US3265597A

US3265597A - Anodizing process and electrolyte

Info

Publication number: US3265597A
Application number: US293920A
Authority: US
Inventors: Neunzig Heinrich; Rohrig Veit
Original assignee: Vereinigte Aluminium Werke AG
Current assignee: Vereinigte Aluminium Werke AG
Priority date: 1962-06-16
Filing date: 1963-06-04
Publication date: 1966-08-09
Anticipated expiration: 1983-08-09
Also published as: LU43691A1; NL121366C; BE632655A; DE1178272B; NL293256A; DK102887C; GB973391A

Description

layer.

United States Patent 11 Claims. in. 2a4 ss The present invention relates to an anodizing process and, more particularly, to a method of producing a colored oxide layer on an aluminum-containing metal body.

In recent times it has frequently been found desirable to use as building elements, or as sheathing for the outer Walls of buildings, aluminum or aluminum alloy bodies which should not reflect a substantial amount of light and preferably should be of a color which may range from the color of dull old silver to the color of smoke or to deep dark brown and even black. The color of the aluminum or the aluminum alloy element must be capable of withstanding atmospheric influences and, furthermore, the surface of the metal elements must possess a sufficiently high corrosion resistance so as to be able to withstand the chemical and physical attacks to which, for instance, outer building walls and the like are exposed. For this reason, it is desirable to form a protective surface layer on the aluminum or aluminum alloy body or structural element, preferably by anodic oxidation.

Conventionally, such metal elements formed of aluminum or aluminum alloys are subjected to anodic oxidation, for instance, in accordance with the direct current-sulfuric acid method, and the thus formed oxide layer is then dyed with inorganic or organic dyestuffs or by application of chromogeni-c compounds.

It also has been suggested to produce a colored surface by carrying out anodic oxidation of the aluminum or aluminum-alloy body with the same immersed in certain electrolytes which will cause formation of a colored oxide The color of such anodic oxide layers is also influenced by the composition of the aluminum, i.e., its purity, or the alloy composition, and generally is sufficiently light-fast and weather resistant for the intended purpose. Thus, it has been proposed to carry out the anodic oxidation of aluminum and aluminum alloys in a bath of oxalic acid and in this manner to produce oxide layers having a yellowish to brownish-yellow color. It also has been reported that anodic oxide coatings on aluminum and aluminum' alloy bodies which were produced in a malonic acid bath will generally have a dark ocher or brown color.

According to another method, it has been proposed to achieve the desired dull coloration of the aluminum or aluminum alloy body by anodic oxidation of the same in an electrolyte which consists of an aqueous solution of sulfosalicyclic acid with additions of sulfuric acid or metal sulfates. In this case, it has been found desirable to use concentrations of between 5 and 50% sulfosalicyclic acid and between 0.1 percent and 15% sulfuric acid, or an equivalent proportion of a metal sulfate.

However, the last mentioned method causes a rather strong development of hydrogen gas which also affects the electrolyte since after proceeding with the anodic oxidation for only a short period of time, a change in the color tone is observed and, furthermore, the color layer becomes "ice less and less uniform. Only by introducing additional amounts of sulfosalicyclic acid has it been possible to obtain again the same color tone as was obtained at the start of the anodic oxidation, and a short while after introduction of the additional amount of sulfosalicyclic acid there will be again a change in color tone and lesser uniformity of the color layer. Thus, this method does not possess the desired reliability.

A further disadvantage of the last mentioned method is the fact that at the generally required current densities of at least between about 3 and 4 amperes per one hundred cm. the final voltage will be very high, in fact, up to about volts.

It is therefore an object of the present invention to overcome the above discussed difficulties and disadvantages of prior art methods for the production of colored oxide layers on aluminum and aluminum alloy bodies.

It is another object of the present invention to provide a method which permits in a simple and economical manner to produce colored anodic oxide layers which will be stable and of high uniformity.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention contemplates a method of producing a colored oxide layer on an aluminum-containing metal body, comprising the step of subjecting the metal body to anodic oxidation in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and metal sulfates, sulfosalicyclic acid, and at least one substance selected from the group consisting of maleic acid and maleic acid anhydride.

According to the present invention, the anodic treatment or anodic oxidation of the aluminum or aluminum alloy body is carried out in an electrolyte which consists of an aqueous solution of sulfosalicyclic acid, maleic acid and/or maleic acid anhydride and which also contains either sulfuric acid or a water soluble metal sulfate.

