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Sulfide scale catalysis of copper corrosion
PII: S0043-1354(00)00025-7
Wat. Res. Vol. 34, No. 10, pp. 2798±2808, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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SULFIDE SCALE CATALYSIS OF COPPER CORROSION S. JACOBS1 and M. EDWARDS2* 1
US Environmental Protection Agency, Indian Programs Oce, 75 Hawthorne Street (CMD-3), San Francisco, CA 94105-3901, USA and 2Department of Civil Engineering, Virginia Tech, Blacksburg, VA 24061-0246, USA (First received 1 February 1999; accepted in revised form 1 August 1999)
AbstractÐThe presence of soluble sul®des in a low alkalinity simulated drinking water increased copper pipe corrosion rates by more than one order of magnitude at pH 6.5 and more than two orders of magnitude at pH 9.2. Sul®des caused formation of a thick, black porous scale layer on pipes that did not signi®cantly reduce pipe corrosion rates even after 9 months of exposure. In fact, both the anodic and cathodic reactions were catalyzed when sul®de containing scale was smeared onto a new copper pipe surface. Sul®de scales have a unique ability to accelerate copper corrosion even at pH > 9.0, with potentially devastating consequences for copper tube performance in potable water applications. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐcopper, corrosion, sul®des, drinking water
INTRODUCTION
In an attempt to better understand rapid brass corrosion failures in polluted seawater, considerable research has been conducted on corrosion catalysis from soluble sul®des. That work demonstrated that soluble sul®des can increase overall corrosion rates through acceleration of the anodic, cathodic, or both reactions at the brass surface depending on the speci®c circumstance (Table 1). Corrosion rates as high as 400 mA/cm2 (Kato et al., 1984) have been reported, with signi®cant adverse eects at soluble sul®de concentrations as low as 7 mg/l (Gudas and Hack, 1979a). When sul®des are absent in potable water or seawater, protective copper oxide or other scale layers typically form on brass or copper which markedly decrease the rate of corrosion (AWWA, 1996). If sul®des are present, however, a thick black, poorly adherent scale forms which is composed primarily of Cu2S, although CuS, Cu2O, and non-stoichiometric copper sul®de species such as Cu1.8S have also been reported (McNeil et al., 1991, 1993; Mor and Beccaria, 1974, 1975; Syrett 1981). It is thought that the structural defects, vacancies, and the porous structure of this copper sul®de scale, which are routinely exploited in semiconductor and photovoltaic applications (Grozdanov and Najdoski, 1995; Tributsch, 1984), somehow render less protection to the underlying metal surface than scales formed in the absence of sul®des. Thus, adverse eects of sul*Author to whom all correspondence should be addressed.
®des are not solely due to direct corrosion catalysis from soluble sul®des, but might also be due to the type of scale that forms. This possibility was clearly con®rmed by Kato et al., (1984), who demonstrated that mechanical removal of the black sul®de scale returned brass corrosion rates to normal levels. Do sul®des play a similar role in the corrosion of pure copper tubes in potable water? Although no fundamental research on this subject is yet available which provides mechanistic insight, it is logical to expect that they would. For example, in the previously cited brass corrosion literature, Al-Hajji and Reda (1994) found that corrosion rates in seawater increased with the percentage of copper in the alloy (Table 1). Moreover, Syrett (1977) determined that sul®des accelerated copper corrosion weight losses during 48-h duration experiments in pure water in the presence of oxygen. More recently, Jacobs et al. (1998) compiled signi®cant anecdotal evidence that water utilities with sul®de in their water experience increased frequency of copper corrosion problems, as evidenced by higher than expected Environmental Protection Agency ``action levels'' for copper at the consumers tap and rapid copper pitting tube failure in homes. The Jacobs study also reproduced high copper corrosion rates in the laboratory by exposure of copper to sul®des and further demonstrated that these problems could not be stopped immediately by de-aeration, maintaining a chlorine residual, or super-chorination. Mechanical removal of the black scale layer was eective in slowing the rate of attack. This work is aimed at de®ning the mechanisms
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Sul®de scale catalysis of copper corrosion
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centrations of 5.5 2 2.1 mg/l and 4.0 2 1.7 mg/l at pH 6.5 and 9.2 respectively. All experiments were conducted at a temperature of 22238C.
by which sul®de accelerates copper corrosion in potable waters. In contrast to all previous work in the literature including that with seawater, experiments were run for nine months as opposed to a few days, because longer term behavior is key to copper pipe performance in homes and short term trends have been known to reverse with a few months aging (Edwards et al., 1994a). In addition, a lower pH (6.5) and higher pH (9.5) range was tested to span those commonly encountered for copper in practice.
Coupon preparation and exposure Copper coupons were 5/8 in diameter nominal copper couplings with an internal surface area of 20.0 cm2 and an actual inner diameter of 3/4 in. Each coupon was soaked in a 0.1 N NaOH solution for 3 min and rinsed ®ve times in deionized-distilled water prior to use. The coupons were exposed to the solutions in a recirculating loop using 16-l reservoirs, centrifugal pumps, and ¯exible Nalgene tubing (Fig. 1). The coupons were electrically isolated from each other by the plastic tubing spacers. Water was circulated for 30 min every 12 h at a constant ¯ow of 1.0 GPM with stagnation at all other times. After seven months of exposure, four fresh coupons were added in series with the aged coupons. Electrochemical measurements were then taken from both the new and aged coupons to determine long and short-term eects of sul®de exposure.
MATERIALS AND METHODS
Synthetic solution preparation Four waters were routinely synthesized for use during long-term coupon exposure. The base solution was designed to simulate waters in which soft water type III pitting occurs (Edwards et al., 1994b), with 0.4 mM chloride, 0.25 mM sulfate, and 0.3 mM bicarbonate in Milli-Q deionized water (Millipore Corp.). The water was ®lter sterilized using a 0.2 mm pore size ®lter before adding these anionic constituents as reagent grade sodium salts. The 2 2 experimental matrix included base solution with and without 5 mg/l sul®des at pH 9.2 and 6.5. A pH-stat maintained pH 9.2 in the presence of sul®des, whereas manual addition of acid or base was sucient to maintain pH within 20.2±0.3 pH units in the three other systems. Fresh bulk solutions were prepared and replaced each month. Sul®des were dosed as Na2S9H2O at a concentration of 5 mg/l as S twice a week. Towards the beginning of the experiment, sul®des were always detectable with concentrations around 0.4 mg/l measured even three days after dosing. However, after seven months of running the experiment, sul®des were completely consumed in less than 24 h. Soluble sul®de levels were non-detectable when electrochemical measurements were taken, with the exception of the ®rst three corrosion rate measurements after 4, 24, and 48 h of exposure. A preliminary examination for bacterial activity in the apparatus after six months of exposure con®rmed that bacteria were present, but at extremely low density (<110 cells/mm2). This is consistent with the very low levels of organic carbon present in the simulated tap water (<0.1 mg/l DOC) as determined using Standard Method 5310C (APHA, 1998), and the associated microbial stability of the solution. In some experiments, oxygen concentrations were controlled by bubbling industrial grade oxygen or nitrogen through the solutions or, more typically, by equilibration with the atmosphere. The solutions without sul®des had an average oxygen concentration of 6.5 2 0.9 mg/l, whereas solutions with sul®des had average oxygen con-
Electrochemical corrosion rate measurements and weight loss Corrosion rates were measured using the Reiber electrochemical cell described elsewhere (Edwards and Ferguson, 1993; Reiber, 1989). The counter electrode is a pure platinum wire ®xed along the central axis of the copper pipe specimen and equidistant from all points on the pipe surface, and the reference electrode was silver/silver chloride. The corrosion current (Icorr) and potential (Ecorr vs AgCl) were determined using the Gamry Instruments, Inc. CMS100 Corrosion Measurement System. During each corrosion rate measurement, the coupon was subjected to a potentiodynamic scan ranging between ÿ100 mV to +100 mV about the open circuit potential (Eoc). The potentiodynamic scans did not always produce curves with classic Tafel behavior. Consequently, corrosion rates were always determined manually using the corrosion potential and the cathodic Tafel slope. To select a scan range, preliminary tests were conducted using sequential voltage perturbations of 200 mV (ÿ100 mV to +100 mV), 600 mV, and 200 mV in each solution. Because the corrosion rate measurement using 600 mV perturbation was only slightly dierent than using the 200 mV sweeps, a scan range of 200 mV was used. A perturbation of less than 200 mV was insucient to accurately determine corrosion rates (Jacobs, 1997). The average and standard deviation on four replicate copper coupons were routinely determined, with results after eight months of exposure presented to illustrate typical results (Table 2). The standard deviation is highest for the corrosion rate measurements, but the average corrosion rate for the coupons are signi®cantly dierent at 95% con®dence. Weight loss was determined after mechanical removal of scale from the metal surface using a syn-
Table 1. Synthesis of previous short-term brass corrosion studies in the presence of sul®des and sea water Key conclusions Higher corrosion rate as alloy copper content increases. Sul®des catalyze oxygen reduction reaction. Sul®des catalyze copper oxidation reaction. Sul®des catalyze the reduction and oxidation reaction. Copper exposed to sul®des returns to normal corrosion rates after sul®des are removed from the water. Oxygen must be present in the water along with sul®des to produce accelerated corrosion rates. a
Not reported.
