Dicarboxylic acid mono-(2-hydroxydodecyl)-esters, their salts and their use as corrosion inhibitors in aqueous systems

Dicarboxylic acid mono-(2-hydroxydodecyl)-esters corresponding to the following general formula ##STR1## and salts thereof corresponding to the following general formula ##STR2## wherein A represents the radicals ##STR3## and M represents an alkali metal or ammonium, the use of these compounds as corrosion inhibitors in aqueous systems either by themselves or in combination with one or more complexing agents in concentrations of from 1 to 100 ppm, optionally in the presence of other scale inhibitors, dispersants, non-ferrous metal inhibitors, and/or microbicides.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to dicarboxylic acid mono-(2-hydroxydodecyl)-esters, 
to salts thereof and the use thereof as corrosion inhibitors in aqueous 
systems. 
2. Discussion of Related Art 
Water-carrying plants, such as a steam-generating plant, heating systems, 
cooling water circuits and closed waterline systems are always exposed to 
the corrosive effect of the water circulating in them which is directed 
primarily against the base metals of the particular systems, for example 
steel, brass, aluminum, zinc or galvanized steel. The risk of corrosion is 
further increased by the fact that high temperatures generally prevail in 
such plants and the circulating water contains constituents which also 
chemically promote the corrosive attack on the particular materials. 
Accordingly, chemicals which are intended to guard against or completely 
prevent corrosion have long been added as corrosion inhibitors to the 
water circulating in the afore-mentioned systems. In this connection, 
particularly good results have been obtained with phosphorus-containing 
compounds, for example phosphonic acids or inorganic phosphates, which are 
optionally combined with zinc salts. Hitherto, the effectiveness of 
combinations such as these has been entirely satisfactory. 
However, the recently discovered relationship between a high phosphate 
content of natural waters and eutrophication, which has even resulted in 
legal stipulations relating to the constituents of raw water systems of 
water-carrying plants, has led to the requirement that such raw waters be 
substantially or completely free from phosphorus-containing compounds. In 
addition, the phosphonic acids or inorganic phosphates mentioned, as 
corrosion-inhibiting additives to raw water, have the further disadvantage 
from a practical and technical point of view that they also promote 
increased biological growth within the cooling systems so that 
microbicides must also be added to the systems to inhibit such growth. 
Since relatively hard waters are also occasionally used in raw-water 
circulation systems of the type herein, the use of phosphate-containing 
corrosion inhibitors additionally leads to the formation of apatite or 
apatite-like deposits which, like the known boiler scale, considerably 
complicate the transfer of heat and hence lead very quickly to operational 
disturbances. In addition, such deposits are extremely difficult to 
remove, particularly in closed circuit systems. 
Problems also arise where zinc salts are used in combination with 
phosphorus-containing corrosion inhibitors of the type under 
consideration. Zinc salts are generally known to be very toxic to fish so 
that the waters of the type herein must not under any circumstances enter 
the effluent. In addition, the self-purifying power of natural waters is 
distinctly inhibited at zinc concentrations of only 0.1 ppm and higher. 
Further, the use of combinations of zinc salts with phosphonic acids or 
phosphates generally leads at relatively high pH values (pH&gt;8.0) to 
increased siltation of the raw-water circulation system through the 
precipitation of zinc hydroxide. 
The corrosion-inhibiting effect of dicarboxylic acid semi-amides and 
derivatives thereof, particularly succinic acid semi-amides, is known from 
German Patent Application No. 33 00 874. Unfortunately, the disadvantage 
of using those compounds is their inadequate solubility in water. 
Thus, an object of the present invention is to provide substantially 
phosphorus- and zinc-free corrosion inhibitors for aqueous systems which 
are effective at only low concentrations, are easy to produce and which 
premanently or completely prevent the corrosion of numerous materials of 
the type used in plant construction. The compounds used should be harmless 
in their behavior with respect to the environment and should satisfy the 
legal requirements imposed in that connection, particularly in regard to 
their toxicity. 
DESCRIPTION OF THE INVENTION 
Other than in the operating examples, or where otherwise indicated, all 
numbers expressing quantities of ingredients or reaction conditions used 
herein are to be understood as modified in all instances by the term 
"about". 
It has now surprisingly been found that particular dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters and salts thereof, namely the corresponding 
monoesters of succinic acid, glutaric acid, itaconic acid and phthalic 
acid, are capable of effectively inhibiting the corrosion of metals in 
aqueous systems, especially in raw-water systems, without falling short of 
ecological requirements. 