Preferably, the concentration of sulfosalicyclic acid in the aqueous electrolyte solution will be between 1 and 5% and most preferably between 3 and 5%, the concentration of the maleic acid and or maleic acid anhydride between 0.5 and 1.5% and most preferably about 1%, and sulfuric acid and/ or the metal sulfate will be present in an amount equivalent to a concentration of between 0.1 and 1% most preferably about 0.5% of sulfuric acid.

The metal sulfate which fully or in part may replace the sulfuric acid of the electrolyte Will be chosen from those metal sulfates which are water soluble. Furthermore, metal sulfates which contain a metal which in the presence of aluminum may be precipitated from the solution should not be introduced since such metal sulfates would give cause to local corrosion and an uneven color tone. Of the easily available water soluble metal sulfates, preferably manganese sulfate, iron sulfate and zinc sulfate may be advantageously used. It is also possible to operate in accordance with the present invention with an electrolyte containing aluminum sulfate, provided that the concentration of the total aluminum dissolved in the electrolyte will not exceed between about 2.5 and 3 grams per liter. The concentration of up to 2.5 and 3 grams per liter is to include the aluminum which is introduced in the form of aluminum sulfate as Well as the aluminum which during anodic oxidation of the aluminum or the aluminum alloy body may be dissolved by the electrolyte.

In addition to this special limitation of the concentration of aluminum in the electrolyte solution, the concentration of sulfate ions in the electrolyte solution should be maintained so as to correspond to the above-mentioned limits of sulfuric acid concentration.

Generally, the replacement of sulfuric acid in the electrolyte with a suitable metal sulfate, for instance manganese sulfate, will not affect the color of the anodic oxide layer.

In order to avoid an undesirable increase in the concentration of the metal ions in the electrolyte solution, particularly if anodic oxidation is carried out for a prolonged period of time, it is of course possible in a manner known per se to control the concentration of the metal ions by means of a suitable cation exchanger, for instance, by repeated passage of the electrolyte solution through such exchanger, or by continuous circulation of the electrolyte therethrough, and in this manner to maintain the desired concentration of metal ions in the electrolyte bath notwithstanding the dissolution of metal from the aluminum or aluminum alloy body.

Anodic oxidation according to the present invention is preferably carried out at ambient temperature, i.e., a temperature of between about 15 and 25 C. However, it is also possible that in consequence of a quick passage of the aluminum or aluminum alloy bodies exposed to anodic oxidation or higher temperature of the room, the temperature of the electrolyte may increase to, for instance, 30 or 35 C. or even up to about 40 C. On the other hand, at temperatures considerably below 15 C. the time required for obtaining the desired anodic oxide layer would be prolonged and thereby the process would become more and more uneconomical.

It is desirable to operate at an electrolyte temperature of below 40 C. or at least a temperature not exceeding 40 C. Higher temperatures than 40 C. are undesirable because the anodic oxide layer formed at such higher temperatures will be of lesser adherence to the aluminum or aluminum alloy body, could be more easily rubbed off, and, generally, an anodic oxide layer formed at higher temperatures will be less resistant to chemical and mechanical attacks. The color tone will tend to become somewhat lighter with increasing electrolyte temperatures.

It is therefore broadly preferred to carry out the anodic oxidation within a temperature range of between about and 40 C. and most preferably within a temperature range of between and 25 C.

At a bath temperature of about C., anodic oxidation according to the present invention can be carried out advantageously at a current density of between 1 and 5, preferably between 1.5 and 3 amperes per 100 cm. Thereby, a potential of between 30 and up to a maximum of about 70 volts will be reached.

The electrolyte according to the present invention differs qualitatively from the electrolytes of the prior art discussed hereinabove by containing maleic acid or maleic acid anhydride. The present invention is preferably carried out at relatively low concentrations of sulfosalicylic acid and sulfuric acid or metal sulfate. The desired dull old silver, smoky or deep-dark brown to black colors can be obtained on aluminum or aluminum alloy bodies in accordance with the present invention, whereby the specific color tone at a given bath temperature and otherwise equal operating conditions will depend on the composition of the aluminum or aluminum alloy body. Even when carrying out anodic oxidation according to the present invention with a bath used already for a longer time, the color layer obtained thereby will remain uniformly the same as the color layer obtained by use of a freshly prepared electrolytic bath. No hydrogen gas-formation Will 4 be observed in carrying out the anodic oxidation according to the present invention.

