effect of annealing on the stress corros

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Effect of Annealing on the Stress Corrosion Cracking ofr-Brass in Aqueous

Electrolytes Containing Aggressive Ions

Nageh K. Allam,*,, Ahmed Abdel Nazeer, and Elsayed A. Ashour

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, and Physical

Chemistry Department, National Research Center, Dokki, Cairo 12622, Egypt

A continuing challenge in materials design is the achievement of greater operational efficiency throughimprovements in performance criterion, particularly, high strength and service lifetime characteristics. In thisstudy, we report on the effect of heat treatment (annealing) on the stress corrosion cracking (SCC) behaviorofR-brass (71Cu-28Zn) in aqueous electrolytes containing some aggressive anions (Cl-, ClO3

-, NO2-, and

SO42-). The annealed specimens suffered from SCC in all tested electrolytes under the anodic polarization

conditions while they were susceptible to SCC only in NaNO2 and Na2SO4 solutions under the open circuitcondition. The susceptibility toward SCC was found to decrease considerably in NaCl, KClO3, and Na2SO4solutions and very slightly in NaNO2 solutions upon annealing the specimens.

Introduction

Brasses are widely used in different industries as condensers,

heat exchanger tube materials, and tubes in sugar juice evapora-

tors and distillation-type desalination plants.1 During operation,

these parts are subject to different stresses besides the influence

of some electrolytic solutions containing chlorides,2 as well as

oxyanions,3 which sometimes lead to failure of these parts by

stress corrosion cracking (SCC).4 There are many procedures

and treatments reported in the literature to protect metals and

alloys against attack in many industrial environments.2,5,6

Among them, heat treatment (annealing) is expected to be a

promising tool.7 Annealing of metals and alloys is generally

made for the following purposes:7 (1) to improve the mechanical

properties; (2) to improve machinability; (3) to increase ductility;

(4) to remove chemical nonuniformity, i.e., achieving the proper

stoichiometry; (5) to alter the microstructure and develop a

structure that is more desirable for hardening; and (6) to relieveinternal stresses, etc. For example, as a result of annealing at

low temperatures, the atoms that have been displaced from their

regular positions by cold working begin to move back to their

equilibrium positions allowing internal stresses to be relieved,

i.e., permits crystal recovery.7

Because the selection of a specific brass alloy for a certain

application is usually based on laboratory test results that were

obtained in the simulated environment, the purpose of the

present work was to study the effect of heat treatment (anneal-

ing) on the SCC behavior ofR-brass (71Cu-28Zn) in aqueous

electrolytes containing aggressive anions (Cl-, ClO3-, NO2

-,

and SO42-). Two sets of SCC experiments were carried out:

one set using the as-received specimens and the other one usingannealed specimens.

2. Experimental Section

The material used was R-brass of the following chemical

composition: 71.7 wt % Cu, 28.284 wt % Zn, 0.006 wt % Pb,

and 0.01 wt % Fe. The mechanical properties are given as

follows: ultimate tensile strength (UTS), 283 N mm-2 (28.8 kg

mm-2); yield strength (YS), 216 N mm-2 (22.0 kg mm-2);

Vickers hardness (VH), 600 N mm-2 (61.0 kg mm-2); and

elongation, 80%. A constant strain rate technique was used at

a constant strain rate of 1.5 10-

5 s-

1, as also recently reportedin ref 5. The tensile test specimens were designed to have the

following dimensions:

Before conducting the test, the specimens (as received, as

well as annealed at 600 C for 30 min with heating and cooling

rates of 5 C/min) were polished with 320, 600, and 1000 SiC

grit paper, 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 ( 1

C) in naturally aerated 0.1 M, 0.5 M, and 1.0 M aqueous

electrolytes of NaCl (pH 6.5), NaNO2 (pH 7.8), KClO3 (pH

6.6), and Na2SO4 (pH 6.8). The stress tests (duplicates) were

carried out at open circuit potential (OCP) and under different

applied anodic potentials (200 and 300 mVNHE). The potential

was controlled using a Wenking potentiostat L.T.73. The failed

specimens were immediately removed from the solution after

failure. The upper part of the specimen was cut 1 cm from the

crack tip and inspected via scanning electron microscopy (SEM).

3. Results and DiscussionFigure 1a shows the stress-time results obtained in 1 M NaCl

solutions for the 71Cu-28Zn alloy under OCP and at relatively

high anodic potential (300 mV NHE) at a strain rate of 3 10-5

s-1 for both as-received and annealed specimens. It can be seen

that both the time to failure and the failure stress of the annealed

specimen are higher than those of the as-received specimens.

However, the surface appearance and the mode of failure of

the failed specimens are the same in both cases. On the other

hand, Figure 1b shows the stress-strain curves obtained under

the same conditions as in Figure 1a. The curve shows the same

conventional shape obtained for other alloys4,5 and is character-

ized by an increase in strain with increasing stress until the yield

* To whom correspondence should be addressed. E-mail:[email protected], [email protected].

