AU642478B2

AU642478B2 - Conversion treatment method and composition for aluminum and aluminum alloys

Info

Publication number: AU642478B2
Application number: AU73370/91A
Authority: AU
Inventors: Kazuya Nakada; Masayuki Yoshida
Original assignee: Henkel Corp
Current assignee: Henkel Corp
Priority date: 1990-02-21
Filing date: 1991-02-13
Publication date: 1993-10-21
Anticipated expiration: 2011-02-13
Also published as: AU7337091A; CA2072592A1; WO1991013186A1; DE69103139T2; EP0516700B1; BR9106049A; DE69103139D1; EP0516700A1

Description

CONVERSION TREATMENT METHOD AND COMPOSITION FOR ALUMINUM

AND ALUMINUM ALLOYS

TECHNICAL FIELD

The present invention relates to a novel conversion treatment solution for aluminum and aluminum alloys which imparts an excellent corrosion resistance and paint adher- ence to the surface of aluminum and aluminum alloys prior to their being painted and to a process of treating sur¬ faces with such a solution. The conversion treatment solu¬ tion is particularly well suited for application to the surface of, for example, the lid material for beverage cans (i.e., can end stock) and the like. BACKGROUND ART

Conversion treatment solutions for aluminum and alum¬ inum alloys may be roughly classified into chromate-type treatments and nonchromate-type treatments. Typical exam- pies of chromate-type treatments are chromic acid/chromate treatments and phosphoric acid/chromate treatments. Chrom¬ ic acid/chromate treatments came into practical application in about 1950, and are still widely used at present on, for example, the fin material of heat exchangers. The princi- pal components of this type of conversion treatment solu¬ tion are chromic acid (Cr0₃) and hydrofluoric acid (HF) , and an accelerator may also be present. A film which con¬ tains some quantity of hexavalent chromium is formed.

The phosphoric acid/chromate conversion treatment is disclosed in United States Patent Number 2,438,877. This conversion treatment solution is composed of chromic acid

(CrO_) , phosphoric acid (H₃PO.) , and hydrofluoric acid

(HF) . The principal component of the resulting film is hydrated chromium phosphate (CrP0₄»4H₂0) . Since this film does not contain much if any hexavalent chromium, it is widely used at present as a paint undercoating treatment for beverage cans and the associated lid stock.

Nonchromate-type treatments are recognized in the art as a distinct category from the chromate-type treatment so- lutions explained above, and are exemplified by the inven- tion disclosed in Japanese Patent Application Laid Open [Kokai] Number 52-131937 [131,937/77]. The treatment solu¬ tion disclosed therein comprises an acidic (pH approximate¬ ly 1.0 to 4.0) aqueous coating solution which contains zir- conium or titanium or a mixture thereof as well as phos¬ phate and fluoride. Treatment with the disclosed conver¬ sion treatment solution produces on the aluminum surface a conversion film whose main component is zirconium and/or titanium oxide. Although the absence of hexavalent chrom- ium is an advantage of the nonchromate-type treatment solu¬ tion, this type of treatment solution nevertheless suffers from a corrosion resistance and paint adherence inferior to those for chromate-type treatments.

Aluminum alloy, in sheet or coil form, is widely used after painting for beverage can lid material, i.e., can end stock. It is subjected to a conversion treatment in order to raise the corrosion resistance and paint adherence, and the phosphoric acid/chromate treatment is employed in al¬ most all commercial can lid manufacturing in Japan. The phosphoric acid/chromate conversion treatment of can end stock generally employs a treatment solution which contains 10.0 to 40.0 g/L phosphate ion, 2.0 to 4.0 g/L hexavalent chromium, and 0.7 to 1.5 g/L fluoride ion. At present, vinyl chloride paint is generally used to coat can end stock. Thus, the production of can ends normally in¬ cludes a phosphoric acid/chromate treatment of aluminum alloy in coil or sheet form, followed by coating with a vinyl chloride paint and then forming.

