Inhibitive effects of benzotriazole on the stress corrosion cracking of α-brass in nitrite solution

Corrosion Science, Vol. 36, No. 2, pp. 221 23(L 1994

( ~

Pergamon

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0(110-938X/94 $6.00 + 0.0(~

INHIBITIVE EFFECTS OF BENZOTRIAZOLE ON STRESS CORROSION CRACKING OF c~-BRASS NITRITE SOLUTION

THE IN

SAYED M . SAYED, ELSAYED A . ASHOUR a n d BADR G . ATEYA* National Research Centre, Dokki, Cairo, Egypt * Faculty of Science, Cairo University, Cairo, Egypt

Abstract--The addition of benzotriazole (BTA) to 0.1 M sodium nitrite solution produces an inhibitive effect on the stress corrosion cracking (SCC) of ~z-brass under applied anodic potential. The maximum stress and the time to failure increased with the B'FA concentration. The mode of cracking changed from transgranular, in nitritc solution alone or in the presence of low concentrations of BTA, to ductile failure at higher concentrations of BTA. Some electrochemical measurements were obtained to complement the SCC mcasuremcnts. The results are interpreted in the light of the inhibiting effects of BTA on dezincification, and hence the subsequent SCC of the brass. INTRODUCTION

BENZOTRIAZOLE ( B T A ) is an effective inhibitor for the corrosion of copper and its alloys. Ever since the early works of Cotton et al. 1,2 there has been a continuing interest in the mechanism of inhibition and in the various aspects of the interaction of BTA with the oxide-free and oxide-covered copper surfaces. 3 7 BTA was shown to inhibit the stress corrosion cracking (SCC) of a-brass in Mattsson's solution.S There is also some evidence that it inhibits dezincification. 9 It is widely believed that the mechanism of inhibition of general corrosion involves the formation of a thin highly stable insoluble C u - B T A film which protects the metal from the corrosive environment. The SCC of c~-brass in nitrite solution is a well documented phenomenon which attracts considerable attention.l°-14 The purpose of this paper is to evaluate the efficiency of BTA as an inhibitor for the SCC of ct-brass in this medium and to clarify the mechanism of its inhibitive action. EXPERIMENTAL

METHOD

The material used was commercial a-brass of the following chemical composition: 71.7% Cu. 28.0% Zn, 0.(I06% Pb and 0.(Jl% Fe. The mechanical properties arc: UTS = 2 8 3 N m m 2 (28.Skgmm =). Y.S.=216Nmm 2 ( 2 2 . 0 k g m m 2), hardness (VHN)=600Nmm 2 ( 6 1 . 0 k g m m 2) and elongation = 80%. A constant strain rate technique was used at a constant strain rate of 1.5 x 10 s s J. The tensile test specimens, 200 mm in length, were machined to give a gauge length of 35 mm and a width of 6 ram. Before conducting the tests, the specimens were polished with 320, 600 and 800 SiC grit papcr. degreased with acetone and coated with paraffin wax so that only the gauge length was exposed to the test solution. The experiments were carried out at room temperature (24 + I°C) in aeratcd 0. I M NaNO, solution and in the presence of BTA ( 10 -3 to 2 x 10 2 M). The stress tests were carried out under an applied anodic potential of 300 m V ( N H E ) , a-Brass is known to be susccptiblc to SCC at this potential. M The potential was controlled using a Wenking Potentiostat L.T. 73. Potentiostatic polarization expcrimcnts were carried out on unstressed specimens after they were equilibrated in solution for 40 min. The potentials were measured using a saturated calomcl reference electrode and reported against the normal hydrogen Manuscript received 10 December 1992; in amended form 25 June 1993. 221

222

S . M . SAYED, E. A. ASHOUR and B. G. ATEYA

electrode. Some runs were performed through a program of potential increase rate of 20 m V min -1 while others were performed at steady state, i.e. the potential was increased only after the current transient reached its stationary value. The cell used was a 200 ml cylindrical glass cell, closed by upper and lower stoppers, through which the ends of the specimen protruded. A platinum sheet was used as a counter electrode. The failed specimens were immediately removed from the solution after failure. The upper part of the specimen was cut at 1 cm from the crack tip and subjected to SEM using J E O L , JSM-T20 (Japan). EXPERIMENTAL

