Ind. Eng. Chem. Res. 2010, 49, 9529–9533
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Effect of Annealing on the Stress Corrosion Cracking of r-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 through improvements in performance criterion, particularly, high strength and service lifetime characteristics. In this study, we report on the effect of heat treatment (annealing) on the stress corrosion cracking (SCC) behavior of R-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 circuit condition. The susceptibility toward SCC was found to decrease considerably in NaCl, KClO3, and Na2SO4 solutions 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 evaporators 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 relieve internal 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 (annealing) on the SCC behavior of R-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 using annealed 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 * To whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. † School of Chemistry and Biochemistry, Georgia Institute of Technology. ‡ Physical Chemistry Department, National Research Center.
mm-2); yield strength (YS), 216 N mm-2 (22.0 kg mm-2); Vicker’s 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 reported in 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 Discussion Figure 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 characterized by an increase in strain with increasing stress until the yield
10.1021/ie101603w 2010 American Chemical Society Published on Web 08/24/2010
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quantitative phenomenological expression for the susceptibility (S) to SCC as follows:5 S ) [(1 - r)(1 - τ)]1/2
(1)
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 conditions undergoes SCC at the OCP as well as under anodic polarization. The mode of cracking transforms from transgranular 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 Figure 2). 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 Cu:Zn f Zn2+ + 2e-
Figure 1. Effect of annealing on the SCC of 71Cu-28Zn brass alloy in 1 M NaCl: (a) stress-time and (b) stress-strain relations under a strain rate of 1.5 × 10-5 s-1 at room temperature (24 ( 1 °C).
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 τ ) tf (sol.)/tf (air) and the maximum stress ratio r ) σmax(sol)/ σmax(air). The authors previously showed that both r and τ can be combined in a
(2)
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 of r-Brass in Chloride Solutions Time to Failure, tf (h:min) Maximum Stress Ratio, r concentration (M) E (mVH) 1 0.1 a
OCP 300 OCP 300
as-received
annealed
as-received
annealed
10:20 9:10 10:30 9:50
13:00 11:00 13:20 12:00
0.84 0.72 0.85 0.78
1.00 0.80 0.98 0.88
Susceptibility, Sa
τ
Mode of Failureb
as-received annealed as-received annealed as-received annealed 0.86 0.76 0.88 0.82
1.08 0.92 1.11 1
0.15 0.47 0.14 0.43
0 0.13 N/A 0
TC TC + IC TC TC + IC
NC TC + IC NC TC + IC
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 of r-Brass in Chlorate Solutions Time to Failure, tf (h:min) Maximum Stress Ratio, r concentration (M) E (mVH) 1 0.1
a
OCP 300 OCP 200 300
as-received
annealed
as-received
annealed
13:00 10:00 14:00 13:00 10:10
16:20 10:50 16:10 14:50 11:00
0.85 0.66 0.90 0.82 0.70
1.00 0.75 1.00 0.92 0.78
Susceptibility, Sa
τ
Mode of Failureb
as-received annealed as-received annealed as-received annealed 1.08 0.83 1.17 1.08 0.85
1.36 0.90 1.35 1.24 0.92
N/A ) undefined as tf (solution) > tf (air). b Acronym legend: NC, no SCC; IC, intergranular SCC.
N/A 0.24 N/A N/A 0.21
0 0.16 N/A N/A 0.135
IC IC IC IC IC
NC IC NC IC IC
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Table 3. Effect of Annealing on the SCC Behavior of r-Brass in Nitrite Solutions Time to Failure, tf (h:min) Maximum Stress Ratio, r concentration (M) E (mVH) 1 0.1
b
OCP 200 300 OCP 200 300
as-received
annealed
as-received
annealed
8:20 5:20 3:40 9:20 6:50 3:30
9:00 5:40 3:50 9:40 8:00 3:50
0.80 0.62 0.48 0.88 0.80 0.52
0.85 0.65 0.52 0.90 0.82 0.55
Susceptibility, S
τ
Mode of Failureb
as-received annealed as-received annealed as-received annealed 0.69 0.44 0.31 0.78 0.57 0.29
0.75 0.47 0.32 0.81 0.67 0.32
0.25 0.46 0.60 0.16 0.29 0.58
0.19 0.43 0.57 0.14 0.24 0.55
IC IC + TC TC IC IC + TC TC
IC IC + TC TC IC IC + TC TC
Acronym legend: TC, transgranular SCC; IC, intergranular SCC.
