Ind. Eng. Chem. Res. 1997, 36, 2903-2909
2903
Corrosion Protection of Mild Steel by a Calcite Layer Shani Keysar, David Hasson,* Rafi Semiat, and Dan Bramson Department of Chemical Engineering, TechnionsIsrael Institute of Technology, Technion City, Haifa 32 000, Israel
Artificial calcite lining represents a novel technique for the rehabilitation of water mains. Calcite linings, similar to the commonly used cement-mortar linings, are of porous nature. The corrosion protection mechanisms of calcite linings were investigated by polarization techniques, complemented by scanning electron microscopy observations and energy dispersive spectrometry analyses. The effect of time on lining durability was examined by exposure of coated mild steel (1020) coupons in a controlled flowing water system. Iron ion distributions in calcite sections, before and after exposure to water flow, indicate that the calcite corrosion protection mechanism is based mainly on accumulation of corrosion products inside the lining. At the calcite-metal interface, a protective film is formed. At the calcite-water interface, the calcite structure, being alkaline with respect to the water, promotes precipitation of the iron ions and blockage of the pores near that interface. The formation of this protective oxide film was evident from the polarization data which showed that both the anodic Tafel constant and the polarization resistance increase with time. This paper also examines the applicability of potentiodynamic and linear polarization techniques for the characterization of the corrosion protection of a porous lining, such as calcite. It is shown that these convenient electrochemical techniques provide reliable and meaningful corrosion protection information. Introduction The role of CaCO3 in corrosion and its control in water distribution systems evokes widespread interest from several points of view. The iron oxide layer formed on a pipe surface in contact with water having even a minor amount of Ca2+ and HCO3- ions, contains some CaCO3. It is widely believed that CaCO3 formation represents a cathodic inhibition mechanism that serves to protect the pipe surface from corrosion (Stumm, 1956; McClanahan and Mancy, 1974; Snoeyink and Kuch, 1985; Legrand and Leroy, 1990). Water quality parameters and flow conditions conducive to protective CaCO3 films have been described in the literature (Snoeyink and Kuch, 1985). However, it is far from clear under which conditions the presence of CaCO3 film gives a significant measure of corrosion control. The actual mechanism of corrosion protection is still uncertain. According to Stumm (1956), the inhibitory capability of CaCO3 occurs only when it is capable of blocking anodic sites of corroding iron and this is particularly effective in neutral waters of high alkalinity. McClanahan and Mancy (1974), studied the quality (especially permeability) of CaCO3 films artificially deposited by an electrochemical reaction occurring on a rotating disk electrode. They found that the best, least permeable, films were deposited from hard water. Buffer capacity was a major variable. Feigenbaum et al. (1978b) examined the microstructure of natural scale layers formed in potable water supply mains having moderate to high hardness. The scale was composed mostly of the iron compounds geothite [R-FeO(OH)], siderite [FeCO3], and magnetite [Fe3O4], mixed with calcite [CaCO3]. There was a wide variation in the Fe:Ca mass ratio in the 15 scale deposits examined. The Ca-rich scale tended to be denser and more electrically resistant; i.e., it had a better corrosion protection capability. Scales poor in calcite, but rich in siderite, still had a fairly high resistance. Scales poor * Corresponding author: tel, 972-4-8292936; fax, 972-48230476; e-mail,
[email protected]. S0888-5885(96)00277-1 CCC: $14.00
in carbonates of Fe and Ca but rich in magnetite showed low resistance. Feigenbaum et al. (1978b) found that the layer nearest to the pipe was rich in Fe compounds. Calcite appeared mostly in the outer zone. On the other hand Stumm (1959) found that CaCO3 precipitated in the greatest proportion in the zone nearest to the metal surface. He took this finding as additional evidence that CaCO3 deposition was largely controlled by electrochemical processes on the metal surface. Sontheimer et al. (1981) carried out an extensive study on the morphology and protective nature of naturally formed scale deposits. They too reached different conclusions from those observed by Feigenbaum et al. (1978a). While Feigenbaum et al. noted that the Ca deficient oxide films were porous and nonprotective, Sontheimer et al. (1981) found that layers poor in