Influence of calcium hypochlorite on the corrosion ...

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Influence of calcium hypochlorite on the corrosion behaviour of metal
components of residential water pipes

R. LEIVA-GARCÍA, M.J. MUÑOZ-PORTERO, J. GARCÍA-ANTON*
Ingeniería Electroquímica y Corrosión (IEC), Departamento de Ingeniería Química y Nuclear

Universidad Politécnica de Valencia
Camino de Vera s/n, 46022 Valencia, tel: +34963877632, Fax: +34963877639

SPAIN
*[email protected] http://www.upv.es

Abstract: The aim of this work is to study the influence of a water disinfectant (calcium hypochlorite) on the
corrosion resistance of different metals used in residential distribution pipes (copper, lead, and galvanised iron).
Corrosion in residential distribution systems may produce different problems, such as pipe breaks or water
quality deterioration. Therefore, among the different parameters of the tap water, it is interesting to study the
effect of disinfectants used in drinking water on the corrosion of metallic pipes. Potentiodynamic curves were
carried out for copper, lead, and zinc in calcium hypochlorite solutions at 25 ºC at different concentration.
According to the results, the presence of this disinfectant increases the corrosion rate of metallic pipes.
Key-Words: - disinfectants; drinking water; copper; lead; zinc; residential water pipes; corrosion

1 Introduction

Corrosion in residential distribution systems for drinking water can cause several problems. Some of the
consequences of internal corrosion are pipe breaks, overflows, clogging of pipes with corrosion products and,
the worst effect for consumers, which is water quality deterioration [1]. Corrosion products detected at the
consumers’ taps produce colour, bad taste and odour; they could even cause health problems, depending on the
pipe materials.

Corrosion processes consist of a series of electrochemical reactions occurring at the metal surface in contact
with water and its constituents [2, 3]. Corrosivity of particular water depends on its chemical properties and
physical characteristics, as well as the nature of the pipe material [4]. Different materials can be found in the
residential distribution pipes. Before 1970 lead piping was popular in plumbing systems, due to its corrosion
resistance; therefore, some old houses may have lead pipes. Galvanised iron is another material employed in
old residential distribution systems. Nowadays, copper is widely used for tubing and piping in the distribution
systems of drinking water throughout the world owing to its excellent corrosion resistance and ease of
installation.

The effects of free chlorine on the corrosion in drinking water pipes have been studied in detail, since
chlorine is a very common disinfectant used in drinking water distribution systems [5-7]. Calcium hypochlorite
can be used as a disinfectant agent according to the UNE-EN 900:1999 standard [8]. That substance reacts in
water producing calcium hydroxide and hypochlorous acid. On the other hand, the calcium hypochlorite is an
oxidant and, then, it can be corrosive for some metals, being the responsible of corrosion problems in metallic
pipes. Therefore, the aim of this work is to study the corrosion effects of calcium hypochlorite on the corrosion
resistance of different materials in residential pipes (copper, lead, and zinc).

2 Experimental Procedure

2.1 Materials
Working electrodes were made of copper, lead, and zinc (galvanised iron), in order to simulate materials
present in the residential pipes. Working electrodes were cylindrically shaped and covered with Teflon. In this

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way, only an area of 0.5 cm2 was exposed to the electrolyte. Prior to the electrochemical tests, the samples were
wet abraded from 220 SiC (Silicon Carbide) grit to a 4000 SiC grit finish, and finally, rinsed with distilled
water and dried with hot air.

2.2 Polarization potentiodynamic curves
Polarization potentiodynamic curves were carried out in aqueous calcium hypochlorite solutions with different
concentrations (from 0 to 10 ppm). Solutions were prepared from well water without previous water treatment
processes. In all cases, tests were repeated at least three times. The scans presented in this paper are one of the
most representative curves. The cell used was a vertical glass cell with various inlets and a thermostatic shirt,
which maintains constant the temperature at 25 ºC. The potential was measured against an Ag/AgCl with KCl
3M reference electrode. The counter electrode was made of platinum.

