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Effect of sulfide ions on the corrosion behaviour of Al–brass and Cu10Ni alloys in salt water
Materials Chemistry and Physics 78 (2003) 825–834
Effect of sulfide ions on the corrosion behaviour of Al–brass and Cu10Ni alloys in salt water S.M. Sayed∗ , E.A. Ashour, G.I. Youssef Department of Physical Chemistry, National Research Centre, Dokki, Cairo, Egypt Received 27 February 2002; received in revised form 25 June 2002; accepted 13 August 2002
Abstract The corrosion behaviour of modified Al–brass (MA72) and Cu10Ni alloys was investigated in 3.5% NaCl in absence and presence of different concentrations of sulfide ions. The results indicated that the Cu10Ni alloy demonstrated better corrosion resistance in 3.5% NaCl solution. The corrosion of the alloys has increased in presence of sulfide ions. The increase of sulfide ions (500, and 1000 ppm) resulted a pitting corrosion in case of Cu10Ni and intergranular corrosion of Al–brass. The detrimental effect of sulfide on the corrosion resistance increases with the increase of sulfide ion concentrations. The results of the weight variations and potentiostatic polarization are in agreement with the potential–time measurements that the sulfide ions in polluted seawater promotes the corrosion of Cu10Ni than Al–brass. The formation of Cu2 S interferes with the protecting oxide film and reduces its corrosion resistance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Corrosion resistance; Al–brass; Multistage flash
1. Introduction Copper base alloys have been used to fabricate structures and components exposed to seawater and other marine environments. This is due to the fact that they have an attractive combination of properties, e.g. good corrosion resistance, good machinability, high thermal and electrical conductivity and resistance to biofouling [1–3]. For instance, copper–nickel and aluminum–brass alloys have been used extensively in water distribution systems, water treatment units, condensers and heat exchanger, where fresh or salt water is used for cooling. These units are often the critical components for processing plants in electricity generating, oil and chemical industries and multistage flash (MSF) in the desalination systems. However, these alloys suffer greatly accelerated corrosion when exposed to seawater polluted with sulfide [4–12]. Sulfide pollution of seawater at the coastal areas can occur from industrial waste discharge, biological and bacteriological process in sea water (seaweed, marine organisms or microorganisms, sulfide-reducing bacteria) [4,13,14]. It was found that corrosion rate of Cu-base alloys increases by a factor of 10–30 when seawater contains sulfur compounds as impurities [15]. Small quantities ∗ Corresponding author. E-mail address: [email protected] (S.M. Sayed).
of Ni, Cr, and Mn may be added to Al–brass as well as cupronickel alloys with Al, Cr, Fe, and Mn, to modify their mechanical properties or corrosion resistance [16]. The objective of this paper is to study and compare the corrosion behaviour of modified Al–brass (MA72) and Cu10Ni alloy in 3.5% NaCl and the role of the presence of different concentrations of sodium sulfide on their corrosion behaviour.
2. Experimental procedure Test electrodes were prepared from Al–brass (MA72) and Cu10Ni (CD706) sheets of 2 mm thickness having the following chemical composition: • Al–brass: 71.62% Cu, 3.58% Al, 1.24% Ni, 0.034% Si, 0.038% Fe, the rest is Zn. • Cu10Ni: 88.12% Cu, 1.42% Fe, 0.38% Mn, 0.13% Zn, 0.01% Pb, the rest is Ni. The electrodes were cut in the form of flag-shaped small rectangles of 1 cm × 1.5 cm × 0.2 cm. The electrodes were polished successively with 320, 400 and 600 grade silicon carbide papers, degreased with acetone and flushed with distilled water. A thin small neck was covered with paraffin wax. A conventional three-electrode cell was used
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Fig. 1. Potential–time curves of Al–brass in 3.5% NaCl solution in absence and presence of different concentrations of Na2 S.
Fig. 2. Potential–time curves of Cu10Ni in 3.5% NaCl solution in absence and presence of different concentrations of Na2 S.
S.M. Sayed et al. / Materials Chemistry and Physics 78 (2003) 825–834
with a saturated calomel reference electrode and a platinum sheet counter electrode. The potential was controlled using a Wenking potentiostat L.T.73. Polarization curves were obtained by changing the potential at a rate of 20 mV min−1 . Measurements of weight changes were performed on rectangular coupons (2 cm × 5 cm × 0.2 cm). They were polished and cleaned as described above, dried and weighed (W1 ). They were allowed to corrode in aerated 3.5% NaCl with different concentrations of Na2 S (500, and 1000 ppm) for 30 days at 24 ± 1 ◦ C. At the end of a run, the specimens were rinsed with distilled water, dried and weighed (W2 ). The weight loss was determined from the relation W= W1 − W2 . The measurements of the weight changes were repeated twice for two alloys at time interval. The corroded surfaces of the tested alloys were inspected by scanning electron microscopy (SEM) using a JOEL, JSM-T 20 (Japan). Sodium sulfate was obtained from Riedel-de Haen. Sodium chloride was Analar grade and the water was doubly distilled. 2.1. X-ray diffraction analysis The corroded alloys surfaces were subjected to X-ray diffraction (X-ray diffractometer, Philips PW-1390 with a Cu-tube, Cu K␣1 , λ = 1.54051 cm).