A beverage can thus normally consists of a can end formed from aluminum alloy coil or sheft treated as de¬ scribed above and of a can body filled with, for example, juice or beer. Depending on its contents, the can may be subjected to sterilization at relatively high temperatures after filling. If it is, steam is formed from vaporiza- tion of the contents, the steam penetrates through the paint film, and the permeated steam then condenses at the interface between the paint film and conversion film. As a result, sterilization tends to reduce the adherence of the paint film. In particular, when a section of the can end is opened by the easy-open method, defects (enamel feathering) can be generated in the opened region due to peeling or exfoliation of the paint film. DESCRIPTION OF THE INVENTION Problem to Be Solved by the Invention

Increasing the adhesion of paint to aluminum and its alloys, particularly aluminum and its alloys used in form- ing beverage can ends to be used for cans requiring high temperature sterilization of the contents, is the major problem addressed by this invention. Summary of the Invention

As a concrete means for solving the problems described hereinbefore for the prior art, the present invention in¬ troduces an aqueous conversion treatment solution for alum¬ inum and aluminum alloys which is characterized in that its pH is in the range from 1.0 to 3.0 and in that it compris¬ es, or preferably consists essentially of, water and 5.0 to 40.0 grams per liter ("g/L") of phosphate ions, 1.0 to 4.0 g/L of hexavalent chromium (in the form of chromium con¬ taining anions) , 0.1 to 2.0 g/L of fluoride ions, and a complex fluoride ion component selected from the group consisting of (i) 4.0 to 15.0 g/L of fluosilicate ion, (ii) 0.5 to 3.0 g/L of fluoborate ion, (iii) 2.0 to 8.0 g/L of fluozirconate ions, and (iv) 2.0 to 8.0 g/L of fluotitanate ions. This conversion treatment solution is capable of forming a highly paint-adherent conversion film which im¬ parts an excellent corrosion resistance to the surface of aluminum and aluminum alloys. In other words, the present invention seeks to offer a conversion treatment solution which imparts an excellent corrosion resistance and paint adherence to the surface of aluminum and aluminum alloy prior to their being painted. Details of Preferred Embodiments of the Invention

The conversion treatment solution of the present invention is an acidic treatment solution which contains complex fluoride ion, phosphate ion, hexavalent chromium, and fluoride ion as its essential components.

The complex fluoride ions are selected from fluosili- cate (SiF, —2) ions, fluotitanate (TiF_g—2), fluozirconate

—2 —2 ^ZrF ₆ ' ' ^and fl^uo*^DOrate (^BF ₄ ) ions, and may be added in the form of fluosilicic acid, fluoboric acid, fluozirconic acid, fluotitanic acid, or any soluble salt thereof. Mix¬ tures of these ions may also be used. A range of 4.0 to 15.0 g/L is preferred for the fluosilicate ion. Values less than 4.0 g/L cannot normally generate good paint ad¬ herence, while values exceeding 15.0 g/L may cause substan¬ tial etching of an aluminum surface and prevent the forma¬ tion of a satisfactory film. A range of 0.5 to 3.0 g/L is preferred for the fluoborate ion. Values less than 0.5 g/L again cannot usually generate a good paint adherence, while values in excess of 3.0 g/L increase waste water pollution and are uneconomical. A range of 2.0 to 8.0 g/L is pre¬ ferred for fluozirconate ions, fluotitanate ions, or mix¬ tures of these two ions. Concentrations of these two com- plex fluoride ions that are less than 2.0 g/L cannot usu¬ ally generate good paint adherence, while concentrations exceeding 8.0 g/L cause substantial etching and usually prevent the formation of a satisfactory film.

Phosphoric acid (H_PO.) is the preferred source for the phosphate ion, and the preferred phosphoric acid con¬ tent falls into the range of 5.0 to 40.0 g/L. When this value is less than 5.0 g/L, the resulting film will nor¬ mally contain only small quantities of chromium phosphate and the paint adherence may be inadequate. While good films are formed at concentrations exceeding 40.0 g/L, the cost of the treatment solution is also increased and the economics become less favorable.

