GB2026550A - Compositions for use in treating metal surfaces - Google Patents

Abstract

A solution which contains at least 25 g of oxalic acid per 100 ml of a solvent comprising N- methylpyrrolidone, dimethyl sulphoxide or N,N-dimethylformamide and, optionally, water. Such solutions are suitable for dilution with water to give compositions useful in cleaning rust from metal surfaces.

Description

SPECIFICATION Compositions for use in treating metal surfaces Oxalic acid is well known for its property of rapidly removing rust, rust stains and other iron-containing deposits at ambient temperatures. For example, it is known to use such solutions to clean oxidised iron brake dust which becomes impregnated into the painted surfaces of carriages and other railway rolling stock. The train is driven slowly through a spray system which covers the area concerned with the solution, and this processing is followed by water rinsing.

Relatively low concentrations of oxalic acid in the solution are usually effective. However, oxalic acid suffers from the disadvantage that it is a solid which is difficult to handle and to make up into operating solutions, particularly as it is poisonous. It would be preferable if a liquid product could be provided in concentrated form which could be diluted at the point of use. Other acids, such as hydrochloric acid, could be provided in concentrated liquid form but few are as effective as oxalic acid. Moreover, materials such as hydrochloric acid suffer from disadvantages such as fuming from the concentrated acid and a great tendency to promote corrosion.

Unfortunately, oxalic acid has a very limited solubility in water. Thus at 1 5 C its solubility in water is only 9.5 9/1 00my. It is known that its solubility is greater in alcohols and glycols, but the stability of such solutions is poor owing to chemical reaction, viz.

esterification, taking place between the compounds containing OH groups and the oxalic acid. It has therefore been possible to make reasonably concentrated stable solutions of oxalic acid for dilution at the point of use.

We have now found that concentrated (i.e. at least 259/100 ml) solutions of oxalic acid can be provided in certain organic solvents which are completely miscible with water. These solvents are N-methylpyrrolidone (NMP), dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF). NMP is preferred since it has the lowest toxicity of these solvents. By way of example, we have made stable solutions of oxalic acid at 40% w/w in NMP, 45% in DMSO and 38% in DMF, at ambient temperatures (18"C).

It will be appreciated that the solutions of this invention are more concentrated than previously known stable solutions of oxalic acid. Thus they are particularly suitablefortransportation and, owing to the miscibility of the solvent with water, can easily be diluted for use. The novel solutions have a low freezing point and this is an additional advantage.

After prolonged storage of the compositions described above, at-100C, no signal of crystallisation or freezing was observed.

For the purposes of transport, the solutions will generally be provided in a container having a vol ume of at least 1, usually at least 5, and preferably at least 10 litres. For use, the solutions of this invention will generally be diluted so that the concentration of oxalic acid is below 259/100 ml, e.g. from 1 to 5% w/v. For example, 2% w/v is suitable when the diluted composition is to be applied by spraying.

Additional components which may be included in the solutions of this invention include conventional sequestering agents, surfacta nts and inhibitors.

Suitable soluble sequestering agents include poly (allyl acetate maleic anhydride) and citric acid. Surfactants may aid the wetting and penetration of deposits, and inhibitors may prevent or decrease possible attack by the acid on the metal being treated. Further, the solutions may contain water as part of the formulation if desired, and other solvents which may aid the cleaning process can also be incorporated provided that such materials do not decrease the solubility of oxalic acid in the solution below the acceptable level of 25g/100ml.

According to a further aspect of this invention, a process for removing iron-containing deposits such as rust from a surface comprises contacting the surface with a solution of the invention after dilution with water. NMP, DMSO and DMF are all capable of dissolving the grease and oil which will usually be present, in addition to rust, on those surfaces which are suitable for treatment using the novel solutions.

1. A solution which contains at least 25 g of oxalic acid per 100 ml of a solvent comprising N-methylpyrrolidone, dimethyl sulphoxide or N,Ndimethylformamide.

2. A solution according to claim 1 in which the solvent comprises N-methylpyrrolidone.

3. A solution according to claim 1 or claim 2 in which the solvent additionally comprises water.

4. Asolution according to claim 1 substantially as herein described.

5. A container having a volume of at least one litre and containing a solution according to any preceding claim.

6. A solution according to any of claims 1 to 4 which has been diluted with water to an oxalic acid concentration of below 25 9/100 ml.