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3. Results and discussion 3.1. Electrochemical measurements The potential–time measurements of both Al–brass and Cu10Ni alloys in 3.5% NaCl solutions containing different concentrations of Na2 S are shown in Figs. 1 and 2. The curves show in general that the steady-state potential is shifted to the more negative direction due to the presence of sulfide and the shift increases with the increase of sulfide concentration. The negative shift of the steady-state (st-st) potential in presence of sulfide ions indicates that the corrosion of the two alloys is under anodic control. Fig. 3 shows the steady-state potential values of Al–brass and Cu10Ni alloys in 3.5% NaCl solutions in absence and presence of different concentrations of Na2 S. The negative shift of the steady-state potential reveals that the reactivity of Al–brass in absence of sulfide is more than Cu10Ni and become almost equal in presence of 100 ppm sulfide. At higher concentrations of Na2 S (>100 ppm) the steady-state potential of Cu10Ni alloy becomes more negative than Al–brass alloy. This will be correlated with the corrosion rate measurements, as shown below. Potentiostatic polarization measurements for Al–brass and Cu10Ni alloys in 3.5% NaCl solution in absence and presence of different
Fig. 3. The steady-state potential values of Al–brass and Cu10Ni alloys in absence and presence of different concentrations of Na2 S.
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Fig. 4. Potentiostatic polarization curves of Al–brass in 3.5% NaCl in absence and presence of different concentrations of Na2 S.
concentrations of Na2 S were carried out. The curves obtained for Al–brass are shown in Fig. 4. All the curves have the same general trend except the curve pertaining to the test solution containing 1000 ppm sulfide. The anodic parts of these curves start from the free corrosion potential by a very fast and abrupt increase of current with increase of potential until a peak of maximum current is attained at ≈0 mV (SCE). After reaching the peak of the maximum current, the current drops to a minimum value at a potential value of ≈50 mV (SCE) and then the current increases gradually with the increase of potential to form a plateau of almost limited current. In case of test solution containing 1000 ppm sulfide, the curve starts at a considerably more negative free corrosion potential and shows a region of fluctuations in the current value covering a wide range of potential (from −900 to −400 mV (SCE)) before reaching the peak of the maximum current. Observation of the curves of Fig. 4 reveals that the presence of sulfide shifts the free corrosion potential and the first region of the anodic curves towards more negative values. Fig. 5 illustrates the potentiostatic polarization curves of Cu10Ni alloy in 3.5% NaCl in absence and presence of different concentrations of sodium sulfide. It is obvious that the presence of sulfide causes a considerable shift of the free corrosion potential towards more negative values. The curves representing the solution containing
500 and 1000 ppm sulfide ions demonstrates more than one small peak at different potentials before attaining the peak of the maximum current which occurs at the same potential (≈−50 mV (SCE)) in all curves. The curves of Figs. 4 and 5 indicate that the presence of sulfide ions increases the reactivity of both the two tested alloys in 3.5% NaCl solution, i.e. decreases the corrosion resistance of the alloys. It is clear that anodic corrosion current decreases slightly as the sulfide concentration increases as shown in Figs. 4 and 5. This is in agreement with the results have been reported by El-Domiaty and Alhajji [17]. These observations find interpretation in the following: (i) the effects of both chloride and sulfide ions depend on the electrode potential (which is a measure of the oxidizing power of the electrolyte in absence of potential control), (ii) the increase of the current may be attributed to the anodic oxidation of the sulfide ions. The anodic oxidation of sulfide ions is a complex process which might lead to the formation of polysulfide, elemental sulfur, thiosulfate, etc. according to the following reactions [18]: 3HS− → S3 2− + 3H+ + 4e− , E 0 = 0.097 V
(NHE)
(1)
HS− → S + H+ + 2e− , E 0 = −0.065 V
(NHE)
(2)
S.M. Sayed et al. / Materials Chemistry and Physics 78 (2003) 825–834
Fig. 5. Potentiostatic polarization curves Cu10Ni in 3.5% NaCl in absence and presence of different concentrations of Na2 S.
Fig. 6. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl solution.
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Fig. 7. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl + 500 ppm Na2 S.
