Pergamon Corrosion Science, Vol. 39, No. 1, pp. 77-93, 1997

Copyright 0 1996 Elsevler Science Ltd Printed in Great Britain. All rights reserved

001&938X/97 $17.00+0.00

PII: !3001&938X(96)00107-2

IRON CORROSION IN DRINKING WATER DISTRIBUTION SYSTEMS-SURFACE COMPLEXATION ASPECTS

A. SANDER,* B. BERGHULT,? E. AHLBERG,? A. ELFSTR6M BROO,? E. LIND JOHANSSON* and T. HEDBERG*

*Department of Sanitary Engineering, Chalmers University of Technology, S-412 96 GBteborg, Sweden t Department of Inorganic Chemistry, Giiteborg University, S-412 96 Goteborg, Sweden

Abstract-Calculations on the surface complexation at an iron oxide surface in water containing carbonate and calcium ions were performed as a function of pH. The results are compared with experimental results from electrochemical measurements, coupon tests and with data collected from the pipe networks in different municipalities. A model is presented where the corrosion rate is shown to depend on the nature and concentration of surface complexes. The model applies both to the initial corrosion as determined by electrochemical experiments in the laboratory and to the steady state situation in the distribution networks. Copyright 0 1996 Elsevier Science Ltd

Keywords: A. iron, B. potentiodynamic, C. rust.

INTRODUCTION

Corrosion in distribution systems for drinking water causes considerable investment for the municipalities world-wide.‘,’ The consequences of internal corrosion are pipe breaks, overflows, clogging of pipes with corrosion products and, the major effect for the consumers, water quality deterioration. Corrosion products detected at the consumers’ taps cause colour, bad taste and odour and, eventually, health problems depending on the pipe materials. Pipe materials are also important when the sludge from the sewage treatment plants is valued as fertiliser. Sludge containing corrosion products with heavy metals is unsuitable to use as soil improver.

Distinguishing between water quality problems and corrosion rate is important. Although the total mass of corrosion products inside the pipes is a function of the corrosion rate integrated over time, water quality deterioration primarily depends on the precipitation and dissolution properties of the corrosion products.

Many investigations describe how to reduce corrosion problems in specific networks.3-6 To be able to avoid corrosion problems more generally and in a longer perspective, the mechanisms for corrosion coupled to different water qualities and different pipe materials have to be better understood than today.

Several theories have been proposed since the beginning of the century, to explain why certain water qualities appear to be less corrosive than others. Tillmans et d7 observed protective layers of crystalline calcium carbonate if the water was saturated with respect to this compound. Earlier the effect on corrosion of increasing the alkalinity was said to be due

Manuscript received 23 May 1996.

77

78 A. Sander et al.

to the increased pH value. Tillmans et al. concluded that for hard water any pH-rising method is appropriate for a protective calcite-rust scale to develop. For very soft waters the concentrations of carbonate and calcium must be considered as well as the pH value. Langlier’ further developed this theory and introduced the Langlier Saturation Index (LSI) as a tool to achieve equilibrium for calcium carbonate. This index indicates whether the water is supersaturated or undersaturated with respect to calcium carbonate, calcite. In a slightly supersaturated water a thin film of calcium carbonate would protect the pipe from further corrosion. Later, several investigations showed that the Langlier index is insufficient to predict the formation of protective scales on iron.‘-*’ For example, precipitation can occur in undersaturated water since the pH value at the pipe surface differs from the pH in the water due to oxygen reduction. Moreover, other solids like siderite, FeC03, reach saturation before the saturation for calcite is achieved. Sontheimer et al.‘* referred to the buffer capacity as the most important factor for the formation of protective scales in the so- called siderite model. When the buffer capacity is high, the precipitation of corrosion products is slower and the scales become more protective. Iron(I1) ions will form siderite instead of less protective iron hydroxide, Fe(OH)*, goethite, cl-FeOOH, or other iron products. Siderite then oxidises to magnetite, Fej04, or pseudomorphous goethite, while retaining a crystal line structure. These scales are stable, dense and protective.