Polarization curves were carried out in aerated solutions. Before each polarization experiment, the open
circuit potential (OCP) was recorded for one hour; the OCP value reported here was the arithmetic mean of the
last five minutes recorded values. After the OCP test, the specimen potential was reduced progressively to –
1000 mVAg/AgCl; this potential was maintained constant for 300 s in order to create reproducible initial
conditions. Then, the working electrode potential was scanned from – 1000 mVAg/AgCl (-1500 mVAg/AgCl in the
zinc tests) to 1000 mVAg/AgCl, using a scan rate of 0.1667 mV/s [9], which is slow enough to appreciate the
changes that occur due to the corrosion process; Therefore, polarization curves were recorded from the cathodic
to the anodic direction. Corrosion potential (Ecorr) and corrosion current density (icorr) were estimated from these
curves and information about the general electrochemical behaviour of the materials was also obtained.

3 Results and discussion

Figure 1 shows the OCP values of copper, lead, and zinc in the different calcium hypochlorite solutions at 25
ºC. There is not a noticeable effect of the disinfectant on the OCP values for all the tested material, since their
values that remain constant in all the cases.

With regard to the corrosion trend of the tested pipe materials, copper presents the highest OCP values; thus,
the lowest tendency to the corrosion attack, lead presents intermediate values, and zinc shows the lowest OCP
values and the highest trend to corrosion processes. Therefore, copper shows the lowest susceptibility to
corrosion, being one of the reasons to use this material in the water pipes.

0.2
0.0

OCP (VAg/AgCl) -0.2
-0.4
-0.6

-0.8

-1.0 2 4 6 8 10 12
0 Concentration of calcium hypochlorite (ppm)

copper lead zinc

Fig. 1. Open Circuit potential values of copper, lead, and zinc in the different calcium hypochlorite solutions at
25 ºC.

Figure 2 shows the potentiodynamic curves of copper, lead, and zinc in the different calcium hypochlorite
solutions at 25 ºC.

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-1
-1
-2 -2
-3 -3
-4 -4

-5 -5
log |i| (A/cm2)
log |i| (A/cm2)
-6 -6

-7 -7

-8 -8

-9 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -9 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-1.0 0 ppm Potential (VAg/AgCl) 10 ppm -1.0 0 ppm Potential (VAg/AgCl) 10 ppm

2 ppm 4 ppm 6 ppm 8 ppm 2 ppm 4 ppm 6 ppm 8 ppm

a) copper b) lead

1

0

-1

log |i| (A/cm2) -2

-3

-4

-5

-6

-7

-8
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Potential (VAg/AgCl)
0 ppm 2 ppm 4 ppm 6 ppm 8 ppm 10 ppm

c) zinc
Fig 2. Potentiodynamic curves for copper, lead, and zinc in the different calcium hypochlorite solutions at 25
ºC.

In the potentiodynamic curves of zinc, the current density values of the cathodic branch increases with the

disinfectant concentration. In the case of lead, some anodic peaks can be observed in the anodic branch, which

can be related to passivation attempts.

From the polarisation potentiodynamic curves, corrosion potential (Ecorr) and corrosion current density
(icorr),were obtained for the different materials and concentrations of calcium hypochlorite. Fig. 3 and 4 show
the corrosion potential values and corrosion current density values of copper, lead, and zinc in the different

calcium hypochlorite solutions.

In the same way that the OCP evolution, there is no influence of the disinfectant on the corrosion potential

of the tested metals. On the other hand, zinc presents the lowest corrosion potential and copper has the most

noble one of all the tested materials. Hence, copper and zinc have the best and the worst behaviour against the

corrosion, respectively.

Corrosion current density, values are low for all the materials tested without disinfectant, the lowest
corrosion current density is obtained for copper (0.39 µA/cm2 for copper, 2.47 µA/cm2 for lead, and 3.48
µA/cm2 for zinc). When the calcium hypochlorite is added, an increase in the experimental corrosion current

density is observed for all the tested materials, this increase is greater in the zinc.