calcite but rich in siderite or magnetite gave the best corrosion protection. They proposed a model in which the key to a protective scale was the formation of siderite. Dense protective structures were considered to be evolved through the decomposition of the siderite into magnetite and geothite. Interest in the mechanism of corrosion protection by CaCO3 was motivated in this study by a further consideration. A chemical lining process proposed by Hasson and Karmon (1984) enables in situ controlled deposition of an adherent calcite coating (about 800 µm thick) on a pipe surface. This technology has certain advantageous features for water main rehabilitation, as compared with the commonly practiced cement-mortar lining technology. Such artificial calcite linings would be applied, for instance, to rehabilitate old graphitized cast iron pipes. This raises the question of whether chemically deposited CaCO3 layers have a corrosion protective capability. Calcite layers are essentially porous, as are also the widely used cement-mortar linings. The corrosion protection mechanism of porous structures has been little studied. The immediate aim of the present work was to characterize the corrosion protection properties of such porous linings. © 1997 American Chemical Society
2904 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
Electrochemical Characterization of the Protective Nature of Linings. In the last 20 years, electrochemical measurements for characterizing corrosion under protective films have been widely used. However, interpretation of the results is still a debatable subject. This is more so in the case of porous coatings which have been scantily studied by polarization measurements. Therefore, in the present work, the applicability of electrochemical techniques for characterizing the corrosion protective abilities of porous linings was examined. Electrochemical measurements of coatings are based on both dc and ac techniques (Haynes and Babion, 1983; Heitz and Kreysa, 1986). dc techniques cannot be applied to impermeable organic films and low conductivity media, but they are the simplest to use in cases where the film is permeable and the medium is conducting, as in the present work. Potentiodynamic polarization and linear polarization are among the widely used dc techniques for studying the corrosion protection ability of coatings (Bacon et al., 1948; Wormwell and Brasher 1949; Clay, 1969; Tomashov et al., 1964; Boxall and Fraunhofer, 1973; Rajagopalan et al., 1975; Devay et al., 1979; Snoeyink and Kuch, 1985; Fontana, 1986). Useful parameters measured by these techniques are the anodic (ba) and cathodic (bc) Tafel coefficients, their harmonic mean (µb), and the polarization resistance (Rp), given respectively by
ba ) 2.3RT/RzF
(1)
bc ) -2.3RT/(1 - R)zF
(2)
[
(3)
1 1 1 ) 2.3 + µb ba bc
]
Rp ) ∆E/∆i ) µb/icorr
(4)
where R is the symmetry factor, z the ion valence, F the Faraday constant, and icorr the corrosion current. An electrochemical mechanism likely to occur in aerated water involves both charge transfer and diffusion control. In this case the total overpotential in a potentiodynamic scan is given by (Newman, 1973; West, 1985)
ηCT + ηC ) -
{
}
ic/i0 1 RT ln - [ln(1 - (ic/i0)] zF (1 - R)
(5)
and the Tafel slope for the cathodic reaction is
dE/d(ln ic) ) (-RT/zF)[(ln ic/i0)/(1 - R) 1 + exp(-zFηCT/RT)] (6) where ηCT is the charge transfer overpotential, ηC the polarization overpotential, ic the cathodic current, and i0 the equilibrium current. Experimental Section Calcite coatings were formed on pipe walls by circulating a supersaturated solution with respect to CaCO3 through a 2 in. diameter piping system, described elsewhere (Hasson and Karmon, 1984). The lining solution was composed of dissolved calcium and carbonate ions to which were added a polyphosphate (in order to inhibit CaCO3 bulk precipitation) and sulfite (in order to protect the pipe walls against corrosion). The coated specimens, used for studying the corrosion protection provided by the calcite, were prepared by
Table 1. Composition of Synthetic Water Circulated in the Corrosion Loop
Ca2+ (ppm as CaCO3) mean value range total alkalinity (ppm as CaCO3) mean value range pH mean value range saturation pH temperature (°C) mean value range conductivity (mΩ-1/cm) mean value range average Langelier index
0-29 days
30-65 days
66-130 days
106 80-120
99 75-130
96 80-106
76 50-120
77 108-110
79 68-110
7.93 7.90-8.15 7.83
7.95 7.70-8.15 7.80
8.00 7.9-9.1 7.86
33.6 34.4-38.8
34.1 31.0-37.7
34.3 30.0-36.2
1.8 1.3-2.2 +0.10
1.6 1.3-1.9 +0.15
2.0 1.8-2.1 +0.14
