Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF INHIBITING CORROSION USING
HALO-BENZOTRIAZOLES
FIELD OF THE INVENTION
The present invention relates to the control of corrosion in
aqueous systems. More particularly, the present invention relates to the
inhibition of corrosion of steel and copper alloys in aqueous systems
through application of halo-benzotriazoles to the aqueous system.
BACKGROUND OF THE INVENTION
The use of triazoles for inhibiting the corrosion of copper and iron
alloys in a wide variety of aqueous and non-aqueous systems is well
known. In industrial cooling water systems, benzotriazole and tolyltriazole
are used most often. Tolyltriazole is generally preferred because of its
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lower cost. Triazoles are film forming materials that provide efficient
coverage of metal or metal oxide surtaces in a system thereby providing
protection against corrosive elements present in an aqueous system. In
addition to the film forming tendency of various azoles, they also
precipitate soluble, divalent copper ions. The precipitation prevents
transport of copper ions to ferrous surfaces, where galvanic reactions
between copper ions and iron atoms leads to pitting corrosion of the
ferrous metal.
While the use of azoles for corrosion inhibition is widespread,
there are drawbacks to their use, specifically with tolyltriazole. The most
important drawbacks are experienced when azoles are used in
combination with oxidizing halogens. Oxidizing halogens such as
elemental chlorine, bromine, their hypohalous acids, or their alkaline
solutions (i.e., solutions of hypochlorite or hypobromite ion) are the most
common materials used to control microbiological growth in cooling water
systems. When copper or iron alloys that have previously been protected
with azoles are exposed to an oxidizing halogen, corrosion protection
breaks down. After breakdown, it is difficult to form new protective films
in tolyltriazole treated cooling systems that are being chlorinated,
particularly continuously chlorinated. Very high dosages of tolyltriazole
are frequently applied in an attempt to improve performance, often with
limited success.
The degradation of protection of azole films in the presence of
oxidizing halogens is well-documented in the literature. For example, R.
Holm, et al., concluded that hypochlorite penetrates an intact triazole film,
leading to higher corrosion rates, and that secondly, hypochlorite attacks
the prefilmed triazole surface, disrupting or degrading the film (53rd
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Annual Meeting of the International Water Conference, Paper No.
IWC-92-40, 1992). Lu, et al., also studied interactions of triazole films
with hypochlorite on copper and copper alloy surfaces ("Effects of
Halogenation on Yellow Metal Corrosion: Inhibition by Triazoles",
Corrosion, 50, 422 (1994)). Lu, et al., concluded:
(a) prefilmed tolyltriazole on copper and brass surfaces
undergoes decomposition during chlorination;
(b) the stability of prefilmed tolyltriazole on copper and brass to
NaOCI was improved when tolyltriazole was added to the
hypochlorite solution;
(c) clean (i.e., non-prefilmed) copper surfaces did not develop
good protective films when placed in solutions containing
mixtures of tolyltriazole and NaOCI.
Thus, the combination of tolyltriazole with NaOCI did not produce
a composition capable of efficient film formation and corrosion inhibition.
The nature of the reaction products when azoles are exposed to
oxidizing halogens in a cooling water system is not clear. The literature
teaches that a compound is formed when chlorine and tolyltriazole are
combined in cooling waters, and that it responds to analytical tests for
chlorine. For example, Vanderpool, et al., state that chlorine reacts
reversibly with tolyltriazole to produce N-chloro-tolyltriazole. They
specifically state, "presumably this compound is not itself an inhibitor."
Rather, they teach that it is readily hydrolyzed to the original tolyltriazole
and hypochlorous acid so that free tolyltriazole becomes available for
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corrosion inhibition ("Improving the Corrosion Inhibitor Efficiency of
Tolyltriazole in the Presence of Chlorine and Bromine", NACE
Corrosionl87, Paper No. 157 (1987)). Hollander and May stated they
were able to isolate 1-chloro-tolyltriazole from stored, more highly
concentrated solutions, but they also teach that "at low concentrations
(less than 10 mg/L) rapid hydrolysis made it impossible to isolate the
chloro adducts." Based upon proton NMR analysis, the material
Hollander and May isolated was chloro-tolyltriazole.
