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dry cyclic conditions
Corrosion Science 47 (2005) 1370–1383 www.elsevier.com/locate/corsci
Atmospheric corrosion of copper under wet/dry cyclic conditions Gamal A. EL-Mahdy
*
Department of Metallurgical System Engineering, Yonsei University, 134-Shinchon-dong, Seodaemun-Ku, Seoul, 120-749, South Korea Received 3 June 2003; accepted 21 July 2004 Available online 14 October 2004
Abstract The polarization resistance of copper subjected to NaCl and an ammonium sulfate solution under wet/dry cycling conditions was monitored using an EIS impedance technique. The copper samples were exposed to 1 h of immersion using different solutions of pH, temperature and surface orientation and 7 h of drying. The copper plates corroded more substantially on the skyward side than those for a ground ward side. The degree of protection copper oxide provides decrease in an acidic medium (pH 4) more than in a neutral medium (pH 7). The corrosion rate of copper increases rapidly during the initial stages of exposure then decreases slowly and eventually attains the steady state during the last stages of exposure. The corrosion products were analyzed using X-ray diffraction. The corrosion mechanism for copper studied under wet/dry cyclic conditions was found to proceed under the dissolution–precipitation mechanism. 2004 Elsevier Ltd. All rights reserved. Keywords: Copper; EIS; Atmospheric corrosion; XRD
1. Introduction The atmospheric corrosion of copper and its alloys has become a subject of great interest due to numerous applications in related structural, architectural, electrical * Permanent address: Chemistry Department, Faculty of Science, Helwan University, Ain-Helwan, Cairo, Egypt.
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.07.034
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
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and electronics usage. The effect of NaCl in combination with O3 on the atmospheric corrosion of copper was investigated [1]. The corrosion products formed after four weeks of exposure were characterized qualitatively by using X-ray diffraction and quantitatively evaluated by gravimetric and ion chromatography measurements of the leaching solution. The corrosive effect of NaCl was strong in purely humid air and in air containing O3 or SO2. The corrosion rate was proportional to the amount of chloride added and increased along with relative humidity. The corrosion of copper studied under various chloride conditions [2–5] resulted generally in reaction products such as cuprous, cupric oxides, cuprous hydroxide and cuprous chloride. Recently Chimielova et al. [6] identified the corrosion products precipitated on copper surface after four days treatment in sodium chloride, sodium/magnesium and sodium/calcium chloride solutions while using a X-ray diffraction powder analysis. The corrosion products identified were Cu2(OH)3Cl, Cu2O and CuCl2. The effect of submicron (NH4)2SO4 particles on the atmospheric corrosion of copper was investigated at varying relative humidities and temperatures (300 and 373 K) [7–9]. Laboratory simulations have shown that (NH4)2SO4 particles led to the corrosion products found in natural patinas. Corrosion under condensed water and cyclic wet and dry conditions pose significant problems for many applications, such as building and bridges, which are strongly dependent on the duration and conditions of daily dry and wet cycles. The presence of NaCl in a marine environment has a strong corrosive effect toward copper under thin electrolyte layers and in alternating wet/dry cyclic conditions. The application of conventional electrochemical techniques used for monitoring the atmospheric corrosion of metals is quite difficult due to the presence of thin electrolyte layers created during the drying process. A high solution resistance and a nonuniform current distribution over the working electrode are the main limitations observed for the inapplicability. It has been established that AC impedance technique was successively applied to monitor the corrosion rate of the steel [10,11], coated steel [12,13] and metals [14–16] under wet/dry cyclic conditions and with thin electrolyte layers. In all of these studies, a two-electrode cell configuration was employed. There have no been studies conducted on the atmospheric corrosion of copper under wet/dry cyclic conditions. The aim of the work is to monitor the corrosion rate of copper during cyclic wet/dry conditions and in a chloride containing environment together with the variations of surface orientation, temperature and pH of solution. The work will also extend to investigate the effects of an ammonium sulfate solution as well as to suggest a mechanism for copper corrosion experienced under wet/dry cyclic conditions.
2. Experimental procedure 2.1. Material and electrode preparation Two-electrode cell configuration were fabricated from copper sheet (99.99) 10 mm (width) · 10 mm (length) and embedded parallel in an epoxy resin with 0.1 mm
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 40 mm 0.5 mm Peripheral bank Electrodes
0.1mm
10 mm
0.5 mm 10 mm
Electrodes
Epoxy
Top View
Wire for connection
Transverse cross-sectional View
Fig. 1. Schematic diagram of the cell used in an AC impedance monitoring experiment conducted under wet/dry cyclic conditions.
separation as shown in Fig. 1. The electrode was polished on SiC-paper in ethanol to 1000 mesh, then washed in ethanol using an ultrasonic agitation. The sample was put in a desiccator until the start of the exposure process. 2.2. Atmospheric exposure The cell containing the two electrodes was fixed horizontally on an acrylic vessel with dimensions of 8 cm · 8 cm · 8 cm with the aid of a water leveler. In all experiments, the cell was placed with the metal surface facing upwards except for the specimen, which was oriented in a downward direction and placed inside a temperature–humidity controlled chamber. The cell was subjected to 1 h of immersion in NaCl or ammonium sulfate solution. The test solution was introduced from an external tank through an inflow magnetic valve programmed to deliver a pre-determined volume used to cover the test specimen. The solution was drained through an outflow magnetic valve after 1 h of immersion and the solution on the surface was left to dry for 7 h at 60% RH and 303 K. The volume of the electrolyte used during immersion period was the same in all experiments (1 h of immersion). Both the inflow and the outflow magnetic valves were controlled by using a cyclic on–off timer control. The wet/dry cycles were conducted for a total period of 96 h. To ensure the constancy of electrolyte layer thickness at the onset of each dry cycle, a peripheral bank whose thickness is 0.5 mm (Mylar sheet) was fixed on top of the electrode surface as shown in Fig. 1. This provides an initial thickness of the electrolyte layer of about 0.5 mm in all experiments remained during the drying process except for the specimen oriented in the downward direction, because there is no electrolyte re-
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mained over the specimen during drying owing to the surface orientation. The only wetting period the specimen experiences is the first hour of immersion during exposure of the downward oriented specimen. 2.3. Monitoring of the polarization resistance The impedance measurement was conducted at 10 kHz (ZH) and 10 mHz (ZL) using a Solartron 1280, which was controlled by using a computer through GPIB interface. The polarization resistance Rp was determined by subtracting the high frequency impedance measured at 10 kHz (ZH) from the low frequency impedance measured at 10 mHz (ZL). Two-electrode cell configuration was used for monitoring the polarization resistance. The reciprocal value of the polarization resistance was proportional to the corrosion rate [10–16]. 2.4. Material characterization The corrosion products were analyzed by using a Shimadzu (Japan) XD-3A dif˚ ). The fractometer measured with CuKa monochromatic radiation (k = 1.5405 A measurement was conducted in a step scanning mode (2/min) from h = 30 to h = 80. X-ray diffraction data was taken directly from the surface of the copper after exposure for 96 h under wet/dry cyclic conditions.
