US3510351A - Method for etching and cleaning of objects and plants,particularly tube systems and boiler plants,consisting of iron or steel - Google Patents

Description

United States Patent 3 510 351 METHOD FOR ETCHIIIG iAND CLEANING OF OB- JECTS AND PLANTS, PARTICULARLY TUBE SYSTEMS AND BOILER PLANTS, CONSISTING OF IRON 0R STEEL Paul van Dillen, Vlaardingen, Netherlands, and Eva Vollmer, Hilden, Germany, assignors of one-half each to Siemens Aktiengeselischaft, Berlin, Germany, and Borg Service G.m.b.H., Dusseldorf, Germany, both corporations of Germany No Drawing. Filed Nov. 26, 1965, Ser, No. 510,105 Claims priority, application Germany, Nov. 27, 1964,

Int. Cl. B08!) 9/00; C02b /06; C23g 1/02 US. Cl. 134-22 6 Claims ABSTRACT OF THE DISCLOSURE Our invention relates to a method for etching and chemically cleaning objects and plants consisting of iron and/or steel, particularly tube systems and boiler plants.

As a rule, boiler plants must be etched before placing them in operation. The interior surfaces of the tube systems, aside from any boiler scale formed thereon, tend to rust because of atmospheric humidity. The welding and bending work done during assembling and installing causes scaling or soiling. Any such foreign substances must be removed to assure satisfactory boiler and turbine performance. This removal is done by the etching treatment. If the narrow boiler tubes are not etched or not cleaned in due time, they become clogged by iron sludge resulting from rust or scale. This may cause tearing or exploding of the tubes. Etching also affords a more rapid starting-up of the plant, because the steam purity required for turbine operation can be reached within a few hours, whereas without etching several months may be required before this steam quality is achieved.

It is further known from patents such as Heitmann et al., Patent No. 3,085,915, to apply chemical cleaning to boiler plants, tube systems, vessels, heat exchangers or other objects of iron and/or steel whose efficiency is impaired by coat formation or corrosion, the cleaning then permits more or less re-establishing the original operating qualities.

The etching or cleaning solution, usually composed of one or several acids with additions, is supposed to be inexpensive and to be so constituted that it does not subject the etched or cleaned tube inner surfaces to an attack greater than 20 g./m. This amount is the approximate permissible maximum of chemically removed material, regardless of whether the etching or cleaning operation is completed within a few hours or extended over several days. The additions to the etchant are preferably temperature resistant to the extent required for utilizing the favorable cleaning effect in the temperature range of 20 C. up to about 200 C.

It is known to employ etching or cleaning solutions having an acid content of 1% to 10% and auxiliary substances and inhibitors. The inhibitor serves to reduce the acid attack upon the pure metal without aifecting the dissolution rate, for example of the iron oxides. The

3,510,351 Patented May 5, 1970 inhibitor may consist, for example, of organic amines, unsaturated dioles, pure or substituted coke-tar products. It has also been proposed to use hydrazine or hydroxylamine condensation products as inhibitors. The manifold effects of these and other substances acting as inhibitors are known; these subtances, however, cannot be used in the entire pH- and temperature range, since, in some cases, the above-mentioned total corrosion limit of 20 g./m. can be preserved only within limited ranges.

According to the novel method of our invention, we employ etching solutions composed of mixtures of hydroxyl-group containing organic acids which, on the one hand, dissolve rust and scale and, on the other hand, maintain the dissolved iron as a complex compound in solution. We have found it particularly advantageous to use synergistic mixtures of known complex forming substances, among others for example, citric acid, hydroxyacetic acid, gluconic acid, heptagluconic acid, malonic acid derivative, tartaric acid, polyphosphates, ethylenediaminetetracetic acid, methyl tartronic acid or other organic acid, which may also be employed in the form of the ammonium salts or organic amines, with or without addition of mineral acids and/ or organic acids for adjusting to the desired pH value, particularly at temperatures at which the dissolution of metal oxides and other depositions takes place.

