CA2083454A1 - No-rinse process - Google Patents

Abstract

ABSTRACT

A method for forming a no-rinse coating on aluminum and aluminum-containing metals is disclosed. The method provides for the addition of fluoride containing compounds to an aqueous solution of multivalent chromium compounds. The resulting solution, when applied to the aluminum and aluminum-containing metals, provides for increased corrosion protection.

Description

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IMPROYED NO-RINSE PROCESS

FIELD OF THE INVENTION

The present invention relates to improved methods for treating aluminum-conta1ning substrates to 1mprove corrosion resistance.

BACKGROUND OF THE INVENTION

Aluminum and aluminum-alloy metals are painted to improve corrosion resistance and for aesthetic purposes. In order to promote adhesion of paint to the aluminum surface, the IO metal must be cleaned and properly prepared.

In a typical process, the aluminum is first cleaned to remove the oils and dirt accumulated during fabrication. The aluminum is then rinsed with water to remove any residual chemicals used in the cleaning step. These steps can be . . , , 2~3~

repeated if necessary to produce a clean surface. After the cleaner and rinse stages, the surface is chemically treated. A
rinse stage follows to remove excess chemicals prior to painting.

The surface quality of the aluminum or alloy, parti~ularly with regards to corrosion prevention and the adhesion of subse-quently applied covering layers, can be substantially improved by coatings such as these. These coatings can also assist in wear, lubrication, stress corrosion cracking and erosion.

Historically, hexavalent chromium compounds were fre-quently chosen as the coating compositions. However, due to thetoxic nature of these compounds, the rinse waters accumulating these compounds requ;re elaborate effluent treatment.

No-rinse processes have successfully replaced rinsable chromate processes in many applications. In no-rinse processes, the metal surface is cleaned in a first stage to remo~e oil, dirt and other residues. Any residues of chemicals from this first stage are removed by rinsing with water. In the following stage of the process, the clean metal surface is wetted with the aqueous no-rinse bath solut;on which is not rinsed off, but instead is dri~d in situ on the me-tal surface and converted there into a solid film of the bath constituents. Thus, chromium contaminated rinse waters are eliminated. Surface quality, particularly in regard to corrosion prevention and the adhesion of subsequently applied covering layers, can be substantially improved by coatings such as these.

SUMMARY OF THE INVENTION

The present invention relates to improved methods of ap-plying a no-rinse coating to aluminum and aluminum-bearing metals.
The methods provide for contacting the aluminum substrate to be treated with an aqueous solution of multivalent chromium compounds and fluoride containing compounds.

DESCRIPTION OF T~E RELATED ART

Aqueous chromium-containing no-rinse solutions are known to improve corrosion resistance for aluminum and aluminum con--taining metals. U.S. Patent No. 4,475,957, Sander, October 1984 teaches an acidic aqueous no-rinse sol~tion comprising hexavalent chromium, trivalent chromium, phosphoric acid and a poly(alkylene-oxy) polymer. This composition can be applied to aluminum and various steel alloys to apply a corrosion resistant surface.

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U.S. Patent No. 4,921,552, Sander et al., May 1990 dis-closes a non-chromate conversion coating solution for aluminum-containing alloys. Acidic aqueous solutions of polyacryiic acid, dihydrohexafluo zirconic acid and hydrofluoric acid are dried in place to provide the siccative caating.

U.S. Patent No. 2,936,254, Newhard et al., May 1960 teaches d coating bath for aluminum containing ions of fluoride, hexavalent chromium and phosphate. This bath is further charac-terized by an addition of sodium and potassium and also by con-taining dissolved aluminum to ensure any precipitate formed toconsist of essentially K2NaAlF6.

U.S. Patent No. 4,435,223, Dollman, March 1984 discloses - a process for cleaning an aluminum based surface. The acidic solutions which are free of hydrofluoric acid and other fluorides comprises sulfuric acid, orthophosphoric acid and a surfactant.

U.S. Patent No. 4,801,337, Higgins, January 1989 teaches compositions and processes for forming a corrosion-resistant complex oxide coating on a zinc or zinc alloy surface. The com-positions comprise an aqueous alkaline solution containing at least 3 complexed polyvalent metals One of the metals need be trivalent chromium.

U.S. Patent No. 4,146,410, Reinhold, March 197g discloses compositions and methods for imparting a chromate conversion 2~3~

coating on aluminum containing surfaces. The composition comprises an acidic aqueous solution of zinc, hexavalent chromium, fluorine and molybdate.