pH range a
7.0±7.5 6.5±8.6 6.5±8.6 7.8±8.3 3.5±8.2
References Al-Hajji and Reda (1993a,b) Gudas and Hack (1979a,b), Kato et al. (1984), de Sanchez and Schirin (1982) MacDonald et al. (1979), Eiselsteing et al. (1983) Mor and Beccaria (1975) Gudas and Hack (1979a,b), Syrett (1981), Eiselstein et al. (1983), Mor and Beccaria (1974) Syrett et al. (1979), Syrett and Wing, (1980), Eiselstein et al. (1983), Bates and Popplewell (1975), Syrett (1977)
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Fig. 1. Experimental set-up. thetic scouring pad that did not signi®cantly disturb the underlying copper metal (Jacobs, 1997; Reiber et al., 1996).
RESULTS AND DISCUSSION
Experimental results are presented in four sections. The ®rst describes the direct eect of sul®des on copper corrosion rates as determined electrochemically and by weight loss. After describing the scale morphology, experiments were conducted to better de®ne the mechanism of corrosion catalysis from sul®des. The ®nal section highlights practical implications of the work in terms of copper tube longevity. Eect of sul®de on corrosion rates The presence of sul®des increased electrochemically estimated copper corrosion rates by about one and two orders of magnitude at pH 6.5 and 9.2, respectively (Fig. 2). Accelerated electrochemical corrosion rates were noted from the ®rst measurement after 4 h of exposure, and rates did not decrease signi®cantly over the entire 260 days of exposure. The average corrosion rate for copper coupons not exposed to sul®des was 1.0 mA/cm2 at pH 6.5 and 0.15 mA/cm2 at pH 9.2. These results are consistent with measurements in typical drinking waters at similar pHs (Edwards and Ferguson, 1993). However, the copper exposed to sul®des at pH 9.2
had corrosion rates as high as 18 mA/cm2 with an average rate of 10 mA/cm2. Corrosion rates of copper at pH 6.5 with sul®des were comparable to those at higher pH, with an average rate of 11 mA/ cm2 and measurements as high as 14 mA/cm2. In fact, over eight months of exposure, the average electrochemical corrosion rates for the two systems with sul®des at dierent pH were not statistically dierent. Although these corrosion rates are much lower than those recorded in the presence of sul®de and sea water (up to 400 mA/cm2), to the authors' knowledge they are the highest rates ever recorded for copper in drinking water or simulated drinking water. There are many possible sources of error in the electrochemical determination of corrosion rates, especially in the presence of thick scale layers (Edwards and Ferguson, 1993). Indeed, Syrett (1977) previously noted that estimates of corrosion rate based on linear polarization were 30 times less than those determined based on more reliable weight loss measurements for a 48-h experiment with sul®des in pure water. To scrutinize the general accuracy of the more sophisticated electrochemical approach used in this work, cumulative weight loss was measured and compared to that predicted by integrated electrochemical measurements at the end of the nine month experiment. Weight loss is the standard against which other measures of corrosion rate are compared (Reiber et al., 1996).
Table 2. Representative mean and standard deviation of electrochemical measurements on four replicate samples Solutions
Icorr (mA/cm2)
Ecorr (mV)
Cathodic slope (V/decade)
Anodic slope (V/decade)
pH pH pH pH
0.9820.17 11.124.2 0.07520.072 4.921.4
7325.7 7527.5 8522.3 64217
ÿ0.1720.0097 ÿ0.1620.031 ÿ0.07520.012 ÿ0.1220.025
0.07420.0068 0.07420.012 0.07320.0054 0.08220.053
6.5, 6.5, 9.2, 9.2,
no sul®des with sul®des no sul®des with sul®des
Sul®de scale catalysis of copper corrosion
The equation to predict weight loss for a two electron transfer is: Weight loss g Icorr A=cm 2 20 cm 2 1 C=A s 63:5 g=mol Cu 96, 485 C=mol e ÿ 2 mol e ÿ =mol Cu Dt s The highest possible prediction of weight loss from electrochemical measurements results if a 1 electron transfer reaction (i.e., Cu0 4 Cu+1) is assumed, whereas the lowest possible prediction assumes a 2 electron transfer (i.e., Cu0 4 Cu+2). These assumptions are necessary only to make a comparison between the two methods, and should not be construed to provide fundamental mechanistic insights into electron transfer processes. The average weight loss for the copper pipe coupons that were exposed to sul®des was 11% (1.82 g) at pH 6.5 and 5.2% (0.89 g) after nine months of experimentation using the coupons at pH 9.2
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(Fig. 3). For the coupons that had no history of sul®de exposure, the average weight loss was 0.44% (0.076 g) at pH 6.5 and ÿ0.25% (ÿ0.043 g) at pH 9.2 over the same time period. The estimates at pH 6.5 with sul®des and pH 9.2 without sul®des were not signi®cantly dierent from that obtained by weight loss when assuming a 2 electron transfer, whereas the electrochemical techniques clearly underestimated weight loss by a factor of 2 at pH 9.2 with sul®des or at pH 6.5 without sul®des. This level of agreement between methods is considered quite good for corrosion studies, especially when considering that the predicted weight loss is based on electrochemical measures during ¯ow conditions only, whereas the actual weight loss is a cumulative measure based on corrosion during some ¯ow but mostly stagnant conditions (AWWA, 1996). Moreover, the fact that trends observed in the weight loss data were the same as those obtained from the electrochemical techniques strongly supports the general validity of the electrochemical measures.
Fig. 2. Corrosion rate of copper coupons in the presence and absence of sul®des at pH 6.5 (A) and pH 9.2 (B). Error bars indicate 90% con®dence intervals based on quadruplicate replicates.
