Corrosion Science 52 (2010) 3492–3503

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Corrosion Science

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Increase of the concentration of dissolved copper in drinking water systemsdue to flow-induced nanoparticle release from surface corrosion by-products

Ignacio T. Vargas, Juan P. Pavissich, Tomás E. Olivares, Gustavo A. Jeria, Rodrigo A. Cienfuegos,Pablo A. Pastén, Gonzalo E. Pizarro *

Departamento de Ingeniería Hidráulica y Ambiental, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 September 2009Accepted 28 June 2010Available online 1 July 2010

Keywords:A. CopperB. SEMB. AFMB. XRD

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.06.027

* Corresponding author. Tel.: +56 2 354 4872; fax:E-mail address: gpizarro@ing.puc.cl (G.E. Pizarro).

Standard measurements of dissolved copper are made by filtering water samples through 0.45 lmpore-size membranes. However, the surface of corroding metallic surfaces may be covered by topo-graphic features < 0.2 lm and structures that can be detached into the bulk water as nano-sized particles.A SEM, EDX, and AFM characterization of a corroding pipe after flow events revealed surface cavities,detached particles and attached particles with sizes between 0.05 and 0.2 lm. Our findings show thatthe release of colloidal and nanoparticles of corrosion by-products into the water can result in an increaseof the dissolved copper measurements.

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1. Introduction

For 200 years copper pipes have been used for domestic waterservices around the world [1]. Nowadays it is the most used pipingmaterial in household drinking water systems because its high cor-rosion resistance [2]. There are considerable experiences and scien-tific understanding on the properties of this noble material [3].Since the first copper pipe, there have been a large number of man-ufacturing developments to obtain better material performances inorder to decrease the concentration of copper released into the tapwater.

Although copper is an essential metal for the human diet, insome cases the ingestion of copper and long-term overexposurecan generate acute and chronic health effects including gastroin-testinal diseases and liver damage [4]. The World Health Organiza-tion (WHO) recommends 2 mg/L as a maximum concentrationvalue for drinking water [5]. This value is based on gastrointestinalepidemiologic studies conducted on populations under controlledexposures. Several of those studies used copper sulfate salts toadd dissolved copper into the drinking water [6–8]. As a result,over the last decade, research on drinking water supply systemsand copper pipeline corrosion has focused on determining andmodeling the processes that control the release of copper intothe water [9–12]. In fact, based on experimental measurements,models have been developed to estimate the concentration of sol-uble copper released into the tap water [13,14].

ll rights reserved.

+56 2 354 5876.

In general, the release of soluble copper into the water is con-trolled by three processes: (1) an electrochemical process that in-volves two half-reactions, one anodic (metallic copper oxidation)and one cathodic (dissolved oxygen reduction) [15]; (2) a scale for-mation process associated with thermodynamic equilibrium con-ditions, affected by pH, dissolved oxygen (DO), temperature, andpresence of ions [3,16,17]; and (3) a dissolution of solid corrosionby-products and release of dissolved copper into the bulk water[3,18].

Current knowledge of copper corrosion supports the theory that,for new pipe systems, soluble corrosion by-product release intowater is controlled by the solubility of cupric hydroxide(Cu(OH)2(s))[10,19]. For aged pipe systems, the transition to less sol-uble and stable phases is catalyzed by the presence of anions in water[20,21]. In cold and low mineral waters cupric hydroxide ages totenorite (CuO(s)), however, in water with high dissolved inorganiccarbon concentration, ages to malachite (Cu(OH)2�CuCO3(s)) [3].Malachite dominates the solid phase speciation of Cu(II) for pHsbetween 5 and 9 [22].

Fluid flow can also affect corrosion by changing chemical andmechanical conditions at the metal–liquid interface. Flow-inducedcorrosion has been traditionally classified in four different types:mass-transport-controlled corrosion, phase-transport-controlledcorrosion, erosion–corrosion, and cavitation corrosion [23–25].Mass-transport-controlled corrosion is related to the increasedrate of mass-transport due to the flow velocity profile whichcontributes to increase the amount of corrosive species reachingthe metal surface, or alternatively, by enhancing the removal ofdissolved corrosion by-products from the solid phase. Phase-trans-port-controlled corrosion occurs when a liquid phase containing

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the corrosion agent gets in contact with the metal surface. Erosion–corrosion is associated to the mechanical removal of protective lay-ers from the metal surface by high-velocity turbulent flowsthrough shear stresses applied on solid boundaries. Cavitation cor-rosion takes place when liquid pressure drops below the vapourpressure, generating an implosion of gaseous bubbles that createsimpulsive forces capable of removing material from the solid phase[18,25,26]. In addition, there is evidence that mechanical removalof nanoparticles from the metal surface can also occur even forlow velocity flows. This process seems to be similar to the so-callederosion–corrosion, but acting at a smaller scale where shear stres-ses might be capable of sloughing micro and nanoparticles fromcorrosion by-products [18,25,26]. Similarly, fluid flow can enhanceconcentration gradients that facilitate desorption of labile copperbonded to organic moieties attached to the metallic surface [18].Therefore, for domestic pipe systems where flow velocities arerather low, it is important to identify the mechanisms that controlthe release of dissolved and particulate copper into tap water,especially because the health implications of particulate copperare poorly characterized.

