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Changes of the corrosion potential of iron instagnation and flow conditions and theirrelationship with metal release

Massimiliano Fabbricino a,*, Gregory V. Korshin b

a Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio 21,

80125 Naples, Italyb Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195-2700, United

States

a r t i c l e i n f o

Article history:

Received 7 March 2014

Received in revised form

25 May 2014

Accepted 29 May 2014

Available online 7 June 2014

Keywords:

Iron

Metal release

Corrosion potential

Sulfate

Chloride

Online monitoring

* Corresponding author.E-mail addresses: fabbrici@unina.it (M. F

http://dx.doi.org/10.1016/j.watres.2014.05.0530043-1354/© 2014 Elsevier Ltd. All rights rese

a b s t r a c t

This study examined the behavior of corrosion potential (Ecorr) of iron exposed to drinking

water during episodes of stagnation and flow. These measurements showed that during

stagnation episodes, Ecorr values decrease prominently and consistently. This decrease is

initially rapid but it becomes slower as the stagnation time increases. During flow episodes,

the Ecorr values increase and reach a quasi-steady state. Experiments with varying con-

centrations of dissolved oxygen showed that the decrease of Ecorr values characteristic for

stagnation is likely to be associated with the consumption of dissolved oxygen by the

exposed metal. The corrosion potential of iron and its changes during stagnation were

sensitive to the concentrations of sulfate and chloride ions. Measurements of iron release

showed that both the absolute values of Ecorr measured prior to or after stagnation episodes

were well correlated with the logarithms of concentrations of total iron. The slope of this

dependence showed that the observed correlations between Ecorr values and Fe concen-

trations corresponded to the coupling between the oxidant consumption and changes of Fe

redox status. These results demonstrate that in situ Ecorr measurements can be a sensitive

method with which to ascertain effects of hydrodynamic conditions and short-term vari-

ations of water chemistry on metal release and corrosion in drinking water. This approach

is valuable practically because Ecorr measurements are precise, can be carried out in situ

with any desired time resolution, do not affect the state of exposed surface in any extent

and can be carried out with readily available equipment.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Corrosion of metals causes massive damage to water distri-

bution networks and deterioration of drinking water quality

abbricino), korshin@u.wa

rved.

(Ahammed and Melchers, 1996; Ahammed, 1998; Aleksyuk,

2002; Caleyo et al., 2002; Netto et al., 2005; Cosham et al.,

2007; Netto et al., 2007; Teixeria et al., 2008; Li et al., 2009;

Nuhi et al., 2011). The latter process is associated with the

release of solutes and particles that affect aesthetic and

shington.edu (G.V. Korshin).

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6 137

organoleptic water characteristics (Lin et al., 2001; Critchley

et al., 2004; Dietrich et al., 2004; Rogers et al., 2004) while the

leaching of heavy metals such as copper and lead from

household plumbing, service lines and other components

exposed to drinking water has been associated with adverse

health effects in exposed populations (Korshin et al., 2000;

Zietz et al., 2001, 2003; Dietrich et al., 2004; Edwards, 2004;

Kim and Herrera, 2010; Peng et al., 2010; Gonzalez et al.,

2013). In addition, trace-level contaminants such as arsenic,

vanadium and others tend to accumulate in corrosion scales

where their concentrations are greatly increased compared to

those in ambient water. As a result, instability of the corrosion

solids and their colloidal mobilization can result in exposures

of water consumers to increased levels of these contaminants

(Schock et al., 2008; Gerke et al., 2010; Peng et al., 2010; Peng

and Korshin, 2011; Yang et al., 2012).

The fundamental need to maintain the integrity of water

distribution networks and ensure the safety of drinking water

provided by them necessitates that corrosion and metal

release in drinking water be suppressed and responses of

these processes to changes of water chemistry and other

conditions in the systems (e.g., variations of temperature, flow

rates) be understood and controlled. Given the inherent

complexity of corrosion and metal release processes in com-

bination with diverse water chemistries and variable expo-

sure conditions typical for drinking water distribution

systems and household plumbing, meeting these goals re-

quires that approaches to quantify corrosion and metal

release processes in realistic conditions be employed.

