EP0310103A1

EP0310103A1 - Pretreatment process for aluminium

Info

Publication number: EP0310103A1
Application number: EP88116151A
Authority: EP
Inventors: Joseph Bunczk
Original assignee: Amchem Products Inc; Henkel Corp
Current assignee: Henkel Corp
Priority date: 1987-10-01
Filing date: 1988-09-30
Publication date: 1989-04-05
Also published as: JPH01165778A

Abstract

A process for pretreating an aluminum or aluminum alloy surface prior to applying an organic coating thereto comprising contacting the surface with an acidic cleaning solution containing sulfuric acid, phosphoric acid, hydrofluoric acid, or a mixture thereof, and a nonionic or anionic surface-active agent, rinsing the surface with water, contacting the surface with a corrosion resistant acidic coating solution containing trivalent chromium and hexavalent chromium,and drying the treated surface.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a process for preparing aluminum and aluminum alloy surfaces in which the treated surface is particularly well-suited for application thereto of an organic coating. More particularly, the present invention relates to a process for preparing aluminum and aluminum alloy surfaces which are capable of forming environmentally stable bonds with an organic coating such as paint and the product produced thereby.
Aluminum and its various alloys are widely used in sheet and panel form and in a multitude of different structural forms, many of these requiring a paint finish. There are a number of methods in commerical use for preparing the surfaces of aluminum and its alloys so that they may be painted. However, there in a need for a pretreatment method that will serve to prepare the surfaces of aluminum and its alloys to provide an excellent paint pretreatment and also to provide a reliably strong environmentally stable bond with the paint. While some methods have been used to accomplish these requirements for a pretreatment process, improved painting quality, durability and consistency, and the ability of such a process to provide organic coating bonds with improved strength and environmental stability continues to be sought.

DISCUSSION OF RELATED ART

Various pretreatment methods have been developed and are in commercial use for providing painted aluminum or aluminum alloy surfaces. Typically, these methods comprise the steps of vapor degreasing the aluminum, cleaning the aluminum with an alkaline cleaner, rinsing, deoxidizing the aluminum, rinsing, forming a chemical conversion coating on the aluminum to which an organic coating will adhere and then rinsing and drying before painting. For example, such methods are disclosed in "Standard Recommended Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting" ASTM D 1730, and "Voluntary Specification for High Performance Organic Coatings on Architectural Extrusions and Panels", Architectural Aluminum Manufacturers Association, Publication No. AAMA 605.2.
Another method that is used commercially to prepare the surfaces of aluminum and its alloys for painting comprises the steps of alkaline cleaning the aluminum, rinsing with hot water, acid cleaning with a non-aggressive acid, rinsing, caustic etching, rinsing, desmutting, rinsing, forming a chromate-phosphate conversion coating, and then rinsing and drying before painting. While this method provides commercially usable painted surfaces, efforts have continued to improve and upgrade the processing conditions and to develop a pretreatment method for preparing aluminum and aluminum alloy surfaces which are capable of forming long-term environmentally stable bonds with an organic coating such as a paint, even under harsh conditions.
In addition, due to chromium waste constraints and such waste treatment requirements, it is desirable to provide an aluminum processing system wherein chromate and/or chromic acid is not rinsed from the treated surfaces or disposed into sewage systems.

DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term "about".
Accordingly, the process of this invention comprises the steps of treating an aluminum or aluminum alloy surface by contacting said surface with an acidic cleaning solution selected from sulfuric acid, phosphoric acid, hydrofluoric acid, or a mixture thereof, and a nonionic or anionic surfactant for a time sufficient to remove the soils therefrom and etch the aluminum surface, rinsing the surface with tap water, contacting the surface with an acidic corrosion resistant coating solution comprising chromium salts in both the trivalent and hexavalent states, with or without the presence of amorphous, fumed silica until the surface is completely covered with the coating solution, and drying the treated surface.
In one embodiment of the afore-described steps, where the nature of the aluminum or aluminum alloy surface is such that it is heavily contaminated, for example, as with grease, oil, or rust, prior to practicing the afore-mentioned steps it is preferred to initially treat the aluminum surface with an alkaline cleaning solution comprising an alkali metal hydroxide and a complexing agent, followed by rinsing the surface with tap water, and then treating the aluminum surface as described above.
When the treated surface is dry, an organic coating such as a paint may be applied to the surface and the coating is cured or allowed to dry. It has been found that an aluminum or aluminum alloy surface thus-treated provides an excellent base for the organic coating and results in an article having an environmentally stable bond with the coating. After coating, such as by painting, the article exhibits long-term durability under severe environmental conditions.
More specifically, when the aluminum or aluminum alloy surface is heavily contaminated, it is subjected to a cleaning process in preparation for the further surface treatment. In such event, the cleaning solution preferably comprises alkali metal hydroxide such as sodium hydroxide and/or potassium hydroxide and a complexing agent for metals or mineral salts. The complexing agent is preferably an alkali metal salt or a hydroxycarboxylic acid such as sodium gluconate, sodium glucoheptonate, and the like. Such suitable alkaline cleaners are commercially available from Amchem Products, Inc., Ambler, Pa., under the trade names Ridoline® 34, Ridoline® 35, and Ridoline® 38 cleaner.
Satisfactory results are generally obtained when the afore-mentioned alkaline cleaner is used to clean a heavily soiled aluminum or aluminum alloy surface at a concentration of between about 1.6 and about 16.3 g./l. of alkali metal hydroxide and about 0.12 to about 1.2 g./l. of said complexing agent. Preferably, the alkaline cleaner is used at a concentration of between about 4.9 and about 11.4 g./l. of alkali metal hydroxide and about 0.36 to about 0.85 g./l. of the complexing agent because more consistent cleaning results are obtained. Optimum results are usually obtained when the alkaline cleaner is used at a concentration of between about 6.5 and about 9.8 g./l. of alkali metal hydroxide and about 0.6 g./l. of the complexing agent.
The temperature at which satisfactory results are obtained with the alkaline cleaning solution may be between about 100°F and about 150°F. However, the temperature of the alkaline cleaning solution is preferably between about 120°F and about 140°F because faster and better cleaning results are obtained.
Satisfactory cleaning of the aluminum or aluminum alloy surface is provided when it is contacted with the alkaline cleaning solution for between about 15 seconds and about 3 minutes. preferably, the contact time of the aluminum and aluminum alloy surface with the alkaline cleaning solution is between about 45 seconds and about 2 minutes because more thorough cleaning results are assured.
Contact of the aluminum or aluminum alloy surface may be by any suitable conventional method such as by spray application or by immersion in the alkaline cleaning solution. Preferably, the alkaline cleaning solution is applied with power spray processing equipment at a pressure of between about 5 psi and about 15 psi. The alkaline cleaning solution is readily adaptable to automatic bath control using LINEGUARD® electronic control equipment which is available from Amchem Products, Inc., Ambler, Pa. The alkaline cleaner solution may also be manually controlled using a cleaner titration test set known as AMCHEM® Chemical Test Set 2615 or its equivalent, and a specific gravity determination using a hydrometer. The chemical test set and hydrometer may also be used to monitor the bath concentration/condition when LINEGUARD® electronic control equipment is used.
After the aluminum or aluminum alloy suface has been cleaned with the alkaline cleaning solution and rinsed with tap water, the surface of the aluminum article is contacted with an acidic cleaning solution which may be free of fluorides or one which contains fluorides. The acidic cleaning solution may comprise sulfuric acid, phosphoric acid, or hydrofluoric acid, and a nonionic surface-active agent selected from a modified oxyethylated straight-chain alcohol having an HLB value of between about 10 and about 12, a polyethylene glycol rosin ester, and an alkyl polyoxyalkylene ether, or an anionic surfactant selected from an alkyl sodium sulfate such as 2-ethylhexylsulfate. Suitable acidic cleaners for this purpose are available from Amchem Products, Inc., Ambler, Pa., under the under the tradenames Ridoline® 124, Ridoline® 124/120E, Ridoline® 200 Brite, and Ridoline® 2001 Cleaner.
Satisfactory results are generally obtained when the afore-mentioned acidic cleaner is used to clean an aluminum or aluminum alloy surface at a concentration of between about 1.5 and about 11.9 g./l. of sulfuric acid (66°Be) and about 0.3 to about 2.6 g./l. of said nonionic surface-active agent. Preferably, the acidic cleaner is used at a concentration of between about 3 and about 9 g./l. of sulfuric acid (66°Be) and about 0.6 to about 2 g./l. of said nonionic surface active agent because more efficient cleaning results are obtained.
The temperature at which satisfactory results are obtained with the acidic cleaner may be between about 100 and about 180°F. However, the temperature of the acidic cleaning solution is preferably between about 125 and about 140°F because efficient cleaning results are obtained and cleaning tank corrosion is minimized compared to operating at higher temperatures .
