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Drinking water distribution — The effect of natural organic matter (NOM) on the corrosion of iron and copper
e>
Pergamon
Wal Sci Tech Vol. 40, No.9, pp. 17-24, 1999 e 1999
Publishedby ElsevierScienceLtd on behalfof the IAWQ Pnnted in Great Britain.All nghts reserved 0273-1223/99 520.00 + 0.00
PH: S0273-1223(99)00635-6
DRINKING WATER DISTRIBUTION - THE EFFECT OF NATURAL ORGANIC MATTER (NOM) ON THE CORROSION OF IRON AND COPPER A. Elfstrom Broo*, B. Berghult* and T. Hedberg** • Department ofChemistry, Goteborg University. S-4/2 96 Goteborg, Sweden •• Department ofSanitary Engineering. Chalmers University of Technology, S-4/2 96 Gbteborg, Sweden
ABSTRACT The objective was to study the influence of natural organic matter (NOM) on the corrosion and by-product release of iron and copper. The corrosion was studied using potentiodynarnic sweeps, coupon tests and field measurements in different Swedish municipalities. For Iron it was found that the corrosion rate decreases in the presence of NOM. The results are explained in terms of surface complexation and a good correlation is found between theoretical calculations and experimental results. In the case of copper It was found that NOM both increases the corrosion rate and the content of copper in the water after one night of stagnation. The results are compared with equilibrium calculations and a good correlation is achieved. CO 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved
KEYWORDS By-product release; copper; corrosion; equilibrium calculations; iron; NOM; surface complexation. INTRODUCTION Today's efforts to produce the ultimate drinking water put several demands on the sanitary system. From the consumer's point of view the water should be tasty and odour free, colourless, cold and free from toxic substances. In addition, the drinking water producer must be able to use the raw water available to produce a drinking water that will not deteriorate during distribution. As the water reaches the sewage treatment plant it must be possible to process the water to a quality that can be recycled to nature. Further, in the sustainable society the sludge should be used as a fertiliser without contaminants. In order to fulfil all these demands each part of the system must be taken into consideration. Usually the organic matter is thought of as an undesired contaminant in the water, being a nutritive supply for microbiological activity. Further, during the oxidising steps in the water treatment process toxic organic SUbstances may be produced and distributed. However, the organic matter also provides a shield against toxic substances in the water. As strong complexing agents the organic matter may form non-toxic complexes with otherwise undesirable pollutants in the water, such as metal ions. Neither is the effect of NOM unambiguous in the case of corrosion. In this investigation the effect of NOM on the corrosion and by-product release of distribution pipes made from iron and copper has been examined. Models for calculating the corrosion and by-product release have so far only considered the effect of the inorganic water quality parameters - pH, alkalinity, calcium content 17
18
A. ELFSTROM BROO et al.
and inorganic salts (Elfstrom Broo et al.• 1997; Sander et al .• 1997). These models have now been extended to the effect ofNOM. METHODS Electrochemical measurements Slow scan potentiodynamic sweeps, PDS. were carried out using a Radiometer (Copenhagen) VoltaLab PGZ 301 potentiostat. A conventional three-electrode electrochemical cell was used. The cell consisted of a Methrom titration vessel. The vessel was equipped with a rotating disc electrode device of own manufacture. Rotating disc electrodes with a surface area of 0.2 cm2 were prepared from pure copper rods, Specpure'" (Johnson Matthey Ltd). The electrodes were moulded in epoxy. exposing only the circular disc surface. Potentials were measured against a silver chloride reference electrode (AglAgCI, sat KCI, 197 mV vs NHE) and a cylindrical platinum net was used as counter electrode. Prior to each recorded sweep the ohmic drop between the working and the reference electrode was measured and the potential was corrected with respect to this drop. All experiments were performed at room temperature. The copper electrodes were wet polished on 1000 and 4000 mesh Carborundum paper (Struers), followed by rinsing in double-distilled water in an ultrasonic bath and immediately transferred to the cell. Coupon tests in stagnant water Iron and copper coupons were exposed to synthetic or natural waters with different water qualities with respect to alkalinity, pH, calcium, chloride and NOM content. The coupons were made of pure iron and pure copper. Before exposure to the waters the coupons were wet polished on 500 mesh Carborundum papers (Struers), rinsed in ethanol in an ultrasonic bath followed by rinsing in double-distilled water. The metal (iron and copper) concentrations were measured on a daily basis and at the end of the test weight loss was measured and corrosion products examined. Field measurements Iron samples were taken from hydrants connected to iron pipes. The hydrants were chosen so that two were located at main pipes. two at the end of the distribution network and two in between. To achieve reproducible results the hydrants were tapped at times of low consumption with a flow rate of 20 Ilmin for 10 min before sampling. Copper samples were taken in public and residential buildings. The sample points were randomly chosen and the samples were taken after stagnation (overnight). For both iron and copper six samples were taken in each municipality and