attaching small disk-shaped coupons on the inner surface of the 2 in. diameter pipe through which the calcite lining solution was circulated. The disk-shaped coupons (25 mm in diameter, 5 mm thick) were made of mild steel (1020) and their corrosion susceptibility was increased to a controlled high level by exposure to a solution of 5% HCl at room temperature for half an hour, just before the calcite lining run. Under the experimental conditions adopted, the rate of calcite layer growth was in the range of 25-60 µm/h. Layer thickness increased linearly with time. To study the effect of layer thickness, two groups of coupons were prepared: coupons having a thickness in the range of 300-550 µm and coupons having a thickness in the range of 700-900 µm. Measurements of the lining thickness were carried out with a thickness measuring probe based on a magnetic induction principle (Minitest 2000, ElectroPhysik, Germany). Its reported accuracy is 1% of the measurement. The lining density was calculated from thickness, area, and weight measurements. A corrosion loop, described elsewhere (Hasson et al., 1989) was used to study the durability of the linings when exposed to continuous flow of water for long periods of time. The circulating water simulated the composition of the Potomac Water Plant in Washington, DC, the only difference being in the enhancement of water conductivity (1.8-2.0 mΩ-1/cm as compared to 1.1 mΩ-1/cm in Potomac River water). Table 1 summarizes water analyses and temperatures during the various stages of the durability experiments. It is seen that water composition did not vary significantly throughout the 130 days of the experiments. Electrochemical measurements were carried out using the computerized Corrosion Measurements System 273/ 342 of PARC & EG&G. All tests were performed with water having a composition similar to that given in Table1 and at comparable temperatures (around 34 °C). The electrochemical measurements consisted of Tafel plots, linear polarization, and cyclic polarization plots. The data were analyzed using the Parcalc procedure (software developed by PARC & EG&G) to provide measurements of the corrosion potential, the corrosion current, the Tafel constants, and the polarization resistance. Results and Discussion Validity of the Polarization Technique for Corrosion Measurements of Lined Metal Specimens.
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2905 Table 2. Average (Harmonic Mean) Tafel Constants (µb) no. of specimens
type of specimen unlined mild steel, smooth surface calcite-lined mild steel unlined mild steel calcite-lined mild steel
surface treatment
µb (mV)
23
5% HCl
79 ( 6
28 11 31
5% HCl 78 ( 20 controlled roughening 88 ( 28 controlled roughening 87 ( 28
The presence of a coating on a metal substrate represents a different electrochemical environment as compared to that of a bare metal. This raises the problem of interpretation of polarization data of coated metals (Walter, 1986). One of the tests applied in this study to check the validity of the polarization technique for porous coatings was to compare results of linear polarization data of lined steel coupons to those of bare steel coupons. In analyzing corrosion measurements of bare steel coupons, Stumm (1959) observed that the Tafel parameters remained substantially constant. This led him to the conclusion that the product of corrosion current and polarization resistance is essentially constant:
icorrRp )
1 = constant µb
(7)
This relationship was tested in this study on a very large number of specimens, both freshly lined and unlined metal coupons, with and without metal surface roughening (Table 2). In every case, the linear relationship predicted by eq 7 between icorr and reciprocal polarization resistance 1/Rp was obtained, as demonstrated in Figure 1. Values of µb for the different linings were all closely similar, in the range of 80-90 mV, as compared to a value of 34 mV measured by Stumm (1959). The applicability of eq 7 for both lined and unlined coupons supports the use of the polarization technique for characterization of the corrosion protection provided by linings. Another problem raised by the presence of a lining is discrimination of the actual value of the internal polarization resistance from the measured overall resistance, which includes the external ohmic resistance, due to the lining. This issue is well analyzed by Mansfeld (1976). When an external resistance exists in the cell, eq 7 becomes
Rp′ ) µb/icorr + RΩ ) Rp + RΩ
(8)
Mansfeld showed that the size of the error ∆ depends on the size of the electrode and the relation between the ohmic resistance and the polarization resistance
∆ ) RΩS/Rp
Figure 1. Relation between corrosion current and reciprocal Rp for different types of metal.