2HS− + 3H2 O → S2 O3 2− + 8H+ + 8e− , E = 0.200 V 0
(NHE)
(3)
It has long been realized that the formation of elemental sulfur on the electrode surface leads to its passivation. Some of these results point to passivation of the electrode surface following the sudden increase in current. The current abruptly increases with potential at values near the ocp. At cathodic potentials to Ecorr , the current is a measure of the oxygen reduction reaction (cathodic reaction in aerated seawater). O2 + 2H2 O + 4e− → 4OH− (4) However, generally, the pollution of seawater with sulfide results in corrosion problems with Cu-base alloys due to the formation of Cu2 S. The formation of black cuprous sulfide (unstable) occurs via 2Cu + S2− → Cu2 S + 2e−
(5)
According to Tromans and Sun [19], Cu is also involved in this reaction: 2Cu + H2 O → Cu2 O + 2H+ + 2e−
(6)
A comparison of the polarization curves of Al–brass and Cu10Ni in 3.5% NaCl solution free of sulfide ions is shown
in Fig. 6. It is clear that the free corrosion potential and both the anodic and cathodic branches of the curve of Cu10Ni alloy are located at less negative potentials with respect to the curve of Al–brass. The peaks of the maximum current occur almost at the same potential value but the peak current is higher in case of Al–brass. In presence of 500 ppm sulfide (Fig. 7) a different situation is observed. In this case the free corrosion potential and both the anodic and cathodic parts of the polarization curve of Cu10Ni alloy are located at more negative potentials than those of Al–brass. Fig. 8 shows the polarization curves in presence of 1000 ppm sulfide ions. It can be observed from the curves that while the free corrosion potentials of the two alloys are nearly the same, the anodic branches of the curves demonstrate different behaviour. In the range from free corrosion potential to −200 mV, the curve of Al–brass alloy illustrates higher current values than that of Cu10Ni, while in the potential range from −200 mV to the end of the curves, Al–brass records lower current values. 3.2. Corrosion rate measurements The corrosion rate measurements of Al–brass and Cu10Ni alloys in 3.4% NaCl in absence and presence of sulfide ions
S.M. Sayed et al. / Materials Chemistry and Physics 78 (2003) 825–834
Fig. 8. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl + 1000 ppm Na2 S.
Fig. 9. Effect of sulfide concentrations on the weight variations of Cu10Ni and Al–brass in 3.5% NaCl for 30 days.
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Table 1 The corrosion rate and steady-state (st-st) potential values of Cu10Ni and Al–brass alloys in 3.5% NaCl in absence and presence of sulfide ion. Sulfide concentration (ppm)
0 500 1000
Cu10Ni
Al–brass
st-st potential mV (SCE)
Corrosion rate (mpy)
st-st potential mV (SCE)
Corrosion rate (mpy)
−217 −572 −807
1.82 5.91 7.73
−256 −433 −398
2.3 4.22 5.86
(500, and 1000 ppm) was calculated from the polarization curves. Table 1 illustrates a comparison between steady-state potential values and corrosion rate values. It is obvious that as the sulfide concentration increases, the corrosion rate increases. Pickering and coworkers [11] suggested that the higher corrosion rates of Cu-9.4Ni-1.7Fe in sulfide contaminated 3.4% NaCl solution are due to a highly defective Cu2 O layer containing Cu2 S which permit rapid ionic and electronic transport through it. The Cu2 S is less protective than Cu2 O.
Fig. 10. SEM micrographs of the corroded surfaces in 3.5% NaCl after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
3.3. Weight variations and metallographic investigations The change of weight was determined for Al–brass and Cu10Ni alloys in 3.4% NaCl in absence and presence of sulfide ions (500, and 1000 ppm) after immersion for 30 days. The average changes in weights are shown in Fig. 9. It is obvious from Fig. 9 that in absence of sulfide the Cu10Ni has showed a large amount of weight gain and the appearance
Fig. 11. SEM micrographs of the corroded surfaces in 3.5% NaCl + 500 ppm sulfide ion after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
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of an adhesive green film on the surface of the specimens, while Al–brass alloy recorded a very small amount of loss in weight and the appearance of the corroded surface still bright yellow without any corrosion products on it. Examination of the corroded surfaces by SEM revealed that in case of Cu10Ni alloy a dark colored film covered with porous light green precipitates of corrosion products are formed as shown in Fig. 10a. In case of Al–brass (Fig. 10b) the surface of the specimens still bright but localized corrosion in the form of a fine dark spots appear among the scratches. In presence of 500 ppm sulfide ions, the Cu10Ni showed a considerable amount of weight loss while Al–brass recorded amount of weight-gain. After washing by brushing under running water to remove the loose corrosion products, the Cu10Ni alloy demonstrated the same original color without brightness. The Al–brass alloy showed the appearance of a brown adhesive film which could not be removed by washing. The micrograph of Fig. 11a shows that the surface of Cu10Ni alloy have some roughness and charac-
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terized in some locations by the occurrence of pits filled with corrosion products in the form of flakes suffering from many cracks as shown in Fig. 11b. Moreover, small area of intergranular corrosion appears beneath the cracked location of corrosion product layer. In case of the test solution containing 1000 ppm sulfide, the two alloys recorded weight loss, but the corrosion of Cu10Ni is higher than that of Al–brass as shown in Fig. 9. The SEM micrograph of Fig. 12a shows that the Cu10Ni alloy suffer from general as well as pitting corrosion while Al–brass show the occurrence of general corrosion in the form of a thick brown layer of corrosion products and intergranular corrosion appears beneath this layer at the cracked locations as shown in Fig. 12b. 3.4. X-ray diffraction analysis The X-ray measurements revealed significant findings: (i) The corrosion products which form on Cu10Ni under free corrosion conditions in the test solution containing sulfide ions are Cu2 O, Cu2 (OH)3 Cl, Cu2 S. (ii) CuCl forms only at anodic potentials, considerably higher than the free corrosion potential. The corrosion products for the corroded Al–brass surfaces could not be identified by X-ray diffraction technique, this might be the case if the corrosion products were in the form of a film that is too thin to be detected by the X-ray.