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S. Jacobs and M. Edwards
Scale morphology A thick black scale was present on the coupons exposed to sul®des at both pH 6.5 and 9.2. After thorough rinsing with deionized water, these scales tested positive for sul®des using the sodium azide sul®de test (Feigl and Anger, 1972). When wet, the black scale had a slimy texture, but after drying, resembled a ®ne powder at pH 6.5 and had the macroscopic appearance of crushed black velvet at pH 9.2. The sul®de scale at both pHs ¯aked o the coupons when disturbed by changes in ¯ow or with vibration during exposure. The scale at pH 6.5 detached the most readily and in the largest pieces. Upon drying, the underlying scale was orange with a powdery appearance at pH 6.5 and was gray and slightly shiny, similar in appearance to graphite at pH 9.2. Under the SEM (Fig. 4), the black sul®de scales were globular and resembled tufts of cauli¯ower, similar to those observed by Eisenmann (1979) on brass. The inner orange scale present at pH 6.5 seemed to be constructed from thin ¯at ¯akes. The scale on the coupons exposed to water at pH 6.5 without sul®des appeared crystalline in form, while that formed at pH 9.2 without sul®des was smoothest and most uniform in appearance. The layering was especially apparent in a cross-sectional view of a coupon exposed to sul®des at pH 9.2 (Fig. 5). The outer and inner layer of the scale at pH 6.5 were examined using a KEVEX 0700 E.D.S. X-ray ¯uorescence (XRF) instrument. There were very small peaks in both scale layers for calcium and iron and large peaks at sulfur and copper. The sulfur peak was signi®cantly larger in the outermost scale layer. Cross-sections of the sul®de scale from pH 6.5 and 9.2 were examined under a JEOL 8600
electron microprobe. The scale at pH 9.2 had a uniform density and had an atomic composition of about 55% Cu, 35% S, and 10% O. The scale at pH 6.5 was composed of two primary materials of dierent density. Inner scale and the skeleton within the large tufts of the outer scale was composed of approximately 40% Cu, 10% S, and 50% O. The outer round balls of the scale at pH 6.5 have the highest sulfur content with a composition of about 55% Cu, 40% S, and 5% O. This material was Xray amorphous. After careful mechanical removal of the scale layers, the inner surface of the coupons exposed to the solution at pH 9.2 without sul®des had the appearance of a new copper coupon. The coupons at pH 6.5 without sul®des had thin shallow pits scattered over the surface, but was relatively free of corrosion over the rest of the surface. The coupons exposed to sul®des at pH 6.5 had uniform corrosion over the entire inner surface of the coupon, while the coupons exposed to sul®des at pH 9.2 exhibited severe localized corrosion with visible pits over much of the surface. Mechanistic insights into sul®de-induced copper corrosion Mechanistic insights to the sul®de-induced corrosion process can be gained from examination of the electrochemical measurements of corrosion rate and potential, as well as anodic and cathodic reaction rates. The anodic reaction rate describes the rate at which the oxidation of copper is occurring in response to an electrochemical perturbation, whereas the cathodic rate describes the rate of oxygen reduction reaction. Although the corrosion rates increased by orders of magnitude for copper coupons exposed to sul®des, the average corrosion
Fig. 3. Predicted and actual weight loss of copper coupons. Error bars represent the 90% con®dence interval. Actual weight loss measured after 9 months of experimentation.
Sul®de scale catalysis of copper corrosion
potential did not signi®cantly increase at a pH of 6.5 and decreased by only about 20 mV at a pH of 9.2. This relatively small change in corrosion potential, in combination with the large increase in corrosion rate, indicates that both the anodic and cathodic reaction rates have been catalyzed (Fig. 6). After nine months, the aged coupons that had been exposed to sul®des were placed in an exposure loop without sul®des for an additional two months.
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After two months of exposure to water without sul®des, the anodic reaction rates had decreased dramatically at both pH 6.5 and 9.2. However, there was only a slight decrease in the cathodic reaction rates. Consequently, the overall corrosion rate was still accelerated by about an order of magnitude when compared to coupons at the same pH that had never been exposed to sul®des. Because the corrosion rates remained acceler-
Fig. 4. SEM photographs of copper scales magni®ed 1500 times.
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S. Jacobs and M. Edwards
Fig. 5. Cross section of the copper metal and sul®de scale at pH 9.2 magni®ed 150 times.
ated after a long absence of soluble sul®des, the elevated corrosion rate cannot be attributed to the sul®des in solution, but must be due to the scale formed when the sul®des are present. After three months of exposure to the sul®de solutions, this scale was mechanically removed from the coupons used for short-term testing. These coupons were then placed in water without sul®des. The corrosion rate of the scraped coupons at pH 9.2 decreased by almost 80% when measured 2 h after the scale was removed. The corrosion rate at pH 6.5 decreased by less than 10% 2 h after removing the scale, but decreased by over 80% over 6 days. After seven weeks, the corrosion rate of the mechanically cleaned coupons at both pH were comparable to those that had never been exposed to sul®des. As a novel test of the corrosion accelerating properties of the sul®de scale, about 1 g of wet solids were scraped from the surface of an ``old'' coupon and then smeared onto a fresh copper coupon. After drying over night, this coupon was then
Fig. 6. Anodic and cathodic reaction rates of copper coupons at pH 6.5 (A) and pH 9.2 (B).
Sul®de scale catalysis of copper corrosion
exposed for 2 h to water without sul®des at pH 9.2 and compared to a fresh coupon without the smeared scale. The experiment was repeated using the scale-smeared and fresh coupons at pH 6.5. Interestingly, the corrosion rates of the scalesmeared coupon at pH 6.5 and 9.2 were nearly equivalent to those observed after eight months of sul®de exposure at the same pH (Fig. 7). Both anodic and cathodic reaction rates were accelerated to nearly the same extent for the scale-smeared coupons as for coupons aged in the presence of sul®des. This result clearly supports the corrosion catalyzing nature of the scale itself ®rst described by Kato et al. (1984) for copper nickel alloys, as opposed to a mechanism of catalysis through sulfur species sorbed to the metal surface described for nickel alloys in Marcus (1995). To examine the eects of oxygen concentration on the corrosion rate of coupons with previous exposure to sul®des, corrosion rate measurements were made at high (16 mg/l) and low (<0.25 mg/l) oxygen concentration without sul®des in the water. These experiments were conducted at both pH 9.2 and 6.5 on coupons with sul®de scale and on coupons which the sul®de scale had been removed 24 h previously. For coupons that had been mechanically cleaned, deaeration reduced the corrosion rate by a factor of 3 as might be expected due to a slowing of the rate limiting cathodic reaction (AWWA, 1996). In contrast, reduced oxygen concentrations have nearly no eect on the elevated corrosion rate of coupons with the sul®de scale intact. This catalysis of oxygen reduction by sul®de scale might explain how long term weight loss, during the mostly stagnant pipe exposure, could be close to weight loss estimated electrochemically based solely on measurements during ¯ow. Without the sul®de scale, a high corrosion rate is not expected to be sustained on copper at low dissolved oxygen due to
2805
reduced rates of oxygen reduction and transfer (AWWA, 1996). Practical implications for copper tube longevity It is useful to compare the adverse impacts of sul®des to other known water quality factors that in¯uence copper corrosion rates. To make an order of magnitude estimate of copper pipe life, data were synthesized for 21 dierent waters in which wellcontrolled copper corrosion experiments were continued for longer than six months (Table 3). Reported data on weight loss and corrosion rates were used to estimate the time required to eat away 1/3 of the 0.042 in pipe wall thickness on a typical 5/8 in nominal Type L copper tube. This is clearly an overly optimistic criteria for time to pipe failure for pitting corrosion and overly pessimistic for uniform corrosion. Estimates of pipe lifetime based on by-product release data were made by extrapolating metal release rates measured during the initial moments of stagnation over the entire pipe life (Edwards and Ferguson, 1993). The net result is that, under the long-term test conditions where chlorine, pH, alkalinity or inhibitors were varied, estimated copper tube lifetimes exceed 400 years (Fig. 8). This is consistent with the proven performance of copper in a wide range of domestic plumbing applications. The exceptions to this rule were tests at pH less than 6.5 under continuous ¯ow conditions or in the short term presence of high sustained chlorine residuals, for which estimated pipe lifetime can decrease to as low as 7± 30 years. In marked contrast, even at pH 9.2 and with mostly stagnant conditions, predicted pipe lifetimes in the presence of sul®des are between 1 and 3 years. It is also worth noting that, under some extremely unusual conditions, copper pipes have failed in as few as three months after installation due to
Fig. 7. Corrosion rates of fresh coupons smeared with sul®de scale are similar to the corrosion rate of coupons with 8 months of exposure to sul®des. Error bars indicate the 90% con®dence interval.
