GB2122598A - Processes and compositions for resisting corrosion - Google Patents

Description

SPECIFICATION Processes and compositions for resisting corrosion It is normal to include corrosion preventing additives in an aqueous liquid so as to provide protection from corrosion of metallic surfaces during their contact with the liquid or during storage after contact with the liquid. It is frequently necessary that the corrosion preventing additive should provide protection for a combination of metals. For instance in order to avoid corrosion in closed heating or cooling cycles, or in apparatus containing hydraulic fluids, it is necessary to protect steel and/or cast iron as well as aluminium, copper, brass, zinc and other metals and metal alloys. Corrosion-preventing additives must be readily soluble and stable as well as insensitive to high temperatures in water of varying hardness. They should give protection even in low doses. They should not be poisonous, polluting or irritating to the skin and should not have a strong odour. Various water-soluble treatment liquids are known for protecting individual metals from corrosion. For example, sodium nitrite or other alkaline solutions of nitrite are used for steel and grey cast iron. Organic substances such as benzoates, boric acid esters, soaps, salts of amino acids and of sulphonamido carboxylic acids are mentioned in the specialist literature. Acids are used predominantly as alkanol amine salts. European Patent No. 0 020 042 describes a corrosion prevention agent containing 5 to 20% by weight of a C8-20 aliphatic monobasic acid, 0 to 4% by weight of lubricant, 10 to 35% by weight of aromatic mono or polycarboxylic acid and an amine which forms a water-soluble salt with the aliphatic and aromatic acid. According to U.S.Patent No. 3 573 225, a corrosion-preventing agent containing 50 to 100 parts of a salt of a C6-18 saturated carboxylic acid with a C6-12 amine, 20 to 200 parts of alkali metal benzoate and 1 to 50 parts of alkanol amide is used as reaction product from ethanol amine and a C6-18 saturated fatty acid. German Offenlegungsschrift No. 26 14 234 discloses an aqueous treatment liquid for protecting metal surfaces from corrosion and which contains a reaction product of C6-10 aliphatic carboxylic acids and of polyhydroxycarboxylic acids such as gluconic acid or tartaric acid with an alkanol amine. Although these liquids for protecting metals from corrosion can reduce the corrosion of steel and cast iron, they do not satisfactorily protect other metals. For instance, aluminium generally requires treatment with chromates or silicates and these are stable in solution only at high pH and in soft water. Zinc requires chromates. U.S. Patent 4 093 780 proposes the use of esters of thioglycollic acid for preventing the formation of white rust, but these esters are unsatisfactory because of their odour and their low solubility in water. They are not suitable for protecting other metals. Corrosion protecting agents based on mercapto benzothiazole or benzotriazole are conventional for copper and brass. But they are less effective in mixtures for protecting steel. Comparatively little material about the protection of multi metal systems has been published. A review is given, for example, by J. Weber "Die Inhibierung der Korrosion in industriellen Kuhlysystemen" in Werkstoff und Korrosion 30 (1979), pages 713 to 722. According to this, only chromates are effective for combinations such as steel, cast iron, aluminium, copper, brass, zinc, but they cannot be used industrially due to their toxicity. Oil emulsions have been added to aqueous treatment liquids for protecting multi metal systems, for example in hydraulics or in the cooling of large combustion engines. However, they have many disadvantages which limit their use. The emulsion breaks down in hard water, the concentrate is inflammable and discharge is not possible without first separating the oil. The present invention relates to the discovery of a composition which is suitable, when in the form of a aqueous solution, for protecting metal surfaces from corrosion and in particular which is suitable for protecting a combination of metal surfaces from corrosion. Thus the composition generally used for protecting a combination including at least two, and usually more, metals selected from iron, aluminium, zinc, copper and brass. The composition comprises (a) at least one C6-10 aliphatic carboxylic acid, (b) at least one C6-8 polyhydroxycarboxylic acid and (c) at least one aromatic monocarboxylic acid, each of the acids being present as a salt with a salt-forming cation. These compositions avoid the more serious disadvantages discussed above and the defined composition surprisingly exhibits a synergistic effect since decidedly better corrosion prevention values are achieved than with one of the constituents alone or in combination with one other constituent. It is normally recommended to use organic acid corrosion inhibitors in the form of alkanolamine salts but we surprisingly find in the invention that best results are obtained ifthe salt forming cations are predominantly alkali ions, generaly sodium. Indeed to achieve optimum corrosion prevention, the content of ammonia ions should not exceed 20% of the total cation content. The molar ratio of the individual components can vary within wide limits and depends mainly on the metals to be protected but also on the concentration of the composition and the other process conditions. Preferably the molar ratio of (a) to (b) is 10:1 to 1:5, most preferably 4:1 to 1:2; the molar ratio of (a) to (c) is 5:1 to 1:5, most preferably 2:1 to 1:2, and the molar ratio of (b) to (c) is 5:1 to 1:10, most preferably 2:1 to 1:4. Particularly good results are achieved if the aqueous treatment liquid contains as (a) salt of caprylic acid and/or ethylhexanic acid, as (b) salt of gluconic acid and/or heptonic acid and as (c) salt of benzoic acid. A further improvement in properties can be achieved if the composition contains non-ionogenic surfactant having a turbidity point below 30[deg]C. Such surfactants are water-soluble at low temperatures. On reaching the turbidity point, the solutions begin to become turbid and the foaming of the surfactant solution is reduced considerably. The wetting of the metal surface is improved by these surfactants so thatthe treatment liquid can also act on moderately greasy or soiled parts. Polyglycol ethers of fatty alcohols, fatty amines and polyamines such as polyglycol esters of fatty acids, which have been reacted with ethylene oxide and propylene oxide, are particularly suitable. High molecular block polymers from polypropylene glycol condensed with polyethylene oxide and from polyethylene glycol condensed with propylene oxide have demonstrated the optimum effect. Corrosion inhibitors known per sefor individual metals can also be introduced into the compositions providing that they are compatible with each other in aqueous solution and do not impairthe corrosion prevention with other metals. Such corrosion inhibitors include, for example, boric acid esters, salts of higher carboxylic acids, salts of amino acids, salts of sulphonamido carboxylic acids and fatty acid alkanol amids. Compositions conaining mercaptobenzothiazole, benzotriazole and the derivatives thereof are particularly advantageous for the treatment of copper or brass. In order to protect the aqueous treatment liquids from an attack by micro-organisms, it is frequently advantageous to add suitable biocides such as phenol drivatives, formaldehyde-releasing compounds, triazines and quatenary ammonium compounds.