Chromic acid (Cr0₃) is the preferred source for the hexavalent chromium, and the preferred chromic acid content is that which will result in a concentration of its stoi- chio etric equivalent as hexavalent chromium in the range from 1.0 to 4.0 g/L. Values less than 1 g/L result in an inferior corrosion resistance because a satisfactory con¬ version film is not formed. Values in excess of 4.0 g/L can cause increased pollution from and/or pollution abate¬ ment cost for waste water from the treatment solution and thus create environmental and economic problems.

The fluoride ion content is an important component for controlling the film growth rate of the conversion film. The fluoride ion source may be, for example, hydrofluoric acid (HF) , sodium fluoride (NaF) , potassium fluoride (KF) , and the like. The fluoride ion concentration in the con¬ version solutions was determined as follows: An ion-selec¬ tive electrode (Fluorine F-125 electrode, reference HS- 305DP from Toa Denpa Kogyo Kabushiki Kaisha) and an ion meter (Type IM-40S from Toa Denpa Kogyo Kabushiki Kaisha) were used. For calibration, standard solutions were pre¬ pared by adding a specified quantity of hydrofluoric acid (for example, 0.1 g/L, 1 g/L, or 10 g/L) to 5 g/L chromic acid and 15 g/L phosphoric acid and by adjusting the pH to 2.0 with phosphoric acid or sodium hydroxide. (The fluor- ide ion concentration was assumed to correspond to the tot¬ al quantity of fluorine from hydrofluoric acid addition) . The meter readings obtained with these solutions of known fluoride ion concentration were then determined and plotted against the fluoride ion concentrations to generate a cali- bration curve. The pH of the conversion solution itself was adjusted to 2.0 using phosphoric acid or sodium hydrox¬ ide and then measured using the fluorine ion meter, and the measured value was converted to the fluoride ion concentra¬ tion by reference to the calibration curve. The preferred range for the fluoride ion concentration is 0.1 to 2.0 g/L. At values less than 0.1 g/L, the growth rate of the conversion film is slow, so that long treatment times must be used in order to obtain satisfactory conver¬ sion films and the productivity is therefore low. Rapid growth rates are encountered at values in excess of 2.0 g/L; this results in large film weights and an undesirable loss of the metallic luster of the workpiece. As a conse- quence, the preferred concentration range is 0.1 to 2.0 g/L; the particularly preferred range is 0.4 to 1.0 g/L.

The pH of this conversion treatment solution should be in the range of 1.0 to 3.0 and may conveniently be adjusted into that range through the use of an acid arbitrarily se¬ lected from acids such as phosphoric acid, nitric acid, and hydrochloric acid or a base arbitrarily selected from bases such as sodium hydroxide, ammonium hydroxide, and the like. A pH below 1.0 causes substantial etching and therefore in- terferes with coat formation. A pH in excess of 3.0 usu¬ ally results in weak etching so that a uniform film cannot be formed.

The use of the conversion treatment solution of the present invention in treatment processes is another embod- iment of this invention and will now be considered in more detail. The conversion treatment solution of the present invention can be used as a substitute for the currently widely used phosphoric acid/chromate treatment solutions. A preliminary surface cleaning must usually be carried out when the conversion treatment solution of the present in¬ vention is used for the conversion treatment of the surface of aluminum or aluminum alloy. The cleaning method in this case may consist of treatment with an acidic, alkaline, or solvent-based cleaning solution or some combination there- of. As necessary or desired, the aluminum or aluminum al¬ loy surface may be etched with alkali or acid after clean¬ ing. Either immersion or spray treatment may be used as the method for treatment with solution according to the present invention. The weight of the resulting conversion film is governed by such factors as the treatment tempera¬ ture and treatment time. The temperature of the treatment solution should preferably fall into the range from room temperature (about 20 degrees Centigrade) to 70 degrees Centigrade and more preferably falls into the range from 35 to 55 degrees Centigrade. Treatment times in the range of 1 to 90 seconds are preferred. As with phosphoric acid/ chromate films, the conversion film weight is normally evaluated based on the deposition of chromium, zirconium, and/or titanium. The quantity of deposition of each of the three metals, when present at all, preferably falls within

₂ the range of 5 to 50 mg/m , and should be adjusted in ac- cordance with the required degree of corrosion resistance. The deposition of chromium, titanium, and/or zirconium can be controlled by suitably adjusting the treatment tempera¬ ture and treatment time.