3. Results and discussions 3.1. Exposure at different surface orientation The polarization resistance (Rp) was monitored under 1 h of immersion and 7 h of drying in 0.01 M NaCl solution at 60% RH and 303 K temperature for samples exposed to both skyward and downward directions. The corrosion rate is proportional to the reciprocal of polarization resistance and can be calculated using the Stern– Geary equation [17]: Corrosion rate ¼ K=Rp ;
ð1Þ
K ¼ ðba bc Þ=2:303ðba þ bc Þ;
ð2Þ
where ba and bc represent anodic and cathodic Tafel slope, respectively. The value of K is a function of metal and electrolyte [12,15] and can be assumed to be constant for a given metal/electrolyte system. Therefore, the average of the reciprocal of polarization resistance per cycle (ARPR) was evaluated to represent the average corrosion rate per cycle and calculated from 1/Rp according to the following equation:
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
Average of reciprocal of polarization resistance per cycle ðARPRÞ Z t ¼ 1=Rp dt;
ð3Þ
0
where t is the time of wetness (TOW) per cycle and 1/Rp is the reciprocal polarization resistance. The time of wetness (TOW), which can be estimated from using the 1/Rp data versus exposure time as the total elapsed time, from the wetting period to the time just before the completion of surface drying (1/Rp dropped to a value of zero during complete drying of surface). Fig. 2 shows that the reciprocal of polarization resistance (1/Rp) which is proportional to the corrosion rate [10–16], increases with the immersion time during the initial stage then attains a steady state during the last stage of exposure. Fig. 3 shows that ARPR, of the skyward-oriented sample is higher than those for a downwardoriented sample. The ARPR of the skyward-oriented sample, increases gradually and reaches its maximum during the eighth cycle then attained a steady state during the subsequent four cycles. In the case of downward-oriented sample, the ARPR increases slowly and reaches its maximum during the third cycle and attains a steady
0.00018 Facing downward
0.00015 0.00012 0.00009
1/Rp, Ω-1 cm-2
0.00006 0.00003 0.00000 0.00018 Facing upward
0.00015 0.00012 0.00009 0.00006 0.00003 0.00000 0
24
48
72
96
Exposure time (hour) Fig. 2. Changes of 1/Rp with exposure time of a copper sample facing in both a downward and upward direction measured under wet/dry cyclic conditions in 0.01 M NaCl solution.
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
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0.0030 upward downward
0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
2
4
6
8
10
12
14
16
Exposure wet/dry cycle
Fig. 3. Variation of average corrosion charge with cycle number of the copper sample facing downward and upward direction.
state during the subsequent six cycles. This result can be attributed to the difference in the time of wetness (tTOW) which is 4 and 1 h per cycle for both skyward and downward sample, respectively. The amount of electrolyte retained on the surface of the skyward sample during the drying process stimulates the corrosion process occurring under thin electrolyte layers. The electrolyte evaporates and concentrates during thinning process leading to a shorter path for oxygen diffusion, which is the rate-determining step in the corrosion process under thin electrolyte layers. In the groundward sample the corrosion process occurs only during the immersion period of each cycle. The surface is passivated and lowers the corrosion process during the subsequent cycles. One part of the corrosion products was washed away from the surface during the next cycle and the other retained it. The steady state observed during the last stages of copper corrosion under wet/dry cycling conditions was attributed to the entire covering of the surface with corrosion products, which inhibits the continuity of the corrosion process. X-ray diffraction indicated the presence of Cu2O, Cu2(OH)3Cl and Cu2Cl as corrosion products. The main constituent corrosion product in case of groundward sample is Cu2O. 3.2. Influence of temperature The effect of temperature was investigated in 0.05 M NaCl solution at two different temperatures (293 and 313 K) at 60% RH under 1 h of immersion time and during 7 h of drying as shown in Fig. 4. The sample exposed was facing upward during all investigations. The reciprocal of polarization resistance at 313 K is higher than that at 293 K. The 1/Rp increases with exposure time then decreases during the last stages at both temperatures. The calculated ARPR vs. cycle number is presented in Fig. 5. The differences in behavior at high and low temperature may be attributed to the formation of more protective corrosion products at low temperature. The results
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 0.0030 T = 293 K
0.0025 0.0020 0.0015
1/Rp, Ω-1 cm-2
0.0010 0.0005 0.0000 0.0030 T = 313 K 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
24
48
72
96
Exposure time (hour)
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
Fig. 4. Monitoring results of 1/Rp vs. exposure time for copper measured under wet/dry cyclic conditions in a 0.05 M NaCl at 60% RH, and at two different temperatures.
