EP3774672A1

EP3774672A1 - Method for preventing the formation of white rust on a zinc-coated steel surface

Info

Publication number: EP3774672A1
Application number: EP19713071.9A
Authority: EP
Inventors: Stéphanie Mourier; Eric TOLEDO
Original assignee: Suez Groupe SAS
Current assignee: Vigie Groupe SAS
Priority date: 2018-03-29
Filing date: 2019-03-29
Publication date: 2021-02-17
Also published as: CA3095231A1; WO2019185906A1; FR3079528A1; US20210017652A1

Abstract

The invention relates to a method for preventing the formation of white rust on a steel surface coated at least partially with zinc comprising a) bringing said surface, preferably under thermal load, into contact with an aqueous composition, the pH of which is between 6.5 and 8.5 comprising at least one organic acid of formula (I): R-X-OH wherein X represents C(O) or S(O)2, and R represents an organic chain. The invention also relates to a composition, and also to a cooling tower treated by the method according to the invention.

Description

PROCESS FOR PREVENTING THE FORMATION OF WHITE RUST ON A ZINC-COATED STEEL SURFACE

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of preventing the formation of white rust on a zinc coated steel surface (galvanized steel), said surface being used in particular as a heat exchanger in a cooling tower. The invention also relates to a cooling tower with a zinc-coated steel surface treated by such a method.

BACKGROUND

An air-cooling tower is a device used to cool a liquid, which is usually water, by contact heat exchange - typically by direct contact - with the ambient air. Such a tower has the function of discharging to the outside environment the heat from systems such as a refrigeration unit or an industrial process.

[0003] There are essentially two types of towers. The first category of cooling tower is called "closed primary circuit" or "closed tower" type, as shown in Figure 1. The second category is called "open primary circuit" or "open tower". For these two types of tower, the calories are evacuated by evaporation of a part of the water. In the case of a so-called "closed" tower 1 (FIG. 1), the hot fluid to be cooled circulates in a tubular bundle 11, cooled by water spraying 12. One advantage of a closed tower is that it limits pollution due to the process to which the tower is connected. In the case of a tower called "open 2" (Figure 2), the hot water to be cooled 10 is sprayed in the upper part of the tower, and circulates in a zone 13 where the water / air contact surface is maximized so to increase the heat transfer. The towers also include walls (14) and a pond (15).

[0004] The cooling towers generally comprise galvanized steel elements. These are in particular heat exchange zones such as tubular bundles (closed tower) or the basin.

As indicated above, galvanized steel is a steel covered with a layer of zinc which protects the steel from corrosion in three ways: 1) by the formation of a physical screen between steel and the external environment, 2) in slowing the corrosion phenomena (the oxidation kinetics of zinc is much lower than that of steel), and 3) playing a sacrificial role in case of discontinuity (scratching for example) of the zinc layer (effect of stack between steel and zinc due to the lower dissolution potential of zinc compared to that of iron). However, the protection offered by the galvanizing layer is fully effective once the zinc covered with a passivation layer.

Thus, the galvanized steel elements cooling towers, including heat exchangers such as tubular bundles (closed towers) or the basin (open towers) are vulnerable to corrosion in the presence of water at the first implementation. service of said towers. Indeed, the zinc layer, which is not yet passivated, can then undergo corrosion, resulting in a reduction in the life of the tower. This corrosion is

Generally fast and localized, and is manifested by the formation of white or gray crystals of hydroxide, oxides and / or zinc carbonates, generally grouped under the generic name of "white rust" or "white corrosion" ".

White rust usually appears on a new tower when put into service, and is accelerated by submission to a thermal load. In other words, the first weeks of exposure of water-galvanized steel are crucial and must be carefully monitored to prevent and / or prevent the formation of white rust.

As indicated above, the zinc is itself stabilized by the formation of an amorphous and non-evolutive passivation layer which is formed on its surface, composed in particular of hydroxide, oxides, zinc carbonates and / or mixed species. The process of forming such a passivation layer is complex, and requires precise monitoring of certain physico-chemical parameters, in particular pH.

A solution for forming the passivation layer is so-called natural passivation. It consists of circulating water without thermal load in the installation while respecting certain physicochemical parameters. The duration of the process, about two months, is however not compatible with the constraints of exploitation: the industrialist is, most of the time, forced to start his cooling tower immediately under thermal load. [001 1] However, the pH of the water contained in a cooling tower is not constant when the tower is in operation due to the degassing of C0 ₂ (in other words, because of the "stripping" phenomenon of C0 ₂ ).

It will be appreciated that it is known from the prior art that strong mineral acids, such as sulfuric, nitric or hydrochloric acid, tend to promote corrosion rather than passivation. The prior art nevertheless discloses several methods of passivation of galvanized steel for cooling tower, including the addition of acid in the water to be refrigerated.

Also known are processes comprising the addition of chromate-based corrosion inhibitors, as described in particular in US Pat. No. 3,352,793. However, because of their toxicity, chromates are no longer used today.