analysed at a certified laboratory. Equilibrium calculations A computer program, SOLGASWATER (Eriksson, 1979) was used for the calculations of the surface complexation of iron. NOM was incorporated in the calculations by using a simple model suggested by Stumm and Morgan (1996). The concentration for the humic surface complex is derived from the total organic carbon content (TOC). The same program was used for the equilibrium calculations in the case of copper. The effect of NOM was accounted for by applying a five-site model for the complexing action of humic and fulvic acids derived by Cabaniss and Shuman (1988). In the model a combination of five different ligands is used, each described by a concentration and equilibrium constant. The concentration for the five sites for the water samples are calculated by taken the value for the TOC content and multiplying it with fitting parameters given by the authors. RESULTS AND DISCUSSION Corrosion is observed by the consumer as by-product release: The water may be coloured from iron corrosion products or it may contain soluble copper complexes giving other problems, such as discolouring of the hair after washing. Too high contents of copper may even be hazardous to health. However, it is important to distinguish between the by-product release and the actual corrosion process. Although the
Drinking water distribution
19
corrosion products are formed through the corrosion process, the by-product release is mainly determined by dissolution / precipitation equilibria. The by-products may contain soluble and insoluble species of different kinds depending on water quality. Thus, corrosion investigations must include both the by-product release and the corrosion process. Surface complexation The inside of the pipes in the drinking water distribution systems is usually covered with corrosion products, decreasing the corrosion rate. How much depends on the properties of the coverage. In contact with water the oxide surface forms surface complexes where hydrogen ions form complexes with the oxide while hydroxide ions form complexes with metal ions. This results in a surface covered with hydroxide ions. The surface groups may then exchange with other ions present in the water, e.g. carbonic acid, hydrogen carbonate and calcium, forming other surface complexes. By the action of surface complexes the dissolution of the corrosion products may be facilitated, but the same surface complexes may also hinder corrosion to take place through the inhomogeneous oxide layer. The dissolution mechanism seems to be relevant in the case of copper while the mechanism involving corrosion protection is dominating at the iron surface (Elfstrom Broo et al., 1997; Sander et al., 1997). Which surface complexes are formed depends on the water quality and different surface complexes are more or less protective. On the basis of equilibrium calculations of the surface complexation it has been possible to identify which surface complexes do not protect the iron surface. The corrosion rate is proportional to the surface concentration of these complexes (eFeOH+and eFeC03Ca+, where e denotes the surface). Complexes in solution
I
The solubility of the corrosion products is also dependent on the water quality. Soluble complexes like cupric carbonate increases the solubility of copper corrosion products, but this is not enough to explain the high copper concentrations found in stagnant water. The copper content is usually much higher than would be expected in relation to the hardly soluble corrosion products, cuprite, tenorite and malachite. Instead the content of copper is determined by an equilibrium between soluble copper complexes and the meta stable solid phase cupric hydroxide (Werner et aI., 1994; Meyer and Edwards, 1994; Edwards et aI., 1996). During the night oxygen is consumed due to the corrosion process and to microbiological activity. The resulting change in redox potential leads to a situation where new solid corrosion products are stable and a rearrangement will take place. By the action of surface complexes the dissolution of the less stable products is facilitated, while the precipitation of the new products is slower with one exception, the formation of solid cupric hydroxide. In the case of iron most corrosion products reaching the consumer are in the form of small particles (Sander et al., 1996). Since the amount of solid corrosion products is much higher than the amount of soluble, Variation in solubility due to water quality is of less importance. Further, since iron is a harmless metal, a situation with fewer particles and more soluble corrosion products would even be preferred. The effect of NOM on the corrosion of iron In the presence of NOM the corrosion rate on iron is decreased. This can be explained by the action of surface complexes involving organic matter. In figure I the corrosion rate as measured in coupon tests by means of weight loss and corrosion product release are presented as a function of the COD Mn• It can be seen that a good correlation is achieved between the two ways of measuring corrosion implying that most Corrosion products leave the surface and in a distribution network these products will reach the consumer. Thus, in the case of iron there is no major difference between corrosion rate and by-product release. In the same figure the theoretical model extended with a surface complex for NOM is plotted. The NOM is accounted for as an organic two protonic model acid with surface complex constants according to Stumm and Morgan (1996). The concentration of the model acid was empirically estimated.
20
A. ELFSTROM BROO et al.
'C' 0,14 III
~
~
0,12 0,1 . .-
...--.6.
