Corrosion Prevention With Sodium Silicate.....8
-
Upload
victor-martinez -
Category
Documents
-
view
123 -
download
3
Transcript of Corrosion Prevention With Sodium Silicate.....8
CORROSION PREVENTION WITH SODIUM SILICATE1
Nausha Asrar, Anees U. Malik, Shahreer AhmedResearch And Development Center
Saline Water Conversion CorporationP. O. Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia
SUMMARY
Corrosion of steel structures is a common problem in everyday life. Use of carbon steel
or mild steel as a construction material is taken as an onus despite its poor corrosion
resistance properties at ambient temperature. This is because of its abundance, easy
availability, low cost, good formability and workability and remarkable strength. The
corrosion in steel structures is generally controlled by 3 methods, viz., cathodic
protection, coating or use of an inhibitor. For pipe lines, storage tanks or similar
structures, silicate or silicate based materials have been used as inhibitor for more than
70 years. Silicate in water provides protection to iron and steel by prohibiting its
anodic dissolution.
In this project, tests were carried out to study the corrosion prevention properties of
Sodium silicate supplied by Adwan Chemical Industries. Immersion and
electrochemical tests of steels in tap water with and without addition of Sodium Silicate
have been carried out under static (stagnant) and dynamic (flowing) conditions. In
order to simulate the in situ condition, tests were also carried out in a corrosion test
loop.
These tests were carried out in the corrosion laboratory using polished carbon steel
coupons. Corrosion rates were determined by weight loss method. Polished coupons
were immersed in static and dynamic tap water containing different amounts of sodium
silicate ranging between 3 - 200 ppm.
Results obtained show positive effect of sodium silicate in controlling the corrosion of
carbon steel immersed in tap water. It has also been observed that sodium silicate is
more effective in controlling the corrosion of carbon steel in dynamic water than in
1 Issued as Technical Report N0.TR3804/EVP95013 in July 1998.
1856
static water. However, in dynamic tap water, saturated with dissolved oxygen, optimum
concentration required to provide maximum corrosion inhibition, appears to be quite
high (> 100 ppm). As the inhibitive effect of Sodium Silicate depends upon dissolved
oxygen, its dosing rate should always be optimized in the system to be protected.
1. INTRODUCTION
1.1 Background
In July 1995, ADWAN Chemical Industries, Riyadh approached Department of
Research and Development for testing their product, Sodium Silicate, in prevention of
corrosion in steel pipe lines. The R&D Center Al-Jubail was asked to asses the
feasibility of such testing and if it is in affirmative then to submit an Evaluation Project
Proposal to the Department of R&D, Riyadh.
Subsequently a project proposal was submitted to Riyadh and an agreement was signed
between SWCC and ADWAN Chemical Industries Co. Ltd. on March 26, 1996
(7.11.1416H) to carry out the testing of Sodium Silicate at R&D Center, AL-Jubail.
1.2 Sodium Silicate as a Corrosion Inhibitor
Sodium silicates are the most commonly used commercially available alkali silicates.
These materials are usually manufactured as glasses and if dissolved in water, form
viscous alkaline solutions. The soluble silicates, called water glass, have been used in
cements, coatings, water treatment and ore benefication. These applications are based
on the ability of silicates to form gels or to react with multivalent metal ions in solutions
or oxide surface [Encyclopedia, 1988].
Sodium silicate has been used for the inhibition of corrosion of steel for more than 70
years. The general principle lies in feeding dissolved silicate to the metal water pipe
where a protective film is formed on the inner surface of the pipe. Sodium silicate has
been used for prevention of corrosion in water heating pipelines, centralized heat supply
1857
system, steam boilers, industrial plant circulated municipal water, water transmission
steel pipelines, etc., [Akol’zin, A. P. et al 1971; Varfolomeev, M. Yu et al 1972;
Akol’zin, A. P. et al 1974, 1981].
According to Robinson et al [1987] the basic mechanism of rust control by silicate is
not explainable by silica iron complex as envisaged by earlier investigators [Dart, J. et
al 1970, 1972]. A colloidal solution formation is more consistent with the facts. The
influence of pH on the corrosion inhibition of iron and mild steel by sodium silicate is
studied and it has been found that dissolution of sodium silicate in water generally
increases the pH and the latter appears to play a pivotal role in the inhibition of
corrosion [Lahodny, S. et al 1981]. A positive effect due to soluble silicate species
resulting from pH change could be distinguished. Armstrong [1994] reported that in
unbuffered solutions, sodium silicate is effective as a corrosion inhibitor for iron by
inhibiting the anodic dissolution of the metal. However, the reduction of the corrosion
rate in the pH range 9.6-11.6 can be accounted for, entirely by the change in pH caused
by the addition of sodium silicate. Shams El-Din et al [1991] in a recent study showed
that the corrosion of rusted steel in conditioned potable water was decreased by a
maximum of 47% in the presence of 16 ppm Na2SiO3. However, once protection has
been achieved, 4 ppm was the minimum amount to sustain a state of permanent
inhibition.