In principle, overall rates of corrosion can be obtained via

weight loss or electrochemical (EC) measurements. The

complication with weight loss measurements is that while

they may be adequate for iron-based materials, they require

long exposure periods, cannot be used to examine effects of

short-term water chemistry variations and have a limited

applicability in the case of corrosion of copper- and lead-

containing materials.

Alternatively, overall corrosion rates can be estimated by

means of fitting, typically using the ButlereVolmer equation

or its variants (Flitt and Schweinsberg, 2005; McCafferty, 2005;

Mansfeld, 2005; Zhang et al., 2009), relationships between

experimentallymeasured currents through the exposedmetal

surface and its electrochemical (EC) potential which in these

measurements is varied using an external device (a poten-

tiostat). While EC measurements can be done in situ, external

polarization of the metal surfaces that is necessary for

potentiodynamic experiments may alter properties of the

examined surfaces. Even a larger complication is that the

interpretation of the data obtained in sweeps of EC potential

or other EC methods such EC impedance spectroscopy (EIS)

may be ambiguous due to the occurrence of processes unac-

counted for in the ButlereVolmer equation, for instance mass

transfer limitations, multiple parallel EC and chemical re-

actions and others. Per se, the EC measurements are also not

useful for predicting rates of metal release (they determine

only those of the electrochemical oxidation of the substrate)

from exposed surfaces, nor can they ascertain short-term

changes of metal release associated with, for instance, water

stagnation in water distribution networks or household

plumbing.

Another limitation of EC measurements of corrosion rates

in drinking water is that they do not provide unambiguous

information concerning the stability of surface-scales accu-

mulated as a result of metal corrosion on the exposed sur-

faces. These scales tend to have complex mineralogical

characteristics and are spatially non-uniform, especially in

the case of corrosion of iron and lead. Because physico-

chemical properties of these scales are strongly affected by

the chemistry of ambient water, they can become unstable in

response to changes of its properties (Sander et al., 1996, 1997;

Broo et al., 1997; Volk et al., 2000; Boulay and Edwards, 2001;

Sarin et al., 2001, 2004; D’Antonio et al., 2008; Nawrocki

et al., 2010).

Given the complexity of predicting responses of corrosion

processes and metal release to long- and short-term varia-

tions of drinking water properties and flow conditions, it is

critical to monitor these responses in real time (Volk et al.,

2000; Sancy et al., 2010; Ishii and Boyer, 2011). In principle,

this can be done by water sampling using designated sam-

pling locations in a water distribution system or using corro-

sion rigs of varying complexities. These options provide data

that can be interpreted in the context of applicable regulations

but their use requires complicated exposure and sampling

systems and ex situ analyses for metals of interest.

The extent of these limitations supports a notion that at

least ideally, metal release and corrosion processes should be

monitored and quantified using an approach based on in situ

measurements of a surrogate parameter or a system of pa-

rameters that can be measured with high precision and suf-

ficient ease in real time without use of expensive equipment

or ambiguous data processing techniques. Such parameter(s)

should be also intrinsically tied to (rather than to be purely

statistically correlated with) the occurrence and intensity of

processes that play a fundamental role in corrosion andmetal

release.

Below, we present data demonstrating that in situ moni-

toring of changes of the corrosion potential (otherwise

referred to as open circuit potential, as it is measured in the

absence of any external EC polarization) of a metal surface

exposed to drinking water and its changes in response to

variations of exposure conditions may be a viable alternative

approach for onlinemonitoring ofmetal release and corrosion

phenomena. To demonstrate this, we examined the behavior

of the corrosion potential (denoted henceforth as Ecorr) of iron

exposed to water containing varying concentrations of back-

ground salts. We also correlate phenomena observed during

episodes of water stagnation and flowwith the extent ofmetal

release in the examined systems.