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S. Jacobs and M. Edwards Table 3. Synthesis of copper corrosion rate information
pH Alkalinity (mg/l as CaCO3) 7.2 7.2 7.2 7.2 7.2 7.2 7.8 7.8 7.8 7.8 7.8 7.8 9.2 9.2 9.2 6.5 6.5 9.2 9.2 6.0 7.0
15 15 15 45 45 45 300 300 300 45 45 45 45 45 45 15 15 15 15 0 15
Other noteworthy constituent (mg/l)
Symbol in Fig. 8
7.2/15 Ortho-P: 1 mg/l as P 7.2/15/P Hexametaphosphate: 1 mg/l as P 7.2/15/HP 7.2/45 Ortho-P: 1 mg/l as P 7.2/45/P Hexametaphosphate: 1 mg/l as P 7.2/45/HP 7.2/300 Ortho-P: 1 mg/l as P 7.2/300/P Hexametaphosphate: 1 mg/l as P 7.2/300/HP 7.8/45 Ortho-P: 1 mg/l as P 7.8/45/P Hexametaphosphate: 1 mg/l as P 7.8/45/HP 9.2/45 Ortho-P: 1 mg/l as P 9.2/45/P Hexametaphosphate: 1 mg/l as P 9.2/45/HP 6.5/15 6.5/15/H2S H2S: 1.0 9.2/15 H2S: 1.0 9.2/15/H2S 6.0 7.0/Cl2 Cl2: 0.9
Corrosion Test Flow measurea duration condb (years) BP: 0.78 BP: 0.13 BP: 0.35 BP: 0.99 BP: 0.30 BP: 0.57 BP: 0.33 BP: 0.42 BP: 0.84 BP: 0.44 BP: 0.16 BP: 0.27 BP: 0.08 BP: 0.07 BP: 0.07 EC: 0.98 EC: 11.0 EC: 0.07 EC: 4.92 WL: 0.46 EC: 2.2
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 0.73 0.73 0.73 0.73 0.52 > 0.5
S S S S S S S S S S S S S S S S S S S F F
Reference
Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi This work This work This work This work Benjamin et al. (1990) Reiber (1989)
et et et et et et et et et et et et et et et
al. al. al. al. al. al. al. al. al. al. al. al. al. al. al.
(1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996)
a
Corrosion measure is the reported data from the indicated reference used to estimate pipe life. WL=weight loss (milli-inches per year); BP=By-product release (mg/l) after 6 h stagnation, EC=electrochemical corrosion rate (mA/cm2). Experimental ¯ow condition: S=mostly stagnant, F=continuous ¯ow.
b
pitting corrosion (Edwards and Ferguson, 1993). To date, this rapid failure has never been reproduced under laboratory controlled conditions. In this work, the general sul®de exposure from solutions did not lead to the formation of distinct pits despite the very high non-uniform corrosion rates observed. However, it seems possible that highly localized production of sul®des, perhaps in the presence of isolated colonies of sulfate reducing bacteria, could somehow concentrate sul®de catalytic eects and cause pit formation. Future research should examine this possibility in greater detail, since there is strong anecdotal evidence that sul®des induce pitting in practice (Jacobs et al., 1998).
As a ®nal point, and as was noted in previous work (Jacobs et al., 1998), it is not yet possible to ascertain the extent to which sul®des aect copper corrosion in practical situations. At one extreme, since sulfate-reducing bacteria appear to be present in all distribution systems and sul®de-induced corrosion problems are initiated at concentrations at least as low as 0.007 mg/l (Gudas and Hack, 1979b), it is possible that sul®des could be a major cause of copper corrosion problems even if they are not present in the raw water. On the other hand, due to eects of other constituents in drinking water not tested in this work which might inhibit sul®de-induced corrosion problems, is possible that
Fig. 8. Rough estimates for time to pipe failure for water qualities synthesized in Table 3. Note the log time scale.
Sul®de scale catalysis of copper corrosion
problems from sul®des might be con®ned to a very narrow range of circumstances even when they are present. Additional research on this subject is necessary to resolve this issue and possible remediation of sul®de induced corrosion problems. In particular, techniques such as electrochemical impedance spectroscopy (EIS) might be useful for examining copper corrosion mechanisms in the presence of thick scale layers, and a more thorough investigation of sul®de's eects on copper corrosion by-product release are desireable. CONCLUSIONS
. The presence of sul®des in drinking water can be expected to increase corrosion rates by about 1 and 2 orders of magnitude at pH 6.5 and 9.2, respectively. Passivation does not occur even after nine months of exposure. . The general validity of electrochemical corrosion rate measures in the presence of sul®des were con®rmed by satisfactory comparison to gravimetric weight loss. Both anodic and cathodic copper corrosion reactions are catalyzed by sul®des. . The accelerated corrosion can be produced by a catalytic layer of sul®de scale on pipe. When this scale is removed and the copper metal is placed in water without sul®des, the corrosion rate returns to normal levels. When this scale is smeared on a fresh copper pipe surface, the resulting corrosion rate in water without sul®des is immediately comparable to that obtained after months of exposure to soluble sul®des. . When compared to other water quality factors known to in¯uence copper corrosion rates including pH, alkalinity, chlorine and inhibitors, sul®des pose the greatest threat to copper tube longevity. AcknowledgementsÐThe ®rst author was supported by a fellowship from the National Science Foundation. This work was supported by the National Science Foundation (NSF) under grant no. BCS-9309078. The opinions, ®ndings, and conclusions or recommendations are those of the authors and do not necessarily re¯ect the views of the NSF. REFERENCES
Al-Hajji J. N. and Reda M. R. (1993aa) Corrosion of CuNi alloys in sul®de-polluted seawater. Corrosion 49, 809±820. Al-Hajji J. N. and Reda M. R. (1994) The con¯icting roles of complexing agents on the corrosion of coper± nickel alloys in sul®de polluted seawater. J. Electrochem. Soc. 141, 1432±1439. Al-Hajji J. N. and Reda M. R. (1993bb) The corrosion of copper±nickel alloys in sul®de-polluted seawater: the eect of sul®de concentration. Corros. Sci. 34, 163±177. AWWA (1996) Internal Corrosion of Water Distribution Systems, Second ed. American Water Works Association, Denver, CO.
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APHA (1998) Standard Methods for the Examination of Water and Wastewater, 20th ed. United Book Press Inc, Baltimore, Maryland. Bates J. F. and Popplewell J. M. (1975) Corrosion of condenser tube alloys in sul®de contaminated brine. Corrosion 31, 269±275. Benjamin M. M., Reiber S. H., Ferguson J. F., Vanderwer E. A. and Miller M. W. (1990) Chemistry of Corrosion Inhibitors in Potable Water. American Water Works Association, Denver, CO. de Sanchez S. R. and Schirin D. J. (1982) The ¯ow corrosion mechanism of copper based alloys in sea water in the presence of sulphide contamination. Corros. Sci. 22, 585±607. Edwards M. and Ferguson J. F. (1993) Accelerated testing of copper corroison. J. AWWA 85, 105±113. Edwards M., Holm T. and McNeill L. S. (1999) Final report to AWWARF. Application of phosphate-based corrosion inhibitors in drinking water treatment. In preparation. Edwards M., Rehring J. and Meyer T. (1994a) Inorganic anions and copper pitting. Corrosion 50, 366±372. Edwards M., Ferguson J. F. and Reiber S. H. (1994b) The pitting corrosion of copper. J. AWWA 86, 74±90. Eiselstein L. E., Syrett B. C., Wing S. S. and Caligiuri R. D. (1983) The accelerated corrosion of Cu-Ni alloys in sulphide-polluted seawater: mechanism no. 2. Corros. Sci. 23, 223±239. Eisenmann E. T. (1979) Rapid data acquisition in the study of slow electrode processes by means of simultaneous potentiostatsÐthe sul®dation of copper. Corrosion 35, 12±16. Feigl F. and Anger V. (1972) Spot Tests in Inorganic Analysis. Elsevier Publishing Company, New York, NY. Grozdanov I. and Najdoski M. (1995) Optical and electrical properties of copper sul®de ®lms of variable composition. Journal of Solid State Chemistry 114, 469±475. Gudas J. P. and Hack H. P. (1979b) Parametric evaluation of susceptibility of Cu-Ni alloys to sul®de induced corrosion in sea water. Corrosion 35, 259±264. Gudas J. P. and Hack H. P. (1979a) Sul®de induced corrosion of copper nickel alloys. Corrosion 35, 67±73. Hidmi L., Gladwell D. and Edwards M. (1996) Role of inhibitors in copper corrosion. Poster Session. In AWWA National Conference. American Water Works Association Water Quality Technology Conference in Boston, MA. Jacobs S. (1997) Sul®de-induced corrosion of copper in drinking water. University of Colorado at Boulder Masters Thesis. Jacobs S., Reiber S. and Edwards M. (1998) Sul®deinduced copper corrosion. JAWWA 90, 62±73. Kato C., Pickering H. W. and Castle J. E. (1984) Eect of sul®de on the corrosion of Cu-9.4Ni-1.7Fe alloy in aqueous NaCl solution. J. Electrochem. Soc. 131(6), 1225± 1229. MacDonald D. D., Syrett B. C. and Wing S. S. (1979) The corrosion of Cu-Ni alloys 706 and 715 in ¯owing sea waterÐII. Eect of dissolved sul®de. Corrosion 35, 367±378. Marcus P. (1995) Sulfur-assisted corrosion mechanisms and the role of alloyed elements. In Corrosion Mechanisms in Theory and Practice, eds P. Marcus and J. Oudar. Marcel Dekker Inc, NY. McNeil M. B., Amos A. L. and Woods T. L. (1993) Adherence of sul®de mineral layers produced by corrosion of copper alloys. Corrosion 49, 755±758. McNeil M. B., Jones J. M. and Little B. J. (1991) Production of sul®de minerals by sulfate-reducing bacteria during microbiologically in¯uenced corrosion of copper. Corrosion 47, 674±677. Mor E. D. and Beccaria A. M. (1975) Behaviour of copper in arti®cial sea water containing sul®des. Br. Corros. J. 10, 33±38.
