- Email: [email protected]
WITHDRAWN: Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 Materials and Design xxx (2013) xxx–xxx 1
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
2
Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution
3 4 5
Q1
6
Amjad Saleh El-Amoush ⇑, Ayman Zamil, Dahman Jaber, Nidal Ismail Al-Balqa Applied University, College of Engineering, Materials and Metallurgical Eng., P.O. Box 7181, Al-Salt 19117, Jordan
7 8
a r t i c l e
1 6 0 1 11 12 13 14 15
i n f o
Article history: Received 28 August 2012 Accepted 4 February 2013 Available online xxxx
a b s t r a c t The stress corrosion cracking (SCC) of the pre-immersed tin brass heat exchanger tube was investigated using slow strain rate testing (SSRT) in an environment containing ammonia. Tubular specimens were exposed at room temperature to this environment for different times prior to straining. The brass tube operated for longer time exhibited more severe SSC cracking and a larger dezincified area than that of the tube operated for shorter time. Intergranular SSC was observed in the fracture surface of the specimen pre-immersed for short time, whereas both intergranular and transgranular SCC were observed at the fracture surface of the specimen pre-immersed for longer time. Furthermore, many pits were observed in the fracture surface of the material. A dezincification had occurred after SSRT and the severity of a dezincified area through the tube depended on the pre-immersion conditions applied to tube. Stress– strain diagrams showed a decrease in tensile properties of the tin brass tube due to the formation of SCC. A remarkable decrease of the ultimate tensile strength (UTS) and the fracture strain of tin brass specimens was found due to SCC. Ó 2013 Published by Elsevier Ltd.
17
among Cu–Ni, Cu–Al, and was attributed to the presence of a mixed passive film of Cu2O and ZnO [1]. The SCC behavior and the mechanism of dezincification of brass in water have been widely investigated [2,3]. The passive film or dezincification layer was found to generate large tensile stress parallel to the applied stress during SCC [4]. Brass is fabricated as pipes and tubes for transporting supply water, condenser systems and heat exchanger due to its excellent corrosion resistance [5–7]. Copper alloys containing 20–40% Zn are highly susceptible to SCC [8]. It was observed that the accelerating effect of the strain rate was higher for intergranular stress corrosion cracking (IGSCC) than for transgranular stress corrosion cracking (TGSCC), which is attributed to the increased crack propagation rate [9]. The present work was undertaken to characterize the stress corrosion cracking of the tin brass heat exchanger tube by immersing the material into ammoniacal solution for various periods before performing the slow strain rate tests.
55
2. Experimental procedure
73
The material used in this study was a commercial tin brass heat exchanger tube provided by the Jordan Petroleum Refinery Company. The material was received in the form of tubing, of 20 mm outside diameter and 2 mm wall thickness. The chemical composition of the material as measured by energy dispersive X-ray (EDX) is shown in Fig 1. Slow strain rate tests (SSRTs) were carried out at a constant strain rate of 1 10 6/s in Mattsson’s solution according
74
18 19 20 21 22 23 24 25 26 27 28 29
30 31
1. Introduction
32
In the Jordan Petroleum Refinery, tin brass heat exchanger tubes are used to transfer heat from combustion gases to liquids of various types. The tubes failed after many years of service under the action of the liquids and their pressure. Stress corrosion cracking (SCC) begins when small cracks develop on the inside and outside surface of the heat exchanger tube. These cracks are initially not visible to the eye and are most commonly found in colonies, with all of the cracks positioned in the same direction. Over a period of years, these individual cracks may lengthen and deepen and the cracks within a colony may join together to form longer cracks. Since SCC develops slowly, it can exist on a heat exchanger tube for many years without causing problems. But if a crack becomes large enough, eventually the heat exchanger tube will fail and will either leak or rupture. SCC is generally considered to be the most complex of all corrosion types. Cracking can have a transgranular or intergranular morphology. Multiple variables affect stress corrosion cracking phenomenon, such as stress level, alloy composition, microstructure, concentration of corrosive species, surface finish, micro-environmental surface effects, temperature, electrochemical potential, etc. Further complications are initiation and propagation phases, and the observation that in some cases cracks initiate at the base of corrosion pits. The Cu–Zn alloy had the highest pitting potential
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
Q2
⇑ Corresponding author. Tel.: +962 5 3491111; fax: +962 5 3530465. E-mail address: [email protected] (A.S. El-Amoush). 0261-3069/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.matdes.2013.02.005
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
75 76 77 78 79 80
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 2
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
97
3. Results and discussion