....
........
';' 0,08
~c:
0,06
.~ 0,04
e... 0,02 o ~
•
weight loss
~
corrosion products
-model
0
o
1
2
3
4
COD(Mn) (mg 011) Figure I. The corrosion rate of iron from coupon tests and the theoretical model as a function of the content of natural organic matter.
Good correlation is also achieved between experimental and theoretical results and the same is observed for the field measurements (Figure 2). In one point a substantial deviation is observed. However, in this municipality the distribution system was exposed to extreme water pressure variations during the measuring period.
..... Median values from the field
250 .- 200 ::::
1--'- Theoretical model
Cl :::L
I
:::- 150 c
.fl c
8 e ,g
100 50
o M1
M2
M3
M4
M5
M6
M7
M8
M9 M10
Municipality Figure 2. Median values from field measurements in different municipalities and the theoretical model.
The theoretical model can be used to illustrate the influence of the water quality parameters on the corrosion process (and the corrosion product release). In Figure 3 the model is plotted as a function of pH for two different water qualities, both having a TOe concentration of 2 mg/l. It can be seen that for low alkalinity waters addition of calcium reduces the iron corrosion rate at high pH levels, while the effect is smaller and opposite in high alkalinity waters. Low pH values are preferable for all alkalinities. Specifically looking at the effect of NOM, figure 4, the reduction of the iron corrosion rate gives curves with the same shape. The maximum reduction is gained in the same region where the highest corrosion rate is observed and the effect is larger in low alkalinity waters. If the concentration of NOM is doubled the effect is also almost doubled. Interesting to note is that pH values corresponding to maximum corrosion rate reduction caused by NOM is in the close vicinity of the pH values recommended according to Langelier's index. The fact that Langelier's index became so widely spread in spite of its poor relevance may be an effect of NOM corrosion rate reduction.
Drinking water distribution
- 0 mM Ca2+ 0.5 mM HC03·
0,8
e
.g
0,7
'1 mM
Ii ~ 0,6 u 0,5 G) "Cc:"C o 0 Gl 04 E'iii'!!! '
2mM
.2
e '"
- 0 mM Ca2+ 5 mM HC03 '1 mM
~ e ~ 0,3 ;; 0 ~ I!!
o
U
21
2mM
c: 0,2
.!
0,1
~
o 6,5
6
7,5
7
8
8,5
9
pH Figure 3. Theoretical model for the iron corrosion rate in drinking water with 0.5 (grey) and 5 (black) mM total carbonate as a function of pH and calcium content and a TOe concentration of2 mg/l.
- 0 mM Ca2+ 0.5 mM HC03· '1 mM 2mM
- 0 mM Ca2+ 5 mM HC03· '1 mM 2mM
6
6,5
7
7,5
8
8,5
9
pH Figure 4. Theoretical calculation of the effect of NOM on the iron corrosion rate release in drinking water with 0.5 (grey) and 5 (black) mM total carbonate as a function of pH and calcium content and a TOC concentration of 2mgll.
The effect of NOM on the corrosion of copper As previously discussed, the copper concentration in stagnant water is primarily a function of equilibria between the meta stable solid cupric hydroxide and copper complexes in solution. Since copper forms strong complexes with NOM, the solubility of copper corrosion products increases with increasing NOM content. This has been accounted for in the model for copper in stagnant water by adding a model for the complexing properties of NOM derived by Cabaniss and Shuman, 1988. In Figure 5 the median values for the copper Content in stagnant water in different municipalities are plotted together with theoretical estimations based On the water quality in each municipality (Elfstrom Broo et 01., 1998). As can be seen from the figure, the values match well in most municipalities. Some deviations may be due to kinetic limitations, i.e. the Concentrations have not yet reached the equilibrium values since more time than one night stagnation is needed.
22
A. ELFSTROMBROO et 01.
-
~ 2500 :::I.
~ 2000 c
.ec
• Model
1500
...8 8. go
o Median value
1000
o
500 0 +-"
'--r-"~'--r-"
M1 M2
'--r-"
M3 M4 M5 M6
'-r-"--'-r-"
M7 M8
'-r-"--...,
M9 M10
Municipality Figure 5. Median copper content in stagnant water from field measurementsand theoretical estimations for different municipalities.
When it comes to the corrosion at the copper surface. the process may be divided into three parts. One initial part. where the corrosion rate is constant. one second part where the corrosion rate decreases with time and one third part where the copper content in the water is constant at a value set by the equilibrium with the meta stable cupric hydroxide. In this third part the corrosion rate is very low. During the initial part mainly one parameter seems to be determining for the corrosion rate. the chloride concentration (Figure 6).