Another observation is that a very characteristic odor is present
whenever tolyltriazole and chlorine are combined in cooling waters.
In contrast, the present authors have shown that chloro
tolyltriazole does not respond to analytical tests for chlorine, despite
extended boiling. And solutions of chloro-tolyltriazole, surprisingly, do
not produce the characteristic odor. Thus chloro-tolyltriazole is clearly
different from the tolyltriazole-chlorine reaction product that forms in-situ
in cooling water systems.
There are also references in the literature to 5-chlorobenzotriazole
(i.e., CAS number [94-97-3]). In "The Water Drop", Volume I No. 2, 1985,
Puckorius & Associates state that chlorinated tolyltriazole is effective as a
corrosion inhibitor and cite R.P. Carr as a reference. A literature review
of published work by Carr indicates that he actually teaches that
reactions between tolyltriazole and chlorine do not occur under cooling
water conditions ("The Performance of Tolyltriazole in the Presence of
Sodium Hypochlorite Under Simulated Field Conditions", NACE
Corrosion/83 Paper No. 283, 1983). In this Corrosion/83 paper, Carr
does discuss the inhibiting action of a chloro-azole but it is a reference to
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earlier literature and specifically to the action of 5-chlorobenzotriazole
and related aryl substituted azoles in sulfuric acid solutions ("Effects of
Substituted Benzotriazole on the Electrochemical Behavior of Copper in
H2S04", Wu et al., Corrosion, Volume 37, No. 4, 223 (1981 )). Since the
5 1985 Puckorius reference, there has been widespread use of tolyltriazole
in chlorinated cooling systems with well established performance
difficulties, indicating a continuing, unsolved problem in the art.
Other problems are well-known when tolyltriazole and oxidizing
halogens are combined in cooling waters. These include a loss in the
extent of precipitation of transition metal ions such as copper, thus
leading to improved transport and galvanic corrosion, a change in the
response of the standard spectrophotometric test for tolyltriazole, leading
to unintentional overfeed, and the objectionable odor mentioned above.
This odor can be sensed even when the cooling water originally
contained 1 ppm tolyltriazole, or less. Since cooling water often passes
over cooling towers, evaporation and drift release the objectionable odor
to the local environment.
The present inventors believe that the odorous material is
N-chloro-tolyltriazole, that it forms OCI- reversibly with tolyltriazole in
dilute solution, and that it is absent in the final product when the reaction
is run in concentrated solution, i.e., tolyltriazole + OCI-~ N-chloro-
tolyltriazole- (intermediate) -~ chloro-tolyltriazole. The present inventors
have found no evidence of reversion of chloro-tolyltriazole to either the
odorous intermediate or to tolyltriazole. Nor is there any evidence of
reactions between hypochlorite and chloro-tolyltriazole in dilute aqueous
solutions.
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SUMMARY OF THE INVENTION
The present inventors have discovered that halo-benzotriazoles
such as chloro-tolyltriazole and bromo-tolyltriazole are more effective
than tolyltriazole in inhibiting corrosion in aqueous systems. The halo-
benzotriazoles are substantially more effective than tolyltriazole in the
presence of chlorine. Furthermore, when chloro-tolyltriazole is exposed
to chlorine, an objectionable odor does not form and the quantity of
chlorine that is required to produce a residual in the aqueous system is
reduced.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present inventors have discovered that halo-benzotriazoles
such as chloro-tolyltriazole and bromo-tolyltriazole are more effective
than tolyltriazole in inhibiting corrosion in aqueous systems. The halo-
benzotriazoles are substantially more effective corrosion inhibitors than
tolyltriazole in the presence of chlorine. The efficacy of the present
invention is surprising given the prior knowledge that chlorination of an
azole treated system leads to degradation of corrosion inhibition
performance. Furthermore, the halo-benzotriazoles of the present
invention are not subject to the formation of objectionable odors when
exposed to chlorine as is tolyltriazole, the quantity of chlorine that is
required to produce a residual in the aqueous system is notably reduced
in comparison to systems treated with tolyltriazole, and the treatment is
effective in the presence of sulfide ions.