Kuch’3,‘4 discussed the red water problems and emphasised the importance of stagnation vs high flow velocities in iron pipes. During stagnation or low flow rates oxygen will be depleted due to the corrosion reactions. The lack of oxygen will cause reduction and dissolution of formed corrosion products. At higher flow rates these reduced products will be reoxidised and transported with the water causing red water problems at

the consumers’ tap. The siderite model is not applicable in very soft waters. Ferguson,” Fiksdal and

Blekkan16 and Fiksdali7 agree with the earlier models that the calcium content is a very important factor for developing protective calcite scales in such waters.

Sander et aI.‘* proposed a surface complexation model to explain the corrosion dependence on the content of calcium and the carbonate system in both soft and harder waters. In the model surface complexes of the present species, e.g. carbonic acid and calcium ions, determines the rate of corrosion. In the present paper theoretical calculations were made on the surface complexation at an iron oxide surface. Both initial corrosion as obtained by electrochemical experiments and the steady state situation in the distribution network is shown to depend on the surface complexation.

Many metals exposed to aqueous solution will corrode to form some kind of layer at the surface. The properties and composition of the layer is dependent on the metal and the water chemistry. Thus a three layered structure will result, metal/film/solution, and the continuation of the corrosion process will be dependent on the processes at the metal/film and film/solution interfaces. Also the properties of the film may be important for the corrosion process. The surface complexation model highlight on the processes at the film/ solution interface. By combining information from surface complexation and corrosion rate determinations the rate limiting processes for the corrosion can be evaluated.

Surface complexation The acid-base properties and the adsorption of metal ions and ligands on solids in

aqueous solutions have been subject to numerous investigations during the last two decades. The aim has been to gain insight into the dissolution-precipitation and adsorption reactions

Fe corrosion in drinking water distribution systems 19

that are thought to play a vital role, for example, in the chemistry of natural waters, in colloid chemistry and in corrosion science. It has, for example, been shown that the dissolution rate of oxides is proportional to the concentration of surface complexes. Thus acid catalysed dissolution involves protonation of the surface while ligand promoted dissolution involves surface complexes with the ligand. The development of the solid-water interface chemistry has been reviewed in recent books19-22 and much experimental information is now available for sorption reactions at room temperature, particularly on hydrous ferric oxide.20723,24

Fundamental to the surface complexation theory are the following assumptions:

1. Sorption of ions involves site-specific binding at surface functional groups. 2. These reactions can be described using mass law equations, similar to complex

formation in solution. 3. The variable electrostatic effects that influence surface reactions can be accounted for

by including a coulombic term in the surface complexation constants. The coulombic term is derived from the electrochemical double layer theory.

The main difference compared to earlier models is the primacy given the specific chemical interaction at the surface over electrostatic effects, and in the assignment of the surface charge to sorption reactions themselves. The charge at the surface is balanced by a charge in solution adjacent to the surface and is calculated from the theory of the electrochemical double layer.

When a metal oxide is exposed to water or its vapour, surface hydroxyl groups are formed by dissociative adsorption of water molecules. These groups are amphoteric and can therefore be protonated and deprotonated. Surface complexation of cations by hydrous oxides involves the formation of bonds with these surface hydroxyl groups and the release of protons from the surface. Thus, one might expect a strong pH dependence. As the pH is increased, cation sorption on hydrous oxides increases from 0 to 100% over a narrow pH range. Specific sorption of anions of hydrous oxides occurs via ligand exchange reactions in which hydroxyl surface groups are replaced by the sorbing ions.

On surfaces like sulphides, carbonates, phosphates and similar compounds the dissociation of water will yield two types of surface sites, the surface hydroxyl group and an anion site that can be deprotonated and absorb cations in a similar way as the surface hydroxyl group.