Accordingly, the present invention relates to dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters corresponding to the formula 
##STR4## 
and to salts thereof corresponding to the following general formula 
##STR5## 
wherein A represents the radicals 
##STR6## 
and M represents an alkali metal or ammonium. 
Accordingly, general formulae (I) and (II) as defined above encompass the 
following compounds: succinic acid mono-(2-hydroxydodecyl)-ester, glutaric 
acid mono-(2-hydroxydodecyl)-ester, itaconic acid 
mono-(2-hydroxydodecyl)-ester, phthalic acid 
mono-(2-hydroxydodecyl)-ester, and salts thereof, preferably the sodium, 
potassium or ammonium salts and particularly the sodium salts. 
In addition, the invention relates to the use of dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters (I) and/or salts thereof (II) in 
concentrations of from 1 to 100 ppm as corrosion inhibitors in aqueous 
systems, optionally in the presence of other scale inhibitors and/or 
dispersants and/or non-ferrous metal inhibitors and/or microbicides known 
per se. 
The invention also relates to the use of dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters (I) and/or water-soluble salts (II) thereof 
in combination with one or more complexing agents selected from the group 
consisting of ethylenediamine tetraacetic acid, nitrilotriacetic acid, 
citric acid, phosphoric acid esters of ethoxylated sugars and also 
phosphonic acid and water-soluble salts of these acids, particularly the 
sodium salts, optionally in the presence of other scale inhibitors and/or 
dispersants and/or non-ferrous metal inhibitors and/or microbicides known 
per se as corrosion inhibitors in aqueous systems, the concentration of 
the mixture of (I) and/or (II) and the complexing agents in aqueous 
solution being in the range from 1 to 100 ppm and the weight ratio of (I) 
and/or (II) to the complexing agents being in the range of from 5:1 to 
1:5. 
The high effectiveness of the compounds herein as corrosion inhibitors is 
the more surprising since dicarboxylic acid mono-(2-hydroxyalkyl)-esters, 
in which the ester alkyl groups contain fewer than 12 or more than 12 
carbon atoms, and dicarboxylic acid monoalkyl esters which do not contain 
a hydroxyl group in the ester alkyl group show little, if any, 
corrosion-inhibiting effect in raw-water circulation systems. 
The corrosion inhibitors according to this invention comprise dicarboxylic 
acid mono-(2-hydroxydodecyl)-esters corresponding to the following formula 
##STR7## 
and water-soluble salts thereof corresponding to the following general 
formula 
##STR8## 
In general formulae (I) and (II), A represents the radicals 
##STR9## 
and M represents an alkali metal or ammonium, preferably sodium, potassium 
or ammonium. The resulting salts (II) all show good solubility in water. 
Preferred salts of formula (II) are the sodium salts (M=Na). 
Accordingly, the corrosion inhibitors used in accordance with this 
invention include the following compounds: succinic acid 
mono-(2-hydroxydodecyl)-ester, glutaric acid 
mono-(2-hydroxydodecyl)-ester, itaconic acid 
mono-(2-hydroxydodecyl)-ester, phthalic acid 
mono-(2-hydroxydodecyl)-esters and salts thereof as defined above. Of 
these, the corresponding monoesters of itaconic acid and glutaric acid and 
salts thereof are preferred for the use according to the invention. 
The compounds (I) according to the invention may readily be obtained in 
high yields, for example by reacting (a) either the corresponding 
dicarboxylic acids, i.e. succinic acid, glutaric acid, itaconic acid or 
phthalic acid, with 1,2-epoxydodecane, or (b) the anhydrides of these 
dicarboxylic acids with 1,2-dodecane diol, in a molar ratio of the 
reactants of 1:1 in accordance with the following reaction scheme: 
##STR10## 
The semi-esters (I) formed by esterification between the acid anhydride or 
the dicarboxylic acid and the alcohol component may then be neutralized by 
reaction with an alkali metal or ammonium hydroxide, salts (II) being 
formed in accordance with the above reaction scheme. 
According to the invention, the dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters (I) and their salts (II), in which A and M 
are as defined above, are used either individually or in admixture with 
one another as corrosion inhibitors. The high corrosion-inhibiting effect 
of the esters (I) by themselves without any other addition is remarkable 
and of particular advantage for the use according to the invention. For 
use in accordance with the invention as corrosion inhibitors in aqueous 
systems, the concentration of the dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters (I) and/or their salts (II) is in the range 
of from 1 to 100 g/m.sup.3, i.e. in the range of from 1 to 100 ppm. A 
preferred concentration range is from 10 to 50 ppm of the above-mentioned 
compounds (I) and/or (II). 