This study was focused on the corrosion of iron due to

several reasons. First, iron is one of the most important ma-

terials present in drinking water distribution systems (Lin

et al., 2001; Tang et al., 2006; Sancy et al., 2010; �Swietlik

et al., 2012; Yang et al., 2012). The corrosion of iron results in

massive amounts of lost non-revenue water and damage to

the infrastructure that amount to up to several billions dollars

per year. Iron release affects taste and color of drinking water

and excessive exposure to it can be harmful (Brittenham, 2003;

Mora et al., 2009). Solid iron corrosion products can be rich in

toxic elements such as arsenic (Copeland et al., 2007; Kim

et al., 2011; Peng and Korshin, 2011) and release of such

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6138

products can lead to adverse health effects. Finally, scales

formed on exposed iron surfaces are colonized by microor-

ganisms forming biofilms whose properties are critical in

controlling the microbiological stability of drinking water

(Volk et al., 2000; McNeill and Edwards, 2001; Lee and

Newman, 2003; Lehtola et al., 2004; Little et al., 2007.)

The corrosion of iron, formation of surface solids and iron

release are sensitive to multiple water chemistry parameters

such as concentrations of dissolved oxygen, chloride, sulfate,

carbonate, natural organic matter and others (Sander et al.,

1996, 1997; Broo et al., 1999; Shi et al., 2000; Volk et al., 2000;

Appenzeller et al., 2001; Mc Neil and Edwards, 2001; Sarin

et al., 2004; Choi et al., 2005; Tang et al., 2006; Shams El Din,

2009; Nawrocki et al., 2010; Peng et al., 2010; Yang et al.,

2012; Liang et al., 2013; Liu et al., 2013; Peng et al., 2013).

Corrosion scales formed on iron exposed to drinking water

tend to form several distinct layers whose structure and

composition is affected both by the chemistry of ambient

water and diffusion processes at the involved interfaces and

through the solid phases (Sander et al., 1997). Some compo-

nents of corrosion scales formed on iron surfaces, notably

green rusts, are particularly sensitive to variations of levels of

common background anions such as chloride and sulfate

(Refait et al., 2003; Sarin et al., 2004; Nawrocki et al., 2010;�Swietlik et al., 2012). Green rusts are mixed layered double

hydroxides in which positive charges of the layers formed by

Fe2þ and Fe3þ ions are balanced by negative charges of the

interlayer anions. The sensitivity of green rusts to the type of

the intercalated anions results in their potentially strong

response to variations of sulfate and chloride concentrations.

Sulfate and chloride ions can also form soluble complexes

with Fe(II) and Fe(III) ions thus affecting the stability of the

surface solids (Lasheen et al., 2008). In general, higher levels of

sulfate and chloride tend to result in increased rates of iron

corrosion and release (Hedberg and Johansson, 1987; Sander

et al., 1996; Veleva et al., 1998; Lin et al., 2001; Refait et al.,

2003; Sarin et al., 2004; Imran et al., 2005, 2006; Nawrocki

et al., 2010; Yang et al., 2012) although more complex trends

have also been observed (Bondietti et al., 1993; McNeill and

Edwards, 2001).

These observations support the point that the occurrence

and significance of potentially rapidly changing concentra-

tions of sulfate, chloride and other water components need to

be monitored to elucidate the response of iron corrosion and

release to water quality variations. Below, we present

Corrosion ciron electr

centrifugal pump

Ag/Age

recirculation tankperistaltic

pump

sampler

Fig. 1 e General schematic of th

experimental datasets that provide information concerning

effects of some of these water chemistry parameters of the

values of Ecorr of iron exposed to drinking water.

2. Materials and methods

The study used two corrosion cells exposed to water whose

composition underwent a series of short-term changes during

the experiments. Main elements of the experimental system

are shown in Fig. 1. Each of the two setups employed in this

study contained a flow-through corrosion cell equippedwith a

two parallel iron plates, a reference Ag/AgCl electrode and a 2-

L polycarbonate recirculation tank which was open to the air.