The solution of the prior art generally implemented today is the use of organophosphorus products, as described for example in US Patent 6,468,470 or WO2017 / 180328. In practice, the effectiveness of this type of process is called into question because the passivation obtained is still insufficient and the rates of corrosion excessive.

Furthermore, the patent application WO2016 / 003483 discloses a method for treating aqueous systems, intended in particular to limit the corrosion of said systems, comprising the use of hydroxycarboxylic acids or their salts, as alternatives to the use phosphorus derivatives. According to one variant, the aqueous compositions used may comprise para-toluenesulphonic acid (p-TSA), but this is used as a "chemical label", more specifically as a fluorophore (see D1, page 7, paragraphs [0029] - [0030]), and not as anti-corrosion agent, specifically aimed at preventing the formation of white rust.

No. 5,866,042 discloses methods and compositions for inhibiting the corrosion of iron-based metals in contact with aqueous systems, using a copolymer of (meth) acrylic acid and 2-acetoacetoxyethylmethacrylate, which does not enter in the definition of formula (I) of the present invention.

The patent application CN101736337 discloses a method for passivating galvanized steel without resorting to chromates. Nevertheless, the method of CN101736337 is an "electroplating" type treatment. This process is fundamentally different from the methods and uses of the present invention, since it involves treating the steel prior to manufacture of metal devices for containing water, and not during their commissioning. In any case, the single composition comprising a sulfonic acid is described in Example 13, and it is noted that the pH of the passivation bath is adjusted to a value of 6.

Finally, the patent application US 2016/0017200 discloses heat transfer fluids comprising carboxylates. It does not disclose compositions or methods for preventing and / or preventing the occurrence of white rust on a steel surface coated at least partially with zinc

There is therefore a need for methods for preventing and / or combating the degradation of the zinc layer during the first weeks of operation of the cooling tower, in particular under thermal load, which are easy to implement industrially, economically viable, relatively fast, not harmful to the environment, and which do not promote the proliferation of bacteria.

SUMMARY OF THE INVENTION

The invention aims to overcome all or part of the problems mentioned above by proposing a solution to combat the degradation of the zinc layer during the first weeks of operation of the cooling tower. The solution consists in the implementation of a conditioning of water able to prevent and / or inhibit the corrosion of zinc and able to form a protective layer, called passivation, on the zinc-coated surface of the cooling tower, and this especially during normal operation, especially under thermal load.

For the purposes of the present invention, a "galvanized steel surface" is understood as a steel surface covered, at least partially, with a thin layer of zinc, generally applied by electrochemical process. According to one embodiment, the steel is entirely covered with a thin layer of zinc. The thickness of the zinc layer varies according to the intended uses. It is generally between 5 and 500 μm, in particular between 50 and 300 μm. In general, it is 80 pm.

The galvanized steel surface is typically located in the heat exchange areas (upper part of the tower). These include heat exchangers, such as tubular bundles (closed towers). Cooling towers may include other parts with galvanized steel surfaces, such as walls and / or basin (lower part of the tower).

For the purposes of the present invention, a "thermal load" means that a hot fluid is circulated in the tower and causes a temperature differential in the heat exchange zone, particularly in heat exchangers such as tubular bundles (closed tower), and leads to evaporation of water. There is thus a temperature differential between the water sprayed in the upper part of the tower (hot part) and the water contained in the basin (cold part). This temperature difference is generally between 5 ° C and 20 ° C, especially 8 ° C and 15 ° C, typically it is 10 ° C.

For this purpose, the invention relates to a method for preventing and / or preventing the appearance of white rust on a steel surface at least partially covered with zinc, comprising a) contacting said surface with an aqueous composition whose pH is between 6.5 and 8.5 comprising at least one organic acid of formula (I):

R-X-OH (I)

wherein X is C (O) or S (O) ₂ , preferably X is S (O) ₂ , and R is an organic chain, especially:

a linear or branched C ₁ -C ₂ alkyl, optionally substituted with one or more (in particular from 1 to 3) groups chosen from a halogen, OH,

COOH, an aryl or heteroaryl group, said aryl or heteroaryl group being itself optionally substituted by a halogen, OH, linear or branched C ₁ -C ₄ alkyl group, or a COOH group; or

an aryl or heteroaryl group, optionally substituted by a halogen, OH, linear or branched C ₁ -C ₄ alkyl or COOH group;

it being understood that when X represents CO, the C ₁ -C ₁₂ alkyl does not comprise more than 2 COOH groups and preferably the C ₁ -C ₁₂ alkyl does not contain more than one COOH substituent.

This method has the effect of bringing the galvanized steel surface into contact with water at a pH between 6.5 and 8.5 in order to form the so-called passivation protective layer on the surface of the zinc.

For the purposes of the present invention, a "Ci-Ci ₂ alkyl _" means a saturated monovalent hydrocarbon chain, linear or branched, comprising 1 to 12, preferably 1 to 6, in particular 1 to 4 carbon atoms. For exemple, mention may be made of methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl or hexyl groups. Preferably, it is methyl and ethyl groups.