Keeping in view, the great versatility of sodium silicate in preventing corrosion of iron
and steels and its promising application in carbon steel and galvanized steel pipelines, it
was proposed to carry out testing of this chemical in corroded or rusted steel water
pipelines. In this study corrosion prevention properties of sodium silicate, supplied by
Adwan Chemicals Industries, was tested. Immersion and electrochemical studies of
steels in tap water, with and without sodium silicate, were carried out under static
(stagnant) and dynamic (flow) conditions.
1.3 What are Soluble Silicates?
The most common method of preparing soluble sodium or potassium silicates involves
the fusion of silica sand and the appropriate carbonate at high temperatures in a furnace
1858
to produce a water soluble glass. The reaction that takes place in the furnace is as
follows, where Me is either sodium or potassium and x is the weight ratio:
Me2CO3 + x SiO2 → Me2O . xSiO2 + CO2↑ (1)
Silicate products with weight ratios of silica (SiO2) to alkali (Na2O or K2O) of up to 4.0
or even higher can be produced. The highest practical ratio is about 3.8. The fused
glasses are dissolved in water to produce syrupy solutions, with solid contents from
about 30 to 50%.
The most common commercial liquid sodium silicate is a product having a weight ratio
of silica to alkali (as Na2O) of 3.22, and with 37 to 38% solids. The ratio can be
reduced by adding sodium hydroxide, to give a series of liquid products, or, in the low
ratio range, a number of solid hydrous or anhydrous materials. A wide range of liquid
forms are available, with SiO2:Na2O ratios from 3.8 to 1.6, and solid contents from
about 25 to 55%.
Because of the variation in properties over the range of products, it is important to
specify the silicate ratio and concentration when discussing an application. The terms
sodium silicate, soluble silicate, or water glass are not explicit enough. Furthermore, in
water treatment applications, it should be clear whether dosages are expressed in terms
of silica (SiO2), silicate solids, or sodium silicate solution as received.
The liquids are similar in that they are all complex solutions of silicate anions, together
with hydroxyl and sodium ions, but the properties differ depending on the ratio and
concentration. The distribution of siliceous species shifts toward higher order polymers
as the ratio of SiO2 to Na2O increases. This is accompanied by a rapid decrease in free
alkalinity and an increase in the colloidal nature of the product.
The stability or solubility domains for various species in equilibrium with amorphous
silica are shown in a plot by Stumm and Morgan (Figure 1) [1970]. The areas of
importance in this concentration-pH diagram are: the monomeric region (Region I),
where the mononuclear species, such as Si(OH)4, Si(OH)3-, and SiO2(OH)2
2-
predominate; the multimeric region (Region II), where silicate polymers are stable; and
the insolubility region (Region III), where amorphous silica precipitates. The
1859
commercial silicate products are found in Region II. Silica species seen in Region I are
normally present in natural water.
Several chemical reactions are characteristic of commercial silicate solutions. The
concentrated solutions are strong alkalis, with pH values from about 11.0 to 13.0. The
pH values of diluted silicate solutions, as a function of alkali contained by the silicate,
are shown in Figure 2. For the lower loadings, the pH values range between about 9 and
10. However, concentrations typically used for potable water treatment which are in the
ppm range, the pH is not affected greatly and, in fact, has a value nearly equal to that of
untreated water.
When the alkalinity is decreased or removed by neutralization, ion exchange or dialysis,
the siliceous species will polymerize at a rate dependent on the silica concentration and
pH value. At SiO2 concentrations of 0.5% or greater, the solution will pass through a
colloidal solution state and ultimately form a solid hydrous gel (Region III, Figure 1).
In the low dosages often used in water applications, the opposite reaction takes place
over a wide pH range, i.e., more polynuclear forms found in Region II and III
depolymerize the mononuclear form. At low to medium pH values, depolymerization
occurs over a period of hours to days, depending on silica concentration [Lange, K. R. et
al 1968].
Katsanis, Krumrine, and Falcone [1983] showed the time dependence of initial
precipitation on the silica concentration and pH value (Figure 3). The pH range of
minimum stability for the silicate solutions is 6 to 8. With aging, precipitates form over
wider and wider ranges of silica concentrations and pH values, but after 24 h there is
little additional precipitation in this pH region. At lower pH values, however, larger
changes continue until the equilibrium conditions shown in Figure 1 are satisfied.
Another reaction of interest to water chemists is the combination of the silica anions
with multivalent cations to form insoluble hydrous metal silicates. This very general
reaction, occurring with alkaline earth and other di- and trivalent cations, is used in the
manufacture of pigments and fillers, soil grouting, waste treatment, metal casting, and
many other industrial processes.
1860
1.4 The Nature of Corrosion Inhibition by Silicate
Silicates inhibit corrosion by forming a thin silicate film on the surfaces of metal water
pipes as the silicate is fed through them. The films are almost invisible and do not
increase in thickness with time. Once the film is formed, the protection is maintained as
long as silicate treatment is continued; if stopped, the protection is gradually lost; if
damaged, the film is self-healing, as long as the silicate feed is continued. The preferred
product for this use is the 3:22 SiO2:Na2O weight ratio. A dosage of 24 to 25 ppm as
silica (60 to 63 gal silicate solution per million gal of water) is recommended for the
first month or two, to form film as quickly as possible. Feed rate can then be reduced to
a maintenance dosage of 8 to 10 ppm SiO2 (20 to 25 gal/MG); maintenance dosages as
low as 4 to 5 ppm SiO2 have been used successfully. For lower pH waters, a more
alkaline silicate should be used, such as the 2.00 weight ratio, to reduce acidity and
thereby enhance the formation of silicate films.