Satisfactory cleaning of the aluminum or aluminum alloy surface is provided when it is contacted with the acidic cleaning solution for between about 15 seconds and about 45 seconds. Preferably, the contact time of the aluminum and aluminum alloy surface with the acidic cleaning solution is between about 1 minute and 3 minutes because such assures total soil removal and satisfactorily prepares the substrate for further treatment.
Contact of the aluminum or aluminum alloy surface may be by any suitable conventional method such as by spray application or by immersion in the acidic cleaning solution. Preferably, the acidic cleaning solution is applied with power spray processing equipment. The acidic cleaning solution is also readily adaptable to automatic bath control using LINEGUARD® electronic control equipment which is available from Amchem Products, Inc., Ambler, Pa. The acidic cleaner solution may also be manually controlled using a cleaner titration test set known as AMCHEM® Chemical Test Set 6233 or its equivalent, and a specific gravity determination using a hydrometer. The chemical test set and hydrometer may also be used to monitor the bath concentration/condition when LINEGUARD® electronic control equipment is used. The equipment for the acidic cleaning solution should be constructed of stainless steel such as Type 316, rubber lined mild steel, or other suitable acid resistant materials.
After the aluminum or aluminum alloy surface has been cleaned with the acidic cleaning solution and rinsed with tap water, the surface of the aluminum article is contacted with an acidic coating solution which is used to produce a corrosion resistant coating on the aluminum or aluminum alloy surface. The acidic coating solution preferably comprises trivalent chromium, hexavalent chromium, and an amorphous, fumed silica dispersion. Suitable acidic coating products for this purpose are available from Amchem Products, Inc., Ambler, Pa., under the tradenames Alodine® NR-2375 and Alodine® NR-2010 Coating Chemical.
Satisfactory results are generally obtained when the afore-mentioned acidic coating product is used to treat the aluminum or aluminum alloy surface at a concentration of between about 0.01 and about 0.7 g./l. of trivalent chromium salt, about 0.02 to about 1.5 g./l. of hexavalent chromium salt, and about 0.03 to about 3.3 g./l. of said amorphous fumed silica. Preferably, the coating product is used at a concentration of between about 0.4 and about 0.5 g./l. of trivalent chromium salt, about 0.9 to about 1.2 g./l. of hexavalent chromium salt, and about 1.9 to about 2.6 g./l. of said silica because a more corrosion resistant coating is obtained. The coating solution should have a pH of about 3.0.
The temperature at which satisfactory results are obtained with the coating product may be between about 70 and about 90°F. However, the temperature of the coating product solution is preferably between about 90 and about 130°F because more efficient coating results are obtained.
Satisfactory coating of the aluminum or aluminum alloy surface is provided when it is contacted with the corrosion resistant coating solution for between about 5 seconds and about 30 seconds. Preferably, the contact time of the aluminum or aluminum alloy surface with the coating solution is between about 30 seconds and about 2 minutes because more uniform coating deposition is obtained.
Contact of the aluminum or aluminum alloy surface may be by any suitable conventional method such as by spray application or by immersion in the corrosion resistant coating solution. Preferably, the coating solution is applied with power spray processing equipment. The corrosion resistant coating solution may be manually controlled by an Alodine® coating chemical titration which enables the determination of required solution replenishment. This chemical titration may be accomplished using Amchem® Chemical Test Set 2266 or its equivalent.
After application to the aluminum or aluminum alloy surface, the wet film of corrosion resistant coating solution is dried on the metal surface, without rinsing, prior to applying an organic coating thereto such as paint. The afore-mentioned process steps and the corrosion resistant coating provides excellent paint bonding properties to the metal surface and affords underfilm corrosion protection. For best results, the treated parts coming from the corrosion resistant coating solution should be dried in an indirectly fired oven or by such means which will not contaminate the metal surface with fumes, oil, gases, or other contaminant. If manual handling of the dried, unpainted parts is necessary, the operators should wear clean cotton gloves or equivalent protective gear.
The process of this invention may be used to prepare an aluminum or aluminum alloy surface whether it be in the form of sheets, extrusions, or castings for painting. Moreover, the process may be performed in either a batch or continuous manner by either a spray or immersion mode. Preferably, the process is performed in an automated sequence from one spray tank to the other spray tank.
The invention will be more clearly understood by reference to the following examples, which are set forth as being merely illustrative of the invention and which are not intended in any manner to be limitative thereof.
In the examples, a number of tests and evaluations were performed and these are described as follows. These test procedures and performance requirements serve to evaluate pigmented organic coatings applied to aluminum extrusions for window, doors, and similar products made from aluminum extrusions. They provide a good level or performance in terms of film integrity, exterior weatherability and general appearance over a period of many years. The terms "film" and "coating" are used interchangeably herein and are defined as meaning the layer of pigmented organic material applied to cover the metal surface after curing and/or drying.
The following described tests may be found in Publication No. AAMA 603.8-85 titled "Voluntary Performance Requirements And Test Procedures For Pigmented Organic Coatings On Extruded Aluminum" by the Architectural Aluminum Manufacturers Association (AAMA), 2700 River Road, Des Plaines, Illinois 60018.

