Note: Descriptions are shown in the official language in which they were submitted.
CA 02252060 1998-11-23
H-1135
METHOD AND COMPOSITION FOR CORROSION AND
DEPOSITION INHIBITION IN AQUEOUS SYSTEMS
FIELD OF THE INVENTION
The present invention is directed to a composition and method for
inhibiting corrosion and deposition in aqueous systems. More
particularly, the present invention relates to a stable, neutral pH, divalent
transition metal containing corrosion and deposition inhibiting treatment
for aqueous systems, such as industrial water systems.
BACKGROUND OF THE INVENTION
The ability of zinc to inhibit corrosion of ferrous metals is well
known. Accordingly, soluble zinc salts are vital ingredients of many
corrosion treatment programs. An art recognized problem associated
with zinc-containing treatments, particularly in cooling water, is the
uncontrolled precipitation of zinc salts. For example, the use of
orthophosphate in combination with zinc as a cooling water treatment is
known. When orthophosphate and zinc are both present in an aqueous
system, zinc phosphate precipitation becomes a concern. Precipitation of
zinc in other forms, for example, zinc hydroxide or zinc silicate can also
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occur. The solubility of the various salts, that is, the retention of the
respective salt constituents in ionic form depends upon such factors as
water temperature, pH and ion concentration. In alkaline waters,
particularly above about pH 7.5 dissolved zinc tends to deposit out or
drop out. Zinc salts are also known to be unstable in neutral or alkaline
water and will precipitate with phosphates. Thus, if any of these
conditions are present, the aqueous medium becomes prone to zinc
precipitation. With the formation of zinc scale, many of the surfaces in
contact with the aqueous medium will foul and the amount of effective
(soluble) corrosion inhibitor present in the aqueous medium can be
significantly reduced.
Attempts to control the stability problems commonly encountered
in zinc-based water treatments have typically involved pH adjustment or
stabilizers which tend to reduce corrosion inhibition efficacy. Most often
alkalinity or acidity is employed to achieve even minimal stability for zinc-
based treatments. In most cases of blends of zinc and polymers,
precipitates (accelerated at low temperature, i.e., less than about 45°
F)
composed primarily of zinc and calcium complexes form. Many additives
have been investigated such as chelants and threshold inhibitors to
alleviate precipitation problems in such products. Attempts to stabilize a
neutral pH formulation with these types of additives have resulted in
failure or stabilized formulations which have significantly reduced
corrosion inhibition efficacy. The loss of corrosion inhibition efficacy is
believed to be primarily due to insoluble zinc complex formation prior to
product use.
Zincates are generally observed to be soluble and stable at low pH
values (less than about pH 5) and often under highly alkaline (pH greater
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than 12) conditions. When the zincates are used in combination with
polymeric materials which are stable at higher pH's such as poly (ether
carboxylates) which are stable at pH's greater than 5, the expected pH
range for stability of the mixture is pH 4 to 5. A neutral aqueous mixture
would not be expected to show stability. The stability problems related to
treatment component matrix and final product pH have made field
application of certain zincate corrosion/deposition inhibitors difficult.
Scaling of feed lines and equipment is not uncommon. The use of
alkalinity or acidity to attempt to enhance product stability has been of
limited success.
SUMMARY OF THE INVENTION
The present invention is directed to a divalent transition metal
based corrosion/deposition inhibition mixture which is substantially
phosphate free and stable at neutral pH. The present invention provides
a transition salt and polymer containing corrosionldeposition inhibitor
combination which incorporates an aliphatic hydroxy carboxylic acid as a
stabilizer. The inclusion of the stabilizer provides for neutral pH stability
of the combination as well as enhancing the corrosion inhibition efficacy
of the combination. The preferred transition metal saltlpolymer
combination includes a water soluble divalent transition metal salt, a low
molecular weight water soluble acrylate copolymer and a poly (ether
carboxylate). This combination is stabilized at neutral pH by the addition
of an aliphatic hydroxycarboxylic acid. The resulting corrosion/deposition
inhibition combination is stable at neutral pH and exhibits enhanced
corrosion inhibition efficacy.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a substantially phosphate-free
neutral pH , stable transition metal salt/polymer corrosion/deposition
inhibition combination for treatment of aqueous systems. The
combination of the present invention avoids the use of acidity or alkalinity
to provide product stability. The stability of the combination of the
present invention avoids the field application problems of prior transition
metal salt-based treatment combinations.
According to the present invention, a corrosion/deposition
inhibiting combination for metal surfaces exposed to an aqueous system
comprises (i) a water soluble divalent treatment metal salt, and a
polymer component preferably (ii) a low molecular weight acrylate
copolymer, and (iii) a poly (ether carboxylate) along with (iv) an aliphatic
hydroxycarboxylic acid stabilizer. It was discovered that the inclusion of
an aliphatic hydroxycarboxylic acid in the divalent transition metal
saltlpolymer mixture provided for stability at neutral pH while also
enhancing corrosion inhibition.
The water soluble divalent transition metal salt of the present
invention can include any metal salt which provides a divalent ion upon
disassociation in water. A preferred example are water soluble zinc salts
which provide Zn+2 ions in aqueous solution. Convenient sources of such
salts includes zinc oxide, zinc chloride, and zinc sulfate. The preferred
source of the Zn+z ion is zinc sulfate.
The polymer component of the present invention includes any
conventional corrosion/deposition inhibiting polymer or copolymer.