10
Q)
8
t!~
c ca
Gl
6
e
4
00
2
o o
.>. cn~
:::I.
•
•
-. •
0
0
2
•
468 Chloride content (mM)
10
12
Figure 6. Initial corrosion rate. as determinedby potentiodynarmc sweeps and coupon measurements.as a function of the chlonde content.
It can be seen from the figure. that low chloride content « 2 mM) inhibits the corrosion. implying that the chloride ions take an active part in the reaction mechanism. At higher chloride concentrations. the corrosion rate increases with increasing chloride concentration.
>: 2 ca
•
'C
~ 1,5
•
-
1
~
°of---..,.---..,..---.,...--..,.----,
.§.
•• •
'C
;;. 0,5
o
0,5
1
1,5
2
2,5
log([C02]/IJM) Figure 7. Corrosion rate measured as the increase in copper concentrationas a function of the logarithmof the content of free carbon dioxide from coupon measurements.
Dnnking water distribution
23
During the second part of the corrosion process in which the corrosion rate decreases with time. also other parameters become important for the reaction rate. In a synthetic water containing only the inorganic compounds of a drinking water, the corrosion rate is proportional to the logarithm of the free carbon dioxide content (Figure 7). The figure represents results from coupon measurements. In order to study the reactions the ratio between the surface area and the water volume is small. In the pipe this ratio is much larger, which in tum means that what takes days in the coupon measurements may proceed in some hours in the pipe. When natural organic matter is added to the water the corrosion rate is increased. In figure 8 the corrosion rate is compensated for the contribution from free carbon dioxide. It can be seen that the corrected corrosion rate is proportional to the content of NOM.
_ 0,025
.c
~
•
0,02
Cl
.§. 0,015
-
:...... :E 0,01
•
::::l
~ 0,005 "C
0 ..._ _-..&.
o
1
......_ _....1
2
3
....._ _......
4
5
TOe (mgtl) Figure 8. Corrosion rate measured as the increase in copper concentration as a function of the content of NOM (as TOCj from coupon measurements.
This implies that the rate during the second part of the corrosion process may be written as d[Cu]
--;j/ =k, • Jg[co,l + k,
• [NOM]
Whether or not this is the right expression or the two terms are coupled in some way may not be concluded from the measurements. However, the expression matches the experimental data and it can be concluded that the content of NOM in the water is important both for the corrosion process and for the solubility of corrosion products. CONCLUSIONS In the presence of NOM the corrosion rate on iron is decreased due to surface complexation. Most iron corrosion products are solid and will sooner or later reach the consumer. Thus, the small increase in dissolved iron due to an increased NOM content will not seriously affect the positive effect of a lower corrosion rate. The concentration of copper corrosion products in stagnant water is a function of equilibria between the meta stable cupric hydroxide and dissolved copper complexes. Since NOM contains strong compexing groups. the solubility of copper corrosion products is increased by NOM. NOM also increases the corrosion rate on copper.
A. ELFSTROM BROO et al.
24
As for most water quality parameters, NOM has opposite effects on the corrosion situation for the two materials iron and copper. This implies the need for new strategies concerning material selection in water distribution systems REFERENCES Cabaniss, S. E. and Shuman, M. S. (1988). Geochim. Cosmochim. Acta. 52, 185. Edwards, M., Schock, M. R. and. Meyer, T. E. (1996) . .1. AWWA. 88(33), 8\. Elfstrom Broo, A., Berghult, B. and Hedberg, T. (1997). Corros. Sci.. 39, 1119. Elfstrom Broo, A., Berghult, B. and Hedberg, T. (1998). Corros. Sci., 40,1479. Eriksson, G. (1979) Anal. Chtm. Acta., 112, 375. Meyer, T. E. and. Edwards, M. (1994). Criucal Issues in Waterand Wastewater Treatment. p. 9, Proc, 1994 National Conference on Environmental Engmeenng (eds J. N. Ryan and M. Edwards), Boulder, Colorado. Sander, A., Berghult, B., Elfstrom Broo, A., Lind Johansson, E. and Hedberg, T. (1996). Corros. Sci.• 38,443. Sander, A., Berghult, B., Ahlberg, E., Elfstrom Broo, A., Lind Johansson, E. and Hedberg, T. (1997). Corros. Sci., 39, 77. Stumm, Wand Morgan, J. J. (1996). Aquatic Chemistry-Chemical Equiltbna and Rates in Natural Waters. 3rd edn, J. Wiley and Sons, Inc., New York. Werner W., Sontheimer, H., Groj3, H-J., Gerlach M. and Horvath, D. (1994). -Gwf-Wasser / Abwasser, 135,92.