It was discovered that the ex-situ preparation of a halo-
benzotriazole provided a corrosion inhibitor which exhibited a surprising
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and unexpected activity when compared to a treatment comprising a
mixture of a benzotriazole and a halogen. The results of the studies of
the present invention clearly show that mere mixtures of a benzotriazole .
and a halogen in a cooling water system do not provide the corrosion
inhibiting effect of the addition of a halo-benzotriazole prepared ex-situ.
As further evidence of the surprising activity of an ex-situ prepared halo-
benzotriazole, the present inventors found that the chlorine demand of a
system treated in accordance with the present invention was significantly
reduced. Furthermore, in systems treated in accordance with the present
invention the objectionable odor common to systems treated with a
triazole and halogen was absent.
The halo-benzotriazoles of the present invention can include
chloro-, fluoro-, bromo- and iodo- as well as haloalkyl (trifluoromethyl)
benzotriazoles. Preferred are chloro-tolyltriazole and bromo-tolyltriazole.
The azole may include tolyltriazole, benzotriazole, butylbenzotriazole,
mercaptobenzothiazole and the like. The preferred azole is tolyltriazole.
The preferred benzotriazole, tolyltriazole, is such that the preferred
halo-benzotriazole is chloro-tolyltriazole or bromo-tolyltriazole. The
preparation of the preferred chloro-tolyltriazole can be by any suitable
means. Examples of preparation methods include but are not limited to
reactions with hypochlorite, N-chlorosuccinimide, and other chlorinating
agents. A method of forming chloro-tolyltriazole is through the reaction
of tolyltriazole with hypochlorite, in which case the final reaction mixture
is an alkaline solution that can be used with or without further
modification. Alternatively, chloro-tolyltriazole can be formed through the
reaction of tolyltriazole with hypochlorite in acetic acid solutions, (i.e.,
hypochlorous acid) and then isolated as a solid. For convenience of
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application, the solid can be redissolved in alcohols such as methanol or
2-propanol, aqueous solutions of alcohols or strong alkaline solutions
such as sodium hydroxide or potassium hydroxide.
The preparation of bromo-tolyltriazole can be by any suitable
means. Examples of preparation methods include but are not limited to
reactions with hypobromite, bromine, and other brominating agents. A
method of forming bromo-tolyltriazole is through the reaction of tolyltria-
zole with bromine in an aqueous solution and then isolating it as a solid.
For convenience of application, the solid can be dissolved in a strong
alkaline solution such as sodium hydroxide or potassium hydroxide.
In treating an aqueous system in accordance with the present
invention, the chloro-tolyltriazole (hereinafter CI-TTA) is preferably fed
continuously to the water. A preferred treatment concentration ranges
from about 0.5 to 10 parts per million, most preferably at about 3 parts
per million. Continuous feed is not, however, a requirement. The
chloro-tolyltriazole can be fed at a concentration sufficient to form a
protective film and thereafter feed can be discontinued for extended
periods of time.
The halo-benzotriazole treatment of the present invention can be
used in combination with other corrosion and/or deposit inhibiting
treatments known in the art including but not limited to phosphates,
phosphonates, acrylic homo- and copolymers, chelants, and oximes.
The present invention will now be further described with reference
to a number of specific examples which are to be regarded solely as
illustrative and not as restricting the scope of the present invention.
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Examples
Example 1
The preparation of the solid samples was as follows:
Tolyltriazole (hereinafter TTA) (30 g, 0.225 mol) was dissolved in
aqueous acetic acid (60 mL, 1:1 ratio) by heating to 32°C. Sodium
hypochlorite (366 g, 5.25% sodium hypochlorite as a bleach solution)
was added while maintaining the reaction temperature at ~ 20°C.
Following the addition, the reaction mixture was stirred at room
temperature for 24 hours. A sticky precipitate formed during this time.
The solid was filtered and taken into methylene chloride. The solid that
did not dissolve was filtered and identified as a mixture of CI-TTA with
minor amounts of TTA and dichloro-tolyltriazole (di-CI-TTA). The
methylene chloride was removed to obtain a yellow solid which was
identified as a mixture of CI-TTA with minor amounts of di-CI-TTA.