Dissolution Even though surface complexation relies on thermodynamic grounds, the surface

complexation concept has been used to formulate kinetic steady-state models for dissolution of solids. The coordination environment of the metal changes in the dissolution reaction of an oxide. For example, when goethite is dissolved, iron in the crystalline lattice exchanges its oxygen ligand for water or another ligand, L. There is a considerable amount of empirical data available on surface reactivity in terms of dissolution rates. The concept of surface complexation has successfully been used to explain the dissolution kinetics of oxides and minerals.25

For a surface controlled dissolution the reaction occurs schematically in the following sequence:

surface sites + reactants(H+, OH- or ligands) fast surface species (1)

80 A. Sander et al.

surface sites SlOW detachment of meta,F metal ion (aq) + surface sites (2)

Although each sequence may consist of a series of smaller reaction steps, the rate law of surface-controlled dissolution is based on the idea: (i) that the attachment of the reactants to the surface sites is fast, (ii) that the subsequent detachment of the metal species from the surface of the crystalline lattice into the solution is slow and thus rate limiting, and (iii) that the original surface sites are continuously reconstituted.

The dissolution reaction is initiated by the surface coordination with H+, OH- and ligands, which polarises, weakens and tends to break the metal-oxygen bonds in the lattice of the surface. Since the detachment of the metal cation is rate limiting the steady-state approach leads to a constant dissolution rate over time, where the rate of dissolution is proportional to the particular surface species. The same conclusion is reached if we treat the reaction sequence according to the transition state theory. The particular surface species that has formed from the interaction of H+, OH- or ligands with surface sites is the precursor of the activated complex. In simple terms, the rate laws for the ligand promoted, RL, and the proton-promoted, RH, dissolution rate can be given by:

RL = kL{- SL} (3)

RH = kH(= SOH,+}j (4)

The overall rate law for the dissolution is given by the sum of the individual reaction rates assuming that the dissolution occurs in parallel at the various metal centres. In a similar way dissolution of mixed oxides and other solids can be treated by assuming that the rate depends on the surface complex of the individual components in relation to the composition. l9

In the present paper a brief description of the concept of surface complexation is given with the emphasis on the corrosion of oxide covered metals. Model calculations of the surface complexation at the goethite-water interface in the presence of calcium and carbonate are compared to corrosion rate determinations in laboratory experiments and field investigations.

EXPERIMENTAL METHOD

Electrochemical measurements: instrumentation, cells and electrodes Slow scan potentiodynamic sweeps, PDS, were carried out using a Hewlett Packard

synthesiser 3325A and a Princeton Applied Research potentiostat/galvanostat 363. A Hewlett Packard multimeter 3478A collected the data and a Hewlett Packard microcomputer HP 85/86 controlled the system and analysed the data. Computer programs for system control and data analysis were specially developed.

A conventional three-electrode electrochemical cell was used. The cell consisted of a Metrohm titration vessel (150 ml) with a specially designed lid. The vessel was equipped with a rotating disc electrode device of own manufacture as working electrode. Rotating disc electrodes with a surface area of 0.2 cm2 were prepared from pure iron rods 99.9985% (Johnson Matthey Ltd). The electrodes were moulded in epoxy, exposing only the circular disc surface. The potential of the working electrode was measured against a silver chloride reference electrode (Ag/AgCl, sat KCl, 197 mV vs the normal hydrogen electrode, NHE).

Fe corrosion in drinking water distribution systems 81

After 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. A cylindrical platinum net was used as counter electrode. All experiments were performed at room temperature under a blanket of clean air.

The iron electrodes were wet polished on 1000 and 4000 mesh Carborundum papers (Struers), rinsed in double-distilled water and immediately transferred to the electrochemical cell. The electrode was pre-treated by a cycle procedure as follows: The scan was started close to the corrosion potential, E = - 3.50 mV(Ag/AgCl). A sweep was made in a positive direction, with a sweep rate of 5 mV/s, to E = 600 mV, reversed to E= - 1000 mV and back to E= - 350 mV. The rotation rate remained constant at 50 rps. This procedure was repeated five times before the scan was recorded.