Where the compounds (I) and/or (II) are used as corrosion inhibitors for 
metals, the systems involved are substantially aqueous systems of the type 
encountered in a water-carrying plant, such as steam-generating plants, 
heating systems, cooling water circuits and waterline systems. The 
compounds mentioned may be used with advantage in raw water systems. 
In practice, the corrosive behavior of raw water is influenced to a large 
extent by whether deposit-forming clouding agents are present therein or 
can be formed by corrosion from the water-carrying parts of the plant. 
Agents such as these are formed for example through precipitation by water 
hardness (calcium carbonate), or through precipitation by clays and iron 
hydroxides. Another object of using dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters and/or salts thereof in accordance with 
this invention is also to prevent the formation of deposits from 
substances such as these and hence to improve the behavior of the raw 
water in the sense of a further inhibition of corrosion. Accordingly, it 
is generally of advantage to add to the circulating water not only 
dicarboxylic acid mono-(2-hydroxydodecyl)-esters (I) and/or salts thereof 
(II), but also a scale inhibitor and/or dispersant and/or non-ferrous 
metal inhibitor and/or microbicide known per se for this purpose. The 
addition of agents such as these is not absolutely essential to the 
inhibition of corrosion per se, but may further improve the behavior of 
the raw water in the circulation system. 
Accordingly, polyacrylates and/or copolymers of acrylic acid and/or 
methacrylic acid and/or derivatives thereof having an average molecular 
weight of from 500 to 4000 and/or ethylene oxide-propylene oxide block 
copolymers having an average molecular weight of from 500 to 3000 and an 
ethylene oxide-to-propylene oxide ratio of from 10:90 to 30:70 have proven 
to be particularly suitable scale inhibitors and/or dispersants. Scale 
inhibitors and dispersants such as these are used in combination with 
dicarboxylic acid mono-(2-hydroxydodecyl)-esters (I) and/or salts thereof 
(II) in quantities of from 1 to 50 g/m.sup.3 (1 to 50 ppm) and preferably 
in quantities of from 3 to 10 ppm. 
Depending on the field of application in which the corrosion inhibitors (I) 
and/or (II) are used in accordance with the invention, it may be of 
further advantage to use inhibitors for nonferrous metals as further 
additives known per se for this purpose. Where dicarboxyic acid 
mono-(2-hydroxydodecyl)-esters and/or water-soluble salts thereof are used 
in accordance with the invention, 3-heptyl-5-amino-1,2,4-triazole, 
benzimidazole, benzotriazole and/or tolyl triazole are preferably 
dissolved in the raw water as non-ferrous metal inhibitors. The 
non-ferrous metal inhibitors are present in concentrations of from 0.1 to 
5 g/m.sup.3 (0.1 to 5 ppm). 
It may also be of advantage to add microbicides or biocides in quantities 
of from 1 to 100 g/m.sup.3 (1 to 100 ppm) to the raw waters in addition to 
the components mentioned above. In this case, particularly suitable 
microbicides or biocides, the use of which is known from the prior art, 
include glutaraldehyde, glyoxal or alkyl oligoamides, preferably in the 
form of a reaction product of dodecyl propylenediamine and 
.epsilon.-caprolactam in a molar ratio of 1:2. 
The dicarboxylic acid mono-(2-hydroxydodecyl)-esters (I) or their 
water-soluble salts (II) used in accordance with this invention as 
corrosion inhibitors for metals have the advantage over comparable 
compounds used as corrosion inhibitors or even, in regard to chemical 
structure, over completely different corrosion inhibitors, in that they 
are easy to produce on an industrial scale, for example by the method 
described above, and develop a remarkably high corrosion-inhibiting effect 
at only low concentrations in the aqueous systems used. This effect is 
mainly independent of the pH-value of the aqueous system. In addition, 
they have no adverse, particularly toxic, effects and may therefore be 
safely used even in waters which are ultimately run off from the 
above-mentioned systems into the environment. Further, by comparison with 
phosphorus-containing corrosion inhibitors, they do not lead to the 
eutrophication of waters. Still further, where the corrosion inhibitors 
according to the invention are used, there is no need to use any zinc 
salts which pollute the effluent because of their toxicity to fish. 
