EA041664B1 - METHOD FOR INHIBITING CORROSION OF METAL SURFACES - Google Patents

Description

The present invention relates to a method for inhibiting corrosion of a metal surface M, wherein said metal surface M is made substantially of carbon steel, comprising treating said metal surface M with polyethyleneimine R in an aqueous medium.

Metal corrosion is an almost ubiquitous phenomenon on all surfaces that come into contact with water. Among the surfaces where corrosion inhibition is particularly important are circuits like cooling or heating circuits that are in constant contact with water.

Numerous approaches to improve the corrosion behavior of metal surfaces have been proposed by specialists. Because many approaches have been described for inhibiting corrosion of stainless steel, there is still a need for effective methods for inhibiting corrosion of carbon steel.

A large number of corrosion inhibitors have been developed and are being used in various systems depending on the medium being treated, the type of surface that is susceptible to corrosion, the type of corrosion encountered, and the conditions under which the medium is exposed. The effectiveness and usefulness of a corrosion inhibitor under one set of conditions is often not the same under another set of conditions. Many applications, like those in industrial wastewater treatment systems, use a combination of more than one corrosion inhibitor along with other additives such as antiscale agents, biocides, and polymeric dispersants.

Schweinsberg et al., Corrosion Science, 37, 1995, 975-985 describe polyvinylpyrrolidone and polyethyleneimine as steel corrosion inhibitors in phosphoric acid.

US 2010/0143632 discloses coating mixtures comprising polyethyleneimine in combination with poly(meth)acrylic acid or its salts to control steel corrosion.

EP 2194095 A1 discloses mixtures of poly(methyl)acrylate polyethyleneimines which are used in the coating of metal surfaces.

It is an object of this invention to provide a method for inhibiting corrosion of metal surfaces made of carbon steel that is effective at a substantially neutral pH.

This problem is solved by a method for inhibiting corrosion of a metal surface M, wherein said metal surface M is made substantially of carbon steel, including treating said water with polyethyleneimine R in an aqueous medium.

In the context of this invention, the term corrosion inhibition means reducing or preventing the formation of metal compounds on a metal surface, in particular on a steel surface. Examples of steel surface corrosion include the formation of iron oxide or iron hydroxides by the reaction of iron with oxygen in the presence of water, also called rusting.

The methods of this invention are suitable for protecting metal surfaces M, including carbon steel. Carbon steel in the context of this invention may mean steel that is not stainless steel. Stainless steel is understood to mean steel that is not easily corroded, rusted or deteriorated in water. Stainless steel has a chromium content of at least 10.5% by weight.

Preferably, the metal surfaces M comprise carbon steel in which the main impurity component is carbon in the range of 0.12-2.0% by weight.

Even more preferably, the metal surface M contains carbon steel, in which the main intercalated impurity component is carbon in the range of 0.12-2.0 wt.% and which should not contain more than 1.65 wt.% manganese, 0.6 wt.% silicon and 0.6 wt.% copper. All percentages given in this invention are wt.%, unless otherwise specified.

In one embodiment, the metal surface M comprises mild steel, also referred to as mild steel or light steel.

Mild steel contains about 0.05-0.30% carbon.

Higher carbon steels normally have a carbon content in the range of 0.30-2%.

Medium carbon steels have a carbon content in the range of 0.31-0.59%.

High carbon steels have a carbon content of 0.6-0.99%.

Ultra high carbon steels have a carbon content of 1.0-2.0%.

The metal surface M can in principle be part of any apparatus which is in contact with water.

In one preferred embodiment, the metal surface M is part of an apparatus in which water circulates.

In one preferred embodiment, the metal surface M is part of a cooling circuit, a heating circuit, or other forms of energy exchange circuits.

In one particularly preferred embodiment, the metal surface M is part of a cooling circuit. Examples of refrigeration loops include closed refrigeration systems, open refrigeration loops, and open recirculating refrigeration systems, including cooling towers and evaporator condensers.

Open cooling loops, for example, are found in open tower cooling loops and operate on the principle of evaporative cooling. They are also sometimes referred to as single-pass coolers because the cooling water passes through the system to remove heat and then exchanges heat with the natural environment.

In open recirculating cooling loops, cooling water is continuously recirculated between the heat source and the cooling tower.

Closed cooling loops pass a cooling fluid (in many cases an aqueous medium, rough cooling water) through a bundle of tubes that are sprayed with clean water and blown through with a fan.