According to our current knowledge there is not a study thatsimulates the dissolution of copper particles in the gastric fluid,however a recent work of lead contamination in drinking watersystems shows that gastric fluid can easily dissolve lead particles,with the associated health impact of soluble lead released intothe human body [27]. On the other hand, Taylor et al. [28] foundthrough a modeling study that corrosion and dissolution potentialsof copper nanoparticles are dependent on the size and shape of theparticle. Thus, the typical thermodynamic values calculated formetals and minerals (corrosion by-products) are not necessarilyaccurate for small particles.

The standard measurement of dissolved copper is made by filter-ing the water sample through a membrane with a pore-size of0.45 lm [29]. However, a passivating film of copper carbonatehydroxides, such as malachite, growing over the metallic surface isformed by the aggregation of structures with a size less than0.2 lm that could be detached into the water due to flow [30]. Thus,the standard definition of dissolved copper includes both soluble cu-prous and cupric species, together with particulated copper. To avoidthis inaccuracy, operational definitions have been used to describethe size distribution for particulate species in drinking water[31,32]. McNeill and Edwards [31] divide dissolved copper into col-loidal (0.1 lm < [Cu] < 0.45 lm) and soluble copper ([Cu] < 0.1 lm).Using this definition, (nanoparticles) particles < 0.1 lm could be con-founded with soluble copper (Table 1). Interestingly, even thoughthe release of particulate copper has been reported [33] and itsdetachment can be linked with the hydrodynamic conditions ofthe piping system [18], the effect of flow-stagnation events onthe detachment of micro and nanoparticles of copper corrosionby-products has been poorly considered.

Our work focuses on the effect of hydrodynamic conditions onthe detachment of copper corrosion by-product nanoparticles un-der abiotic conditions. This paper presents evidence of the detach-ment of nano and micro copper carbonate hydroxide structuresformed on the inner surface of copper pipes, induced by the shearstress produced by the fluid flow, which increases the concentra-tion of dissolved copper in water.

Table 1Operational definition of size distribution for utility surveys [31].

Size Operational definition

Total Total digested unfiltered [metal]Soluble [metal] < 0.1 lmColloidal 0.1 lm < [metal] < 0.45 lmParticulate [metal] > 0.45 lm

2. Materials and methods

Flushing experiments were conducted using a single-pass labo-ratory system, which consisted of a 1 m long copper pipe with aninternal diameter of 1.95 cm and 0.3 L of volume, preceded by aPVC pipe and a Cole-Parmer model N� 7553-75 peristaltic pumpconnected to a water tank. The copper pipes were preconditionedin a three steps protocol: (1) the pipes were filled with NaOH(0.1 M) to dissolve all oxides present on the inner surface of thepipe. (2) After two minutes with sodium hydroxide, the pipes werethoroughly rinsed three times with tap water to remove metalliccopper particles. (3) The pipes were thoroughly rinsed three timeswith MiliQ water to clean the inner surface. This protocol wasadapted from previous copper corrosion studies [31,34]. Fourflushing experiments (two with new pipes and two with 20 weekold pipes) were conducted at 19 ± 1 �C. Synthetic water was pre-pared using MilliQ water and NaHCO3 (chemical grade, 99.7% pure,Merck KGaA, Darmstadt, Germany) to adjust the amount of car-bonate to 14 mM of HCO3

� and pH 8.5. This synthetic water ishighly purified and not representative of the potable water thatcopper pipes are usually exposed to. In addition, the level of NaH-CO3 added is high for potable water and was used to induce the for-mation of a copper carbonate hydroxide film. Prior to the flushingexperiments, the synthetic water was held stagnant within the sys-tem for 8 h. Although stagnation time in household systems variesaccording to local characteristics, the European and US standardsestablish a regulatory level within 6–12 h [35]. The aging processof the 20 week old pipes consisted of daily changes of water at aflow rate of 0.4 L/min, over a 45 min period to approximate thestandard of daily consumption pattern of a kitchen tap in a fourperson house (stagnation time 97% and flow 3% of the day) accord-ing to the German pipe rig standard [3]. The average water temper-ature during the stagnation was 20 ± 1 �C. Additionally, during theaging process pH and DO were monitored though a HQ40d multimeter (HACH Company, Loveland, CO).

