Note: Descriptions are shown in the official language in which they were submitted.
1204372
This invention relates to methods and compositions
for simultaneously removing iron and copper scales from ferrous
metal surfaces.
In the operation of high pressure steam generating
equipment utilized in electric power generation and other
applications, the interiors of boiler tubes generally always
gradually become encrusted with scale deposits consisting
primarily of ferric oxide, e.g., magnetite (Fe304) and
hematite (Fe203). Copper oxide scale is also usually present
and copper metal is often plated directly onto the boiler tube
walls.
Removal of iron and copper scales from boiler tubes
and other scale-containing ferrous metal surfaces has been
accomplished heretofore by contacting the scale-containing
surfaces with acidic formulations to dissolve the scales.
One such acidic composition which has found wide usage in
removing iron scales from industrial boiler and other heating
surfaces is an aqueous mixture of hydroxyacetic acid and formic
acid. However, the hydroxyacetic-formic acid mixture has here-
tofore had to be used at high temperatures, i.e., about 200Fwith constant agitation in order to efficiently remove the
scales. Because of the high temperatures involved, copper
complexing chemicals have not been included in the composition,
and consequently, a separate step has been required for removing
copper scales. That is, the aqueous mixture of hydroxyacetic
and formic acids has been removed from contact with a scale-
containing surface after iron scales thereon have been dissolved
and a second composition containing a copper complexor has then
been brought into contact with the scale-containing surface at
a lower temperature to remove copper scales. This two-step
procedure has been necessitated by the fact that the copper
~J~
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complexors degrade and are ineffective at temperatures above
about 160F.
By the present invention methods and compositions
for simultaneously removing iron and copper scales from
ferrous metal surfaces are provided. In accordance with the
invention, scale-containing surfaces are contacted with the
compositions at temperatures in the range of from about 75F
to about 150~F and maintained in such contact for a time
period sufficient to dissolve the scales.
The scale-removing compositions of the present
invention are comprised of water, an organic chelating acid
or mixture of organic chelating acids which dissolve iron, a
reducing agent selected from the group consisting of erythorbic
acid, alkali metal salts of erythorbic acid, ammonium salts of
erythorbic acid and mixtures thereof and a copper complexing
compound selected from the group consisting of thiourea,
hexahydropyrimidine-2-thione and mixtures thereof.
Any organic acid or mixture of organic acids having
low pH (a pH of less than 7 at room temperature) which chelate
iron can be used in the compositions of this invention.
Examples of suitable such acids are hydroxyacetic acid, formic
aeid, malic acid, eitric acid, ethylenediaminetetraacetic acid
(EDTA), nitrilotriacetic acid, and mixtures of such acids. Of
the various organic iron chelating acids which can be used, a
mixture of hydroxyacetic acid and formic acid is preferred.
The organic iron chelating acid or acids utilized
in the aqueous scale-removing compositions of this invention
are preferably present in an amount in the range of from about
1% to about 10% by weight of the compositions. The preferred
acids, i.e., hydroxyacetic acid and formic acid are preferably
present in the aqueous compositions in a weight ratio of
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hydroxyacetic to formic acid of about 2:1.
The erythorbic acid and/or salt reducing agent
functions in the compositions of this invention to increase
the rate of dissolution of iron by the organic chelating
acid or acids utilized whereby iron scales can be effectively
removed from ferrous surfaces by the compositions at low
temperatures, i.e., temperatures in the range of from about
75F to about 150F. This in turn allows copper complexing
compounds to be included in the compositions whereby both
iron and copper scales are simultaneously removed by the
compositions. As indicated above, the reducing agents
utilized in the compositions are selected from the group
consisting of erythorbic acid, alkali metal salts of erythorbic
acid, ammonlum salts of erythorbic acid and mixtures thereof
and are included in the compositions in an amount in the
range of from about 0.25% to about 5% by weight of the com-
positions. Most preferably, the reducing agent is sodium
erythorbate and is present in the aqueous compositions in an
amount of about 1% by weight of the compositions.
