Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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12~162~
THE TREATMENT OF AQUEOUS SYSTEMS
The present invention relates to the
tre3tment of aqueous systems and, more particularly,
to reducing or eliminating corrosion in aqueous
systems.
Many different types of material have been
employed to prevent corrosion in aqueous systems. ~hese
include inorganic salts such as nitrites and chromates,
inorganic mono- and polyphosphates and
certain water-soluble polymers including naturally
occurring materials such as lignins and starches
as well as synthetic materials such as polyacrylates.
Particular problems arise in cooling systems
which are subject to intermittent operation or periodic
shut-down. This is because the majority of corrosion
inhibitors and the like only function effectively when
the cooling system is in motion. Indeed, the only
materials which have so far proved to be at all effective
for systems involviny periodic shut-down are the nitrites
and, to a less extent, the chromates. Unfortunately,
however, while nitrites are effective they have to be used
in quite hi~h concentrations; amounts as much às
1000 ppm are not uncommon. Such amounts present disposal
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12116~
problems because these inorganic nitrites are quite
toxic. Thus the maximum nitrogen content permitted
by the World Health Organisation in drinking water is
equivalent to only 45 mg/l of sodium nitrite.
However, such qua:~tities of nitrite are ineffective for
use as a corrosion inhibita,r in cooling systems subject
to intermitt~nt operation.
It has now been found, according to the present
invention, that it is possible to obtain effective
corrosion inhibition if an inorganic nitrite, even in
"non-toxic" amounts, that is to say less than 45 ppm, is
used in com~ination in speci~ic proportions and concentrations
with a particular class of phosphonate. In fact, a
synergistic effect has surprisingly been found.
According to the present invention there is
provided a method of controlling inhibition in aqueous
systems which comprises adding to the aqueous system at
least one water soluble inorganic nitrite and at least one
phosphonate as defined below, the concentration of phosphonate
exceeding 5 ppm (wt/volume e.g. S mg/litre) and the weight ratio oi
nitrite to phosphonate being from 90:10 to 5:95, in general
from 80:20 to 10:90.
Preferred ratios are between 5:1 to 2:1, especially
4.5:1 to 3:1, and most especially about 4:1.
This combination is particularly useful where
very corrosive conditions exist in the cooling system, for
~21 ~ 6Z4
example in base exchanged water and very low water
hardness systems.
The phosphonates for use in the present invention
are those which contain at least 2 acid groups which are
carboxylic and phosphonic alcid groups, such that at
least one acid group is a carboxylic acid group and at
least one acid group is a phosph~nic acid group, at least
the said 2 acid groups being attached to carbon atoms.
The preferred pha,sphonates possess the general
formula:
0 R
Il I
(~O)2P - C - COOH
CH2 ~ COOH
wherein R is hydrogen, alkyl, alkenyl or alkynyl having
up to 4 carbon atoms; phenyl, cycloalkyl having 3 to 6
carbon atoms, benzyl; phenethyl or
Rl R"
CH CH R " '
wherein R' is hydrogen, alkyl having 1 to 4 carbon atoms
or carboxyl, R" is hydrogen or methyl and R'l' is carboxyl
or phospllonate. 2-Phosphonobutane-1,2,4-tricarboxylic acid,
a commercially available material, is particularly
preferred. Another preferred material is 2,4-diphosphono-
butane-1,2-dicarboxylic acid. These phosphonates can be
obtained by processes well known in the art, for example
lZ~16~4
-- 4 --
as des~ribed in British Specification No 1 282 078.
While it is possible to add the materials
separately it will generally be more convenient to
incorporate them together :in the form of a composition.
Accordingly, the present invention also provides a
composition suitable for addition to water to reduce
or prevent corrosion which comprises at least one water
soluble inorganic nitrite and at lea~t one phosphonate
as defined above, the weight ratio of nitrite to
phosphonate being from 90::L0 to 5:95, with the proviso
that if the said ratio is from 90:10 to 5:1 the
compo~ition also contains at least one other water
treatment additive.
Typically, the water-soluble nitrite is
sodium nitrite but other alkali metal nitrites and also
calcium nitrite are also suitable~
As indicated above, by incorporating the
specified phosphonate with the inorganic nitrite in the
specified amounts it is possible to obtain effective
corrosion inhibition even though the concentration of
nitrite is less than 45 ppm. Indeed, amounts as little
as 10 ppm have been found to be effective. Preferably,
the nitrite is present in the system in an amount from 10
to 45 ppm especially 30 to 45 ppm, and most pr~ferably
about 45 ppm. It will be appreciated, though, that in
situations where health is not a factor concentrations
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-- 5 --
of up to 50 ppm or even, say, 100 ppm can be useful.
The amount of phosphonate used will generally be less
than that of the nitrite in order to keep costs down and,
in general, amoun*s above 5 up to 50 ppm, particularly
up to 30 ppm are suitable, amounts from 10 to 15 ppm,
especially about :L2 ppm, being preferred, thereby keeping
down the phosphorus content in the water so as to reduce
disposal problems. Amounts~ in excess of about 30 ppm
are generally commercially unviable.