0.040 0.035
T = 313 K T = 293 K
0.030 0.025 0.020 0.015 0.010 0.005 0.000 0
2
4
6
8
10
12
Exposure wet / dry cycle
Fig. 5. Influence of temperature on the average corrosion charge of copper in a 0.05 M NaCl solution.
may be explained on the basis of diffusion process of oxygen, which was enhanced at a higher temperature during the drying process under thin electrolyte layers when
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observed with the rising of temperature, while the reduction of oxygen is a rate-determining step in the corrosion process under thin electrolyte. Hence, the corrosion rate of copper increases with an increase in temperature. In addition, the dissolution of the protective oxide followed by copper metal loss is enhanced by an increase in temperature during the immersion period (anodically control in bulk solution) which leads to an increase in the observed corrosion rate. The cuprite formed at the lower temperature was more protective than that measured at 313 K. It seems that the property of cuprite changed to become less protective during exposure experienced at high temperature. Thus creating a greater tendency for ion diffusion into the corrosion layer. NaCl causes a breakdown of the passive film (mainly cuprite) due to the formation of soluble species such as CuCl2. The average reciprocal of polarization resistance observed at both temperature increases and together with the cycle number, reaches its maximum during the third cycle and attains a steady state during the last seven cycles with different steady state values. The compounds Cu2O and Cu2(OH)3Cl and Cu2Cl were identified as corrosion products by using X-ray diffraction. It is clear that the ARPR presented in Fig. 3 (0.01 M NaCl) increases gradually as the exposure cycle increases then eventually attains the steady state during the ninth cycle. However, in Fig. 5 (0.05 M NaCl) ARPR increases rapidly to a maximum value in the third cycle then decreases slowly until reaches its steady state during the subsequent sixth cycle. The difference in corrosion behavior can be attributed to the difference in the amount of corrosion products precipitated over the entire copper surface. In Fig. 3, while the concentration is lower than in Fig. 5, the corrosion rate proceeds slowly and the amount of corrosion products partially covered the copper surface until it reaches a steady state during the ninth cycle, while the surface is entirely covered with corrosion products. In Fig. 5 the corrosion rate increases rapidly due to a higher concentration of NaCl which leads to an increase in the amount of corrosion products until it attains a maximum in the third cycle. The observed decrease in the corrosion rate measured during fourth and fifth cycles can be attributed to the dissolution process of some precipitated corrosion products, which are loosely bound to the copper surface. The ARPR reaches its steady state during the sixth cycle due to the entire covering of the surface with the reformation of corrosion products. It can be concluded that the higher the concentration of NaCl, the faster the steady state occurs due to the larger amount of corrosion products precipitated over the copper surface. 3.3. Exposure at different pH To study the influence of pH on the atmospheric corrosion of copper, two experiments were conducted in 0.025 M NaCl solution. The copper was subjected to 1 h of immersion and 7 h of drying at 298 K and 60% RH. Both samples were exposed facing upward. The data depicted in Fig. 6 indicated that the overall amount of copper corrosion increases with the lowering of the solution pH. The ARPR per cycle was calculated and presented in Fig. 7. The values of ARPR, at low pH are higher than those at pH 7. The results imply that pH is an important parameter to be considered
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 0.00040 pH = 7 0.00032
0.00024
0.00016
1/Rp, Ω-1 cm-2
0.00008
0.00000 0.00040 pH = 4 0.00032
0.00024
0.00016
0.00008
0.00000 0
24
48
72
96
Exposure time (hour)
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
Fig. 6. Variation of 1/Rp with exposure time for copper subjected to 0.025 M NaCl at two different pH solutions under wet/dry cyclic conditions.
0.0040 0.0035
pH = 4 pH = 7
0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
2
4
6
8
10
12
14
16
Exposure wet / dry cycle
Fig. 7. Variation of average corrosion charge with exposure cycle number measured at two different pH solutions.
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for Cu corrosion under wet/dry cycling conditions. Hydrogen ions can promote Cu corrosion as described in the equation below: 2Cu þ 4Hþ þ O2 ¼ 2Cu2þ þ 2H2 O
ð4Þ
At pH 7 a possible passivation process on the copper surface may decrease the corrosion rate. The morphology of the deposited corrosion products was distributed heterogeneously, showing some paths through which copper ions released and chloride ions diffused. The corrosion rates of copper exposed to acidic chloride solution are initially higher than those observed in the neutral solution and decrease with increasing exposure cycles. The ions of Cu(I) are oxidized by atmospheric oxygen into Cu(II) and the latter ion plays an important role in copper corrosion. Through hydrolysis of the Cu(II) ions, basic copper chloride is formed in a neutral medium (paratacamite) 4Cu2þ þ 2Cl þ 6H2 O ¼ Cu4 Cl2 ðOHÞ6 þ 6OH
ð5Þ
The main corrosion products identified by using XRD-data after 96 h exposure was cuprite, paratacamite, and clinoatacamite (Cu2(OH)3Cl) in neutral and natokite (CuCl) in acidic solution. Copper surface formed a protective passive film of cuprite in the neutral medium [18,19], however in the acidic medium cuprite was not stable. 3.4. Corrosion behavior of copper in ammonium sulfate solution The polarization resistance was conducted with exposure time in 0.05 M ammonium sulfate solution, under wet/dry cyclic conditions at 303 K and 60% RH as shown in Fig. 8. The surface of copper was exposed facing upward. The ARPR values were calculated using Eq. (1) and traced versus the cycle number as displayed in Fig. 9. The ARPR increases during the initial stages due to the dissolution of copper as Cu(NH3)2+ and attains its maximum value during the third cycle. The ARPR
0.0020
1/Rp, Ω-1 cm-2
0.0015
0.0010
0.0005
0.0000 0
24
48
72
96
Exposure time (hour) Fig. 8. Changes in the 1/Rp with time for copper exposed to alternating conditions, of wet/dry cyclic conditions and in a 0.05 M ammonium sulfate solution at 303 K and 60% RH.