Pergamon
Wal Sci Tech Vol. 40, No.9, pp. 17-24, 1999 e 1999
Publishedby ElsevierScienceLtd on behalfof the IAWQ Pnnted in Great Britain.All nghts reserved 0273-1223/99 520.00 + 0.00
PH: S0273-1223(99)00635-6
DRINKING WATER DISTRIBUTION - THE EFFECT OF NATURAL ORGANIC MATTER (NOM) ON THE CORROSION OF IRON AND COPPER A. Elfstrom Broo*, B. Berghult* and T. Hedberg** • Department ofChemistry, Goteborg University. S-4/2 96 Goteborg, Sweden •• Department ofSanitary Engineering. Chalmers University of Technology, S-4/2 96 Gbteborg, Sweden
ABSTRACT The objective was to study the influence of natural organic matter (NOM) on the corrosion and by-product release of iron and copper. The corrosion was studied using potentiodynarnic sweeps, coupon tests and field measurements in different Swedish municipalities. For Iron it was found that the corrosion rate decreases in the presence of NOM. The results are explained in terms of surface complexation and a good correlation is found between theoretical calculations and experimental results. In the case of copper It was found that NOM both increases the corrosion rate and the content of copper in the water after one night of stagnation. The results are compared with equilibrium calculations and a good correlation is achieved. CO 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved
KEYWORDS By-product release; copper; corrosion; equilibrium calculations; iron; NOM; surface complexation. INTRODUCTION Today's efforts to produce the ultimate drinking water put several demands on the sanitary system. From the consumer's point of view the water should be tasty and odour free, colourless, cold and free from toxic substances. In addition, the drinking water producer must be able to use the raw water available to produce a drinking water that will not deteriorate during distribution. As the water reaches the sewage treatment plant it must be possible to process the water to a quality that can be recycled to nature. Further, in the sustainable society the sludge should be used as a fertiliser without contaminants. In order to fulfil all these demands each part of the system must be taken into consideration. Usually the organic matter is thought of as an undesired contaminant in the water, being a nutritive supply for microbiological activity. Further, during the oxidising steps in the water treatment process toxic organic SUbstances may be produced and distributed. However, the organic matter also provides a shield against toxic substances in the water. As strong complexing agents the organic matter may form non-toxic complexes with otherwise undesirable pollutants in the water, such as metal ions. Neither is the effect of NOM unambiguous in the case of corrosion. In this investigation the effect of NOM on the corrosion and by-product release of distribution pipes made from iron and copper has been examined. Models for calculating the corrosion and by-product release have so far only considered the effect of the inorganic water quality parameters - pH, alkalinity, calcium content 17
18
A. ELFSTROM BROO et al.
and inorganic salts (Elfstrom Broo et al.• 1997; Sander et al .• 1997). These models have now been extended to the effect ofNOM. METHODS Electrochemical measurements Slow scan potentiodynamic sweeps, PDS. were carried out using a Radiometer (Copenhagen) VoltaLab PGZ 301 potentiostat. A conventional three-electrode electrochemical cell was used. The cell consisted of a Methrom titration vessel. The vessel was equipped with a rotating disc electrode device of own manufacture. Rotating disc electrodes with a surface area of 0.2 cm2 were prepared from pure copper rods, Specpure'" (Johnson Matthey Ltd). The electrodes were moulded in epoxy. exposing only the circular disc surface. Potentials were measured against a silver chloride reference electrode (AglAgCI, sat KCI, 197 mV vs NHE) and a cylindrical platinum net was used as counter electrode. Prior to each recorded sweep the ohmic drop between the working and the reference electrode was measured and the potential was corrected with respect to this drop. All experiments were performed at room temperature. The copper electrodes were wet polished on 1000 and 4000 mesh Carborundum paper (Struers), followed by rinsing in double-distilled water in an ultrasonic bath and immediately transferred to the cell. Coupon tests in stagnant water Iron and copper coupons were exposed to synthetic or natural waters with different water qualities with respect to alkalinity, pH, calcium, chloride and NOM content. The coupons were made of pure iron and pure copper. Before exposure to the waters the coupons were wet polished on 500 mesh Carborundum papers (Struers), rinsed in ethanol in an ultrasonic bath followed by rinsing in double-distilled water. The metal (iron and copper) concentrations were measured on a daily basis and at the end of the test weight loss was measured and corrosion products examined. Field measurements Iron samples were taken from hydrants connected to iron pipes. The hydrants were chosen so that two were located at main pipes. two at the end of the distribution network and two in between. To achieve reproducible results the hydrants were tapped at times of low consumption with a flow rate of 20 Ilmin for 10 min before sampling. Copper samples were taken in public and residential buildings. The sample points were randomly chosen and the samples were taken after stagnation (overnight). For both iron and copper six samples were taken in each municipality and analysed at a certified laboratory. Equilibrium calculations A computer program, SOLGASWATER (Eriksson, 1979) was used for the calculations of the surface complexation of iron. NOM was incorporated in the calculations by using a simple model suggested by Stumm and Morgan (1996). The concentration for the humic surface complex is derived from the total organic carbon content (TOC). The same program was used for the equilibrium calculations in the case of copper. The effect of NOM was accounted for by applying a five-site model for the complexing action of humic and fulvic acids derived by Cabaniss and Shuman (1988). In the model a combination of five different ligands is used, each described by a concentration and equilibrium constant. The concentration for the five sites for the water samples are calculated by taken the value for the TOC content and multiplying it with fitting parameters given by the authors. RESULTS AND DISCUSSION Corrosion is observed by the consumer as by-product release: The water may be coloured from iron corrosion products or it may contain soluble copper complexes giving other problems, such as discolouring of the hair after washing. Too high contents of copper may even be hazardous to health. However, it is important to distinguish between the by-product release and the actual corrosion process. Although the
Drinking water distribution
19
corrosion products are formed through the corrosion process, the by-product release is mainly determined by dissolution / precipitation equilibria. The by-products may contain soluble and insoluble species of different kinds depending on water quality. Thus, corrosion investigations must include both the by-product release and the corrosion process. Surface complexation The inside of the pipes in the drinking water distribution systems is usually covered with corrosion products, decreasing the corrosion rate. How much depends on the properties of the coverage. In contact with water the oxide surface forms surface complexes where hydrogen ions form complexes with the oxide while hydroxide ions form complexes with metal ions. This results in a surface covered with hydroxide ions. The surface groups may then exchange with other ions present in the water, e.g. carbonic acid, hydrogen carbonate and calcium, forming other surface complexes. By the action of surface complexes the dissolution of the corrosion products may be facilitated, but the same surface complexes may also hinder corrosion to take place through the inhomogeneous oxide layer. The dissolution mechanism seems to be relevant in the case of copper while the mechanism involving corrosion protection is dominating at the iron surface (Elfstrom Broo et al., 1997; Sander et al., 1997). Which surface complexes are formed depends on the water quality and different surface complexes are more or less protective. On the basis of equilibrium calculations of the surface complexation it has been possible to identify which surface complexes do not protect the iron surface. The corrosion rate is proportional to the surface concentration of these complexes (eFeOH+and eFeC03Ca+, where e denotes the surface). Complexes in solution
I
The solubility of the corrosion products is also dependent on the water quality. Soluble complexes like cupric carbonate increases the solubility of copper corrosion products, but this is not enough to explain the high copper concentrations found in stagnant water. The copper content is usually much higher than would be expected in relation to the hardly soluble corrosion products, cuprite, tenorite and malachite. Instead the content of copper is determined by an equilibrium between soluble copper complexes and the meta stable solid phase cupric hydroxide (Werner et aI., 1994; Meyer and Edwards, 1994; Edwards et aI., 1996). During the night oxygen is consumed due to the corrosion process and to microbiological activity. The resulting change in redox potential leads to a situation where new solid corrosion products are stable and a rearrangement will take place. By the action of surface complexes the dissolution of the less stable products is facilitated, while the precipitation of the new products is slower with one exception, the formation of solid cupric hydroxide. In the case of iron most corrosion products reaching the consumer are in the form of small particles (Sander et al., 1996). Since the amount of solid corrosion products is much higher than the amount of soluble, Variation in solubility due to water quality is of less importance. Further, since iron is a harmless metal, a situation with fewer particles and more soluble corrosion products would even be preferred. The effect of NOM on the corrosion of iron In the presence of NOM the corrosion rate on iron is decreased. This can be explained by the action of surface complexes involving organic matter. In figure I the corrosion rate as measured in coupon tests by means of weight loss and corrosion product release are presented as a function of the COD Mn• It can be seen that a good correlation is achieved between the two ways of measuring corrosion implying that most Corrosion products leave the surface and in a distribution network these products will reach the consumer. Thus, in the case of iron there is no major difference between corrosion rate and by-product release. In the same figure the theoretical model extended with a surface complex for NOM is plotted. The NOM is accounted for as an organic two protonic model acid with surface complex constants according to Stumm and Morgan (1996). The concentration of the model acid was empirically estimated.
20
A. ELFSTROM BROO et al.
'C' 0,14 III
~
~
0,12 0,1 . .-
...--.6.