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Preferably, the polymer component is a combination of a poly(ether
carboxylate) and a low molecular weight acrylate copolymer. The
preferred poly (ether carboxylate) according to the present invention is
preferably a polyepoxysuccinic acid of the general formula:
5
R R Formula I
HO ~ C - CO ~ H
~ n
0=C C= O
I
O O
I I
M M
wherein n ranges from about 2 - 11, M is hydrogen or a water soluble
cation such as Na+, NH4+, or K+ and R is hydrogen, C 1-4 alkyl or C1-4
substituted alkyl. Preferably R is hydrogen.
The preferred low molecular weight acrylate copolymer according
to the present invention is preferably a polymer characterized by the
structural formula:
R~ R, Formula II
I
CH2 - C CH2 - C
C=O x CH2 y
3o I I
Rz O
I
R3
I
(XZ)a
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wherein R, is H or lower alkyl (C, - Cs); R2 is OH, OM' or NHz; M' is a
water soluble cation; Rs is a hydroxy substituted alkyl or alkalene radical
having from 1 to 6 carbon atoms or a non-substituted alkyl or alkalene
radical having from 1 to about 6 carbon atoms; X, when present is an
anionic radical selected from the group consisting of SOs, POs, POa and
COO; Z, when present is hydrogen or any water soluble cation or cations
which together counterbalance the valence of the anionic radical; a is 0
or 1.
The number average molecular weight of the water soluble
copolymer of Formula II may fall within the range of about 1,000 to about
500,000 with the range of about 1,500 to about 10,000 being preferred.
The molar ratio X:Y of the monomer of Formula II may fall within the
range of about 30:1 to about 1:20 with the X:Y molar ratio range from
about 10:1 to about 1:5 being preferred. A preferred polymer is:
H H Formula III
CH2 - i CHz - i
C - p x i Hi Y
OH O
CH2
CHOH
SOsM'
wherein M' is the same as given in Formula II above. This polymer
(Formula III) is referred to as acrylic acidlallyl hydroxy propyl sulfonate
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ether (AAIAHPSE). The IUPAC nomenclature for AHPSE is 1-propane
sulfonic acid, 2-hydroxy-3-(2-propenyl oxy)-mono sodium salt.
The alphatic hydroxy carboxylic acid according to the present
invention preferably has a carbon number of 7 or less. The material may
be selected from the group gluconic acid, succinic acid, citric acid, malefic
acid, glutaric acid, adipic acid, and mixtures thereof. Preferably the
alphatic hydroxy carboxylic acid is citric acid.
The combination of a water soluble divalent transition metal salt, a
low molecular weight acrylic copolymer, a poly (ether carboxylate), and
an aliphatic hydroxy carboxylic acid of the present invention is adjusted to
a neutral pH by the addition of an alkalyzing agent such as KOH.
A neutral pH, stable combination of the present invention may be
used in combination with other water treatment agents including
corrosion inhibitors, conventional scale and contamination inhibitors,
metal ion sequestering agents, biocides and other conventional water
treatment agents.
Generally, the corrosionldeposition inhibiting combination of the
present invention is added to an aqueous system to be treated at
concentrations ranging from about 5 ppm to about 500 ppm as product
Preferably the treatment concentration of the combination of the present
invention ranges from 5 ppm to about 200 ppm. Typical treatment
concentrations for the individual preferred components of the
combination of the present invention are as follows: from about 0.2 to 10
parts per million water soluble divalent transition metal salt; from about 2
to about 20 parts per million low molecular weight acrylic copolymers;
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from about 2 to about 20 parts per million poly (ether carboxylate); and
from about 0.5 to about 20 parts per million aliphatic hydroxy carboxylic
acid. The ratio of the aliphatic hydroxy carboxylic acid to the divalent
metal salt is preferably about 2:1. The ratio of the water soluble acrylate
copolymer to the divalent transition metal salt (on an active basis)
preferably ranges from about 3:1 to about 6:1. The poly (ether
carboxylate) is used in a polymer to active divalent metal in a ratio in a
range from about 3:1 to about 8:1.
Example
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.
Testing was undertaken in a laboratory scale recirculating heat
transfer apparatus. The corrosion inhibiting activity of a treatment
comprising 6 parts per million polyexpoxysuccinic acid, 8 parts per million
acrylic acidlallyl hydroxy propyl sulfonate ether; 1.5 parts per million as
zinc sulfate monohydrate when applied at a 100 parts per million dosage
both with and without about 5% citric acid. These treatment combinations
were added to a recirculating cooling water containing 400 parts per
million Ca, 150 parts per million Mg, 210 parts per million M-alkalinity (as
CaC03), 2 parts per million orthophosphate and 51 parts per million Si02
at pH 8.6 in a specific conductance of about 2,200 microohms. The
metallurgy tested included low carbon steel (LCS) coupons and an LCS
heat transfer probe which generated a heat flux of 8,000 BTU/square feet
per hour. The bulk water temperature was 120°F, and water velocity was
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at 2.8 feet per second. Retention time was 1.4 days. Table 1
summarizes the test results.
TABLE 1
__________________LCS Appearance-_________________
LCS Coupon Pitting Heat Transfer Pitting
Product (mpy) De- nsity D_ epth Density Depth
Without Citric Acid 0.5 1 x 104'm2 NIA 1 x 104'm2 26 micron
With Citric Acid 0.2 clean NIA clean NIA
The data in Table 1 shows that the addition of an aliphatic
hydroxycarboxylic acid to the treatment combination enhanced the overall
corrosion inhibition as well as decreasing the pitting corrosion. The
aliphatic hydroxycarboxylic acid also allows adjustment to neutral pH with
no adverse impact on product stability.
While this 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 construed to
cover all such obvious forms and modifications which are within the true
spirit and scope of the present invention.