Electrochemical measurements: basic considerations Electrochemical measurements in diluted solutions have to be made under consideration

of the basic assumptions for the technique used. One item of interest is the migration. A rough calculation of the current distribution in the most diluted solution reveals that less than 4% of the total current is carried by the reaction participants. Since the results follow the same trend as for the solutions with much higher ionic strength, the migration effect has been assumed to be of minor importance. A similar question arises concerning double layer effects. Double layer effects on corrosion rate estimations have been discussed,26 and if present large errors may appear. To estimate the quality of the results obtained by the electrochemical techniques, independent corrosion measurements were made using a test rig where no electrochemical techniques were applied (coupon tests).

Coupon tests in stagnant water Iron coupons were exposed to double-distilled water with varying additions of hydrogen

carbonate and calcium in the same range as for the PDS measurements. Three test periods of 12-l 5 days were performed. The iron coupons were 22 x 37 x 2 mm3 and made of pure iron. They were wet polished on 500 and 1000 mesh Carborundum papers (Struers), rinsed in ethanol in an ultrasonic bath followed by rinsing in double-distilled water and immediately transferred to the beakers. The beakers were mounted in a water bath at a constant temperature of 30°C.

The water volume was 400 ml at the start of the test. Each analysis of the iron content required between 0.5-10ml of water. This volume decrease was accounted for when calculating the final iron concentrations.

Iron concentrations were analysed each day during the test period. Micropipettes, 200 and 5000 ml, were used to transfer water samples from the beakers. The accuracy of the pipettes was + 1% (normally 0.6%) and the reproducibility 0.3%. A Hach instrument (DR/ 700 Colorimeter, Module 50.01) was used for iron analyses. The instrument gives accurate values between 0.02-5.0mg/l Fe. In the present investigation the iron content never fell below 0.30 mg/l. Some samples had to be diluted to fit within the range of the analyser.

At the end of the test the weight loss of the coupons was measured and the corrosion

products examined.

Electrolytes Electrolytes, synthetic drinking water, were prepared from double-distilled water and

sodium hydrogen carbonate (NaHC03, p.a.), in concentrations between 0.5mM and

82 A. Sander et al.

Table 1. Constants of surface and solution reactions used in the calculations

Surface or solution reaction ‘ogK Reference

H+ + CO’- = HCO; 10.33 29 2H+ + CO:- * H2CO3 16.68 29 Ca2+ + Hz0 * CaOH+ + H+ - 12.6 30 Ca2+ + H+ + CO:- = CaHCO: 11.59 31

Ca2+ + CO:- S CaCOs(s) 8.42 31 s FeOH + H+ * = FeOH: 7.47 23 s FeOH = E FeO- + H+ -9.51 23 z FeOH + Ca2+ + E FeOCa+ + Hf -5.85 20

E FeOH + 2H+ + CO:- + G FeCOsH + Hz0 20.78 29 z FeOH + H+ + CO:- + - FeCOy + H20 12.71 29 E FeOH + H+ + Ca2+ + CO:- + FeCOsCa+ = H20 15.69 best guess

10 mM. Clean air in equilibrium with the electrolyte concentration of carbon dioxide was bubbled through the electrolyte for 30min before each measurement to saturate the electrolyte with respect to air. The pH was adjusted using 0.1 M nitric acid. Different concentrations of nitrates were tested and showed no effect on the corrosion rate within this concentration range. Sodium was presumed not to influence the corrosion rate. Calcium

was added as Ca(NO&.

Field investigations Samples were taken from six hydrants in each municipality. The hydrants were

connected to iron pipes and chosen so that two were located at main pipes, two at the end of the distribution network and two in between. Samples were taken four times (different seasons) in each municipality. In order to achieve reproducible results the hydrants were tapped at times of low consumption with a flow rate of 20 l/min for 10 min before sampling. 27 The samples were analysed at a certified laboratory.

