Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~.o5'~86
This invention relates to a method for control of
corrosion in an aqueous sy~tem, such as boiler water
systems, due to dissolved oxygen by adding to the aqueous
system, i.e. the boiler water, an effective amount of an
oxygen scavenger.
Dissolved oxygen is objectionable in water used for
industrial purposes becuase of the corrosive effect on
iron and steel with which the water comes in contact. In
cold and hot water lines, failure of piping may occur and
the lines may become blocked with the products of corrosion.
"Red water" and iron stains may result from iron brought
into solution by the corrosive attack of dissolved oxygen.
Increased temperatures and low pH values have been known
to accelerate oxygen attack.
In boiler systems, corrosion may result in feed lines,
heaters, economizers, boilers, steam and return lines.
Correspondingly, in cooling water systems corrosion of heat
exchangers, engine jackets, pumps and piping may result.
Dissolved oxygen in water is a principal factor influencing
corrosion.
Elimination of the corrosive effect of dissolved
oxygen can be accomplished by both direct and indirect
means. The direct means involves actual removal of
dissolved oxy~en from the water by mechanical or chemical
deaeration. Such direct action is usually applied to boiler
feedwater systems. Indirect means are employed where the
removal of oxygen is not feasible either from a technical
or economic standpoint, such as in open recirculating
cooling water systems. In such cases, corrosion inhibitors
which exhibit a passivating influence on the metal surface
are used.
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Sodium sulfate has been the usual oxygen scavenger.
At cold water temperatures, catalyzed sodium sulfite is
sometimes required fn order to secure sufficiently rapid
reaction between sulfite and oxygen.
Use of chromate salt is an example of one indirect
means for elimination of the corrosive effects of oxygen.
By formation of a mixed oxide film on the metal surface,
the surface is rendered passive to oxygen attack. A con-
tinuous supply, however, of such a passivating agent onto
the metal surface is required in order to correct defects
in the film. Because of the high concentration of pass-
ivating agent required for the prevention of corrosive
attack, this method of treatment is not commonly used for
high pressure boiler systems.
Another means for preventing attack of dissolved
~n;~ .
oxygen involves addition of alkaline treatment ~
The theory behind this treatment relies on the formation of
a thin film of calcium carbonate for preventing contact
of dissolved oxygen with the surface to be protected. If
not carefully controlled, heavy scale may develop and,
therefore, this method of treatment, similar to the use
of sodium silicate, is limited.
Research toward increasing the speed of the oxygen-
sulfite reaction has found that certain materials act as
catalysts in speeding this reaction. The most suitable
catalysts are heavy metal cations having two or more val-
ences. Iron, copper, cobalt, nickel and manganese are
among the more effective catalytic aids to the oxygen-
sulfite reaction. Combinations of several of these heavy
metal cations have proved effective in providing a
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continuously active influence on the speed of reaction.
As boiler operating pressures have increasea, two
disadvantages in the u~e of ~odium sulfite as a chemical
deaerant have become evident. The product of the sodium
sulfite-oxygen reaction is ~odium sulfate (Na2S04) which
increases the dissolved solids concentration of the boiler
water. While the increase in d~ssolved solids in low or
medium pressure boilers is generally of little con~equence,
it can be significant in high pressure boilers. Also, at
high pressure, the sulfite in the boiler tends to decompose
to form acidic gases, sulfur dioxide (S02) and hydrogen
sulfide (H2S) which can contribute to corrosion in the
return system.
Hydrazine, which does not possess these disadvantages
for high pressure operation, can remove aissolved oxygen.
The advantage of hydrazine is that the decomposition proaucts
are ammonia and nitrogen. The ammonia is alkaline and,
therefore, will not attack steel. However, if present in
sufficient quantity, it can attack copper bearing alloys
when oxygen is present. With proper application, the con-
centration of ammonia in the steam can be controlled so
that the danger of attack of copper bearing alloys will be
minimized. At the same time, the ammonia will neutralize
carbon dioxide so that return line corroqion due to carbon
dioxide will be reduced. Hydrazine, however, is a toxic
liquid and, therefore, must be handled with unusa~lcca~
The speed of the oxygen-hydrazine reaction is exceedingly
slow at ambient temperatures.
Thus, although a number of useful oxygen ~cavenger~
have been known for control of corrosion in boiler water
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sy~tems, their use involves severe difficulties.