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) optical micrograph of a sample anodized in 0.1 M NaCl. Note that anodization was carried out at 300 mVNHE in 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.
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 Figures 3a 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
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Figure 4. Optical micrographs of (a) as-received and (b) annealed brass samples tested in 0.5 M NaClO3 at 300 mV.
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 NaNO2 solutions are summarized in Table 3. The data presented in this table indicate that annealing of the specimens results in a slight increase 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 KClO3 solutions. 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 is often 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 intergranular to 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 NaNO2 solutions. 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 Na2SO4 solutions (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 under anodic polarization conditions. The mode of failure is
Figure 5. Optical micrographs showing examples of SCC of annealed R-brass specimens tested in 1 M NaNO2 (a) at OCP and (b) at 300 mVH. Table 4. Effect of Annealing on the SCC Behavior of r-Brass in Sulfate Solutions Time to Failure, tf (h:min) Maximum Stress Ratio, r concentration (M) E (mVH) 1 0.1
a
OCP 200 300 OCP 200 300
as-received
annealed
as-received
annealed
11:00 10:30 7:00 11:40 11:00 9:40
12:10 11:50 7:30 12:30 12:00 10:50
0.78 0.73 0.72 0.90 0.89 0.74
0.92 0.87 0.84 0.95 0.93 0.87
N/A ) undefined as tf (solution) > tf (air). b IC ) intergranular SCC.
Susceptibility, Sa
τ
Mode of Failureb
as-received annealed as-received annealed as-received annealed 0.92 0.88 0.58 0.98 0.92 0.81
1.01 0.99 0.63 1.04 1 0.90
0.14 0.18 0.34 0.05 0.09 0.23
N/A 0.04 0.24 N/A 0 0.11
IC IC IC IC IC IC
IC IC IC IC IC IC
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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.
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 Na2SO4 solutions 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 conditions. However, the annealed specimens undergo SCC in all tested electrolytes under the anodic polarization conditions, while they are susceptible to SCC only in NaNO2 and Na2SO4 solutions under the OCP. Furthermore, the susceptibility toward SCC decreases considerably in NaCl, KClO3, and Na2SO4 solutions and very slightly in NaNO2 solutions upon annealing the specimens. Generally, the annealing of R-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 available free of charge via the Internet at http://pubs.acs.org
Literature Cited (1) Davis, J. R. Copper and Copper Alloys; ASM International: Materials Park, OH, 2001. (2) Fe´ron, D.; Saclay, C. E. A. Corrosion BehaViour and Protection of Copper 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 70Cu30Zn brass in chlorate solutions. J. Corros. Sci. 1983, 23, 753–762. (4) Jones, R. H. Stress-Corrosion Cracking: Materials Performance and EValuation; ASM International: Materials Park, OH, 1992. (5) Allam, N. K.; Ashour, E. A. Electrochemical and stress corrosion cracking behavior of 67Cu-33Zn alloy in aqueous electrolytes containing chloride and nitrite ions: Effect of di-sodium hydrogen phosphate (DSHP). Mater. Sci. Eng., B 2009, 156 (1), 84–89. (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 and polluted environments. J. Appl. Electrochem. 2009, 39 (7), 961–969. (7) Rajan, T. V. Heat Treatment: Principles and Techniques; PrenticeHall of India Pvt Ltd.: New Delhi, India, 2007. (8) Fenelon, A. M.; Breslin, C. B. An electrochemical study of the formation of benzotriazole surface-films on copper, zinc and copper-zinc alloy. J. Appl. Electrochem. 2001, 31, 509–516. (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 of R-brass in Mattsson’s solution under potentiostatic conditions. Corros. Sci. 1972, 12, 217. (12) Mattson, E. Stress corrosion in brass considered against the background of potential/pH diagrams. Electrochim. Acta 1961, 3, 279–291. (13) Alvarez, M. G.; Lapitz, P.; Ferna´ndez, S. A.; Galvele, J. R. Passivity breakdown and stress corrosion cracking of R-brass in sodium nitrite solutions. Corros. Sci. 2005, 47 (7), 1643–1652. (14) Hoar, T. P.; Booker, C. J. L. The electrochemistry of the stresscorrosion cracking of alpha brass. Corros. Sci. 1965, 5, 821.
ReceiVed for reView March 28, 2010 ReVised manuscript receiVed August 18, 2010 Accepted August 18, 2010 IE101603W