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
1380
0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0
2
4
6
8
10
12
14
16
Exposure wet/dry cycle
Fig. 9. Plots of the average corrosion charge for copper with exposure cycle number and in an ammonium sulfate solution conducted at 303 K and 60% RH.
starts to decrease slowly reaching a steady state during the subsequent five cycles. Cuprite is the major corrosion product formed in the early stages of atmospheric corrosion of copper occurring in a natural environment. The basic copper sulfate Cu3(SO4)(OH)4 (antertlerite) and Cu4(SO4)(OH)6 (bronchatite) compounds formed during the latter stages of corrosion as revealed from X-ray diffraction analysis. Fig. 10 represents the magnified plot of the second cycle displayed in Fig. 9 and can be divided into three distinct regions. Experimentation indicates that the corrosion rate is high during the onset of region I (1 h of immersion) due to dissolution of the precipitated ammonium sulfate formed in the preceding cycle and during the drying process of the first cycle, leading to an increase in the concentration of ammonium sulfate, which results in an increase in the corrosion rate in region I. The
0.0036
Time of wetness
Onset of drying period
0.0018 0.0012 0.0006
III
II
I
Complete dryness of surface
0.0024
1-hr immersion
1/Rp, Ω-1 cm-2
0.0030
0.0000 8
9
10
11
12
13
14
15
16
Exposure time (hour) Fig. 10. Monitoring results for a single wet/dry cycle (second cycle in Fig. 9) showing the variations of 1/Rp versus the exposure time.
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observed decrease in the corrosion rate within the region I was attributed to the passivation of the copper surface with corrosion products. The overall corrosion rate increases gradually as the drying time progresses in region II due to an increase in the ammonium sulfate concentration as a result of electrolyte evaporation. In addition, the diffusion of oxygen increases as the thickness of electrolyte decreases as the drying time increases, leading to an increase in the corrosion rate under thin electrolyte layers. The sharp decrease in the corrosion rate during the end of region II was attributed to the complete drying of the surface and 1/Rp dropped to a value of zero. There was no corrosion process during region III as a result of complete the dryness of the surface. Fig. 10 shows an estimation of the time of wetness (TOW), measured from 1/Rp data versus exposure time as the total elapsed time, from the wetting period (region I) to the time just before the complete surface drying period and at the end of region II. 3.5. Mechanism of copper corrosion The mechanism of copper corrosion experienced under wet/dry cyclic conditions seems to proceed under the dissolution–precipitation mechanism. The first step is the dissolution process of the air-formed oxide film followed by the dissolution of copper leads to a high corrosion rate observed during the initial stages of exposure. The dissolution process proceeds under a wet period (1 h of immersion) and continues under the thin electrolyte layers, and during drying period. The dissolution process increases at high temperature and low pH due to the enhancement of the diffusion process of chloride ions, which causes a breakdown of the film formed on the copper surface. The breakdown of the film is also enhanced at low pH = 4 due to the dissolution of the passive film and formation of copper (1) chloride, which is described as follows: 1=2Cu2 O þ 2Cl þ Hþ ! CuCl2 þ 1=2H2 O
ð6Þ
A small fraction of the corrosion products is retained on the copper surface during the initial exposure. During the immersion period, one part of the corrosion products formed on freely exposed copper will remain on the surface whereas another part will be released and washed off. The degree of protection depends on the stability of the formed corrosion products. During the last stages of exposure the amount of corrosion products and the degree of protection increases concurrently with exposure time due to the precipitation of thick corrosion products. Hence, the corrosion rates decrease and approach a steady state value. Decreasing the corrosion rates along with exposure time was attributed to the continuous build up of protective corrosion products on the copper surface. In an ammonium sulfate solution, the rate of corrosion is high during the initial stages, then decreases slowly and attains a steady state during the last stages of exposure. The corrosion rate increases during the initial stages of exposure due to the formation CuðNH3 Þþ 2 then decreases during the final stages of corrosion, due to the formation of insoluble basic copper sulfate, antlerite, brochantite and posnjakite.
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The cathodic reaction occurring during the atmospheric corrosion of copper is the reduction of oxygen: O2 þ 2H2 O þ 4e ¼ 4OH
ð7Þ
4. Conclusions From the results observed during monitoring the polarization resistance of copper under wet/dry cyclic conditions, in chloride and ammonium sulfate containing environments, the following conclusions were drawn: 1. The application of AC impedance and the experimental set up used in this investigation have proven to be a successful tool as well as a useful arrangement when used for monitoring the atmospheric corrosion of copper under periodic wet/dry conditions. 2. The presence of the thin electrolyte layers during the drying period enhances the corrosion process of copper in the upward-oriented sample and the corrosion rates are higher than those of downward-oriented specimen. 3. Exposure of copper tested in a marine environment indicated that NaCl has a strong effect on copper corrosion. The mechanism of atmospheric corrosion of copper under wet/dry cyclic conditions proceeds under dissolution–precipitation mechanism. The corrosion rate of copper increases rapidly during the initial stages of exposure then decreases slowly and eventually attains a steady state during the last stages of exposure. 4. The inhibitive action of copper-oxide decreased more gradually in an acidic medium (pH 4) than in a neutral medium.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
H. Strandberg, G. Johansson, J. Electrochem. Soc. 145 (4) (1998) 1093. H.P. Lee, K. Nobe, A.J. Pearlstien, J. Electrochem. Soc. 132 (5) (1985) 1031. C.H. Bonfigilo, H.C. Albava, O.A. Cobo, Corros. Sci. 13 (1973) 717. H.P. Lee, K. Nobe, J. Electrochem. Soc. 133 (10) (1986) 2035. F. King, C.D. Litke, M.J. Quinn, D.M. LeNeveu, Corros. Sci. 37 (5) (1995) 833. M. Chimielova, J. Seidlerova, Z. Weiss, Corros. Sci. 45 (5) (2003) 883. R.E. Lobnig, R.P. Frankenthal, D.J. Siconolfi, J.D. Sinclair, J. Electrochem. Soc. 140 (7) (1993) 1902. R.E. Lobnig, R.P. Frankenthal, D.J. Siconolfi, J.D. Sinclair, M. Stratmann, J. Electrochem. Soc. 141 (11) (1994) 2935. R.E. Lobnig, C.A. Jankaski, J. Electrochem. Soc. 145 (3) (1998) 946. A. Nishikata, T. Takahashi, H. Bao-rong, T. Tsuru, Zairo Kankyo 43 (1994) 225. A. Nishikata, Y. Yamashita, H. Katayama, T. Tsuru, A. Usami, T. Tanabe, Corros. Sci. 37 (1995) 2059. G.A. EL-Mahdy, A. Nishikata, T. Tsuru, Corros. Sci. 42 (2000) 183. G.A. EL-Mahdy, A. Nishikata, T. Tsuru, Corros. Sci. 42 (2000) 1509. A. Nishikata, S. Kumagai, T. Tsuru, Corros. Sci. 43 (1994) 109.