It has now been found that by practice of the present
invention, there is provided a new improved method for con-
trol of corrosion in boiler water systems and other aque~us
systems, which overcomes many of the disadvantages of
prior art systems.
Generally stated, the present invention provides a
method for control of corrosion in an aqueous system
by means of oxygen scavengers having the general formula:
Rl
~ - o - R3
R2~
wherein Rl, R2 and R3 are either the same or different and
are selected from the group consisting of hydrogen, lower
alkyl, preferably having between 1 to 8 carbon atoms, and
aryl, preferably phenyl, benzyl and tolyl, and water-
soluble salts thereof. Specific examples of oxygen sca-
vengers usefully employed herein include hydroxylamine,
oxygen-substituted and nitrogen-substituted derivatives,
and the water-soluble salts thereof, such as the chloride,
sulfate, acid sulfa~e, phosphate and sulfite. These
materials are added to boiler water in an effective
amount, such a 0.001 to 500 parts per million part of
water. Preferably, amounts of 0.01 to 50 parts per million
are adequate and are thus preferred.
Practice of the present invention will become more
apparent from the following non-limiting examples.
Example 1
The effectiveness of hydroxylamine and its salts a~
an oxygen scavenger was investigated under experimental
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boiler conditions, i.e. 26 kg/cm2 and 228C.
During the test, the boiler feedwater was saturated
with dissolved oxygen by continuous aeration. The dissolved
oxygen in the feedwater ranged from 9 to 10 mg/l (as 2)
The boiler steam was condensed through a heat exchanger
producing a condensate temperature of 130C. ~he conden-
sate was then passed through a chamber in which an oxygen
probe was inserted to monitor the dissolved oxygen. A
blank run without an oxygen scavenger was first conducted
until a constant level of dissolved oxygen was attained.
~n~e the initial dissolved oxy~en reading was established,
the oxygen scavenger being tested was fed into the boiler.
The reduction of the dissolved oxygen in the condensate
was then recorded.
Other boiler water treatment chemicals, such as
sodium hydroxide and disodium phosphate, for alkalinity
and calcium hardness controls were also added during
experimental runs. The feedwater contained 10 ppm
(as CaCO3) total hardness.
At a dosage of 60 ppm active in the feedwater of
oxygen scavengers, the following results were obtained.
TABLE l
Dissolved Oxygen % Reduction
(mg/~) of Dissolved
Oxy~en Scavenqer in the Condensate OxYqen
Initial Final
Sodium sulfite (Na2SO3)3.00 0.095 96.8
Hydrazine (N2H4) 3-75 0.10 97.3
Hydroxylamine hydrochloride -
(NH2OH HCl) 3.40 0.06 98.2
N, N-Diethylhydroxylamine
(C2H~)2NOH 3.70 0.06 98.4
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Exam~le 2
Using the method of Example 1, hydrazine and hydroxyl-
amine were compared in the following tables at various
dosages in the feedwater.
TABLE 2 - HYDRAZINE
Initial Condensate Dissolved Oxygen: 3.30 mg/l (as 2)
DosageFinal Dissolved aæygen% Reduction
In the Feed,in the Condensate,of Dissolved
mg/l, as N2H4 mg/1 as 2 Oxygen
2.1 2.60 21.2
4.2 2.00 39.4
6.3 1.70 48.5
8.4 1.50 54.4
10.5 1.15 65.2
12.6 0.88 73.3
16.8 0.24 92.7
18.9 0.08 97.8
Exam~le 3
The procedure of Example 1 was repeated, except using
hydroxylamine. The following results were recorded:
TABLE 3 - HYDROXYLAMI~E
Initial Condensate Dissolved Oxygen: 3.75 mg/l (as 2)
DoaageFinal Dissolved Oxygen% Reduction
in the Feed,in the Condensate,of Dissolved
mg/1 as NH2OH mg/1 as 2 Oxygen
:
2.85 3.05 18.7 `'
5.7 1.90 49.3
8.57 1.10 70.7
11.4 0.49 86.9
14.2 0.21 94.4
17.1 0.09 97.6
Exam~le 4
The procedure of Examp ~ was repeated, except using
N,N-diethylhydroxylamine. The following results were
recorded:
TABLE 4 - N,N-DIETHYLHYDROXYLAMINE
Initial Condensate Dissolved Oxygen: 3.70 mg/l (as 2)
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DosageFinal Dissolved Oxygen % Reduction
in the Feed,in the Condensate, of Di~solved
mg/l ag (C2H5)2 mg/l as 2 Oxygen
12 1.20 67.6
18 0.41 88.9
24 0.27 92.7
0.06 98 .4
ExamPle 5
The oxygen scavenging activity of hydroxyl~mmonium
acid sulfate (HAS) was measured at 20C. A do~age of lO~o ..