Further still, there are none of the deposits of zinc hydroxide which 
normally occur where zinc salts are used in raw-water systems. Another 
important advantage is the fact that they may readily be used with other 
additives known per se, such as scale inhibitors and dispersants, 
non-ferrous metal inhibitors or biocides, and in conjunction with such 
additives show further improved corrosion-inhibiting behavior. 
Accordingly, where ecological aspects, i.e. the complete freedom of zinc 
and phosphorus from the aqueous cooling water or raw water systems, are 
important factors, the sole use of the esters corresponding to formula (I) 
and/or their salts corresponding to general formula (II), preferably the 
sodium salts, corresponds to a particularly preferred and advantageous 
embodiment of the present invention which, in addition, is distinctly 
better in regard to corrosion-inhibiting effectiveness than conventional 
complexing agents. It has surprisingly been found in accordance with this 
invention that the combination of complexing agents used for corrosion 
inhibitors with the esters (I) according to the invention and/or their 
salts (II) leads to a distinct improvement in corrosion inhibition. This 
is of advantage particularly when a small phosphorus content arises such 
as, for example, from the presence of phosphonic acids as complexing 
agents can be tolerated, for example in closed cooling systems. 
Accordingly, another preferred embodiment of the invention is 
characterized in that dicarboxylic acid mono-(2-hydroxydodecyl)-esters (I) 
and/or water-soluble salts (II), of which the sodium salt is particularly 
preferred to the other alkali metal salts and to the ammonium salt, may be 
used either individually or even in admixture with one another. 
Complexing agents selected from the group consisting of ethylenediamine 
tetraacetic acid, nitrilotriacetic acid, citric acid, phosphoric acid 
esters of ethoxylated sugars and also phosphonic acid and water-soluble 
salts of these acids, particularly the sodium salts, are suitable for 
preferred combinations such as these. Of the phosphoric acid esters of 
ethoxylated sugars, esters of sugars having a degree of ethoxylation of 
from 1 to 10 and preferably of from 1 to 5 are suitable. The sugars are 
selected from the group consisting of sorbitol, mannitol, glucose and 
mixtures of 2 or 3 of these sugars in any quantitative ratio. 
The phosphonic acids used may be any of the phosphonic acids suitable for 
the purposes of complexing, phosphonic acids selected from the group 
consisting of 1-hydroxyethane-1,1-diphosphonic acid, 
amino-tris-(methylenephosphonic acid) and 
2-phosphonobutane-1,2,4-tricarboxylic acid and also the water-soluble 
salts of such phosphonic acids being particularly suitable. They may be 
used herein either individually or in admixture with one another. 
The concentration of the combination of dicarboxylic acid 
mono-(2-dihydroxydodecyl)-ester (I) or water-soluble salts thereof (II) on 
the one hand, and one or more complexing agents from the above-mentioned 
group on the other hand in the aqueous solution is in the range of from 1 
to 100 ppm and preferably in the range of from 2 to 60 ppm. According to 
the invention, the ratio of the component ester (I) and/or salt thereof 
(II) to complexing agent is in the range of from 5:1 to 1:5, the range 
from 2:1 to 1:2 being particularly preferred. 
Although a small phosphorus content in the corrosion inhibiting 
combinations has to be expected (in the case of the phosphoric acid esters 
of ethoxylated sugars or the phosphonic acids), it is nevertheless clear 
that the combination of one or more compounds (I) and/or (II) with one of 
the above-mentioned complexing agents produces a further significant 
improvement in the corrosion inhibition values. In cases where 
combinations such as these are used, the corrosion-inhibiting effect is 
again substantially independent of the pH value because the acid form of 
the esters, i.e. the compound (I), is directly formed in the acidic pH 
range, for example at a pH value of 6.5, while the alkali metal or 
ammonium salts of the esters, i.e. the compounds (II), are present at 
alkaline pH values, for example at a pH value of 8.2. Other additives 
typically used in systems of the type herein may also readily be added 
with advantage to the combinations according to the invention of compounds 
(I) and/or (II) with one or more of the above-mentioned complexing agents, 
with the result that the inhibition of corrosion may be further improved 
in conjunction with such additives. Additives of the type in question, 
include, for example, scale inhibitors and/or dispersants, non-ferrous 
metal inhibitors or biocides from the above-mentioned groups.

The invention is illustrated by the following examples. 
EXAMPLE I 
In this example the corrosion-inhibiting behavior of compounds (I) and (II) 
was determined. 