The tested pipes were then flushed at two flow rates which arerepresentative of domestic pipe systems under laminar and transi-tion to turbulent conditions: 0.4 and 3.0 L/min (Re = 423 and 3174,respectively). Mean flow velocity within the pipe is respectively2.2 cm/s and 16.7 cm/s. The wall shear stress can be estimated[36] as:

sb ¼f8qU2 ð1Þ

where q is the water density, U is the mean flow velocity and f is theDarcy–Weisbach friction factor. The fiction factor for laminar flowsis computed from:

F ¼ 64Re

ð2Þ

while in turbulent flows, the Colebrook–White formula provides agood estimate:

1ffiffiffif

p ¼ �2 log�=D3:7þ 2:51

Reffiffiffif

p !

ð3Þ

where e is the wall roughness height (e � 0.2 mm for copper pipes[36]) and D is the pipe diameter. For the investigated conditions,the wall shear stress is estimated at 0.0094 N/m2 in the laminarcase, and at 0.18 N/m2 in the transition case, using the Colebrook–White formula (3). Even if the shear stress for the second case isestimated using formula (3), which is valid for turbulent conditions,this value should represent an upper limit for the friction factor intransition regime. Nevertheless, it is important to emphasize thatestimated wall shear stresses are an order of magnitude belowthe critical values for erosion–corrosion reported by Efird [25].

Table 2Conditions for the flow-stagnation experiments.

Experiment Flow (L/min) Reynolds number Pipe age pH [HCO3�] (mM) Temperature during stagnation (�C)

1 0.4 423 (laminar flow) New 8.50 14 19 ± 12 3 3174 (transition flow) New 8.50 14 20 ± 13 0.4 423 (laminar flow) 20 weeks 8.55 14 18 ± 14 3 3174 (transition flow) 20 weeks 8.50 14 18 ± 1

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Details of flushing experiments are presented in Table 2 whileFig. 1a shows the experimental recirculation set-up used in theaging process. One aged pipe (pipe ‘‘A” in Fig. 1a) was tested inthe subsequent flushing experiments, and the other (‘‘B” inFig. 1a) was used for surface analyses to characterize the inner pipesurface of the aged pipe (Fig. 1b).

2.1. Copper release measurements

During flushing, 10 sequential water samples of 15 mL weretaken from the end of each tested copper pipe to determine copperconcentration until approximately 4.5 L of water were extractedfrom the pipe, following the methodology described by Calle et al.[18]. Dissolved and total copper concentrations were measured withan ICP-MS spectrometer. Dissolved copper was measured aftermembrane filtration through 0.4 and 0.2 lm pore-size cellulose ace-tate membrane (isopore) (Fig. 1c). The filter apparatus consisted of a20 mL plastic syringe, 25 mm diameter filter, and a polycarbonatesupport. The filtration method used was tested and compared in pre-vious corrosion studies [19,34]. After flushing experiments, watersamples were acidified to pH < 2 with concentrated nitric acid andstored at room temperature to dissolve copper particles. Nitric acid

Fig. 1. Experimental set-up and the procedure used for filtration-ultracentrifugation.membrane filtration and latter ultracentrifugation of water samples.

has been used in previous studies of particulate and colloidal metalsin drinking water systems to adjust the pH and dissolve solid species[27,31]. The mass of total and dissolved copper released was esti-mated by integrating the copper concentration evolution versusthe volume of water extracted from the pipe.

2.2. Surface and particle analysis

Several 1 � 1 cm copper coupons cut from the middle section offlushed and unflushed pipes (pipes A and B, respectively in Fig. 1)and isopore membranes were aseptically cut in a laminar flowchamber (Labtech, model LCB-0122H) for microscopic analysis(SEM-EDX and AFM). The samples were kept hydrated before prep-aration for microscopic examination. The coupons were treatedwith critical point drying and coated with a thin gold film. Thistreatment of the sample allows the preservation of the morphologyof hydrated structures, such as microbial cells [37,38] and clays[39]. This treatment has successfully been used in previous studiesof microbially induced corrosion [18,40] where the preservation ofmicrobial cells and corrosion by-product structures were needed.To capture particles lower than 0.05 lm (nanoparticles), aftermembrane filtration (by 0.4, 0.2 and 0.05 lm pore-size), 50 mL of