The copper complexing compounds utilized in the
compositions of this invention are selected from the group
consisting of thiourea, hexahydropyrimidine-2-thione and
mixtures of such compounds. The copper complexor or mixture
is included in the aqueous composition in an amount in the
range of from about 0.25% to about 3% by weight of the com-
positions. Preferably, the copper complexor is a mixture of
hexahydropyrimidine-2-thione and thiourea consisting of 60
parts by weight hexahydropyrimidine-2-thione and 40 parts by
weight thiourea present in the aqueous composition in an amount
of about 1% by weight.
A particularly preferred composition of this invention
is comprised of water, hydroxyacetic acid present in the com-
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position in an amount of about 2% by wei~ht of the composition,formic acid present in the composition in an amount of about
1% by weight of the composition, sodium erythorbate present
in the composition in an amount of about 1% by weight, and
a mixture of 60 parts by weight hexahydropyrimidine-2-thione
and 40 parts by weight thiourea present in the composition
in an amount of about 1% by weight~
Various ferrous metal corrosion inhibitors can be
included in the compositions of this invention, as for example,
dibutyl thiourea, quaternary alkyl pyridinium salts, alkyl-
benzene sulfonate and heavy aromatic naphtha. The most pre-
ferred ferrous metal corrosion inhibitor for use in accordance
with this invention is a low chloride inhibitor mixture com-
prised of 15% by weight heavy aromatic naphtha-, 40% by weight
ethylene glycol, 8% by weight dibutyl thiourea, 12% by weight
acetic acid, 10% by weight alkyl pyridine, 10% by weight non-
ionic ethoxylated alcohol and 5% by weight ethoxylated amine.
The corrosion inhibitor is preferably included in the aqueous
compositi~n in an amou~t in the range of from about .05% to
about 6% by volume of the composition.
In carrying out the methods of the present invention
for simultaneously removing iron and copper scales from ferrous
surfaces, a composition of the present invention is brought
into contact with an iron and copper scale-containing ferrous
metal surface at a temperature and for a time sufficient for
the scales to be dissolved by the composltion and thereby
removed from the surface. The composition containing the
dissolved scales is removed from contact with the surface and
disposed of in the usual manner whereby pollution of the en-
vironment does not result.
As mentioned above, the temperature of the aqueouscomposition during the contact of the scale-containing sur-
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faces can be as low as 75F while still efficiently removingscale from the surfaces up to as high as about 150F. At
temperatures above about 150F, degradation of the copper
complexors begins to take place. The most preferred contact
temperature is about 140F.
- As is well understood by those skilled in the art,
the cleaning compositions can be brought into contact with
the scale-containing surfaces in a static condition, or as
is preferred, the compositions can be circulated over the
surfaces. The compositions effectively dissolve deposits
containing iron and copper at temperatures in the range of
from about 75F to about 150F in a single stage treatment.
In order to facilitate a clear understanding of the
methods and compositions of the present invention, the follow-
ing examples are given.
Example 1
Aqueous solutions containing 2% by weight hydroxy-
acetic acid, 1% by weight formic acid and 0.1% by volume of a
corrosion inhibitor are prepared. One hundred milliliter
portions of the solutions are placed in glass beakers, 2 grams
of powdered iron oxide (technical grade magnetite) are added
thereto and dry pre-weighed 1020 mild steel corrosion coupons
are placed in the solutions. Various quantities of sodium
erythorbate are added to some of the test solutions, the
solutions are heated to the temperatures given in Table I
below and the solutions are maintained at such temperatures
for time periods of six hours. During the six-hour periods
the test solutions are stirred for one minute each hour and
at the termination of the six hour periods, the solutions
are analyzed for dissolved iron (by atomic absorption analysis)
and the weight losses of the corrosion coupons are determined.
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The results of these tes-ts are given in Table I below.
The corrosion inhibitor is a commercially available
mixture comprised cf 15% by weight heavy aromatic naphtha,
40% by weight ethylene glycol, 8% by weight dibutyl thiourea,
12% by weight acetic acid, 10% by weight alkyl pyridine, 10%
by weight nonionic ethoxylated alcohol and 5% by weight
ethoxylated amine.