Unless noted, this latter solid was used in the following Examples.
Example 2
A slurry of TTA (50 g, 0.376 mol) in 25 g of water was warmed to
35°C. Sodium hypochlorite (27.9 g, 0.376 mol, added as 226.8 g of a
12.3% sodium hypochlorite solution) was added over a period of 2 hours.
After the addition, the reaction was kept at 45°C for one hour.
During the
addition the pH of the reaction mixture increased to 12 and the solids
dissolved. The final product was analyzed by ~ H and ~3C NMR and LC-
UV and found to be composed of 81.9% CI-TTA, 8.8% residual TTA, and
9.3% di-CI-TTA based on the relative areas in the UV spectra.
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On dilution to 1 to 100 ppm azole, with or without pH adjustment to
about 7.2, there was no odor from the halo-benzotriazole solution of the
present invention.
5 Example 3
In the schemes below, TTA was present at 100 ppm, in contrast to
Example 2 where the initial slurry contained about 200,000 ppm. "x"
denotes a stoichiometric ratio.
Scheme 1
pH=7 slow
TTA+1.1xNaOCI<---------->odorous solution --------->CI-TTA(ppt odorous)
Scheme 2
pH=11.8 pH=8.2
TTA+1.1xNaOCI<------------->solution, no odor----------->CI-TTA(ppt odorous)
Example 4
CI-TTA, prepared as a solid according to Example 1, was dissolved
in methanol and charged to a simulated cooling water solution. The
solution contained 319 ppm Ca (calculated as CaC03), 7 ppm Mg
(calculated as CaC03), 190 ppm NaHC03, 882 ppm Na2S04, 1184 ppm
NaCI, 5 ppm CI-TTA, and 2.4 ppm of hydroxyethylidene diphosphonic
acid (HEDP). Hypochlorite was absent. The solution was maintained at
120°F by an admiralty brass heater tube and at pH=7.2 to 7.5 by a pH
controller equipped to feed sulfuric acid on demand. The solution was
recirculated past the heater and past both admiralty and copper/nickel
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alloy corrosion rate meters (CRM). After 1 hour the solution was drained
and replaced by an identical solution with no CI-TTA. This solution was
fed to overflow which replenished the system with fresh solution at a rate ,
of about 4% by volume per hour. This system was maintained under
these conditions continuously until the bright admiralty tube was
tarnished, at which point the experiment was terminated. Comparisons
were made to identical experiments with TTA and benzotriazole.
TABLEI
Admiralty Tube Appearance
Pretreatment 40 hours 94 hours 336 hours
CI-TTA Bright Bright Tarnished
TTA Bright Tarnished
Benzotriazole Tarnished
TABLE II
Admiralty CuINi
Corrosion Rate (mpy) Corrosion Rate (mpy)
Pretreatment 40 hrs. 94 hrs. 336 hrs. 40 hrs. 94 hrs. 336 hrs.
C I-TTA 0. 2 0. 3 0. 5 0. 7 0.4 0. 8
TTA <0.1 2.2 * N/A 5.2
Benzotriazole 1.3 * * 2.0
*Experiment previously terminated.
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Example 5
Corrosion tests were carried out in the apparatus described in
Example 4 with water containing 500 ppm Ca, 250 ppm Mg, 25 ppm Malk,
15 ppm o-P04, 3 ppm tetrapotassium pyrophosphate, 10 ppm of a 3:1, low
molecular weight, acrylic acid/allyl 2-hydroxypropyl sulfonate ether
copolymer, 2.4 ppm HEDP, a 3ppm of either CI-TTA or TTA. The pH was
maintained at 7.2 with a blended mixture of air and carbon dioxide at
120°
F for 18 hours. Electrochemical corrosion rates were measured using
admiralty brass (ADM) and low carbon steel (LCS) working electrodes. All
tests also had both admiralty and LCS coupons in contact with the
solution. The method differed from Example 4 in that the azole was fed
continuously at 3 ppm during these experiments. The azole was supplied
by dissolving the solid in potassium hydroxide solution and then diluting it
into the feedwater for the system. Each experiment was duplicated: once
with an admiralty brass heated tube, and once with a low carbon steel
heated tube. Corrosion rates were measured as in Example 4 from
admiralty and LCS working electrodes, and by weight changes of admiralty
and LCS coupons. Rates for the coupons were measured for the initial
day of each run, and a "differential" rate was calculated for the remaining
days of the run by offsetting the initial rate from the overall rate.
