). The double layer charging currents were recorded by a digital storage oscilloscope and the charge was calculated by computer.

EXPERIMENTAL RESULTS AND DISCUSSION

The effects o f ageing potent ia l and ageing time after deal loying on the fracture m o d e of A g - A u alloys are shown in Fig. 1. All specimens showed intergranular brittle fracture immedia te ly after dealloying, at each ageing potential . The fracture m o d e at 750 m V ( S C E ) was still brittle after 90 min. Judging f rom the polar izat ion curve of A g - 2 0 a t % A u alloy in 1 M H C 1 0 4 at r o o m tempera ture , the critical potent ial for deal loying (Ec) is be tween 700 and 750 mV(SCE) . Therefore , a brittle fracture can be p roduced because macro-dea l loying is still occurr ing during ageing above 750 m V ( S C E ) . I f deal loying is fast enough, SCC can occur even with high coarsening rates, for instance in chloride solutions, but may tend to vanish below a critical deal loying rate. On the o ther hand, the fracture mode changed f rom brittle to ductile with t ime below 700 m V ( S C E ) where there was no persistent dealloying. The brittle/ductile transit ion t ime was quite different above and below 600 m V ( S C E ) . A partially ductile fracture surface was observed after only 10 rain ageing and the fracture m o d e changed comple te ly to ductile after 30 min for ageing potentials be tween 600 and 700 m V ( S C E ) ; however , be low 550 m V ( S C E ) a partially brittle fracture surface was obta ined even after 90 rain ageing. The deal loyed layer thickness at the grain b o u n d a r y was no more than 3 ~tm in every case, as de te rmined by SEM and E D X . Residual silver in the layer was < 1 % .

Some investigations on the recons t ruc ted electrode surface of A u show that the double layer capaci tances of Au(100) and Au(111) increase rapidly above 600 m V ( S C E ) in 0.01 M H C 1 0 4 solution. 11,12 This increase of the capaci tance may be due to the adsorpt ion o f specific ions, in this case C10 4. These kinds of adsorpt ion increase the surface self-diffusivity. Consequent ly , the coarsening rate of deal loyed layers becomes faster and the rapid coarsening reduces the ability of a crack in the deal loyed layer to penet ra te into the substrate.

To evaluate the difference in coarsening rates as a funct ion of potential , the double layer capaci tance measuremen t s by a potent ia l step me thod were carried out during ageing at 0 V, 550 and 700 m V ( S C E ) . The test results are shown in Fig. 2.

As Kelly et al. ment ioned elsewhere, 6 the pore size can be es t imated f rom the charge passed during a 50 m V step, which is p ropor t iona l to the total surface area of

* Analytical reagent grade perchloric acid (chloride: max. 3 ppm) and triple-distilled water were used.

416 M. SAITO, G. S. SMITH and R. C. NEWMAN

pores in the dealloyed layer. The log charge-log time curves were almost the same at 0 V and 550 mV(SCE). The slopes of both curves became about -0.3 after 1 h ageing. That is, Q/Qo varies as t -°3 and the pore size (on a rod and channel model) varies as t 03. Coarsening processes controlled by surface diffusion generally have t °25 kinetics, 13 so the basic process of coarsening is considered to be surface diffusion. Exponents closer to 0.25 were reported by Kelly et al. 6

Coarsening occurred much more rapidly at 700 mV(SCE) than at 0 V and 550 mV(SCE). The specific adsorption of perchlorate accelerated the coarsening above 600 mV(SCE) as mentioned before.

Rapid tensile tests were conducted in 1 M HC104 containing different concen- tration of chloride (as HC1) and pyridine so that the effects of surface-active species on the coarsening could be studied. The resulting SEM micrographs are shown in Fig. 3. When the specimens were aged for 5 min at 0 V(SCE), the fracture surface was completely brittle in the pure HC10 4 (a); however, some ductile fracture occurred in the solution containing 1 x 10-3M HC1 (b) and the fracture surface became completely ductile in the solution containing 1 x 10 .2 M HC1 (c). When the solution contained 1 x 10-2M pyridine as well as 1 x 10-2M HC1, most of the fracture surface became brittle again (d).