At the levels of SiO2 treatment recommended, figures 1 and 3 show the precipitation of
silica is not likely throughout the entire pH scale. However, those treatments have been
shown to be quite effective, it is reasonable to assume that the thin silicate film on the
metal pipe is formed by an adsorption mechanism.
In old water lines, where corrosion and scale are present, silicate treatment may displace
the accumulated deposits and release the rust [Stericker, W. et al 1938]. There may be
temporary red water problem, but subsequently the pipes will be as cleaner as they
originally were. It is advantageous to start the silicate treatment immediately following
mechanical cleaning and flushing of the pipes if possible.
1.5 Theory of Silicate Inhibition
The silicates are considered anodic inhibitors, initially forming a film on anodic areas.
Lehraman and Schuldener [1951] investigated the mechanism of the process and
concluded that the deposition of the film depends on the presence of small amounts of
corrosion products on the metal surfaces.
1861
At the anodic areas, the oxidation of a metal occurs:
metal → metaln+ + ne- (2)
For ferrous metal pipes:
Feo + 2H2O → Fe(OH)2 + 2H+ + 2e- (3)
In the presence of oxygen, the ferrous ions will be oxidized to ferric:
2Fe++ + ½O2 + 2H+ → 2Fe+++ + H2O (4)
The negatively charged siliceous species can react with the metallic cations to form the
protective film. However, with continuous feeding of the silicates, the films extend to
cathodic areas [Lehrman 1964]. This is important, because as Scheider and Stumm
[1964] point out, the reduction in the free surface for anodic action, without a
corresponding reduction in the surface available for cathodic action, will lead to an
increase in current density at any unprotected anodic sites and result in accelerated
corrosion and pitting at these sites.
For this reason, it is important to use a maintenance dosage that is high enough to
preserve the film throughout the entire system. Maintenance dosages as low as 4 ppm
and lower dosages should be tried gradually.
Microscopic and X-ray examinations of the films show two layers, with most of the
silica in the surface layer adjacent to the water. Most of the silica is amorphous. When
the hydrous metal oxide or metal silicate has been covered with a silica layer, further
growth is halted, which accounts for the observation that continued silicate feed does
not cause a build-up of the films.
2. OBJECTIVES
• To study the effect of sodium silicate on corroded rusted steel pipe lines.
• To measure the corrosion rate of carbon steel in tap water in absence and
presence of sodium silicate.
1862
• To determine the optimum concentration of Sodium Silicate dosing for
corrosion inhibition of the rusted carbon steel.
• To determine the minimum concentration of Sodium Silicate dosing for
maintaining the state of permanent inhibition.
3. EXPERIMENTAL
3.1 Immersion Test
The corrosion tests on carbon steel has been carried out in tap water under static and
dynamic conditions. The composition of the tap water used during the experiments is
given in Table-1.
3.1.1 Coupon Preparation
Commercial grade carbon steel containing 0.3% carbon was used for the tests. Carbon
steel coupons of 100 x 40 x 4 mm size with a hole of 6 mm ϕ near the edge were cut
from a sheet. Coupons were machined and abraded sequentially with silicon carbide
papers of grades 180, 320 and 600. Polished coupons were washed, degreased with
ethyl alcohol and dried up. After taking the initial weight and dimension, coupons were
hanged in the silicate solutions of different concentrations. In order to avoid galvanic
and crevice corrosion, coupons were loosely tightened in the holes using Teflon nuts
and bolts.
3.1.2 Sodium Silicate Solution Preparation
Sodium silicate solutions of required concentrations were prepared as follows:
Chemical Composition:
Na2O : 9.8 %
SiO2 : 31.2 %
Ratio : 3.18
Sp. Gr. (at 20 oC) : 1.44
1863
Normality =
= = 7.47 N Amount of silicate present in 1 litre = 7.47 x 60.0855 = 448.838 g.
= 448838 ppm
1000 ppm Stock. Solution
Volume of silicate in one litre = 1000 x 1000 = 2.22 ml 448838
or = 2.22 x 1.44
= 3.208 gm.
Stock solution was further diluted to prepare solutions of required concentrations.
3.1.3 Calculation of Corrosion Rate and % Inhibition
Corrosion rates were calculated according to the following equation:
534 × WCorrosion Rate (mpy) = ------------------
D × A × t
Where;
W = Weight loss (mg)
D = Density of coupon material (g/ cm3)
A = Area (inch2)
t = Time of exposure (Hr.)
In order to evaluate the effect of inhibitor, percentage inhibition was calculated
according to the following equation:
(C.R.)TR - (C.R.)UTR
% Inhibition = -------------------------- × 100 (C.R.)UTR
Where;
1.44 x 31.2 x 100060.0855 x 100
Sp.Gr. x % Purity x 1000 Mol. Wt. x 100
1864
(C.R.)UTR = Corrosion rate in untreated tap water
(C.R.)TR = Corrosion rate in treated water
Percentage of corrosion inhibition by silicate treatment of tap water in static, dynamic
and loop tests is given in Table - 2.