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G.A. EL-Mahdy, Corrosion 59 (2003) 505. G.A. EL-Mahdy, Kwang B. Kim, Electrochim. Acta 49 (2004) 1937. M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, Pergamon, New York, 1966, p. 388. [19] J.D. Narin, S.G. Skennerton, A. Atrens, J. Mater. Sci. 38 (2003) 995.
Atmospheric corrosion of copper under wet/dry cyclic conditions Gamal A. EL-Mahdy
*
Department of Metallurgical System Engineering, Yonsei University, 134-Shinchon-dong, Seodaemun-Ku, Seoul, 120-749, South Korea Received 3 June 2003; accepted 21 July 2004 Available online 14 October 2004
Abstract The polarization resistance of copper subjected to NaCl and an ammonium sulfate solution under wet/dry cycling conditions was monitored using an EIS impedance technique. The copper samples were exposed to 1 h of immersion using different solutions of pH, temperature and surface orientation and 7 h of drying. The copper plates corroded more substantially on the skyward side than those for a ground ward side. The degree of protection copper oxide provides decrease in an acidic medium (pH 4) more than in a neutral medium (pH 7). The corrosion rate of copper increases rapidly during the initial stages of exposure then decreases slowly and eventually attains the steady state during the last stages of exposure. The corrosion products were analyzed using X-ray diffraction. The corrosion mechanism for copper studied under wet/dry cyclic conditions was found to proceed under the dissolution–precipitation mechanism. 2004 Elsevier Ltd. All rights reserved. Keywords: Copper; EIS; Atmospheric corrosion; XRD
1. Introduction The atmospheric corrosion of copper and its alloys has become a subject of great interest due to numerous applications in related structural, architectural, electrical * Permanent address: Chemistry Department, Faculty of Science, Helwan University, Ain-Helwan, Cairo, Egypt.
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.07.034
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
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and electronics usage. The effect of NaCl in combination with O3 on the atmospheric corrosion of copper was investigated [1]. The corrosion products formed after four weeks of exposure were characterized qualitatively by using X-ray diffraction and quantitatively evaluated by gravimetric and ion chromatography measurements of the leaching solution. The corrosive effect of NaCl was strong in purely humid air and in air containing O3 or SO2. The corrosion rate was proportional to the amount of chloride added and increased along with relative humidity. The corrosion of copper studied under various chloride conditions [2–5] resulted generally in reaction products such as cuprous, cupric oxides, cuprous hydroxide and cuprous chloride. Recently Chimielova et al. [6] identified the corrosion products precipitated on copper surface after four days treatment in sodium chloride, sodium/magnesium and sodium/calcium chloride solutions while using a X-ray diffraction powder analysis. The corrosion products identified were Cu2(OH)3Cl, Cu2O and CuCl2. The effect of submicron (NH4)2SO4 particles on the atmospheric corrosion of copper was investigated at varying relative humidities and temperatures (300 and 373 K) [7–9]. Laboratory simulations have shown that (NH4)2SO4 particles led to the corrosion products found in natural patinas. Corrosion under condensed water and cyclic wet and dry conditions pose significant problems for many applications, such as building and bridges, which are strongly dependent on the duration and conditions of daily dry and wet cycles. The presence of NaCl in a marine environment has a strong corrosive effect toward copper under thin electrolyte layers and in alternating wet/dry cyclic conditions. The application of conventional electrochemical techniques used for monitoring the atmospheric corrosion of metals is quite difficult due to the presence of thin electrolyte layers created during the drying process. A high solution resistance and a nonuniform current distribution over the working electrode are the main limitations observed for the inapplicability. It has been established that AC impedance technique was successively applied to monitor the corrosion rate of the steel [10,11], coated steel [12,13] and metals [14–16] under wet/dry cyclic conditions and with thin electrolyte layers. In all of these studies, a two-electrode cell configuration was employed. There have no been studies conducted on the atmospheric corrosion of copper under wet/dry cyclic conditions. The aim of the work is to monitor the corrosion rate of copper during cyclic wet/dry conditions and in a chloride containing environment together with the variations of surface orientation, temperature and pH of solution. The work will also extend to investigate the effects of an ammonium sulfate solution as well as to suggest a mechanism for copper corrosion experienced under wet/dry cyclic conditions.
2. Experimental procedure 2.1. Material and electrode preparation Two-electrode cell configuration were fabricated from copper sheet (99.99) 10 mm (width) · 10 mm (length) and embedded parallel in an epoxy resin with 0.1 mm
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 40 mm 0.5 mm Peripheral bank Electrodes
0.1mm
10 mm
0.5 mm 10 mm
Electrodes
Epoxy
Top View
Wire for connection
Transverse cross-sectional View
Fig. 1. Schematic diagram of the cell used in an AC impedance monitoring experiment conducted under wet/dry cyclic conditions.