was observed. Moreover, there are many pits distributed on both surfaces of these specimens. However, larger pits are clearly seen on the surface of the sample taken after 10 years of operation as can be seen from Fig. 3. This indicates that the latter sample has been subjected to more severe corrosion conditions which resulted from more oil pressure and long period of exposure to it. To further investigate the mode of failure of the tube during the service the samples taken after 1 and 10 years of operation were fractured in air. The fracture surfaces of the above samples are shown in Fig. 4. Few and small cracks are observed at the fracture surfaces of the sample taken after 1 year of operation (Fig. 4a) while much and larger cracks are clearly seen at the other samples 10 years’ operated (Fig. 4b). These observations clearly indicate the effect of the service period on the failure mode of tin brass heat exchanger tube and crack development. Moreover, dezincification was observed on the inner wall surfaces of the material after 1 and 2 years of service (Fig. 5). The 1 year operated sample exhibited a less dezincified area, while the 10 years’ one exhibited a larger dezincified area. Two mechanisms have been proposed for this phenomenon, copper and zinc dissolve in the alloy leading to redeposit of copper on the surface. The other mechanism suggested that zinc selectively dissolves leaving the more noble elements i.e. copper in porous mass [11]. It has been found that the failure of the brass after a long service in the corrosive medium was caused by dezincification. The latter condition resulted in the formation of large pits [12–14]. Copper–zinc alloys containing more than 15% Zn are susceptible to dezincification. In the dezincification of brass selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide (analyses of dezincified areas usually indicate 90–95% Cu with the remainder being copper oxide). Corrosion of a similar nature continues beneath the primary corrosion layer resulting in the gradual replacement of sound brass by weak, porous copper. Dezincification also appears to be one of the principle factors in the SCC of copper–zinc–tin alloys. The preferential dissolution or loss of zinc at the fracture interface during SCC results in the corrosion products having a higher concentration of zinc than the adjacent alloy [15]. Unless arrested, dealloying eventually penetrates the metal, weakening it structurally and allowing liquids or gases to leak through the porous mass in the remaining structure. The Cu–Zn solid solution is more susceptible to dealloying in NaCl than Cu–Al solid solution [16] (see Table 1). The cross sections of fracture surfaces of the specimens pre-immersed in the solution and underwent SSRT show similar phenomenon. Similarly, the pre-immersed specimen for longer time exhibited more severe pitting as can be seen from Fig. 6. The pits formed during immersion is found to be due to dezincification of the alloy [17], which manifests it as a selective leaching of the brass. EDX analysis of different areas within the fracture surface revealed that zinc dissolved preferentially in the marked B area (Fig. 6a and corresponding EDX analysis, Figs. 7 and 8). The results clearly indicate that the pits were formed due to the dezincification as can be obtained from the chemical compositions of the pit fracture area which revealed zinc deficiency (Table 2).
98
3.1. Microscopic observations
3.2. SSRT behavior and fractographic examination
162
99
SEM micrographs of two samples taken from the tin brass heat exchanger tube after 1 and 10 years of operation are shown in Fig. 2a and b respectively. Short IGSCC and TGSCC cracks are initiated at the surface of the sample after 1 year of operation as can be seen clearly from Fig. 2a. Extensive IGSCC and TGSCC cracks are observed on the surface of the sample after 10 years of operation as shown in Fig. 2b. The well-developed cracks observed on the surface of the former sample were attributed to the combined action of stress resulted from the oil pressure and exposing the material to it. Longer the exposure to the oil, more severe the cracking
Stress–strain diagrams of the tin brass heat exchanger tube were obtained during exposure to the solution using slow strain rate tensile testing SSRT. The tensile properties of the specimens Q8 pulled in a solution were found to decrease as the pre-immersion period increased as can be seen clearly from Fig. 9. The UTS of the specimen immersed for 24 h before SSRT was decreased from approximately 293 MPa (refers to the specimen pulled by SSRT and without pre-immersion) to 277.5 MPa. More severe degradation in the UTS was found in the specimens immersed in the solution for longer time, i.e. 48 and 96 h (Table 3).
163
Fig. 1. Schematic of the Teflon beaker with the specimen and surrounded with the solution used in SSRT.
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
100 101 102 103 104 105 106 107 108
to ASTM:G37 which consists of 1.0 M/l NH4OH, and 0.05 M/l Cu+2 Q3 added as CuSO45H2O [10]. The pH of this solution was 10. SSRTwere performed on specimens of full-size tubular sections with 25 cm gauge length according to ASTM:E8 methods for tension testing. In order to permit the testing machine jaws to grip these specimens properly, metal plugs were inserted into the end of these specimens. The tests were carried out in a Teflon beaker containing Mattsson’s solution. The specimens were pre-immersed in a solution for 24, 48 and 96 h at a freely-corroding potential before straining. The schematic of the Teflon beaker with the specimen and surrounded with the solution is shown in Fig. 1. The ultimate tensile strength (UTS) and the fracture strain were obtained from the stress–strain diagrams of SSRT. The fracture surfaces were cleaned with alcohol using an ultrasonic cleaning machine and examined using an optical and scanning electron microscope (SEM).