U.S. Patent No. 4,266,988, Krippes, May 1981 teaches compositions and methods for depositing chromate on ferrous and non-ferrous metal surfaces. The compositions comprise chromic acid, hydrofluoric acid, fluoboric acid, sulfuric acid, hydrofluo-silicic acid and an additive selected from the group consisting of zinc oxide, magnesium oxide, magnesium hydroxide, aluminum sulfate and aluminum hydroxide.

U.S. Patent No. 4,761,189, Mady et al., August 1988 teaches a process for treating a clean metal surface for subsequent appli-cation of an organic coating. Sequentially, this process involves adding to the metal surface an aqueous solution of trivalent chromium ions, fluoride ions, a homopolymer or copolymer of acrylic acid or methacrylic acid, and phosphate ions.

The metal is removed from the aqueous solution and the ~ater is removed from the aqueous solution remaining on the metal.
The remaining thin film is heated to render the film water insoluble.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for methods of forming a no-rinse coating on the surfaces of aluminum containing metals 2~83~

wherein said surfaces are contacted by an aqueous solution of multivalent chromium compounds comprising including in said aqueous solution a fluoride containing compound.

By "no-rinse" coating is meant a coating that is not rinsed off but allowed to dry in place on the aluminum surface.
The coating formed by the present invention provides for increased corrosion resistance under an organic coating for the aluminum containing metal. The present inventor has found that the addition of the fluoride containing compound to a traditional chromium coating program provides for a surprising increase in corrosion resistance.

A no-rinse coating solution is described in U.S. Patent No. 4,475,957, which is wholly incorporated by reference herein.
The multivalent chromium compounds can include trivalent chromium and hexavalent chromium both of which would function in a no-rinse coating solution.

The fluoride containing compounds which are useful in thè
present invention are any which can provide the fluoride, F-, ion to the aqueous solution. Preferred compounds include ammonium fluoride and hydrofluoric acid. Ammonium bifluoride and sodium fluoride are also anticipated to be effective in the present invention. Mixtures of fluoride containing compounds also prove effective.

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The methods of the present invention provide an effective no-~inse coating. The ability to dry in place obviates prior art problems with regeneration of recycled solution and buildup of aluminum ions in the solution.

It should be understood that the multivalent chromium aqueous solution and fluoride containing compound may be added separately to the bath, thus avoiding the preparation of a concentrate. Either method is within the purview of this invention. The preferred method of application is by spraying the inventive solution onto the aluminum containing metal. The inventors anticipate that the inventive solution may be employed by immersing the aluminum containing metal in the solution. This would mean that a higher level of fluoride ions need be present in the solution.

The concentration level of the fluoride ion in the aqueous, preferably deionized water, solution are dependent on a number of variables. The pH, temperature, type of aluminum alloy, composition of the cleaning solution, contact time with the no-rinse solution and the shape of the article being treated may affect the concentration level of fluoride ions in the - aqueous solution. Preferably from about 1 to about 1000 parts per million fluoride ion is added to the multivalent chromium aqueous system.

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The contact time between the aluminum containing metal and the aqueous solution of the present invention can vary with regard to the thickness of the desired coating. Preferably, 30 seconds spray of the aqueous solution will provide adequate coating for corrosion resistance.

The aluminum containing metals that can be treated by the present invention include commercially pure aluminum and alloys - such as 5052, Alclad 2024 and 3003, 6061, ~063 and 7075. These are meant to be representative alloys and not limiting as to the aluminum containing metals which can be treated.

The invention will now be further described with reference to a number of specific examples which are to be regarded solely as illustrative, and not as restricting the scope of the invention.

EXAMPLES

3003 alloy aluminum panels were pretreated according to the following process:

Staqe _ _ Treatment Step 1 2X Betz Klee ~ 149 alkaline wash, 20 second spray at 120F
2 Tap water rinse 3 2% Betz Klee ~ 175 acid wash, 20 second spray at 85F

4 Tap water rinse 8% Permatrea ~ 630 no rinse, 30 second spray at ambient 6 Oven dry at 225F (5-10 minutes) Betz Klee ~ 175 and Permatrea ~ 630 are registered trademarks of Betz Laboratories, Inc.

To eva~uate the efficacy of adding fluoride ions to the no-rinse aqueous solution, a 50/50 mixture of 36% N~4F/20% HF
was added to the no-rinse bath. The concentrations achieved were 100,500 and 1000 parts per million as fluoride ion. The panels so treated were subsequently painted with a Morton bronze extrusion paint for a 1 mil dry film thickness.