(9)
where S is the electrode surface. In the presence of a nonconducting coating the error ∆ can be appreciable. In the case of the calcite lining examined here, it is reasonable to assume that, because of the porous nature of the lining, the dominating ohmic resistance is that of the electrolytes in the pores. This resistance is not expected to be high in view of the relatively modest dissolved salt content in drinking waters. The above hypothesis was tested on lined coupons by the “current interrupt” option (Heitz and Kreysa, 1986). Figure 2 shows an example of the ohmic resistance measured (during a Tafel scan) by the current interrupt
Figure 2. Calcite lining ohmic resistance during potentiodynamic scan.
technique in which the “wet resistance” is in the range of 30-40 Ω, equivalent to 50-70 Ω cm2. Results from several such repeat runs indicate that the “wet” resistance of the lining was in the range of 0.03-0.4 kΩ cm2. According to literature data, the “dry” ohmic resistance for natural scales consisting mainly of calcite is around 5 MΩ cm2 (Feigenbaum et al., 1978a). Comparison of the “wet” and “dry” resistances supports the above hypothesis that the measured ohmic resistance for a wet coating represents the resistance of the electrolyte in the pores. Measured polarization resistances in this work were in the range of 3-24 kΩ cm2 compared to ohmic resistances in the range of 0.03-0.4 kΩ cm2. Thus the error ∆ due to ohmic resistance was small, of the order of 0.1-14%. Characterization of Corrosion Protection Properties of Freshly Lined Coupons. Table 3 lists the thickness and density of linings prepared in four different lining runs. These runs provided coupons having a varying lining thicknesses and a varying lining densities. The accuracy in the determination of lining density from weight and thickness measurements was estimated to be around (0.05 g/cm3. As noted before, all mild steel coupons, unlined and lined, were pretreated with HCl to increase their corrosion susceptibility. Table 3 also gives corrosion results measured on the freshly lined coupons as well as on bare metal coupons.
2906 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 3. Corrosion Current and Corrosion Potential of Freshly Lined Mild Steel Coupons expt no.
no. of specimens
mean lining thickness (µm)
mean lining density (g/cm3)
corrosion currenta (µA/cm2)
corrosion potentiala (V vs SCE)
3 1 2 4 3 1 2 4 bare metal
5 2 4 6 1 2 2 2 22
850 750 820 700-900 400 300 550 370
2.45 2.35 2.50 2.50 2.45 2.30 2.40 2.60
26.0 ( 6.6 27.8 7.8 ( 1.9 17.2 ( 6 29.9 32.4 27.8 7.9 81.8 ( 14.4
-0.61 ( 0.04 -0.64 -0.58 ( 0.05 -0.68 ( 0.05 -0.61 -0.68 -0.71 -0.71 -0.63 ( 0.04
a
Mean value and ( standard deviation. Table 4. Change in the Tafel Constant with Time time (days)
specimen
ba (mV)a
bc (mV)a
µb (mV)a
0 0 29 29 65 65 130 130
bare lined bare lined bare lined bare lined
325 ( 108 500 ( 108 331 ( 11 701 ( 130 496 ( 49 616 ( 150 444 ( 40 498 ( 53
422 ( 113 322 ( 110 284 ( 15 396 ( 60 433 ( 58 380 ( 50 380 ( 82 444 ( 40
79 ( 6 78 ( 31 66 ( 1 108 ( 9 100 ( 12 102 ( 6 86 ( 13 102 ( 2
a
Figure 3. Relation between corrosion current and calcite lining density.