(a) Aging experimental set-up. (b) Flushing experimental set-up. (c) Sequential

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water was ultracentrifuged (Beckman L-80, SW40 rotor) for 24 h at30,000 rpm and room temperature (Fig. 1c). After ultracentrifuga-tion, the supernatant was carefully withdrawn and 10 lL of samplewas extracted from the bottom of the ultracentrifugation tube anddeposited over a 0.05 lm pore-size membrane filter (isopore) formicroscopic observation and elemental analysis. The procedureused to capture particles was adapted from the methodologydeveloped by Lienemann et al. [41] for the examination of colloidalmaterial by transmission electron microscopy. Scanning electronmicroscopy (SEM) and energy dispersive X-ray spectroscopy(EDX) were used to study the morphology and elemental composi-tion of the structures formed on the inner surface of copper pipesand particles detached into the water. A LEO 1420VP scanning elec-tron microscope coupled to an Oxford 7424 solid-state detectorwas utilized for microscopic analyses. Atomic Force Microscopy(AFM) was used to study the topography and granular structuresformed on the inner surface of the 20 weeks-aged copper pipeflushed with a transition flow (3.0 L/min). A custom made AFMmicroscope with a height precision of 0.2 nm (z axis) and previ-ously employed in surface studies [42,43] was used to study thecorrosion by-products surface. X-ray diffraction (XRD) was usedto identify the nature of the corrosion by-products formed on thepipe inner surface. XRD analysis was carried out on scales ex-tracted from the tested pipes. The pipe surface was previouslyrinsed with MilliQ water to remove the excess of salts and thengently removed with a spatula to recover and place the detachedmaterial into a centrifuge tube. Before the analysis the samplewas dried at 40 �C for 18 h. The corrosion by-products were iden-tified using a Bruker D8 Advance diffractometer with a 40 kV/30 mA copper cathode and a Sol-X detector. The experiments wereperformed continuously from 5� to 100� with a scan step size of0.02�.

2.3. Thermodynamic calculations

Thermodynamic equilibrium calculations were made using thesoftware ChemEQL [44] to predict the maximum concentration ofdissolved copper as a function of water quality parameters of theflushing experiments. Thermodynamic data for cupric species wereobtained from the literature [10,19]. Temperature corrections ofthe thermodynamic parameters were made using the Van’t Hoffrelationship [18]. Soluble copper concentration can be expressedas the sum of the soluble cupric species, assuming that a solidphase is formed [10,34,45].

3. Results

The results of the flushing experiments are organized in threesections: (1) copper release measurements, depicted by curves ofcopper mass versus the volume of water extracted from the testedpipes; (2) surface analyses that include a detailed multi-methodcharacterization of the pipes before and after the flushing experi-ments and; (3) particle observations, where the results of microand nanoparticles captured by the sequential filtering procedureare shown.

3.1. Copper release measurements

Results of flushing experiments under laminar (0.4 L/min) andtransition (3.0 L/min) flow conditions show that the copper releasedecays as the volume of water transported through the pipe in-creases (Fig. 2).

For 8 h stagnant new pipes, where the solid corrosion by-prod-ucts film is not completely developed [30], significant differencesbetween total and dissolved copper were registered (Fig. 2a). These

differences suggest the presence of particulate copper, probablyassociated to large particles (>0.4 lm) sloughed from the pipe’ssurface after a flow event. Although the amount of copper particleslarger than 0.4 lm is small, the difference in the mass of total anddissolved copper released is considerable (Fig. 2a). The ratio be-tween total and dissolved copper for the first 0.3 L (volume ofwater stagnant in 1 m pipe) varied between 1.9 (laminar flow)and 5.9 (transition flow).

In contrast, for aged pipes data do not show systematic differ-ences between total and dissolved copper (Fig. 2b). The results sug-gest that all copper is present as dissolved copper and/or copperparticles smaller than 0.2 lm. Thus, it is reasonable to speculatethat the difference between new and aged pipe experiments isassociated with the age and mechanical strength of the corrosionby-products film. In addition, for aged pipes, the mass of copper re-leased computed for all copper concentration curves are signifi-cantly greater than the mass of copper predicted by a plug-flowbehavior. However, it is probable that for all flushing experiments,especially for experiments with larger flows that generate highershear stress on the surface, copper particles < 0.2 lm increase dis-solved copper concentration.

3.2. Surface analysis

Scanning electron microscopy (SEM) analysis of the inner sur-face of 8 h stagnant new and aged pipes showed differences inthe morphology before and after transition-flow flushing experi-ments. Before the flushing event, the surface of new pipes showeda homogeneous morphology, presenting a granular aggregated filmof solid corrosion by-products that covers the metallic copper sur-face (Fig. 3a). In contrast, after flushing, the mechanical effects ofthe flow conditions on the new pipe surface are evident, with cra-ters resulting from granular formations detaching from the undev-eloped surface (Fig. 3b). A closer analysis of these granularformations suggest that they are formed by the aggregation ofsmaller features < 0.2 lm that can potentially be detached fromthe surface, thus leaving cavities (Fig. 3c). Interestingly, the cavitiesobserved after flux events have the same size as the structuresmentioned before, estimated between 0.05 and 0.2 lm (Fig. 3d).

SEM micrographs of aged pipe inner surface show a developedfilm of corrosion by-products scales. These surfaces seemed to bemore granular and heterogeneous than new pipe surfaces. Differ-ences between before and after flux were also evidenced. More-over, structures and cavities identified on new pipe surfaces werealso observed in aged pipes (Fig. 4).