Particularly preferably, the metal surface M is part of an open cooling circuit or an open recirculating cooling system.

Methods according to the invention include treating said metal surface M with at least one polyethyleneimine R in an aqueous medium.

Polyethyleneimine R (PEI) is a polymer with repeating units containing an amino group and two aliphatic carbon spacers CH2CH2. Linear polyethyleneimines contain all secondary amines (with the exception of the terminal positions), in contrast to branched polyethyleneimines P, which contain primary, secondary and tertiary amino groups. Polyethyleneimines P may also be in dendritic form, which is fully branched.

The composition of polyethyleneimine in one embodiment may be represented by the following general molecular formula:

- (CH ₂ -CH ₂ -NH) _η -, with 10< n <105.

Suitable polyethyleneimine P may be linear, branched or dendritic. Preferably the polyethyleneimine R is branched.

Suitable polyethyleneimines are, for example, those available from BASF under the trade name Lupasol®.

Branched polyethyleneimine R can be obtained by ring-opening polymerization of aziridine.

In one embodiment, polyethyleneimine R is prepared as disclosed in US 2014/163199 in [0014]-[0031].

In another embodiment, polyethyleneimine R is prepared as disclosed in US 8697834 col 2, ln 53 to col 9 ln 26.

Polyethyleneimine R normally has a molecular weight Mw of 500 to 2,000,000 g/mol (Mw determined by GPC (Gel Permeation Chromatography) at pH 4.5; solvent THF (tetrahydrofuran), Mw determined by comparison with a polystyrene standard).

In one embodiment, the polyethyleneimine P has an average molecular weight Mb of 500 to 1500.

In another embodiment, the polyethyleneimine P has an average molecular weight Mw of 1501 to 50,000.

In yet another embodiment, the polyethyleneimine P has an average molecular weight Mw of 50,001 to 2,000,000.

Preferably, the polyethyleneimine P has an average molecular weight Mw of 600 to 1000, more preferably 700 to 900. The polyethyleneimine P normally has a viscosity of more than 3000 (water content 1% according to German standard DIN 53715, K. Fischer; concentration 99% according to ISO 3251; viscosities here according to Brookfield, ISO 2555 and given in [mPa-s]).

In one embodiment, polyethyleneimine P has a viscosity of 3000 to 10000, preferably 4000 to 9000 (water content 1% according to German standard DIN 53715, K. Fischer; concentration 99% according to ISO 3251).

In yet another embodiment, polyethyleneimine R has a viscosity of 10,001 to 100,000 (water content 1% according to DIN 53715, K. Fischer; concentration 99% according to ISO 3251).

In another embodiment, polyethyleneimine P has a viscosity greater than 100,000 or 200,000 (water content 1% according to DIN 53715, K. Fischer; concentration 99% according to ISO 3251).

Preferably polyethyleneimine P has a viscosity of 3000 to 7000, more preferably 4000 to 6000 (water content 1% according to DIN 53715, K. Fischer; concentration 99% according to ISO 3251).

In one embodiment, polyethyleneimine R has a viscosity of 100 to 500, preferably 200 to 450 (water content 1% according to DIN 53715, K. Fischer; concentration 99% according to ISO 3251). In yet another embodiment, polyethyleneimine P has a viscosity of 501 to 15000 (water content 50% according to DIN 53715, K. Fischer; concentration 50% according to ISO 3251).

In another embodiment, polyethyleneimine P has a viscosity higher than 15001 (water content 50% according to DIN 53715, K. Fischer; concentration 50% according to ISO 3251).

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In yet another embodiment, polyethyleneimine P has a viscosity of 15001 (water content 50% according to

DIN 53715, K. Fischer; concentration 50% according to ISO 3251) up to 800 (water content 76% according to

DIN 53715, K. Fischer; concentration 24% according to ISO 3251).

Preferably polyethyleneimine R has a viscosity of 200 to 450 (water content 50% according to

DIN 53715, K. Fischer; concentration 50% according to ISO 3251).

In accordance with the methods of the present invention, the metal surface M is treated with polyethyleneimine R in an aqueous medium. In the case of open, open recirculating or closed cooling circuits, this aqueous medium can be, for example, the cooling fluid of said cooling circuit.

Aqueous medium in the context of this invention may mean a liquid which contains at least 60 wt.%, preferably at least 70 wt.%, more preferably at least 90 wt.% and most preferably at least 99 wt.% water.