The water was recirculated using a centrifugal pump. The

recirculation loop contained a flow meter and a valve to con-

trol the flow rate. All elements of the system were connected

using Tygon tubing. Periodic sampling of water in the recir-

culation tank was done using a peristaltic pump that with-

drew small water aliquots from the recirculation tank. The

centrifugal and peristaltic pumps were controlled by a pro-

grammable timer that used to run a predetermined sequence

of flow and stagnation conditions in the cell and to withdraw

water samples with desired frequency. Water samples with-

drawn from the system using the peristaltic pump were

acidified with high purity nitric acid and analyzed for iron

concentration using a PerkinElmer DRC-e Elan ICP/MS spec-

trometer. The calibration of the ICP/MS instrument and other

related procedures were carried as described in Peng and

Korshin 2011.

The iron cells had two rectangular 50 � 140 mm grey cast

iron plates with a 16 mm (5/800) thickness. This material was

purchased from McMaster-Carr. The plates were separated

with a 2 mm rubber seal. Each plate had a 28.2 cm2 area

exposed to ambient water, and the total exposed iron surface

was 56.4 cm2. A gel-filled Ag/AgCl reference electrode (model

A57193, Beckman Coulter Inc.) was placed in the center of the

upper plate. The tip of the reference electrode was positioned

flush with the metal surface. The potential of the surface vs.

the reference electrode was recorded every 10 s using a high

impedance multi-channel digital data acquisition unit (Agi-

lent Technologies Model 34970A) connected to a computer

that was used to store and process the data.

Initial exposures to Seattle tap water were performed

to allow the metal surfaces to develop surface scales.

Flow meter

ell with odes

PC

High impedance digital multimeter

Cl reference lectrode

e experimental apparatus.

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6 139

The exposures of the two cells were initiated at different times

(ca. 2 months difference) to allow the metal surfaces in one of

the cells to develop more extensive scales than those in the

other. Thus the two corrosion cells allowed comparing re-

sponses of relatively freshly exposed and more extensively

corroded surfaces.

The initial exposureswere followed by tests to examine the

generate data concerning effects of flow and stagnation con-

ditions of the corrosion potential of iron and examine the

reproducibility of the observed effects. These tests allowed

establishing a 4 h flow followed by a 4 h stagnation sequence

that was deemed to be adequately representative of condi-

tions in drinking water distribution systems or household

plumbing.

All experiments were carried out with Seattle tap water

that was recirculated in the system. The water initially con-

tained ca. 0.9 mg L�1 active chlorine but this oxidant dis-

appeared quickly after the beginning of each recirculation

test. Water in the recirculation tanks was exchanged daily.

Seattle tap water has lowmineralization, with background

sulfate and chloride concentrations of 3 and 2 mg L�1,

respectively. The alkalinity and hardness of the water are ca.

20 and 25 mg L�1 as CaCO3, respectively. The baseline water

conductivity is ca. 60 mS cm�1.

Further experiments addressed effects of variation of

concentrations of dissolved oxygen, chloride and sulfate.

Concentrations of NaCl or Na2SO4, used in the experiments

are listed in Table 1. In some experiments, the concentration

of dissolved oxygen in the ambient water was increased by

bubbling oxygen gas through the recirculation tank. In other

experiments the water was deoxygenated by bubbling nitro-

gen or adding requisite amounts of 0.1 mol L�1 sodium sulfite

stock solution to the recirculation tank. The pH of the water

was checked during recirculation and was found to be always

stable.

Between any successive tests that employed a different

water chemistry, the cells were flushed with the tap water for

2 h and then for 1 h by water with the desired chemical

composition. The water used for flushing was discarded.

Following this, measurements of Ecorr values and metal

release were resumed. When experiments with variations of

any controlled water chemistry parameter were finished and

before the commencement of a next series of experiments, all

plastic tubing, fittings and flow-meters were replaced and the

Table 1 e Characteristics of used solutions.

NaCl Na2SO4

mol L�1 mS cm�1 pH mol L�1 mS cm�1 pH

0.00005 73 6.5 0.00001 73 6.5

0.00068 153 6.5 0.00010 89 6.5

0.00102 197 6.5 0.00021 111 6.5

0.00136 230 6.5 0.00032 126 6.5

0.00170 261 6.5 0.00042 154 6.5

0.00205 287 6.5 0.00052 179 6.5

0.00273 395 6.5 0.00074 191 6.5

0.00341 456 6.6 0.00095 212 6.6

0.00411 492 6.6 0.00116 273 6.6

0.00479 565 6.7 0.00137 311 6.7

recirculation tankswere acidwashed and rinsed several times

with tap water.