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TABLE III
CRM Corrosion Rates: Values at end of six days (mpy)
LCS Heated Surface ADM Heated Surface
CI-TTA TTA CI-TTA TTA
LCS 0.2 0.4 0.45 0.75
ADM 0.00 0.00 0.05 0.07
TABLE IV
Gravimetric Coupon Corrosion Rates (mpy)
(First day and differential rates)
LCS Heated Surface ADM Heated
Surface
CI-TTA TTA CI-TTA TTA
Day 1 LCS 4.6 3.0 3.4 2.9
Day 6 LCS (diff.) 0.25 0.33 0.25 0.25
Day 1 ADM 1.9 2.1 1.6 1.8
Day 6 ADM (diff.) 0.00 0.20 0.00 0.10
Example 6
The method of Example 5 was followed, except a solution of sodium
hypochlorite was added after 20 hours and continued for an additional 72
hours. The feed rate of the sodium hypochlorite was controlled to produce
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a "chlorine residual" of about 0.1 to 0.3 ppm as C12 using a standard DPD
spectrophotometric test on the recirculating water. For the experiment with
CI-TTA, the feed rate of the sodium hypochlorite was about 30% of that ,
required for TTA. For TTA, the characteristic odor was detected
immediately after the first hypochlorite was added. With CI-TTA, there
was no odor upon initiating hypochlorite addition, and only a trace was
sensed just prior to concluding the four day run.
TABLE V
CRM Corrosion Rates: Values at 90 hour mark (mpy)
LCS Heated Surface
CI-TTA TTA
LCS 0.5 2.3
ADM 0.06 0.02
TABLE VI
Gravimetric Corrosion Rates (mpy)
LCS Heated Surface
CI-TTA TTA
Day 2 to 4 LCS 1.1 2.6
Day 4 LCS (diff.) 0.4 1.4
Day2to4ADM 1.1 1.2
Day 4 ADM (diff.) 0.15 0.85
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Example 7
Solutions of azole at 6 ppm were made in deionized water, and the
5 pH was adjusted to 7Ø Cu+2 ion was added (0.1 ppm from cupric
sulfate) and the pH was again adjusted to 7Ø A sample was digested
with nitric acid, analyzed for copper, and a second sample was filtered
(0.2 micron pore size), digested, and analyzed for copper. The ratio was
expressed as "% soluble Cu":
TABLE VII
Sample % Soluble Cu
TTA 15
TTA+N a0 C I 90
C I-TTA 13
Example 8
Admiralty brass corrosion coupons and working electrodes were
coated with a sulfide layer by exposing the metal to a sodium sulfide
solution for 18 hours. These samples were rinsed and dried. Corrosion
tests were carried out in aqueous solutions in stirred beakers containing
500 ppm Ca, 250 ppm Mg, 25 ppm Malk, 15 ppm o-P04, 3 ppm tetra-
potassium pyrophosphate, 10 ppm of a 3:1, low molecular weight, acrylic
acid/allyl 2-hydroxypropyl sulfonate ether copolymer, 2.4 ppm HEDP,
and the pH was maintained at 7.2 with a blended mixture of air and carbon
dioxide at 120°F for 18 hours. Electrochemical corrosion rates were
meas-
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16
ured using admiralty brass or low carbon steel working electrodes. All tests
also had both admiralty and LCS coupons in contact with the solution.
Each solution was tested with and without addition of sodium
hypochlorite (added after 1 hour exposure). In a separate, but otherwise
identical experiment, clean low carbon steel working electrodes were
used in place of the sulfide-exposed admiralty brass, but the sulfide-
exposed brass coupons were present as a source of copper. At the
conclusion of the experiment, a sample of the supernatant solution was
taken and analyzed for copper. Analyses were taken with and without
filtration through a 0.2 micron membrane filter.
