GB2118159A

GB2118159A - The treatment of aqueous systems

Info

Publication number: GB2118159A
Application number: GB08211321A
Authority: GB
Inventors: Brian Greaves; Alan Marshall
Original assignee: Dearborn Chemicals Ltd
Current assignee: Grace Dearborn Ltd
Priority date: 1982-04-20
Filing date: 1982-04-20
Publication date: 1983-10-26
Also published as: SE8302161D0; FR2525241B1; SE8302161L; JPS58189380A; IT1164190B; ES521607A0; MY8600404A; PH19173A; GB2118159B; CA1211624A; DE3314008A1; IT8320678A0; ES8406391A1; FR2525241A1

Abstract

A method of inhibiting corrosion in an aqueous system comprises adding to the system a water soluble inorganic nitrite and an organic phosphonate which contains at least one carboxylic acid group and at least one phosphonic acid group the said two acid groups being attached to carbon atoms, the concentration of phosphonate exceeding 5 ppm weight per volume and the weight ratio of nitrite to phosphonate being from 90:10 to 5:95.

Description

SPECIFICATION The treatment of aqueous systems The present invention relates to the treatment of aqueous systems and, more particularly, to reducing or eliminating corrosion in aqueous systems.

Many different types of material have been employed to prevent corrosion in aqueous systems.

These include inorganic salts such as nitrites and chromates, inorganic mono- and polyphosphates and certain water-soluble polymers including naturally occurring materials such as lignins and starches as well as synthetic materials such as polyacrylates.

Particular problems arise in cooling systems which are subject to intermittent operation or periodic shut-down. This is because the majority of corrosion inhibitors and the like only function effectively when the cooling system is in motion. Indeed, the only materials which have so far proved to be at all effective for systems involving periodic shut-down are the nitrites and, to a less extent, the chromates. Unfortunately, however, while nitrites are effective they have to be used in quite high concentrations; amounts as much as 1000 ppm are not uncommon. Such amounts present disposal problems because these inorganic nitrites are quite toxic. Thus the maximum nitrogen content permitted by the World Health Organisation in drinking water is equivalent to onlv 45 mq/l of sodium nitrite.However, such quantities of nitrite are ineffective for use as a corrosion inhibitor in cooling systems subject to intermittent operation.

It has now been found, according to the present invention, that it is possible to obtain effective corrosion inhibition if an inorganic nitrite, even in "non-toxic" amounts, that is to say less than 45 ppm, is used in combination in specific proportions and concentrations with a particular class of phosphonate.

In fact, a synergistic effect has surprisingly been found.

According to the present invention there is provided a method of controlling inhibition in aqueous systems which comprises adding to the aqueous system at least one water soluble inorganic nitrite and at least one phosphonate as defined below, the concentration of phosphonate exceeding 5 ppm (wt/volume e.g. 5 mg/litre) and the weight ratio of nitrite to phosphonate being from 90:10 to 5:95, in general from 80:20 to 10:90.

Preferred ratios are between 5:1 to 2:1, especially 4.5:1 to 3:1, and most especially about 4:1.

This combination is particularly useful where very corrosive conditions exist in the cooling system, for example in base exchanged water and very low water hardness systems.

The phosphonates for use in the present invention are those which contain at least 2 acid groups which are carboxylic and phosphonic acid groups, such that at least one acid group is a carboxylic acid group and at least one acid group is a phosphonic acid group, at least the said 2 acid groups being attached to carbon atoms.

The preferred phosphonates possess the general formula:

wherein R is hydrogen, alkyl, alkenyl or alkynyl having up to 4 carbon atoms: phenyl; cycloalkyl having 3 to 6 carbon atoms: benzyi; phenethyl or

wherein R' is hydrogen, alkyl having 1 to 4 carbon atoms or carboxyl, R" is hydrogen or methyl and R"' is carboxyl or phosphonate. 2-Phosphonobutane-1 ,2,4-tricarboxylic acid, a commercially available material, is particularly preferred. Another preferred material is 2,4-diphosphonobutane-1 ,2-dicarboxylic acid. These phosphonates can be obtained by processes well known in the art, for example as described in British Specification No. 1 282 078.

While it is possible to add the materials separately it will generally be more convenient to incorporate them together in the form of a composition. Accordingly, the present invention also provides a composition suitable for addition to water to reduce or prevent corrosion which comprises at least one water soluble inorganic nitrite and at least one phosphonate as defined above, the weight ratio of nitrite to phosphonate being from 90:10 to 5:95, with the proviso that if the said ratio is from 90:10 to 5:1 the composition also contains at least one other water treatment additive.

Typically, the water-soluble nitrite is sodium nitrite but other alkali metal nitrites and also calcium nitrite are also suitable.