excess HAS (as NH2OH) was uqed, based on 1 ppm ~H2OH
required per 1 ppm dissolved oxygen. The pH of the testing
solution was adjusted by using dilute sodium hydroxide
solution. The following results were recorded:
TABLE 5 - HYDROXYLAMMONIUM ACID SULFATE AND ~ -
OXYGEN REACTION RATE AT 20C
Time in Minutes PH = 9 to 11 Dissolved Oxygen, mg/liter
pH above 11
0 8.50 8.50
2 7.30 3.25
4 6.90 0.005 ~ ,
6.30 0.005
5.30 0.001
4.50 .
2. 80 o . ooo
0.90 . `
Example 6
The oxygen scavenging activity of N,N-diethylhydroxyl-
amine (DEHA) was determined at 20C without a catalyst and
with a catalyst copper carbonate in a ratio of DEHA to
catalyst of 100:1 parts by weight. A dosage of lOYo excess
DEHA was used, based on 1 ppm of DEHA required per 1 ppm
of di~solved oxygen. The pH of the testing solution was
adjuqted by uqing dilute ~odium hydroxide solution. The
following results were recorded:
TABLE 6 - N,N-DIETHYLHYDROXY1AMINE AND OXYGEN
REACTION RATE AT 20C AND PH 11
, .. . . . .
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Time in DISSOLVED OXYGEN, mg/liter
Minutes No Catalvst coPPer Carbonate as CatalYst
0 8.60 8.70
7.30 3.70
6.65 2~50
5.90 l.g5
5.75 1.40
5.10 0.40
4.60 0.10
Example 7
The oxygen scavenging activity of hydrazine was
determined at 20C. A dosage of 10% excess hydrazine
was used, based on 1 ppm of hydrazine required per 1
ppm of dissolved oxygen. The pH of the testing solution
was adjusted by using dilute sodium hydroxide solution.
The following results were recorded:
TABLE 7 - HYDRAZINE AND OXYGEN REACTION RATE AT
20C and pH 11
Time in Minutes Dissolved OxYqen, mq/liter
0 8.40 s
8.30
8.30
8.20
8.10
8.00
7.60
As shown in Examples 1, 3, 4, 5 and 6, excellent
oxygen scavenging activity was demonstrated by hydroxyl-
amine, hydroxylamine hydrochloride, hydroxylammonium
acid sulfate, and N, N-diethylhydroxylamine. By comparing
Examples 5 and 6 with Example 7, the advantage of
hydroxylammonium acid sulfate and N,N-diethylhydroxyl-
amine over hydrazine is evident. The following compounds
according to this invention show similar unexpected
oxygen scavenging activities when tested by the procedure
described in Example 1.
ExamPle No.
8 Hydroxylamine phosphate
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9 N-Ethylhydroxylamine
10 N,N-Dimethylhydroxylamine
11 O-Methylhydroxylamine
12 O-Hexylhydroxylamine
13 N-Heptylhydroxylamine
14 N,N-Dipropylhydroxylamine
15 O-Methyl N,N-diethylhydroxylamine
16 N-Octylhydroxylamine
17 O-Ethyl N,N-dimethylhydroxylamine
18 N,N-Diethylhydroxylamine hydrochloride
19 N-Methyl N-ethylhydroxylamine
20 O-Methylhydroxyl~sine phosphate
21 N-Butylhydroxylamine
22 N-Benzylhydroxylamine ~ -Benzylhydroxylamine)
23 O-Benzylhydroxylamine (~ -Benzylhydroxylamine)
24 N,N-Diethylhydroxylamine acetate
The presently used hydroxylamines may be catalyzed
using any of a number of well known catalyst used in :.
sodium sulfite or hydrazine boiler water treatment
methods. Alkali metal hydroxides, water soluble metal
salts, hydroquinone, and benzoquinone are also found to
be useful catalysts.
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