3.1.4 Immersion in Static Water
Polished coupons were immersed in tap water containing 0, 5, 10, 15, 25, 35, 45, 100,
150 and 200 ppm of silicate. Glass beakers of 1000 cc were used to immerse the
coupons for the period of 14, 28 and 42 days. Silicate solutions were daily replenished
by fresh ones. In order to avoid experimental errors, two coupons were immersed in
each solution. After immersion coupons were taken out, their condition was observed
visually and photographed. After visual observations coupons were cleaned in a
ultrasonic bath. Coupons were dried and their weights were taken to determine the
weight loss and corrosion rate (Table 2).
3.1.5 Immersion in Dynamic Water
Coupons were immersed in silicate solutions of above mentioned concentrations stirred
with magnetic stirrer with 1300 rpm (velocity = 2 m/ sec). Duration of the test and
weight loss measurement procedures were the same as in case of static water. Results
are given in Table 2.
While studying the effect of 0, 5, 10, and 15 ppm of silicate, coupons were exposed for
14, 28 and 42 days. During these tests it was observed that the corrosion behavior of
carbon steel showed similar trend during all the three durations. It does not show any
remarkable difference in the corrosion behavior during longer duration (42 days).
Similar observations were made during the use of 25, 35 and 45 ppm of silicate when
the tests were carried out for 14 and 28 days. In view of this, other experiments were
carried out only for 14 days.
3.2 Corrosion Loop Test
1865
In order to study the inhibitive effect of silicate in flowing tap water, tests were carried
out in a dynamic corrosion test loop. The test loop assembly is comprised of a plastic
tank of 200 liter capacity, stainless steel pump, temperature, pressure, flow rate and
dissolved oxygen measuring system (Figure 4). All the pipes and fittings of the loop were
of PVC. At one end of the pipeline of the loop, two polished and initially weighed
carbon steel coupons of known dimension (100 × 40 × 4 mm) were fixed on retractable
coupon holder. In order to measure the dissolved oxygen and temperature of the
recirculating water, the probe of Dissolved Oxygen Monitor (Yellow Springs
Instrument Co., Model – 56) was connected with the water flowing in the pipeline and
on-line monitoring of dissolved oxygen was done. Coupons were allowed to oxidize for
24 hours in the loop. After 24 hours, the required amount of silicate was added to the
200 liters of the tap water recirculating in the loop. Tests were carried out at 35 °C in 0,
3, 5, 10 and 15 ppm of silicate solutions for 160 hours. The flow rate was maintained at
80 GPM (2.5 m/ Sec). After 160 hours of exposure, coupons were taken out, cleaned in
ultrasonic bath, dried and finally weighed to know the weight loss/ area. The corrosion
rate is calculated, from the obtained weight loss, time of exposure and original exposed
surface area of the material, by the formula mentioned in section 2.1.4.
3.3 Open Circuit Potential (OCP) Measurement
While carrying out the loop test, OCP measurement was also carried out under the same
condition and concentration of silicate. OCP measurement was carried out to study the
change in potential of the carbon steel immersed in flowing water with time and
concentration of silicate. The OCP curve pattern gives an idea about the protective
behavior of the oxide scales formed on the metal surface.
For this study, probe electrodes (for Petrolite Corrosion Meter) of 1018 MS material
and 44 × 6 mm dimension were abraded with 600 SiC paper and finally mirror polished
with emery paper. Polished electrode was washed with ethyl alcohol and dried. Polished
electrodes were inserted into the pipe line of the loop with the help of a tightly fitted
rubber cork and connected to the plotter for on-line measurement of change in potential
against calomel electrode. OCP of the carbon steel electrodes were plotted for 160 hours
at a chart speed of 2 cm/ hr. All the electrodes were preoxidized for 24 hours in tap
water. Silicate was added after 24 hours to make the solution of required concentration.
1866
To maintain a situation of the constant dosing, water was replenished after every 24
hours.
4. RESULTS AND DISCUSSION
4.1 Immersion Test
The corrosion data of the steel in water under static and dynamic conditions (Table 2)
indicate that the trend of the corrosion inhibition by different concentrations of silicate
did not change with time. However, the magnitude of corrosion rate has increased with
the treatment period. This behavior indicates that the rate of formation and thickening of
the passive film decrease with time, and in the present condition the above mentioned
concentration of silicate is unable to passivate the steel coupon. Figure 6 shows typical
plots of corrosion rate vs silicate concentration in tap water for a 14 day test.
On the basis of the study of the corrosion rate of carbon steel immersed in the tap water
containing 5 - 200 ppm of sodium silicate, following generalizations can be made:
∗ In static tap water containing up to 100 ppm silicate, corrosion of carbon
steel was always less than that in the dynamic water.
∗ Under dynamic conditions, the tap water containing more than 100 ppm
silicate shows protection.