As indicated above, by incorporating the specified phosphonate with the inorganic nitrite in the specified amounts it is possible to obtain effective corrosion inhibition even though the concentration of nitrite is less than 45 ppm. Indeed, amounts as little as 1 U ppm have been tound to be effective.

Preferably, the nitrite is present in the system in an amount from 10 to 45 ppm especially 30 to 45 ppm, and most preferably about 45 ppm. It will be appreciated, though, that in situations where health is not a factor concentrations of up to 50 ppm or even, say, 100 ppm can be useful. The amount of phosphonate used will generally be less than that of the nitrite in order to keep costs down and, in general, amounts above 5 up to 50 ppm, particularly up to 30 ppm are suitable, amounts from 10 to 15 ppm, especially about 12 ppm, being preferred, thereby keeping down the phosphorus content in the water so as to reduce disposal problems. Amounts in excess of about 30 ppm are generally commercially unviable.

Phosphonates other than those of formula (I), in general, do not provide advantageous results and should, therefore, generally not be used in the system.

It has further been found that the presence of a water-soluble organic polymer in the system can further inhibit corrosion and, indeed, in certain cases an additional synergistic effect is found.

In general, the polymers suitable for use in the present invention are vinyl addition products possessing recurring units of the general formula:

wherein R, represents hydrogen or alkyl of 1 to 4 carbon atoms, X represents C00H and Z represents hydrogen or COOH or X and Z together represent --COO--O---CO-. The preferred polymers are those of methacrylic acid i.e. where R1 is methyl and Z is hydrogen and acrylic acid i.e. where R, and Z are both hydrogen. In addition phosphinopolyacrylates and methacrylates can be used; these materials can be obtained by polymerisation of (meth)acrylic acid with hypo-phosphorous acid.In general, the molecular weight of the polymers is from 500 to 190 000 and the preferred polymethacrylic acid has a molecular weight of about 5 000 and the preferred polyacrylic acid a molecular weight of about 1 000. It will, of course, be appreciated that the polymers used may be copolymers containing recurring units derived from other vinyl monomers.

Not only does the presence of polymer further reduce corrosion but since the polymers are, in general, less expensive than the phosphonates used, by incorporating polymer and, in particular, by replacing some of the phosphonate by polymer it is possible further to reduce the cost of the additives.

Of course, the polymer can be added to the system separately but it will, in general, be incorporated in a composition with the nitrite and phosphonate.

Although the formulae of the phosphonate and polymer have been given in terms of the free acid it is to be understood that these materials can be used in the form of an inorganic or organic salt, in particular an alkali metal, salt such as sodium or potassium, ammonium or a lower amine salt as well as zinc or other salts. In general, however, the use of alkali metal salts is preferred.

Typically, the polymer is used in an amount from 0.5 to 50 ppm, the preferred amount being from 2to 10ppm.

It will be appreciated that other low toxic materials conventionally used in water treatment can be added to the system and/or the composition including silicates, inorganic phosphates and polyphosphates and lignin derivatives as well as heterocyclic compounds for corrosion inhibition of yellow metals, such as benzotriazole, tolyltriazole and mercaptobenzothiazole.

The compositions of the present invention will normally be in the form of an aqueous solution but other possible forms include powders and briquettes.

Typically the solutions will contain 20 to 50 weight percent, especially 25 to 35 weight percent, nitrite, 2.5 to 20 weight percent, especially 5 to 10 weight percent, phosphonate and, optionally, 1 to 10 weight percent, especially 1.5 to 3 weight percent, polymer. Concentrations of other additives, such as benzotriazole, which can be present, are suitably from 0.1 to 5 weight percent, especially 0.5 to 1.5 weight percent; in order to bring the benzotriazole into solution the pH should be increased to, say, 12 to 12.5 with caustic soda. A particularly preferred formulation contains: 30 weight % sodium nitrite, 1 6 weight % 2-phosphonobutane-1 ,2,4-tricarboxylic acid (50% active), 5 weight % polyacrylic acid (33% active) and 1 weight % benzotriazole with sufficient caustic soda to bring the pH of the aqueous solution to 12 to 12.5. Naturally the amount of the formulation used will depend upon the nature of the system and of the water but with this specific ffrmulation amounts from 100 to 200, especially 1 50 to 200, ppm are generally suitable.

The following examples further illustrate the present invention. In these examples two different types of tests were employed, namely a circulatory test and a test to simulate intermittent flow operations.

In the circulatory test a laboratory test apparatus was used in which water is circulated etc. by means of a pump from a reservoir maintained at a temperature of 540C with a heater and thermostat.