Table I
Magnetite Dissolution Tests in Aqueous Hydroxyacetic-
Formic Acid Solutions with and without Sodium Erythorbate
Sodiuml Mild Steel - Magnetite2
Test Erythorbate Temperature Corrosio2n Rate Dissolved
No. % F lbs/ft /day Grams Percent
1 0 120 0.002 0.015 0.75
2 0 140 0.003 0.030 1.5
3 0 160 0.003 0.023 1.15
4 0 180 0.003 0.037 1.85
1 120 0.001 0.214 10.7
6 1 140 0.001 0.265 13.25
7 1 160 0.002 0.344 17.2
8 1 180 0.003 0.400 20.0
9 2 120 0.001 0.237 11.85
2 140 0.001 0.355 17.75
11 2 160 0.002 0.419 20.95
12 2 180 0.003 0.451 22.55
1 The percent number indicated is the number of grams of sodium
erythorbate per 100 milliliters of solution.
Amount of dissolved magnetite is determined by atomic
absorption analysis of spent solvent. This value is
corrected for coupon weight loss.
From Table I it is readily apparent that an aqueous
solution of hydroxyacetic and formic acids dissolves a greater
quantity of magnetite at higher temperatures. However, it is
also readily apparent that the inclusion of sodiu~ erythorbate
in an aqueous solution of hydroxyacetic and formic acids brings
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about an increase in the dissolution of magnetite. For example,
at 120F the inclusion of 1% by weight sodium erythorbate
results in a 14-fold increase in the quantity of magnetite
dissolved (tests 1 and 5). A 2% concentration of sodium
erythorbate results in a 16-fold increase in the dissolution
of magnetite (tests 1 and 9). At 140F, a solution without
sodium erythorbate dissolves twice as much magnetite as does
the same solution at 120F (tests 1 and 2). The same aqueous
solution with 1% sodium erythorbate at 140F results in an
18-fold increase in magnetite dissolution as compared to the
solution without sodium erythorbate at lZ0F (tests 1 and ~).
Thus, the inclusion of sodium erythorbate in the aqueous
hydroxyacetic-formic acid solutions brings about an increase
in the rate of dissolution of magnetite which is far greater
than the effect of heat alone on the rate of dissolution.
-- Example 2
One hundred milliliter portions of aqueous solutions
containing 2% by weight hydroxyacetic acid, 1% by weight formic
acid and 0.1% by volume of the corrosion inhibitor described in
Example 1 are placed in glass beakers. Sodium erythorbate
and/or copper complexor are combined with some of the solutions
and 2 grams of powdered iron oxide (technical grade magnetite)
and 0.1 gram of copper powder are combined with the solutions.
Dry pre-weighed 1020 mild steel corrosion coupons are placed
in the solutions and the solutions are heated and maintained
at temperatures of 140F for time periods of six hours with
one minute of stirring each hour. At the termination of the
six-hour test periods, the solutions are analyzed (by atomic
absorption analysis) for dissolved iron and copper and the
weight losses of the corrosion coupons are determined.
The results of these tests are given in Table II below.
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1;Z(~4372
Table II
Copper and Magnetite Dissolution in Aqueous
Hydroxyacetic-Formic Acid Solutions with and
without Sodium Erythorbate and Copper Complexor
at 140F for 6 Hours
Additive Results
. . _ _ . . . _ .
Sodium Copper Magnetite Copper
Test Erythorbate Complexor Dissolved Dissolved
No. % % Grams Percent Percent
13 0 0 0.029 1.45 0.0010
14 0 1% "A" 0.045 2.25 0.0240
0 1% "A" 0.037 1.85 0.0215
16 1 0 0.270 13.5 0.0015
17 1 1% "A" 0.334 16.7 0.0235
18 1 1% Thiourea 0.456 22.8 0.0223
19 2 1% "A" 0.363 18.15 0.0247
2 1% Thiourea 0.373 18.65 0.0222
1 Copper complexor "A" is a mixture consisting of 60% by weight
hexahydropyrimidine-2-thione and 40% by weight thiourea.
Amounts of dissolved magnetite and copper are determined by
atomic absorption analysis of spent solvent. The value of
dissolved magnetite is corrected for coupon weight loss.
3 The percent number indicated is the number of grams of
additive per 100 milliliters of solution.