TABLE VIII
Admiralty Low Carbon
Brass Steel
NaOCI Corrosion Corrosion Copper (ppm)
Azole (ppm) Rate (mpy) Rate (mpy) Unfiltered Filtered
none 0 1.01 5.4 0.354 0.103
3 ppm TTA 0 0.07 1.2 0.014 0.014
3 ppm CI-TTA 0 0.06,0.05 1.0 0.005 0.004
none 2.0 2.09 5.2 0.417 0.059
3 ppm TTA 2.0 0.45 2.6 0.133 0.066
3 ppm CI-TTA 2.0 0.13 1.7 0.086 0.039
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Example 9
A synthetic sea water was formulated from deionized water plus
1010 ppm Ca as CaC03, 5226 ppm Mg (as CaC03), 18971 ppm CI, 2660
ppm S04, 117 ppm M-alkalinity (as CaC03), 5 ppm azole (see below), and
the pH was maintained at 7.8 with a blended mixture of air and carbon
dioxide at 100°F.
Admiralty brass electrodes were exposed to this medium for 1 hour
and then they were transferred to identical water with no azole present.
Electrochemical corrosion rates were measured for 18 hours.
TABLE IX
Mean Electrochemical
Azole Corrosion Rate (mpy)
Benzotriazole 40
5-Butylbenzotriazole 15
Tolyltriazole 6
Chloro-tolyltriazole 3.2
Example 10
Sodium hypochlorite (12.2%, 204.9 g, 0.336 mol) was added over
90 minutes to a stirring slurry of benzotriazole (40 g, 0.336 mol) in 30 g of
water at room temperature. Following the addition, the reaction mixture
was held at 45-50°C for one hour. Upon cooling, a precipitate formed. A
clear yellow solution was obtained after adjusting the pH to 11. The final
product was analyzed by LC/MS and ~3C and ~H NMR and found to be
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composed of 54.6% chloro-benzotriazole (CI-BZT), 23.9% residual
benzotriazole, and 21.5% di-chloro-benzotriazole (di-CI-BZT).
Example 11
Bromine (12.5 g, 0.078 mol) was added to a stirring slurry of TTA
(10 g, 0.075 mol) in 66 g of water in a reactor protected from light, while
maintaining the temperature at <25°C. After the addition, the reaction
mixture was held at 35-40°C for one hour. Upon cooling, adjusting the
pH
to 11-12 did not produce a clear solution. The small amount of precipitate
that formed upon standing was removed by filtration, the pH of the filtrate
was adjusted to neutral, and the resulting precipitate filtered. This solid
was characterized by LC/MS and ~ 3C and ~ H NMR and found to be
composed of 90.5% bromo-tolyltriazole (Br-TTA), 4.9% residual TTA, and
4.2% di-bromo-TTA.
Example 12
The method of Example 8 was followed, using samples from
Examples 2, 11 and 12 at 1 to 4 ppm total actives. The following were the
18 hour averaged electrochemical corrosion rates:
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TABLE X
Average
Conc. Corrosion
Azole (ppm) Source NaOCI Rate (mpy)
CI-BZT 1 Ex.11 none 0.21
2 0.09
4 0.03
CI-BZT 1 Ex. 11 2 ppm 0.55
2 0.25
4 0.09
CI-TTA 1 Ex.2 none 0.14
2 0.09
4 0.08
CI-TTA 1 Ex. 2 2 ppm 0.58
2 0.24
4 0.09
Br-TTA 1 Ex.12 none 0.17
2 0.11
4 0.07
Br-TTA 1 Ex. 12 2 ppm 0.45
2 0.16
4 0.09
TTA 1 none 0.13
2 0.14
4 (n/a)
TTA 1 2 ppm (n/a)
2 0.45
4 0.27
The above t the halo-benzotriazoles
examples of the
show tha
present ion inhibitorseven in the presence
invention
are effective
corros
of chlorine.
CA 02238082 1998-06-23
While the present invention has been described with respect to
particular embodiments thereof, it is apparent that numerous other forms
and modifications of this invention will be obvious to those skilled in the
art. The appended claims and this invention generally should be
5 construed to cover all such obvious forms and modifications which are
within the true spirit and scope of the present invention.