According to this invention, the treatment of a metal surface M with polyethyleneimine R is carried out in an aqueous medium, with said metal surface M in continuous contact with said aqueous medium containing polyethyleneimine R. The methods of this invention do not involve the use of a coating containing polyethyleneimine R. Coating in this context means a polymeric coating with an average thickness of at least 200 nm, which contains a polymeric binder other than polyethyleneimine R and, if necessary, other additives. Examples of such coatings are, for example, paints or varnishes. In cooling circuits such as open or closed cooling circuits, polyethyleneimine R is normally contained in the cooling fluid.

The expression in continuous contact in this context means that said metal surface M is not only in contact with said aqueous medium containing polyethyleneimine R, but is also once or for specific and short periods of time, for example, in contact, for coating from the aquatic environment. Preferably, said metal surface M is in contact with said aqueous medium continuously for an extended period of time, such as more than a day, a week, a month or a year. For example, the metal surface M is part of a cooling system, said aqueous medium comprising polyethyleneimine R may be cooling water, and said metal surface M is in continuous contact with said aqueous medium when said cooling system is in operation.

In open cooling circuits, suitable cooling fluids are normally aqueous media which contain at least 95 wt%, preferably at least 99 wt% water.

In closed cooling circuits, suitable cooling fluids are normally aqueous media which contain at least 60 wt%, preferably at least 70 wt% water. In one embodiment, suitable cooling fluids in closed cooling circuits contain at least 95 wt.% or at least 99 wt.% water. Coolant fluids in closed cooling circuits may contain from 0.1 to 40%, preferably from 5 to 30% by weight of liquid freezing point depressants like ethylene glycol.

Normally, the concentration of polyethyleneimine R in said aqueous medium is from 1 to 1000 ppm by weight.

In open and open recirculating cooling circuits, the concentration of polyethyleneimine R is preferably 2 to 100 ppm, more preferably 3 to 50 ppm and particularly preferably 5 to 20 ppm by weight.

In closed systems, the concentration of polyethyleneimine R is in one embodiment preferably from 1 to 100 ppm by weight, more preferably from 3 to 50 ppm and particularly preferably from 5 to 20 ppm by weight.

In another embodiment, the concentration of polyethyleneimine R in closed systems is from 101 to 1000 ppm, more preferably from 200 to 500 ppm by weight.

In open refrigeration circuits and open recirculating refrigeration circuits, cooling is achieved by evaporating water and the evaporating water is continuously replaced with fresh water (replenishment water). Said replenishment water may be, for example, river water, lake water, tap water, brackish water, or deionized or distilled water. In one embodiment, such make-up water is pre-treated before being introduced into the cooling circuit.

Preferably, such make-up water contains polyethyleneimine R in an amount necessary to maintain the concentration of polyethyleneimine R in the aqueous medium (cooling fluid) at a substantially constant level. Preferably, the concentration of polyethyleneimine R in the cooling fluid differs by no more than 25% by weight above the average concentration.

In closed cooling circuits, only a small amount of cooling fluid is used up and is refreshed or replaced sporadically. If the cooling fluid in such a cooling circuit is refreshed or replaced, the newly introduced cooling fluid will normally contain the amount of polyethyleneimine R required to maintain the concentration of polyethyleneimine R in the aqueous medium in the cooling fluid at a substantially constant level. Preferably, the concentration of polyethyleneimine R in the cooling fluid differs by no more than 25% by weight above the average concentration.

Preferably, the treatment of the metal surface M with polyethyleneimine R is carried out at a pH of 6.0 to 9.0, more preferably 6.5 to 8.5.

The method according to the invention can in principle be carried out using an aqueous medium of any hardness.

In one embodiment, the total hardness of the aqueous medium for CaCO ₃ is from 1 to 100 mg/L. In another embodiment, the total hardness of the aquatic environment for CaCO ₃ is from 101 to 1000 mg/l. In yet another embodiment, the total hardness of the aquatic environment for CaCO ₃ is more than 1000 mg/l, the total amount of hardness, which is the sum of calcium and magnesium, is expressed as mg/l of calcium carbonate (CaCO ₃ ).

An unexpected result of this invention is that polyethyleneimine has proved to be an effective corrosion inhibitor for carbon steel surfaces at pH values from 6.0 to 9.0.

In one embodiment, polyethyleneimine R is applied to the metal surface M in combination with other corrosion inhibitors.