3. Results and discussion

3.1. Effect of hydrodynamic conditions

Measurements of the corrosion potential of iron exposed to

the baseline water quality indicated that Ecorr values exhibited

pronounced and reproducible changes during episodes of

stagnation and flow (Fig. 2). This figure demonstrates that

during flow episodes, the corrosion potential rose gradually

from its value measured at the end of a preceding stagnation

episode. That increase wasmost prominent during first one to

2 h of flow. Following this, Ecorr values stabilized although they

continued to increase slowly until the end of flow episode

which was 4 h in our experiments. The absolute values of the

observed potentials were somewhat higher than those re-

ported for Evian water with varying concentrations of free

chlorine (e.g., Frateur et al., 1999) or Karlsruhe drinking water

with varying concentration of dissolved oxygen (Kuch, 1988).

In general, the observed Ecorr valueswere close to those largely

dominated by redox transition involving carbonate-

containing green rusts (their general formula is FeII4-FeIII2(OH)12CO3) formed on the electrode surface (G�enin et al.,

2006).

When the flow of water through the system was inter-

rupted, the corrosion potential decreased almost immediately

by ca. 20 mV. This was likely to be caused by nearly-

instantaneous consumption of oxygen in the layer of solu-

tion adjacent to the metal surface. After this initial rapid

change, the corrosion potential continued decreasing more

gradually as shown in Fig. 2. In the baseline water quality, the

overall decrease of the corrosion potential during a 4-h stag-

nation episode was ca. 90 mV.

This trend was similar for both cells used in this study

although the absolute values of their corrosion potentials

measured, for instance, during flow episodes were different

(�0.05 V and �0.17 V). The existence of differences of the ab-

solute values of Ecorr measured for the cells was not surprising

since the two cells had different amounts and, potentially,

properties of corrosion solids formed on the surface of iron.

Notwithstanding the observed differences in the absolute Ecorrvalues, their changes induced by alternating stagnation and

flow conditions were practically identical for both cells and

highly reproducible from one experiment to another as

demonstrated in Fig. 2.

3.2. Effect of sulfate and chloride concentration

Furthermeasurements showed that the corrosion potential of

iron was sensitive to variations of concentrations of sulfate

and chloride. As mentioned in the introductory section, such

variations are likely to occur in any distribution system

especially in those that use water sources with different

background chemistries (e.g., groundwater and surface water,

or their blends with desalinated water) (Broo et al., 1999;

Appenzeller et al., 2001; Choi et al., 2005; Tang et al., 2006;

Fig. 2 e Behavior of the corrosion potential of iron in Seattle tap water during episodes of stagnation and flow.

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6140

Shams El Din, 2009; Peng et al., 2010; Liang et al., 2013; Liu

et al., 2013; Peng et al., 2013).

Figs. 3e5 demonstrate that increases of NaCl or Na2SO4

concentrations led to several effects. The absolute values of

the corrosion potential of iron during both flowand stagnation

conditions decreased as the concentration of sodium chloride

increased (Fig. 3). The observed decreases of the corrosion

potential in the presence of either chloride or sulfate are

consistent with the notion that the redox properties of the

exposed surfaces are controlled by carbonate-, chloride- and

sulfate-containing green rusts (G�enin et al., 2006); the hy-

pothesis concerning the presence of these solids will

Fig. 3 e Effects of sodium chloride concentration on the corrosion

ultimately need to be confirmed by means of relevant struc-

tural measurements.

While the behavior of relative changes of Ecorr values dur-

ing the flow conditions did not undergo considerable changes

at varying NaCl or Na2SO4 levels, the time profiles of Ecorrduring stagnation were affected by the concentrations of

these salts. The decrease of the corrosion potential (calculated

vs. its value at the end of the preceding flow episode) within

less than a minute after the interruption of flow was most

prominent in the baseline water chemistry without additions

of NaCl or sodium sulfate (Figs. 4A and 5A). Increases of NaCl

concentration lead to a slower change of Ecorr values for

of potential of iron during episodes of stagnation and flow.