4. Conclusions
Fig. 12. SEM micrographs of the corroded surfaces in 3.5% NaCl + 1000 ppm sulfide ion after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
1. The steady-state potentials for the Cu10Ni and Al–brass are shifted towards more active values by the effect of sulfide ions, but the steady-state potential shift in case of Cu10Ni is more than that of Al–brass. 2. The potentiostatic polarization measurements indicated that the presence of sulfide ions increases the reactivity of both the two tested alloys in 3.5% NaCl solution, i.e. decreases the corrosion resistance of the alloys. 3. Cu10Ni alloy showed a high weight gain while Al–brass has a few amount of weight loss in 3.5% NaCl. 4. The weight loss of Cu10Ni in 3.5% NaCl + 1000 ppm Na2 S was larger about eight times than Al–brass. 5. The metallographic SEM examinations indicated the occurrence of general corrosion of both alloys. However, pitting corrosion in case of Cu10Ni and intergranular corrosion in case of Al–brass were observed in presence of sulfide ions. 6. The results indicated that the Cu10Ni alloy demonstrated better corrosion resistance in 3.5% NaCl solution but in high concentrations of sulfide ions polluted salt water, Cu10Ni becomes less corrosion resistance than Al–brass. 7. The formation of Cu2 S interferes with the protecting oxide film and reduces its corrosion resistance.
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References [1] E.G. West, Copper and Its Alloys, Ellis Horwood, New York, 1982, p. 155. [2] R.B. Ross, Metallic Materials Specification Handbook, 3rd. ed., E. and F.N. Spon, London, 1968, p. 139. [3] C. Manfredi, S. Simison, S.R. de Sanchez, Corrosion 43 (1987) 458. [4] J.P. Gudas, H.P. Hack, Corrosion 35 (1979) 67. [5] L.E. Eiselstein, B.C. Syrett, S.S. Wing, R.D. Caligiuri, Corros. Sci. 23 (1983) 223. [6] B.C. Syrett, Corrosion 33 (1977) 257. [7] B.C. Syrett, Corros. Sci. 21 (1981) 187. [8] J.N. Alhajji, M.R. Reda, J. Electrochem. Soc. 142 (1995) 2944. [9] K. Habib, A. Amin, Desalination 85 (1992) 275.
[10] D.R. Lenard, J.G. Moores, G.E. Morin, Br. Corros. J. 24 (1989) 19. [11] C. Kato, H.W. Pickering, J.E. Castle, J. Electrochem. Soc. 131 (1984) 1225. [12] J.N. Alhajji, M.R. Reda, J. Electrochem. Soc. 141 (1994) 1432. [13] F.P. Ijsseltng, Br. Corros. J. 24 (1989) 55. [14] B.C. Syrett, Corrosion, vol. 33, Chicago, IL, 1980, p. 80. [15] T.S. Lee, H.P. Hack, D.G. Tipton, in: Proceedings of the 5th International Congress on Marine Corrosion and Fouling, 1980. [16] B.C. Syrett, Corrosion 33 (1977) 275. [17] A. El-Domiaty, J.N. Alhajji, J. Mater. Eng. Perform. 6 (1997) 534. [18] G. Valensi, J. van Muylder, M. Pourbaix, in: M. Pourbaix (Ed.), Proceedings of the Atlas of Electrochemical Equilibria in Aqueous Media, NACE, Houston, TX, 1974, p. 545. [19] D. Tromans, R.H. Sun, J. Electrochem. Soc. 138 (1991) 3235.