Energy dispersive X-ray spectroscopy (EDX) analyses of the ob-served structures (point A in Fig. 3c) reveal an elemental composi-tion of C = 31%, O = 11%, and Cu = 58% (atomic percentages). Thisresult suggests that these structures are copper carbonatehydroxides.

Additionally, X-ray diffraction (XRD) analysis of these scales ex-tracted from the inner surface of the pipe reveals that the greenishfilm observed by light microscopy is mainly composed by mala-chite (Fig. 5).

Atomic force microscopy (AFM) analysis shows differences be-tween the inner surface of a new clean pipe (as control) and a20 week old pipe (Fig. 6). While the clean pipe (control) showeda very smooth surface with granular formations about 1 lm(Fig. 6a), probably due to a cuprite thin film of oxides formed byatmospheric corrosion [3], the aged pipe showed a porous surfaceof malachite with granular formation about 0.1 lm (Fig. 6b).Additionally, Fig. 6 reinforces the hypothesis that for aged pipesthe malachite film is formed by the aggregation of structures(Fig. 7a). A closer analysis (Fig. 7b) confirms that these structureshave a size between 0.05 and 0.2 lm. A cross-sectional analysiswas performed on one of the cavities previously observed through

Fig. 3. Scanning electron micrographs of the inner surface of new copper pipes before and after flow. (a) New copper pipe inner surface without flushing. (b) New copper pipeinner surface after flushing. (c) Magnification of the new copper inner surface without flushing, showing a structure about 0.2 lm (point A). (d) Magnification of the newcopper inner surface after flushing, showing structures and cavities about 0.2 lm.

Fig. 2. Mass measured for total and dissolved copper (through membranes of 0.4 and 0.2 lm) during flushing. 1 m long copper pipes were used for the flushing testsperformed under two flow conditions: laminar (0.4 L/min) and transition (3.0 L/min) flow. For new pipe experiments particles > 0.4 lm increase the mass of copper release(a). For aged pipe experiments, no systematic and significant differences were observed between total and dissolved copper, however high shear stress due to the flowconditions and detachment of particles < 0.2 lm were observed (b).

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Fig. 4. Scanning electron micrographs of the inner surface of aged copper pipes (20 weeks) before and after flushing. (a) Micrograph of an aged copper pipe inner surfacebefore flushing, showing presence of structures about 0.2 lm. (b) Micrograph of an aged copper pipe inner surface after flushing, showing cavities and structures about0.2 lm.

Fig. 5. X-ray diffraction (XRD) pattern of the scales extracted from the tested pipe surface. XRD analysis shows that malachite, tenorite and cuprite form the corrosion by-products film observed by SEM and AFM. The identification of malachite supports the hypothesis that copper carbonate hydroxide features observed by EDX analysis areprobably detached as colloidal malachite and nanoparticles of malachite due to hydrodynamic conditions imposed by the flow of water.

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SEM and identified as a cavity left by a detached structure (Fig. 7c),the AFM topography results support the observation that cavitiesand the structures have sizes between 0.05 and 0.2 lm.

3.3. Particle observations

SEM-EDX analysis made on filtering membranes suggests thepresence of copper carbonate hydroxides particles (assumed tobe malachite) detached during the flushing experiments (Figs. 9and 10).

In agreement with the copper release measurements for newpipe flushing experiments under laminar and transition flow, cop-per particles > 0.4 lm (large particles) were detected (Figs. 8a and9a). The presence of those large particles explains the differencebetween total and filtered copper observed previously (Fig. 2a).Even though the amount of copper particles between 0.4 and0.2 lm was negligible (Figs. 7b–8b), particles < 0.2 lm weredetected (Figs. 8e and 9e).

For the 20 weeks-aged pipe flushing, particulate matter wasretained by the three pore-size membranes (0.4, 0.2, and0.05 lm) but no large particles were observed (Figs. 8 and 9). Par-ticulate matter captured on 0.4 and 0.2 lm pore-size membranesseemed to be associated with the aggregation of smaller featuresdetached from the aged pipe surface.

Although, the morphology of malachite structures is associatedwith the stagnation conditions (e.g. dissolved inorganic carbon,temperature) and age, different configurations are formed by theaggregation of structures < 0.2 lm [30]. It is important to note thatfor all flushing experiments, particles < 0.2 lm were observed. Thisinformation is in agreement with the post-flow SEM micrographsand AFM pipe surface analysis (Figs. 3, 4 and 7) that show cavitiesof similar size.

EDX analysis of particulate matter captured by serial filtration andlater by ultracentrifugation shows that all of them are formed by car-bon, oxygen, and copper in similar atomic percentages (Table 3).Although the background from cellulose acetate membranes distorts

Fig. 6. Atomic force microscopy images of the inner surface of copper pipes. Granular formations (about 0.1 lm) observed on the aged pipe are due to the corrosion by-products formed during the aging time. (a) Clean new pipe with a soft surface used as control. (b) Aged pipe (20 week) with a porous surface. In both images z axis is in nm.

