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

Abstract

A process for preventing the formation of white rust on a steel surface at least partially coated with zinc includes 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:

RXOH (I) wherein X represents C(O) or S(O)2, and R represents an organic chain. A composition and also to a cooling tower treated by the process are provided.

Claims

1. A process for preventing the formation of white rust on a steel surface at least partially coated with zinc, comprising a) bringing said surface 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):
RXOH (I) wherein X represents S(O).sub.2, and R represents: a linear or branched C.sub.1-C.sub.12 alkyl, optionally substituted with one or more groups chosen from a halogen, OH, COOH or an aryl or heteroaryl group, said aryl or heteroaryl group being itself optionally substituted with a halogen, OH, a linear or branched C.sub.1-C.sub.4 alkyl or a COOH group; or an aryl or heteroaryl group, optionally substituted with a halogen, OH, linear or branched C.sub.1-C.sub.4 alkyl or COOH group.

2. The process as claimed in claim 1, further comprising the following steps: b) measuring the pH of said composition brought into contact with the surface, and c) depending on the result of the measurment 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. The process as claimed in claim 1, wherein the pH of the aqueous composition is between 7.0 and 8.0, preferentially between 7.5 and 8.0, most preferably between 7.8 and 8.0.

4. The process as claimed in claim 1, wherein the composition has a TH between 8 F. and 30 F., and a TA of greater than or equal to 8 F., and a conductivity of less than 2000 S/cm.

5. The process as claimed in claim 1, wherein the steel surface at least partially coated with zinc is subjected to a temperature rise.

6. The process as claimed in claim 1, wherein X is SO.sub.2.

7. The process as claimed in claim 1, wherein R is CH.sub.3.

8. The process as claimed in claim 1, wherein the aqueous composition further comprises a phosphorus compound, preferably a phosphating agent, for example hexametaphosphate.

9. The use of an organic acid of formula (I) as defined in claim 1 to stop the appearance of white rust on a steel surface at least partially coated with zinc brought into contact with water in order to obtain a solution having a pH at a desired value lying between 6.5 and 8.5.

10. The use of an aqueous composition comprising an organic acid of formula (I) as defined in claim 1 to prevent and/or stop the appearance of white rust on a steel surface at least partially coated with zinc, said composition having a pH lying between 6.5 and 8.5, preferentially between 7 and 8, preferentially between 7.5 and 8, preferentially between 7.8 and 8.

11. The aqueous use as claimed in claim 10, wherein characterized in that the acid of formula (I) is methanesulfonic acid.

12. The aqueous use as claimed in claim 10, wherein the aqueous composition further comprises a phosphorus compound, preferably a phosphating agent, for example hexametaphosphate.

13. The aqueous use as claimed in claim 10, wherein the aqueous composition consists of water, acid of formula (I) as defined in and optionally an additive chosen from a scale or corrosion inhibitor, a biocide and mixtures thereof, said aqueous composition having a pH between 6.5 and 8.5.

14. The aqueous use as claimed in claim 10, wherein the aqueous composition comprises at least 95% by weight of water, in particular at least 99% by weight of water, relative to the total weight of the composition.

15. The use as claimed in claim 10, wherein it has a TH of between 8 F. and 30 F., and a TA of greater than or equal to 8 F., and a conductivity of less than 2000 S/cm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0104] The invention will be better understood and other advantages will be become apparent on reading the detailed description of an embodiment given by way of example, which description is illustrated by the appending drawing, wherein:

[0105] FIG. 1 schematically represents a closed-type cooling tower according to the prior art,

[0106] FIG. 2 schematically represents an open-type cooling tower according to the prior art,

[0107] FIG. 3 schematically represents a cooling tower treated by the process according to the invention.

[0108] For the sake of clarity, the same components bear the same references in the various figures.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0109] FIG. 1 schematically represents a closed-type cooling tower according to the prior art, FIG. 2 schematically represents an open-type cooling tower according to the prior art and both were presented in the introduction.

[0110] In one advantageous embodiment, the process according to the invention comprises a step of bringing said surface into contact with an aqueous composition, the pH of which is between 6.5 and 8.5 comprising methanesulfonic acid.