Effect of sulfide ions on the corrosion behaviour of Al–brass and Cu10Ni alloys in salt water S.M. Sayed∗ , E.A. Ashour, G.I. Youssef Department of Physical Chemistry, National Research Centre, Dokki, Cairo, Egypt Received 27 February 2002; received in revised form 25 June 2002; accepted 13 August 2002
Abstract The corrosion behaviour of modified Al–brass (MA72) and Cu10Ni alloys was investigated in 3.5% NaCl in absence and presence of different concentrations of sulfide ions. The results indicated that the Cu10Ni alloy demonstrated better corrosion resistance in 3.5% NaCl solution. The corrosion of the alloys has increased in presence of sulfide ions. The increase of sulfide ions (500, and 1000 ppm) resulted a pitting corrosion in case of Cu10Ni and intergranular corrosion of Al–brass. The detrimental effect of sulfide on the corrosion resistance increases with the increase of sulfide ion concentrations. The results of the weight variations and potentiostatic polarization are in agreement with the potential–time measurements that the sulfide ions in polluted seawater promotes the corrosion of Cu10Ni than Al–brass. The formation of Cu2 S interferes with the protecting oxide film and reduces its corrosion resistance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Corrosion resistance; Al–brass; Multistage flash
1. Introduction Copper base alloys have been used to fabricate structures and components exposed to seawater and other marine environments. This is due to the fact that they have an attractive combination of properties, e.g. good corrosion resistance, good machinability, high thermal and electrical conductivity and resistance to biofouling [1–3]. For instance, copper–nickel and aluminum–brass alloys have been used extensively in water distribution systems, water treatment units, condensers and heat exchanger, where fresh or salt water is used for cooling. These units are often the critical components for processing plants in electricity generating, oil and chemical industries and multistage flash (MSF) in the desalination systems. However, these alloys suffer greatly accelerated corrosion when exposed to seawater polluted with sulfide [4–12]. Sulfide pollution of seawater at the coastal areas can occur from industrial waste discharge, biological and bacteriological process in sea water (seaweed, marine organisms or microorganisms, sulfide-reducing bacteria) [4,13,14]. It was found that corrosion rate of Cu-base alloys increases by a factor of 10–30 when seawater contains sulfur compounds as impurities [15]. Small quantities ∗ Corresponding author. E-mail address: [email protected] (S.M. Sayed).
of Ni, Cr, and Mn may be added to Al–brass as well as cupronickel alloys with Al, Cr, Fe, and Mn, to modify their mechanical properties or corrosion resistance [16]. The objective of this paper is to study and compare the corrosion behaviour of modified Al–brass (MA72) and Cu10Ni alloy in 3.5% NaCl and the role of the presence of different concentrations of sodium sulfide on their corrosion behaviour.
2. Experimental procedure Test electrodes were prepared from Al–brass (MA72) and Cu10Ni (CD706) sheets of 2 mm thickness having the following chemical composition: • Al–brass: 71.62% Cu, 3.58% Al, 1.24% Ni, 0.034% Si, 0.038% Fe, the rest is Zn. • Cu10Ni: 88.12% Cu, 1.42% Fe, 0.38% Mn, 0.13% Zn, 0.01% Pb, the rest is Ni. The electrodes were cut in the form of flag-shaped small rectangles of 1 cm × 1.5 cm × 0.2 cm. The electrodes were polished successively with 320, 400 and 600 grade silicon carbide papers, degreased with acetone and flushed with distilled water. A thin small neck was covered with paraffin wax. A conventional three-electrode cell was used
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 1 1 - X
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Fig. 1. Potential–time curves of Al–brass in 3.5% NaCl solution in absence and presence of different concentrations of Na2 S.
Fig. 2. Potential–time curves of Cu10Ni in 3.5% NaCl solution in absence and presence of different concentrations of Na2 S.
S.M. Sayed et al. / Materials Chemistry and Physics 78 (2003) 825–834
with a saturated calomel reference electrode and a platinum sheet counter electrode. The potential was controlled using a Wenking potentiostat L.T.73. Polarization curves were obtained by changing the potential at a rate of 20 mV min−1 . Measurements of weight changes were performed on rectangular coupons (2 cm × 5 cm × 0.2 cm). They were polished and cleaned as described above, dried and weighed (W1 ). They were allowed to corrode in aerated 3.5% NaCl with different concentrations of Na2 S (500, and 1000 ppm) for 30 days at 24 ± 1 ◦ C. At the end of a run, the specimens were rinsed with distilled water, dried and weighed (W2 ). The weight loss was determined from the relation W= W1 − W2 . The measurements of the weight changes were repeated twice for two alloys at time interval. The corroded surfaces of the tested alloys were inspected by scanning electron microscopy (SEM) using a JOEL, JSM-T 20 (Japan). Sodium sulfate was obtained from Riedel-de Haen. Sodium chloride was Analar grade and the water was doubly distilled. 2.1. X-ray diffraction analysis The corroded alloys surfaces were subjected to X-ray diffraction (X-ray diffractometer, Philips PW-1390 with a Cu-tube, Cu K␣1 , λ = 1.54051 cm).
