GB1577923A - Corrosion intibitor composition - Google Patents

Description

(54) CORROSION INHIBITOR COMPOSITION (71) We, HEMLAB AG, a Swiss company of Langenjohnstrasse 9, 7000 Chur, Switzerland, do hereby declare this invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to corrosion inhibition, particularly to a method and composition for inhibiting the corrosion of steel articles contacted by water and more particularly to improved compositions suitable for addition to cooling water circulating systems such as are found in steelworks, power stations and heat exchangers.

Two problems caused by cooling water in contact with steel are corrosion of the metal and a tendency, especially in hard water areas, for scale and sludge to be deposited on the metal.

It has now been found that corrosion of steel articles contacted by water may be inhibited, especially when the water has a certain pH and temperature, by the use of a composition comprising an aqueous solution of orthophosphoric acid and an alkanolamine.

According to this invention there is provided a composition for inhibiting the corrosion of steel which composition comprises an aqueous solution of orthophosphoric acid and an alkanolamine, and which includes a minor proportion of a polyacrylate.

The invention also provides a method of inhibiting the corrosion of steel articles contacted by water, which method comprises adding to the water a composition according to the invention.

The preferred alkanolamine is triethanolamine, although mono- or diethanolamine may be used separately or together, with or without triethanolamine. Other alkanolamines may be selected from diethylethanolamine, phenylethanolamine, dimethylethanolamine, mono-, di- or tri-isopropanolamine and mixtures thereof and may be blended with the above.

We have found that the inclusion of a minor proportion of polyacrylate improves the effectiveness of the composition at higher temperatures. This is particularly so in hard waters for as temperature and hardness salt concentration increase so does the likelihood of corrosion cell formation. Higher temperatures generally also lead to higher cycles of concentration thus increasing scaling potential through increased hardness salt concentration. The presence of the polyacrylate prevents scale formation and hence inhibits corrosion cell formation thus contributing to the overall lowering of corrosion. The polyacrylate may have a molecular weight of from 1000 to 50,000, preferably from 2000 to 20,000, and may be a polymerised acrylate or methacrylate or a mixture of these. Preferably the composition includes 2 to 25 percent by weight, more preferably 2 to 12 percent by weight, of polyacrylate.

When the composition is used to treat a scale forming water the composition may contain a minor proportion of other additives for example, a scale mobiliser or dispersant such as tannin, a lignosulphate or a phosphonate; a corrosion inhibitor for copper such as benzotriazole, sodium mercaptobenzothiazole, or an gamine, together with small amounts of an alkali such as sodium hydroxide, potassium hydroxide and ammonium hydroxide; or an antifoaming agent such as a silicone or fatty acid amide derivative. Thus the composition may include up to 25 percent by weight of a corrosion inhibitor for copper and up to 20 parts by weight of an alkali metal or alkaline earth metal hydroxide and may include up to 10 percent by weight of an antifoaming agent.

Compositions according to the invention preferably comprise (percentage by weight): orthophosphoric acid (in the form of an 85% solution) 10-50 water 10-20 alkanolamine 40-80 polyacrylate up to 50 corrosion inhibitor for copper 0-25 alkali metal or alkaline earth metal hydroxide 0-20 antifoaming agent 0-10.

Preferably the compositions comprise (percentage by weight):orthophosphoric acid (in the form of an 85% solution) 10-50 water 10-20 alkanolamine 40-80 polyacrylate 5-25.

Most preferred compositions according to the invention comprise (percentage by weight):alkanolamine 50-60 orthosphosphoric acid 19-25 polyacrylate 3-6 corrosion inhibitor for copper (e.g. an amine) 1-5 water 10-15 together with alkali and anti-foaming agent if required.

Generally, the compositions of the invention may be used at concentrations of from 5 to 1000 parts by weight per million parts by volume of water. The compositions of the invention can provide effective inhibition of corrosion of steel in systems where the pH of the water is from 6 - 12 more suitablv from 7 to 10.5. The compositions may be used in systems using soft water having a total hardness of less than 25 ppm (CaCO3) which typically have a pH of 9 to 12. Soft waters in the form of base exchange softened or demineralised water will generally have a pH in the narrower range of 10 to 12. The compositions may also be used in systems with hard waters, which have hardnesses above 25 ppm (CaCO3).

The corrosion inhibitor may be used in both recirculating water systems and once-through water systems. Re-circulating systems include both open systems, e.g. cooling towers for heat exchangers in food factories. and closed systems. e.g. cooling hoods on steel convertors in steelworks. An example of a once-through water system is the water cooling system for condensers in Power Stations.

The following Examples. in which all parts are by weight, will serve to illustrate the invention. In the Example the orthophosphoric acid was used in the form of an 85CHo solution.

Example I A composition was made up of: orthosphosphoric acid (S.G. 1.65) 20 water 12 triethanolamine (S.G. 1.12) 56 polyacrylate (M.Wt. = 5000) 12 This composition was evaluated in a laboratory corrosion trial in which mains tap water of medium hardness and pH of 7 to 9 was maintained at 60"C (a relatively high temperature for cooling systems) and a simulated flow rate of 0.5 - 2ft/sec. The degree of corrosion was measured in milligrams per sq. decimeter per day (mdd) and the results obtained are reported below.