Sodium silicate has been found very effective in controlling corrosion of carbon steel by
many investigators [Akol’zin, A. P. et al 1971; Varfolomeev, M. Y. et al 1972;
Akol’zin, A. P. et al 1974, 1981; Robinson, R. B. et al 1987; Dart, J. et al 1970, 1972;
Lahodny, O. et al 1981] and the optimum quantity of sodium silicate required to
suppress the corrosion has been cited as less than 10 ppm. The present results show that
under static immersion conditions, silicate fails to suppress the corrosion rates even at
concentrations as high as 200 ppm. Under dynamic immersion conditions, corrosion
rates appear to decrease at silicate dosing higher than 100 ppm. There is lowering down
of corrosion rates by 17.8 and 71.0 % at silicate concentrations of 150 and 200 ppm,
respectively.
1867
It is important to consider that above immersion tests were carried out in beaker, where
the water gets saturated with oxygen (6 ppm dissolved oxygen in static condition and
8.5 ppm dissolved oxygen in dynamic condition). Oxygen concentration may also affect
the performance of silicate. Also, it is evident from the above results that silicate is
more effective in flowing water than stagnant one. It is a pointer towards the fact that
corrosion prevention mechanism of silicate does depend on the flow rate of the water. It
is also to be emphasized that the dynamic tests carried out in beaker are far away from
the real situation of water flowing in a pipe line. In view of this, it was thought
worthwhile to carry out a close circuit loop test where the flow rate was 80 GPM (flow
velocity 2.5 m/ sec.) and the dissolved oxygen will be comparatively less.
4.2 Loop Test
In the loop test corrosion rate was determined at a flow rate of 2.5 m/ sec. which is not
much different from the flow rate maintained in the beaker during dynamic immersion
test. However, the dissolved oxygen concentration in the loop test was much lower (5.5
ppm) than that found during the dynamic immersion test (8.5 ppm).
Change in pH during the loop test was also monitored. The pH of the fresh tap water
was found in the range of 7.2 - 7.5, but after exposing the carbon steel coupons for five
days, the pH of the water was shifting towards more basic range, showing a value of
8.2. The alkalinity of the tap water could be due to the presence of rust (FeOOH) in the
water. The pH pattern of silicate treated tap water at different silicate concentrations
was as follows:
3 ppm Silicate Solution pH = 8.0
5 ppm Silicate Solution pH = 8.2
10 ppm Silicate Solution pH = 8.4
15 ppm Silicate Solution pH = 8.5
The alkalinity does not show any considerable rise with time.
1868
The results of corrosion test loop showed in general, relatively high corrosion as
compared to that observed during dynamic and static immersion tests. But inspite of
very high corrosion rates, during loop tests, silicate was found very effective in
inhibiting the corrosion of carbon steel. Corrosion rates observed after treating the tap
water with silicate are given in table 2. The plots of corrosion rates against the silicate
concentration, show that the addition of silicate in the range of 3 - 15 ppm inhibits the
corrosion by >45% (Figure 7). However, out of the above mentioned concentrations, 5
ppm appeared to be the most effective amount which can inhibit the corrosion by more
than 62% (Figure 8). It is also evident from Figure 7 that an increase in the concentration of
silicate above 5 ppm does not increase the corrosion protection.
Comparing the corrosion behavior of carbon steels in sodium silicate treated tap water
tested under three different conditions e.g., static immersion, dynamic immersion and
dynamic loop, the results of loop test appear to be of far reaching importance. In loop
tests, corrosion rates are although more due to high flow velocity, silicate is far more
effective in inhibiting the corrosion than other two types of tests.
In the classical approach, the corrosion of iron is represented by the anodic reaction:
Fe → Fe 2+ + 2e- ------(i)
And by cathodic reaction:
2 H+ + 2 e- → H2 ------(ii)
In the presence of oxygen depolarization takes place:
½O2 + H2O + 2e- → 2OH- ------(iii)
The effect of velocity of water on steel is largely determined by oxygen diffusion and
film formation.
The rate of corrosion is generally controlled by oxygen depolarization. The rate of
diffusion of oxygen to the metallic surface depends on the rate of diffusion through the
corrosion product films which act as a barrier to the passage of oxygen. For instance,
one of such well known films is of Fe(OH)3 formed by the reaction:
1869
2 Fe + 2H2O + O2 → 2 Fe(OH)2 ----(iv)
2 Fe(OH)2 + H2O + ½ O2 → 2 Fe(OH)3 ----(v)
In addition to the hydrated oxides, a layer of magnetite (Fe3O4) or FeO.Fe2O3 often
forms between iron oxides (FeO) and hematite (Fe2O3). Actually, the various oxides and
hydroxides of iron form a rather complicated system of compounds. The compound
FeOOH has been found to exist in three different crystal forms plus an amorphous form
[Metal Hand Book, 1987]. The occurrence of the various oxide species is dependent on
pH, oxygen availability and the composition of the steel.
Rust formation in open atmosphere:
4 Fe + H2O + 3 O2 → 2 Fe2O3 . H2O
Rust formation in water:
It generally forms two layers:
∝ FeOOH + γ FeOOH (Outer, loose, crystalline)
FeOOH + Fe3O4 (Inner, dense, crystalline)
The actual nature of the rust film is important because FeOOH seems to be more
adherent than hydrated Fe3O4 and Fe2O3 [Metal Hand Book, 1987], and therefore, more
likely to show the corrosive attack, but the higher oxides and oxy-hydroxides are more
prone to spallation.