In the case of zinc, corrosion current density values increase strongly as the disinfectant concentration is
high, the value being 25.27 µA/cm2 in the 10 ppm of calcium hypochlorite. With regard to lead behaviour, the

corrosion current density is greater as the disinfectant concentration increases, the current density reaching 9.49
µA/cm2 for the concentration of 10 ppm. On the other hand, in the case of copper the increase in the corrosion

current density with the presence of disinfectant is very low, that is a corrosion current density value around 0.6
µA/cm2 in all the calcium hypochlorite solutions. Therefore, the calcium hypochlorite has a harmful effect on

the corrosion resistance of metallic pipes; this effect is greater in the case of zinc that undergoes a higher

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increase in the corrosion current density with the increase in the disinfectant concentration. These results is in
agreement with observations obtained by others researchers about the effect of other disinfectants used in water
treatment on the metallic pipes [6, 10-16].

Corrosion Potential (VAg/AgCl) 0.0

-0.2

-0.4

-0.6

-0.8

-1.0 2 4 6 8 10 12
0 Concentration of calcium hypochlorite (ppm)

copper lead zinc

Fig. 3. Corrosion potential values of copper, lead, and zinc in the different calcium hypochlorite solutions at 25
ºC.

30Corrosion current density (µA/cm2)

25

20

15

10

5

0
0 2 4 6 8 10 12
Concentration of calcium hypochlorite (ppm)
copper lead zinc

Fig. 4. Corrosion current density values of copper, lead, and zinc in the different calcium hypochlorite solutions
at 25 ºC.

Corrosion current density is directly related with corrosion rate. Equation 1 expresses the corrosion rate in

mm/year.

Corrosion rate (mm/year) = k × A × icorr (1)
n×D

where:

A is the atomic weight,

n is the number of interchange electrons,
D is the density of material (g/cm3),

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k is the relation between depth and time (0.00327),
icorr is the value of the corrosion current density (A/cm2).

Table 1 summarises the results of the corrosion rate expressed in mm/year for copper, lead, and zinc in the
different calcium hypochlorite solutions.

Table 1. Corrosion rates values of copper, lead, and zinc in the different calcium hypochlorite solutions at 25
ºC.

Corrosion rate (mm/year)

Concentration (ppm) copper lead zinc
0 0.040
0.045 0.074

2 0.059 0.159 0.091

4 0.059 0.165 0.218

6 0.060 0.223 0.272

8 0.061 0.230 0.280

10 0.059 0.248 0.378

According to the values showed in Table 1, the effect of the disinfectant on the thickness loss of copper
pipes is low, with a small increment of 0.014 mm/ year between the tests with and without disinfectant. In the
case of lead, there is an important increase in the material loss when the calcium hypochlorite is added, this loss
increases with the disinfectant concentration. With regards to zinc, the corrosion rate of zinc grows in the
presence of calcium hypochlorite and this effect is higher with concentration. When the zinc film is lost in the
galvanised iron pipes, the corrosion of iron take place, leading to the presence of iron ions in water which
colour could reduce its quality.

Therefore, copper pipes present the best corrosion resistance in the presence of calcium hypochlorite and
zinc has the worst corrosion behaviour. Then, the main problems during this treatment will be in the oldest
pipes made of lead and zinc (galvanised iron), that have less corrosion resistance than copper in presence of
disinfectants. Thus, when the calcium hypochlorite is used, it is convenient to control its use, as far as possible,
in areas with old residential distribution systems.

4 Conclusions

The main conclusions of this work can be summarised as follows:

- Copper presents the lowest trend to corrosion of the three tested materials. On the other hand, zinc
shows the highest trend to corrosion and lead has an intermediate behaviour.

- Copper presents the highest corrosion potential of the three tested materials. On the other hand, zinc
shows the lowest corrosion potential and lead has an intermediate value.

- Copper has the lowest corrosion rate of the three tested materials in the presence of calcium
hypochlorite.

- In the case of lead and zinc, the addition of calcium hypochlorite produces an increase in the corrosion
rate of both metals. Therefore, when calcium hypochlorite is used, it is convenient to control the
treatment conditions, as far as possible, in areas with old residential distribution systems.

Acknowledgements
We wish to express our gratitude to FEDER, and to Dr. Asunción Jaime for her translation assistance.

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