The water passes through a glass tube assembly holding the metal test specimens (flow rate 2 ft/sec) and then is returned to the reservoir entraining air as it does so in order to keep the water saturated with oxygen as it would be in a typical open recirculating cooling system.

Water lost by evaporation (concentration factor 1.7) is replaced from an elevated tank through a float control to maintain a constant volume in the system.

In each test treatment is applied at three times normal dose for 24 hours in order to passivate the metals; then the water is diiuted to the normal dose for the remainder of the test. Each test is for a minimum of 3 days, the test specimens being cleaned before and after each run to find the weight loss which is then calculated to show the average corrosion rate in mils (0.025 mm) per year.

The water used in the tests was Widnes mains water, 75 ppm calcium hardness, 90 ppm. M.

alkalinity.

In the following tests the combined dosage of additives in the system was 30 ppm. "In line" or "High flow" represents the corrosion rate in the tube while "low flow/static areas" represents the corrosion rate in the body of the reservoir.

The results clearly show that the chosen phosphonates in combination with the nitrite do exhibit synergism.

Tests at 540C

Corrosion rate, mils/year Additive composition, % by weight (mm/year) 2-Phosphonobutane Mild Steel (in low Sodium Nitrite 1 2,4-tricarboxylic acid Mild Steel (in Line) flow/static areas) 100 v -- 17.0 (0.425) 43.0 (1.075) 85 15 10.0 (0.25) 32.0 (0.8) 66.67 33.33 5.0 (0.125) 14.0 (0.35).

33.33 66.67 2.0 (0.05) 2.0 (0.05) 15 85 2.0 (0.05) 3.0 (0.075) 100 4.0 (0.1) 6.0 (0.15) Further tests were carried out in a similar manner, with the following results:

Additive, ppm (in system) Corrosion rate, mils/year (mm/year) Ex. Mild Steel Mild Steel Brass No. NaNO2 PBTC PAA BZT TTZ (High flow rate) (Low flow rate) (High flow rate) Tests carried out at 54 C 1 - - - - - 56.0 (1.4) 37.0 (0.925) 1.0 (0.025) 2 25 5 - - - 10.0 (0.25) 32.0 (0.8) 3 20 10 - - - 5.0 (0.125) 14.0 (0.35) 4 10 20 - - - 2.0 (0.05) 2.0 (0.05) 5 5 25 - - - 2.0 (0.05) 3.0 (0.075) 6 40 8 - - - 3.4 (0.085) 6.2 (0.155) 7 40 8 3.0 0.75 0.75 3.1 (0.0775) 5.4 (0.135) 0.4 (0.01) 8 40 6 3.0 0.75 0.75 5.3 (0.1325) 21.4 (0.535) 0.2 (0.005) 9 30 4.5 3.0 0.75 0.75 17.5 (0.4375) 31.5 (0.7875) 0.1 (0.0025) 10 | 40 | 8 | 2.5 | 0.75 | 0.75 | 3.4 (0.085) | 4.3 (0.1075) | 0.1 (0.0025) 11 45 12 2.5 0.75 0.75 2.7 (0.0675) 1.9 (0.0475) 0.1 (0.0025) Additive, ppm (in system) Corrosion rate, mils/year (mm/year) Ex. Mild Steel Mild Steel Brass No. NaNO2 PBTC PAA BZT TTZ Z (High ftow rate) (Low flow rate) (High flow rate) Tests at 60 C 12 46 13.5 2.5 1.5 - 2.1 2.1 (0.0525) 2.5 (0.0625) 0.2 (0.005) Test at 600C 13 45 12 2.5 0.75 0.75 3.2 (0.08) 4.1 (0.1025) 0.1 (0.0025) Test carried out at 54 C; base exchanged Widnes mains water 14 45 12 2.5 0.75 0.75 2.5 (0.0625) 1.7 (0.0425) 0.1 (0.0025) 15 T - - - - 89.0 (2.225) 62.0(1.55) 1.0(0.025) PBTC = 2-phosphobutane-1,2,4-tricarboxylic acid BZT = Benzotriazole PAA = polyacrylic acid - molecular weight 1000 TTZ = Tolyl triazole The above examples illustrate the usefulness of the present invention. Examples 7-11 (inclusive) show the effect of varying the phosphonate ratio and phosphonate concentration; it can be seen from Example 9 that when the concentration of phosphonate is reduced below the permitted minimum, the corrosion rate increases significantly. Examples 6 and 7 show the effect of the addition of other additives. Examples 12, 13 and 14 illustrate the effectiveness of the nitrite/phosphonate combination as a corrosion inhibitor under very severe corrosive conditions.

