IL25944A - Process for cleaning metal surfaces - Google Patents

Description

ISRAEL Patents Form No, 3 PATENTS AND DESIGNS ORDINANCE *77/66 SPECIFICATION "PROCESS FOR CLEANING METAL SURFACES" 1131)0 ·»ΠΒΕ ^Wl*? Τ»*7Πη WE, BCRG HOLDING A.G., a Swiss Body Corporate of Gubelstrasse 5» 2>ug, Switzerland, do hereby declare the nature of this invention and in what manner the same is to be performed, to This invention relates to the cleaning of, metal surfaces, particularly those of production plant and pipe systems such as boiler and refinery installations.

The surfaces of unused metal., plant are often contaminated with undesirable materials such as mill scale. The chemical removal of such materials is known generally as pro-service cleaning. In addition metal surfaces can become coated in use with imputities which can reduce the efficiency of operation* The removal of such impurities is -known generally as maintenance cleaning. The invention relates to both types of cleaning.

Hitherto most cleaning processes have been carried out using aqueous solutions of strong mfaeral acids such as sulphuric and hydrochloric acids.

Their use is attended by certain disadvantages in that while they attack scale deposits they also attack or corrode the metal, and in that they give off corrosive and irritating vapours. White it is , possible to inhibit the oorrosivity of such acids, . which is related to their low pH values (see Todtg Korrosion, Berlin 155,p.118), by adding corrosion inhibitors, for example urotropin or, at temperatures above kO°C, more expensive and complex inhibitors such as polyethylene oxide condensates, it is still not possible, to operate at high temperatures, e.g. above 90°C, without corrosion being a serious problem.

Also the presence of certain oxidising agents can cause an inhibitor to fall in its action with the result that the metal surface is seriously damaged by corrosion* A further disadvantage of the use of mineral acids is that they are active only up to certain pH values* As the pH rises with the progressive neutralisation of the acid and dissolution of the scale, so certain neutralisation products tend to precipitate out* For ·.· example, when fiydrochloric acid solutions ar*» ueed to clean iron articles, the ferric hydroxide formed tends to precipitate o at pH values above 3·5· Such a precipitate may accumulate in awkward places in the plant and its removal can cause problems and may require special treatment. To avoid this it is customary to operate with higher concentrations of acid to give reserves of acid sufficient to maintain a low pH but the cleaning process then becomes inefficient in terms of utilisation of the cleaning agent. Also the use of consistently low pH values means that corrosion remains a serious problem.

It has also been proposed to use organic carboxylic acids as pioltl-iag gents. Examples of such acids are acetic, citric, tartaric and glycollic acids. These acids can form complexes with the metal compounds formed during the cleaning process and the problem of precipitation of compounds such as ferric hydroxide thus does not normally arise. For this reason it is not necessary to provide for reserves of organic acids and the cleaning process con to this extent be regarded as more efficient. In addition these organic acids are less corrosive to metals, as reflected by their pk values or dissociation constants, and they arc consequently less difficult to inhibit than are mineral acids. However, such organic acids are expensive, are not always readily available and some of them than mineral acids have higher molecular weights/so that large quantities of them may be needed.

Cther non-oxidising inorganic acids than mineral acids have been used, chief among which is sulphamic acid. Such acids can have the advantage that they corrode metals less severely and are non-volatile and non-toxic. However, their molecular weights or equivalent weights are often high and this may mean that larger quantities of them are required than of hydrochloric acid. For example, hydrochloric acid in a concentration of lie-will dissolve almost three times as much scale as the same concentration of sulphamic acid under the same conditions. This is despite the fact that the utilisation of hydrochloric acid is only about 60% complete (as the hydrochloric acid requires a reserve concentration to prevent precipitation of ferric hydroxide at pH values above 3·5)? whereas that of the sulphamic acid is about ?S% complete. The greater utilisation of sulphamic acid is due to the formation of complexes, such as( NE^SO^O and even, in the alkaline region, NaO-SC^- H^ Fe, most of which NaO-S02-NH ^ do not precipitate. However, the thermal stability of these complexes is not great and if a solution containing them is boiled they dissociate into metal hydroxides and free sulphamic acid. Since the easiest and most convenient way to descale a boiler is to use it G a boiler and boil the cleaning solution this lack of stability is a serious disadvantage. vJo have now found that by using sulphamic acid in association with certain aliphatic hydroxycarboxylic acids it is possible to obtain cleaning solutions which not only exhibit synergism in their ability to dissolve metal oxide scales but also do not precipitate insoluble metal hydroxides on boiling.