From Table II it can be seen that the addition of a
copper complexor to an aqueous solution of hydroxyacetic acid,
formic acid and sodium erythorbate does not diminish the
ability of the solution to dissolve magnetite at 140F (see
tests 6 and 10 of Table I and 17, 18, 19 and 20 of Table II)
but, in fact, increases the amount of magnetite dissolved. The
addition of the copper complexor to an aqueous solution of
hydroxyacetic and formic acids in the absence of sodium
erythorbate (tests 13, 14 and 15), produces a small increase
in the dissolution of magnetite, but the dissolution of copper
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is greatly increased. When sodium erythorbate and copper
complexor are both present in the aqueous solution (tests
17, 18, 19 and 20), greatly improved dissolution of both
magnetite and copper are obtained as compared to when
neither additive or only one is present, i.e., tests 2, 6,
10, 13, 14, 15 and 16~
Example 3
Small sections of boiler tube containing iron and
copper scale are placed in glass beakers, each of which con-
tains 225 milliliters of an aqueous solution containing 2%by weight hydroxyacetic acid, 1% by weight formic acid and
0.1% by volume of the corrosion inhibitor described in Example
1. Some of the solutions also contain sodium erythorbate and
copper complexor. The solutions are heated to temperatures of
140F and maintained at such temperatures for time periods of
24 hours. At the terminations of the 24-hour test periods,
the solutions are analyzed by atomic absorption analysis for
iron and copper content and the boiler tube sections are
inspected for the presence of scale.
The test sections of boiler tube are cut from two
boiler tube samples designated herein as boiler tube sample A
and boiler tube sample B. The scale on boiler tube sample A
consists of magnetite, copper metal and hydroxyapatite
[Ca5tP04)30H]. The scale on boiler tube sample B consists
of magnetite, copper and nickel.
The results of these tests are set forth in Table
III below.
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Table III
Scale Removal by Aqueous Hydroxyacetic-Formic Acid
Solutions with and without Sodium Erythorbate and
Copper Complexor at 140F for 24 Hours
Formulation Results
Boiler Sodium Copper
Tube Test Erythorbate Complexor Tube
Sample No. % % _ % Cu % Fe Appearance
A 21 0 0 0.00019 0.28 copper pl~ted
A 22 0 1 0.0078 0.45 not clean 3
A 23 1 1 0.0093 0.39 tube clean
B 24 0 0 0.0010 0.39 copper plated
B 25 0 1 0.0280 0.33 hot side
not clean2
B 26 1 1 0.0230 0.29 90% clean4
A mixture consisting of 60% by weight hexahydropyrimidine-2-
thione and 40% by weight thiourea.
All copper removed but about 40% iron oxide was still on
tube section.
3 All scale removed.
All copper removed, but about 10% iron oxide was still on
tube section.
.
From Table III it can be seen that the formulation
containing sodium erythorbate and copper complexor is effective
in removing iron and copper scale.
Example 4
One hundred milliliter portions of an aqueous solution
containing 2% by weight hydroxyacetic acid, 1% by weight formic
acid and 0.1% by volume of the corro3ion inhibitor described in
Example 1 are placed in three beakers. Two grams of powdered
iron oxide are combined with each solution. One percent by
weight sodium erythorbate and 1% by weight copper complexor
(60% by weight hexahydropyrimidine-2-thione and 40% by weight
thiourea) are combined with the first solution which is main-
tained at a temperature of 75F for six hours with one minute
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of stirring each hour. One percent by weight copper complexor
only is added to the second solution which is also maintained
at 75F for a six-hour time period. The third solution is
heated to 190F and is maintained at such temperature for
a six~hour time period. At the terminations of the six-hour
time periods, the solutions are analyzed for dissolved iron.
The results in these tests are shown in Table IV below.
Table IV
Iron Dissolution in Aqueous Hydroxyacetic-Formic
Acid Solutions with and without Sodium Erythorbate
and Copper Complexor
Sodium CopperMagnetite
Test Erythorbate ComplexorTemperature Dissolved
No. % % F %
27 1 1 75 0.026
28 0 1 75 0.012
29 0 0 190 0.027
2 The percent number indicated is the number of grams of
additive per 100 milliliters of solution.
Amounts of dissolved magnetite and copper are determined
by atomic absorption analysis of spent solvent. The value
of dissolved magnetite is corrected for coupon weight loss.
~ = .
From Table IV it can be seen that the solution con-
taining sodium erythorbate and copper complexor ef,fectively
dissolves magnetite at 75F.