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3. Results and discussion 3.1. Electrochemical measurements The potential–time measurements of both Al–brass and Cu10Ni alloys in 3.5% NaCl solutions containing different concentrations of Na2 S are shown in Figs. 1 and 2. The curves show in general that the steady-state potential is shifted to the more negative direction due to the presence of sulfide and the shift increases with the increase of sulfide concentration. The negative shift of the steady-state (st-st) potential in presence of sulfide ions indicates that the corrosion of the two alloys is under anodic control. Fig. 3 shows the steady-state potential values of Al–brass and Cu10Ni alloys in 3.5% NaCl solutions in absence and presence of different concentrations of Na2 S. The negative shift of the steady-state potential reveals that the reactivity of Al–brass in absence of sulfide is more than Cu10Ni and become almost equal in presence of 100 ppm sulfide. At higher concentrations of Na2 S (>100 ppm) the steady-state potential of Cu10Ni alloy becomes more negative than Al–brass alloy. This will be correlated with the corrosion rate measurements, as shown below. Potentiostatic polarization measurements for Al–brass and Cu10Ni alloys in 3.5% NaCl solution in absence and presence of different
Fig. 3. The steady-state potential values of Al–brass and Cu10Ni alloys in absence and presence of different concentrations of Na2 S.
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Fig. 4. Potentiostatic polarization curves of Al–brass in 3.5% NaCl in absence and presence of different concentrations of Na2 S.
concentrations of Na2 S were carried out. The curves obtained for Al–brass are shown in Fig. 4. All the curves have the same general trend except the curve pertaining to the test solution containing 1000 ppm sulfide. The anodic parts of these curves start from the free corrosion potential by a very fast and abrupt increase of current with increase of potential until a peak of maximum current is attained at ≈0 mV (SCE). After reaching the peak of the maximum current, the current drops to a minimum value at a potential value of ≈50 mV (SCE) and then the current increases gradually with the increase of potential to form a plateau of almost limited current. In case of test solution containing 1000 ppm sulfide, the curve starts at a considerably more negative free corrosion potential and shows a region of fluctuations in the current value covering a wide range of potential (from −900 to −400 mV (SCE)) before reaching the peak of the maximum current. Observation of the curves of Fig. 4 reveals that the presence of sulfide shifts the free corrosion potential and the first region of the anodic curves towards more negative values. Fig. 5 illustrates the potentiostatic polarization curves of Cu10Ni alloy in 3.5% NaCl in absence and presence of different concentrations of sodium sulfide. It is obvious that the presence of sulfide causes a considerable shift of the free corrosion potential towards more negative values. The curves representing the solution containing
500 and 1000 ppm sulfide ions demonstrates more than one small peak at different potentials before attaining the peak of the maximum current which occurs at the same potential (≈−50 mV (SCE)) in all curves. The curves of Figs. 4 and 5 indicate that the presence of sulfide ions increases the reactivity of both the two tested alloys in 3.5% NaCl solution, i.e. decreases the corrosion resistance of the alloys. It is clear that anodic corrosion current decreases slightly as the sulfide concentration increases as shown in Figs. 4 and 5. This is in agreement with the results have been reported by El-Domiaty and Alhajji [17]. These observations find interpretation in the following: (i) the effects of both chloride and sulfide ions depend on the electrode potential (which is a measure of the oxidizing power of the electrolyte in absence of potential control), (ii) the increase of the current may be attributed to the anodic oxidation of the sulfide ions. The anodic oxidation of sulfide ions is a complex process which might lead to the formation of polysulfide, elemental sulfur, thiosulfate, etc. according to the following reactions [18]: 3HS− → S3 2− + 3H+ + 4e− , E 0 = 0.097 V
(NHE)
(1)
HS− → S + H+ + 2e− , E 0 = −0.065 V
(NHE)
(2)
S.M. Sayed et al. / Materials Chemistry and Physics 78 (2003) 825–834
Fig. 5. Potentiostatic polarization curves Cu10Ni in 3.5% NaCl in absence and presence of different concentrations of Na2 S.
Fig. 6. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl solution.
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Fig. 7. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl + 500 ppm Na2 S.