The rust layer, therefore, provides resistance to the diffusion of oxygen. The first
resistance is offered to the passage of oxygen from the fluid to the porous corrosion
product film. The second resistance is applied by the porous solid layer filled with fluid.
1870
Cathodic reaction proceeds very rapidly at the metal surface because of its low
activation energy. The concentration of oxygen at the metal surface is therefore, very
small as compared to the bulk concentration. The main resistance to the mass transfer is
offered by Fe(OH)3. At higher velocities, the resistance to the mass transfer is decreased
and the rate of corrosion is increased. However, this has also been reported that the rate
of corrosion is not only related to the velocity of the water but it also depends on the
type of corrosion product and temperature. For instance, Butler and Stroud [1961] has
reported a maxima in the corrosion rate at a velocity of 1 m/ sec. as compared to that at
2 m/ sec. while temperature is 25 °C (Figure 8). But at 55 °C, corrosion rate increases with
velocity when measured at the above said velocities. At further higher temperature of 76
°C, the corrosion rate again decreases with rise in velocity. This is attributed to the more
compactness of the corrosion product at higher temperatures which makes it more
resistant at higher velocities [Whiteman, G. E. et al 1924]. Thus, oxygen diffusivity,
temperature and corrosion product play an important role in the corrosion of carbon
steel and the rate of corrosion depends on the dominance of one or more factors.
Although increasing oxygen concentration at first accentuates corrosion of iron in water,
as one expects from equation (iii), it is found that beyond a critical concentration the
corrosion rate may drop again to low values [Groesbeck, E. et al 1931].
Regardless of the actual pH of water between pH 4 and 9.5, the surface of iron is always
in contact with an alkaline saturated solution of hydrous ferrous oxide. Since the
corrosion product film next to the iron is essentially unchanged by external conditions
within the above range of pH, the corrosion rate is altered only by change in dissolved
oxygen and velocity of water. Also, so far the effect of pH is concerned, it is well
known that the corrosion rate of iron in aerated solution is independent of pH ranging
between 4 and 10, at a pH > 10 the corrosion rate decreases rapidly with increasing pH
[Whiteman, G. E. et al 1924]. Dissolution of Na2SiO3 in water generally increases the
pH since silicic acid H2SiO3 is a weak acid with pK1 = 9.5 and pK2 = 12.7 and the
following reaction occurs:
SiO32- + H2O ⇔ H.SiO3
- + OH
1871
As after addingNa2SiO3 in the range of 3 - 15 ppm the pH of the solution does not
exceed pH 9, while the corrosion rate is considerably suppressed, it appears that
Na2SiO3 is controlling the corrosion of steel not by changing the pH but by some other
mechanism.
High concentration of dissolved oxygen (8.5 ppm) and velocity of water (∼ 2 m/ Sec.)
appear to be the cause of higher corrosion rate observed during the dynamic immersion
test carried out at 25 °C. The analysis of the corrosion product shows that the rust
formed during the static and dynamic immersion tests carried out in the beakers was
hydrated loose ϒ-Fe2O3 and Fe3O4 which give comparatively less protection to the steel
from corrosion [Metal Hand Book, 1987]. No remarkable effect of the inhibitor addition
during static and dynamic immersion tests can be attributed to the formation of the
above mentioned loose hydrated oxides which may be due to result of the comparatively
high concentration of dissolved oxygen.
The remarkable suppression in the corrosion rate of carbon steel in 3 - 15 ppm of
Na2SiO3 solution observed during loop test is an interesting and important aspect of this
study. It has been discussed by many authors that the nature of the oxide scales formed
on the steel surface plays an important role in determining the inhibitive effect of
Na2SiO3 in water [24 - 32]. Chen et al [1991], postulated that a layered structure of
Fe2O3/FeO/Fe is responsible for the lowering of the corrosion rate of iron in contact
with both aerated and deaerated Silicate solutions. Presence of FeO in the rust formed
on the steel coupons exposed in the loop was confirmed by X-ray diffraction analysis.
Formation of FeO along with ϒ-Fe2O3 is expected due to low dissolved oxygen (5.5
ppm) as compared to the dynamic test (8.5 ppm) or also due to higher flow rate.
It is also very important to determine the optimum concentration of the Na2SiO3 which
should be fed continuously. This is because of the fact that after covering the anodic
site, the film of the inhibitor may extend to the cathodic areas. As Schneider and Stumm
reported [1964], any reduction in free surface for anodic action without a corresponding
reduction in the surface available for cathodic action, will lead to an increase in current
density at any unprotected anodic sites and result in accelerated corrosion and pitting at
these sites.1872
These results indicate that it is very important to use a maintained dosage of Silicate
which could be most suitable to preserve the protective film throughout the system.
Also, excess aeration of the flowing water should be avoided to enhance the inhibitive
effect of Na2SiO3. This findings disagree with the contention of Katasanis et al [1986]
that Silicate treatment would be more effective with high oxygen content.
4.3 Open Circuit Potential (OCP) Test
The open circuit potentials of clean and rusted C-steel coupons were followed as a
function of time in the untreated and silicate treated tap water until a constant value was
attained. At the moment of immersion in water, the clean coupons registered potentials
ranging between -450 to -480 mV vs SCE. These potentials slowly drifted towards more
negative values and attained constancy in the range of -570 mV to -632 mV, after
around 15 - 20 hours of immersion. The negative shift in potential is indicative of the
destruction of the air-formed pre-immersion oxide film, and the attack on the metal.