Furthermore, surprisingly, it has been found that when proceeding in accordance with the present invention it is possible to obtain colorations of the aluminum or aluminum alloy body which are similar to those obtained with prior art methods, however, by forming an anodic'oxide layer which need to be only about half as thick as the anodic oxide layers required according to these prior art methods. Deep dark to deep black colors are obtained according to the present invention by forming anodic oxide layers having a thickness not exceeding 30 millimicrons, and, on the other hand, by forming in accordance with the present invention a thicker layer, i.e., anodic oxide layers having a thickness of more than 30 millimicrons, no further deepening of the color will take place. It follows that, apart from the reliability of the present process and the other advantages of the same, generally it will suflice to produce anodic oxide layers having a thickness of 10 to 30 millimicrons, particularly since such layers are sufficiently resistant against chemical corrosion. By forming such oxide layers, the further advantage is achieved that relatively high end voltages are avoided.

However, it is also possible to produce thicker layers, for instance of 40 to millimicrons on bodies of aluminum or aluminum alloys of higher electrical conductivity as for instance of pure aluminum or aluminum magnesium alloy, but this often has the disadvantage of too high end voltages.

Furthermore, and generally contrary to the conditions which prevail in the above-discussed prior art methods Which usually require current densities considerably higher than 1.5 amperes per cm. it is possible according to the method of the present invention in many cases to obtain a deep dark or even a black oxide layer with current densities as low as one ampere per 100 cm. or preferably about 1.5 amperes per 100 cm. Direct or alternating current may be used.

In a general manner, the current density is primarily dependent on the quality of the aluminum or aluminum alloy. Certain aluminum alloys are of higher conductivity than others. Those of lesser conductivity are preferably treated at lower current densities, since otherwise the final voltage would be too high and this would lead to localized excessive load conditions. Furthermore, on weakened portions of the aluminum or alloy body it could then happen that local corrosions and uneven color tones would occur. By using a lesser current density, the length of time for which the aluminum body is subjected to anodic oxidation must be correspondingly increased, as described in some of the examples following further below. On the other hand, aluminum alloys which are of higher electric conductivity may be anodized at greater current densities, such as for instance, between 3 and 4 amperes per 100 cm. The foregoing holds also true for unalloyed pure aluminum. Aluminum alloys of relatively low conductivity are preferably electrolytically oxidized at a current density of between 1 and up to at most 2 amperes per 100 cm. The foregoing presumes that the oxide layer formed by anodic oxidation is to be of a thickness of at least about 20 millimicrons and at most 30 millimicrons. Only in the case of metal bodies of very high conductivity it may also be desired to form an oxide layer having a thickness of about 40 millimicrons.

The examples following hereinbelow are given as illustrative only, of the present invention, without, however, limiting the invention to the specific details of the examples.

Examples 120 are summarized in Table I. The description of the aluminum or aluminum alloy composition in Table I corresponds to that in the German ofiicial standards DIN 1712, Sheet 1, edition of 1961 and DIN 1725, Sheet 1, edition of 1961. These standards are de scribed in Tables III and IV further below.

TABLE I.-ANODIC OXIDATION OF ALUMINUM AND ALUMINUM ALLOYS AT 20 C.

Current Thickdensity, Treating Final ness of Ex. Material Shape Electrolyte, percent by weight, balance water amp/100 period, voltage, Oxide Appearance cm. minutes volts Layer,

1 AlMg3 Sheet 5% sllllfosaligylic acid, 0.5% sulfuric acid, 1% 2 30 32-40 20 Yellowish to silver.

ma eic aci 2 AlMg3 3 30 34-61 30 Darlobrown. 3 AlMgSi0.5 1. 5 40 36-43 20 Yellowish to old silver. 4 AlMgSiOfi-.. 3 20 41-52 20 solmewhat darker than 5 AlMgSi0.5 3 30 43-64 30 Dark-brown to black. 6 AlMgSil- 1. 5 40 40-62 20 Deep-black. 7 AlMg3 3 30 30 Dark-brown.

maleic acid. 8 AlMg3 do 1% sulfosalicylic acid, 0. sulfuric acid, 1% 3 30 30 Light-bronze.

maleic acid. 9 do do 5% sulfosalicylic acid, 0.75% manganese sul- 3 30 30 Dark-brown.

fate 1% maleic acid. 10 d0 do 5% sulfosalicylic acid, 0.6% aluminum sulfate, 3 30 30 Do.