[0111] In this embodiment, the process 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 to a value within the target pH range (at least between 6.5 and 8.5) as a function of the result of the measurement obtained in step b), in particular by addition of methanesulfonic acid, the pH having a tendency to increase naturally owing to the CO.sub.2 stripping phenomenon. These steps are preferably carried out continuously. The measurement and adjustment steps allow good control of the pH so that the composition has a pH equal to or substantially equal to the desired value(s), or lying within a target range. Owing to these steps, the conditions required in terms of pH for the formation of the passivation layer are ensured. A pH control device also makes it possible to avoid any overconsumption of acid.

[0112] Owing to the relatively low pKa of the methanesulfonic acid (pKa=1.9), the pH may by regulated by moderate additions of this acid, without supplementary supply of salts of harmful strong acids.

[0113] The risk of the pH decreasing too greatly after addition of acid is prevented by the fact that the acid is added gradually in water-diluted form, the adjustment being controlled by a regulator.

[0114] Once the passivation layer has formed, the zinc is protected. The adjustment of organic acid of formula (I) as defined above is no longer necessary. A preventative anticorrosion treatment, not comprising the addition of acid of formula (I), is then preferably applied, in accordance in particular with the specifications of the cooling tower manufacturers.

[0115] In this embodiment, preferably the composition further comprises hexametaphosphate, in particular sold by SUEZ under the name AQUALEAD8005. The composition advantageously has a polyphosphate content between 5 g/m.sup.3 and 100 g/m.sup.3, preferably between 5 g/m.sup.3 and 50 g/m.sup.3, typically of 20 g/m.sup.3. The composition may also comprise an anti-scaling additive and/or a biocide.

[0116] In this embodiment, the composition has a TH between 8 F. and 30 F., and a TA greater than or equal to 8 F. and a conductivity of less than or equal to 2400 or 2000 pS/cm. Preferably, these parameters are measured continuously and adjusted in order to be maintained within the target value ranges.

[0117] FIG. 3 schematically represents a cooling tower 130 treated by the process according to the invention. The cooling tower 130 comprises at least one steel surface 131 coated, at least partially, with zinc. Represented are the air intake (f), the air outlet (a), the hot fluid inlet (b) and the cooled fluid outlet (c), and also the recirculation pump (d) and the splash guard (e).

[0118] Treated by the process according to the invention as described above, the surface 131 has a protective layer referred to as a passivation layer.

EXAMPLES

[0119] Tests were carried out on a pilot cooling tower over a minimum operating period of four weeks. The test conditions are summarized in the table below:

TABLE-US-00001 Makeup water quality Municipal water (TH 28, TA 20, pH 7.8) Tower power 28 kW Circuit volume 160 liters Flow rate 900 liters/hour Exchanger skin temperature 70 C. (regulated) Tower outlet temperature 25 C. (regulated) Concentration factor Between 1.1 and 1.3 Analyses Monitoring of the following parameters (makeup and circuit) Parameter: TH, TA, Cl (chlorides in mg/L), pH, SO.sub.4.sup.2 (sulfates in mg/L), SiO.sub.2 (silica in mg/L), phosphates (in mg/L), Zn (zinc in mg/L) Treatment Isothiazolinone-type biocide treatment with (examples 1, 2 and 3) or without (example 4) anti-scaling and anticorrosion treatment Acid control Example 1 = 7.5 (Circuit pH setpoint) Example 2 = 7.8 Example 3 = 8.0 Monitoring of the corrosion LPR (low (or linear) polarization resistance) method on a mild steel probe. Visual observation of the galvanized steel tubes and plates in the packing and the basin of the cooling tower (CT). Metallographic analysis of the control tubes

[0120] The TH is maintained at a value between 8 F. and 30 F., the TA 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 pS/cm. The tests were carried out with various acids.

Comparative Example 1

[0121] Firstly, tests were carried out with citric acid. The quality of the film formed under these conditions is mediocre: an onset of generalized corrosion of the zinc layer (galvanized layer) took place but (early stage) with a loss of thickness of the galvanized layer of around 30 m. The underlying steel tube is intact and does not exhibit any significant damage, but the passivation layer thus obtained does not make it possible to prevent corrosion. Moreover, the bacterial count is off target. Citric acid does not therefore make it possible to solve the problem of the invention.