Fig. 4 e Effects of sodium chloride concentration on

changes of corrosion potential of iron during stagnation

episodes. (A) stagnation time up to 5 min and (B)

stagnation time up to 60 min.Fig. 5 e Effects of sodium sulfate concentration on changes

of corrosion potential of iron during stagnation episodes.

(A) stagnation time up to 5 min and (B) stagnation time up

to 60 min.

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6 141

stagnation times less than 2 min but for longer stagnation

times the decrease of Ecorr wasmore prominent at higher NaCl

concentrations. This effect was evident for stagnation times

up to ca. 60 min (Figs. 4B and 5B). For longer stagnation times,

changes of Ecorr vs. time tended to be close for NaCl concen-

trations above ca. 0.0014 mol L�1. For sulfate, trends of the

time profiles of Ecorr values at increasing Na2SO4 concentra-

tions were similar compared to those observed for sodium

chloride.

As mentioned above, the observed decreases of the Ecorrvalues of iron at increasing concentrations of chloride and

sulfate are consistent with the presence of green rusts on the

surface processes but the nature and overall significance of

these effects need to be studied in more detail in experiments

with varying concentrations of carbonate, pH and other pa-

rameters of the solution chemistry that are supposed to cause

theoretically predictable changes on the composition of sur-

face scales.

3.3. Effect of dissolved oxygen

On the other hand, changes of the Ecorr values consistently

observed during stagnation episodes may be hypothesized to

reflect the consumption of dissolved oxygen as a result of

oxidation of the exposed metal, similarly to what was

reported in Kuch 1988. While presenting a detailed theoretical

discussion concerning the effects that take place in the

ambient water and on the electrode surface during each

stagnation episode goes beyond the scope of this paper, suf-

fice it mention here that, fundamentally, the oxidation of iron

surface is coupled with the reduction of dissolved oxygen

present in the ambient water in the cell and/or in the pores of

the corrosion solids that cover the exposed surface. If the rate

of corrosion is sufficiently high, the dissolved oxygen in these

compartments will be noticeably reduced and the values of

Ecorr that are defined by the EC properties of the surface per se

and the concentration of dissolved oxygen at the surface may

exhibit changes reflecting the consumption of the oxidant and

other concurrent processes.

To test this hypothesis, measurements of Ecorr values at

varying levels of dissolved oxygen in the ambient water were

carried out. In one of the experiments, the water that was

recirculated through the cells was saturated with oxygen

which was continuously bubbled through the water reservoir.

This resulted in a consistent increase of the corrosion poten-

tials of iron during both flow and stagnation conditions (Fig. 6).

The behavior of Ecorr during stagnationwas in this case similar

to that observed to the background oxygen concentration but

Fig. 6 e Effects of oxygen and sulfite of the corrosion potential of iron exposed to Seattle tap water.

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6142

during the first 2 min of stagnation the corrosion potential in

water saturated with oxygen decreased more rapidly

compared with its change at the ambient dissolved oxygen

concentration.

The corrosion potential of iron strongly responded to the

removal of dissolved oxygen caused by the addition of the

oxygen scavenger sodium sulfite (Fig. 6). When the amount of

sodium sulfite corresponding to a 50% removal of the dis-

solved oxygen present in the recirculating water was added,

the corrosion potential measured during a flow episode

decreased by ca. 20mVwithin 10min after the injection of the

oxygen scavenger. Increase of sodium sulfite concentration to

a level corresponding to a 100% removal of the dissolved ox-

ygen resulted in a rapid decrease of the corrosion potential

that continued throughout the flow episode. The extent of the

changes of Ecorr values in these conditions was comparable to

that observed during flow conditions in the experiments re-

ported by Kuch 1988 for Kalsruhe drinking water in which the

concentration of oxygen was varied from close to zero to ca.