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Mor E. D. and Beccaria A. M. (1974) In¯uence of sul®des on the products of corrosion of copper in polluted sea water. Corrosion 30, 354±356. Reiber S., Ryder R. A. and Wagner I. (1996) Corrosion assessment technologies. In Internal Corrosion of Water Distribution Systems, Second ed. American Water Works Association, Denver, CO. Reiber S. (1989) Copper plumbing surfaces: an electrochemical study. JAWWA 81, 114±122. Syrett B. C. (1977) Accelerated corrosion of copper in ¯owing pure water contaminated with oxygen and sul®de. Corrosion 33, 257±262.
Syrett B. C. (1981) The mechanism of accelerated corrosion of copper±nickel alloys in sulphide-polluted seawater. Corros. Sci. 21, 187±209. Syrett B. C. and Wing S. S. (1980) Eect of ¯ow on corrosion of copper±nickel alloys in aerated sea water and in sul®de-polluted sea water. Corrosion 36, 73±85. Tributsch H. (1984) Metal sul®des in photovoltaic, photoelectrochemical and solar biological energy conversion. In Sulfur, its Signi®cance for Chemistry, for the Geo-, Bio- and Cosmosphere and Technology, eds A. Muller, B. Krebs, Elsevier Science Publishers, Amsterdam. Studies in Inorganic Chemistry, 5, 277±310.
Wat. Res. Vol. 34, No. 10, pp. 2798±2808, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
SULFIDE SCALE CATALYSIS OF COPPER CORROSION S. JACOBS1 and M. EDWARDS2* 1
US Environmental Protection Agency, Indian Programs Oce, 75 Hawthorne Street (CMD-3), San Francisco, CA 94105-3901, USA and 2Department of Civil Engineering, Virginia Tech, Blacksburg, VA 24061-0246, USA (First received 1 February 1999; accepted in revised form 1 August 1999)
AbstractÐThe presence of soluble sul®des in a low alkalinity simulated drinking water increased copper pipe corrosion rates by more than one order of magnitude at pH 6.5 and more than two orders of magnitude at pH 9.2. Sul®des caused formation of a thick, black porous scale layer on pipes that did not signi®cantly reduce pipe corrosion rates even after 9 months of exposure. In fact, both the anodic and cathodic reactions were catalyzed when sul®de containing scale was smeared onto a new copper pipe surface. Sul®de scales have a unique ability to accelerate copper corrosion even at pH > 9.0, with potentially devastating consequences for copper tube performance in potable water applications. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐcopper, corrosion, sul®des, drinking water
INTRODUCTION
In an attempt to better understand rapid brass corrosion failures in polluted seawater, considerable research has been conducted on corrosion catalysis from soluble sul®des. That work demonstrated that soluble sul®des can increase overall corrosion rates through acceleration of the anodic, cathodic, or both reactions at the brass surface depending on the speci®c circumstance (Table 1). Corrosion rates as high as 400 mA/cm2 (Kato et al., 1984) have been reported, with signi®cant adverse eects at soluble sul®de concentrations as low as 7 mg/l (Gudas and Hack, 1979a). When sul®des are absent in potable water or seawater, protective copper oxide or other scale layers typically form on brass or copper which markedly decrease the rate of corrosion (AWWA, 1996). If sul®des are present, however, a thick black, poorly adherent scale forms which is composed primarily of Cu2S, although CuS, Cu2O, and non-stoichiometric copper sul®de species such as Cu1.8S have also been reported (McNeil et al., 1991, 1993; Mor and Beccaria, 1974, 1975; Syrett 1981). It is thought that the structural defects, vacancies, and the porous structure of this copper sul®de scale, which are routinely exploited in semiconductor and photovoltaic applications (Grozdanov and Najdoski, 1995; Tributsch, 1984), somehow render less protection to the underlying metal surface than scales formed in the absence of sul®des. Thus, adverse eects of sul*Author to whom all correspondence should be addressed.
®des are not solely due to direct corrosion catalysis from soluble sul®des, but might also be due to the type of scale that forms. This possibility was clearly con®rmed by Kato et al., (1984), who demonstrated that mechanical removal of the black sul®de scale returned brass corrosion rates to normal levels. Do sul®des play a similar role in the corrosion of pure copper tubes in potable water? Although no fundamental research on this subject is yet available which provides mechanistic insight, it is logical to expect that they would. For example, in the previously cited brass corrosion literature, Al-Hajji and Reda (1994) found that corrosion rates in seawater increased with the percentage of copper in the alloy (Table 1). Moreover, Syrett (1977) determined that sul®des accelerated copper corrosion weight losses during 48-h duration experiments in pure water in the presence of oxygen. More recently, Jacobs et al. (1998) compiled signi®cant anecdotal evidence that water utilities with sul®de in their water experience increased frequency of copper corrosion problems, as evidenced by higher than expected Environmental Protection Agency ``action levels'' for copper at the consumers tap and rapid copper pitting tube failure in homes. The Jacobs study also reproduced high copper corrosion rates in the laboratory by exposure of copper to sul®des and further demonstrated that these problems could not be stopped immediately by de-aeration, maintaining a chlorine residual, or super-chorination. Mechanical removal of the black scale layer was eective in slowing the rate of attack. This work is aimed at de®ning the mechanisms
2798
Sul®de scale catalysis of copper corrosion
2799
centrations of 5.5 2 2.1 mg/l and 4.0 2 1.7 mg/l at pH 6.5 and 9.2 respectively. All experiments were conducted at a temperature of 22238C.
by which sul®de accelerates copper corrosion in potable waters. In contrast to all previous work in the literature including that with seawater, experiments were run for nine months as opposed to a few days, because longer term behavior is key to copper pipe performance in homes and short term trends have been known to reverse with a few months aging (Edwards et al., 1994a). In addition, a lower pH (6.5) and higher pH (9.5) range was tested to span those commonly encountered for copper in practice.
Coupon preparation and exposure Copper coupons were 5/8 in diameter nominal copper couplings with an internal surface area of 20.0 cm2 and an actual inner diameter of 3/4 in. Each coupon was soaked in a 0.1 N NaOH solution for 3 min and rinsed ®ve times in deionized-distilled water prior to use. The coupons were exposed to the solutions in a recirculating loop using 16-l reservoirs, centrifugal pumps, and ¯exible Nalgene tubing (Fig. 1). The coupons were electrically isolated from each other by the plastic tubing spacers. Water was circulated for 30 min every 12 h at a constant ¯ow of 1.0 GPM with stagnation at all other times. After seven months of exposure, four fresh coupons were added in series with the aged coupons. Electrochemical measurements were then taken from both the new and aged coupons to determine long and short-term eects of sul®de exposure.