2HS− + 3H2 O → S2 O3 2− + 8H+ + 8e− , E = 0.200 V 0
(NHE)
(3)
It has long been realized that the formation of elemental sulfur on the electrode surface leads to its passivation. Some of these results point to passivation of the electrode surface following the sudden increase in current. The current abruptly increases with potential at values near the ocp. At cathodic potentials to Ecorr , the current is a measure of the oxygen reduction reaction (cathodic reaction in aerated seawater). O2 + 2H2 O + 4e− → 4OH− (4) However, generally, the pollution of seawater with sulfide results in corrosion problems with Cu-base alloys due to the formation of Cu2 S. The formation of black cuprous sulfide (unstable) occurs via 2Cu + S2− → Cu2 S + 2e−
(5)
According to Tromans and Sun [19], Cu is also involved in this reaction: 2Cu + H2 O → Cu2 O + 2H+ + 2e−
(6)
A comparison of the polarization curves of Al–brass and Cu10Ni in 3.5% NaCl solution free of sulfide ions is shown
in Fig. 6. It is clear that the free corrosion potential and both the anodic and cathodic branches of the curve of Cu10Ni alloy are located at less negative potentials with respect to the curve of Al–brass. The peaks of the maximum current occur almost at the same potential value but the peak current is higher in case of Al–brass. In presence of 500 ppm sulfide (Fig. 7) a different situation is observed. In this case the free corrosion potential and both the anodic and cathodic parts of the polarization curve of Cu10Ni alloy are located at more negative potentials than those of Al–brass. Fig. 8 shows the polarization curves in presence of 1000 ppm sulfide ions. It can be observed from the curves that while the free corrosion potentials of the two alloys are nearly the same, the anodic branches of the curves demonstrate different behaviour. In the range from free corrosion potential to −200 mV, the curve of Al–brass alloy illustrates higher current values than that of Cu10Ni, while in the potential range from −200 mV to the end of the curves, Al–brass records lower current values. 3.2. Corrosion rate measurements The corrosion rate measurements of Al–brass and Cu10Ni alloys in 3.4% NaCl in absence and presence of sulfide ions
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Fig. 8. Potentiostatic polarization curves Cu10Ni and Al–brass alloys in 3.5% NaCl + 1000 ppm Na2 S.
Fig. 9. Effect of sulfide concentrations on the weight variations of Cu10Ni and Al–brass in 3.5% NaCl for 30 days.
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Table 1 The corrosion rate and steady-state (st-st) potential values of Cu10Ni and Al–brass alloys in 3.5% NaCl in absence and presence of sulfide ion. Sulfide concentration (ppm)
0 500 1000
Cu10Ni
Al–brass
st-st potential mV (SCE)
Corrosion rate (mpy)
st-st potential mV (SCE)
Corrosion rate (mpy)
−217 −572 −807
1.82 5.91 7.73
−256 −433 −398
2.3 4.22 5.86
(500, and 1000 ppm) was calculated from the polarization curves. Table 1 illustrates a comparison between steady-state potential values and corrosion rate values. It is obvious that as the sulfide concentration increases, the corrosion rate increases. Pickering and coworkers [11] suggested that the higher corrosion rates of Cu-9.4Ni-1.7Fe in sulfide contaminated 3.4% NaCl solution are due to a highly defective Cu2 O layer containing Cu2 S which permit rapid ionic and electronic transport through it. The Cu2 S is less protective than Cu2 O.
Fig. 10. SEM micrographs of the corroded surfaces in 3.5% NaCl after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
3.3. Weight variations and metallographic investigations The change of weight was determined for Al–brass and Cu10Ni alloys in 3.4% NaCl in absence and presence of sulfide ions (500, and 1000 ppm) after immersion for 30 days. The average changes in weights are shown in Fig. 9. It is obvious from Fig. 9 that in absence of sulfide the Cu10Ni has showed a large amount of weight gain and the appearance
Fig. 11. SEM micrographs of the corroded surfaces in 3.5% NaCl + 500 ppm sulfide ion after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
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of an adhesive green film on the surface of the specimens, while Al–brass alloy recorded a very small amount of loss in weight and the appearance of the corroded surface still bright yellow without any corrosion products on it. Examination of the corroded surfaces by SEM revealed that in case of Cu10Ni alloy a dark colored film covered with porous light green precipitates of corrosion products are formed as shown in Fig. 10a. In case of Al–brass (Fig. 10b) the surface of the specimens still bright but localized corrosion in the form of a fine dark spots appear among the scratches. In presence of 500 ppm sulfide ions, the Cu10Ni showed a considerable amount of weight loss while Al–brass recorded amount of weight-gain. After washing by brushing under running water to remove the loose corrosion products, the Cu10Ni alloy demonstrated the same original color without brightness. The Al–brass alloy showed the appearance of a brown adhesive film which could not be removed by washing. The micrograph of Fig. 11a shows that the surface of Cu10Ni alloy have some roughness and charac-
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terized in some locations by the occurrence of pits filled with corrosion products in the form of flakes suffering from many cracks as shown in Fig. 11b. Moreover, small area of intergranular corrosion appears beneath the cracked location of corrosion product layer. In case of the test solution containing 1000 ppm sulfide, the two alloys recorded weight loss, but the corrosion of Cu10Ni is higher than that of Al–brass as shown in Fig. 9. The SEM micrograph of Fig. 12a shows that the Cu10Ni alloy suffer from general as well as pitting corrosion while Al–brass show the occurrence of general corrosion in the form of a thick brown layer of corrosion products and intergranular corrosion appears beneath this layer at the cracked locations as shown in Fig. 12b. 3.4. X-ray diffraction analysis The X-ray measurements revealed significant findings: (i) The corrosion products which form on Cu10Ni under free corrosion conditions in the test solution containing sulfide ions are Cu2 O, Cu2 (OH)3 Cl, Cu2 S. (ii) CuCl forms only at anodic potentials, considerably higher than the free corrosion potential. The corrosion products for the corroded Al–brass surfaces could not be identified by X-ray diffraction technique, this might be the case if the corrosion products were in the form of a film that is too thin to be detected by the X-ray.