Computer program for equilibrium calculations

A computer program, SOLGASWATER, was used for the calculations of the surface complexation.28

EXPERIMENTAL RESULTS AND DISCUSSION

In a previous paper18 surface complexation was proposed as a possible explanation for the dependence of iron corrosion on the carbonate system and the calcium ion content in drinking water. In this paper calculations have been made on the surface complexing properties of the system and correlations have been made to experimental data collected from electrochemical experiments, coupon tests and data from the water piping collected in different municipalities.

Theoretical calculations In Table 1 the equilibrium constants used are presented. Since the goethite surface has

been vigorously studied,20~23~24~29-3’ good estimations of the constants are available in most cases. Otherwise, best guesses have been used. All reactions in the system are pH-dependent.

Fe corrosion in drinking water distribution systems 83

Therefore, the calculations have been made with pH as the independent variable. However, in order to improve the visibility the results are in most cases presented as a function of the content of free carbon dioxide, i.e. the sum of dissolved carbon dioxide and deprotonated carbonic acid.

In Fig. 1 the different surface complexes are plotted vs pH in the range from 6 to 9 for a typical soft water with 0.5 mM total carbonate concentration and 0.4 mM calcium content (a), and for a harder water with 5 mM total carbonate concentration and 2 mM calcium (b). The total surface concentration is in all calculations set to 1 x lo-* M. This concentration is calculated as the total number of surface sites divided by the total water volume. The value is about typical in comparison with a water distribution pipe, but calculations show that there is no dependence on the surface concentration for the complexing properties as long as the surface concentration is highly limiting in the system. It can be seen that different surface species are dominating in the two waters, and that the formation of the species are very pH dependent. The precipitation of calcite (calcium carbonate) is presented on a separate axis in the figures. The low alkalinity water needs a pH of about 8.7 for the precipitation to occur while in the high alkalinity water calcite starts to precipitate slightly above pH 7. This precipitation can take place at any surface in the system or even in the water phase and is thereby not primarily determining for the corrosion rate. However, it affects the complex formation as can be seen in the figures, where sharp kinks are obtained for some complexes at a pH where the precipitation starts.

One possible explanation for the surface complex interaction on the corrosion process is that the surface complexes dissolve the oxide at the surface, as discussed in the introduction, and open the way for further corrosion. The corrosion will in turn yield new iron ions forming passivating corrosion products and a steady state situation may be reached. In this case the corrosion rate would be proportional to the dissolution rate of the corrosion products.

Another explanation for the corrosion dependence on the surface complexation is based on the assumption that the corrosion reaction proceeds through the oxide film. This is possible if the oxide is inhomogeneous. Corrosion can occur at the metal surface and reactants and products can be transported through pores and cracks in the oxide layer. Depending on which surface complex present at the corrosion site the process will proceed with different rates. Thus, in this case the corrosion rate is dependent on physical, chemical and electrochemical properties of the complexes instead of the dissolution rate, and the apparent rate constants are primarily functions of these properties.

Both mechanisms may be valid at the same time but, as can be seen in the following, the second mechanism seems to be dominating at least on a long term basis.

Electrochemical measurements The electrochemical data on the initial corrosion rate show a slight dependence on the

free carbon dioxide content of the water. Lower corrosion rates were obtained for higher carbon dioxide concentrations. There are also differences depending on the calcium content, but somewhat more confusing. For low alkalinity waters an increase in calcium content

Fig. 1. Surface complex concentration as a function of pH (a) for a typical soft water with 0.5 mM total carbonate

concentration and 0.4 mM calcium content and (b) for a harder water with 5 mM total carbonate concentration and 2 mM calcium content. The concentration of solid calcium carbonate is plotted on a separate axis.

84

1 ,OOE-O8

8,00E-O9

_ 8,00E-09 L

: ‘G P s ii s

$ !J :, 4,00E-09

2,00E-O9

O,OOE+OO

A. Sander et al.

1,20E-O4

9,00E-O5

1,OOE-OS

I,OOE+OO

-a- C for >FeOH

--b C for >FeOHZ +

-A- C for >FeO-

-x- C for >FeOCa+

+ C for sFeCO3H

* C for >FeC03-

+ C for >FeCOSCa+

- C for CaCOS(s)

6 6,6 7 7,5 8 8,5 9

PH

Fig. 1.

