GB2137657A - Sealing anodized surfaces of aluminium work pieces - Google Patents

Abstract

Anodized surfaces of work pieces made of aluminum and its alloys are sealed in an aqueous solution of 0.01 to 5% flourine or a flourine compound, the solution having a temperature of from 25 DEG to 60 DEG and a pH from 3 to 7.5. Sealing can be accomplished more economically and faster than before. Besides, the treated work pieces do not require any follow up treatment.

Description

SPECIFICATION Compound for the cold-fixing of anodized surfaces of work pieces made of aluminum and their alloys This invention relates to a compound for the cold-fixing of anodized surfaces of work pieces made of aluminum and its alloys.

To obtain a physical-chemical stabilization of work pieces made of aluminum, the surfaces of these work pieces are anodized, that is to say, they are artificially oxidized in order to achieve the formation of an inert layer of aluminum oxide on the surfaces.

This layer has the purpose of protecting the subjacent metallic surface against the action of atmospheric substances and chemical components. This layer consists of an inferior protecting layer, which sticks directly to the basic metal and which consists of monohydrated aluminum oxide, as well as a superior protective layer which consists of trihydrated crystals of the aluminum oxide.

The crystals of this last-mentioned layer reveal a porosity which permits the passage of atmospheric constituents and which thus, inside the aluminum trihydrate, can implement their destructive effect which can lead all the way to destruction of entire zones of the upper protective layer. Any coloring that might possibly be applied to the aluminum work pieces will therefore fade very quickly and the natural coloring of the work pieces experiences rapid decomposition. The work piece reveals a surface decomposed due to corrosion and with scars. It is therefore necessary to treat the surface of anodized aluminum work pieces for the fixing of the protective oxide layer and the color layer applied upon it.

This treatment is known abroad by the term of "colmatage" (French) and "sealing" (English). One can usually achieve the fixing of the trihydrated oxide by dipping the anodized aluminum work pieces in a bath consisting of metallic salts that are heated to 95-100"C; through this treatment one achieves the conversion of the aluminum trihydrate into monohydrate which is considerably more stable and furthermore at the same time we can fill up the pores of the uppermost layer.

Another conventional treatment method consists in the anodized aluminum work pieces into a bath which contains dimineralized water and which is almost at boiling temperature. One variation consists in the fact that the work pieces are exposed to a saturated vapor atmosphere. In this case one merely achieves the closing of the pores in the uppermost protective layer due to a conversion of the trihydrate into monohydrate; but the pores are not filled up.

The methods described reveal numerous disadvantages: The first disadvantage is to be seen in the considerable energy expenditure for heating the baths and for maintaining the bath temperature near the boiling point of water throughout the entire treatment phase. These cases involve tubs with a capacity of up to 30,000 liters of liquid which must be heated to 95-100"C and which must constantly be kept at this temperature.

The conventional treatment steps furthermore are very long-drawn-out; we usually need 50-60 minutes to fix an aluminum oxide layer with a thickness of 20 microns and that considerably restricts the utilization of the system (plant). Another disadvantage of known method consists in the fact that there is a flower-like deposit of inorganic salts on the treated surfaces of the work piece. The presence of this material, which represents a byproduct of the aluminum oxide fixing process, is extremely undesirable because it is necessary to remove this product and to polish and clean the work pieces.

Another disadvantage of the known methods is to be seen in the separation of considerable and harmful vapor quantities from the baths which have been heated to the boiling temperature of the water. This separation of vapors impairs the ecological balance in the work rooms.

It is the purpose of this invention to prevent the above-described disadvantages of the known solutions.

The invention consists in the fact that the compound consists of an aqueous solution that contains 0.01-5% by weight fluorine.

The considerable energy expenditure for heating the known solutions is avoided due to this invention. Besides, no vapor is generated so that the atmospheric conditions in the work-rooms are considerably improved. The processing times are furthermore also reduced. Finally, we prevent a powder layer from settling on the surface of the work piece.

According to another feature of the invention, the dissolved fluorine reveals an oxidation number of - 1 and is present in the form of ions, groups, or their combinations in the form of solutions.

According to another feature of the invention, the solution reveals 0.01-5% by weight of one or more of the following ions or ion groups: Nickel-ion, BO3-, BF4-, Co2+, NH4+, NQ-,PQ3-, SQ2-, SiO32-, SiF62-, Cl-, C1Q-.

According to another feature of the invention, these ions come from salts whose cations are alkaline metals of the first and of the second group or transition metals.

The pH value of the solution must be kept between 3 and 7.5 during the treatment phase according to the invention. The method according to the invention for the cold-fixing of the anodized surface of aluminum work pieces is also a part of the invention. The method consists in the fact that the work pieces are dipped into a bath for several minutes, preferably 3-1 5 minutes, a bath consisting of a solution of 0.01-5% by weight of fluorine in water, and which is kept at a temperature of about 25-60"C and has a pH value between 3 and 7.5.