4. Conclusions
Fig. 12. SEM micrographs of the corroded surfaces in 3.5% NaCl + 1000 ppm sulfide ion after immersion for 30 days for (a) Cu10Ni, (b) Al–brass.
1. The steady-state potentials for the Cu10Ni and Al–brass are shifted towards more active values by the effect of sulfide ions, but the steady-state potential shift in case of Cu10Ni is more than that of Al–brass. 2. The potentiostatic polarization measurements indicated that the presence of sulfide ions increases the reactivity of both the two tested alloys in 3.5% NaCl solution, i.e. decreases the corrosion resistance of the alloys. 3. Cu10Ni alloy showed a high weight gain while Al–brass has a few amount of weight loss in 3.5% NaCl. 4. The weight loss of Cu10Ni in 3.5% NaCl + 1000 ppm Na2 S was larger about eight times than Al–brass. 5. The metallographic SEM examinations indicated the occurrence of general corrosion of both alloys. However, pitting corrosion in case of Cu10Ni and intergranular corrosion in case of Al–brass were observed in presence of sulfide ions. 6. The results indicated that the Cu10Ni alloy demonstrated better corrosion resistance in 3.5% NaCl solution but in high concentrations of sulfide ions polluted salt water, Cu10Ni becomes less corrosion resistance than Al–brass. 7. The formation of Cu2 S interferes with the protecting oxide film and reduces its corrosion resistance.
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References [1] E.G. West, Copper and Its Alloys, Ellis Horwood, New York, 1982, p. 155. [2] R.B. Ross, Metallic Materials Specification Handbook, 3rd. ed., E. and F.N. Spon, London, 1968, p. 139. [3] C. Manfredi, S. Simison, S.R. de Sanchez, Corrosion 43 (1987) 458. [4] J.P. Gudas, H.P. Hack, Corrosion 35 (1979) 67. [5] L.E. Eiselstein, B.C. Syrett, S.S. Wing, R.D. Caligiuri, Corros. Sci. 23 (1983) 223. [6] B.C. Syrett, Corrosion 33 (1977) 257. [7] B.C. Syrett, Corros. Sci. 21 (1981) 187. [8] J.N. Alhajji, M.R. Reda, J. Electrochem. Soc. 142 (1995) 2944. [9] K. Habib, A. Amin, Desalination 85 (1992) 275.
[10] D.R. Lenard, J.G. Moores, G.E. Morin, Br. Corros. J. 24 (1989) 19. [11] C. Kato, H.W. Pickering, J.E. Castle, J. Electrochem. Soc. 131 (1984) 1225. [12] J.N. Alhajji, M.R. Reda, J. Electrochem. Soc. 141 (1994) 1432. [13] F.P. Ijsseltng, Br. Corros. J. 24 (1989) 55. [14] B.C. Syrett, Corrosion, vol. 33, Chicago, IL, 1980, p. 80. [15] T.S. Lee, H.P. Hack, D.G. Tipton, in: Proceedings of the 5th International Congress on Marine Corrosion and Fouling, 1980. [16] B.C. Syrett, Corrosion 33 (1977) 275. [17] A. El-Domiaty, J.N. Alhajji, J. Mater. Eng. Perform. 6 (1997) 534. [18] G. Valensi, J. van Muylder, M. Pourbaix, in: M. Pourbaix (Ed.), Proceedings of the Atlas of Electrochemical Equilibria in Aqueous Media, NACE, Houston, TX, 1974, p. 545. [19] D. Tromans, R.H. Sun, J. Electrochem. Soc. 138 (1991) 3235.