By "aryl". for the purposes of the present invention, an aromatic monocyclic or bicyclic hydrocarbon group, preferably comprising from 6 to 10 carbon atoms, and comprising one or more contiguous rings, for example a phenyl or naphthyl group. Advantageously, it is phenyl.

By "heteroaryl" is meant, within the meaning of the present invention, a monocyclic or bicyclic aromatic group comprising 5 to 10 ring atoms including one or more heteroatoms, advantageously 1 to 4 and even more advantageously 1 or 2, chosen from the sulfur, nitrogen and oxygen atoms, the other cyclic atoms being carbon atoms. Examples of heteroaryl groups are furyl, thienyl, pyrrolyl, pyridinyl, pyrimidinyl, pyrazolyl, imidazolyl, triazolyl, tetrazolyl, indyl, quinoline or isoquinoline groups.

For the purposes of the present invention, the term "halogen atom" means the fluorine, chlorine, bromine and iodine atoms, preferably the fluorine atom.

Typically, in the method of the invention, the steel surface at least partially covered with zinc is subjected to a thermal load (or temperature rise), especially during step a). This rise in temperature results from bringing said surface into contact with a hot or fluid fluid to be cooled, said hot fluid generally being at a temperature of between 40 and 100 ° C., in particular between 50 and 90 ° C.

Typically, in a closed tower, the hot fluid is a refrigerant such as ammonia, which is generally at a temperature of about 80 ° C, especially between 75 ° C and 85 ° C. In an open tower, the hot fluid is typically industrial water at a temperature generally between 35 ° C and 50 ° C.

Under thermal load the composition has a temperature between 15 ° C and 40 ° C, especially between 25 ° C and 35 ° C, depending on the season and the measuring point in the tower, the water being more hot in the upper part of the tower, especially in the heat exchanger in the lower part of the tower, and in particular in the basin. Advantageously, R represents linear or branched C ₁ -C ₆ alkyl, preferably linear, optionally substituted by one or more groups (in particular from 1 to 3 groups) chosen from a halogen, OH, COOH, an aryl group, said aryl group being itself optionally substituted by a halogen, OH, or COOH group, it being understood that the C ₁ -C ₁₂ alkyl does not comprise more than 2 COOH groups when X represents CO. Preferably, R represents linear or branched C ₁ -C ₆ alkyl, preferably linear, optionally substituted by one or two groups chosen from a halogen, OH, COOH and a phenyl group, said phenyl group being itself optionally substituted by a halogen, OH, or COOH group.

Preferably, R represents linear or branched C ₁ -C ₆ alkyl, preferably linear, optionally substituted by one or two groups selected from a fluorine, OH or COOH atom. Most preferably, R is unsubstituted linear or branched C ₁ -C ₆ alkyl, and especially R is CH ₃ .

According to a first embodiment, X represents S0 ₂ . This embodiment is advantageous because the sulphonic acids have a lower pKa than the corresponding carboxylic acids, which makes it possible, to obtain a pH of the identical composition, to add a lesser amount of acid. In this embodiment, R may represent a tolyl group (the acid of formula (I) is then advantageously para-toluenesulfonic acid), a methyl or a trifluoromethyl. Advantageously, in this embodiment, R is CH ₃ , that is to say that the acid of formula (I) is methanesulfonic acid. Methanesulfonic acid is particularly advantageous because it dissociates almost completely in water. This results in ease of pH regulation by moderate additions of this acid.

According to another embodiment of the invention, X represents CO. In this embodiment, the acid of formula (I) is a carboxylic acid, such as acetic acid. It should be noted that citric acid is excluded from the scope of the invention, since, although it is effective in promoting the passivation layer and therefore makes it possible to prevent the formation of white rust, it has been shown that citric acid promotes bacterial proliferation.

For the purposes of the present invention, an "aqueous composition" is composed mainly or essentially of water, and may contain one or more additives. In particular, the aqueous composition comprises at least 95% by weight of water, especially at least 99% by weight of water relative to the total weight of the composition.

The aqueous composition of the invention may in particular be water for industrial use, whose pH is adjusted to a value between 6.5 and 8.5 by addition of organic acid of formula (I) as defined above or below, and to which are optionally added one or more additives. It may be industrial water, but also city water (possibly softened and / or pretreated), drilling water and / or surface water (including lake water, rainwater).

According to a particular embodiment, the aqueous composition of the invention is devoid of chromates.

According to a particular embodiment, the aqueous composition of the invention is devoid of tannins. "Tannin" refers to a polyphenolic compound comprising several phenolic groups (i.e. phenyl groups substituted by at least one OH group). An example of tannin is tannic acid (CAS No. 1401-55-4 or 18483-17-5).

According to a particular embodiment, the aqueous composition is devoid of surfactant, especially the aqueous composition is free of amphoteric surfactant.

According to a particular embodiment, the aqueous composition of the invention further comprises a scale inhibitor and / or a corrosion inhibitor and / or a biocide.

The scale inhibitor can prevent or limit scale formation. It acts by dispersion or crystalline distortion. It is for example an acrylic polymer, which acts by dispersion or a phosphonate, which acts by crystalline distortion.