Comparative Example 2

[0122] Another test was carried out with nitric acid HNO.sub.3. At the end of the test period, the thickness of the zinc layer (galvanized layer) was partially consumed and an onset of corrosion of the steel of the test tube was observed. This confirms that a strong inorganic (mineral) acid such as nitric acid does not make it possible to form the protective passivation layer, but on the contrary leads to the corrosion of the galvanized steel.

[0123] Examples according to the invention: Several tests were carried out with methanesulfonic acid (of formula RXOH where X is SO.sub.2 and R is CH.sub.3) and a target pH value of 7.5 (example 1) and of 7.8 (example 2). These tests showed the partial formation of the stable passivation layer after two weeks of testing, combined with a loss of thickness of the galvanized layer of around 30 pm. Unlike comparative example 1, the loss of thickness of the zinc layer is stabilized at this stage, owing to the good quality of the passivation layer formed. The underlying steel tube does not exhibit any significant damage. The bacterial proliferation remains controlled. The passivation layer appears uniform after two months of testing. The analyses carried out on the film by infrared (IR) spectroscopic analysis reveal the presence of stable zinc oxide forms.

[0124] A complementary test carried out with a target pH value of 8.0 (example 3) gives similar results. Such a target pH value further makes it possible to moderate the consumption of methanesulfonic acid during the implementation of the process.

[0125] Another test was carried out in the absence of phosphorus additive (example 4). It is then observed that the passivation layer forms more slowly than in examples 1, 2 and 3.

Supplementary Example 1

Carrying out a Chemical Passivation Under Load Over 4 Weeks During the Startup of a Baltimore Cooling Tower

[0126] The Baltimore cooling tower passivated in this test is an evaporative condenser (fluid used in the primary circuit=NH.sub.3), model VXC-221 R series no. H180200201 (Dieue sur Meuse), of which the exchange body (tube bundles) is made of galvanized steel. Purging is carried out if a predetermined conductivity threshold (measured using a conductivity probe) is exceeded.

[0127] During this test, the makeup water used is a re-hardened water so as to achieve a value greater than 8 F. in the circuit. Moreover, the pH is adjusted so that the alkalinity TA is greater than 8 F. in the circuit.

[0128] During the passivation period, which extends over a period of 4 to 8 weeks after startup of the tower, the following additives are added to the makeup water: [0129] Phosphating agent: Aqualead PO 8005, at a dose of 70 to 100 g/m.sup.3 in the circuit. This product is injected by a specific pump, independent of the device for adding methanesulfonic acid. [0130] Methanesulfonic acid: Aqualead PA 065/DPIA16-0003, over a period of 4 weeks so as to maintain a regulated pH of 7.8 to 8.2 in the circuit, the target pH being 7.8.

[0131] During the passivation period, the parameters of the water used in the circuit are the following:

TABLE-US-00002 Characteristics PASSIVATION Duration 4 weeks Characteristics of TH: 6.2 f., TA: 20.1 f., the makeup water Cl.sup.: 18.5 mg/L, (mixed water - on C: 560 S/cm, pH = 7.4 average over the period) Characteristics of TH: 18 f., TA: 13.3 f., the circuit water (on Cl.sup.: 64 mg/L, average over the period) C: 1529 S/cm, pH = 8.1 pH control pH: 7.9 by injection of methanesulfonic acid Average DPIA16-0003 dosage 593 g/m.sup.3 Average PO 8005 (makeup) dosage 25 g/m.sup.3 Biocide BC 08 Water consumption 489 m.sup.3

[0132] The analytical monitoring is carried out each week in order 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 is remotely monitored. In the case of starting up a tower of this model under load, a natural increase in the pH up to 8.3 in the circuit is usually observed. It is therefore necessary to inject a passivating and acidic product in order to be within the optimum interval of 7.5<pH<8 for the passivation of the galvanized steel. Via control, the target pH is 7.8. The appearance and the thickness of the galvanized steel tube bundles are observed regularly during the chemical passivation. The shiny tubes at the start (D0) gradually become gray and dull (D0+4 weeks) (visual observation). Moreover, it is observed that the average thickness over 3 points measured by permascope does not decrease: at D0, this thickness is 65 m, and changes to 68 m at D0+4 weeks.