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
109 110 111 112 113 114 115 116
Q4
117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138
Q5
139 140 141 142 143 144 145 146 147 148
Q6
149 150 151
Q7
152 153 154 155 156 157 158 159 160 161
164 165 166 167 168 169 170 171 172
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
3
Fig. 2. Optical micrographs of tin brass heat exchanger tube: (a) 1 year operated and (b) 10 year operated.
Fig. 3. Microscopic examination of tin brass heat exchanger tube: (a) 1 year operated and (b) 10 year operated.
Fig. 4. Dezincification layers of the tin brass heat exchanger tube specimens: (a) 1 year operated and (b) 10 year operated.
173 174 175
The percentage decreases of UTS and fracture strain as a function of immersion time before SSRT were determined referring to the specimen pulled without pre-immersion procedure. As can
be seen from Fig. 10, the specimen pre-immersed for 96 h shows the lowest value of percentage decrease in UTS, i.e. 29% as compared to those of other specimens pre-immersed for shorter time
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
176 177 178
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 4
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Fig. 5. Fracture surfaces of the tin brass heat exchanger tube specimens: (a) 1 year operated and (b) 10 year operated.
Table 1 Chemical composition of the tin brass heat exchanger tube. Element Wt%
Cu 71.06
Zn 26.88
Sn 1.55
Si 0.35
Fe 0.16
and then underwent SSRT. This demonstrates the effect of preimmersion on the UTS, consequently, as the immersion time increases, the UTS decreases further. Similar finding was observed for the fracture strain behavior. Again the percentage decrease of fracture strain for the specimen immersed for 96 h exhibited the lowest value i.e. 32% among other specimens immersed for shorter
Fig. 6. Fracture surfaces of cross sections of the tin brass heat exchanger tube specimens that underwent SSRT after pre-immersion in the solution for: (a) 24, (b) 48 and (c) 96 h.
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
179 180 181 182 183 184
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 5
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Percentage decrease of UTS, %
50.0
Fig. 7. EDX analysis of area A in Fig. 6a.
40.0
30.0
20.0
10.0
0.0 0
20
40
60
80
100
Pre-Immersion time, hrs Fig. 10. Percentage decrease in UTS of the tin brass heat exchanger tube specimens as a function of pre-immersion time.
50.0
Percentage decrease of fracture strain, %
40.0
Fig. 8. EDX analysis of area B in Fig. 6a.
Table 2 EDX analysis of areas A and B in Fig. 6a. Area in Fig. 4a
Cu
Zn
Sn
A B
99.19 70.52
0.43 26.08
0.61 1.39
30.0
20.0
10.0
0.0 0
20
40
60
80
100
Pre-Immersion time, hrs Fig. 11. Percentage decrease in fracture strain of the tin brass heat exchanger tube specimens as a function of pre-immersion time.
Fig. 9. Stress–strain diagram of the specimens that underwent SSRT after preimmersion in the solution for: (a) 0, (b) 24, (c) 48 and (d) 96 h (the strain rate was 10 6 mm/s). Table 3 UTS and fracture strain obtained from SSRT. Pre-immersed time (h)
0 24 48 96
UTS (MPa)
Fracture strain (%)
Test 1
Test 2
Avg.
Test 1
Test 2
Avg.
292.5 278 244.8 207.7
293.7 276.2 240.8 208.5
293.1 277.1 242.8 208.1
1.74 1.67 1.27 1.14
1.72 1.63 1.27 1.08
1.73 1.65 1.27 1.11
time before SSRT (Fig. 11). The remarkable decrease in the fracture stress and the fracture strain of the tin brass during SSRT in a solution is due to SCC. Fig. 12 shows the fracture surfaces of tin brass heat exchanger tube after SSRT. The tube specimens exhibited severe stress corrosion cracking in a solution at a freely-corroding potential. The pre-immersed specimen for 24 h exhibited intergranular stress corrosion cracks (Fig. 12a) while the other specimens pre-immersed for longer times (i.e. 48 and 96 h) showed transgranular stress corrosion cracks at their fracture surfaces (Fig. 12b and c). Extensive transgranular SSC was observed at the fracture surface of the specimen pre-immersed for 96 h which clearly demonstrate the effect of pre-immersion on the SSC mode. The change of SSC mode from intergranular to transgranular could be attributed to the different concentrations of cupric complex ions in a solution. This is probably due to the fact that the pre-immersion for a longer period resulted in a higher cupric ion concentration leading to a change of SSC mode from intergranular to transgranular one. It was found that a critical concentration of cupric complex ions is necessary for the occurrence of transgranular SCC [18]. Other possible reason for the different effect on SSRT, between intergranular and transgranular SCC, is that grain boundaries act as barriers for the free movement of dislocations, leading to dislocation pileups [19].