The corrosion inhibitor is particularly intended to prevent and / or limit corrosion of the metal parts of the cooling tower which do not include a zinc layer. It allows to act additionally in a preventive way against corrosion. The corrosion inhibitor is preferably a phosphorus corrosion inhibitor such as orthophosphate.

The biocidal agent makes it possible to prevent or limit the proliferation of bacteria. It is chosen from oxidizing biocidal agents, such as hypochorite sodium and chlorine dioxide, and non-oxidizing biocidal agents such as isothiazolinones.

The method according to the invention may also comprise one or more in combination among the following steps:

- Addition in the aqueous composition of a corrosion inhibitor,

- Addition in the aqueous composition of a scaling inhibitor,

- Addition in the aqueous composition of a biocide.

The corrosion inhibitor, the scale inhibitor and the biocide are advantageously as defined above.

In an advantageous embodiment, the composition further comprises a phosphorus compound. The phosphorus compound may also have other properties, such as anticorrosion properties or scaling inhibiting properties or properties of phosphating agent, and be already known for the conditioning of cooling towers. Such phosphorus compounds, in combination with the acid of formula (I), provides a synergistic effect. Without wishing to be bound by the theory, it appears that such phosphorus compounds make it possible to improve and / or accelerate the formation of the passivation layer.

Preferably, said phosphorus compound is a phosphating agent. For example, it may be hexametaphosphate, in particular marketed by SUEZ under the name AQUALEAD® PO 8005. The composition advantageously has a polyphosphate content of between 5 g / m ³ and 100 g / m ³ , preferably between 5 g / m ³ and 50 g / m ³ , typically 20 g / m ³ .

According to a particular embodiment, the aqueous composition consists of water, in particular water for industrial use, and of acid of formula (I) as defined above and below, and optionally of an additive among a scaling inhibitor, a corrosion inhibitor, a biocide and mixtures thereof, said aqueous composition having a pH of between 6.5 and 8.5.

Preferably, the pH of the aqueous composition is between 6.5 inclusive and 8.5 excluded. Advantageously, the pH of the aqueous composition is between 7.0 and 8.0, preferably between 7.5 and 8.0, most preferably between 7.8 and 8.0.

The aqueous composition is also characterized by its hydrotimetric titre and its complete alkalimetric titre. The "hydrotimetric title" (TH) or "hardness of the water" is a parameter well known to those skilled in the art. It corresponds to the concentration of calcium and magnesium ions and is often expressed in French degrees (symbol ° f or ° fH). TH measurement methods are well known to those skilled in the art. Generally, one distinguishes the permanent hardness and the temporary hardness, the sum of the two being the total hardness. The hardness is determined by a complexometric determination by EDTA.

The "total alkalinity titer" (TAC) is the quantity used to measure the level of hydroxides, carbonates and bicarbonates of a water. The unit of the TAC is the French degree (° f or ° fH).

TAC measurement methods are well known to those skilled in the art. For example, one can titrate the water to be analyzed with an acid in the presence of two colored indicators, one at 8.2 (phenolphthalein or Thymol blue) and the other at 4.4 (helianthine). Such an assay makes it possible to determine all the hydroxide, carbonate and bicarbonate ions initially present.

Advantageously, in the process of the invention, the composition has a TH between 8 ° F and 30 ° f, and a TAC greater than or equal to 8 ° f.

The composition is also characterized by its conductivity, which is preferably less than or equal to 2400 pS / cm 2, more preferably less than or equal to 2000 pS / cm 2.

The amount of acid to be added depends on the amount of the TAC (complete alkalimetric titer) to neutralize knowing that the make-up water cooling towers have varying qualities. In the example of a sulfonic acid, especially methanesulfonic acid, the pH range required for a TAC of 10 ° f is obtained with a convenient dosage of the acid generally between 160 mg / L and 1200 mg / L the total volume of the circuit.

According to a particularly advantageous embodiment, the composition has a TH between 8 ° F and 30 ° f, and a TAC greater than or equal to 8 ° f and a conductivity less than or equal to 2400 pS / cm. Preferably, these parameters are measured continuously and adjusted to be maintained in the target ranges by methods well known to those skilled in the art.

When the galvanized steel surface is a cooling tower, or part of such a tower, then step a) can be performed according to several embodiments. According to the first embodiment, step a) comprises: a1) bringing said surface into contact with water, and a2) adding at least one organic acid of formula (I) as defined above or below, so that the water exhibits a pH between 6.5 and

8.5.

According to this first embodiment, said steel surface at least partially covered with zinc is first put in contact with water whose pH is not controlled, then the pH of the water is adjusted by adding an acid of formula (I) so as to be between 6.5 and 8.5.

Alternatively, said steel surface coated at least partially with zinc is put directly in contact with the aqueous composition having a pH of between 6.5 and 8.5. The pH of the aqueous composition in contact with the steel surface is thus controlled from the outset.