School of Chemistry and Biochemistry, Georgia Institute ofTechnology.

Physical Chemistry Department, National Research Center.

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stress point is reached, followed by a slight gradual increase in

the form of plateau until it reaches a maximum, after which the

strain begins to decline to reach the point of failure. The

behavior obtained in air is also included for comparison and

the results are summarized in Table 1.

The susceptibility of the alloy to SCC was measured by the

ratios of both the time to failure ) tf (sol.)/tf (air) and the

maximum stress ratio r ) max(sol)/ max(air). The authors

previously showed that both r and can be combined in a

quantitative phenomenological expression for the susceptibility

(S) to SCC as follows:5

Generally, the results indicate that the annealed specimens

are less susceptible to SCC, compared to the as-received

specimens. These observations are the results from tests

conducted at the open-circuit condition (OPC), as well as under

anodic polarization (see Tables 1-4). It is worthwhile to

mention that SCC was not observed in the case of the annealed

specimens that were tested in NaCl solutions at the OCP

condition where the failure was ductile, indicating merely

mechanical failure. However, R-brass in the as-received condi-

tions undergoes SCC at the OCP as well as under anodic

polarization. The mode of cracking transforms from transgranu-

lar at OCP to mixed intergranular and transgranular observed

under anodic polarization conditions (see Figures 2a and 2b).

Transgranular cracking was associated with pitting corrosion

(see Figure 2c). It is possible that pitting, with the help of tensile

stress, may create intense anodic sites and lead to transgranular

cracking. Under anodic polarization, corrosion will be intensified

locally through the pores of the CuCl film formed.8 Microscopic

inspection revealed the occurrence of dezincification (see Figure2). Thus, it is suggested that dezincification may occur through

the pores of the film, which enlarge with time and may lead to

an embrittled zone at the grain boundary and the appearance of

intergranular cracking. Upon dezincification, R-brass undergoes

the following reaction:9,10

Then, after the outer surface of the brass is depleted of zinc

and enriched in copper, the brass undergoes simultaneous

dissolution of both zinc and copper.10 Also, it was shown that

the dezincification increases as the oxidizing power of the salt

solutions increases.3

Similar results were obtained when the tests were carried out

in KClO3 solutions, as shown in Table 2. Note that SCC does

not occur in 1.0 and 0.1 M KClO3 solutions at OCP for the

annealed specimens, whereas it does occur in the case of the

as-received specimens under the same conditions. Under anodic

polarization conditions, on the other hand, SCC was observed

in all cases. However, under the same applied anodic potential,

both the time to failure and the stress ratio are lower for the

as-received specimens, compared to the annealed samples. The

Table 1. Effect of Annealing on the SCC Behavior ofr-Brass in Chloride Solutions

Time to Failure, tf (h:min) Maximum Stress Ratio,r Susceptibility, Sa Mode of Failureb

concentration (M) E(mVH) as-received annealed as-received annealed as-received annealed as-received annealed as-received annealed

1 OCP 10:20 13:00 0.84 1.00 0.86 1.08 0.15 0 TC NC300 9:10 11:00 0.72 0.80 0.76 0.92 0.47 0.13 TC + IC TC + IC

0.1 OCP 10:30 13:20 0.85 0.98 0.88 1.11 0.14 N/A TC NC

300 9:50 12:00 0.78 0.88 0.82 1 0.43 0 TC + IC TC + IC

a N/A ) undefined as tf(solution) > tf(air). b Acronym legend: NC, no SCC; TC, transgranular SCC; IC, intergranular SCC.

Table 2. Effect of Annealing on the SCC Behavior ofr-Brass in Chlorate Solutions



1 OCP 13:00 16:20 0.85 1.00 1.08 1.36 N/A 0 IC NC

300 10:00 10:50 0.66 0.75 0.83 0.90 0.24 0.16 IC IC0.1 OCP 14:00 16:10 0.90 1.00 1.17 1.35 N/A N/A IC NC

200 13:00 14:50 0.82 0.92 1.08 1.24 N/A N/A IC IC

300 10:10 11:00 0.70 0.78 0.85 0.92 0.21 0.135 IC ICa N/A ) undefined as tf(solution) > tf(air).

b Acronym legend: NC, no SCC; IC, intergranular SCC.

Figure 1. Effect of annealing on the SCC of 71Cu-28Zn brass alloy in 1M NaCl: (a) stress-time and (b) stress-strain relations under a strain rateof 1.5 10-5 s-1 at room temperature (24 (1 C).