[0133] Conclusion: The passivation was validated visually and corroborated by the absence of appearance of white rust. The device for adding phosphating agent and methanesulfonic acid was removed, and the tower now operates routinely according to the supplier's recommendations.

Supplementary Example 2

Carrying out a Chemical Passivation Under Load Over 4 Weeks During the Startup of a Baltimore Cooling Tower

[0134] The Baltimore cooling tower passivated in this test is an evaporative condenser (fluid used in the primary circuit=NH.sub.3+glycol water), model CXVE-340 1012 201 series no. H17 07 71701 (Honfleur), of which the exchange body (tube bundles) is made of galvanized steel. Purging is carried out if a predetermined conductivity threshold (measured using a conductivity probe) is exceeded.

[0135] During this test, the makeup water used is a re-hardened water so as to achieve a value greater than 8 F. in the circuit. Moreover, the pH is adjusted so that the alkalinity TA is greater than 8 F. in the circuit.

[0136] During the passivation period, which extends over a period of 4 to 8 weeks after startup of the tower, the following additives are added to the makeup water: [0137] Phosphatinq agent: Aqualead PO 8005, at a dose of 70 to 100 g/m.sup.3 in the circuit. This product is injected by a specific pump, independent of the device for adding methanesulfonic acid. [0138] Methanesulfonic acid: Aqualead PA 065/DPIA16-0003, over a period of 4 weeks so as to maintain a regulated pH of 7.8 to 8.2 in the circuit, the target pH being 7.8.

[0139] During the passivation period, the parameters of the water used in the circuit are the following:

TABLE-US-00003 Characteristics PASSIVATION Start date/Stop date From 16 Feb. 2018 to 12 Apr. 2018 Characteristics of TH: 5.4 f., TA: 22.4 f., the makeup water (mixed water - Cl.sup.: 37 mg/L, on average over the period) C: 568 S/cm, pH = 7.8 Characteristics of TH: 10.3 f., TA: 7.5 f., the circuit water (on Cl.sup.: 81 mg/L, average over the period) C: 1192 S/cm, pH = 7.8, ox = 0.68 mg/LCl.sub.2/L pH control pH: 7.9 by injection of organic acid Average DPIA16-0003 dosage 400 g/m.sup.3 Average PO 8005 dosage 71.6 g/m.sup.3 Average PO 8005 (makeup) dosage 50 g/m.sup.3 for a target Rc at 2 Biocide BC 16C Water consumption 101 m.sup.3 Fluids NH.sub.3, GLYCOL WATER

[0140] The analytical monitoring is carried out each week in order 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 is remotely monitored. In the case of starting up a tower of this model under load, a natural increase in the pH up to 8.3 in the circuit is usually observed. It is therefore necessary to inject a passivating and acidic product in order to be within the optimum interval of 7.5<pH<8 for the passivation of the galvanized steel. Via control, the target pH is 7.8. The appearance of the galvanized steel tube bundles are observed regularly during the chemical passivation. The shiny tubes at the start (D0) gradually become gray and dull (D0+4 weeks) (visual observation).

[0141] Conclusion: The passivation was validated visually and corroborated by the absence of appearance of white rust. The device for adding phosphating agent and methanesulfonic acid was removed, and the tower now operates routinely according to the supplier's recommendations.

[0142] The device for regulating the addition of methanesulfonic acid as a function of the pH used in the two supplementary examples 1 and 2 above comprises a pH measurement probe, a metering pump, a metering tank and the containment thereof. The water from the circuit is diverted to an inlet point of the regulating device. The water from the circuit is drawn off at a draw-off point connected to a flow detector. A pH probe measures the pH of the circuit water drawn off. A second pH probe, referred to as a safety probe may also be present. Depending on the result of the pH measurement, the pH is adjusted, if need be, within a range of predetermined values as explained above, by addition of an acid of formula (I), methanesulfonic acid in the two supplementary examples 1 and 2, at an injection point. The metering pump is connected to the injection point on the one hand, and to the metering tank and the containment thereof on the other hand. A dedicated apparatus is connected between the pH probe and the metering pump to manage the amount of acid to be injected as a function of the pH measurement. Lastly, the device comprises an outlet point downstream of the injection point and the water is sent back to the circuit.