Advantageously, the method according to the invention further comprises the following successive steps:

b) measuring the pH of said composition contacted with the surface, and c) depending on the result of the measurement obtained in step b) adjusting the pH of said composition contacted with the surface to a value between 6 , 5 and 8.5, in particular by addition of acid of formula (I) as defined above or below.

Because of the stripping phenomenon C0 ₂ ( "C0 ₂ stripping"), pH naturally tends to increase. Thus, the adjustment of the pH is generally carried out by adding an acid of formula (I) as defined above or hereinafter. Thus, except operating accident, we usually encounter two cases:

- either the pH measured in step b) is equal to the target value or included in the target range, then the adjustment step c) is not necessary;

or the pH measured in step b) is greater than the target value or the upper limit of the target pH range; in this case the sufficient amount of acid of formula (I) is added to obtain the target pH value or to return to the targeted pH range (i.e.

8.5).

According to a particular embodiment, the amount of acid of formula (I) added for the adjustment of step c) is slaved to the measurement of step b) via an automaton. According to a particular embodiment, the succession of steps b) and c) is repeated at predetermined time intervals until complete passivation of the zinc layer. In particular, steps b) and c) can be repeated at regular time intervals, for example every week, or every 3 days, or every day, until complete passivation of the zinc layer (especially during 5 days). at 8 weeks).

According to a preferred embodiment, steps b) and c) are implemented in a continuous manner.

Preferably, the pH of the aqueous composition is adjusted during step c) to a value between 6.5 inclusive and 8.5 excluded. Advantageously, the pH of the aqueous composition is adjusted during step c) to a value in the range 7.0 - 8.0, preferably the range 7.5 - 8.0, most preferably the range. 7.8 - 8.0.

The process comprising the successive steps a), b) and c) is preferably carried out so as to maintain the pH of the aqueous composition at a value within the targeted pH range (i.e. 6.5 and 8.5) until complete passivation of the zinc layer. In general, the process of the invention is carried out for a period of approximately 2 months, especially between 3 weeks and 2 months, for example between 4 and 6 weeks.

The raw galvanized steel has a shiny appearance. Passivated galvanized steel is dull because of the passivation layer. The formation of the passivation layer is followed by:

• regular observations of the surface of galvanized steel beams (photos before, during and at the end of the chemical passivation),

· The laying of corrosion coupons in galvanized steel (on lyre, immersion in the basin or on the surface of the tubes),

• the permascope measurement of the zinc thickness on the tubular bundles.

Passivation is considered complete when the appearance of galvanized steel has become dull and gray over the entire exposed surface.

Once the complete passivation of the zinc layer is obtained, it is possible to stop the implementation of the process. The passivated galvanized steel layer can then be brought into direct contact with an acid-free composition of formula (I), advantageously meeting the specifications of the tower manufacturer, particularly in terms of TH, TAC, pH and conductivity.

The invention also relates to the use of an organic acid of formula (I) as defined above (both in its general definition and in its preferred embodiments, advantageous or particular), to prevent the occurrence of white rust on a steel surface at least partially covered with zinc brought into contact with water to obtain a solution having a pH at a desired value between 6.5 and 8.5. The use of such an organic acid of formula (I) with water allows the formation of the passivation layer on the surface and thus prevents the formation of white rust. Preferably, the organic acid of formula (I) is methanesulfonic acid.

The invention also relates to an aqueous composition comprising an organic acid of formula (I) as defined above (both in its general definition and in its preferred, advantageous or particular embodiments), the composition having an pH between 6.5 and 8.5 (in particular between 6.5 and 8.5 excluded), preferably between 7 and 8, preferably between 7.5 and 8, preferably between 7.8 and 8. The composition of the The invention is useful for preventing and / or preventing the occurrence of white rust on a steel surface at least partially covered with zinc.

[0065] Preferably, the organic acid of formula (I) is methanesulphonic acid.

According to an advantageous embodiment, the aqueous composition of the invention further comprises a scale inhibitor and / or a corrosion inhibitor and / or a biocide, which may be as described above.

Advantageously, the aqueous composition is devoid of strong mineral acid, such as sulfuric acid or nitric acid or hydrochloric acid. Thus, preferably, the composition of the invention is devoid of sulfuric acid, nitric acid and hydrochloric acid.

According to a particular embodiment, the aqueous composition of the invention is devoid of tannins. According to a particular embodiment, the aqueous composition is devoid of surfactant, especially the aqueous composition is free of amphoteric surfactant.

According to a particular embodiment, the aqueous composition consists of water, especially water for industrial use, of acid of formula (I) as defined above and below, and optionally of an additive among a scaling inhibitor (for example a polymer, a phosphonate), a corrosion inhibitor, a biocide (such as an oxidizing biocidal agent, especially chosen from sodium hypochorite and chlorine dioxide, or a non-oxidizing biocidal agent such as isothiazolinones), and mixtures thereof, said aqueous composition having a pH of between 6.5 and 8.5.

In particular, the aqueous composition comprises at least 95% by weight of water, especially at least 99% by weight of water relative to the total weight of the composition.