S) [(1 - r)(1 - )]1/2

(1)

Cu:Zn f Zn2++ 2e

-(2)

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mode of failure is always intergranular (in all cases), but

cracking was more severe in the case of the as-received

specimens, compared to the annealed sepcimens (see Figures3a and 3b). These observations indicate that the as-received

specimens are more susceptible to SCC than the annealed

samples. On the other hand, microscopic investigations indicate

also the occurrence of dezincification in the failed specimens,regardless of whether it was annealed or as-received (see Figure

Figure 2. (a) SEM image of the as-received sample anodized in 1 M NaCl, (b) SEM image of the annealed sample anodized in 1 M NaCl, and (c) opticalmicrograph of a sample anodized in 0.1 M NaCl. Note that anodization was carried out at 300 mVNHEin all cases.

Figure 3. SEM micrographs showing examples of the fractures in 0.1 M KClO3 solutions at 200 mV for (a) as-received and (b) annealed specimens.

Table 3. Effect of Annealing on the SCC Behavior ofr-Brass in Nitrite Solutions

Time to Failure, tf (h:min) Maximum Stress Ratio,r Susceptibility, S Mode of Failureb


1 OCP 8:20 9:00 0.80 0.85 0.69 0.75 0.25 0.19 IC IC200 5:20 5:40 0.62 0.65 0.44 0.47 0.46 0.43 IC + TC IC + TC

300 3:40 3:50 0.48 0.52 0.31 0.32 0.60 0.57 TC TC

0.1 OCP 9:20 9:40 0.88 0.90 0.78 0.81 0.16 0.14 IC IC200 6:50 8:00 0.80 0.82 0.57 0.67 0.29 0.24 IC + TC IC + TC

300 3:30 3:50 0.52 0.55 0.29 0.32 0.58 0.55 TC TC

b Acronym legend: TC, transgranular SCC; IC, intergranular SCC.

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4). The effect of annealing appears as a reduction in the

susceptibility of the alloy to SCC, because of the relief of the

residual stresses by annealing.4,7

The results obtained in the case of NaNO 2 solutions are

summarized in Table 3. The data presented in this table

indicate that annealing of the specimens results in a slightincrease in the time to failure (tf) and stress ratio (r),

compared to the as-received specimens (i.e., a decrease in

the susceptibility S of R-brass to SCC). The effect in the

case of NaNO2 solutions was very slight, in comparison to

the effect observed in the case of NaCl and KClO3solutions.

The results also show that the alloy is strongly susceptible

to SCC in NaNO2 solutions, whether it was in the annealed

or the as-received conditions, and the severity of cracking

increases as the solution concentration increases and with

anodic polarization (see Figure 5). Anodic polarization can

be expected to shorten the initiation time to fracture and isoften found to increase the crack propagation rate.2,4,11

Fractographic inspection shows that the mode of failure

changes from intergranular at OCP to transgranular by anodic

polarization. Relating the transformation of the mode of

failure to the boundaries of the Pourbaix diagram for the

Cu-H2O system11,12 indicates the association of intergranular

cracking with the Cu2O domain with transgranular cracking

at potentials in the stability domain of CuO, with a mixed

mode of cracking being observed in tests conducted near the

boundaries of these two oxides (200 mVH). Sircar et al.11

reported that anodic polarization caused a minimum in

cracking for annealed specimens, which was accompanied

by a transition of the mode of cracking from intergranularto transgranular. Also, Alvarez et al.13 suggested that anodic

dissolution processes are controlling the SCC of brass in

nitrite solutions. The present results are in agreement with

those results, and it seems that the annealing process has no

(or a very slight) effect on the SCC behavior of R-brass in

NaNO2solutions. Thus, it may be suggested that film rupture

in the presence of tensile stress creates very active anodic

sites from which the crack initiates and propagates with time

until failure occurs.

The same trend was observed for samples tested in Na2SO4solutions (see Table 4). The results indicate that the

susceptibility of R-brass to SCC decreases upon annealing

and that this phenomenon occurs at the OCP, as well as underanodic polarization conditions. The mode of failure is

Figure 5. Optical micrographs showing examples of SCC of annealed R-brass specimens tested in 1 M NaNO 2 (a) at OCP and (b) at 300 mV H.

Table 4. Effect of Annealing on the SCC Behavior ofr-Brass in Sulfate Solutions



1 OCP 11:00 12:10 0.78 0.92 0.92 1.01 0.14 N/A IC IC

200 10:30 11:50 0.73 0.87 0.88 0.99 0.18 0.04 IC IC

300 7:00 7:30 0.72 0.84 0.58 0.63 0.34 0.24 IC IC0.1 OCP 11:40 12:30 0.90 0.95 0.98 1.04 0.05 N/A IC IC

200 11:00 12:00 0.89 0.93 0.92 1 0.09 0 IC IC

300 9:40 10:50 0.74 0.87 0.81 0.90 0.23 0.11 IC ICa N/A ) undefined as tf(solution) > tf(air).

b IC ) intergranular SCC.