MATERIALS AND METHODS
Synthetic solution preparation Four waters were routinely synthesized for use during long-term coupon exposure. The base solution was designed to simulate waters in which soft water type III pitting occurs (Edwards et al., 1994b), with 0.4 mM chloride, 0.25 mM sulfate, and 0.3 mM bicarbonate in Milli-Q deionized water (Millipore Corp.). The water was ®lter sterilized using a 0.2 mm pore size ®lter before adding these anionic constituents as reagent grade sodium salts. The 2 2 experimental matrix included base solution with and without 5 mg/l sul®des at pH 9.2 and 6.5. A pH-stat maintained pH 9.2 in the presence of sul®des, whereas manual addition of acid or base was sucient to maintain pH within 20.2±0.3 pH units in the three other systems. Fresh bulk solutions were prepared and replaced each month. Sul®des were dosed as Na2S9H2O at a concentration of 5 mg/l as S twice a week. Towards the beginning of the experiment, sul®des were always detectable with concentrations around 0.4 mg/l measured even three days after dosing. However, after seven months of running the experiment, sul®des were completely consumed in less than 24 h. Soluble sul®de levels were non-detectable when electrochemical measurements were taken, with the exception of the ®rst three corrosion rate measurements after 4, 24, and 48 h of exposure. A preliminary examination for bacterial activity in the apparatus after six months of exposure con®rmed that bacteria were present, but at extremely low density (<110 cells/mm2). This is consistent with the very low levels of organic carbon present in the simulated tap water (<0.1 mg/l DOC) as determined using Standard Method 5310C (APHA, 1998), and the associated microbial stability of the solution. In some experiments, oxygen concentrations were controlled by bubbling industrial grade oxygen or nitrogen through the solutions or, more typically, by equilibration with the atmosphere. The solutions without sul®des had an average oxygen concentration of 6.5 2 0.9 mg/l, whereas solutions with sul®des had average oxygen con-
Electrochemical corrosion rate measurements and weight loss Corrosion rates were measured using the Reiber electrochemical cell described elsewhere (Edwards and Ferguson, 1993; Reiber, 1989). The counter electrode is a pure platinum wire ®xed along the central axis of the copper pipe specimen and equidistant from all points on the pipe surface, and the reference electrode was silver/silver chloride. The corrosion current (Icorr) and potential (Ecorr vs AgCl) were determined using the Gamry Instruments, Inc. CMS100 Corrosion Measurement System. During each corrosion rate measurement, the coupon was subjected to a potentiodynamic scan ranging between ÿ100 mV to +100 mV about the open circuit potential (Eoc). The potentiodynamic scans did not always produce curves with classic Tafel behavior. Consequently, corrosion rates were always determined manually using the corrosion potential and the cathodic Tafel slope. To select a scan range, preliminary tests were conducted using sequential voltage perturbations of 200 mV (ÿ100 mV to +100 mV), 600 mV, and 200 mV in each solution. Because the corrosion rate measurement using 600 mV perturbation was only slightly dierent than using the 200 mV sweeps, a scan range of 200 mV was used. A perturbation of less than 200 mV was insucient to accurately determine corrosion rates (Jacobs, 1997). The average and standard deviation on four replicate copper coupons were routinely determined, with results after eight months of exposure presented to illustrate typical results (Table 2). The standard deviation is highest for the corrosion rate measurements, but the average corrosion rate for the coupons are signi®cantly dierent at 95% con®dence. Weight loss was determined after mechanical removal of scale from the metal surface using a syn-
Table 1. Synthesis of previous short-term brass corrosion studies in the presence of sul®des and sea water Key conclusions Higher corrosion rate as alloy copper content increases. Sul®des catalyze oxygen reduction reaction. Sul®des catalyze copper oxidation reaction. Sul®des catalyze the reduction and oxidation reaction. Copper exposed to sul®des returns to normal corrosion rates after sul®des are removed from the water. Oxygen must be present in the water along with sul®des to produce accelerated corrosion rates. a
Not reported.
pH range a
7.0±7.5 6.5±8.6 6.5±8.6 7.8±8.3 3.5±8.2
References Al-Hajji and Reda (1993a,b) Gudas and Hack (1979a,b), Kato et al. (1984), de Sanchez and Schirin (1982) MacDonald et al. (1979), Eiselsteing et al. (1983) Mor and Beccaria (1975) Gudas and Hack (1979a,b), Syrett (1981), Eiselstein et al. (1983), Mor and Beccaria (1974) Syrett et al. (1979), Syrett and Wing, (1980), Eiselstein et al. (1983), Bates and Popplewell (1975), Syrett (1977)
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Fig. 1. Experimental set-up. thetic scouring pad that did not signi®cantly disturb the underlying copper metal (Jacobs, 1997; Reiber et al., 1996).
RESULTS AND DISCUSSION
Experimental results are presented in four sections. The ®rst describes the direct eect of sul®des on copper corrosion rates as determined electrochemically and by weight loss. After describing the scale morphology, experiments were conducted to better de®ne the mechanism of corrosion catalysis from sul®des. The ®nal section highlights practical implications of the work in terms of copper tube longevity. Eect of sul®de on corrosion rates The presence of sul®des increased electrochemically estimated copper corrosion rates by about one and two orders of magnitude at pH 6.5 and 9.2, respectively (Fig. 2). Accelerated electrochemical corrosion rates were noted from the ®rst measurement after 4 h of exposure, and rates did not decrease signi®cantly over the entire 260 days of exposure. The average corrosion rate for copper coupons not exposed to sul®des was 1.0 mA/cm2 at pH 6.5 and 0.15 mA/cm2 at pH 9.2. These results are consistent with measurements in typical drinking waters at similar pHs (Edwards and Ferguson, 1993). However, the copper exposed to sul®des at pH 9.2
had corrosion rates as high as 18 mA/cm2 with an average rate of 10 mA/cm2. Corrosion rates of copper at pH 6.5 with sul®des were comparable to those at higher pH, with an average rate of 11 mA/ cm2 and measurements as high as 14 mA/cm2. In fact, over eight months of exposure, the average electrochemical corrosion rates for the two systems with sul®des at dierent pH were not statistically dierent. Although these corrosion rates are much lower than those recorded in the presence of sul®de and sea water (up to 400 mA/cm2), to the authors' knowledge they are the highest rates ever recorded for copper in drinking water or simulated drinking water. There are many possible sources of error in the electrochemical determination of corrosion rates, especially in the presence of thick scale layers (Edwards and Ferguson, 1993). Indeed, Syrett (1977) previously noted that estimates of corrosion rate based on linear polarization were 30 times less than those determined based on more reliable weight loss measurements for a 48-h experiment with sul®des in pure water. To scrutinize the general accuracy of the more sophisticated electrochemical approach used in this work, cumulative weight loss was measured and compared to that predicted by integrated electrochemical measurements at the end of the nine month experiment. Weight loss is the standard against which other measures of corrosion rate are compared (Reiber et al., 1996).