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 6
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Fig. 12. Fractographic examination of SSC fracture surfaces of the tin brass heat exchanger tube specimens that underwent SSRT after pre-immersion in the solution for: (a) 24, (b) 48 and (c) 96 h. 209
4. Conclusions
210
The following conclusions can be made from the failure analysis and SSRT of the tin brass heat exchanger tube:
211 212 213 214 215 216 217 218 219 220 221 222
(1) The brass tube operated for longer time exhibited more severe SSC cracking and larger dezincified area than that of the tube operated for shorter time. (2) The pre-immersion period has a pronounced effect on the SSC cracking mode. The change from intergranular SSC to transgranular one is associated with longer time of preimmersion. (3) Failure analysis of the brass tube revealed that dezincification had occurred after SSRT and the severity of a dezincified area through the tube depended on the pre-immersion conditions.
223 224 225 226 227 228 229 230 231 232
References [1] Alfantazi AM, Ahmed TM, Tromans D. Corrosion behavior of copper alloys in chloride media. Mater Des 2009;30:2425–30. [2] Kolotyrkin VA, Chervyakov VN, Pschelnikov AP, Losev VV. Electrochemical behavior of brass under conditions of stress corrosion cracking. Zashch Met 1995;31:220–3. [3] Polunin AV, Pchelnikov AP, Losev VV, Marshakov IK. Electrochemical studies of the kinetics and mechanism of brass dezincification. Electrochim Acta 1982;27:467.
[4] Hong Lu, Kewi, Gao, Wuyang Chu. Determination of tensile stress induced by dezincification layer during corrosion for brass. Corros Sci 1998;40:1663–70. [5] Xia Z, Szklarska-Smialowska Z. Pitting of admiralty brass. Corrosion 1990;46:85–8. [6] Plewes JT, Loiacono DN. Free-cutting copper alloys contain no lead. Adv Mater Process 1991;10:23–7. [7] Brooks CR. Heat treatment, structure and properties of nonferrous alloys. Materials Park(OH): ASM International; 1982. [8] Thompson DH, Tracey AW. Influence of composition on the stress-corrosion cracking of some copper-base alloys. Trans Amer Inst Min Met Eng 1949;185:100–44. [9] Serebrinsky SA, Duffó GS, Galvele JR. Effect of strain rate on stress corrosion crack velocity: difference between intergranular and transgranular cracking. Corros Sci 1999;41:191–5. [10] McEvily AJ, Bond AP. On the initiation and growth of stress corrosion cracks in tarnished brass. Electrochem Soc 1965;112:131–8. [11] William TB, Roch JS. Corrosion failures. Failure analysis and prevention. ASM Metals handbook. Materials Park: ASM International; 1975. [12] Shalaby HM. Failure investigation of Muntz tube sheet and Ti tubes of surface condenser. Eng Fail Anal 2006;13:780–8. [13] Russo SG, Henderson MJ, Hinton BRW. Corrosion of an aircraft radar antenna waveguide. Eng Fail Anal 2002;9:423–34. [14] Carlo M, Andrea G, Mattia B. Analysis of the factors involved in failure of a brass sleeve mounted on an electro-valve. Eng Fail Anal 2010;17:431–9. [15] Garyerick Linda. Corrosion in the petrochemical industry. Materials Park(OH): ASM International; 2000. [16] Pryor MJ. The dealloying of a Cu–8.9Al solid solution. Electrochem Soc 1983:1625–7. [17] Qu S, Yao G, Tian JF, Zhang ZF. Failure analysis of the brass tubes in a lubricating oil cooler. Eng Fail Anal 2011;18(8):2232–9. [18] Pugh EN. The theory of stress corrosion cracking in alloys. Brussels: NATO; 1971. [19] Hirth JP, Lothe J. Theory of dislocations. Malabar (FL): Krieger Publishing Co.; 1992.