1% maleic acid. 11 do do 5% sulfosalicylic acid, 0.1% sulfuric acid, 1% 3 30 30 Brownish-grey.

maleic acid. 12 do do 5% sulfosalicylic acid, 0.5% sulfuric acid, 0.5% 3 30 30 Light-brown.

maleic acid. 13 do d0 5% sulfpsalicylic acid, 0.5% sulfuric acid, 1.5% 3 30 30 Deep-dark brown.

* ma 910 8.01 14 do do 1% sulfosalicylic acid, 0.1% sulfuric acid, 0.5% 3 30 30 Light-grey.

maleic acid. 15 do do 5% suliosalicylic acid, 0.25% sulfuric acid, 3 30 30 Dark-gbrown.

0.3% aluminum sulfate, 1% maleic acid. 16 do do 5% sulfosalicylic acid, 0.5% sulfuric acid, 1% 3 30 30 D0.

maleic acid anhydrite. 17 do. do 5% sulfosalicvlic acid, 0.5% sulfuric acid, 0.5% 3 30 30 Do.

maleic acid, 0.5% maleic acid anhydrite. 18 A1999 d0 5% sulfosalicylic acid, 0.5% sulfuric acid, 1% 3 30 30 Do.

maleic acid. 19 AlMn d0 .d0 3 30 30 Nearly-black. 20 AlMgl d0 do 3 30 30 Light-bronze.

serve to illustrate the effects of the temperature of the electrolytic bath on the color of the anodic oxide layer.

Example 14 shows the results obtained at substantially the lower limit of concentration of all components of the TABLE II.-APPEARANCE OF ALUMINUM ALLOY BODY ANODIZED AT DIFFERENT TEMPERATURES Current Treating Thick- Tempera- Ex. Material Shape Electrolyte, percent by weight, balance density, period, ness of turc, 0. Appearance water amp. /100 minutes Oxide 0111. Layer,

AlMg3 Sheet 5% sulfosalicylic acid, 0.5% sulfuric acid, 3 30 30 10 Dark-brown.

1% maleic acid. d0 3 30 30 15 Do. do 3 30 30 Do.

do 3 30 30 Light-brown. -..-d0 8 30 30 New silver.

Generally, all aluminum alloys which may be anodically oxidized are also capable of being treated in accordance with the present invention. However, aluminum alloys which contain more than 1% copper should be exelectrolytic bath. In this manner a rather light colored surface is obtained. It is not advisable to reduce the concentration of the constituents of the electrolytic bath below those given in Example 14, since at lower concentrations the body surface does not become dark and at the same time an attack at the grain boundaries of the material is to be seen.

The upper limit of concentration of the individual components of the electrolytic bath may be somewhat higher than given in the examples in Table I. Thus, for instance it is possible to operate with higher concentrations than 5% of sulfosalicylic acid. However, for economic reasons, it is undesirable to increase the consumption of this relatively expensive acid. It also would be possible to increase the concentration of maleic acid, for instance to about 2%, without affecting the color tone. However, depending on the overall conditions of concentration in the bath, there will be a certain danger that at concentrations higher than 1.5% maleic acid may crystallize and this would result in an uneven coloration of the metal body.

Examples 21-25 which are summarized in Table II will eluded since due to the strong IC-dlSSOllltlOll which occurs with such alloys of high copper content, it is not possible to obtain a continuous colored oxide layer.

Since the method of the present invention is primarily used for the treatment of aluminum and aluminum alloy bodies for architectural purposes, such aluminum and aluminum alloy bodies are of primary interest which may be anodized so as to achieve a decorative effect thereby. These alloys include in addition to AlMg3, AlMgSi0.5 and AlMgSil which were used in the examples, also alloys such as AlMgl and AlMgZ. Furthermore, also AlMn as well as pure aluminum of a purity of 99.0, 99.3, 99.5 and 99.9 may be advantageously treated according to the present invention. To use aluminum of a higher purity than 99.9% would not be practical for economic reasons. The composition of the aluminum of varying purity and of the aluminum alloys described hereinabove by their '5 DIN designations will be found in the following Tab es III and IV.