Fe corrosion in drinking water distribution systems 85

1 ,OOE-O8

8,OOE-09

2 8*00E-Og

0’ ‘S b f : s

8 g

’ 4,00E-O9

2,0QEQ9

O,OOE+OO

2,00E-O3

1,50E-O3

i,OOE-O4

D.OOE+OO

-6- C for >FeOH

--C C for >FeOHZ +

-i+ C for >FeO-

--)t C for >FeOCa+

+ C for >FeC03H

* C for zFeCO3-

+ C for >FeCOSCa+

- C for CaCOS(s)

6 6,5 7 7,s 8 8,s 9

PH

Fig. 1. continued

86 A. Sander et al.

gives a lower corrosion rate, while no effect on the corrosion rate was obtained for high alkalinity waters when the calcium content was increased. All these results are discussed in a previous paper.”

The surface complex model was applied to these results and each total carbonate concentration was separately analysed. Since the surface was wet ground and pre-swept to both anodic and cathodic potentials for several times before each recorded sweep (see Experimental Method Section), no well defined oxide layer was present at the surface. It was seen that the corrosion rate correlated to the sum of surface complexes except for those involving calcium. Two complexes were less dominant than the others, the hydrated complexes =FeOHz and =FeHCO3. As previously mentioned surface complexation may promote dissolution of the oxide layer. Under the assumption that the corrosion mechanism involving oxide dissolution is dominating one would expect an even lower contribution from these hydrated surface complexes at the current pH values (6 < pH < 9) due to the power exponent in the rate expression for proton-promoted dissolution (see Introduction, equation (4)). Thus, there are reasons to believe that either the mechanism involving corrosion through the inhomogeneous oxide layer or both mechanisms are determining for the rate during the initial stage of iron corrosion in drinking water. The fact that the calcium complexes seem to inhibit the surface may in that case mean that the dissolution rate is lower for these complexes. However, in order not to overinterpret the results only linear terms were applied for the interpretation giving an approximate rate function according to equation (5):

r,,,, o< [z FeOH] + [= FeO-] + [= FeCOJ + 0.8([= FeOHl] + [z FeHC03]) (5)

In Fig. 2(a)-(d) the norrnalised experimental results and the theoretical data are plotted together as a function of the logarithm of the free carbon dioxide content for each total carbonate concentration. In Fig. 2(e) all data are plotted and normalised together. The mean error in percent of the difference between the highest and the lowest experimental data is 11.7 and the error standard deviation in percent of the same interval is 14.8. Thus, we do not claim that this is the best theoretical approximation for the system, but we do mean that it gives strong evidence for the applicability of the surface complex model on the system studied.

Field measurements The difference between the initial corrosion rate on an iron sample and the corrosion

product release from the distribution network to the drinking water is often pointed out.3-6 The sum of the corrosion products in the pipes are of course a function of the corrosion rate if no iron is present in the water from the water works, but the products released to and transported with the water may primarily be a function of the dissolution/precipitation properties of the corrosion products. Therefore, it is reasonable to believe that another theoretical function has to be applied to the data from the field measurements where the corrosion product release is measured.

Data were collected from a number of Swedish municipalities and one Norwegian municipality.

Collecting water samples along public pipe nets is, of course, afflicted with several sources of error. Due to differences in pressure and flow rate, the status of the hydrant connection to the pipe, the succession of different pipe materials affecting the water quality

Fe corrosion in drinking water distribution systems 87

-5 43 496 44 42 4

log (free carbon dioxide/M)

oi H .z 0.90

8 t 0 * 0,80

X

Qad 0

X X

X El X0

-5 45 4 -3,s

log (free carbon dioxide/M)

-3

0 3 1,2Q -

0 u t l,lQ - ”

g c l,QO 8 q

e 5 *$ 0,9Q - 0

0 t s o,ao 4

-5 -45 4 -3,s

log (free carbon dioxide/M)

Fig. 2. Normalised corrosion rate and the theoretical model as function of the free carbon dioxide concentration for certain total carbonate concentrations and varying calcium content (a-d), and all

different total carbonate concentrations plotted together (e).