Q9
233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
No. of Pages 6, Model 5G
6 March 2013 Materials and Design xxx (2013) xxx–xxx 1
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
2
Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution
3 4 5
Q1
6
Amjad Saleh El-Amoush ⇑, Ayman Zamil, Dahman Jaber, Nidal Ismail Al-Balqa Applied University, College of Engineering, Materials and Metallurgical Eng., P.O. Box 7181, Al-Salt 19117, Jordan
7 8
a r t i c l e
1 6 0 1 11 12 13 14 15
i n f o
Article history: Received 28 August 2012 Accepted 4 February 2013 Available online xxxx
a b s t r a c t The stress corrosion cracking (SCC) of the pre-immersed tin brass heat exchanger tube was investigated using slow strain rate testing (SSRT) in an environment containing ammonia. Tubular specimens were exposed at room temperature to this environment for different times prior to straining. The brass tube operated for longer time exhibited more severe SSC cracking and a larger dezincified area than that of the tube operated for shorter time. Intergranular SSC was observed in the fracture surface of the specimen pre-immersed for short time, whereas both intergranular and transgranular SCC were observed at the fracture surface of the specimen pre-immersed for longer time. Furthermore, many pits were observed in the fracture surface of the material. A dezincification had occurred after SSRT and the severity of a dezincified area through the tube depended on the pre-immersion conditions applied to tube. Stress– strain diagrams showed a decrease in tensile properties of the tin brass tube due to the formation of SCC. A remarkable decrease of the ultimate tensile strength (UTS) and the fracture strain of tin brass specimens was found due to SCC. Ó 2013 Published by Elsevier Ltd.
17
among Cu–Ni, Cu–Al, and was attributed to the presence of a mixed passive film of Cu2O and ZnO [1]. The SCC behavior and the mechanism of dezincification of brass in water have been widely investigated [2,3]. The passive film or dezincification layer was found to generate large tensile stress parallel to the applied stress during SCC [4]. Brass is fabricated as pipes and tubes for transporting supply water, condenser systems and heat exchanger due to its excellent corrosion resistance [5–7]. Copper alloys containing 20–40% Zn are highly susceptible to SCC [8]. It was observed that the accelerating effect of the strain rate was higher for intergranular stress corrosion cracking (IGSCC) than for transgranular stress corrosion cracking (TGSCC), which is attributed to the increased crack propagation rate [9]. The present work was undertaken to characterize the stress corrosion cracking of the tin brass heat exchanger tube by immersing the material into ammoniacal solution for various periods before performing the slow strain rate tests.
55
2. Experimental procedure
73
The material used in this study was a commercial tin brass heat exchanger tube provided by the Jordan Petroleum Refinery Company. The material was received in the form of tubing, of 20 mm outside diameter and 2 mm wall thickness. The chemical composition of the material as measured by energy dispersive X-ray (EDX) is shown in Fig 1. Slow strain rate tests (SSRTs) were carried out at a constant strain rate of 1 10 6/s in Mattsson’s solution according
74
18 19 20 21 22 23 24 25 26 27 28 29
30 31
1. Introduction
32
In the Jordan Petroleum Refinery, tin brass heat exchanger tubes are used to transfer heat from combustion gases to liquids of various types. The tubes failed after many years of service under the action of the liquids and their pressure. Stress corrosion cracking (SCC) begins when small cracks develop on the inside and outside surface of the heat exchanger tube. These cracks are initially not visible to the eye and are most commonly found in colonies, with all of the cracks positioned in the same direction. Over a period of years, these individual cracks may lengthen and deepen and the cracks within a colony may join together to form longer cracks. Since SCC develops slowly, it can exist on a heat exchanger tube for many years without causing problems. But if a crack becomes large enough, eventually the heat exchanger tube will fail and will either leak or rupture. SCC is generally considered to be the most complex of all corrosion types. Cracking can have a transgranular or intergranular morphology. Multiple variables affect stress corrosion cracking phenomenon, such as stress level, alloy composition, microstructure, concentration of corrosive species, surface finish, micro-environmental surface effects, temperature, electrochemical potential, etc. Further complications are initiation and propagation phases, and the observation that in some cases cracks initiate at the base of corrosion pits. The Cu–Zn alloy had the highest pitting potential
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
Q2
⇑ Corresponding author. Tel.: +962 5 3491111; fax: +962 5 3530465. E-mail address: [email protected] (A.S. El-Amoush). 0261-3069/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.matdes.2013.02.005
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
75 76 77 78 79 80
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 2
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
97
3. Results and discussion