The corrosion measurements were carried out with airsaturated water having the composition indicated in Table 1. As is evident from Table 3, the corrosion current is considerably reduced in lined coupons, by a factor of the order of 4-9. In fact, the corrosion current in the unlined coupons is so high that it leads to a diffusional limitation in the cathodic reaction (eq 6). Such a limitation was never observed with the lined coupons. It is also to be noted that the general features of the Tafel plots are typical of general, rather than localized, corrosion. The data of Table 3 show that lining thickness has no significant effect on the magnitude of the corrosion current. This result, obtained on a porous lining, differs from observations on nonporous insulating coatings. In nonporous linings, ohmic resistance increases linearly with coating thickness, leading to a corresponding reduction in corrosion current (Wormwell and Brasher, 1949). As discussed before, calcite lining thickness does not have a significant effect on corrosion current because the ohmic resistance of the lining is governed by the conductivity of the electrolyte filling the pores. Bulk density of the lining, which represents the degree of porosity of the lining, has a profound effect on the corrosion current. It is seen in Figure 3 that increase of bulk lining density from 2.3 g/cm3 ( ≈ 16%) to 2.6 g/cm3 ( ≈ 4%) reduces the corrosion current by a factor of 4-5. Thus, as may be anticipated, corrosion current is roughly proportional to the unlined fraction of the metal. This result conforms to the observations made by Wormwell and Brasher (1949). Durability of the Lining with Time. Prediction of the durability of a calcite lining is obviously a major issue. It raises the question of the effect of lining properties on long time corrosion protection. To obtain
Mean value and ( standard deviation.
such data, lined and unlined specimens of mild steel (1020) were exposed for long periods of time to a continuous flow of water having a controlled composition (Table 1). The effect of lining thickness was studied by exposing specimens of two different thicknesses: thicker linings of 840-900 µm and thinner linings of 310-430 µm. The degree of corrosion protection of the linings in the course of time was determined by Tafel plot measurements, yielding corrosion current, corrosion potential, Tafel constants, and polarization resistance. It has been shown by McClanahan and Mancy (1974) that repeated polarization tests on coupons do not have a drastic effect on the accuracy of the data. As noted later, the magnitudes of Tafel constants were relatively high (Table 4). The values conform to Tafel data measured by Hausler (1977) with mild steel coupons exposed to tap water containing inhibitors. Strictly speaking, under such conditions, Tafel constants should be evaluated from eq 8, but this is impractical. The Tafel constants reported here were evaluated using the corrosion system software, which assumes the simpler Tafel equation (eq 4). As is well-known, the maximum error resulting from this simplification is a deviation of the Tafel constant by a factor of less than 2. Table 4 shows values of the anodic, cathodic, and harmonic mean Tafel constants as a function of time for bare unlined and lined metal. The cathodic Tafel constants of both unlined and lined specimens remain substantially similar. However, the anodic Tafel constants are consistently higher for lined metals. The increase in Tafel constants in lined specimens is due to a concentration polarization effect, i.e., mass transport limitation due to shortage or excess of one of the species (see eq 9). Thus the presence of a calcite lining leads to the formation of an oxide film on the metal. Corrosion Current. Figure 4 shows the variations in corrosion current with time for the thicker and thinner calcite lined specimens as compared to that of a bare metal. The corrosion current is seen to have a very high value for the bare metal. It should be recalled that the bare metal received acid pretreatment to accelerate its corrosion susceptibility. This pretreat-
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2907
Figure 4. Changes in corrosion current with time.
Figure 5. Changes in corrosion potential with time.
ment seems to accelerate the corrosion by two mechanisms. Dissolution creates a more nonuniform surface which is more reactive. Also, residual adsorbed Cl on the surface accelerates the corrosion reactions (Ashley and Burstein, 1991). The corrosion current of the bare metal is seen to remain high, with no evidence of a formation of an oxide film. On the other hand, the same metal covered by a calcite lining displayed a corrosion current 4-9 times smaller throughout the 140-day exposure. Clearly, the presence of a calcite lining induced the formation of an oxide layer on the metalsa result already noted above from the Tafel constant observations. Figure 4 also shows that the thicker calcite lining gave essentially the same corrosion protection as the thinner lining. This result, already noted before on fresh lining, confirms that the more significant parameter is the lining porosity, which determines the degree of metal coverage. It should be noted, however, that the thicker lining is expected to provide a better corrosion protection by providing for a greater probability of pore blockage by corrosion products. As discussed below, this seems to be another important mechanism in the corrosion protection ability of calcite. The probability of pore blockages by corrosion products is greater as the lining becomes thicker. Corrosion Potential. Figure 5 shows that exposure of the lined and unlined coupons to flowing water led to a monotonous increase in corrosion potential with time. No significant difference could be noted among
Figure 6. Variation of polarization resistance with time.