The conversion film formed by the conversion treatment solution according to the present invention when neither zirconium or titanium is present is believed to be chemi¬ cally and physically similar to the film formed by phos¬ phoric acid/chromate treatments, and is composed princi¬ pally of hydrated chromium phosphate (CrP0₄»4H₂0) . When either fluotitanate or fluozirconate is included in the treatment solution, the conversion film usually contains both hydrated chromium phosphate and zirconium oxide (ZrO_) and/or titanium oxide (TiO_) .

Examples

The conversion treatment solution of the present in¬ vention is explained in greater detail below through the use of several illustrative examples. The first group of examples are for solutions containing fluoborate or fluo- silicate ions, and the effectiveness of such solutions rel¬ ative to comparison examples is reported in Table 1.

The substrate for these examples was an aluminum/mag¬ nesium alloy (described in detail in Japanese Industrial Standard {hereinafter "JIS") A5082) . This aluminum alloy was degreased and conversion treated using a small sprayer designed to give spraying conditions identical to those currently encountered in typical spray treatments on com¬ mercial continuous conversion treatment lines for the con¬ version treatment of aluminum alloy coil. Chromium content in the coating deposited by the conversion process was mea¬ sured using a fluorescent X-ray analyzer (Model 3070E from

Rigaku Denki Kogyo) . This conversion treated aluminum al¬ loy sheet was then coated with a can end paint of a poly (vinyl chloride} type to give a paint film thickness of 12 to 14 micrometers, which was then baked at 200 degrees Cen¬ tigrade for 10 minutes before the sheets were subjected to the other tests reported in Table 1.

Salt-spray testing was conducted in order to evaluate the corrosion resistance. Salt-spray testing was conducted in accordance with JIS Z-2371, and the value reported is the time required for the appearance of blistering at a cross form cut in the paint film on the painted test panel. Thus, longer times correspond to a better corrosion resist- ance. Spray times of 2000 hours or more are generally now rated as excellent.

The paint adherence was evaluated as follows The painted test sheet was cut into 5 x 150 millimeter (herein- after "mm") size rectangular strips, which were then hot- press-bonded with polyamide film. The obtained test spec¬ imen was immersed in boiling deionized water for 3 hours, and the peel strength was then evaluated in a 180° peel test. High peel strength values correspond to a better paint adherence, and as a general rule a value of 3.0 kilo¬ grams of force (hereinafter "kgf") per 5 mm width is rated as excellent.

Enamel feathering was evaluated in accordance with the Alcoa method, as described on page 49 of the Lecture Notes from the 73rd Fall Meeting of Keikinzoku Gakkai [Institute of Light Metals of Japan] . This evaluation is based on the maximum residual paint film width after peeling. Thus, smaller residual paint film widths correspond to a more de¬ sirable smaller amount of enamel feathering, and as a gen- eral rule residual widths not exceeding 0.5 mm are rated as excellent. Example 1

The surface of the aluminum alloy was cleaned by rins¬ ing with a hot (70 degrees Centigrade) 4 % aqueous solution of a commercial strongly alkaline degreaser (FINE CLEANER™ 4418 from Nihon Parkerizing Company, Limited) and then with water. This was followed by spraying for 5 seconds with conversion treatment solution 1 heated to 50 degrees Cent¬ igrade, rinsing again with tap water, spraying with deion- ized water (specific resistance > 3,000,000 ohm-cm) for 10 seconds, and finally drying in a hot-air drying oven at 70 degrees Centigrade for 5 minutes. After drying, the con¬ version coated test panel was painted as described above, and the corrosion resistance, paint adherence, and enamel feathering were then evaluated.