RESULTS

AND

DISCUSSION

Stress-time m e a s u r e m e n t s

Figure 1 shows the stress-time curves of a-brass in a 0.1 M NaNO2 solution in the presence of increasing concentrations of B T A under a controlled potential of + 3 0 0 m V ( N H E ) . The curve in 0.1 M NaNOe without B T A is also included for comparison. The results are summarized in Table 1. The stress corrosion susceptibility was measured by the ratios of both the time to failure (r = tf (sol.)/tf (air) and the maximum stresses r = O'max (sol.)/O'ma x (air). As the concentration of B T A increases, both r and r increase. Figures 2-5 display the SEM micrographs of the fracture surfaces obtained under different conditions. Brittle transgranular cracking was observed in the blank solution and in presence of low to moderate concentrations of B T A ( 1 0 - 3 - 1 0 - 2 M ) i.e. Figs 2-4. At a higher concentration of B T A (i.e. 2 x 10 -2 M) the fracture mode changed to ductile (Fig. 5). Polarization curves

Figure 6 illustrates the polarization curves of a-brass in 0.1 M NaNO2 in the

20--

°°°'°% °°

°

°°°° °° °°

.'f.

...'/ 15

--

."/

I

10

0.1 M NO2

0.1 M NO2 + 0.01 M BTA 0.1 M NO2 + 0.02 M BTA

0

5

I

I

I0

15

T i m e (h) FIG.

1.

Stress-time curves of a-brass in nitrite solution in the presence of B T A at 300 m V ( N H E ) .

201.tm

3 FI(~. 2. FIG. 3.

SEM of the fracture surface of or-brass in 0.1 M NaNO~ at 300 m V ( N H E ) . SEM of the fracture surface of a-brass in 0.1 M NaNO2 + 10 3 M BTA.

223

FI6.4. SEM of the fracture surface of a-brass in 0.1 M NaNO 2 + 10 2 M BTA: (a) an area showing cleavage-like transgranular cracking and a fine secondary crack; (b) another area showing transgranular cracking accompanied with a small portion of intergranular cracking.

224

Dimples Cleavage

Fro. 5. SEM of the fracture surface of or-brass in 0.1 M NaNO 2 + 2 x 10 _2 M BTA: (a) a small area of the surface showing cleavage side by side with dimples; (b) the major part of the fracture surface which shows dimples indicating ductile failure.

225

B e n z o t r i a z o l e and S C C of a - b r a s s

TABLE 1.

227

THE EFFECT OF B T A ON ThE SXRESS CORROSION CRACKING PARAMETERS OF ~-BRASS IN 0. [ M N a N O 2 SOLUTION AX 300 m V ( N H E ) T i m e to failurc h min

Cone. Air (1) 0.1 M N O 2 (2) (I.l M N O 2 + (a) 10 3 M B T A (b) l0 2 M B T A (c) 2 × l(I 2 M B T A

11

Stress r a t i o o (sol.)/o (air)

r = tf (sol.)/q (air)

M o d e of failure

Figure

10

1.00

1.0(t

ductile

--

3 20

0.30

0.50

T.C.

2

5 5 5 30 9 20

0.45 0.5(I 0.84

0.5(/ 0.59 0.71

T.C. T.C. ductile

3 4 5

I ' . C . = T r a n s g r a n u l a r cracking.

presence and absence of B T A . These curves were obtained under a program of potential increase rate of 20 m V min 1. The presence of B T A has a significant inhibiting effect on the anodic current at potentials up to about 500 m V ( N H E ) . For example at 300 m V ( N H E ) , 10 -2 M B T A lowers the anodic dissolution current by 10

--

Cu20 Cu(OH) 2

I 10 o --

cu2o Ic.o~

<

10-1

I

I~

_

10-2 -Cu I Cu20

o

0.1MNO 2

•

0 . 1 M NO2 + 2 x 10-2 M BTA

[] 0.1 M NO 2 +

10-2 M BTA

0.1 M NO 2 +

10-3 M BTA

•

10-3 -300

I -100

I 0

t.,w( 100

~300

I

I

I

500

700

900

E (mV(H))

FIG. 6.