Claims

1. A method of controlling corrosion in an aqueous system which comprises adding to the system at least one water soluble inorganic nitrite and at least one phosphonate which contains at least two acid groups which are carboxylic and phosphonic acid groups such that at least one acid group is a carboxylic acid group and at least one acid group is a phosphonic acid group, at least the said two acid groups being attached to the carbon atoms, the concentration of phosphonate exceeding 5 ppm weight per volume and the weight ratio of nitrite to phosphonate being from 90:10 to 5:95.

2. A method according to claim 1 in which the weight of nitrite to phosphonate is from 80:20 to 10:90.

3. A method according to claim 1 or 2 in which the weight ratio of nitrite to phosphonate is from 5:1 to 2:1.

4. A method according to any one of claims 1 to 3 in which the weight ratio of nitrite to phosphonate is from 4.5:1 to 3:1.

5. A method according to any one of the preceding claims in which the aqueous system is a base exchanged water or a water of very low water hardness.

6. A method according to any one of the preceding claims in which the phosphonate has the general formula:

wherein R is hydrogen, alkyl, alkenyl or alkynyl having up to 4 carbon atoms; phenyl; cycloalkyl having 3 to 6 carbon atoms; benzyl; phenethyl or

wherein R' is hydrogen, alkyl having 1 to 4 carbon atoms or carboxyl, R" is hydrogen or methyl and R"' is carboxyl or phosphonate.

7. A method according to claim 6 in which the phosphonate is 2-phosphonobutane-1 2,4- tricarboxylic acid or 2,4-diphosphonobutane-1 2-dicarboxylic acid or a salt thereof.

8. A method according to any one of the preceding claims in which the concentration of nitrite is less than 45 ppm weight per volume.

9. A method according to claim 8 in which the concentration of nitrite is from 10 to less than 45 ppm.

10. A method according to any one of the preceding claims in which the concentration of phosphonate is from 10 to 1 5 ppm weight per volume.

11. A method according to any one of the preceding claims which comprises adding also a water soluble organic polymer.

12. A method according to claim 11 in which the organic polymer possesses recurring units of the general formula:

wherein R, represents hydrogen or alkyl of 1 to 4 carbon atoms, X represents COOH and Z represents hydrogen or C00H or X and Z together represent --COO--O--CO-.

1 3. A method according to claim 1 2 in which the polymer is a polymer of acrylic or methacrylic acid.

14. A method according to claim 11 in which the organic polymer is a phosphonopolyacrylate or methacrylate.

1 5. A method according to any one of claims 11 to 14 in which the polymer has a molecular weight from 500 to 100 000.

16. A method according to any one of claims 11 to 1 5 in which the polymer is added in an amount from 0.5 to 50 ppm weight per volume.

1 7. A method according to claim 16 in which the polymer is added in an amount from 2 to 10 ppm weight per volume.

18. A method according to claim 1 substantially as hereinbefore described.

1 9. A method according to claim 1 substantially as described in any one of Examples 2 to 4, 6 to 8 and lotto 14.

20. A composition suitable for addition to water which comprises at least one water soluble inorganic nitrite and at least one phosphonate as defined in claim 1, the.weight ratio of nitrite to phosphonate being from 90:10 to 5:95, with the proviso that if the said ratio is from 90:10 to 5:1 the composition also contains at least one other water treatment additive.

21. A composition according to claim 20 in which the said other water treatment additive is a silicate, inorganic phosphate or polyphosphate, a lignin derivative or a heterocyclic compound.

22. A composition according to claim 20 or 21 which is in the form of an aqueous solution which contains 20 to 50 weight percent of nitrite and 2.5 to 20 weight percent of phosphonate.

23. A composition according to claim 22 which contains 25 to 35 weight percent of nitrite and 5 to 10 weight percent of phosphonate.

24. A composition according to any one of claims 20 to 23 in which the weight ratio of nitrite to phosphonate is as defined in any one of claims 2 to 4.

25. A composition according to any one of claims 20 to 24 in which the phosphonate is as defined in claim 6 or 7.

26. A composition according to any one of claims 20 to 25 which also contains a water soluble organic polymer.

27. A composition according to claim 26 in which the organic polymer is as defined in any one of claims 12 to 14.

28. A composition according to claim 26 or 27 which contains 1 to 10 weight percent of polymer.

29. A composition according to claim 28 which contains 1.5 to 3 weight percent of polymer.

30. A composition according to any one of claims 20 to 29 which comprises 30 weight percent sodium nitrite, 8 weight percent 2-phosphonobutane-1 2,4-tricarboxylic acid, 1 2/3 weight percent polyacrylic acid and 1 weight percent benzotriazole, the composition being adjusted to a pH of 12 to 12.5 with caustic soda.

31. A composition according to claim 18 substantially as hereinbefore described.