88 A. Sander et al.

-0 1,20 c t ”

f 1919 -

=a# q P.s

OX

g jjl,OO - x q

X0 X Em 5 Cl ‘3 0,90 - 2 ::

B 0,50 ,

-5 495 4 -3,s -3 -2,5

log (t?ee carbon dioxide/M)

log (free carbon dioxide/M)

Fig. 2. continued

before the iron pipe measured on, and many other errors, a wide range of measurements can be obtained in a network. However, a standard procedure for sample collection was chosen in order to minimise these errors and the sample points were chosen in a standard way (see experimental section). Since samples from all points were taken and analysed at four different times in order to increase the reliability the results should be as relevant as possible.

The data was treated in two different ways, the median value for each municipality and the fraction of samples exceeding 100 ug iron/l. These two ways of analysing the data was compared and match quite well, Fig. 3. One consequence of this is that public complaints of red water may be a good indicator for the overall water quality situation in a municipality, as indicated by some authors6

In Fig. 4 the median of iron samples in each municipality, the fraction of iron samples exceeding 100 ug/l and a theoretical attempt are plotted vs the free carbon dioxide content. The theoretical model includes the complexes in equation (6) and is based on the following assumption. The inside surfaces of the iron pipes along a distribution network are covered with corrosion products to a different extent. As discussed earlier there are two possible ways for the corrosion to occur. Either the oxide is dissolved by the surface complexes

Fe corrosion in drinking water distribution systems 89

Y = 0,9952x + 0,004a u2 = o,a241

01

0 095 1 1,s 2

&action of iron samples ~100 pgll, normalized scale

4

295

Fig. 3. Fraction of iron samples exceeding 100 peg/l vs the mean value for field measurements of iron

0 100 200 300 400 500 600 700

free carbon dioxide/MM

Fig. 4. Normalised statistic evaluation of iron samples from different

o median iron concentration rl x fraction of iron samples > 1 OOpgll

a theoretical model

municipalities and the theoretical model as functions of the free carbon dioxide concentration.

making the surface free for further corrosion or the corrosion can proceed through the inhomogeneous oxide layer. Since most complexes seem to inhibit further corrosion it is likely that the dominating corrosion mechanism involves corrosion through the oxide layer. During the corrosion process, Fe*+ ions are produced and siderite can be formed. The siderite may be better attached to the surface than the oxide or be the initial product for

90 A. Sander et al.

0- 0 25 50 75 100

free carbon dioxide/PM

Fig. 5. Total amount of corrosion products as iron for coupon tests and the theoretical model plotted on a normalised axis vs the free carbon dioxide concentration.

further oxidation to stabile oxide products as claimed by some authors. 12P’4332 The current model does not exclude this possibility or the action of calcite for the formation of stable corrosion product layers in low alkalinity waters. There may even be a possibility that the products are formed through the action of surface complexes. However, the corrosion product release to the water does not seem to depend on these products. Instead, the two complexes in equation (6) are decisive for further corrosion through pores and cracks in the oxide layer and less protective than the other complexes. If this is true it would mean that the corrosion product release in a steady state situation is a measure of the corrosion rate in the distribution system, and that very little of the corrosion product will remain in the pipes for a long time.

rcorr 0: [C FeOH] + [= FeC03Cafl

It can be seen that at low free carbon dioxide concentrations the values of the theoretical function in most cases have higher values than those measured. However, it is a well known fact that rust of larger dimensions will remain lying on the bottom of the pipe until the system is flushed due to high consumption or pressure disturbance.