The present invention provides a process for treating metal surfaces to remove surface contamination which comprises applying t* than an aqueous solution of a synergistic mixture of a sulphamic acid or a salt or ester thereof and an aliphatic hydroxycarboxylic acid, preferably the solution compriSSs 0.1 to 10>t by volume of sulphamic acid and from 0.1 to 10% by volume of hydroxycarboxylic acid.

The preferred sulphamic acid is sulphamic acid itself and the preferred hydroxycarboxylic acid is glycollic acid. These acids are preferably combined in the proportions -0 to 9Co by weight of sulphamic and 60 to 10? by weight of glycoljict especially 50 to 80?i by weight of sulphamic and 50 to 20 by weight of glycol<¾xc acids. The synergistic effect of these two acids can be se from the following results of on experiment measuring the weight percentage of 1 gram of mill scale dissolved over 22 hours at 90°C. by cleaning solutions of 3% total concentration of different proportions of the acids:-sulphamic acid acid % scale in % by weight in % by weight dissolved 0 3.0 20 0.6 2 36 1.2 1.8 56 1.8 1.2 75 2 0.6 85 3.0 0 86 These values are completely reproducible. When plotted on a graph it can be seen that the curve is well ibove the straight line connecting the values obtained with 3;. glycollic acid and with 3 of sulphamic acid and which represents the theoretical effect of combining the two acids in different proportions. The mixture thus exhibits synergism. The synergistic effect is not to be found with mixtures of sulphamic acid with other acids v/hich have been mentioned as possible descaling agents. Thus the values for the percentage scale dissolved by a 3 solution of mixtures in different proportions of acetic acid and sulphamic acid when plotted on a graph show very little departure from the straight line connecting the values obtained with acetic acid alone and with sulphamic acid alone. Also not all hydroxy- carboxylic acid/sulphamic acid mixtures will exhibit synergism. Thus mixtures of citric, lactic, and gluconic acids alone with sulphamic acid do not.

Other synergistic mixtures which may be used in the process of the invention can be obtained by replacing some of the glycollic acid in the glycollic acid/sulphamic acid mixture with other hydroxycarboxylic acids. Thus the glycollic acid may be replaced by glycollic acid/citric acid mixtures, for example containing 30>.- to 7Q- of citric acid and mixtures comprising 10 to 25% by weight glycollic acid, 10?t by weight gluconic acid, 70 to 55¾·· by weight citric acid and 10 ethylenediamine tetra- acetic acid.

Another synergistic mixture which may be used comprises sulphamic acid and methyltartronic acid. These acids are preferably combined in the proportions 20-95¾, especially 70-9 ;t methyltartronic and 80-5^, especially 30-10?o, sulphamic acid.

The synergistic effect of these two acids can be seen from the following results of an experiment measuring the weight percentage of 1 gram of a mill scale dissolved over 2k hours at 100°C. by cleaning solutions of total concentration of different proportions of the acids:-sulphamic methyltartronic Tnethyiteii-brenTC acid SEKbpfcxajJOcacid % of scale in % by weight in ;'. by weight dissolved 0 3.0 8 0.6 2 32 1.2 1.8 k6 1.8 1.2 57 2 0.6 82 3.0 0 8k The synergistic effect is also accompanied by an improved thermostability of the products of cleaning · When used cleaning solutions of sulphamic acid alone or glycollic acid alone are boiled metal hydroxides eventually settle out, but with the synergistic mixtures used in the process of the invention this does not happen and it is possible to boil the cleaning solution until it is exhausted without precipitation occurring. Since the cleaning time decreases as the temperature is increased, this means that by using the synergistic mixtures of the invention at temperatures of 100°C. to 150°C. or even above, it is possible to carry out a cleaning operation more quickly and hence put an installation &ack into operation more quickly than has previously been practicable.