Table 2. Representative mean and standard deviation of electrochemical measurements on four replicate samples Solutions
Icorr (mA/cm2)
Ecorr (mV)
Cathodic slope (V/decade)
Anodic slope (V/decade)
pH pH pH pH
0.9820.17 11.124.2 0.07520.072 4.921.4
7325.7 7527.5 8522.3 64217
ÿ0.1720.0097 ÿ0.1620.031 ÿ0.07520.012 ÿ0.1220.025
0.07420.0068 0.07420.012 0.07320.0054 0.08220.053
6.5, 6.5, 9.2, 9.2,
no sul®des with sul®des no sul®des with sul®des
Sul®de scale catalysis of copper corrosion
The equation to predict weight loss for a two electron transfer is: Weight loss g Icorr A=cm 2 20 cm 2 1 C=A s 63:5 g=mol Cu 96, 485 C=mol e ÿ 2 mol e ÿ =mol Cu Dt s The highest possible prediction of weight loss from electrochemical measurements results if a 1 electron transfer reaction (i.e., Cu0 4 Cu+1) is assumed, whereas the lowest possible prediction assumes a 2 electron transfer (i.e., Cu0 4 Cu+2). These assumptions are necessary only to make a comparison between the two methods, and should not be construed to provide fundamental mechanistic insights into electron transfer processes. The average weight loss for the copper pipe coupons that were exposed to sul®des was 11% (1.82 g) at pH 6.5 and 5.2% (0.89 g) after nine months of experimentation using the coupons at pH 9.2
2801
(Fig. 3). For the coupons that had no history of sul®de exposure, the average weight loss was 0.44% (0.076 g) at pH 6.5 and ÿ0.25% (ÿ0.043 g) at pH 9.2 over the same time period. The estimates at pH 6.5 with sul®des and pH 9.2 without sul®des were not signi®cantly dierent from that obtained by weight loss when assuming a 2 electron transfer, whereas the electrochemical techniques clearly underestimated weight loss by a factor of 2 at pH 9.2 with sul®des or at pH 6.5 without sul®des. This level of agreement between methods is considered quite good for corrosion studies, especially when considering that the predicted weight loss is based on electrochemical measures during ¯ow conditions only, whereas the actual weight loss is a cumulative measure based on corrosion during some ¯ow but mostly stagnant conditions (AWWA, 1996). Moreover, the fact that trends observed in the weight loss data were the same as those obtained from the electrochemical techniques strongly supports the general validity of the electrochemical measures.
Fig. 2. Corrosion rate of copper coupons in the presence and absence of sul®des at pH 6.5 (A) and pH 9.2 (B). Error bars indicate 90% con®dence intervals based on quadruplicate replicates.
2802
S. Jacobs and M. Edwards
Scale morphology A thick black scale was present on the coupons exposed to sul®des at both pH 6.5 and 9.2. After thorough rinsing with deionized water, these scales tested positive for sul®des using the sodium azide sul®de test (Feigl and Anger, 1972). When wet, the black scale had a slimy texture, but after drying, resembled a ®ne powder at pH 6.5 and had the macroscopic appearance of crushed black velvet at pH 9.2. The sul®de scale at both pHs ¯aked o the coupons when disturbed by changes in ¯ow or with vibration during exposure. The scale at pH 6.5 detached the most readily and in the largest pieces. Upon drying, the underlying scale was orange with a powdery appearance at pH 6.5 and was gray and slightly shiny, similar in appearance to graphite at pH 9.2. Under the SEM (Fig. 4), the black sul®de scales were globular and resembled tufts of cauli¯ower, similar to those observed by Eisenmann (1979) on brass. The inner orange scale present at pH 6.5 seemed to be constructed from thin ¯at ¯akes. The scale on the coupons exposed to water at pH 6.5 without sul®des appeared crystalline in form, while that formed at pH 9.2 without sul®des was smoothest and most uniform in appearance. The layering was especially apparent in a cross-sectional view of a coupon exposed to sul®des at pH 9.2 (Fig. 5). The outer and inner layer of the scale at pH 6.5 were examined using a KEVEX 0700 E.D.S. X-ray ¯uorescence (XRF) instrument. There were very small peaks in both scale layers for calcium and iron and large peaks at sulfur and copper. The sulfur peak was signi®cantly larger in the outermost scale layer. Cross-sections of the sul®de scale from pH 6.5 and 9.2 were examined under a JEOL 8600
electron microprobe. The scale at pH 9.2 had a uniform density and had an atomic composition of about 55% Cu, 35% S, and 10% O. The scale at pH 6.5 was composed of two primary materials of dierent density. Inner scale and the skeleton within the large tufts of the outer scale was composed of approximately 40% Cu, 10% S, and 50% O. The outer round balls of the scale at pH 6.5 have the highest sulfur content with a composition of about 55% Cu, 40% S, and 5% O. This material was Xray amorphous. After careful mechanical removal of the scale layers, the inner surface of the coupons exposed to the solution at pH 9.2 without sul®des had the appearance of a new copper coupon. The coupons at pH 6.5 without sul®des had thin shallow pits scattered over the surface, but was relatively free of corrosion over the rest of the surface. The coupons exposed to sul®des at pH 6.5 had uniform corrosion over the entire inner surface of the coupon, while the coupons exposed to sul®des at pH 9.2 exhibited severe localized corrosion with visible pits over much of the surface. Mechanistic insights into sul®de-induced copper corrosion Mechanistic insights to the sul®de-induced corrosion process can be gained from examination of the electrochemical measurements of corrosion rate and potential, as well as anodic and cathodic reaction rates. The anodic reaction rate describes the rate at which the oxidation of copper is occurring in response to an electrochemical perturbation, whereas the cathodic rate describes the rate of oxygen reduction reaction. Although the corrosion rates increased by orders of magnitude for copper coupons exposed to sul®des, the average corrosion
Fig. 3. Predicted and actual weight loss of copper coupons. Error bars represent the 90% con®dence interval. Actual weight loss measured after 9 months of experimentation.
Sul®de scale catalysis of copper corrosion
potential did not signi®cantly increase at a pH of 6.5 and decreased by only about 20 mV at a pH of 9.2. This relatively small change in corrosion potential, in combination with the large increase in corrosion rate, indicates that both the anodic and cathodic reaction rates have been catalyzed (Fig. 6). After nine months, the aged coupons that had been exposed to sul®des were placed in an exposure loop without sul®des for an additional two months.
2803
After two months of exposure to water without sul®des, the anodic reaction rates had decreased dramatically at both pH 6.5 and 9.2. However, there was only a slight decrease in the cathodic reaction rates. Consequently, the overall corrosion rate was still accelerated by about an order of magnitude when compared to coupons at the same pH that had never been exposed to sul®des. Because the corrosion rates remained acceler-
Fig. 4. SEM photographs of copper scales magni®ed 1500 times.
2804
S. Jacobs and M. Edwards
Fig. 5. Cross section of the copper metal and sul®de scale at pH 9.2 magni®ed 150 times.
ated after a long absence of soluble sul®des, the elevated corrosion rate cannot be attributed to the sul®des in solution, but must be due to the scale formed when the sul®des are present. After three months of exposure to the sul®de solutions, this scale was mechanically removed from the coupons used for short-term testing. These coupons were then placed in water without sul®des. The corrosion rate of the scraped coupons at pH 9.2 decreased by almost 80% when measured 2 h after the scale was removed. The corrosion rate at pH 6.5 decreased by less than 10% 2 h after removing the scale, but decreased by over 80% over 6 days. After seven weeks, the corrosion rate of the mechanically cleaned coupons at both pH were comparable to those that had never been exposed to sul®des. As a novel test of the corrosion accelerating properties of the sul®de scale, about 1 g of wet solids were scraped from the surface of an ``old'' coupon and then smeared onto a fresh copper coupon. After drying over night, this coupon was then
Fig. 6. Anodic and cathodic reaction rates of copper coupons at pH 6.5 (A) and pH 9.2 (B).