Coupon tests

The amount of corrosion products produced in the coupon tests would be expected to follow the same theoretical expression as was found in the networks. In Fig. 5 this is shown. Two points do not match the model. These points are results from water with low total carbonate content (0.5 mM) and no calcium. On these coupons, extensive pitting corrosion was observed. The pitting corrosion does not follow the same model since the corrosion is located to specific points at the surface with very high corrosion activity. The surrounding material was almost unaffected and the total corrosion rate was therefore lower. However, the consequences of pitting corrosion is normally worse.

Fe corrosion in drinking water distribution systems

6

PH

Ca2+ (mM)

5 and 5 mM total carbonate content

Fig. 6. Theoretical model for the initial corrosion rate vs uH and calcium content for two total carbonate concentrations, 0.5 and 5 mM.

It is interesting to note that the total amount of corrosion products matches the theoretical function, not only the corrosion products released to the water. Thus, the iron content of these corrosion products equals the amount of iron produced by corrosion. Therefore, it is possible to believe that the network situation is at steady state and that almost all corrosion products will reach the consumer over time. Of course some clogging will appear at end pipes or at other parts of the network with unfavourable flow situations, but the over all situation would be at steady state.

Consequences of the theoretical model It has already been stated that the specific rate expressions used in this paper are not

claimed to be fundamentally correct. It is interesting, however, to note how well the expressions fit real data. Since no other expressions are available that fit the system better, some speculations can be made on the bases of these expressions. In Fig. 6 the initial corrosion rates for two different total carbonate concentrations are plotted vs pH. As the third independent variable the total calcium content is introduced. It can be seen, that for

the initial corrosion rate, the calcium content has a distinct corrosion reducing effect in the low alkalinity water with high pH, while the effect is very small in the water with high alkalinity. The initial corrosion rate is almost independent of alkalinity if no calcium is present.

In Fig. 7 the expression for the situation in the networks (or coupon tests) are plotted for the same waters. Here it can be seen, that a low pH value would yield a low corrosion rate for all waters. The calcium effect remains in the low alkalinity water, but now the corrosion rate increases slightly with the calcium content in the high alkalinity water. The lowest pH values

giving the low corrosion rate would be interesting if concrete or copper pipes were not present in the systems, but with today’s situation, higher pH values are preferable. Thus, an increase in the calcium content in low alkalinity waters and a decrease in high alkalinity water would yield a slightly better situation in the networks.

In a forthcoming paper the situation in the networks will be discussed in more detail.

92 A. Sander et al

6

Ca (mM) 0.6 and 5 mM total carbonate content

Fig. 7. Theoretical model for the corrosion rate (and the iron content of the water) in a network vs

pH and calcium content for two total carbonate concentrations, 0.5 and 5 mM.

CONCLUSIONS

1. It seems evident that the surface complex model presents a convenient way of expressing iron corrosion in drinking water systems.

2. The primacy given chemical interactions between ions in solution and functional groups of a solid allows for a thermodynamic description of the system. Knowing the acid- base properties and surface complexation constants of a particular solid, it is possible to predict the influence of changes in the water chemistry.

3. The initial corrosion rate on an iron surface in drinking water possibly proceeds both through oxide dissolution and through the inhomogeneous oxide film. Surface complexes dissolve the oxide and make the surface free for further corrosion.

4. The corrosion in the networks does not follow the same rate expression. Most likely the corrosion situation in the networks is dependent on the corrosion rate through the inhomogeneous oxide layers also determined by surface complexation.

5. The formation of siderite or, at higher pH values, calcite may take part in producing stable and dense corrosion product layers at the pipe wall, but does not affect the corrosion process.

6. It is possible that the situation in the networks is at steady state, where both the corrosion rate and the corrosion product release to the water follow the same expression.

Acknowledgements-The authors wish to thank the Swedish Water and Waste Water Works Association for financial support.

REFERENCES

E. Mattsson, Basic Corrosion Technology for Scientists and Engineers, Ellis Horwood Limited, Chichester, U.K. (1989).

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