P o t e n t i o s t a t i c p o l a r i z a t i o n c u r v e s of a - b r a s s in nitrite s o l u t i o n s in the p r e s e n c e of BTA.

228

S.M. SAYED,E. A. ASHOURand B. G. ATEYA

about two orders of magnitude. At higher potentials, i.e. above 500 m V ( N H E ) , BTA loses much of its inhibiting efficiency. Figure 7 illustrates the current transients obtained at a potential of 300 m V ( N H E ) in the absence and presence of B T A (10 -2 M). The presence of BTA suppresses the anodic current starting from very early times, and keeps it constant at this low level for the duration of the experiment. In the nitrite solution, the current decreases with time reaching a stationary value in about 20rain. This current transient bears a similarity to those reported for the selective dissolution of binary alloys 15"t6and for the growth of a protective film. 10,17There is considerable evidence in the literature ~8 that a-brass undergoes preferential dezincification for an initial transient period, during which time the surface is enriched in Cu, before it undergoes simultaneous dissolution involving both components. A useful criterion for selective dissolution may be obtained 19 from a plot of the current versus 1/~t. This is shown in the inset of Fig. 7. A straight line is obtained in the nitrite solution with a positive slope for about 3 min before the current converges on a small value at longer times. This indicates that selective dissolution occurs during the first 3 min, before the onset of simultaneous dissolution at longer times. On the other hand, in the inhibited solution, the current remains at a low constant value independent of time, indicating the absence of selective dissolution. This indicates that B T A inhibits the dissolution of zinc as well as Cu. Furthermore its inhibitive effects on the SCC of the brass result as a consequence of its effects on dezincification.

O.l MN02 80 -- 300 mV /

i0 -I

<=

~o

~ I

10-2

/

"

o 0'IMNO2+10 MBTA,300mV "~

"

/

-

o /

,

oQ_

~

0

L.~-- - - I - - x. - - I - - x " 0.4

<

0.8

1.2

I L6

1 / ~ / t (min-1/2)

v 300 mV 10-3 I 0.1 MNO2+ 10-2 M BTA, 0

0

0

0

0 300

10-4 / 0

I 5

I 10

I 15

I 20

0

0--

mV I 25

I 30

J 35

Time (rain) FIG. 7.

Potentiostatic current-time transients of a-brass in 0.1 M NaNO 2 solution in absence and in presence of 10 2 M BTA. Inset shows a plot of I versus 1/k/~.

Benzotriazolc and SCC of cz-brass

229

At longer times, when the surface is enriched in Cu, the brass surface is involved in several equilibria] i.e. 2Cu + H : O ~ C u 2 0 + 2H + + 2e

(la)

Cu~O + H 2 0 ~ 2CuO + 2H + + 2e

(2a)

Cu20 + 3 H 2 0 ~ 2 C u ( O H ) 2 + 2H + + 2e .

(3a)

The equilibrium potentials of these systems are given, respectively, by:

Ecu:o/cu =

{).461 - 0.059 pH, V ( N H E )

(l b)

Ecuo/cu:o = 0.607 - 0.059 p H , V ( N H E )

(2b)

Ecu(OH}~/cu:o = 0.730 -- {).059 pH, V ( N H E ) .

(3b)

In 0.1 M NaNO2 (pH = 7), the above potentials assume the following values: 0.048, ~). 194 and 0.317 V ( N H E ) , respectively. Since the free corrosion potential in 0.1 M NaNO~ is about 0.150 V ( N H E ) in absence of B T A , it follows that both C u 2 0 and C u O exist on the alloy surface. In the presence of B T A the situation is much more complex. While some authors ~ refer to the facilitating effects of C u 2 0 on the formation of protective C u - B T A complexes, others <7 refer to protective film formation via adsorption of B T A and direct interaction with the bare copper surface. Under such conditions, there exist other competing equilibria involving B F A . For instance on a bare Cu surface one expects an equilibrium such as: (~'7 nCu+nBTA

~[Cu(I)BTAI, ,+he

(4)

This polymeric [Cu(I)BTA],, complex is responsible for inhibiting the corrosion process. Since good inhibition was observed on oxide-covered brass surfaces, it follows that the complex [Cu(I)BTA],, exists also on such surfaces. Due to the stability of this complex, one envisages the occurrence of a displacement reaction such as: n Cu,O + n BTA

+ n H 2 0 ,~- 2[Cu(I)BTA],, + 2 n O H - .