TABLE III.COMPOSITION OF ALUMINUM OF VARYING DEGREE OF PURITY Permissible Constituents in percent Designation Total Si Fe T1 011 Zn Others TABLE IV.COMPOSITION OF ALUMINUM ALLOYS Designation Alloying Constituents Permissible Impurities AlMgl Mg 0.6 to 1 2 Cr Up to 0.05 Mn to 0. Al Balance AlMg2 AlM3 2.6 to 3.3 0 t0 0. 4 Up to 0.05 Balance AlMn 0 8 to 1. 0 to 0.3 Balance AlMgSiO.5 4 to 0.9 3 to 0.7 Balance AlMgSil Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended Within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body to anodic oxidation in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble meta-l sulfates, said substance being present in an amount equivalent to between 0.1% and 1% by Weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5% and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid lanhydride.

2. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body at substantially ambient temperature to anodic oxidation in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said sub- 8 stance being present in an amount equivalent to between 0.1% and 1% by Weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5 and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

3. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body at a temperature of up to 40 C. to \anodic oxidation in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said substance being present in an amount equivalent to between 0.1 and 1% by weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5% and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid .anhydride.

4. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body to anodic oxidation at a current density of between about 1 and 5 amperes per cm. in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said substance being present in an amount equivalent to between 0.1% and 1% by weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5% and 1.5% by Weight of a substance selected from the group consisting of maleic acid and maleic acid 'anhydride.

5. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the steps of subjecting said body to anodic oxidation in an electrolyte consisting essentially of an aqueous solutionof at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said substance being present in an amount equivalent to about 0.5 by weight of sulfuric acid, between 3% and 5% by weight of sulfosalicylic acid, and about 1% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

6. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body to anodic oxidation at a current density of between about 1.5 and 3 amperes per 100 cm. in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, :said substance being present in an amount equivalent to between 0.1% and 1% by weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5% and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

7. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body at a temperature of up to 40 C. to anodic oxidation at a current density of between about 1.5 and 3 amperes per 100 cm? in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said substance being present in an amount equivalent to about 0.5% by weight of sulfuric acid, between 3% and 5% by weight of sulfosalicylic acid, and about 1% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

8. A method of producing a colored oxide layer on a surface of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body at a temperature of between about 15 C. and 25 C. to anodic oxidation in an electrolyte consisting essentially of an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble metal sulfates, said substance being present in an amount equivalent to between 0.1% and 1% by weight of sulfuric acid, between 1% and 5% by weight of sulfosalicylic acid, and between 0.5% and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

9. A method of producing a colored oxide layer on a suri ace of a body formed of a metal selected from the group consisting of aluminum and aluminum alloys, comprising the step of subjecting said body to anodic oxidation in an electrolyte consisting essentially of an aqueous solution containing sulfate ions in an amount equivalent to between 0.1% and 1% by weight of sulfuric acid, between 1% and 5% by weight of su-lfosalicylic acid, and between 0.5% and 1.5% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

10. As a new composition of matter, an aqueous solution of at least one substance selected from the group 25 consisting of sulfuric acid and water-soluble metal Sulfates, said substance being present in an amount equivalent to between 0.1% and 1% by weight of sulfuric acid, Ibetween 1% and 5% by weight of sulos-alicylic acid, and between 0.5 and 1.5 by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

11. As a new composition of matter, an aqueous solution of at least one substance selected from the group consisting of sulfuric acid and water-soluble meta-l sulfates said substance being present in an amount equivalent to about 0.5 by weight of sulfuric acid, between 3% and 5% by weight of sulfosalicylic acid, and about 1% by weight of a substance selected from the group consisting of maleic acid and maleic acid anhydride.

References Cited by the Examiner UNITED STATES PATENTS Re. 25,566 4/1964 Deal et a1. 20458 1,965,682 7/1934 Work 20458 2,963,409 12/1960 Ramirez 20458 3,031,387 4/1962 Deal et al. 204-58 JOHN H. MAC-K, Primary Examiner.

G. KAPLAN, Assistant Examiner.

Claims

1. A METHOD OF PRODUCING A COLORED OXIDE LAYER ON A SURFACE OF A BODY FORMED OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS, COMPRISING THE STEP OF SUBJECTING SAID BODY TO ANODIC OXIDATION IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND WATER-SOLUBLE METAL SULFATES, SAID SUBSTANCE BEING PRESENT IN AN AMOUNT EQUIVALENT TO BETWEEN 0.1% AND 1% BY WEIGHT OF SULFURID ACID, BETWEEN 1% AND 5% BY WEIGHT OF SULFOSALICYLIC ACID, AND BETWEEN 0.5% AND 1.5% BY WEIGHT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF MALEIC ACID AND MALEIC ACID ANHYDRIDE.