Other corrosion inhibitors can be, for example, organic phosphonates such as 1hydroxyethylidene-1,1-diphosphonic acid, amino-tris(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), tetramethylenediamine-tetra(methylenephosphonic acid), hexamethylenediamine-tetra(methylenephosphonic acid ), diethylenetriamine-penta(methylenephosphonic acid), phosphonobutane-tricarboxylic acid, N-(phosphonomethyl)iminodiacetic acid, 2-carboxyethylphosphonic acid, 2-hydroxyphosphonocarboxylic acid, amino-tris(methylenephosphonic acid), K,K-bis(phosphonomethyl)glycine or their salts, or hydroxyphosphonic acid.

Other corrosion inhibitors may further be chromates, molybdates, silicates, nitrites, nitrates, phosphates, polyphosphates, pyrophosphates, orthophosphates or zinc salts, especially zinc phosphates.

In one embodiment, the carbon steel surface is treated with polyethyleneimine at a pH of 6 to 9 in the absence of poly(meth)acrylates such as any poly(meth)acrylic acid or salts thereof.

The aqueous medium comprising polyethyleneimine R may further include other additives such as other corrosion inhibitors, anti-scale agents, chelating agents, anti-freeze agents, detergents, dispersants, biodispersants, biocides, defoamers and defoamers.

Another aspect of this invention relates to the use of polyethyleneimine R as a corrosion inhibitor for metal surfaces M.

The methods according to the invention are very efficient and effective in inhibiting corrosion on M metal surfaces.

The methods according to the invention are also very economical and environmentally friendly. The methods of this invention do not use metal ion corrosion inhibitors like zinc Zn ²⁺ , molybdates and chromates, which are limited by local and general regulations.

The methods according to the invention also do not cause or contribute to scale formation.

Examples

The effectiveness of corrosion inhibition methods was classified according to the guidelines of Bennett P. Boffardi, Ph.D., FNACE. Standards for Corrosion Rates, AWT Analyst, Spring 2000 as follows.

Corrosion rate classification for open recirculating cooling water systems (corrosion rate expressed in milli inches per year (md/g)) for carbon steel:

negligible or excellent: less than or equal to 1, poor or very good: 1 to 3, good: 3 to 5, moderate to moderate: 5 to 8, poor: 8 to 10, very poor to severe: >10.

Corrosion rate classification of closed recirculating cooling water systems (corrosion rate, ppm) for carbon steel:

excellent: less than or equal to 0.2 good: 0.2 to 0.5 moderate: 0.5 to 0.8 poor: 0.8 to 1 very poor to severe: greater than or equal to 1.

Example 1 Application examples in open recirculation systems.

The methods used to test (screen) mild steel corrosion inhibitors are electrochemical measurements using linear polarization resistance (LPR) and Tafel slopes. Gamry potentiostat and Gamry corrosion software are used in electrochemical measurements and in particular Gamry Reference 600 potentiostat, Gamry ECM8 Multiplexer,

3-electrode assembly:

cell according to ALS plate, electrolyte volume: 1 ml,

RE (working electrode): US (carbon steel);

PE (counter electrode): Pt wire;

ES (reference electrode): sat. Ag/AgCl, active area: 0.5 cm ² .

A low-carbon Chemetall Gardobond OS test panel with a chromium content of less than 0.1 wt.% was chosen as the basis in order to represent the quality of the carbon steel installed in the cooling system.

It is available as cold rolled mild steel (MCS) which is then degreased with isopropanol and cleaned with acetone.

The following test protocol was used.

The use of mixed softened and acidified water, typical for open recirculating cooling systems.

Test methodology (T = 22±1°C):

Stabilization of the potential of the open circuit for 1 hour.

LPS measurement.

Measurement of cathodic potentiodynamics (Tafel slope).

Sample replacement.

Stabilization of the potential of the open circuit for 1 hour.

Measurement of LPS (if necessary).

Measurement of anode potentiodynamics (Tafel slope).

Data analysis.

The corresponding standard for calculating Tafel slopes is given in the German DIN ISO 17475:2008.

The corrosion rate is then obtained by combining the Butler-Volmer (Tafel) equation and the Stern-Geary (Stern-Geary) (LPS) equation.