If the amount of scale or oxide deposit to be removed is known at the outset, the calculated amount of cleaning solution can be used and boiling continued until the cleaning solution is exhausted without risk of corrosion, that is to say that a boiler installation may be cleaned with the cleaning solution without recourse to the lower temperatures associated with the use of hydrochloric acid. Vhen the cleaning operation is performed at the boil, normal circulation of the boiler contents is set up in the boiler installation. When hydrochloric acid is used, cleaning at the boil is out of the question because the previously known inhibitors are incapable of inhibiting the corrosion sufficiently for most requirements at these high temperatures. To achieve circulation with hydrochloric acid, acid inlets are provided to the boiler parts to be cleaned and, by means of pumps resistant to the cleaning solution, a circulation of the cleaning solution is set up. All these expedients which are required with hydrochloric (and also sulphuric) acid cleaning can be dispensed with when cleaning is performed at the boil with one of the synergistic mixtures. The saving in expenditure attributable to the expedients which can now be dispensed with is often half the cost of the operation as a whole.

The pH values of cleaning solutions comprising the synergistic mixtures of the invention are initially relatively low; for example a mixture containing, per litre, 2 g of sulphamic acid and 6g of glycollic acid e.s a pH of 1.5» but as the scale is dissolved, the pH value rises along reproducible curves. This shows the dependence of the pH value on the iron content, that is to say that the progress of the descaling operation may readily be followed either by titration with an alkali metal hydroxide solution and indicator, the iron content being determined by one of the known methods, or, as a result of the distinct and reproducible dependence on pH, by continuous pH measuring. The actual performance of the process is considerably facilitated as is its control so that it can be carried out by less experienced personnel.

Vihile plant still contains much scale or rust (that is to say during the initial stages of the process) the pH value is low, for example 1.5, but as dissolution progresses the pH value rises gradually to values higher than the 3«5 maximum of hydrochloric acid solutions and the solution loses its corrosivity while remaining active. In other words, by the time the base metal is reached, the bath has generally become weak and non-corrosive. Serious damage, as may be caused by hydrochloric acid with slack supervision, cannot be caused in such a case. When working on deposits of irregular thickness, acid concentration is still high, and while in other areas there is still scale present, it is desirable to incorporate an inhibitor or sequestering agent or both which may be of any known type, such as ethylene-diamine tetraacetic acid, as a precaution against corrosion.

Other additives can also be incorporated in the cleaning solutions used in the process of the invention. Thus it is possible to add fluorides, for example ammonium bifluoride (which is preferred), ammonium fluoride or sodium fluoride, to enhance the solubility of the scale and to dissolve silicates. Still other additives which may be used include hydrazine and or hydroxylamine which can be added at the end of the pickling process. The cleaning process of the present invention is not dependent on pH values to the same extent as the known process using hydrochloric acid and it is possible to operate in the wide range of pH values of from 0.5 to 9» preferably 1.5 to 6. Surprisingly it has been found that when using a certain amount of a synergistic mixture in accordance with the invention the cleaning bath does not become permanently exhausted when a high pH is reached and that it is possible by adding an acid such as sulphuric to reduce the pH, for example from 6 to P.S low as 1.5t restore the cleaning properties of the solution without causing precipitation or undue corrosion. This means that the amounts of expensive ingredients in cleaning solutions need not be proportional to the amount of scale dissolved.

The plenning solutions of this invention are non-toxic, when exhausted are not necessarily acidic, are substantially non-corrosive and contain no volatile ingredients. In contrast to known plaaning baths containing mineral acids the new cleaning solutions are easy to dilute and drain. Effluent problems as known with mineral acids no longer arise.

The invention is illustrated by the following Examples.

SiAHPLa 1 A rippled deposit has formed during operation in a ^00 t/h power station boiler. It consists of about 90 magnetite and about ^P/o copper oxide with the remainder made up of silica and phosphorus pentoxide.

The deposit can be removed within hours at a temperature of 90°C. with a cleaning solution made up as follows: l6 g/1 sulphamic acid g/1 glycollic acid j5 g/1 ammonium bifluoride 1 ·5 g/l piperidine base Following treatment with this solution, a sample pipe cut out was clean and free from any copper deposit.