was observed. Moreover, there are many pits distributed on both surfaces of these specimens. However, larger pits are clearly seen on the surface of the sample taken after 10 years of operation as can be seen from Fig. 3. This indicates that the latter sample has been subjected to more severe corrosion conditions which resulted from more oil pressure and long period of exposure to it. To further investigate the mode of failure of the tube during the service the samples taken after 1 and 10 years of operation were fractured in air. The fracture surfaces of the above samples are shown in Fig. 4. Few and small cracks are observed at the fracture surfaces of the sample taken after 1 year of operation (Fig. 4a) while much and larger cracks are clearly seen at the other samples 10 years’ operated (Fig. 4b). These observations clearly indicate the effect of the service period on the failure mode of tin brass heat exchanger tube and crack development. Moreover, dezincification was observed on the inner wall surfaces of the material after 1 and 2 years of service (Fig. 5). The 1 year operated sample exhibited a less dezincified area, while the 10 years’ one exhibited a larger dezincified area. Two mechanisms have been proposed for this phenomenon, copper and zinc dissolve in the alloy leading to redeposit of copper on the surface. The other mechanism suggested that zinc selectively dissolves leaving the more noble elements i.e. copper in porous mass [11]. It has been found that the failure of the brass after a long service in the corrosive medium was caused by dezincification. The latter condition resulted in the formation of large pits [12–14]. Copper–zinc alloys containing more than 15% Zn are susceptible to dezincification. In the dezincification of brass selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide (analyses of dezincified areas usually indicate 90–95% Cu with the remainder being copper oxide). Corrosion of a similar nature continues beneath the primary corrosion layer resulting in the gradual replacement of sound brass by weak, porous copper. Dezincification also appears to be one of the principle factors in the SCC of copper–zinc–tin alloys. The preferential dissolution or loss of zinc at the fracture interface during SCC results in the corrosion products having a higher concentration of zinc than the adjacent alloy [15]. Unless arrested, dealloying eventually penetrates the metal, weakening it structurally and allowing liquids or gases to leak through the porous mass in the remaining structure. The Cu–Zn solid solution is more susceptible to dealloying in NaCl than Cu–Al solid solution [16] (see Table 1). The cross sections of fracture surfaces of the specimens pre-immersed in the solution and underwent SSRT show similar phenomenon. Similarly, the pre-immersed specimen for longer time exhibited more severe pitting as can be seen from Fig. 6. The pits formed during immersion is found to be due to dezincification of the alloy [17], which manifests it as a selective leaching of the brass. EDX analysis of different areas within the fracture surface revealed that zinc dissolved preferentially in the marked B area (Fig. 6a and corresponding EDX analysis, Figs. 7 and 8). The results clearly indicate that the pits were formed due to the dezincification as can be obtained from the chemical compositions of the pit fracture area which revealed zinc deficiency (Table 2).
98
3.1. Microscopic observations
3.2. SSRT behavior and fractographic examination
162
99
SEM micrographs of two samples taken from the tin brass heat exchanger tube after 1 and 10 years of operation are shown in Fig. 2a and b respectively. Short IGSCC and TGSCC cracks are initiated at the surface of the sample after 1 year of operation as can be seen clearly from Fig. 2a. Extensive IGSCC and TGSCC cracks are observed on the surface of the sample after 10 years of operation as shown in Fig. 2b. The well-developed cracks observed on the surface of the former sample were attributed to the combined action of stress resulted from the oil pressure and exposing the material to it. Longer the exposure to the oil, more severe the cracking
Stress–strain diagrams of the tin brass heat exchanger tube were obtained during exposure to the solution using slow strain rate tensile testing SSRT. The tensile properties of the specimens Q8 pulled in a solution were found to decrease as the pre-immersion period increased as can be seen clearly from Fig. 9. The UTS of the specimen immersed for 24 h before SSRT was decreased from approximately 293 MPa (refers to the specimen pulled by SSRT and without pre-immersion) to 277.5 MPa. More severe degradation in the UTS was found in the specimens immersed in the solution for longer time, i.e. 48 and 96 h (Table 3).
163
Fig. 1. Schematic of the Teflon beaker with the specimen and surrounded with the solution used in SSRT.
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96
100 101 102 103 104 105 106 107 108
to ASTM:G37 which consists of 1.0 M/l NH4OH, and 0.05 M/l Cu+2 Q3 added as CuSO45H2O [10]. The pH of this solution was 10. SSRTwere performed on specimens of full-size tubular sections with 25 cm gauge length according to ASTM:E8 methods for tension testing. In order to permit the testing machine jaws to grip these specimens properly, metal plugs were inserted into the end of these specimens. The tests were carried out in a Teflon beaker containing Mattsson’s solution. The specimens were pre-immersed in a solution for 24, 48 and 96 h at a freely-corroding potential before straining. The schematic of the Teflon beaker with the specimen and surrounded with the solution is shown in Fig. 1. The ultimate tensile strength (UTS) and the fracture strain were obtained from the stress–strain diagrams of SSRT. The fracture surfaces were cleaned with alcohol using an ultrasonic cleaning machine and examined using an optical and scanning electron microscope (SEM).