Calculation of corrosion rate (CR):

ea from the measurement of the anodic Tafel slope, β _ε from the measurement of the cathodic Tafel slope, β _ε and ea determined according to the Tafel equation,

R _p (resp. I _kopp ) and U _kopp from LPS measurement

CR [mm/year] = β β K EW / (2.3 (β +β ) PAR) ac ac p ea: anodic Tafel slope, β _ε : cathodic Tafel slope,

K: 3272 for mm per year (mm/y),

EW: Fe equivalent weight ^ 27.92 g/eq., p: Fe density ^ 7.87 g/ ^cm3 ,

A: sample area ^ 0.5 cm ² ,

Rp: polarization resistance.

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Table 1

Water quality characteristic of open recirculating cooling systems. Total hardness (OJ) is the sum of calcium hardness and magnesium hardness. Total alkalinity or methyl orange alkalinity (M-Alkalinity) includes the concentration of bicarbonates, carbonates and hydroxides.

Quality Units Softener mixed in Acidified General hardness (coolant) mg/l CaCO ₃ 80 410 OZHSA mg/l CaCO ₃ 80 410 M-Shell mg/l CaCO ₃ 1020 60 Chloride mg/l 77 124 Sulfate mg/l 75 750 pH - 8.3 7.2 Conductivity σ µS/cm 2030 1960

In table. 2 shows the corrosion rate (in µm/yr) in water of a model open recirculating cooling system.

The results in the table. 2 show that the compounds of this invention are as effective as zinc salts in acidified water, but significantly more effective than zinc salts in softener mixed water.

table 2

Corrosion rate (in µm/yr) in a model open recirculating cooling system using various corrosion inhibitors. The viscosity of PEI with MW 800 was 5000 mPa-s (water content 1% according to DIN 53715, K. Fischer; concentration 99% according to ISO 3251). The viscosity of PEI with MW 750000 was 25000 mPa-s (water content 50% according to DIN 53715, K. Fischer; concentration 50% according to ISO 3251). The viscosity of PEI with MW 2000000 was 700 mPa-s (water content 76% according to DIN 53715, K. Fischer; concentration 24% according to ISO 3251)________________

In acidified water In water with added softener without inhibitor 109.1 | 291.0 Zinc sulfate (2 mg/l) 43.0 61.1 Zinc sulfate (10 mg/l) 44.2 61.2 PEI Mv 800 (dosed at 10 mg/l) 48.3 43.0 PEI Mv 750000 (dosed at 10 mg/l) 83.1 17.7 PEI Mv 2000000 (dosed at 10 mg/l) 50.5 41.1

Example 2

Equipment used for example 2:

Gamry Reference 600 potentiostat, Gamry ECM8 Multiplexer,

3-electrode assembly, working electrode: C1010 coupon, counter electrode: graphite rod, reference electrode: saturated calomel electrode (SCE), coupon surface area: 4.75 cm ² .

Test protocol:

h exposure.

Measurements of continuous linear polarization resistance (LIS).

Measurements of cathodic and anode Tafel slope.

Corrosion rate calculation:

From measurements of the anodic Tafel slope,

Ze from cathodic Tafel tilt measurements, R _p from LIS measurements.

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Table 3

Water quality for example 2

Water quality at 50°C Ca as CaCO ₃ (ppm) 300 Md as CaCO ₃ (ppm) 150 Total hardness as CaCO ₃ (ppm) 450 Alkalinity as CaCO ₃ (ppm) 200 Chloride (ppm) 212 Sulfate (ppm) 142 pH 8.3 Langelier Saturation Index (50°C) 1.3 Ryznar index (50°С) 5.7 Addition of FBTK (million shares) 7.3

Table 4

Resultant corrosion rates with PEI at various dosages

without inhibitor 859 µm/g (33.8 ppm) 4 ppm as Zn 775 µm/g (30.5 ppm) Polyethyleneimine used 10 million shares 50 million shares 100 million shares 200 million shares Molecular weight 800 411 µm/g (16.2 ppm) 135 µm/g (5.31 ppm) 56 µm/g (2.2 ppm) 129 µm/g (5.08 ppm) Molecular weight 2000 159 µm/g (6.27 ppm) 167 µm/g (6.56 ppm) 215 µm/g (8.5 ppm) 594 µm/g (23.4 ppm) Molecular weight 25000 458 µm/g (18.02 ppm) 912 µm/g (35.89 ppm) 967 µm/g (38.07 ppm) 902 µm/g (35.51 ppm) Molecular weight 2000000 385 µm/g (15.15 ppm) 158 µm/g (6.21 ppm) 146 µm/g (5.74 ppm) 153 µm/g (6.02 ppm)

md/g = 25.4 µm/year.