Figure 4. Optical micrographs of (a) as-received and (b) annealed brasssamples tested in 0.5 M NaClO3 at 300 mV.

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intergranular in all cases, and the severity of cracking was

found to increase as the concentration increased and with

anodic polarization. Figure 6 compares the morphology of

as-received and annealed fractured samples in 0.1 M Na 2SO4solutions at 300 mV. Under anodic polarization, a black film

was observed on the brass specimens. This film has been

suggested by several authors to be mainly composed of

Cu2O.12,14

Conclusions

The effect of annealing on the stress corrosion cracking (SCC)

behavior of 71Cu-28Zn brass alloy was documented, in

comparison to as-received samples. Our results revealed that

the as-received R-brass specimens are susceptible to SCC in

the test electrolytes (Cl-, ClO3-, NO2

-, SO42-) under the free

corrosion conditions, as well as the anodic polarization condi-

tions. However, the annealed specimens undergo SCC in all

tested electrolytes under the anodic polarization conditions,

while they are susceptible to SCC only in NaNO 2 and Na2SO4solutions under the OCP. Furthermore, the susceptibility toward

SCC decreases considerably in NaCl, KClO3, and Na2SO4solutions and very slightly in NaNO2solutions upon annealing

the specimens.

Generally, the annealing ofR-brass induces a decrease in its

susceptibility to SCC in all tested solutions, which can be

attributed to the fact that annealing relieves the residual and

internal stresses in the alloy, i.e., reduces the overall applied

stresses and also because annealing increases the ductility and

removes chemical nonuniformity.

Supporting Information Available:A photograph showing

the experimental setup is provided. This material is availablefree of charge via the Internet at http://pubs.acs.org

Literature Cited

(1) Davis, J. R.Copper and Copper Alloys; ASM International: MaterialsPark, OH, 2001.

(2) Feron, D.; Saclay, C. E. A.Corrosion BehaViour and Protection ofCopper and Aluminium Alloys in Seawater; Woodhead Publishing Limited:Great Abington, Cambridge, U.K., 2010.

(3) Maria, M.; Scully, J. C. The stress corrosion cracking of 70Cu-30Zn brass in chlorate solutions.J. Corros. Sci. 1983, 23, 753762.

(4) Jones, R. H.Stress-Corrosion Cracking: Materials Performance andEValuation; ASM International: Materials Park, OH, 1992.

(5) Allam, N. K.; Ashour, E. A. Electrochemical and stress corrosioncracking behavior of 67Cu-33Zn alloy in aqueous electrolytes containingchloride and nitrite ions: Effect of di-sodium hydrogen phosphate (DSHP).

Mater. Sci. Eng., B 2009, 156 (1), 8489.(6) Allam, N. K.; Nazeer, A. A.; Ashour, E. A. A review of the effects

of benzotriazole on the corrosion of copper and copper alloys in clean andpolluted environments. J. Appl. Electrochem. 2009, 39 (7), 961969.

(7) Rajan, T. V.Heat Treatment: Principles and Techniques; Prentice-Hall of India Pvt Ltd.: New Delhi, India, 2007.

(8) Fenelon, A. M.; Breslin, C. B. An electrochemical study of theformation of benzotriazole surface-films on copper, zinc and copper-zincalloy. J. Appl. Electrochem. 2001, 31, 509516.

(9) Pickering, H. W. Characteristic features of alloy polarization curves.Corros. Sci. 1983, 23, 1107.

(10) Kaiser, H.Alloy Dissolution in Corrosion Mechanisms; Mansfield,M., Ed.; Marcel Dekker: New York, 1987.

(11) Sircar, S. C.; Chatterjee, U. K.; Zamin, M.; Vijayendra, H. G.Mechanism of SCC ofR-brass in Mattssons solution under potentiostaticconditions. Corros. Sci. 1972, 12, 217.

(12) Mattson, E. Stress corrosion in brass considered against thebackground of potential/pH diagrams.Electrochim. Acta1961,3, 279291.

(13) Alvarez, M. G.; Lapitz, P.; Fernandez, S. A.; Galvele, J. R. Passivitybreakdown and stress corrosion cracking of R-brass in sodium nitritesolutions. Corros. Sci. 2005, 47 (7), 16431652.

(14) Hoar, T. P.; Booker, C. J. L. The electrochemistry of the stress-corrosion cracking of alpha brass. Corros. Sci. 1965, 5, 821.

ReceiVed for reView March 28, 2010ReVised manuscript receiVedAugust 18, 2010

AcceptedAugust 18, 2010

IE101603W

Figure 6. SEM micrographs showing examples of the fractures in 0.1 M Na2SO4 solutions at 300 mV for (a) as-received and (b) annealed specimens.

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