Sul®de scale catalysis of copper corrosion
exposed for 2 h to water without sul®des at pH 9.2 and compared to a fresh coupon without the smeared scale. The experiment was repeated using the scale-smeared and fresh coupons at pH 6.5. Interestingly, the corrosion rates of the scalesmeared coupon at pH 6.5 and 9.2 were nearly equivalent to those observed after eight months of sul®de exposure at the same pH (Fig. 7). Both anodic and cathodic reaction rates were accelerated to nearly the same extent for the scale-smeared coupons as for coupons aged in the presence of sul®des. This result clearly supports the corrosion catalyzing nature of the scale itself ®rst described by Kato et al. (1984) for copper nickel alloys, as opposed to a mechanism of catalysis through sulfur species sorbed to the metal surface described for nickel alloys in Marcus (1995). To examine the eects of oxygen concentration on the corrosion rate of coupons with previous exposure to sul®des, corrosion rate measurements were made at high (16 mg/l) and low (<0.25 mg/l) oxygen concentration without sul®des in the water. These experiments were conducted at both pH 9.2 and 6.5 on coupons with sul®de scale and on coupons which the sul®de scale had been removed 24 h previously. For coupons that had been mechanically cleaned, deaeration reduced the corrosion rate by a factor of 3 as might be expected due to a slowing of the rate limiting cathodic reaction (AWWA, 1996). In contrast, reduced oxygen concentrations have nearly no eect on the elevated corrosion rate of coupons with the sul®de scale intact. This catalysis of oxygen reduction by sul®de scale might explain how long term weight loss, during the mostly stagnant pipe exposure, could be close to weight loss estimated electrochemically based solely on measurements during ¯ow. Without the sul®de scale, a high corrosion rate is not expected to be sustained on copper at low dissolved oxygen due to
2805
reduced rates of oxygen reduction and transfer (AWWA, 1996). Practical implications for copper tube longevity It is useful to compare the adverse impacts of sul®des to other known water quality factors that in¯uence copper corrosion rates. To make an order of magnitude estimate of copper pipe life, data were synthesized for 21 dierent waters in which wellcontrolled copper corrosion experiments were continued for longer than six months (Table 3). Reported data on weight loss and corrosion rates were used to estimate the time required to eat away 1/3 of the 0.042 in pipe wall thickness on a typical 5/8 in nominal Type L copper tube. This is clearly an overly optimistic criteria for time to pipe failure for pitting corrosion and overly pessimistic for uniform corrosion. Estimates of pipe lifetime based on by-product release data were made by extrapolating metal release rates measured during the initial moments of stagnation over the entire pipe life (Edwards and Ferguson, 1993). The net result is that, under the long-term test conditions where chlorine, pH, alkalinity or inhibitors were varied, estimated copper tube lifetimes exceed 400 years (Fig. 8). This is consistent with the proven performance of copper in a wide range of domestic plumbing applications. The exceptions to this rule were tests at pH less than 6.5 under continuous ¯ow conditions or in the short term presence of high sustained chlorine residuals, for which estimated pipe lifetime can decrease to as low as 7± 30 years. In marked contrast, even at pH 9.2 and with mostly stagnant conditions, predicted pipe lifetimes in the presence of sul®des are between 1 and 3 years. It is also worth noting that, under some extremely unusual conditions, copper pipes have failed in as few as three months after installation due to
Fig. 7. Corrosion rates of fresh coupons smeared with sul®de scale are similar to the corrosion rate of coupons with 8 months of exposure to sul®des. Error bars indicate the 90% con®dence interval.
2806
S. Jacobs and M. Edwards Table 3. Synthesis of copper corrosion rate information
pH Alkalinity (mg/l as CaCO3) 7.2 7.2 7.2 7.2 7.2 7.2 7.8 7.8 7.8 7.8 7.8 7.8 9.2 9.2 9.2 6.5 6.5 9.2 9.2 6.0 7.0
15 15 15 45 45 45 300 300 300 45 45 45 45 45 45 15 15 15 15 0 15
Other noteworthy constituent (mg/l)
Symbol in Fig. 8
7.2/15 Ortho-P: 1 mg/l as P 7.2/15/P Hexametaphosphate: 1 mg/l as P 7.2/15/HP 7.2/45 Ortho-P: 1 mg/l as P 7.2/45/P Hexametaphosphate: 1 mg/l as P 7.2/45/HP 7.2/300 Ortho-P: 1 mg/l as P 7.2/300/P Hexametaphosphate: 1 mg/l as P 7.2/300/HP 7.8/45 Ortho-P: 1 mg/l as P 7.8/45/P Hexametaphosphate: 1 mg/l as P 7.8/45/HP 9.2/45 Ortho-P: 1 mg/l as P 9.2/45/P Hexametaphosphate: 1 mg/l as P 9.2/45/HP 6.5/15 6.5/15/H2S H2S: 1.0 9.2/15 H2S: 1.0 9.2/15/H2S 6.0 7.0/Cl2 Cl2: 0.9
Corrosion Test Flow measurea duration condb (years) BP: 0.78 BP: 0.13 BP: 0.35 BP: 0.99 BP: 0.30 BP: 0.57 BP: 0.33 BP: 0.42 BP: 0.84 BP: 0.44 BP: 0.16 BP: 0.27 BP: 0.08 BP: 0.07 BP: 0.07 EC: 0.98 EC: 11.0 EC: 0.07 EC: 4.92 WL: 0.46 EC: 2.2
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 0.73 0.73 0.73 0.73 0.52 > 0.5
S S S S S S S S S S S S S S S S S S S F F
Reference
Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi Edwards et al. (1999), Hidmi This work This work This work This work Benjamin et al. (1990) Reiber (1989)
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(1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996) (1996)
a
Corrosion measure is the reported data from the indicated reference used to estimate pipe life. WL=weight loss (milli-inches per year); BP=By-product release (mg/l) after 6 h stagnation, EC=electrochemical corrosion rate (mA/cm2). Experimental ¯ow condition: S=mostly stagnant, F=continuous ¯ow.
b
pitting corrosion (Edwards and Ferguson, 1993). To date, this rapid failure has never been reproduced under laboratory controlled conditions. In this work, the general sul®de exposure from solutions did not lead to the formation of distinct pits despite the very high non-uniform corrosion rates observed. However, it seems possible that highly localized production of sul®des, perhaps in the presence of isolated colonies of sulfate reducing bacteria, could somehow concentrate sul®de catalytic eects and cause pit formation. Future research should examine this possibility in greater detail, since there is strong anecdotal evidence that sul®des induce pitting in practice (Jacobs et al., 1998).
As a ®nal point, and as was noted in previous work (Jacobs et al., 1998), it is not yet possible to ascertain the extent to which sul®des aect copper corrosion in practical situations. At one extreme, since sulfate-reducing bacteria appear to be present in all distribution systems and sul®de-induced corrosion problems are initiated at concentrations at least as low as 0.007 mg/l (Gudas and Hack, 1979b), it is possible that sul®des could be a major cause of copper corrosion problems even if they are not present in the raw water. On the other hand, due to eects of other constituents in drinking water not tested in this work which might inhibit sul®de-induced corrosion problems, is possible that
Fig. 8. Rough estimates for time to pipe failure for water qualities synthesized in Table 3. Note the log time scale.
Sul®de scale catalysis of copper corrosion
problems from sul®des might be con®ned to a very narrow range of circumstances even when they are present. Additional research on this subject is necessary to resolve this issue and possible remediation of sul®de induced corrosion problems. In particular, techniques such as electrochemical impedance spectroscopy (EIS) might be useful for examining copper corrosion mechanisms in the presence of thick scale layers, and a more thorough investigation of sul®de's eects on copper corrosion by-product release are desireable. CONCLUSIONS
. The presence of sul®des in drinking water can be expected to increase corrosion rates by about 1 and 2 orders of magnitude at pH 6.5 and 9.2, respectively. Passivation does not occur even after nine months of exposure. . The general validity of electrochemical corrosion rate measures in the presence of sul®des were con®rmed by satisfactory comparison to gravimetric weight loss. Both anodic and cathodic copper corrosion reactions are catalyzed by sul®des. . The accelerated corrosion can be produced by a catalytic layer of sul®de scale on pipe. When this scale is removed and the copper metal is placed in water without sul®des, the corrosion rate returns to normal levels. When this scale is smeared on a fresh copper pipe surface, the resulting corrosion rate in water without sul®des is immediately comparable to that obtained after months of exposure to soluble sul®des. . When compared to other water quality factors known to in¯uence copper corrosion rates including pH, alkalinity, chlorine and inhibitors, sul®des pose the greatest threat to copper tube longevity. AcknowledgementsÐThe ®rst author was supported by a fellowship from the National Science Foundation. This work was supported by the National Science Foundation (NSF) under grant no. BCS-9309078. The opinions, ®ndings, and conclusions or recommendations are those of the authors and do not necessarily re¯ect the views of the NSF. REFERENCES
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