Shifting of the potential towards the original OCP is an indication of passivation of the
metal surface. It is evident from Figure 9 that an addition of 3 - 5 ppm of Sodium Silicate
to tap water provides a maximum passivation to the C-steel coupons as the OCP shifts
from ~ -630 mV to ~ -560 mV, while a higher concentration of 10 and 15 ppm of
silicate could not shift the OCP below -609 mV. These results are in consistence with
the findings of the Loop Test and confirms the remarkable corrosion inhibition when 3 -
5 ppm of silicate is added to the flowing tap water.
5. CONCLUSIONS
On the basis of Static and Dynamic Immersion, Loop and Electrochemical Tests, the
following conclusions can be made:
1. Optimum concentration of silicate depends upon the dissolved oxygen
and the rate of flow of the water.
2. Although the corrosion rates of the steel in dynamic condition are
invariably higher than those under static condition but silicate acts
1873
more effectively as an inhibitor when the water is in dynamic
condition.
3. From the loop test it is evident that at ambient temperature, 5 ppm
concentration of silicate is sufficient to suppress the corrosion of
carbon steel by 62% in tap water having a dissolved oxygen level of
5.5 ppm and a flow rate of 2.5 m/Sec.
4. Electrochemical tests indicate that silicate concentration in the range
of 3-5 ppm provides remarkable inhibition to the flowing tap water.
6. RECOMMENDATIONS
1. In order to protect carbon steel pipes, it is recommended to maintain a
continuous dosing of 5 ppm of silicate in tap water because it is
effective in controlling the corrosion of C-steels as shown by the
results of loop tests.
2. Due to the clear effect of oxygen content in normal tap water, the role
of other oxidizing agent contents, such as chlorine, is to be
investigated in a separate study. Such an overall oxidizing agents
effect could lead to a better understanding about silicate dose
optimization.
REFERENCES
1. Akol'zin, A. P. et al (1971) 'Use of sodium silicate for preserving the equipment ofwater heating pipe lines'. Energetik, 7, 8-9.
2. Akol'zin, A. P. et al (1974) 'Use of sodium silicate to prevent corrosion of boiler steel'.Energetik, 6, 12-13.
3. Akol'zin, A. P. et al (1974) 'Use of sodium silicate for protecting heating system fromcorrosion’. Zh. Telplo energy, 35-490.
1874
4. Akol'zin, A. P. et al (1981) 'Protection of boilers from downtime corrosion by sodiumsilicate solution prepared by using hard water'. Prom. Energy, 7, 41-43.
5. Armstrong, R. D. and Zhou, S. (1988) ‘Corrosion Sci.’, 28, 1177.
6. Armstrong, R. D., Peggs, L. and Walsh, A. (1994) 'Behavior of sodium silicate andsodium phosphate (tribasic) as corrosion inhibitor for iron'. J. Applied Electrochemistry,24, 1244.
7. Butler, B. and Strand, G. (1961) ‘British Corrosion Journal’. 1, 110.
8. Chen J. R. and Chao, H. Y. (1991) ‘Surface Sci.’, 247, 352.
9. Dart, J. and Foley, P. D. (1970) 'Preventing iron deposition with sodium silicate'.Journal AWWA, 62, 663.
10. Dart, J. and Foley, P. D. (1972) 'Silicate as Fe, Mn deposition preventive in distributionsystems'. Journal AWWA, 64, 244.
11. Duffeck, E. F. and Mckinney, B. S. (1956), ibid, 103, 645.
12. Encyclopedia of Polymer Science and Engineering, 'Silicates' (1988), 2nd Ed., 15,178.Fujita, N., Matsuura, C. and Ishigure, K. (1989) ‘Corrosion’. 45 (11), 901.
13. Groesbeck, E. and Waldron, L. (1931), Proc. Amer. Soc., Testing Materials, 31, part-II, 279-291.
14. Kalsanis, E. P., Kumrine, P. H. and Falcone, J. S. Jr. (1983) ‘Chemistry of precipitationand scale formation in geological systems’. Presented at International Symposium onOil-field and Geothermal Chemistry, Denver, CO, June, 1- 3.
15. Katsanis, E. P., Esmonde, W. B. and Spencer, R. W. (1986) ‘Matter. Perform.’. 25, 19.
16. Lahodny, O. S. and Kasleau, L. (1981) 'The influence of pH on the inhibition ofcorrosion of iron and mild steel by sodium silicate'. Corrosion Science, 21, 265.
17. Lange, K. R. and Spencer, R. W. (1968) ‘Environmental Science and Technology, 2(3),212.
18. Lehrman, L. and Shuldener, H. L. (1951) ‘Journal of the American Water WorksAssociation’. 43, 175.
19. Lehraman, L. (1964) ‘Journal of the American Water Works Association’. 56(8), 1009.
20. Metal Hand Book (1987),American Society of Metals, 13,511.
21. Pryor, M. J. and Cohen, M. (1953) ‘J. Electrochem. Soc.’, 100, 203.
1875
21
22. Robinson, R. B., Minear, R. A. andHolden, J. M. (1987) 'Effects of several ions on irontreatment by sodium silicate and hypochlorite'. Journal AWWA, 79, 116.