The water may in particular be water for industrial use. It can be industrial water, city water (possibly softened and / or pretreated), drilling water and / or surface water (including lake water, rainwater).

The composition is also characterized by its conductivity, which is preferably less than or equal to 2400 μS / cm, more preferably less than or equal to 2000 μS / cm.

According to an advantageous embodiment, the composition further comprises a phosphorus compound. Preferably, said phosphorus compound is a phosphating agent. For example, it may be hexametaphosphate, in particular marketed by SUEZ under the name AQUALEAD®PO 8005. The composition advantageously has a polyphosphate content of between 5 g / m ³ and 100 g / m ³ , preferably between 5 g / m ³ and 50 g / m ³ , typically 20 g / m ³ . The invention also relates to a cooling tower comprising a steel surface at least partially covered with zinc (thus subject to the appearance of white rust in the presence of water), the surface being treated by the process mentioned above. The resulting technical effect is the formation of the passivation layer in a reduced time to a few weeks. The advantage that This is to prevent the formation of white rust while allowing the tower to operate quickly under thermal load.

Advantageously, the method of the invention allows to obtain the formation of the passivation layer from the commissioning of the cooling tower, under thermal load.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, a description illustrated by the attached drawing in which:

FIG. 1 schematically represents a closed-type air-cooling tower according to the prior art,

FIG. 2 diagrammatically represents an open-type air-cooling tower according to the prior art,

Figure 3 schematically shows a cooling tower treated by the method according to the invention.

For the sake of clarity, the same elements will bear the same references in the different figures. DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1 schematically represents a closed type air-cooling tower according to the prior art, FIG. 2 schematically represents an open-type air-cooling tower according to the prior art and both have been presented in introduction.

In an advantageous embodiment, the method according to the invention comprises a step of bringing said surface into contact with an aqueous composition whose pH is between 6.5 and 8.5, comprising methanesulphonic acid.

In this embodiment, the method comprises a step b) of measuring the pH of said composition in contact with the surface, and a step c) of adjusting the pH of said composition in contact with the surface to the desired value or a value in the target pH range (at least between 6.5 and 8.5) depending on the result of the measurement obtained in step b), in particular by addition of methanesulphonic acid, the pH having a tendency to increase naturally because of stripping phenomenon of C0 ₂ . These steps are preferably performed in a continuous manner. The measurement and adjustment steps allow a good regulation of the pH so that the composition has a pH equal to or substantially equal to the desired value (s), or is in a target range. By these steps, the pH requirements for the formation of the passivation layer are ensured. A pH regulating device also makes it possible to avoid any overconsumption of acid.

Due to the relatively low pKa of methanesulfonic acid (pKa = -1.9), the pH regulation can be carried out by moderate additions of this acid, without additional input of strong, detrimental acid salts.

The risk that the pH decreases too strongly after the addition of acid is removed by the fact that the acid is added gradually in diluted form in water, the adjustment being controlled by a regulator.

Once the passivation layer formed, the zinc is protected. The organic acid adjustment of formula (I) as defined above is no longer necessary. A preventive anticorrosion treatment, not including the addition of acid of formula (I), is then preferably applied, in accordance in particular with the specifications of the manufacturers of cooling towers.

In this embodiment, preferably the composition further comprises hexametaphosphate, in particular marketed by SUEZ under the name AQUALEAD®8005. The composition advantageously has a polyphosphate content of between 5 g / m ³ and 50 g / m ³ , preferably between 5 g / m ³ and 100 g / m ³ , typically 20 g / m ³ . The composition may also include an anti-scale additive and / or a biocide.

In this embodiment, the composition has a TH between

8 ° f and 30 ° f, and a TAC greater than or equal to 8 ° f and a conductivity less than or equal to 2400 or 2000 pS / cm. Preferably, these parameters are measured continuously and adjusted to be maintained in the target ranges.

[0088] Figure 3 schematically shows a cooling tower 130 treated by the method according to the invention. The cooling tower 130 comprises at least one surface 131 of steel covered, at least partially, with zinc. The air suction (f), the air outlet (a), the hot fluid inlet (b) and the cooled fluid outlet (c), as well as the recirculation pump (d) and the droplet (e). Processed by the method according to the invention as described above, the surface 131 has a protective layer called passivation.

EXAMPLES

Tests were conducted on a cooling tower pilot over a minimum operating period of four weeks. The test conditions are summarized in the table below:

The TH is maintained at a value between 8 ° f and 30 ° f, the TAC is maintained at a value greater than or equal to 8 ° f and the conductivity is maintained at a value less than or equal to 2400 μS / cm. . The tests were conducted with different acids.

Comparative Example 1: First, the tests were conducted with citric acid. The quality of the film formed under these conditions is poor: a generalized corrosion start of the zinc layer (galvanized layer) took place but (early stage), with a thickness loss of the galvanized layer of about 30 μm. The underlying steel tube is intact and shows no significant damage, but the passivation layer thus obtained does not prevent corrosion. In addition, the bacterial count is drifting. Citric acid therefore does not solve the problem of the invention.