Three carefully pretreated, i.e. degreased, pickled and dried, test plates 
(material: steel strip St 1203 (DIN 1623); dimensions: 75 mm.times.10 
mm.times.1 mm) were immersed for 6 hours at room temperature in a 1 liter 
glass beaker filled with 800 ml of test water in which a certain quantity 
of the dicarboxylic acid mono-(2-hydroxydodecyl)-ester (I) or its sodium 
salt has been dissolved (cf. Table 1). The aqueous solution was stirred at 
100 r.p.m. during the test. 
On completion of the test, the plates were cleaned to remove corrosion 
products and the weight loss was gravimetrically determined. The corrosion 
inhibition value of the inhibitor according to this invention, based on a 
blank value, was determined from the mean value of three tests in 
accordance with the following formula: 
EQU I (%)=100.multidot.(1-a/b) 
I=corrosion inhibition value, 
a=weight loss of the plate treated with inhibitor, and 
b=weight loss of the plate treated without inhibitor. 
The blank value was determined on plates of the same quality after 
treatment with an aqueous solution which did not contain an inhibitor 
according to the invention. 
The test water used as the corrosive medium had the following analysis: 
8.degree. Gh. (calcium hardness) 
2.degree. Gh. (magnesium hardness) 
1.degree. Gh. (carbonate hardness) 
500 ppm Cl.sup.-. 
Table 1 below shows the results of the corrosion inhibition tests using 
dicarboxylic acid mono-(2-hydroxydodecyl)-esters (DHDE) or sodium salts 
thereof. 
TABLE 1 
__________________________________________________________________________ 
Corrosion inhibition value (I) 
in % 
Dosage 
pH 6.5 pH 8.2 
in ppm 
(acid form) 
(Na salt) 
__________________________________________________________________________ 
##STR11## 30 95 
SHDENasalt 30 94 
##STR12## 30 94 
GHDENasalt 30 93 
##STR13## 30 91 
IHDENasalt 30 92 
##STR14## 30 73 
PHDENasalt 30 85 
__________________________________________________________________________ 
.sup.(1) Succinic acid mono(2-hydroxydodecyl)-ester 
.sup.(2) Glutaric acid mono(2-hydroxydodecyl)-ester 
.sup.(3) Itaconic acid mono(2-hydroxydodecyl)-ester 
.sup.(4) Phthalic acid mono(2-hydroxydodecyl)-ester 
COMISON EXAMPLE I 
Other corrosion inhibitors known per se and compounds structurally, similar 
to the DHDEs were tested as in Example 1 by comparision with DHDE and the 
Na salts of DHDE. The results are set out in Table 2 below 
TABLE 2 
__________________________________________________________________________ 
Corrosion inhibition value (I) 
in % 
Dosage 
pH 6.5 pH 8.2 
in ppm 
(acid form) 
(Na salt) 
__________________________________________________________________________ 
(a) 
##STR15## 30 20 17 
(b) 
##STR16## 30 7 12 
(c) 
##STR17## 30 2 4 
(d) 
##STR18## 30 38 36 
(e) 
##STR19## 30 50 14 
(f) 
##STR20## 30 73 30 
(g) 
HOOC(CH.sub.2).sub.2 COOCH.sub.2 (CH.sub.2).sub.10 CH.sub.3 
30 8 12 
(h) 
HEDP.sup.(1) 30 74 66 
(i) 
ATMP.sup.(2) 30 76 77 
(j) 
HEDP + Zn.sup.++ 30 + 30 
98 90.sup.(3) 
(k) 
ATMP + Zn.sup.++ 30 + 30 
99 99.sup.(3) 
__________________________________________________________________________ 
.sup.(1) 1-hydroxyethane-1,1-diphosphonic acid 
.sup.(2) Aminotris-(methylenephosphonic acid) 
.sup.(3) Zinc hydroxide precipitates 
Result: 
Comparison of the values in Tables 1 and 2 shows that distinctly better 
corrosion inhibition can be obtained with dicarboxylic acid 
mono-(2-hydroxydodecyl)-esters (I) or Na-salts thereof than with the 
compounds structurally similar to the DHDEs (see tests a to g) both in 
mildly acidic and in mildly basic medium. Comparison compounds a) to f) 
are corresponding dicarboxylic acid monoesters containing relatively short 
or relatively long carbon chains in the ester alkyl groups. In the case of 
the comparison compound g), the ester alkyl group, having a chain length 
of 12 carbon atoms, does not contain a hydroxyl group. Better or 
comparable values are obtained by comparison with the corrosion inhibitors 
known from the prior art (see tests h to k), although, where the DHDEs are 
used, no phosphorus or zinc enters the waste water, nor are any deposits 
formed. 