the various coupons. It seems that corrosion potential changes do not always provide reliable information on metal passivation (Boxall and Fraunhofer, 1973; Rajagopalan et al., 1975; Walter, 1986). The presence of surface heterogeneity or, in the case of lined coupons, the occurrence of simultaneous anodic and cathodic polarizations can also be responsible for potential changes. Polarization Resistance. The changes in polarization resistance with time for lined and unlined specimens are depicted in Figure 6. It is seen that the polarization resistance of the bare metal remained constant at a low value. The polarization resistances of the lined metal coupons were considerably higher, by a factor of 4-25 as compared to those of the bare metal. Also, the polarization resistance of the lined coupons did not remain constant for the bare metal but varied considerably during the time of the experiment. These observations can be explained as follows. The polarization resistance of the corrosion product film is actually the sum of the polarization resistance and of the ohmic resistance of the film (Tomashov, 1966). The fact that the polarization resistance of the bare metal did not change with time shows that no significant changes occurred on the metal. Coverage of the metal by calcite lining promoted the formation of an oxide film, thus increasing the polarization resistance with time. The decrease in polarization resistance of the lined coupons stemmed probably from the fact that the passivation did not arrest sufficiently the corrosion processes. The buildup of the corrosion film on the calcite-metal interface probably weakened the calcite adhesion to the metal. This led to a diminished polarization resistance. This is in agreement with the results of Spengler et al. (1993). An analogous phenomenon is known to be one of the causes of failure of paints. Its symptoms are as observed in this worksa reduction in the polarization resistance with time (Bacon et al., 1948; Rajagopalan et al., 1975; Wormwell and Brasher, 1975). Calcite Lining Contamination with Fe Corrosion Products. All above analyses of the electrochemical measurements led to the conclusion that the calcite protection ability is related to the formation of a diffusion barrier film. Electron microscopy scans and energy dispersive spectrometry (EDS) examination of sections of the calcite layers reveal that the calcite gives further corrosion protection by a mechanism of accumulation of iron oxides inside the calcite layer. This
2908 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
a
b
Figure 7. (a, top) Calcite mixed with corrosion products. (b, bottom) Fe mapping of the calcite layer section of part a.
is illustrated in Figure 7a showing a side-cut section of a calcite layer performed with a rotating sharp blade. The exposed calcite layer was covered with a transparent epoxy layer. It is seen in Figure 7a that the calcite layer has a heterogeneous structure. The area delineated by the inverted V is filled with largely uncontaminated calcite while the rest of the field is covered with contaminated calcite. This is evident from the iron mapping made on the same section, as shown in Figure 7b. It is clearly seen that the contamination of the calcite is by Fe corrosion products present in the calcite structure. Data on the corrosion products distribution within calcite layers were obtained by performing EDS analyses of the iron content throughout the calcite layer (Keysar, 1992). Figure 8 shows the phenomenon of corrosion product accumulation in the calcite lining. Calcite linings, exposed to flowing water, were analyzed to give the distribution of the corrosion products across a side section of the lining (from the top surface of the calcite layer, in contact with the flowing water, to the bottom surface, adhering to the metal surface). Figure 8a refers to a calcite lining (approximately 600 µm thick) exposed to water for 29 days. The lower dotted line shows the Fe scan of the freshly prepared lining. It is seen that the calcite lining is iron free, except in the vicinity of the calcite-metal interface. The upper line shows a significant increase in the iron content across the whole calcite lining.