5 mg L�1.

Further increase of sodium sulfite concentrations to a level

corresponding to 200% of the expected dissolved oxygen con-

centration resultedonly in a relatively small transient changeof

Ecorr values which continued to decrease following the trend

established by the injection of the amount of sulfite equivalent

to a 100% of dissolved oxygen. The interruption of the flow and

onset of stagnation resulted in a small transient of Ecorr values

which continued changing like it did during the flow of oxygen-

free water. The experiment with addition of sulfite was then

interrupted to prevent a potentially massive reduction of iron

corrosion products that may occur in strongly reducing condi-

tions.Recirculationwithwatercontainingdissolvedoxygenwas

resumed and after two sequences of flow/stagnation the

corrosionpotential of iron returned to its valueobservedprior to

the introduction of Na2SO3.

These results confirm the hypothesis that the changes of

Ecorr values observed during the stagnation episodes can be

attributed to the consumption of oxygen in the corrosion cell

albeit that process may not be the only mechanism that

manifests itself as changes of Ecorr during stagnation. For

instance, the corrosion potential of iron is likely to be sensitive

to the redox state of solid phases, e.g., Fe2þ and Fe3þ con-

taining solids such as ferrous hydroxide, ferrihydrate, lep-

idocrosite, magnetite, siderite and potentially green rusts

(G�enin et al., 2006; Nawrocki et al., 2010) that may respond to

changes of near-surface concentrations of oxygen as well as

those of chloride, sulfate and other solution components. EC

potentials of metals are also sensitive to the concentration of

the same ion in the solution. Theoretically, the Nernst equa-

tionwhose general form is predicts that the EC potential of the

relevant metal (e.g., Fe) is linearly correlated with the loga-

rithm of activity of the free form of the oxidized form of that

metal (e.g., Fe2þ) but in the case of iron surfaces or generally

metal surface covered with scales, this dependence may not

necessarily be observed. Another complication factor is that

Ecorr values are defined by the EC potentials of both iron and

oxygen couples as well as on the intrinsic EC parameters such

as Tafel slopes that characterize the dependence of the

oxidation and reduction currents vs. the actual potential of

the surface. As a result, the observed Ecorr changes can be

indicative of several concurrent processes, including both the

accumulation of iron oxidation products and also the con-

sumption of dissolved oxygen, as shown in Fig. 6.

3.4. Relationships among corrosion potential and metalrelease

To examine relationships between changes of Ecorr and metal

release, concentrations of total iron found in the recirculation

reservoir immediately after the end of each stagnation

Fig. 8 e Correlations between iron concentrations and

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6 143

episode were measured. These measurements showed that

the total Fe concentrations in the recirculation reservoir ten-

ded to increase with those of chloride or sulfate (Fig. 7)

although incremental increases of the Fe concentrations were

small for Cl and sulfate concentrations exceeding 0.0018 and

0.0007 mol L�1, respectively. The extent of these effects was

comparable for the two examined inorganic ions, and the

observed Fe concentrations were close for the two cells used

in the experiments.

Comparisons of the total Fe concentrations and Ecorr values

or their changes during the stagnation episodes showed that

the absolute values of Ecorr measured either during flow con-

ditions or in the end of the stagnation episode after which Fe

concentrations were measured are were reasonably strongly

correlated (R2 values from 0.72 to 0.79) with logarithms of the

observed Fe concentrations (Fig. 8).

The data shown in Fig. 8 combine the results of measure-

ments carried out for variations of chloride and sulfate ions.

These results show that for Ecorr values measured in the flow

conditions preceding stagnation episodes after which the Fe

concentrations were determined, the correlations between

Ecorr values and Fe release are linear but different for the

examined cells, apparently due to their different histories and

properties of the scales on their surfaces. However, these

correlations become stronger and closer to each other for Ecorrvalues measured at the end of the stagnation episodes after

which Fe concentrations in the recirculation reservoir were

measured.