These different fracture surfaces are considered to be obtained because specific adsorption of the added species affected the coarsening rate of the porosity in the dealloyed layer; chloride ion is known to increase surface diffusivity of gold and pyridine to retard it. 14,15 The effect of pyridine was also observed during ageing for 30 rain at 700 mV(SCE) in 1 M HC104 containing 1 x 10 -2 M pyridine (f). In future work the effects of some other ions such as NO3 and NH2 will be studied.

It might be commented that the transitions in fracture mode shown in Fig. 1 are occurring after times when very little coarsening has occurred according to Fig. 2. In fact, impedance spectra show that the smallest pores coarsen much faster than the average, 6 and since these control the fracture behaviour, it is possible to see transitions where the average pore size has only increased by a factor of 2 but pores with (say) one-third of the average size have been eliminated completely.

In further work 16 similar results have been obtained on Cu-Au alloys, including evidence for transgranular film-induced cleavage at low ageing potentials or even in the dry state.

CONCLUSIONS The coarsening of the dealloyed layer controls the ability of a brittle crack in the

layer to penetrate into the substrate. The potential and time dependence of the coarsening are controlled by surface diffusion; the coarsening is accelerated above 550 mV(SCE), probably by specific adsorption of C104. Chloride accelerates the coarsening and pyridine retards it, in accordance with the effects of these substances on surface self-diffusion of gold.

R E F E R E N C E S 1. K. SIERADZKI and R. C. NEWMAN, Phil. Mag. A 51, 95 (1985). 2. K. SIERADZKI and R. C. NEWMAN, J. phys. Chem. Solids 48, 1101 (1987). 3. A. T. COLE, R. C. NEWMAN and K. SIERAI)ZKI, Corros. Sci. 28,109 (1988). 4. K. SIERADZKI, J. S. KIM, A. T. COLE and R. C. NEWMAN, J. electrochem. Soc. 134, 1635 (1987). 5. R. G. KELLY, A. J. FROST, T. SHAHRABI and R. C. NEWMAN, Metall. Trans. A 22A, 531 (1991). 6. R. G. KELLY, A. J. YOUNG and R. C. NEWMAN, The Characterization of the Coarsening of Dealloyed

Film-induced cleavage model 417

Layers by EIS and Its Correlation with Stress-Corrosion Cracking, Electrochemical Impedance: Analysis and Interpretation, ASTM STP 1188, (eds J. R. SCVLLY, D. C. SILVERMAN and M. W. KENI)I~), 94-112 (1993).

7. J. S. CHEN, M. SALMERON and T. M. DEVINE, Scr. Metall. 26,739 (1992). 8. R. C. NEWMAN, T. SHAr~RABr and K. SIE~ADZKI, Scr. Metall. 23, 7l (1989). 9. J. S. CHEN, M. SALMERON and T. M. DEVINE, Metall. Trans. A (in press).

10. R .E . RICKER, J. L. FINK, J. S. HARRIS and A. J. SHAPIRO, Scr. Metall. 26, 1019 (1992). 11. D. M. KOLB and J. SCHNEIDER, Electrochim. Acta 31,929 (1986). 12. H. AN~ERSTEIN-KozEowsKA, B. E. CONWAY, A. HAMELIN and L. STOICOVIClU, Electrochim. Acta 31,

1051 (1986). 13. B. K. CrIAKRAVERTY, J. Phys. Chem. Solids 28, 2401 (1967). 14. C. ALONSO, R. C. SALVAREZZA, J. M. VARA and A. J. ARWA, Electrochim. Acta 35, 1331 (1990). 15. H. HONBO, S. Str~AWAI~A and K. ITAYA, Anal. Chem. 62, 2424 (1990). 16. R. C. NEWMAN and M. SAITO, Corrosion-Deformation Interactions (eds J.-M. GRAS and T. MA6NIN),

Les Editions de Physique, (in press).