....
........
';' 0,08
~c:
0,06
.~ 0,04
e... 0,02 o ~
•
weight loss
~
corrosion products
-model
0
o
1
2
3
4
COD(Mn) (mg 011) Figure I. The corrosion rate of iron from coupon tests and the theoretical model as a function of the content of natural organic matter.
Good correlation is also achieved between experimental and theoretical results and the same is observed for the field measurements (Figure 2). In one point a substantial deviation is observed. However, in this municipality the distribution system was exposed to extreme water pressure variations during the measuring period.
..... Median values from the field
250 .- 200 ::::
1--'- Theoretical model
Cl :::L
I
:::- 150 c
.fl c
8 e ,g
100 50
o M1
M2
M3
M4
M5
M6
M7
M8
M9 M10
Municipality Figure 2. Median values from field measurements in different municipalities and the theoretical model.
The theoretical model can be used to illustrate the influence of the water quality parameters on the corrosion process (and the corrosion product release). In Figure 3 the model is plotted as a function of pH for two different water qualities, both having a TOe concentration of 2 mg/l. It can be seen that for low alkalinity waters addition of calcium reduces the iron corrosion rate at high pH levels, while the effect is smaller and opposite in high alkalinity waters. Low pH values are preferable for all alkalinities. Specifically looking at the effect of NOM, figure 4, the reduction of the iron corrosion rate gives curves with the same shape. The maximum reduction is gained in the same region where the highest corrosion rate is observed and the effect is larger in low alkalinity waters. If the concentration of NOM is doubled the effect is also almost doubled. Interesting to note is that pH values corresponding to maximum corrosion rate reduction caused by NOM is in the close vicinity of the pH values recommended according to Langelier's index. The fact that Langelier's index became so widely spread in spite of its poor relevance may be an effect of NOM corrosion rate reduction.
Drinking water distribution
- 0 mM Ca2+ 0.5 mM HC03·
0,8
e
.g
0,7
'1 mM
Ii ~ 0,6 u 0,5 G) "Cc:"C o 0 Gl 04 E'iii'!!! '
2mM
.2
e '"
- 0 mM Ca2+ 5 mM HC03 '1 mM
~ e ~ 0,3 ;; 0 ~ I!!
o
U
21
2mM
c: 0,2
.!
0,1
~
o 6,5
6
7,5
7
8
8,5
9
pH Figure 3. Theoretical model for the iron corrosion rate in drinking water with 0.5 (grey) and 5 (black) mM total carbonate as a function of pH and calcium content and a TOe concentration of2 mg/l.
- 0 mM Ca2+ 0.5 mM HC03· '1 mM 2mM
- 0 mM Ca2+ 5 mM HC03· '1 mM 2mM
6
6,5
7
7,5
8
8,5
9
pH Figure 4. Theoretical calculation of the effect of NOM on the iron corrosion rate release in drinking water with 0.5 (grey) and 5 (black) mM total carbonate as a function of pH and calcium content and a TOC concentration of 2mgll.
The effect of NOM on the corrosion of copper As previously discussed, the copper concentration in stagnant water is primarily a function of equilibria between the meta stable solid cupric hydroxide and copper complexes in solution. Since copper forms strong complexes with NOM, the solubility of copper corrosion products increases with increasing NOM content. This has been accounted for in the model for copper in stagnant water by adding a model for the complexing properties of NOM derived by Cabaniss and Shuman, 1988. In Figure 5 the median values for the copper Content in stagnant water in different municipalities are plotted together with theoretical estimations based On the water quality in each municipality (Elfstrom Broo et 01., 1998). As can be seen from the figure, the values match well in most municipalities. Some deviations may be due to kinetic limitations, i.e. the Concentrations have not yet reached the equilibrium values since more time than one night stagnation is needed.
22
A. ELFSTROMBROO et 01.
-
~ 2500 :::I.
~ 2000 c
.ec
• Model
1500
...8 8. go
o Median value
1000
o
500 0 +-"
'--r-"~'--r-"
M1 M2
'--r-"
M3 M4 M5 M6
'-r-"--'-r-"
M7 M8
'-r-"--...,
M9 M10
Municipality Figure 5. Median copper content in stagnant water from field measurementsand theoretical estimations for different municipalities.
When it comes to the corrosion at the copper surface. the process may be divided into three parts. One initial part. where the corrosion rate is constant. one second part where the corrosion rate decreases with time and one third part where the copper content in the water is constant at a value set by the equilibrium with the meta stable cupric hydroxide. In this third part the corrosion rate is very low. During the initial part mainly one parameter seems to be determining for the corrosion rate. the chloride concentration (Figure 6).