Comparative Example 2: Another test was conducted with nitric acid HNO ₃ . At the end of the test period, the thickness of the zinc layer (galvanized layer) was partially consumed and a beginning of corrosion of the steel of the test tube was observed. This confirms that a strong inorganic acid (mineral) such as nitric acid does not form the protective passivation layer, but on the contrary causes corrosion of the galvanized steel.

Examples according to the invention Several tests were conducted with methanesulphonic acid (of formula RXOH where X is SO ₂ and R is CH ₃ ) a target pH value of 7.5 (example 1) and 7 , 8 (Example 2). These tests showed the partial formation of the stable passivation layer after two weeks of testing, associated with a thickness loss of the galvanized layer of about 30 μm. Unlike Comparative Example 1, the loss of thickness of the zinc layer is stabilized at this stage because of the good quality of the passivation layer formed. The underlying steel tube has no significant damage. Bacterial proliferation remains under control. The passivation layer appears uniform after two months of testing. The analyzes carried out on the film by infrared spectroscopic analysis (IR) reveal the presence of stable forms of zinc oxides.

A further test carried out with a target pH value of 8.0 (Example 3) gives similar results. Such a target pH value makes it possible in addition to moderating the consumption of methanesulfonic acid during the implementation of the process.

Another test was carried out in the absence of phosphorus additive (Example 4). It is then found that the passivation layer is formed more slowly than in Examples 1, 2 and 3.

[0096] Additional example 1: Achieving chemical passivation under load for 4 weeks when starting a air-cooling tower

Baltimore: The cooling tower Baltimore passivated in this test is an evaporative condenser (fluid used in the primary circuit = NH ₃ ), model VXC-221 R series No. H180200201 (Dieue sur Meuse), whose exchange body (tubular bundles ) is made of galvanized steel. Bleeding is performed if a predetermined conductivity threshold (measured with a conductivity probe) is exceeded.

In this test, the makeup water used is a "re-cured" water, so as to reach a value greater than 8 ° f in circuit. In addition, the pH is adjusted so that the alkalinity TAC is greater than 8 ° f in circuit.

During the "passivation" period, which is spread over a period of 4 to 8 weeks after starting the tower, the following additives are added to the make-up water:

> Phosphating agent: aqualead PO 8005, at a dose of 70 to 100 g / m3 in circuit. This product is injected by a specific pump, independent of the device for adding methanesulphonic acid.

> Methanesulfonic acid: aqualead PA 065 / DPIA16-0003, during a period of 4 weeks so as to maintain a regulated pH of 7.8 to 8.2 in circuit, the target pH being 7.8.

During the passivation period, the parameters of the water used in the circuit are as follows:

The analytical monitoring is performed weekly to ensure that the physicochemical and microbiological parameters are satisfactory. The average pH during the period is pH = 8.1 (between 7.8 and 8.3) and monitored remotely. In the case of starting a tower of this model under load, there is usually a natural increase in pH up to 8.3 in circuit. It is therefore necessary to inject a passivating and acidic product in order to be in the optimum range of 7.5 <pH <8 for the passivation of galvanized steel. By regulation the target pH is 7.8. The appearance and thickness of galvanized steel tubular bundles are regularly observed during chemical passivation. The shiny tubes at the beginning (J0) become gradually gray and dull (J0 + 4 weeks) (visual observation). In addition, it is found that the average 3-point thickness measured by permascope does not decrease: at 0, this thickness is 65 pm, and goes to 68 pm at 0 + 4 weeks.

Conclusion: The passivation was validated visually and corroborated by the absence of white rust occurrence. The device for adding phosphating agent and methanesulfonic acid has been removed, and the tower now operates routinely according to the supplier's recommendations.

[00102] Additional example 2: Achievement of a chemical passivation under load for 4 weeks when starting a Baltimore cooling tower: The Baltimore cooling tower passivated in this test is an evaporative condenser (fluid used in the primary circuit = NH ₃ + brine), model CXVE-340 1012 201 series no. H17 07 71701 (Honfleur), whose exchange body (tubular bundles) is made of galvanized steel. Bleeding is performed if a predetermined conductivity threshold (measured with a conductivity probe) is exceeded. In this test, the make-up water used is a "re-cured" water, so as to reach a value greater than 8 ° f in circuit. In addition, the pH is adjusted so that the alkalinity TAC is greater than 8 ° f in circuit.

> Phosphating agent: aqualead PO 8005, at a dose of 70 to 100 g / m3 in circuit. This product is injected by a specific pump, independent of the device for adding methanesulphonic acid. > Methanesulfonic acid: aqualead PA 065 / DPIA16-0003, during a period of 4 weeks so as to maintain a regulated pH of 7.8 to 8.2 in circuit, the target pH being 7.8.