separation as shown in Fig. 1. The electrode was polished on SiC-paper in ethanol to 1000 mesh, then washed in ethanol using an ultrasonic agitation. The sample was put in a desiccator until the start of the exposure process. 2.2. Atmospheric exposure The cell containing the two electrodes was fixed horizontally on an acrylic vessel with dimensions of 8 cm · 8 cm · 8 cm with the aid of a water leveler. In all experiments, the cell was placed with the metal surface facing upwards except for the specimen, which was oriented in a downward direction and placed inside a temperature–humidity controlled chamber. The cell was subjected to 1 h of immersion in NaCl or ammonium sulfate solution. The test solution was introduced from an external tank through an inflow magnetic valve programmed to deliver a pre-determined volume used to cover the test specimen. The solution was drained through an outflow magnetic valve after 1 h of immersion and the solution on the surface was left to dry for 7 h at 60% RH and 303 K. The volume of the electrolyte used during immersion period was the same in all experiments (1 h of immersion). Both the inflow and the outflow magnetic valves were controlled by using a cyclic on–off timer control. The wet/dry cycles were conducted for a total period of 96 h. To ensure the constancy of electrolyte layer thickness at the onset of each dry cycle, a peripheral bank whose thickness is 0.5 mm (Mylar sheet) was fixed on top of the electrode surface as shown in Fig. 1. This provides an initial thickness of the electrolyte layer of about 0.5 mm in all experiments remained during the drying process except for the specimen oriented in the downward direction, because there is no electrolyte re-
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
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mained over the specimen during drying owing to the surface orientation. The only wetting period the specimen experiences is the first hour of immersion during exposure of the downward oriented specimen. 2.3. Monitoring of the polarization resistance The impedance measurement was conducted at 10 kHz (ZH) and 10 mHz (ZL) using a Solartron 1280, which was controlled by using a computer through GPIB interface. The polarization resistance Rp was determined by subtracting the high frequency impedance measured at 10 kHz (ZH) from the low frequency impedance measured at 10 mHz (ZL). Two-electrode cell configuration was used for monitoring the polarization resistance. The reciprocal value of the polarization resistance was proportional to the corrosion rate [10–16]. 2.4. Material characterization The corrosion products were analyzed by using a Shimadzu (Japan) XD-3A dif˚ ). The fractometer measured with CuKa monochromatic radiation (k = 1.5405 A measurement was conducted in a step scanning mode (2/min) from h = 30 to h = 80. X-ray diffraction data was taken directly from the surface of the copper after exposure for 96 h under wet/dry cyclic conditions.
3. Results and discussions 3.1. Exposure at different surface orientation The polarization resistance (Rp) was monitored under 1 h of immersion and 7 h of drying in 0.01 M NaCl solution at 60% RH and 303 K temperature for samples exposed to both skyward and downward directions. The corrosion rate is proportional to the reciprocal of polarization resistance and can be calculated using the Stern– Geary equation [17]: Corrosion rate ¼ K=Rp ;
ð1Þ
K ¼ ðba bc Þ=2:303ðba þ bc Þ;
ð2Þ
where ba and bc represent anodic and cathodic Tafel slope, respectively. The value of K is a function of metal and electrolyte [12,15] and can be assumed to be constant for a given metal/electrolyte system. Therefore, the average of the reciprocal of polarization resistance per cycle (ARPR) was evaluated to represent the average corrosion rate per cycle and calculated from 1/Rp according to the following equation:
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
Average of reciprocal of polarization resistance per cycle ðARPRÞ Z t ¼ 1=Rp dt;
ð3Þ
0
where t is the time of wetness (TOW) per cycle and 1/Rp is the reciprocal polarization resistance. The time of wetness (TOW), which can be estimated from using the 1/Rp data versus exposure time as the total elapsed time, from the wetting period to the time just before the completion of surface drying (1/Rp dropped to a value of zero during complete drying of surface). Fig. 2 shows that the reciprocal of polarization resistance (1/Rp) which is proportional to the corrosion rate [10–16], increases with the immersion time during the initial stage then attains a steady state during the last stage of exposure. Fig. 3 shows that ARPR, of the skyward-oriented sample is higher than those for a downwardoriented sample. The ARPR of the skyward-oriented sample, increases gradually and reaches its maximum during the eighth cycle then attained a steady state during the subsequent four cycles. In the case of downward-oriented sample, the ARPR increases slowly and reaches its maximum during the third cycle and attains a steady
0.00018 Facing downward
0.00015 0.00012 0.00009
1/Rp, Ω-1 cm-2
0.00006 0.00003 0.00000 0.00018 Facing upward
0.00015 0.00012 0.00009 0.00006 0.00003 0.00000 0
24
48
72
96
Exposure time (hour) Fig. 2. Changes of 1/Rp with exposure time of a copper sample facing in both a downward and upward direction measured under wet/dry cyclic conditions in 0.01 M NaCl solution.
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
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0.0030 upward downward
0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
2
4
6
8
10
12
14
16
Exposure wet/dry cycle
Fig. 3. Variation of average corrosion charge with cycle number of the copper sample facing downward and upward direction.
state during the subsequent six cycles. This result can be attributed to the difference in the time of wetness (tTOW) which is 4 and 1 h per cycle for both skyward and downward sample, respectively. The amount of electrolyte retained on the surface of the skyward sample during the drying process stimulates the corrosion process occurring under thin electrolyte layers. The electrolyte evaporates and concentrates during thinning process leading to a shorter path for oxygen diffusion, which is the rate-determining step in the corrosion process under thin electrolyte layers. In the groundward sample the corrosion process occurs only during the immersion period of each cycle. The surface is passivated and lowers the corrosion process during the subsequent cycles. One part of the corrosion products was washed away from the surface during the next cycle and the other retained it. The steady state observed during the last stages of copper corrosion under wet/dry cycling conditions was attributed to the entire covering of the surface with corrosion products, which inhibits the continuity of the corrosion process. X-ray diffraction indicated the presence of Cu2O, Cu2(OH)3Cl and Cu2Cl as corrosion products. The main constituent corrosion product in case of groundward sample is Cu2O. 3.2. Influence of temperature The effect of temperature was investigated in 0.05 M NaCl solution at two different temperatures (293 and 313 K) at 60% RH under 1 h of immersion time and during 7 h of drying as shown in Fig. 4. The sample exposed was facing upward during all investigations. The reciprocal of polarization resistance at 313 K is higher than that at 293 K. The 1/Rp increases with exposure time then decreases during the last stages at both temperatures. The calculated ARPR vs. cycle number is presented in Fig. 5. The differences in behavior at high and low temperature may be attributed to the formation of more protective corrosion products at low temperature. The results
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 0.0030 T = 293 K
0.0025 0.0020 0.0015
1/Rp, Ω-1 cm-2
0.0010 0.0005 0.0000 0.0030 T = 313 K 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
24
48
72
96
Exposure time (hour)
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
Fig. 4. Monitoring results of 1/Rp vs. exposure time for copper measured under wet/dry cyclic conditions in a 0.05 M NaCl at 60% RH, and at two different temperatures.