Fig. 7. Atomic force microscopy images of the inner surface of the aged copper pipe after flushing. (a) Granular structures and cavities are observed on the aged copper pipeinner surface. (b) Magnification of the previous image, showing that the structures have a size about 0.2 lm. (c) Image of other zone of the surface, showing cavities. (d) Crosssection topography analysis of the line drawn across the previous image, showing that structures and cavities are about 0.2 lm.

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Fig. 8. Scanning electron microscopy images of membrane surfaces used to filter the flushing experiments water samples and to capture particles. Flow rate of 0.4 L/min(laminar flow). (a) 0.4 lm pore-size membrane (new pipe experiment). (b) 0.4 lm pore-size membrane (aged pipe experiment). (c) 0.2 lm pore-size membrane (new pipeexperiment). (d) 0.2 lm pore-size membrane (aged pipe experiment). (e) 0.05 lm pore-size membrane (new pipe experiment). (f) 0.05 lm pore-size membrane (aged pipeexperiment).

I.T. Vargas et al. / Corrosion Science 52 (2010) 3492–3503 3499

the percentages of carbon and oxygen, the presence of copperindicates that these particles contain copper.

4. Discussion

For non-reactive surfaces, a plug-flow analysis would be suffi-cient to characterize the release of copper during the flushingevent. This non-reactive characteristic would be noted as a suddenincrease followed by an early stabilization in the mass of copperreleased (Fig. 2). However, for pipes coated with a reactive filmof solid corrosion by-products, the ideal plug-flow assumption isnot adequate to describe the release of copper into the water,and the effect of particle detachment must be considered [18](the additional information section contains supplementary flush-ing experiments under more hydrodynamic conditions).

Copper released from the inner surface of the pipe could be:(1) cupric ions released from cuprous oxide (Cu2O) oxidation[15], (2) Cu2+ due to the solubility of solid corrosion by-products,and (3) particulate copper detached by mechanical effects due toadvective mass-transport, and velocity gradients in the boundarylayer responsible for wall shear stresses.

While flushing experiments occur in few minutes (less than 2and 12 min for laminar and transition flows rate respectively), oxy-gen depletion due to copper oxidation takes place within sometens or hundreds of minutes [3]. Thus, it is reasonable to assume

that the concentration of copper in water is linked to stagnationconditions, and during flow the amount of copper oxidized is neg-ligible. Assuming that malachite controls the solubility of the solidcorrosion by-products [22], thermodynamic calculations predictdissolved copper concentrations of 0.04 mg/L. The average initialconcentration of dissolved copper observed in the four flushingexperiments after 0.4 lm pore-size membrane filtering was0.96 mg/L. This is about 22 times the value predicted by thermody-namic considerations. In addition, it is important to keep in mindthat thermodynamic equilibrium is not expected after 8 h of stag-nation [3,46]. Consequently, for the flushing experiments con-ducted in this study the assumption that 0.4 lm pore-sizemembrane filtrated copper corresponds to only dissolved copperis not adequate. The presence of particulate copper is strongly sug-gested and cannot be discarded.

Results of pipe surface SEM analysis suggest that a developedfilm of corrosion by-product scales is formed by the aggregationof structures with a size < 0.2 lm. Thus, during flow conditions,these structures could be detached and incorporated into thewater. Thermodynamic considerations, morphology observations,and X-ray diffraction results strongly suggest that the structurespresented in Figs. 3 and 4 are malachite.

EDX analysis suggests that both particles captured in laminar(point B, Fig. 7f) and transition (point C, Fig. 9f) experiments, and par-ticles observed after ultracentrifugation (points D and E, Fig. 10) arecopper carbonate hydroxides, presumably malachite (Table 3). Thus,

þMCuðps¼0:1Þ � NPðps¼0:1Þ þMCuðps¼0:2Þ � NPðps¼0:2Þ ð4Þ

Fig. 9. Scanning electron microscopy images of membrane surfaces used to filter the flushing experiments water samples and to capture particles. Flow rate of 3 L/min(transition flow). (a) 0.4 lm pore-size membrane (new pipe experiment). (b) 0.4 lm pore-size membrane (aged pipe experiment). (c) 0.2 lm pore-size membrane (new pipeexperiment). (d) 0.2 lm pore-size membrane (aged pipe experiment). (e) 0.05 lm pore-size membrane (new pipe experiment). (f) 0.05 lm pore-size membrane (aged pipeexperiment).

Fig. 10. Scanning electron microscopy images of particles captured after filtration and subsequent ultracentrifugation. Flow rate of 3 L/min (transition flow). (a) New pipeexperiment. (b) Aged pipe experiment.