10
Q)
8
t!~
c ca
Gl
6
e
4
00
2
o o
.>. cn~
:::I.
•
•
-. •
0
0
2
•
468 Chloride content (mM)
10
12
Figure 6. Initial corrosion rate. as determinedby potentiodynarmc sweeps and coupon measurements.as a function of the chlonde content.
It can be seen from the figure. that low chloride content « 2 mM) inhibits the corrosion. implying that the chloride ions take an active part in the reaction mechanism. At higher chloride concentrations. the corrosion rate increases with increasing chloride concentration.
>: 2 ca
•
'C
~ 1,5
•
-
1
~
°of---..,.---..,..---.,...--..,.----,
.§.
•• •
'C
;;. 0,5
o
0,5
1
1,5
2
2,5
log([C02]/IJM) Figure 7. Corrosion rate measured as the increase in copper concentrationas a function of the logarithmof the content of free carbon dioxide from coupon measurements.
Dnnking water distribution
23
During the second part of the corrosion process in which the corrosion rate decreases with time. also other parameters become important for the reaction rate. In a synthetic water containing only the inorganic compounds of a drinking water, the corrosion rate is proportional to the logarithm of the free carbon dioxide content (Figure 7). The figure represents results from coupon measurements. In order to study the reactions the ratio between the surface area and the water volume is small. In the pipe this ratio is much larger, which in tum means that what takes days in the coupon measurements may proceed in some hours in the pipe. When natural organic matter is added to the water the corrosion rate is increased. In figure 8 the corrosion rate is compensated for the contribution from free carbon dioxide. It can be seen that the corrected corrosion rate is proportional to the content of NOM.
_ 0,025
.c
~
•
0,02
Cl
.§. 0,015
-
:...... :E 0,01
•
::::l
~ 0,005 "C
0 ..._ _-..&.
o
1
......_ _....1
2
3
....._ _......
4
5
TOe (mgtl) Figure 8. Corrosion rate measured as the increase in copper concentration as a function of the content of NOM (as TOCj from coupon measurements.
This implies that the rate during the second part of the corrosion process may be written as d[Cu]
--;j/ =k, • Jg[co,l + k,
• [NOM]
Whether or not this is the right expression or the two terms are coupled in some way may not be concluded from the measurements. However, the expression matches the experimental data and it can be concluded that the content of NOM in the water is important both for the corrosion process and for the solubility of corrosion products. CONCLUSIONS In the presence of NOM the corrosion rate on iron is decreased due to surface complexation. Most iron corrosion products are solid and will sooner or later reach the consumer. Thus, the small increase in dissolved iron due to an increased NOM content will not seriously affect the positive effect of a lower corrosion rate. The concentration of copper corrosion products in stagnant water is a function of equilibria between the meta stable cupric hydroxide and dissolved copper complexes. Since NOM contains strong compexing groups. the solubility of copper corrosion products is increased by NOM. NOM also increases the corrosion rate on copper.
A. ELFSTROM BROO et al.
24
As for most water quality parameters, NOM has opposite effects on the corrosion situation for the two materials iron and copper. This implies the need for new strategies concerning material selection in water distribution systems REFERENCES Cabaniss, S. E. and Shuman, M. S. (1988). Geochim. Cosmochim. Acta. 52, 185. Edwards, M., Schock, M. R. and. Meyer, T. E. (1996) . .1. AWWA. 88(33), 8\. Elfstrom Broo, A., Berghult, B. and Hedberg, T. (1997). Corros. Sci.. 39, 1119. Elfstrom Broo, A., Berghult, B. and Hedberg, T. (1998). Corros. Sci., 40,1479. Eriksson, G. (1979) Anal. Chtm. Acta., 112, 375. Meyer, T. E. and. Edwards, M. (1994). Criucal Issues in Waterand Wastewater Treatment. p. 9, Proc, 1994 National Conference on Environmental Engmeenng (eds J. N. Ryan and M. Edwards), Boulder, Colorado. Sander, A., Berghult, B., Elfstrom Broo, A., Lind Johansson, E. and Hedberg, T. (1996). Corros. Sci.• 38,443. Sander, A., Berghult, B., Ahlberg, E., Elfstrom Broo, A., Lind Johansson, E. and Hedberg, T. (1997). Corros. Sci., 39, 77. Stumm, Wand Morgan, J. J. (1996). Aquatic Chemistry-Chemical Equiltbna and Rates in Natural Waters. 3rd edn, J. Wiley and Sons, Inc., New York. Werner W., Sontheimer, H., Groj3, H-J., Gerlach M. and Horvath, D. (1994). -Gwf-Wasser / Abwasser, 135,92.