EXAMPLE II 
This example illustrates the corrosion-inhibiting behavior of a combination 
of ester (I) or its sodium salt (II) with a complexing agent. 
Following the procedure described in Example I, a corresponding number of 
test plates was treated in an aqueous solution containing a combination of 
DHDE (I) or a sodium salt thereof (II) and a complexing agent as 
identified in Table 3. The plates were treated as described in Example I 
and the corrosion inhibition value of the combination used in accordance 
with the invention, based on a blank value, was determined in accordance 
with the formula shown in Example I. 
The results are shown in Table 3 below. 
COMISON EXAMPLE II 
Other corrosion inhibitors known per se were tested, in some cases together 
with zinc salts, for their corrosion-inhibiting behavior in the same way 
as in Example II by comparison with a combination of DHDE or sodium salts 
thereof and complexing agents. The results are shown in Table 4 below. 
TABLE 3 
______________________________________ 
Corrosion inhibition value (I) 
in % 
Dosage pH 6.5 pH 8.2 
Combination in ppm (acid form) 
(Na salt) 
______________________________________ 
SHDE + ATMP 10 + 10 94 95 
SHDE + HEDP 10 + 10 94 84 
SHDE + PBTC.sup.(1) 
10 + 10 91 96 
SHDE + EDTA.sup.(2) 
10 + 10 62 65 
SHDE + NTA.sup.(3) 
10 + 10 64 69 
SHDE + citric acid 
10 + 10 89 75 
SHDE + sorbitol .multidot. 
10 + 10 97 85 
2EO .multidot. H.sub.3 PO.sub.4 
GHDE + ATMP 10 + 10 86 84 
GHDE + HEDP 10 + 10 93 74 
GHDE + PBTC 10 + 10 93 90 
GHDE + citric acid 
10 + 10 77 74 
IHDE + ATMP 10 + 10 72 89 
IHDE + PBTC 10 + 10 78 67 
IHDE + citric acid 
10 + 10 90 78 
PHDE + ATMP 10 + 10 99 73 
PHDE + citric acid 
10 + 10 80 76 
______________________________________ 
.sup.(1) 2-phosphonobutane-1,2,4-tricarboxylic acid 
.sup.(2) Ethylenediamine tetraacetic acid 
.sup.(3) Nitrilotriacetic acid 
TABLE 4 
______________________________________ 
Corrosion inhibition value (I) 
in % 
Dosage pH 6.5 pH 8.2 
in ppm (acid form) 
(Na salt) 
______________________________________ 
ATMP 10 43 44 
HEDP 10 55 35 
PBTC 10 61 50 
EDTA 10 7 8 
NTA 10 18 13 
Citric acid 10 31 55 
Sorbitol .multidot. 2 EO .multidot. H.sub.3 PO.sub.4 
10 61 44 
ATMP + ZnCl.sub.2 
10 + 10 99 97.sup.(1) 
HEDP + ZnCl.sub.2 
10 + 10 98 85.sup.(1) 
PBTC + ZnCl.sub.2 
10 + 10 97 72.sup.(1) 
______________________________________ 
.sup.(1) Zinc hydroxide precipitates 
Result: 
Comparison of the values in Table 3 and 4 shows that, for the same in-use 
concentrations, the corrosion inhibition values for the combination of 
DHDE (I) (at pH 6.5) or sodium salts thereof (at pH 8.2) and complexing 
agents are distinctly higher than the values for the complexing agents 
alone and are comparable with the values obtained with complexing agents 
in combination with zinc salt. However, the latter combination has the 
disadvantage encountered in every case that zinc salts are present in the 
solution and, in addition, deposits of zinc hydroxide are formed, leading 
to siltation of the plant system to be protected. 
EXAMPLE III 
This example illustrates the general preparation of the dicarboxylic acid 
monoesters. 
1 mole of 1,2-dodecanediol and 1 mole of the corresponding anhydride are 
refluxed for 6 hours in 500 ml of toluene. After cooling, the 
corresponding precipitated product is filtered off under suction. 
(a) Succinic acid mono-(2-hydroxydodecyl)-ester acid number: 177, melting 
point: 91.degree. C. 
(b) Glutaric acid mono-(2-hydroxydodecyl)-ester acid number: 182, melting 
point: 78.degree. C. 
(c) Itaconic acid mono-(2-hydroxydodecyl)-ester acid number: 178, melting 
point: 104.degree. C. 
(d) Phthalic acid mono-(2-hydroxydodecyl)-ester acid number: 148, melting 
point: 97.degree. C.