(5)

The equilibrium potentials of equations ( l a ) - ( 3 a ) are marked on Fig. 6 along with the phases existing on the brass surface under different conditions. In the absence of B T A , the current is supported by reactions (1 a)-(3a), depending on the potential. In presence of B T A , the current is supported by reaction (4). At the end of the steady state potentiostatic experiment, the brass which was polarized in uninhibited NaNO2 was covered with a black layer of CuO. Alternatively, a brass sample which received an identical treatment in presence of 10 2 M B T A retained a bright surface. SUMMARY AND CONCLUSIONS Inspection of the above results indicates that B T A inhibits the SCC of a-brass in (I. 1 M NaNO2 at 300 m V ( N H E ) . The presence of B T A leads to the following: (1) Significant increase in the time to failure and in the stress ratio, r = o ....

(sol.)/o

..... (air).

(2) Change of the morphology of the fracture surface from brittle transgranular cracking to ductile failure.

230

S.M. SAYED,E. A. ASHOURand B. G. ATEYA

(3) I n h i b i t i o n o f t h e s e l e c t i v e d i s s o l u t i o n o f t h e a l l o y a n d o f t h e f o r m a t i o n o f c o p p e r o x i d e s o n its s u r f a c e . (4) S i g n i f i c a n t d e c r e a s e in t h e a n o d i c c u r r e n t at p o t e n t i a l s u p t o a b o u t 500 m V ( N H E ) .

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

REFERENCES I. DUGDALEand J. B. COTTON,Corros. Sci. 3, 69 (1963). J. B. CoTToN and I. R. SCHOLES,Br. Corros. J. 2, 1 (1967). T. HASHEMIand C. A. HOGARTH,Electrochim. Acta 33, 1123 (1988). D. CHADWICKand T. HASHEMI,Corros. Sci. 18, 39 (1978). P. G. Fox, G. LEWISand P. J. BODEN, Corros. Sci. 19, 457 (1979). R. YOUDA,H. NISHIHARAand K. ARAMAKLElectrochim. Acta 35, 1011 (1990). D. TROMANSand RU-HONGSUN, J. electrochem. Soc. 138, 3235 (1991). V. ASHWORTH,K. LYTIJand R. P. M. PROCTOR,Corrosion 35, 19 (1979). R. WALKER,Corrosion 29, 290 (1973). A. T. CoEz and R. C. NEWMAN,Corros. Sci. 28, 109 (1988). R. B. REBAK,R. M. CARRANZAand J. R. GALVELE,Corros. Sci. 28, 1089 (1988). J. Yu, R. N. PARKINS,Y. Xu, G. THOMPSONand G. C. WOOD, Corros. Sci. 27, 141 (1987). J. Yu and R. N, PARKINS,Corros. Sci. 27, 159 (1987). V. K. GOUDA, S. M. SAYEDand H. A. E. SAYED,Proc. Int. Congr. Met. Corros., Vol. 1, p. 153, NRCC: Ottawa, Ont. (1984). H. W. PICKERINGand P. J. BYRNE,J. electrochem. Soc. 116, 1492 (1969). H. W. PICKERINGand P. J. BYRNE,J. electrochem. Soc. 118, 209 (1971). M. M. LAZ, R. M. SOUTO,S. GONZALEZ,R. C. SALVAREZZAand A. J. ARVIA,Electrochim. Acta 37, 665 (1992). H. KAISER,Alloy dissolution, in Metallic Corrosion (ed. F. MANSFELD).Marcel Dekker, New York (1987). H. W. PICKER1NGand C. WAGNER,J. electrochem. Soc. 114, 698 (1967).