Conversion treatment solution 1 contained 18.8 g/L of 40% fluosilicic acid (H₂SiF_g) == 7.4 g/L of SiFg²~; 21.3 g/L of 75 % phosphoric acid (H₃P0₄) = 15.5 g/L of P0₄3-. _{5 g /L} of chromic acid (Cr0₃) = 3.0 g/L of Cr ; and 3.0 g/L of 20 % hydrofluoric acid (HF) = 0.6 g/L of F~; the pH was adjusted to 2.0 with ammonium hydroxide after all the other ingredients had been added. Example 2

This was identical to Example 1, except that the Con¬ version treatment solution 2 used contained only 12.5 g/L of 40 % fluosilicic acid = 4.9 g/L of SiF ^* , rather than the larger amount in Conversion treatment solution 1. Example 3

This was identical to Example 2, except that (i) the

Conversion treatment solution 3 used contained only 2.9 g/L of chromic acid = 1.5 g/L of Cr 6+, rather than the larger amount in Conversion treatment solution 2 and (ii) the pH was adjusted to 1.5 with hydrochloric acid rather than to 2.0 with ammonium hydroxide as in Conversion treatment so¬ lution 2. Example 4 This was identical to Example 1, except that the Con¬ version treatment solution 4 used contained 5.0 g/L of 20 % hydrofluoric acid = 1.0 g/L of F~, rather than the small¬ er amount in Conversion treatment solution 1. Example 5 This was identical to Example 1, except that the Con¬ version treatment solution 5 used contained 1.0 g/L of sod¬ ium fluoborate (NaBF₄) = 0.8 g/L of BF ~, instead of the fluosilicic acid used in Conversion treatment solution 1. Example 6 This was identical to Example 5, except that (i) the Conversion treatment solution 6 used contained 2.0 g/L of sodium fluoborate (NaBF₄) = 1.6 g/L of BF ~, rather than the smaller amount in Conversion treatment solution 5 and (ii) the pH was adjusted to 2.5 instead of 2.0. Example 7

This was identical to Example 1, except that the sam¬ ples were spray treated for 10 seconds at 40 degrees Centi- grade rather than for 5 seconds at 50 degrees Centigrade as in Example 1. Example 8

This was identical to Example 1, except that the sam- pies were spray treated for 10 seconds rather than for 5 seconds as in Example 1. Comparison Example 1

This was identical to Example 1, except that the Con¬ version treatment solution 7 used contained only 6.3 g/L of 40 % fluosilicic acid = 2.5 g/L of SiF_g ²~, rather than the larger amount in Conversion treatment solution 1. Comparison Example 2

This was identical to Example 1, except that the Con¬ version treatment solution 8 used contained 40.0 g/L of 40 % fluosilicic acid = 15.8 g/L of SιF_g 2—, rather than the smaller amount in Conversion treatment solution 1. Comparison Example 3

The aluminum alloy was cleaned as in Example 1 and then spray-treated for 5 seconds with a 5 % aqueous solu- tion of a commercial phosphoric acid/chromate treatment concentrate (ALCHROM™ K702 from Nihon Parkerizing Company, Limited) heated to 50 degrees Centigrade. After this treatment, it was rinsed with water, dried, and painted as in Example 1, and its performance was then evaluated. Comparison Example 4

The aluminum alloy was cleaned as in Example 1 and then spray-treated for 30 seconds with a 2 % aqueous solu¬ tion of a commercial non-chromate treatment concentrate (PARCOAT™ K3761 from Nihon Parkerizing Company, Limited) heated to 50 degrees Centigrade. After this treatment, it was rinsed with water, dried, and painted as in Example 1, and its performance was then evaluated.

Another group of examples and comparison examples utilized solutions containing fluozirconate or fluotitanate ions, as described in more detail below. Example 9

This was identical to Example 1, except that the Con- version Solution 9 used contained 20.2 g/L of 20 % aqueous fluozirconic acid (H₂ZrF_g) = 4.0 g/L of ZrF_g _2 instead of the fluosilicic acid used in Conversion Solution 1 in Ex¬ ample 1. Example 10

This was identical to Example 9, except that the Con¬ version Solution 10 used contained 12.6 g/L of 20 % aqueous

_₂ fluozirconic acid (H₂ZrF_g) = 2.5 g/L of ZrF_g instead of the larger amount of fluozirconic acid used in Conversion Solution 9 in Example 9. Example 11