The painted panels were then subjected to 1700 hours neutral salt spray (NSS) testing as per AS~TM B-117. After exposure, the panels were rated per ASTM D-1654. Creepback from the scribe is rated on a O to 10 scale. A rating of 10 indicates no failure while a rating of O indicates that creepback exceeds 5/8 of an inch.

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Ratinq of Scribe Failure Maximum Measurement Rating of Fajlure from Scribe - BY Number (in.) mm o o 10 1/64 0.4 9 1/32 0.8 8 1/16 1.6 7 1/08 3.2 6 3/16 4.8 5 1/04 . 6.4 4 1/02 12.7 2 5/08 15.9 5/08 or more 15.9 or more 0 These results are presented in Table I.

TABLE I
Fluoride (as F-) 1700 Hour NSS
(pDm) ASTM B-117 10~ - 7/9 -/- denotes duplicate panels From these observations, it can be seen that the addition of fluoride ion to the no-rinse solution improves corrosion resis-tance of the aluminum under an organic coating. Fluoride ion in amounts as low as 100 part per million showed improved corrosion resistance. Dosages of 500 and 1000 parts per million pruved 30- almost perflect under the conditi~ns of the test.

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Further testing was performed utilizing various pretreat-ments on 6063 extruded aluminum stock. ~he following pretreatment steps were performed.

Staqe Treatment Step 1 3% Betz Klee ~ 148, 45 second spray at 140F
2 Tap water rinse 3 No rinse treatment 4 10 minutes oven dry at 200 to 225F

10Betz Kleen~i-c a registered trademark of Betz Laboratories, Inc. The no-rinse treatments are delineated in Tables 11, III and IV. After exposure-to the treatments, the panels were first taped, ; as per the AAMA, and then scraped. These results are reported in Tables 11, III and IV.

Morton Bronze Extrusion Paint 1000 Hour Neutral Salt Spray (ASTM B-117) Treatment Ratinq (ASTM D-1654) 8% Permatreat~3630 5/8 208% Permatreat~D630 10 and 500 ppm F-(NH4F/HF) 12% Permatreat~ 630 7 Chrnmium Phosphate 10 2~83~

TABLE III
Morton Bronze Extrus;on Paint 1500 Hour Neutral Salt Spray (ASTM B-117) Treatment Ratinq (ASTM D-1654) 8% Permatreatd~630 8% Permatreat~D 630 10 and 500 ppm F-(NH4F/HF) 12% Permatreat~D630 - 9-10 Chromium Phosphate 10 TABLE IV
Morton Bronze Extrusion Paint 2000 Hour Neutral Salt Spray (ASTM B-117) Treatment Ratinq (ASTM D-1654) 8% Permatrea ~ 630 5/2 8% Permatreat~D630 10/9.5 and 500 ppm F-(NH4F/HF) 12% Permatrea ~ 630 7/1 Chromium Phosphate 10 Permatrea ~ 630 is a registered trademark of Betz Laboratories, Inc.

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The results reported in Tables II, III and IV indicate that the addition of fluoride ion to the aqueous solution of multivalent chromium compounds makes for an improved no-rinse solution and improved corrosion resistance.

Adhesion performance tests were performed on 6063 extruded panel stock. These tests were performed according to AAMA specifi-cations 603.8, 6.4.1.1 and 6.4.1.2. These comprise a cross-hatch of 11 x 11 parallel cuts, 1t16 of an inch apart. Permacel ~ 99 adhesive tape is smoothed down on the painted grid and then pulled off. The B rating is ASTM D-3359 where a 5B represents no ~ailure or pickoff and a OB represents greater than 65~ pickoff. These results are reported in Tables V, Vl, VII and VIII.

TABLE V
Adherence Performance Testing Morton Bronze Extrusion Paint Boiling WaterDetergent Water SoakResistance Treatment Dry 20 Min. 24 Hr.72 Hr.
8% Permatrea ~ 630 5B lB 4B Pass 8% Permatrea ~ 630 5B 4B 4B Pass and 500 ppm F-12% Permatrea-~ 630 5B OB 4B/3BPass Chrome Phosphate 5B 3B 4B Pass ~83~

TABLE VI
Adherence Performance Testing Morton White Paint Boiling WaterDetergent Water SoakResistance - 5 Treatment DrY 20 Min. 24 Hr.72 Hr.
8% Permatreat~630 5B 4B 5B Pass 8% Permatreat~630 5B 4B 5B Pass and 500 ppm F-12% Permatreat~D630 5B 3B 5B Pass Chrome Phosphate 5B 5B 5B Pass TABLE YII
Adherence Performance Testing Akzo White Paint Boiling WaterDetergent Water SoakResistance Treatment Dry 20 Min. 24 Hr.72_Hr.
8% Permatrea ~ 630 5B 3B 5B Pass 8% Permatreat~630 5B 3B 4B/5B Pass and 500 ppm F-12% Permatrea ~ 630 4B 4B 5B Pass Chrome Phosphate SB 4B 5B Pass 208~5~

The results of this test;ng again indicate that the addition of fluoride ions to the aqueous multivalent chromlum solut;on promotes an improved no-rinse coating.