3500 I.T. Vargas et al. / Corrosion Science 52 (2010) 3492–3503

EDX results support the idea that copper particles > 0.2 lm areaggregations of smaller features. As a consequence of hydrodynamicconditions, these particles can be detached from the surface andincorporated into the bulk water.

Assuming that malachite nanoparticles retained in the 0.05 lmpore-size membrane are spheres with diameters of 0.05–0.2 lmand an average density of 4.05 g/cm3 [47], the mass of copper ofeach nanoparticle is between 0.152 and 9.75 ng. According to ourobservations, particles were mainly retained in the central part ofthe filtering membranes. A conservative estimation of the mem-brane effective area, considering the distribution observed, is

0.785 mm2, 0.16% of the total membrane area. Particles werecounted on a selected area of 12 lm2 in the center of the mem-brane that was used to filter the water within a 20 weeks-agedpipe working under laminar flow conditions (Fig. 8f). The numberof particles and the mass of copper associated are presented inTable 4. The estimated mass of copper (EMCu) associated to nano-particles was calculated as:

EMCu ¼ MCuðps¼0:02Þ � NPðps¼0:02Þ þMCuðps¼0:05Þ � NPðps¼0:05Þ

Table 3EDX analysis of corrosion by-products structures and particles detached.

Sample Description Pipe age Flow rate (L/min) Elemental composition (atomic percentages)

C (%) O (%) Cu (%)

A Fig. 3c Corrosion by-product feature New – 31 11 58*

B Fig. 7f Particle (after filtration though 0.2 lm) New 3 95** 3** 2C Fig. 8f Particle (after filtration though 0.2 lm) 20 weeks 3 82** 16** 2D Fig. 9a Ultracentrifuged particle (after filtration though 0.05 lm) New 3 81** 18** 1E Fig. 9b Ultracentrifuged particle (after filtration though 0.05 lm) 20 weeks 3 88** 10** 2

* Metallic copper pipe background distorts the percentages of copper.** Cellulose acetate membranes background distorts the percentages of carbon and oxygen.

Table 4Estimated mass of copper (EMCu) related to nanoparticle size.

Particle size(ps) (lm)

Mass of copper perparticle (MCu) (ng)

No of particlescounted in 12 lm2

No of particles estimatedin 0.785 mm2 (NP)

Estimated mass ofcopper (MCu�NP) (lg)

0.02 0.01 39 2.55 � 106 0.020.05 0.15 11 7.20 � 105 0.110.1 1.2 2 1.31 � 105 0.160.2 9.7 1 6.55 � 105 0.64

I.T. Vargas et al. / Corrosion Science 52 (2010) 3492–3503 3501

where MCu is the mass of copper and NP is the estimated number ofparticles, both per particle size (ps). The numerical subscripts of Mcu

and NP are the particle size captured by the filter membrane (Table4). NP was estimated as the number of particles counted on the ana-lyzed area of SEM images multiplied by the ratio between the mem-brane effective area (i.e. the center of the membrane, estimated byseveral experimental observations to be 0.785 mm2) and the frac-tion of the membrane analyzed by SEM (12 lm2). The estimatedmass of copper related to nanoparticles (EMCu) was calculated tobe 0.93 mg, a value that, in order of magnitude, explains the differ-ence between the total and dissolved mass of copper measured(Fig. 2b).

Although Figs. 7 and 8 show particles > 0.05 lm, it is reasonableto assume that smaller particles are also released into the water.Fig. 10 shows < 0.05 lm particles captured by ultracentrifugationfor flushing experiments with new and aged pipes in transitionflow conditions.

According to the study conducted by Cong et al. [48], the waterchemistry conditions of the experiments (pH 8.5, 14 mM of HCO3

�,and a negligible concentration of Cl� and SO4

2�) could induce pit-ting during stagnation, being an additional mechanism for copperrelease. Pitting and repassivation potentials rise with carbonateconcentration and decrease with the addition of other anions suchas chloride and sulfate. Pitting enhances the amount of copper re-leased from the surface, increasing surface heterogeneity, and pre-venting formation of passivating films of malachite. However, dueto the thermodynamic conditions of the system, malachite struc-tures precipitate on the inner pipe surface, not forming a homoge-neous film, but probably following a patchwork configuration. Theprediction of dissolved copper concentration in water due to mal-achite dissolution (mentioned earlier) is 0.04 mg/L. This value isinsufficient for explaining the average concentration of dissolvedcopper measured in the experiments (0.96 mg/L). Inefficiency ofmalachite passivation based on the formation of a thin film couldexplain higher copper concentration due to metallic copper oxida-tion. However, during the pipe ageing process, pH and DO weremonitored. Over 9 weeks DO consumption and changes in pH werenot observed in the stagnant water. In-situ DO measurements havebeen used for estimating copper pipe’s inner surface passivation[46,49]. If DO is not consumed and pH does not change (due tothe cathodic half-reaction), metallic copper oxidation is not ex-pected. In addition, the ‘‘granular topography” showed by theSEM and AFM images are probably another effect of this precipita-

tion/dissolution cycles. Given the time scales on which the chem-ical reactions occur, it is reasonable to assume steady-state forchemical reactions during flow conditions. Thus, since the surfaceis covered by a heterogeneous and granular film of malachite, itis the most probable mechanism of addition of copper to the wateris the detachment of corrosion by-products particles by shearstress due to fluid flow.