This appears to indicate that the decrease of Ecorr and Fe

release are intrinsically correlated, which is consistent with

Fig. 7 e Total iron concentrations measured during flow

periods as function of concentration of (A) chloride and (B)

sulfate.

corrosion potentials measured in (A) flow conditions and

(B) stagnation conditions.

the view that Fe release is enhanced in conditions when the

dissolved oxygen at the surface of exposed iron surface or

within the surface scales is consumed. The latter process

drives the redox status of Fe solids on the surface towards

higher prevalence of ferrous iron that has higher solubility

and is released during the stagnation episodes. The actual

mechanisms that define the extent and kinetics of changes of

Ecorr values of iron during stagnation may be more complex

and involve the decrease of dissolved oxygen concentrations

at the surface, possible changes of local pH and concentra-

tions of chloride, sulfate and other ions in the reactive zone.

These processes will also strongly depend on the nature of

solid phases formed on exposed iron surface; the properties of

these solids and their sensitivity to long- and short-term

changes of water chemistry are likely to be site specific. The

extent and significance of such site-specificity as well as

intrinsic mechanisms defining the behavior of Ecorr values

presented in this paper need to be addressed in future

research.

4. Conclusions

This study examined the behavior of corrosion potential (Ecorr)

of iron exposed to drinking water with varying composition.

wat e r r e s e a r c h 6 2 ( 2 0 1 4 ) 1 3 6e1 4 6144

Measurements of corrosion potential of iron during episodes

of stagnation and flow showed that Ecorr value exhibit com-

plex but highly reproducible changes. During stagnation epi-

sodes, the corrosion potential of iron decreases noticeably.

This decrease is initially rapid but it becomes progressively

slower as the stagnation time increases. During flow episodes,

the Ecorr values increase and reach a quasi-steady state. Ex-

periments with varying concentrations of dissolved oxygen in

the water showed that the decrease of Ecorr values character-

istic for stagnation conditions may be at least partially asso-

ciated with the consumption of dissolved oxygen by the

exposedmetal. The corrosion potential of iron and its changes

during stagnation episodes were also determined to be sen-

sitive to the concentrations of sulfate and chloride ions.

Measurements of iron release showed that both the absolute

values of Ecorr measured prior to or after stagnation episodes

were well correlated with the logarithms of concentrations of

total iron. The slope of this dependence showed that the

observed correlations between Ecorr values and total Fe con-

centrations were not reflective of the applicability of the

Nernst-type relationship between the electrochemical po-

tentials and metal concentrations but rather they corre-

sponded to the coupling between the oxidant consumption

and metal oxidation which is fundamental for corrosion pro-

cesses at open circuit potentials.

While detailed further examination of the effects of tem-

perature, flow conditions, solution chemistry, for instance pH,

carbonate concentration, NOM, hardness cations etc. is

necessary to gainmore insight into the nature of the observed

phenomena, the data generated in this study demonstrate

that in situ Ecorr measurements can be a sensitive and ulti-

mately powerful method with which to ascertain effects of

hydrodynamic conditions and short-term variations of water

chemistry on metal release and corrosion in drinking water.

This approach can be all the more valuable given that Ecorrmeasurements are precise, can be carried out in situ with any

desired time resolution, do not affect the state of exposed

surface in any extent and do not require any equipment other

than a standard reference electrode and a high-impedance

voltmeter both of which are readily available. This enables

this type monitoring to be carried out at virtually any site in a

drinking water distribution system or in household plumbing.

The implementation of this approach is contingent on the

data of further experiments with iron, its alloys and other

metals typical for drinking water systems, notably copper-

and lead-containing materials, stainless steel and results of

relevant monitoring for systems experiencing variations of

water chemistry, temperature and other conditions affecting

rates of corrosion and metal release.

Acknowledgements

This research was supported by the U.S. National Science

Foundation (grant 0931676). Dr. Fabbricino expresses his deep

gratitude to the Department of Civil and Environmental En-

gineering of the University of Washington and especially to

Prof. Stephen Burgess for the June 2004 Endowed Visiting

Professorship that supported his long-term visit that allowed

carrying out experiments reported in this paper. Views and

conclusions presented in this study do not necessary express

those of the funding agencies.

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