Since the synergistic mixtures result in a bonding ability for metal ions which is much larger than the sum of the individual components, the required quantities, relative to the equivalents, are smaller than the metal-oxide equivalents present. The total quantities of complex forming agents to be used may be successfully limited to a total of 2 to 5 g./l. (grams per liter). In previously operated plants, it is sometimes of advantage to make preliminary tests with tubes in which depositions of respectively different compositions are ascertainable, in order to thus determine a particularly favorable synergistic mixture for the particular plant.

For removing depositions particularly difficult to dissolve, it is sometimes necessary to employ concentrations of up to 5% and more. Furthermore, the required components of such mixtures may be added to the etching solution at a pH value most favorable to the particular depositions.

When utilizing the boiler feed pump for circulating the etching solution, concentrated synergistic mixtures may be injected into the plant shortly behind the feedwater pump.

With the method according to the invention the sludge formation is almost completely prevented. It is known as such that larger or smaller quantities of Fe O or Fe(OH) will precipitate from solutions having a relatively low content of complex forming agents (for example, citric acid and ethylenediaminetetracetic acid), depending upon time, temperature and pH value. With the known methods, however, the precipitated quantities can no longer be dissolved in the etching solution present. That is, the receptivity of the small amount of complexing agents in these solutions becomes exhausted after a certain limited time, whereafter large amounts of F6203 or Fe(OH) will precipitate. The resulting sludge must be drawn from the system from time to time. In contrast thereto, the use, according to the present invention, of a synergistic plural-component mixture of complex forming agents having a good etching effect and special complex forming agents for ferri-ions, results in precipitating as well as dissolving the sludge forming constituents.

In the presence of reducing substances, the predominant share of the dissolved iron is converted to the bivalent form. Only 1 mole of this bivalent iron (ferrous) can be complexed by 1 mole of citric acid, whereas 4 moles of the trivalent ion (ferric) are complexed. Other complex-forming agents, for example, methyltartronic acid, have a considerably more favorable complex-forming ability than citric acid. If oxidizing agents, such as nitrite, are added, the dissolved iron is present in trivalent form so that the complex-forming ability of hydroxylgroup containing organic acids is still further augmented.

Accordingly, a preferred mode of performing the method of the invention resides in using as etching solution, organic acids or mixtures of different organic acids capable of dissolving rust and scale and of maintaining the dissolved iron dissolved as a complex compound, the etching solution being constituted by hydroxyl-group containing organic acids or their salts and by combinations of several hydroxyl-group containing organic acids or their salts. In this manner, the complex-forming ability and the stability of the formed complexes are further augmented. Thus, for example, a combination of citric acid and methyl tartronic acid has a better complexforming ability than the same respective amounts of theindividual components of the combination. By further combination with other hydroxyl-group containing organic acids or their salts with additions of aminoacetic acids, for example, ethylenediaminetetracetic acid, the complex formation can be increased to a further extent.

By thus employing a mixture of different organic acids or their salts, the formation of iron hydroxide sludge can be almost fully prevented; and providing a virtually metallically bare surface which can be converted into a magnetite protective layer by subsequent hot-water operation.

The dissolution rate of rust and scale is essentially affected by the pH value of the etching solution. Dissolution still takes place in the alkaline range. Preferably however, the pH value is kept between 3 and 6. This can be done, particularly when using salts of the mentioned acid mixtures, by adding mineral acids, for ex ample, sulfuric acid, at localities within the circulation at which the pH value has risen too high.

For accelerating the reaction during dissolution of the iron oxides, the etching solution may be given an addition of fluorides. The dissolution rate of rust and scale can be additionally or exclusively increased simply by employing higher temperatures, for example, 100 C. and more.

Success and economy of an etching process largely depend upon the requirement that any resulting sludge will not clog the narrow tubes, for example, in the superheater or preheaters, and that such sludge will be completely eliminated. Since the method of the invention prevents the occurrence of any appreciable amount of sludge, a settling tank for sludge is no longer required.