0.040 0.035
T = 313 K T = 293 K
0.030 0.025 0.020 0.015 0.010 0.005 0.000 0
2
4
6
8
10
12
Exposure wet / dry cycle
Fig. 5. Influence of temperature on the average corrosion charge of copper in a 0.05 M NaCl solution.
may be explained on the basis of diffusion process of oxygen, which was enhanced at a higher temperature during the drying process under thin electrolyte layers when
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observed with the rising of temperature, while the reduction of oxygen is a rate-determining step in the corrosion process under thin electrolyte. Hence, the corrosion rate of copper increases with an increase in temperature. In addition, the dissolution of the protective oxide followed by copper metal loss is enhanced by an increase in temperature during the immersion period (anodically control in bulk solution) which leads to an increase in the observed corrosion rate. The cuprite formed at the lower temperature was more protective than that measured at 313 K. It seems that the property of cuprite changed to become less protective during exposure experienced at high temperature. Thus creating a greater tendency for ion diffusion into the corrosion layer. NaCl causes a breakdown of the passive film (mainly cuprite) due to the formation of soluble species such as CuCl2. The average reciprocal of polarization resistance observed at both temperature increases and together with the cycle number, reaches its maximum during the third cycle and attains a steady state during the last seven cycles with different steady state values. The compounds Cu2O and Cu2(OH)3Cl and Cu2Cl were identified as corrosion products by using X-ray diffraction. It is clear that the ARPR presented in Fig. 3 (0.01 M NaCl) increases gradually as the exposure cycle increases then eventually attains the steady state during the ninth cycle. However, in Fig. 5 (0.05 M NaCl) ARPR increases rapidly to a maximum value in the third cycle then decreases slowly until reaches its steady state during the subsequent sixth cycle. The difference in corrosion behavior can be attributed to the difference in the amount of corrosion products precipitated over the entire copper surface. In Fig. 3, while the concentration is lower than in Fig. 5, the corrosion rate proceeds slowly and the amount of corrosion products partially covered the copper surface until it reaches a steady state during the ninth cycle, while the surface is entirely covered with corrosion products. In Fig. 5 the corrosion rate increases rapidly due to a higher concentration of NaCl which leads to an increase in the amount of corrosion products until it attains a maximum in the third cycle. The observed decrease in the corrosion rate measured during fourth and fifth cycles can be attributed to the dissolution process of some precipitated corrosion products, which are loosely bound to the copper surface. The ARPR reaches its steady state during the sixth cycle due to the entire covering of the surface with the reformation of corrosion products. It can be concluded that the higher the concentration of NaCl, the faster the steady state occurs due to the larger amount of corrosion products precipitated over the copper surface. 3.3. Exposure at different pH To study the influence of pH on the atmospheric corrosion of copper, two experiments were conducted in 0.025 M NaCl solution. The copper was subjected to 1 h of immersion and 7 h of drying at 298 K and 60% RH. Both samples were exposed facing upward. The data depicted in Fig. 6 indicated that the overall amount of copper corrosion increases with the lowering of the solution pH. The ARPR per cycle was calculated and presented in Fig. 7. The values of ARPR, at low pH are higher than those at pH 7. The results imply that pH is an important parameter to be considered
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G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 0.00040 pH = 7 0.00032
0.00024
0.00016
1/Rp, Ω-1 cm-2
0.00008
0.00000 0.00040 pH = 4 0.00032
0.00024
0.00016
0.00008
0.00000 0
24
48
72
96
Exposure time (hour)
Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
Fig. 6. Variation of 1/Rp with exposure time for copper subjected to 0.025 M NaCl at two different pH solutions under wet/dry cyclic conditions.
0.0040 0.0035
pH = 4 pH = 7
0.0030 0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 0
2
4
6
8
10
12
14
16
Exposure wet / dry cycle
Fig. 7. Variation of average corrosion charge with exposure cycle number measured at two different pH solutions.
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383
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for Cu corrosion under wet/dry cycling conditions. Hydrogen ions can promote Cu corrosion as described in the equation below: 2Cu þ 4Hþ þ O2 ¼ 2Cu2þ þ 2H2 O
ð4Þ
At pH 7 a possible passivation process on the copper surface may decrease the corrosion rate. The morphology of the deposited corrosion products was distributed heterogeneously, showing some paths through which copper ions released and chloride ions diffused. The corrosion rates of copper exposed to acidic chloride solution are initially higher than those observed in the neutral solution and decrease with increasing exposure cycles. The ions of Cu(I) are oxidized by atmospheric oxygen into Cu(II) and the latter ion plays an important role in copper corrosion. Through hydrolysis of the Cu(II) ions, basic copper chloride is formed in a neutral medium (paratacamite) 4Cu2þ þ 2Cl þ 6H2 O ¼ Cu4 Cl2 ðOHÞ6 þ 6OH
ð5Þ
The main corrosion products identified by using XRD-data after 96 h exposure was cuprite, paratacamite, and clinoatacamite (Cu2(OH)3Cl) in neutral and natokite (CuCl) in acidic solution. Copper surface formed a protective passive film of cuprite in the neutral medium [18,19], however in the acidic medium cuprite was not stable. 3.4. Corrosion behavior of copper in ammonium sulfate solution The polarization resistance was conducted with exposure time in 0.05 M ammonium sulfate solution, under wet/dry cyclic conditions at 303 K and 60% RH as shown in Fig. 8. The surface of copper was exposed facing upward. The ARPR values were calculated using Eq. (1) and traced versus the cycle number as displayed in Fig. 9. The ARPR increases during the initial stages due to the dissolution of copper as Cu(NH3)2+ and attains its maximum value during the third cycle. The ARPR
0.0020
1/Rp, Ω-1 cm-2
0.0015
0.0010
0.0005
0.0000 0
24
48
72
96
Exposure time (hour) Fig. 8. Changes in the 1/Rp with time for copper exposed to alternating conditions, of wet/dry cyclic conditions and in a 0.05 M ammonium sulfate solution at 303 K and 60% RH.