The analytical monitoring is performed weekly to ensure that the physicochemical and microbiological parameters are satisfactory. The average pH during the period is pH = 8.1 (between 7.8 and 8.3) and followed remotely. In the case of starting a tower of this model under load, there is usually a natural increase in pH up to 8.3 in circuit. It is therefore necessary to inject a passivating and acidic product in order to be in the optimum range of 7.5 <pH <8 for the passivation of galvanized steel. By regulation the target pH is 7.8. The appearance of galvanized steel tubular beams is observed regularly during chemical passivation. The shiny tubes at the beginning (OJ) become gradually gray and dull (OJ + 4 weeks) (visual observation).

Conclusion: The passivation was validated visually and corroborated by the absence of occurrence of white rust. The device for adding phosphating agent and methanesulfonic acid has been removed, and the tower now operates routinely according to the supplier's recommendations.

The device for regulating the addition of methanesulphonic acid as a function of the pH used in the two additional examples 1 and 2 above comprises a pH measuring probe, a dosing pump, a dosing tank and its retention. . The circuit water is derived at an entry point of the regulating device. The water from the circuit is taken at a sampling point connected to a flow detector. A pH probe measures the pH of the withdrawn circuit water. A second so-called safety pH probe may also be present. Depending on the result of the pH measurement, the pH is adjusted, if necessary, within a range of predetermined values as explained above, by adding an acid of formula (I), methanesulfonic acid in the two additional examples 1 and 2, at a point of injection. The dosing pump is connected to the injection point on the one hand and to the dosing tank and its retention on the other. Dedicated equipment is connected between the pH probe and the dosing pump to control the amount of acid to be injected according to the pH measurement. Finally, the device comprises an outlet point downstream of the injection point and the water is redirected to the circuit.

Claims

1. A method for preventing the formation of white rust on a steel surface at least partially covered with zinc, characterized in that it comprises a) bringing said surface into contact with an aqueous composition whose pH is between 6, 5 and 8.5 comprising at least one organic acid of formula (I):

R-X-OH (I)

wherein X is S (O) ₂ , and R is:

a linear or branched C ₁ -C ₁₂ alkyl, optionally substituted with one or more groups chosen from a halogen, OH, COOH, an aryl or heteroaryl group, said aryl or heteroaryl group being itself optionally substituted with a halogen group, OH, a linear or branched C ₁ -C ₄ alkyl, or a COOH group; or

an aryl or heteroaryl group, optionally substituted with a halogen, OH, linear or branched C ₁ -C ₄ alkyl or COOH group.

2. Method according to claim 1, characterized in that it further comprises the following steps:

b) measuring the pH of said composition in contact with the surface, and c) depending on the result of the measurement obtained in step b), adjusting the pH of said composition in contact with the surface to a value between 6 , 5 and 8.5 by addition of acid of formula (I) as defined in claim 1.

3. Method according to any one of claims 1 or 2, characterized in that the pH of the aqueous composition is between 7.0 and 8.0, preferably between 7.5 and 8.0, most preferably between 7.8 and 8.0.

4. Method according to any one of claims 1 to 3, characterized in that the composition has a TH between 8 ° f and 30 ° f, and a TAC greater than or equal to 8 ° f, and a conductivity less than 2000 pS / cm.

5. Method according to any one of claims 1 to 4, characterized in that the steel surface coated at least partially with zinc is subjected to a rise in temperature.

6. Method according to any one of claims 1 to 5, characterized in that X is S0 ₂ .

7. Process according to any one of claims 1 to 6, characterized in that R is CH ₃ .

8. Process according to any one of claims 1 to 7, characterized in that the aqueous composition further comprises a phosphorus compound, preferably a phosphating agent, for example hexametaphosphate.

9. Use of an organic acid of formula (I) as defined in any one of claims 1, 6 and 7, for preventing the appearance of white rust on a steel surface covered at least partially with zinc applied. contact with water to obtain a solution having a pH at a desired value between 6.5 and 8.5.

10. Use of an aqueous composition comprising an organic acid of formula (I) as defined in any one of claims 1, 6 and 7 for preventing and / or preventing the appearance of white rust on a coated steel surface. at least partially zinc,

said composition having a pH of between 6.5 and 8.5, preferably between 7 and 8, preferably between 7.5 and 8, preferably between 7.8 and 8.

1. An aqueous use according to claim 10, characterized in that the acid of formula (I) is methanesulfonic acid.

12. Aqueous use according to claim 10 or 11, characterized in that the aqueous composition further comprises a phosphorus compound, preferably a phosphating agent, for example hexametaphosphate.

13. Aqueous use according to claim 10 or 1 1, characterized in that the aqueous composition consists of water, acid of formula (I) as defined in any one of claims 1, 5 and 6, and optionally an additive selected from an inhibitor scaling or corrosion, a biocide and mixtures thereof, said aqueous composition having a pH between 6.5 and 8.5.

14. Aqueous use according to any one of claims 10 to 13, characterized in that the aqueous composition comprises at least 95% by weight of water, especially at least 99% by weight of water, relative to the total weight of the composition.

15. Use according to any one of claims 10 to 14, characterized in that it has a TH between 8 ° f and 30 ° f, and a TAC greater than or equal to 8 ° f, and a conductivity less than 2000 pS / cm.