The method of the invention substantially eliminates the disadvantages of the known etching and cleaning methods operating with mineral acids and organic acids, such as the necessity of operating with particularly acidresistant etchant-circulation pumps, switching the plant from normal operation to operation with such special etchant pumps, and using large quantities of acid and inhibitor additions. The precipitation of Fe (OH) at elevated temperatures and almost all pH values, usually occurring when using organic acids, is also virtually avoided.

Since the initially chosen optimal pH value may change with the progressing dissolution of coatings, it is sometimes advantageous to correct the pH value in a suitable part of the plant with the aid of organic acids and/or mineral acids, for example sulfuric acid. It can be ascertained from the metal content of the solution during etching, whether or not the initial dosage of complexforming agents is suflicient so that an excessive consumption of chemicals can be avoided.

This method of the invention removes metal oxides from the metal surface by locally selective dissolution and converts the oxides to a stable solution. During the etching or cleaning process, the layers of scale often splitting off at the beginning of the etching or cleaning operation, are virtually entirely dissolved due to the obtaining operating conditions of the plant, namely the effect of flow, temperature, pH value and solubility. The base metal is virtually not attacked. The removal of material from the surface, for example, for a non-alloyed carbon steel, at 75 C. and adjustment of the pH value with ammonia, using a synergistic mixture, is apparent from the following table:

pH-value: Removal q./m. h. 2.0 1.74 3.0 0.81 4.0 0.54 5.0 0.50 6.0 0.50 7.0 0.36 8.0 0.36 9.0 0.26 10.0 0.10

Particularly favorable results, as to the removal of material, can be achieved with the acid of mixtures of glycol acid, heptagluconic acid, malonic acid derivatives and ethylenediaminetetraacetic acid. For example, the following values are thus obtained:

pH-value: Removal gm. h. 2.0 2.8 3.0 0.9 4.0 0.4 5.0 0.3 6.0 0.2 7.0 0.2 8.0 0.2 9.0 0.01

The solubility of 1% etching and cleaning solutions for pure Fe O differs greatly. Thus, the following solutions applied at 100 C. for dissolutions of scale from a tube surface of 1 m. resulted in the amounts of dissolution apparent from the following table:

Removal No. Etchant pH-value (g.)

1 1% sulfuric acid 1. 8 86 2. 1% hydrochloric acid 1. 5 3 1% citric acid+NH4OH 3. 5 22 4- 0.5 dycol acid and 0.5% citr acid 5. 0 25 5 0.6% citric acid and 0.2% ethyleuediaminetetraacetic acid. 7. 5 58 6.. 1% synergistic mixture of glycol acid, heptagluconic acid, malonic acid derivitives and ethylenediaminetetraaeetic acid. 5. O 135 Upon completlon of the treatment with each of the etchants Nos. 3 to 6, the metal surface was excellently passivated.

It has ben found that, generally, a useful synergistic effect occurs only with mixtures of at least three component products here mentioned, and that such multicomponent mixtures result in excellent dissolution of iron oxide at pH values between 1.0 and 9.0. The process is preferably performed at temperatures between 50 C. and C. and a pH value between 4.0 and 8.0. This pH range of the etching and cleaning solution permits using pumps made of normal materials for feeding or circulating the etching and cleaning liquid, such as ordinary boiler feed pumps, booster pumps, condensate pumps or other pumps normally available in the plants.

The possibilities afforded by the invention greatly simplify the etching and cleaning operations in comparison with known processes requiring buffering of the etching solution to define prescribed pH values. The known corrosion danger of ferric-ions in mineral acids is in no way involved.

In some cases it is advantageous to perform the etching or cleaning process under increased pressure in order to permit operating at higher temperatures, for example. During etching, the pressure may be increased or decreased as may be desired.