23. Schneider, C. R. and Stumm, W. (1964) ‘Journal of the American Water WorksAssociation’. 56(5), 621.
24. Shams El-Din, A. M., Saber, T. M. and Salem, A. H. (1991), Proc. Vth Middle EastCorrosion Conference (NACE), Bahrain, October 28-30, 331.
25. Shuldner, H. L. and Sussman, S. S. (1960), ibid, 16, 126.
26. Stericker, W. (1938) ‘Industrial and Engineering Chemistry’. 30 (3), 348.
27. Stumm, W. and Morgan, J. J. (1970) ‘Aquatic chemistry, an introduction emphasizingchemical equilibria in Natural Waters’. Wiley-Interscience, New York, 395.
28. Varfolomeev, M. Yu. and Glyakhov, O. (1972) 'Use of sodium silicate for theprotection of centralized heat supply system from internal corrosion'. Elek. Sta. 10, 31-34.
29. Whiteman, G. E., Russell, R. P. and Alliery, V. J. (1924) ‘Ind. Eng. Chem.’. 16, 665.
30. Wood, J. W., Beecher, J. S. and Laurence, P. S. (1957), Corrosion NACE, 13, 41.
1876
1877
Table 2. Percentare Corrosion Inhibition by Addition of Sodium Silicate in Tap Water During Different Tests
S. No. Type of Corr. Test Corrosion Rate in
Untreated Water (mpy) Silicate
Concentration (ppm)
Corrosion Rate in Silicate Treated Tap
Water (mpy)
% Inhibition
1 Static Immersion Test 1.07 5 1.07 0.0 10 1.04 -2.8 15 1.13 5.6 25 1.55 44.8 35 1.53 43.0 45 1.60 49.5 100 1.76 64.5 150 1.56 45.8 200 1.13 5.6
2 Dynamic Immersion Test 11.48 5 12.58 9.5
10 10.96 -4.5 15 10.96 -4.5 25 13.94 21.4 35 17.15 49.4 45 16.30 42.0 100 12.08 5.2 150 9.42 -17.8 200 3.30 -71.0 3 Loop Test 201.5 3 107.20 -46.9
5 76.15 -62.2 10 109.06 -45.9 15 91.65 -54.5
1878
EFFECT OF Ph ON THE SOLUBILITY OF AMORPHOUS SILICA
log k (250C)1. SiO2 (amorphous) + 2H2O = Si(OH)4 -2.72. Si(OH)4 = Si(OH)3
- + H+ -9.463. SiO(OH)3
- = SiO2(OH)2-2 = H+ -12.56
4. Si(OH)4 = Si4O6(OH)6-2 + 2H+ + 4H2O -12.57
Figure 1. Species in equilibrium with amorphous silica. Diagramcomputed from equilibrium constants (250C). The linesurrounding the shaded area gives the maximum soluble silica.The mononuclear wall represents the lower concentration limit,below which multinuclear silica species area not stable. Innatural waters the dissolved silica is present as monomericsilicic acid.
1879
Figure 2. PH values of sodium silicate solutions at 200C. Numbers onlines are SiO2/Na2O ratio by weight.
Figure 3. Solubility diagrams for 3.22 weight ratio Na-silicate as afunction of pH for aging times up to 1 month at 250C.
1880
CORROSION TEST LOOP
EC = Electrochemical Cell PG = Pressure GaugeFM = Flow Meter RC = Retractable Coupon HolderOM = Oxygen Monitor TI = Temperature IndicatorP = Pump V = Valve
Figure 4. Flow Diagram of Corrosion Test Loop.
EC TI
V
OMV FM
V
V
Drain
FM PGP
RC
RESERVIOR
1881
Figure 5. Corrosion test loop used for the study of corrosion inhibition by sodium silicate
1882
0
2
4
6
8
10
12
14
0 5 10 15
SILICATE CONCENTRATION (PPM)
CORR
OSI
ON
RA
TE (m
py)
DynamicStatic
Figure 6. Corrosion of carbon steel in sodium silicate containing tap water (A14 Day Test Result)
0
50
100
150
200
250
0 3 5 10 15SODIUM SILICATE CONC. (ppm)
CO
RR
OSI
ON
RA
TE (
mpy
)
Figure 7. Corrosion behavior of carbon steel in silicate treated tap waterduring loop test.
1883
3 5 10 15
46.9
62.2
45.9
54.5
0
10
20
30
40
50
60
70
3 5 10 15
Conc. of Silicate (ppm)
% I
nhib
ition
Figure 8. Corrosion inhibition of carbon steel by sodium silicate treatment inloop test.
1884
-675
-650
-625
-600
-575
-550
-525
-500
0 20 40 60 80 100 120 140 160
TEST DURATION (Hr. )
PO
TE
NT
IAL
(-m
V)
Tap Water
Tap Water + 3 ppm
Tap Water + 5ppm
Tap Water + 10 ppm
Tap Water +15 ppm
Figure 9. Change in open circuit potential of carbon steel in sodium silicate treated tap water during loop test.
1885