EXAMPLE 2 A sectional boiler has become coated with a mixture of Fe,0i, and Fe_0, , mixed and attached in some places to 20 copper oxide. Silicates are also present. A coating of this type is completely dissolved within 5 hours at 100°C. with a cleaning solution of: ?:'. sulphamic acid ?. by volume of 56. '· glycollic acid 2, sodium fluoride 0.2%- dibenzyl sulfoxide in water Small weighed sheets made of the same material as the boiler and boiled inside it with the cleaning solution showed a corrosion figure which amounted to 11 gm per sq. m.

EXAMPLE 3 Stainless steel sample pipes were taken from a newly set up atomic reactor and pickled for 5 hours at 60°C. The cleaning solution has the following composition: Nitric acid 6% by volume; sulphamic acid 1% by weight; 'ό by weight of a synergistic mixture of ethylenediamine tetra-acetic acid (10%), citric acid (?0/ό), gluconic acid (1C%), and glycollic acid (10 ); and hydrofluoric acid. After treatment the pipes are clean and passive.

For atomic reactors, for example, where it is customary to use austenitic grades of steel, the synergistic mixtures of sulphamic acid and hydroxycarboxylic acid can be used together in a multi-component system without fear of corrosion resulting from adding oxidising mineral acids, e.g. nitric acid in an amount of up to 15 by volume. Even these mixtures, containing highly corrosive individual components, are harmless to ferritic steels and can be used at temperatures of up to 250°C. Here again efficient, 3afe cleaning operations con be carried out with cheap mixtures of individual components which by themselves are much less suitable.

Claims (3)

1. HAVING NOW particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is: 1. A process for treating metal surfaces to remove surface scale contamination which comprises applying to them an aqueous solution of a synergistic mixture of a sulphamic acid or a salt or ester thereof and an aliphatic hydroxycarboxylic acid.
2. 2. A process as claimed in claim 1, wherein the solution comprises 0.1 to 10 by volume of sulphamic acid and from 0.1 to 10# by volume of hydroxycarboxylic acid.
3. 3. A process according to claim 1 or 2, wherein the metal surfaces are treated with a solution at a temperature of 90°C. or above. k, A process according to claim 3» wherein the temperature of the solution is 100 to 150° C. 5. A process according to any one of the preceding claims wherein the solution comprises also a corrosion inhibitor or sequestering agent or both. 6. A process according to any one of the preceding claims wherein the aqueous solution comprises sulphamic acid itself. 7* A process according to any one of the preceding claims, wherein the aliphatic hydroxycarboxylic acid is glycollie acid. 8. A process according to. claim.7i wherein the aqueous solution comprises gl collie acid and sulphamic acid in a weight ratio of from' 2/3:1 to 9:1. 9. A process according to claim 8, wherein the weight 10. A process according to claim 7» 8 or 9, wherein the aqueous solution comprises from 0.1 to 10# by volume of a mixture comprising (a) 10# by weight of ethylenediamine tetra-acetic acid, (b) 10 to Z% by glycollic acid, (c) 10 of gluconic acid and (d) 70 to 55# of citric acid. 11. A process according to any one of claims 1 to 6, wherein the aliphatic hydroxycarboxylic acid is methyltartronic acid, 12. A process according to claim 11, wherein the aqueous solution comprises methyltartronic acid and sulphamic acid in a weight ratio of l/k:l to 19:1. 13. A process according to claim 12, wherein the weight ratio is from to 9:1. l*t. A process as claimed in any one of the preceding claims, wherein the aqueous solution is at least partially regenerated by adding an acid to reduce the pH value. . 15. A process according to claim 14, wherein when the pH of the aqueous solution has reached a value of 6 sulphuric aoid is added to reduce the pH to a value as low as 1.5. 16. A process as claimed in any one of the preceding claims wherein fluoride ions are present in the aqueous solution. 17. A process as claimed in any one of the preceding claims wherein the fluoride ions are derived from hydrogen fluoride or sodium or ammonium fluoride or ammonium bifluoride. 18. A process according to claim 1, substantially as hereinbefore described. 19· · Metal plant when treated by the process claimed claims 1 to 18» DATED THIS 6th Day of June , 1966.