7. A solution according to claim 6 in which the concentration of oxalic acid is from 1 to 5 w/v.

8. A process for removing iron-containing deposits from a surface, which comprises contacting the surface with a solution according to claim 6 or claim 7.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Compositions for use in treating metal surfaces Oxalic acid is well known for its property of rapidly removing rust, rust stains and other iron-containing deposits at ambient temperatures. For example, it is known to use such solutions to clean oxidised iron brake dust which becomes impregnated into the painted surfaces of carriages and other railway rolling stock. The train is driven slowly through a spray system which covers the area concerned with the solution, and this processing is followed by water rinsing. Relatively low concentrations of oxalic acid in the solution are usually effective. However, oxalic acid suffers from the disadvantage that it is a solid which is difficult to handle and to make up into operating solutions, particularly as it is poisonous. It would be preferable if a liquid product could be provided in concentrated form which could be diluted at the point of use. Other acids, such as hydrochloric acid, could be provided in concentrated liquid form but few are as effective as oxalic acid. Moreover, materials such as hydrochloric acid suffer from disadvantages such as fuming from the concentrated acid and a great tendency to promote corrosion. Unfortunately, oxalic acid has a very limited solubility in water. Thus at 1 5 C its solubility in water is only 9.5 9/1 00my. It is known that its solubility is greater in alcohols and glycols, but the stability of such solutions is poor owing to chemical reaction, viz. esterification, taking place between the compounds containing OH groups and the oxalic acid. It has therefore been possible to make reasonably concentrated stable solutions of oxalic acid for dilution at the point of use. We have now found that concentrated (i.e. at least 259/100 ml) solutions of oxalic acid can be provided in certain organic solvents which are completely miscible with water. These solvents are N-methylpyrrolidone (NMP), dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF). NMP is preferred since it has the lowest toxicity of these solvents. By way of example, we have made stable solutions of oxalic acid at 40% w/w in NMP, 45% in DMSO and 38% in DMF, at ambient temperatures (18"C). It will be appreciated that the solutions of this invention are more concentrated than previously known stable solutions of oxalic acid. Thus they are particularly suitablefortransportation and, owing to the miscibility of the solvent with water, can easily be diluted for use. The novel solutions have a low freezing point and this is an additional advantage. After prolonged storage of the compositions described above, at-100C, no signal of crystallisation or freezing was observed. For the purposes of transport, the solutions will generally be provided in a container having a vol ume of at least 1, usually at least 5, and preferably at least 10 litres. For use, the solutions of this invention will generally be diluted so that the concentration of oxalic acid is below 259/100 ml, e.g. from 1 to 5% w/v. For example, 2% w/v is suitable when the diluted composition is to be applied by spraying. Additional components which may be included in the solutions of this invention include conventional sequestering agents, surfacta nts and inhibitors. Suitable soluble sequestering agents include poly (allyl acetate maleic anhydride) and citric acid. Surfactants may aid the wetting and penetration of deposits, and inhibitors may prevent or decrease possible attack by the acid on the metal being treated. Further, the solutions may contain water as part of the formulation if desired, and other solvents which may aid the cleaning process can also be incorporated provided that such materials do not decrease the solubility of oxalic acid in the solution below the acceptable level of 25g/100ml. According to a further aspect of this invention, a process for removing iron-containing deposits such as rust from a surface comprises contacting the surface with a solution of the invention after dilution with water. NMP, DMSO and DMF are all capable of dissolving the grease and oil which will usually be present, in addition to rust, on those surfaces which are suitable for treatment using the novel solutions. CLAIMS

1. A solution which contains at least 25 g of oxalic acid per 100 ml of a solvent comprising N-methylpyrrolidone, dimethyl sulphoxide or N,Ndimethylformamide.

2. A solution according to claim 1 in which the solvent comprises N-methylpyrrolidone.

3. A solution according to claim 1 or claim 2 in which the solvent additionally comprises water.

4. Asolution according to claim 1 substantially as herein described.

5. A container having a volume of at least one litre and containing a solution according to any preceding claim.

6. A solution according to any of claims 1 to 4 which has been diluted with water to an oxalic acid concentration of below 25 9/100 ml.

7. A solution according to claim 6 in which the concentration of oxalic acid is from 1 to 5 w/v.

8. A process for removing iron-containing deposits from a surface, which comprises contacting the surface with a solution according to claim 6 or claim 7.