The results in the table. 4 show that the synergy of PBT (2-phosphonobutane-1,2,4-tricaboxylic acid) and PEI is significantly more effective than the synergy of PBT and zinc salts.

Example 3

Application examples in closed systems.

Test provision:

glass Gamry cell, measuring instrument Corrater,

2-probe Corrater electrode, coupon with surface area: 4.75 cm ² .

Test protocol:

diving days.

Continuous measurements of linear polarization resistance (LPS).

VA = 200 mV/dec.

ec = 100 mV/dec.

Measurement of LPS within 0.5 h.

Cathodic scan down to -0.2 V with respect to open loop potential.

Wait 1 h, anode scan to 0.2 V with respect to open loop potential.

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Claims (9)

Table 5 Water quality for example 3 Water quality at 70°C Ca as CaCO ₃ (ppm) 20 Md as CaCO ₃ (ppm) 10 Total hardness as CaCO ₃ (ppm) 30 M-alkalinity as CaCO ₃ (ppm) 200 Chloride (ppm) 56 Sulfate (ppm) 35 pH 9.0 Langelier Saturation Index (70°C) 1.4 Ryznar index (70°С) 6.2 Table 6 Synergistic treatment with molybdate for closed systems Treatment* NaOH (%) BASF Antiprex® AD 1020XP (%) Benzotriazole (%) Molybdates (%) PEI (Mw800) Corrosion rate md/g Corrosion rate µm/g A 10.0 1.9 1.7 5.0 1.72 44 IN 10.1 1.9 1.7 5.0 6.7 0.13 3 Table 7 Synergistic processing of PBT for closed systems Treatment* KOH (%) BASF Antiprex® AD 1020XP Benzotriazole (%) FBTK (%) PEI (Mw800) (%) Corrosion rate md/g Corrosion rate µm/g WITH 9.0 2.5 0.5 4.1 0.23 6 D 9.0 2.5 0.5 4.0 5.1 0.20 5 * Treatment applied at 1 g/l. The results shown in table. 6 and 7 demonstrate that the addition of polyethyleneimine to the molybdate-based mixture significantly improves the corrosion protection of carbon steel. CLAIM 1. A method for inhibiting corrosion of a metal surface, wherein said metal surface is in contact with an aqueous medium having a pH of 6.0 to 9.0, said metal surface being made of carbon steel in which the main impurity component is carbon in the range 0 05-2.0 wt.%, including the treatment of the specified aqueous medium in contact with the metal surface with a branched polyethyleneimine having an average molecular weight Mw in the range from 25,000 to 2,000,000 g/mol and an average molar mass Mn from 500 to 25,000 g /mol, moreover, the specified branched polyethyleneimine is present in the specified aqueous medium in an amount of from 1 to 1000 ppm by weight, and moreover, corrosion inhibitors based on metal ions are not used. 2. Method according to claim 1, wherein said metal surface is part of a cooling or heating circuit. 3. The method according to claim 1 or 2, wherein said metal surface is part of an open, open recirculation or closed cooling circuit. 4. A process according to any one of claims 1 to 3, wherein said polyethyleneimine has a Brookfield viscosity of 100 to 15,000 mPa-s in a 50 wt% solution in water. 5. Process according to any one of claims 1 to 4, wherein said aqueous medium is treated with polyethyleneimine in the absence of poly(meth)acrylic acid or its salts. 6. The method according to any one of claims 1 to 5, wherein said polyethyleneimine contains primary, secondary and tertiary amino groups. 7. The method according to any one of claims 1 to 6, wherein said polyethyleneimine is derived from aziridine by ring-opening polymerization. 8. The method according to any one of claims 1 to 7, wherein said carbon steel contains no more than 1.65 wt.% manganese, 0.6 wt.% silicon and 0.6 wt.% copper. 9. Use of a branched polyethyleneimine having an average molecular weight Mw in the range of 25,000 to 2,000,000 g/mol and an average molar mass Mn of 500 to 25,000 g/mol added -8041664 into an aqueous medium in an amount of from 1 to 1000 ppm by weight as a corrosion inhibitor in cooling circuits in contact with an aqueous medium having a pH of 6.0 to 9.0 and which are made of carbon steel, in which the main introduced impurity component is carbon in the range of 0.05-2.0 wt.%, and corrosion inhibitors based on metal ions are not used. Eurasian Patent Organization, EAPO Russia, 109012, Moscow, Maly Cherkassky per., 2