When performing the method of the invention with a reducing medium, it may happen that upon saturation of the complex-forming agents with iron oxides, these oxides may precipitate. Relatively large amounts of iron hydroxides may then precipitate as sludge. Considerable quantities of rinsing Water are needed for eliminating such sludge. According to a further feature of the invention, such precipitation, however, can be avoided by providing for a sudden change in potential shortly prior to saturation.

To this end, an oxidizing agent, for example, alkali and/ or ammonia-nitrite, -nitrate, -hypochlorite, -chlorate,

etc. is added at a pH value at which the oxidation takes place. This has the effect that all ferrous-ions contained in the solution are converted to ferric-ions, whereby the molar ratio for the complex formation is displaced in favor of ferric-ions. Any precipitating iron hydroxides are then again forced to go into solution. In other words, when the complexing agent being employed has become saturated by bivalent iron so that precipitation of bivalent iron takes place, the precipitate is converted to Fe O which can again be dissolved by the complexing agent, since a portion of the precipitated bivalent iron compound is thus converted to the trivalent form which is forced to enter into the solution.

The potential of the synergistic mixture may be changed at a predetermined moment of the etching or cleaning operation by then adding suitable oxidizing agents in the pH range of 2 to 9 at corresponding temperatures. Then, with a constant Fe-content, selectively acting reduction or other reaction agents may also be added to the solution. This processing mode is likewise applicable with other etching and cleaning methods departing in other respects from the above-described method. Synergistic efiects have been ascertained also also in a higher pH range; and this may be utilized to advantage particularly where very thin coatings or special incrustations are involved. Media for increasing the pH value are, for example, NH alkali and similar substances.

Upon completion of the etching operation, the etching solution is preferably rinsed out of the etching system with the fully desalted water, if desired with an addition of hydrazine. After pre-rinsing, the etching circulation is preferably short-circuited, for example, through a device for desalting any condensate present in the plant, for the purpose of expediting the ultimate cleaning. The passivation may be performed after rinsing the system with fully desalted Water. This may be done, for exam- 6 ple, by operating the plant for about 30 hours with fully desalted water and an addition of hydrazine at temperatures above C., thus producing a magnetite protected layer on the inner surface of the tubes.

We claim:

1. In a process for etching and cleaning iron and steel tubes and boiler installations by treatment with a mixture of organic acid which dissolve rust and scale and hold the dissolved iron in solution in the form of a complex, the improvement which consists in using a mixture of glycol acid, heptagluconic acid, a malonic acid derivative, and ethylenediaminetetraacetic acid, at a pH of 4-8 at a temperature of 50-100 C.

2. Process according to claim 1, wherein the treatment is effected in the presence of an oxidizing agent which promotes complex-formation and is selected from the group which consists of alkali metal and ammonium nitrates, nitrites, hypochlorites, and chlorates.

3. Process according to claim 1, wherein a boiler installation is treated, by pumping an aqueous solution of the mixture of acids through the installation by means of a pump forming part of the installation.

4. Process according to claim 2, wherein a boiler installation is treated, by pumping an aqueous solution of the mixture of acids through the installation by means of a pump forming part of the installation.

5. Process according to claim 3, wherein the solution is pumped through the installation by means of the water feed pump.

6. Process according to claim 4, wherein the solution is pumped through the installation by means of the Water feed pump.

References Cited UNITED STATES PATENTS 2,318,559 5/1943 Percival. 2,817,606 12/1957 Barrett 134-22 3,085,915 4/1963 Heitmann et a1. 13422 3,132,975 5/1964 Freud 134-22 X 3,166,444 1/1965 Ehren et al 134--41 X 3,369,934 2/1968 Pollard 1343 1,554,483 9/1925 Bailey et a1. 134-28 2,362,284 11/ 1944 McDonald. 2,992,997 7/1961 Arden et al. 3,003,898 10/1961 Reich 25282 X 3,130,153 4/1964 Keller 252 X JOSEPH SCOVRONEK, Primary Examiner U.S. Cl. X.R.