Further examples for salts and ion groups of fluorine, which can be used in the solution according to the invention, are: potassium fluoride, lithium, boron tetrafluoride, and others.

The concentration solution of these ions can likewise vary between 0.01-5% by weight.

A particularly advantageous acid for the purpose according to the invention is H2SO4.

During the cold-fixing treatment of aluminum, according to the invention at hand, the work pieces, which have been subjected to anodizing, organic dyeing, inorganic dyeing, electrodyeing, or other methods, are washed in water and are dipped into the described fluorine solution. The immersion time varies between 3 and 1 5 minutes. The bath temperature is between 25 and 60'C. It has been observed that the treatment times increase for temperatures outside this range.

The anodized work pieces after dipping into the bathare subjected to drying in order then to be packaged. The fixing of the alumina oxide is completed within a period of 24 hours following treatment.

The end result reveals a shiny, uniform surface which is completely free of powdery deposits of inorganic salts even after a longer period of time and no further final treatment steps are required.

It is assumed that the solution according to the invention will alter the lattice-like distribution of the oxide crystals and will eliminate porosity.

Comparison Example A conventional solution for the treatment of anodized aluminum work pieces was prepared with the use of demineralized water. The bath made with this solution (100 liters) was heated to 98"C and anodized aluminum samples were then dipped into this bath. The measurements made for this treatment and the determinations for the treated work pieces 48 hours after fixation treatment of the work pieces lead to the following results: Treatment time in bath for attainment of oxide layer of 20 microns: 50 minutes.

Bath temperature: 98"C; Required thermal energy: 8,800 kcal; Presence of powdery deposits on samples treated: yes; Required final treatment: yes.

Example According to the Invention To 1 liter of deionized water we added 3 g NaF, 20 g NiSO4 . 6H20, and 1 g NA2SO4.

We keep stirring until dissolution. The solution was filtered and the pH value was adjusted with the addition of diluted H2SO4 to a value of 5.5. The solution thus made was heated to a temperature of 25"C and the aluminum work pieces to be processed were dipped into this solution bath for 6 minutes and were then exposed to air drying. The measurements and examinations of the work pieces performed for this treatment 48 hours after fixation treatment yielded the following results: Treatment time in bath to produce oxide layer of 20 microns on work pieces: 6 minutes; Bath temperature: 25'C; Required thermal energy quantity: 1,500 kcal; Presence of powdery deposits on treated samples: no; Need for final treatment: no.

Claims (14)

1. A compound for the cold-fixing of anodized surfaces of work pieces made of aluminum and their alloys, characterized by an aqueous solution that contains 0.01-5% by weight of fluorine.

2. A compound as claimed in claim 1, wherein the dissolved fluorine is present in the form of ions, groups, and combination of same.

3. A compound as claimed in claim 1 or 2, wherein the dissolved fluorine is derived from salts whose cations are alkaline metals of the first and second group.

4. A compound as claimed in claim 3, wherein said solution comprises a solution of sodium fluoride in demineralized water.

5. A compound as claimed in claim9, wherein the dissolved fluorine is derived from salts whose cations come from transition metals.

6. A compound as claimed in any one of claims 1 to 5, wherein said solution additionally contains 0.01-5% by weight of one or more of the following ions: nickel-ion, BO33-, BF4-, CO2+, NO3-, PO43-, SiO3-, SiF62-, Cl-, C103, NH4+.

7. A compound according to claim 6, wherein said solution contains an inorganic acid in a quantity which brings about a pH value of between 3 and 7.5.

8. A compound as claimed in claim 7 wherein the inorganic acid brings about a pH of between 4.5 and 7.

9. A compound as claimed in claim 8 wherein the temperature of the solution is between 25 and 60"C.

10. A compound for the cold fixing of anodized surfaces as claimed in claim 1 substantially as hereinbefore described and as illustrated in the foregoing Example.

11. A method for the cold-fixing of the anodized surface of work pieces made of aluminum and its alloys, wherein the work pieces are dipped into a bath, said bath containing a solution of 0.01-5% by weight of fluorine in water, being kept at a temperature of between 25 and 60"C and having a pH value between 3 and 7.5.

12. A method as claimed in claim 11, wherein one of the following ions is contained in the bath: nickel-ion, BO3-, BF4-, Co2+, NH4+, NO3, PQ3-, SQ2-, SiO32-, SiF62-, Cl-, Cl03-.

13. A method for cold-fixing anodized surfaces as claimed in claim 11 substantially as hereinbefore described and as illustrated in the foregoing Example.

14. An article of aluminum or an aluminum alloy having a cold-fixed anodized surface whenever produced by a method as claimed in any one of claims 11 to 13.