G.A. EL-Mahdy / Corrosion Science 47 (2005) 1370–1383 Average of Reciprocal of Polarization Resistance per cycle (ARPR), Ω-1 cm-2
1380
0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0
2
4
6
8
10
12
14
16
Exposure wet/dry cycle
Fig. 9. Plots of the average corrosion charge for copper with exposure cycle number and in an ammonium sulfate solution conducted at 303 K and 60% RH.
starts to decrease slowly reaching a steady state during the subsequent five cycles. Cuprite is the major corrosion product formed in the early stages of atmospheric corrosion of copper occurring in a natural environment. The basic copper sulfate Cu3(SO4)(OH)4 (antertlerite) and Cu4(SO4)(OH)6 (bronchatite) compounds formed during the latter stages of corrosion as revealed from X-ray diffraction analysis. Fig. 10 represents the magnified plot of the second cycle displayed in Fig. 9 and can be divided into three distinct regions. Experimentation indicates that the corrosion rate is high during the onset of region I (1 h of immersion) due to dissolution of the precipitated ammonium sulfate formed in the preceding cycle and during the drying process of the first cycle, leading to an increase in the concentration of ammonium sulfate, which results in an increase in the corrosion rate in region I. The
0.0036
Time of wetness
Onset of drying period
0.0018 0.0012 0.0006
III
II
I
Complete dryness of surface
0.0024
1-hr immersion
1/Rp, Ω-1 cm-2
0.0030
0.0000 8
9
10
11
12
13
14
15
16
Exposure time (hour) Fig. 10. Monitoring results for a single wet/dry cycle (second cycle in Fig. 9) showing the variations of 1/Rp versus the exposure time.
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observed decrease in the corrosion rate within the region I was attributed to the passivation of the copper surface with corrosion products. The overall corrosion rate increases gradually as the drying time progresses in region II due to an increase in the ammonium sulfate concentration as a result of electrolyte evaporation. In addition, the diffusion of oxygen increases as the thickness of electrolyte decreases as the drying time increases, leading to an increase in the corrosion rate under thin electrolyte layers. The sharp decrease in the corrosion rate during the end of region II was attributed to the complete drying of the surface and 1/Rp dropped to a value of zero. There was no corrosion process during region III as a result of complete the dryness of the surface. Fig. 10 shows an estimation of the time of wetness (TOW), measured from 1/Rp data versus exposure time as the total elapsed time, from the wetting period (region I) to the time just before the complete surface drying period and at the end of region II. 3.5. Mechanism of copper corrosion The mechanism of copper corrosion experienced under wet/dry cyclic conditions seems to proceed under the dissolution–precipitation mechanism. The first step is the dissolution process of the air-formed oxide film followed by the dissolution of copper leads to a high corrosion rate observed during the initial stages of exposure. The dissolution process proceeds under a wet period (1 h of immersion) and continues under the thin electrolyte layers, and during drying period. The dissolution process increases at high temperature and low pH due to the enhancement of the diffusion process of chloride ions, which causes a breakdown of the film formed on the copper surface. The breakdown of the film is also enhanced at low pH = 4 due to the dissolution of the passive film and formation of copper (1) chloride, which is described as follows: 1=2Cu2 O þ 2Cl þ Hþ ! CuCl2 þ 1=2H2 O
ð6Þ
A small fraction of the corrosion products is retained on the copper surface during the initial exposure. During the immersion period, one part of the corrosion products formed on freely exposed copper will remain on the surface whereas another part will be released and washed off. The degree of protection depends on the stability of the formed corrosion products. During the last stages of exposure the amount of corrosion products and the degree of protection increases concurrently with exposure time due to the precipitation of thick corrosion products. Hence, the corrosion rates decrease and approach a steady state value. Decreasing the corrosion rates along with exposure time was attributed to the continuous build up of protective corrosion products on the copper surface. In an ammonium sulfate solution, the rate of corrosion is high during the initial stages, then decreases slowly and attains a steady state during the last stages of exposure. The corrosion rate increases during the initial stages of exposure due to the formation CuðNH3 Þþ 2 then decreases during the final stages of corrosion, due to the formation of insoluble basic copper sulfate, antlerite, brochantite and posnjakite.
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The cathodic reaction occurring during the atmospheric corrosion of copper is the reduction of oxygen: O2 þ 2H2 O þ 4e ¼ 4OH
ð7Þ
4. Conclusions From the results observed during monitoring the polarization resistance of copper under wet/dry cyclic conditions, in chloride and ammonium sulfate containing environments, the following conclusions were drawn: 1. The application of AC impedance and the experimental set up used in this investigation have proven to be a successful tool as well as a useful arrangement when used for monitoring the atmospheric corrosion of copper under periodic wet/dry conditions. 2. The presence of the thin electrolyte layers during the drying period enhances the corrosion process of copper in the upward-oriented sample and the corrosion rates are higher than those of downward-oriented specimen. 3. Exposure of copper tested in a marine environment indicated that NaCl has a strong effect on copper corrosion. The mechanism of atmospheric corrosion of copper under wet/dry cyclic conditions proceeds under dissolution–precipitation mechanism. The corrosion rate of copper increases rapidly during the initial stages of exposure then decreases slowly and eventually attains a steady state during the last stages of exposure. 4. The inhibitive action of copper-oxide decreased more gradually in an acidic medium (pH 4) than in a neutral medium.
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