The compositions are used in the form of dilute aqueous solutions, generally containing the components (a), (b) and (c) in a total amount of 0.2 to 4%, and preferably 0.5 to 2% by weight, and generally having a pH between 7 and 10, preferably from 8 to 9. The treatment solution can be made merely by mixing into water the desired organic acids and alkalies (or organic acid salts) at temperatures generally between 40 and 90[deg]C. Alternatively, the composition can be initially produced as a solid salt mixture which is then dissolved in water. Preferably, however, the composition is initially produced as an aqueous liquid concentrate, generally containing the specified ingredients (a), (b) and (c) in a concentration of 30 to 80% by weight, and often having a total dissolved solids content of from 30 to 80% by weight. Other desired additives may be present in the concentrate. For instance it may contain from 0.5 to 10%, preferably 1 to 4% by weight of the specified surfactants. In the following examples, the corrosion protection effect of the treatment liquid according to the invention (examples 9 to 12) has been compared with individual components and with other compositions (examples 1 to 8). A modified method has been adopted for testing purposes of the type proposed in "Funften Bericht uber Anforderungen and Prufungen schwer entflammbarer Flussigkeiten zue hydraulischem Kraftubertragung und Steuerung", Luxembourg 1974, published by the Commission of the European Communities. Test plates (100 x 20 x 1 mm) of steel, aluminium, copper, brass (70/30) and zine were half immersed in water at 22.4[deg]dH (adjusted using calcium chloride) with addition of corrosion inhibitors and were stored in glass beakers covered with watch glass for 28 days at a temperature of 35[deg]C. After testing, the samples were rinsed, dried and the weight difference determined. The inhibition value of the additives was calculated by the following formula:

M1 .... removed material in water without addition M2 .... removed material in inhibited solution. The calculated inhibition values for test plates examined in water and in various treatment liquids are compiled in the following Table. The concentration of the individual corrosion inhibitors added are listed in column 3.

The results in the Table show that the compositions of the invention give good corrosion protection to a wide variety of metal combinations including iron, aluminium, zinc, copper, molybdenum and manganese, and the very beneficial results with the sodium salts in examples 9,10 and 11 are particularly surprising.

Claims (19)

1. A composition suitable, when in the form of an aqueous solution, for protecting metal surfaces from corrosion and which comprises (a) at least one C6-10 aliphatic carboxylic acid, (b) at least one C6 - 8 polyhydroxycarboxylic acid and (c) at least one aromatic monocarboxylic acid, and in which each of the acids is present as a salt with salt-forming cation.

2. A composition according to Claim 1 in which the salt forming cations are predominantly alkali ions.

3. A composition according to Claim 2 in which not more than 20% of the salt forming cations are ammonium.

4. The composition according to Claim 2 in which the alkali ions are sodium.

5. A composition according to any preceding Claim in which the molar ratio a:b is 10:1 to 1:5, the molar ratio axis 5:1 to 1:5 and the molar ratio b:c is 5:1 to 1:10.

6. A composition according to Claim in which the molar ratio a:b is 4:1 to 1:2, the molar ratio a:c is 2:1 to 1:2 and the molar ratio b:c is 2:1 to 1:4.

7. A composition according to the preceding claim in which component (a) is selected from caprylic and ethyl hexanic acids.

8. A composition according to any preceding claim in which component (b) is selected from gluconic and heptonic acids.

9. A composition according to any preceding Claim in which component (c) is benzoic acid.

10. A composition according to any preceding Claim including a non ionic surfactant having a turbidity point below 30[deg]C.

11. A composition according to any preceding claim in which the surfactant is a block polymer of propylene glycol with ethylene oxide and/or polyethylene glycol with propylene oxide.

12. A composition according to any preceding Claim in the form of an aqueous treatment solution containing 0.2 to 4% by weight of the components (a), (b) and (c).

13. A composition according to Claim 12 having a pH of 7 to 10.

14. A composition according to Claim 12 or Claim 13 containing 0.5 to 2% by weight ofthe components (a), (b) and (c), and having a pH of 8 to 9.

15. A composition according to any Claims 1 to 11 in the form of an aqueous concentrate containing from 30 to 80% by weight of the components (a), (b) and (c).

16. A method of protecting metal surfaces by contacting the surfaces with a composition according to any Claims 1 to 14 and that is in the form of an aqueous treatment solution.

17. A method according to Claim 16 in which the metal surfaces include a combination of metals selected from iron, aluminium, zinc, copper and brass.

18. A composition according to Claim 1 substantially as herein described with reference to any of the examples.

19. A method according to Claim 16 substantially as herein described with reference to any of the examples.