Figure 8. (a) Fe distribution through a section of a calcite layer after 29 days of exposure to flowing water. (b) Fe distribution through a section of a calcite layer after 130 days of exposure to flowing water.
Figure 8b shows the same layer after 130 days of exposure. The three Fe profiles were taken from the same calcite specimen at different locations. These measurements suggest that Fe accumulation blockage is more significant, both at the water-calcite interface and at the metal-calcite interface. The increased Fe content in the vicinity of the metal calcite interface is due to the diffusion of iron oxides from the metal surface into the calcite. The increased Fe content in the vicinity of the water-calcite interface may be due to several reasons: (a) Chemical precipitation of iron ions diffusing from the solution into the calcite layer because of the local increase in pH at the water-calcite interface. (b) Oxidation of iron ions present inside the calcite layer due to contact with dissolved oxygen diffusing through the calcite-water interface. (c) Adsorption of iron oxide particles present in water on the lining surface. Mechanisms a and b are believed to provide the corrosion protection of cement-mortar linings (Legrand and Leroy, 1990). The high pH induced in the cementmortar pores by the presence of Ca(OH)2 promotes the formation of a protective deposit of magnetic iron oxide. Whatever the mechanism prevailing in the case of calcite linings, it is reasonable to assume that the
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 2909
accumulation of Fe products in the lining pores reduces the oxygen diffusion toward the metal and thus enhances the corrosion protection offered by the calcite layer. Conclusions This study has shown that the corrosion protection of artificially made calcite linings depends on two mechanisms: promotion of a protective oxide film on the metal and accumulation of corrosion products retarding oxygen diffusion. In a freshly formed calcite lining, most of the corrosion protection comes from the reduction of the bare metal areas (corrosion currents decrease with decreasing porosity of the calcite). The cathodic metal surface is insulated from the water and ceases to be an electrode (Legrand and Leroy, 1990). With time, a protective film is formed. Tafel constants and polarization resistances of calcite coated steel were shown to increase with time. Failure of the calcite can occur when the corrosion products (within the calcite layer) develop an internal stress, larger than that of the adhesive bonds between the metal and the calcite. An analogous phenomenon is known to be one of the causes of failure of paints. A symptom indicative of the development of a paint failure is a decreasing polarization resistance (Bacon et al., 1948; Wormwell and Brasher, 1949; Rajagopalan et al., 1975). Such an effect was also noted in this work with calcite linings. Finally, this work shows that the linear polarization and potentiodynamic methods can serve as reliable techniques for characterizing the corrosion protection ability of porous linings such as calcite. Acknowledgment This paper forms part of the research thesis toward the M.Sc. Degree of Shani Keysar at TechnionsIsrael Institute of Technology. The project was supported by funds from Mekoroth-Water Co., the National Council for Research and Development, and the Carylon Foundation, NJ, USA. Literature Cited Ashley, G. W.; Burstein, G. T. Initial Stages of the Anodic Oxidation of Iron in Chloride Solutions. Corrosion 1991, 47, 908. Bacon, R. C.; Smith, J. J.; Rugg, F. M. Electrolytic Resistance in Evaluating Protective Merit of Coating on Metals. Ind. Eng. Chem. 1948, 40, 161. Boxall, J.; von Fraunhofer, J. A. Immersion Characteristics of Zinc Chromate Pigmented Coatings. Paint Manuf. 1973, 43, 13. Clay, H. F. Further Work on Polarisation Testing of Painted Specimens. J. Oil Col. Chem. Assoc. 1969, 52, 158. Devay, J.; Meszaros, L.; Janaszik, F. Electrochemical Techniques to Monitor Performance of Polymer Coatings in Corrosion Protection. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 13. Feigenbaum, C.; Gal-Or, L.; Yahalom, J. Microstructure and Chemical Composition of Natural Scale Layers. Corrosion 1978a, 34, 33. Feigenbaum, C.; Gal-Or, L.; Yahalom, J. Scale Protection Criteria in Natural Water. Corrosion 1978b, 34, 65. Fontana, M. G. Corrosion Engineering, 3rd ed.; McGraw-Hill: New York 1986. Hasson, D.; Karmon, M. Novel Process for Lining Water Mains by Controlled Calcite Deposition. Corros. Prev. Control 1984, 31 (No. 4), 9. Hasson, D.; Keysar, S.; Bramson, D.; Zahavi, J. Corrosion Protection Characteristics of Calcite Lined Iron Water Supply Pipes. 9th European Congress on Corrosion of the European Federation of Corrosion, Proc. publ. by Inst. of Materials, London, Utrecht, Oct. 1989.