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
109 110 111 112 113 114 115 116
Q4
117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138
Q5
139 140 141 142 143 144 145 146 147 148
Q6
149 150 151
Q7
152 153 154 155 156 157 158 159 160 161
164 165 166 167 168 169 170 171 172
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
3
Fig. 2. Optical micrographs of tin brass heat exchanger tube: (a) 1 year operated and (b) 10 year operated.
Fig. 3. Microscopic examination of tin brass heat exchanger tube: (a) 1 year operated and (b) 10 year operated.
Fig. 4. Dezincification layers of the tin brass heat exchanger tube specimens: (a) 1 year operated and (b) 10 year operated.
173 174 175
The percentage decreases of UTS and fracture strain as a function of immersion time before SSRT were determined referring to the specimen pulled without pre-immersion procedure. As can
be seen from Fig. 10, the specimen pre-immersed for 96 h shows the lowest value of percentage decrease in UTS, i.e. 29% as compared to those of other specimens pre-immersed for shorter time
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
176 177 178
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 4
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Fig. 5. Fracture surfaces of the tin brass heat exchanger tube specimens: (a) 1 year operated and (b) 10 year operated.
Table 1 Chemical composition of the tin brass heat exchanger tube. Element Wt%
Cu 71.06
Zn 26.88
Sn 1.55
Si 0.35
Fe 0.16
and then underwent SSRT. This demonstrates the effect of preimmersion on the UTS, consequently, as the immersion time increases, the UTS decreases further. Similar finding was observed for the fracture strain behavior. Again the percentage decrease of fracture strain for the specimen immersed for 96 h exhibited the lowest value i.e. 32% among other specimens immersed for shorter
Fig. 6. Fracture surfaces of cross sections of the tin brass heat exchanger tube specimens that underwent SSRT after pre-immersion in the solution for: (a) 24, (b) 48 and (c) 96 h.
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
179 180 181 182 183 184
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 5
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Percentage decrease of UTS, %
50.0
Fig. 7. EDX analysis of area A in Fig. 6a.
40.0
30.0
20.0
10.0
0.0 0
20
40
60
80
100
Pre-Immersion time, hrs Fig. 10. Percentage decrease in UTS of the tin brass heat exchanger tube specimens as a function of pre-immersion time.
50.0
Percentage decrease of fracture strain, %
40.0
Fig. 8. EDX analysis of area B in Fig. 6a.
Table 2 EDX analysis of areas A and B in Fig. 6a. Area in Fig. 4a
Cu
Zn
Sn
A B
99.19 70.52
0.43 26.08
0.61 1.39
30.0
20.0
10.0
0.0 0
20
40
60
80
100
Pre-Immersion time, hrs Fig. 11. Percentage decrease in fracture strain of the tin brass heat exchanger tube specimens as a function of pre-immersion time.
Fig. 9. Stress–strain diagram of the specimens that underwent SSRT after preimmersion in the solution for: (a) 0, (b) 24, (c) 48 and (d) 96 h (the strain rate was 10 6 mm/s). Table 3 UTS and fracture strain obtained from SSRT. Pre-immersed time (h)
0 24 48 96
UTS (MPa)
Fracture strain (%)
Test 1
Test 2
Avg.
Test 1
Test 2
Avg.
292.5 278 244.8 207.7
293.7 276.2 240.8 208.5
293.1 277.1 242.8 208.1
1.74 1.67 1.27 1.14
1.72 1.63 1.27 1.08
1.73 1.65 1.27 1.11
time before SSRT (Fig. 11). The remarkable decrease in the fracture stress and the fracture strain of the tin brass during SSRT in a solution is due to SCC. Fig. 12 shows the fracture surfaces of tin brass heat exchanger tube after SSRT. The tube specimens exhibited severe stress corrosion cracking in a solution at a freely-corroding potential. The pre-immersed specimen for 24 h exhibited intergranular stress corrosion cracks (Fig. 12a) while the other specimens pre-immersed for longer times (i.e. 48 and 96 h) showed transgranular stress corrosion cracks at their fracture surfaces (Fig. 12b and c). Extensive transgranular SSC was observed at the fracture surface of the specimen pre-immersed for 96 h which clearly demonstrate the effect of pre-immersion on the SSC mode. The change of SSC mode from intergranular to transgranular could be attributed to the different concentrations of cupric complex ions in a solution. This is probably due to the fact that the pre-immersion for a longer period resulted in a higher cupric ion concentration leading to a change of SSC mode from intergranular to transgranular one. It was found that a critical concentration of cupric complex ions is necessary for the occurrence of transgranular SCC [18]. Other possible reason for the different effect on SSRT, between intergranular and transgranular SCC, is that grain boundaries act as barriers for the free movement of dislocations, leading to dislocation pileups [19].