TABLE VIII
5 Adherence Performance Testing Polycron III White Paint Boiling WaterDetergent Water SoakResistance Treatment DrY 20 Min. 24 Hr.72 Hr.
8% Permatreat~630 5B 4B 4B Pass 8% Permatreat~D630 5B 4B 4B Pass and 500 ppm F-12% Permatreat~630 5B 4B 5B Pass Chrome Phosphate SB 4B 5B Pass The results presented in Tables YI, VII and VIII show that the addition of the fluoride containing compound to the aqueous multivalent chromium solution provides the formation of an effec-tive no-rinse coating. Although no better result was achieved9 it is postulated that "better" paints providing strong adherence to the metal surface are too good to show differences.

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Tables IX through XIII represent additional neutral salt spray testing on other paints as per ASTM B-117. The pretreatment steps were the same as those performed for Tables II, III'and IV.

TABLE IX
Morton White Extrusion Paint 1500 Hour Neutral Salt Spray (ASTM B-117) Treatment Ratinq (ASTM D-1654) 8% Permatrea ~ 630 8 8% Permatrea ~ 630 and 500 PPM F- (NH4/HF) 9 12% Permatreat~'630 9 Chromium Phosphate 9 TABLE X
Akzo White Extrusion Paint 1500 Hour Neutral Salt Spray (ASTM B-117) Treatment Ratinq lASTM D-1654) B% Permatreat~630 10 8% Permatrea ~ 630 and 500 PPM F- (NH4/HF) 10 12~ Permatrea ~ 630 10 Chromium Phosphate 10 -2~83~

TABLE XI
Morton White Extrusion Paint 2000 Hour Neutral Salt Spray (AS~M B-117) Treatment Ratinq_~ TM D-1654) 8% Permatreat~D630 10/9 8% Permatreat~D630 and 500 PPM F- (NH4/HF~ 10/9 12% Permatrea ~ 630 10/9 Chromium Phosphate 10/10 TABLE XII
Akzo White Extrusion Paint 2000 Hour Neutral Salt Spray (ASTM B-117?
Treatment Ratinq (ASTM D-1654) 8% Permatrea ~ 630 10/10 8% Permatrea ~ 630 and 500 PPM F- (NH4/HF) 10/10 12% Permatreat~ 630 10/10 Chromium Phosphate 10/10 - 2~3~

~ABLE XIII
Polycron III White Extrusion Paint 2000 Hour Neutral Salt Spray (ASTM B-117) Ireatment Ratinq tASTM D-1654) 8~ Permatreat~D630 10/9 8% Permatreat~D630 and 5~0 PPM F- (NH4/HF)9!9 12% Permatrea ~ 630 10/9 Chromium Phosphate 10/9 These results further indicate the formation of an effective no-rinse coating. Again, where no better results were shown, it is postulated that "better" paints provide strong adherence to the metal surface and need more stringent tests to further demonstrate the efficacy of the invention.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.

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Claims (7)

1. A method of forming a no-rinse coating on the surfaces of aluminum containing metals wherein said surfaces are contacted by an aqueous solution of multivalent chromium compounds comprising including in said aqueous solution a fluoride containing compound.

2. The method as claimed in claim 1 wherein said fluoride containing compound is ammonium fluoride, hydrofluoric acid or mixtures thereof.

3. The method as claimed in claim 2 wherein said fluoride containing compound is ammonium fluoride.

4. The method as claimed in claim 2 wherein said fluoride containing compound is hydrofluoric acid.

5. The method as claimed in claim 2 wherein said fluoride containing compound is added to said aqueous solution in an amount from about 1 part per million to about 1000 parts per million.

6. The method as claimed in claim 1 wherein said aluminum containing metal is contacted with said aqueous solution by spraying said solution onto said aluminum containing metal.

7. The method as claimed in claim 1 wherein said aluminum containing metal is contacted with said aqueous solution by immersing said aluminum containing metal in said solution.