Heitz, E.; Kreysa, G. Principles of Electrochemical Engineering; VCH: Weinheim, Germany, 1986. Hausler, R. H. Practical Experiences With Linear Polarization Measurements. Corrosion 1977, 33, 117. Haynes, G. S.; Babion, R. Laboratory Corrosion Tests and Standards; ASTM Symposium, ASTM Technical Publication; American Society for Testing and Materials: Philadelphia, PA, 1983. Keysar, S. Corrosion Protection Characteristics of Porous Coatings. M.Sc. Dissertation, TechnionsIsrael Institute of Technology, Haifa, Israel, 1992. Legrand, L.; Leroy, P. Prevention of Corrosion and Scaling in Water Supply Systems; Ellis Horwood: England, 1990. Newman, J. Electrochemical systems; Prentice Hall: Englewood Cliffs, NJ, 1973. Mansfeld, F. The Effect of Uncompensated IR Drop on Polarization Resistance Measurements. Corrosion 1976, 32, 143. McClanahan M. A.; Mancy K. H. Effect of pH on Quality of CaCO3 Deposited from Moderately Hard Water. J.sAm. Water Works Assoc. 1974, 66, 49. McClanahan, M. A.; Mancy, K. H. Comparison of Corrosion Rate Measurements on Fresh vs. Previously Polarized Samples. J.sAm. Water Works Assoc. 1974, 66, 461. Rajagopalan, K. S.; Guruviah, S.; Sundaram, M.; Chandrasekharan, V. Electrochemical Methods of Evaluation of Protection by Paints. J. Sci. Ind. Res. 1975, 34, 482. Snoeyink, V. I.; Kuch, A. Principles Of Metallic Corrosion in Water Distribution Systems. Internal Corrosion of Water Distribution Systems. AWWA Research Foundation and DVGW, Denver, CO, 1985. Sontheimer, H.; Kolle, W.; Snoeyink, V. L. The Siderite Model Of The Formation of Corrosion Resistance Scales. J.sAm. Water Works Assoc. 1981, 73, 572. Spengler, E.; Margarit, I. C. P.; Mattos, O. R. On the Relation Between Adherence of a Paint Film and Corrosion Protection. Electrochim. Acta 1993, 38, 1999. Stern, M.; Geary, A. L. Electro Chemical Polarization (I) A Theoretical Analysis of The Shape of Polarization Curves. J. Electrochem. Soc. 1957, 104 (No. 1), 56. Stumm, W. Calcium Carbonate Deposition at Iron Surfaces. J. AWWA. 1956, 56, 82. Stumm, W. Estimation of Corrosion Rates in Water. Ind. Eng. Chem. 1959, 51, 1487. Tomashov, N. D. Theory of Corrosions and Protection of Metals; Macmillan: New York, 1966. Tomashov, N. D.; Mikhailovskii, Y. N.; Leonov V. V. Mechanism Of Electrochemical Corrosion of Metals under Insulating Coatings. Corrosion 1964, 20, 218. Walter, G. W. A Critical Review Of D.C. Electrochemical Tests for Painted Metals. Corros. Sci. 1986, 26 (No. 1), 39. West, J. M. Basic Corrosion and Oxidation, 2nd ed.; Ellis Horwood: Hemel Hempstead, England, 1985. Wormwell, F.; Brasher, D. M. Electrochemical Studies of Protective Coatings on Metals. J. Iron Steel Inst. 1949, 162, 129.
Received for review May 20, 1996 Revised manuscript received October 3, 1996 Accepted November 6, 1996X IE9602771
Abstract published in Advance ACS Abstracts, June 15, 1997. X