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005
185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208
JMAD 5139
No. of Pages 6, Model 5G
6 March 2013 6
A.S. El-Amoush et al. / Materials and Design xxx (2013) xxx–xxx
Fig. 12. Fractographic examination of SSC fracture surfaces of the tin brass heat exchanger tube specimens that underwent SSRT after pre-immersion in the solution for: (a) 24, (b) 48 and (c) 96 h. 209
4. Conclusions
210
The following conclusions can be made from the failure analysis and SSRT of the tin brass heat exchanger tube:
211 212 213 214 215 216 217 218 219 220 221 222
(1) The brass tube operated for longer time exhibited more severe SSC cracking and larger dezincified area than that of the tube operated for shorter time. (2) The pre-immersion period has a pronounced effect on the SSC cracking mode. The change from intergranular SSC to transgranular one is associated with longer time of preimmersion. (3) Failure analysis of the brass tube revealed that dezincification had occurred after SSRT and the severity of a dezincified area through the tube depended on the pre-immersion conditions.
223 224 225 226 227 228 229 230 231 232
References [1] Alfantazi AM, Ahmed TM, Tromans D. Corrosion behavior of copper alloys in chloride media. Mater Des 2009;30:2425–30. [2] Kolotyrkin VA, Chervyakov VN, Pschelnikov AP, Losev VV. Electrochemical behavior of brass under conditions of stress corrosion cracking. Zashch Met 1995;31:220–3. [3] Polunin AV, Pchelnikov AP, Losev VV, Marshakov IK. Electrochemical studies of the kinetics and mechanism of brass dezincification. Electrochim Acta 1982;27:467.
[4] Hong Lu, Kewi, Gao, Wuyang Chu. Determination of tensile stress induced by dezincification layer during corrosion for brass. Corros Sci 1998;40:1663–70. [5] Xia Z, Szklarska-Smialowska Z. Pitting of admiralty brass. Corrosion 1990;46:85–8. [6] Plewes JT, Loiacono DN. Free-cutting copper alloys contain no lead. Adv Mater Process 1991;10:23–7. [7] Brooks CR. Heat treatment, structure and properties of nonferrous alloys. Materials Park(OH): ASM International; 1982. [8] Thompson DH, Tracey AW. Influence of composition on the stress-corrosion cracking of some copper-base alloys. Trans Amer Inst Min Met Eng 1949;185:100–44. [9] Serebrinsky SA, Duffó GS, Galvele JR. Effect of strain rate on stress corrosion crack velocity: difference between intergranular and transgranular cracking. Corros Sci 1999;41:191–5. [10] McEvily AJ, Bond AP. On the initiation and growth of stress corrosion cracks in tarnished brass. Electrochem Soc 1965;112:131–8. [11] William TB, Roch JS. Corrosion failures. Failure analysis and prevention. ASM Metals handbook. Materials Park: ASM International; 1975. [12] Shalaby HM. Failure investigation of Muntz tube sheet and Ti tubes of surface condenser. Eng Fail Anal 2006;13:780–8. [13] Russo SG, Henderson MJ, Hinton BRW. Corrosion of an aircraft radar antenna waveguide. Eng Fail Anal 2002;9:423–34. [14] Carlo M, Andrea G, Mattia B. Analysis of the factors involved in failure of a brass sleeve mounted on an electro-valve. Eng Fail Anal 2010;17:431–9. [15] Garyerick Linda. Corrosion in the petrochemical industry. Materials Park(OH): ASM International; 2000. [16] Pryor MJ. The dealloying of a Cu–8.9Al solid solution. Electrochem Soc 1983:1625–7. [17] Qu S, Yao G, Tian JF, Zhang ZF. Failure analysis of the brass tubes in a lubricating oil cooler. Eng Fail Anal 2011;18(8):2232–9. [18] Pugh EN. The theory of stress corrosion cracking in alloys. Brussels: NATO; 1971. [19] Hirth JP, Lothe J. Theory of dislocations. Malabar (FL): Krieger Publishing Co.; 1992.
Q9
233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267
Please cite this article in press as: El-Amoush AS et al. Stress corrosion cracking of the pre-immersed tin brass heat exchanger tube in an ammoniacal solution. J Mater Design (2013), http://dx.doi.org/10.1016/j.matdes.2013.02.005