This was identical to Example 9, except that the Con¬ version Solution 11 used (i) contained 1.9 g/L of chromic acid = 1.0 g/L of Cr instead of the larger amount of chromic acid in Conversion Solution 1 in Example 1 and (ii) had a pH of 1.5 achieved by adjustment with hydrochloric acid rather than a pH of 2.0 achieved by adjustment with ammonia as in Conversion Solution 9. Example 12 This was identical to Example 11, except that the Con¬ version Solution 11 used contained 5.8 g/L of chromic acid = 3.0 g/L of Cr and 5.0 g/L of 20 % aqueous hydrofluoric acid = 1.0 g/L of F~ ions instead of the smaller amounts of these two constituents used in Conversion Solution 11 in Example 11. Example 13

This was identical to Example 9, except that the Con¬ version Solution 13 used (i) contained 20.3 g/L of aqueous

_.__ fluotitanic acid = 4.0 g/L of TiF_g instead of the fluo- zirconic acid in Conversion Solution 9 in Example 9 and (ii) had a pH of 2.5 achieved by adjustment with sodium hy¬ droxide rather than a pH of 2.0 achieved by adjustment with ammonia as in Conversion Solution 9. Example 14 This was identical to Example 9, except that the Con¬ version Solution 14 used contained 12.7 g/L of 20 % aqueous fluotitanic acid = 1.6 g/L of TiF_g ^"2 and 12.6 g/L of 20 % aqueous fluozirconic acid = 2.5 g/L of ZrF_g- ^_ons instead of the larger amount of fluozirconic acid, with no fluoti¬ tanic acid, used in Conversion Solution 9 in Example 9. Example 15 This was identical to Example 9, except that the sam¬ ples were spray treated for 10 seconds at 40 degrees Centi¬ grade rather than for 5 seconds at 50 degrees Centigrade as in Example 9. Example 16 This was identical to Example 9, except that the sam¬ ples were spray treated for 10 seconds rather than for 5 seconds as in Example 9. Comparison Example 5

This was identical to Example 9, except that the Con- version treatment solution 15 used contained only 5.0 g/L of 20 % fluozirconic acid = 1.0 g/L of ZrF_g ^2", rather than the larger amount in Conversion treatment solution 9.

Comparison Example 6

This was identical to Example 9, except that the Con- version treatment solution 16 used contained 50.0 g/L of 20

2— % fluozirconic acid = 15.8 g/L of ZrF_g , rather than the smaller amount of fluozirconic acid in Conversion treatment solution 9.

Test results from this second group of examples are shown in Table 2, where Comparison Examples 3 and 4 are repeated from Table 1.

Benefit of the Invention

As Tables 1 and 2 make clear, application of the conversion treatment solution of the present invention affords an excellent corrosion resistance and paint adherence as well as an excellent resistance enamel feathering. Table 2: RESULTS OF TESTING WITH FLUOZIRCONATE AND FLUOTITANATE CONTAINING SOLUTIONS

Areal Density Salt Spray Peel Test Alcoa Test in Conversion Test Time, Strength, Residual Hours K c Widt -

Claims

1. An aqueous conversion coating solution characterized in that is has a pH value from 1.0 to 3.0 and comprises:

(A) an amount of phosphate ions that is stoichiometrically equivalent to at least 5.0 g/L of phosphoric acid;

(B) at least 1.0 g/L of hexavalent chromium;

(C) at least 0.1 g/L of fluoride ions; and

(D) a complex fluoride ion component selected from the group consisting of: (i) at least 4.0 g/L of fluosilicate ions,

(ii) at least 0.5 g/L of fluoborate ions,

(iii) at least 2.0 g/L of fluozirconate ions, and

(iv) at least 2.0 g/L of fluotitanate ions.

2. An aqueous solution according to claim 1, characterized in that it comprises:

(A) an amount of phosphate ions that is stoichiometrically equivalent to from 5.0 to 40.0 g/L of phosphoric acid;

(B) from 1.0 to 4.0 g/L of hexavalent chromium;

(C) from 0.1 to 2.0 g/L of fluoride ions; and (D) a complex fluoride ion component selected from the group consisting of:

(i) from 4.0 to 15.0 g/L of fluosilicate ions, (ii) from 0.5 to 3.0 g/L of fluoborate ions, (iii) from 2.0 to 8.0 g/L of fluozirconate ions, and (iv) from 2.0 to 8.0 g/L of fluotitanate ions.