Colloidal copper and copper corrosion by-product nanoparticlesdetached from the surface due to the fluid flow incorporate dis-solved copper (according to the Standard Methods definition) intothe bulk water. Interestingly, as mentioned by Taylor et al. [28]thermodynamic properties of copper nanoparticles are not neces-sarily the same as their analogs large particles, so probably parti-cles in bulk water could be ingested. In this study the authorsfound, that for standard conditions of temperature and pressure,copper nanoparticles present changes in the stability regions,resulting in a reduction in the immunity area (metallic copper do-main) and an expansion of both the passivity and corrosion areas.Hence, according with these findings it is reasonable to assumethat the corrosion and dissolution potentials depend on both thesize and shape of the particle and macroscopic properties are notenough to describe the behavior of small particles.

It is important to note that changes in the potential observed byTaylor et al. [28] were obtained for small clusters of Cu� (between 3and 38 copper atoms) not for colloidal or nanoparticulated corro-sion by-products. Additionally, the environment used for corrodingmetallic copper does not include carbonate and other mechanismssuch as pitting.

The dissolution of copper particles by gastric fluid (pH �1.2 and37 �C) [27] effectively let us assume that copper guidelines are va-lid for total copper concentration and the amount of soluble andcolloidal copper are not relevant. However, there are two aspectsthat need to be addressed. First, as it was previously mentioned,there is not clarity if it is correct to assume that thermodynamicproperties of large particles are applicable to nanoparticles [28], in-deed, the results obtained by Hong et al. [50] in a study of bindingproperties of copper to salivary proteins suggest that, at neutralpH, malachite particles (>0.45 lm) precipitate and soluble copperform protein–copper complexes, and both are retained in themouth. However, colloidal (from 0.1 to 0.45 lm size) copper andnanoparticulated (<0.1 lm size) corrosion by-products (insolubleat neutral pH, Figs. 8–10) could still be ingested and dissolved bythe gastric fluid. A second aspect that is important to keep in mind

3502 I.T. Vargas et al. / Corrosion Science 52 (2010) 3492–3503

is that models of copper speciation, release, and exposition havebeen calibrated with experimental data assuming the measuredconcentration of dissolved copper as soluble copper, underestimat-ing the presence of colloidal or nanoparticulated copper.

However, there are still several issues to be addressed, includ-ing for example, for abiotic corrosion, the release of copper parti-cles under higher flow rates, the influence of different waterquality parameters, and a detailed quantification of the amountof copper released in the form of nanoparticles. On the other side,when there is biocorrosion there are several issues that are stilluncertain such as the influence of a reactive layer (e.g. biofilm)on the porosity and the detachment properties of the solid corro-sion by-products.

5. Conclusions

This work is the first effort aimed at characterizing the effect ofhydrodynamic conditions on the release of copper corrosion by-product nanoparticle into drinking water systems.

Flushing experiments conducted under laminar and transitionto turbulent conditions show that even if the wall shear stress pro-duced by the flow is one order of magnitude smaller than the ero-sion–corrosion threshold values reported by Efird [25] mechanicaldetachment of nanoparticles could occur. Thus, corrosion studiesof the size and structure of passivating layers need to considerthe effect of flow, even below the erosion–corrosion threshold.

The results of flushing experiments, together with a multi-tech-nique surface characterization of the tested pipes, demonstratethat the detachment of copper carbonate hydroxide features, iden-tified as malachite, could result in the release of colloidal and cop-per corrosion by-product nanoparticles into the water. Estimationof the mass of copper particles using SEM images indicate that themass of these particles may explain the difference between totaland dissolved copper mass measured experimentally.

Models of copper release into drinking water will need to con-sider hydrodynamics within the pipe to estimate the mass of cop-per released as nanoparticles from the surface. This might haveprofound impact on the calibration of existing models and alsoon the evaluation of copper exposure to population, since particu-lated copper might not have the same effects as dissolved copper.This is likely to occur with other metallic pipe surfaces that maydevelop corrosion by-products with micro or nanoscale features.

Future work will be aimed at studying the release of colloidaland nanoparticulated copper under higher flow rates, the influenceof different water quality parameters, and the behavior of systemsundergoing microbially influenced corrosion.

Acknowledgements

This research was funded by CONICYT Grant 24080013/2008and FONDECYT project 1080578/2008.

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