Phosphonates other than those of formula (I),
in general, do not provide advantageous results and
should, therefore, generally not be used in the system.
It has further been found that the presence of
a water-soluble organic polymer in the system can further
inhibit corrosion and, indeed, in certain cases an
additional synergistic effect is found.
In general, the polymers suitable for use in the
present invention are vinyl addition products possessing
recurring units of the general formula:
Rl
H
~ C - C
Z X
wherein Rl represents hydrogen or alkyl of 1 to 4
carbon atoms, X represents COOH and Z represents hydrogen
or COOH or X and Z together represent -CO-0-Co-,
1211624
The preferred polymers are those of methacrylic acid
i.e. where Rl is methyl and z is hydrogen and acrylic acid
i.e. where Rl and Z are both hydrogen. In addition phosphinopoly-
-acrylates and methacrylates can be used, these materials can
be obtained by polymerisati.on of (meth)acrylic acid with
hypo-phosphorous ,acid. In general, the molecular weight of
the polymers is from 500 tc, 100 000 and the preferred
polymethacrylic acid has a molecular weight of about
5 000 and the preferred pol.yacrylic acid a molecular weight
of about 1 000. It will, c,f course, be appreciated that
thP polymers used may be copolymers containing recurring
units derived from other vinyl monomers.
Not only does the presence of polymer further
reduce corrosion but since the polymers are, in general,
less expensive than the phosphonates used, by incorporating
polymer and, in particular, by replacing some of the
phosphonate by polymer it is possible further to reduce
the cost of the additives. Of course, the polymer can
be added to the system separately but it will, in general,
be incorporated in a composition with the nitrite and
phosphonate.
Although the formulae of the phosphonate and
polymer have been given in terms of the free acid it is
to be understood that these materials can be used in the
form of an inorganic or or~anic salt, in partio~ar an
alkali metal salt such as sodium or potassium, ammonium or
12~:~L624
a lower amine salt as well as zinc or other salts. In
general, however, the use of alkali metal salts is
preferred.
Typically, the polymer is used in an amount
from 0.5 to 50 pp!m, the preferred amount being from
2 to 10 ppm.
It will be appreciated that other 1~ toxic
materials conventionally us,ed in water treatment can be
added to the system and/or the composition including
silicates, inorganic phosphates and polyphosphates and
lignin derivatives as well as heterocyclic compounds for
corrosion inhibition of yellow metals, such as benzo-
triazole, tolyltriazole and mercaptobenzothiazole.
The compositions of the present invention will
normally be in the form of an aqueous solution but other
possible forms include powders and briquettes.
Typically the solutions will contain 20 to 50
weight percent, es,pecially 25 to 35 weight percent, nitrite,
2.5 to 20 weight percent, especially 5 to 10 weight percent,
2~ phosphonate and, optionally, 1 to 10 weight percent,
especially 1.5 to 3 weight percent, polymer. Concentrations
- of other additives, such as benzotriazole, which can be
present, are suitably from 0.1 to 5 weight percent,
especially 0.5 to l.S weight percent; in order-to bring
the benzotriazole into solution the pH sho~lld be increased
to, say, 12 to 12.5 with caustic soda. A particularly
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-- 8 --
preferred fonmulation contains: 30 weight % sodium
nitrite, 16 weight % 2-phosphonobutane-1,2,4-
tricarboxylic acid (50% active), 5 weight % polyacrylic
acid (33% active) and 1 weight % benzotriazole with
sufficient caustic! soda to ]bring the pH of the aqueous
solution to 12 to 12.5. Naturally the amount of the
formulation used will depend upon the nature of the system
and o~ the water but with t]nis specific formulation
amounts from 100 to 200, especially 150 to 200,ppm are
generally suitable.
The following examples further illustrate the
present invention. In these examples two different types of
tests were employed, namely a circulatory test and a
test to simulate intermittent flow operations.
In the circulatory test a laboratory test
apparatus was used in which water is circulated etc by
means of a pump from a reservoir maintained at a
temperature of 54C with a heater and thermostat. The
water passes through a glass tube assembly holding the
metal test specimens (flow rate 2 ft/sec) and then is
returned to the reservoir entraining air as it does so in order
- to keep the water saturated with oxygen as it would be in
a typical open recirculating cooling system.
Water lost by evaporation tconcentrat-ion factor
1.7) is replaced from an elevated tank through a float
control to maintain a constant volume in the system.
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In each test treatment is applied at three
times normal dose for 24 hours in order to passivate the
metals, then the water is diluted to the normal dose for
the remainder of the test. Each test is for a minimum of
3 days, the test specimens being cleaned before and after
each run to find the weight: loss which is then calculated
to show the average corrosiLon rate in mils (0.025 mm) per
year.
The water used in the tests was Widnes mains
water, 75 ppm calcium har~less, 90 ppm. M. alkalinity.
In the following tests the combined dosage of
additives in the system was 30 ppm. "In line" or
"High flow" represents the corrosion rate in the tube
while "low flow/static areas" represents the corrosion
rate in the body of the reservoir.
The results clearly show that the chosen
phosphonates in combination with the nitrite do exhibit
synergism.
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