Mains Tap Water analysis: pH = 7.5 Total Alkalinity as CaCo3 = 100 ppm Total Hardness = 100 ppm as CaCO3 Calcium Hardness = 75 ppm as CaCO3 Chloride = 40 ppm as Cl Test No. Amount of Composition Corrosion Rate Used (ppm) (mdd) Control - 200 1 25 236 2 150 16 3 200 17 4 250 25 5 300 17 6 500 14 These results illustrate that the optimum addition rate of the inhibitor composition to this water was 150 ppm.

The corrosion inhibitor was evaluated over the pH range of 6 to 10 to determine the relationship between the pH and the corrosion inhibiting properties.

Medium hardness mains water having its natural pH range adjusted for the experimerit with appropriate acid or alkali reagents was treated at the rate of 150 ppm of inhibitor.

Test No. pH adjusted to Corrosion Rate (mdd) Untreated 6 to 10 168 (mean) 1 6.0 120 2 7.0 23 3 8.0 16 4 9.0 19 5 10.0 126 The results show the inhibitor composition was particularly efficient at the pH range of 7 to 9.

Example 2 Example 1 was repeated using the following composition: orthophosphoric acid (S.G. 1.65) 35 water 10 triethanolamine 49 polymethacrylate (M.Wt.=12000) 6 Results comparable to those of Example 1 were obtained.

Example 3 A composition was made up of: orthophosphoric acid (S.G. 1.65) 35 water 10 triethanolamine 48 polyacrylate (M.Wt=12000) 7 This composition was tested in the manner of Example 1 on demineralised water, having a conductivity of less than 0.5 umhos per cm2.

Good corrosion inhibition took place at a dosage of 150 ppm. Tests to determine the optimum pH value indicated that the inhibitor composition was extremely effective at the higher pH values.

Example 4 Example 1 was again repeated using the following composition: orthophosphoric acid (S.G. 1.65) 10 water 10 triethanolamine (S.G. 1.12) 60 polyacrylate (M.Wt = 5000) 20 Results comparable with those of Example 2 were obtained.

The compositions were also evaluated in a separate series of trials using hard water, low hardness water and high alkalinity water at between 35"-60"C and a concentration of between 50-250 ppm, pH 6-10 and the flow rate was 0;25 - 3.0ft/sec. and acceptable corrosion inhibition was obtained.

Typical analyses were: Hard water Total Hardness- = 600 ppm as CaCO3 Calcium Hardness = 450 ppm as CaCO3 Low hardness water Total Hardness = 30 ppm as CaCO3 High Alkalinity water Total Hardness = 400 ppm as CaCO3 Calcium Hardness = 300 ppm as CaCO3 Total Alkalinity = 350 ppm as CaCO3.

WHAT WE CLAIM IS: 1. A composition for inhibiting the corrosion of steel, which composition comprises an aqueous solution or orthophosphoric acid and an alkanolamine, and which includes a minor proportion of a polyacrylate.

2. A composition according to claim 1, wherein the alkanolamine is triethanolamine.

3. A composition according to claim 1, wherein the alkanolamine is monoethanolamine, diethanolamine, diethylethanolamine, phenylethanolamine, dimethylethanolamine, mono-, di- or triisopropanolamine, a mixture of two or more of these alkanolamines together, or a mixture of one or more óf these alkanolamines together with triethanolamine.

4. A composition according to any one of the preceding claims which includes 2 to 25 percent by weight of a polyacrylate.

5. A composition according to claim 4, wherein the polyacrylate has a molecular weight of from 2000 to 20,000 6. A composition according to claim 4 or claim 5 which includes from 2 to 12 percent by weight of polyacrylate.

7. A composition according to any one of the preceding claims which comprises from 10 to 50 percent by weight of orthophosphoric acid, from 10 to 20 percent by weight of water and from 40 to 80 percent by weight of alkanolamine.

8. A composition according to any one of the preceding claims which includes a minor proportion by weight of a scale mobiliser or dispersant.

9. A composition according to any one of the preceding claims which includes up to 25 percent by weight of a corrosion inhibitor for copper and up to 20 percent by weight of an alkali metal or alkaline earth metal hydroxide.

10. A composition according to any one of the preceding claims which includes up to 10 percent by weight of an antifoaming agent.

11. A composition according to claim 9, or claim 9 and claim 10. which comprises from 50 to 60 percent by weight of alkanolamine, from 19 to 25 percent by weight of orthophosphoric acid, from 3 to 6 percent by weight of polyacrylate, from 1 to 5 percent by weight of corrosion inhibitor for copper, and from 10 to 15 percent by weight of water.

12. A composition according to claim 1 and substantially as hereinbefore described in any one of the specific Examples.

13. A method of inhibiting the corrosion of steel articles contacted by water, which method comprises adding to the water a composition according to any one of the preceding

Claims (1)

1. claims.

   14. A method according to claim 13, wherein the composition is employed at a concentration of from 5 to 1000 parts by weight per million parts by volume of water.

   15. A method according to claim 13 or claim 14, wherein the pH of the water is from 7 to 10.5.

   16. A method according to any one of claims 13 to 15, wherein the water is in a re-circulating water system.

   17. A method according to any one of claims 13 to 15, wherein the water is in a once-through water system.