3. An aqueous solution according to claim 2, character¬ ized in that it comprises from 0.4 to 1.0 g/L of fluoride ions.

4. An aqueous solution according to claim 1, character- ized in that it comprises from 0.4 to 1.0 g/L of fluoride ions.

5. A process for treating a surface of aluminum or an aluminum alloy, said process comprising steps of forming a conversion coating on said surface and subsequently over- coating the conversion coated surface with an organic pro- tective coating, characterized in that the improvement com¬ prises forming the conversion coating on said surface by contacting said surface with an aqueous solution having a. pH value from 1.0 to 3.0 and comprising: (A) an amount of phosphate ions that is stoichiometrically equivalent to at least 5.0 g/L of phosphoric acid;

(B) at least 1.0 of hexavalent chromium;

(C) at least 0.1 g/L of fluoride ions; and

(D) a complex fluoride ion component selected from the group consisting of:

(i) at least 4.0 g/L of fluosilicate ions,

(ii) at least 0.5 g/L of fluoborate ions,

(iii) at least 2.0 g/L of fluozirconate ions, and

(iv) at least 2.0 g/L of fluotitanate ions.

6. A process according to claim 5, characterized in that said aqueous solution comprises:

(B) from 1.0 to 4.0 g/L of hexavalent chromium; (C) from 0.1 to 2.0 g/L of fluoride ions; and

(D) a complex fluoride ion component selected from the group consisting of:

(i) from 4.0 to 15.0 g/L of fluosilicate ions, (ii) from 0.5 to 3.0 g/L of fluoborate ions, (ϋi) from 2.0 to 8.0 g/L of fluozirconate ions, and (iv) from 2.0 to 8.0 g/L of fluotitanate ions.

7. A process according to claim 6, characterized in that said aqueous solution comprises from 0.4 to 1.0 g/L of fluoride ions.

8. A process according to claim 5, characterized in that said aqueous solution comprises from 0.4 to 1.0 g/L of fluoride ions.

9. A process according to claim 8, characterized in that the conversion coating formed contains from 5 to 50 milli- grams per square meter of at least one of chromium, zircon¬ ium, or titanium.

10. A process according to claim 7, characterized in that the conversion coating formed contains from 5 to 50 milli¬ grams per square meter of at least one of chromium, zircon¬ ium, or titanium.

11. A process according to claim 6, characterized in that the conversion coating formed contains from 5 to 50 milli¬ grams per square meter of at least one of chromium, zircon¬ ium, or titanium.

12. A process according to claim 5, characterized in that the conversion coating formed contains from 5 to 50 milli¬ grams per square meter of at least one of chromium, zircon¬ ium, or titanium.

13. A process according to claim 12, characterized in that the conversion coating is performed at a temperature in the range from 20 to 70 degrees Centigrade.

14. A process according to claim 11, characterized in that the conversion coating is performed at a temperature in the range from 20 to 70 degrees Centigrade.

15. A process according to claim 10, characterized in that the conversion coating is performed at a temperature in the range from 20 to 70 degrees Centigrade.

16. A process according to claim 9, characterized in that the conversion coating is performed at a temperature in the range from 20 to 70 degrees Centigrade.

17. A process according to claim 16, characterized in that the conversion coating is performed at a temperature in the range from 35 to 55 degrees Centigrade for a contact time in the range from 1 to 90 seconds.

18. A process according to claim 15, characterized in that the conversion coating is performed at a temperature in the range from 35 to 55 degrees Centigrade for a contact time in the range from 1 to 90 seconds.

19. A process according to claim 14, characterized in that the conversion coating is performed at a temperature in the range from 35 to 55 degrees Centigrade for a contact time in the range from 1 to 90 seconds.

20. A process according to claim 13, wherein the conver¬ sion coating is performed at a temperature in the range from 35 to 55 degrees Centigrade for a contact time in the range from 1 to 90 seconds.