1. Film Adhesion (Cross Hatch Tape) Test:

(a) Eleven parallel cuts, 1/16 inch apart, are made through the film, and eleven similar cuts are made at 90° to and crossing the first eleven cuts. A tape such as Permacel 99 or equivalent 3/4 inch wide is applied over the area of cuts by pressing down firmly against the coating to eliminate voids and air pockets. Then, the tape is sharply pulled off at a right angle to the plane of the surface being tested. The test pieces should be at ambient temperature (65-80°F). This test is a measure of dry film adhesion.
(b) For wet film adhesion evaluation, parallel cuts are made through the films as outlined in (a) above. The sample is immersed in distilled or deionized water at 100°F for 24 hours. The sample is removed and wiped dry. Within five minutes, a tape as described in (a) above is applied and then pulled off as described therein.

To pass the film adhesion test, no removal of film under the tape within or outside of the cross-hatched area or blistering anywhere on the wet test specimen should be evident. Any loss of film adhesion is reported as a percentage of squares affected, for example, if 10 squares are lifted, this is reported as 10% failure.

2. Muriatic Acid Resistance:

Ten drops of a 10% by volume solution of muriatic acid (37% commerical grade HC1) in tap water are applied to a coated aluminum specimen and covered with a watch glass, convex side up. The acid solution and the test temperature is at 65-70°F. After 15 minutes exposure, the acid solution is washed off with running tap water. The test is repeated at least four times.
To pass this test, no blistering and no visual change in appearance of the specimen should be evident when examined by the unaided eye.

3. Mortar Resistance:

A mortar is prepared by mixing 75 grams of building lime (ASTM C 207) and 225 grams of dry sand, both passing 10-mesh wire screen, and sufficient water (about 100 grams) to make a soft paste. Wet pats of mortar about 2 square inches in area and 1/2 inch in thickness are immediately applied to coated aluminum specimens which have been aged at least 24 hours after coating. The test specimens are immediately exposed for 24 hours to 100% relative humidity at 100°F. At least four tests are conducted.
To pass the mortar resistance test, the mortar should dislodge easily from the painted surface, and any residue should be removable with a damp cloth. Any lime residue should be easily removed with a 10% muriatic acid solution as described in section 6.6.1.1. of said afore-mentioned AAMA publication. There should be no loss of film adhesion and visual change in appearance when examined by the unaided eye.

4. Detergent Resistance:

A 3% by weight solution of detergent and distilled water is prepared. At least 2 test specimens are immersed in the detergent solution at 100°F for 72 hours. The samples are removed and wiped dry. A tape (Permacel 99 or equivalent, 3/4" wide) is immediately applied by pressing down firmly against the coating to eliminate voids and air pockets. The tape is placed longitudinally along the entire length of the test specimen. If blisters are visible, the blistered area is taped and rated. The tape is then pulled off sharply at a right angle to the plane of the surface being tested.

The detergent composition is as follows::

Technical Grade Reagents	% by Weight
Tetrasodium Pyrophosphate	45
Sodium Sulphate, Anhydrous	23
Sodium Alkylarysulfonate	22
Sodium Metasilicate, Hydrated	8
Sodium Carbonate, Anhydrous	2
	100

To pass the detergent resistance test, there should be no loss in adhesion of film to metal; and no blistering or significant visual change in appearance when examined by the unaided eye.

5. Humidity Resistance:

Samples are exposed in a controlled heat-and-humidity cabinet or 1,000 hours at 100°F and 100% relative humidity with the cabinet operated in accordance with ASTM D 2247.
To pass the humidity resistance test, there should be no formation of blisters to an extent greater than a "few" blisters size no. 8, as shown in figure no. 4, ASTM D 714.

6. Salt Spray Resistance:

Using a sharp knife or blade instrument, the film is scored sufficiently deep to expose the base metal. The sample is exposed for 1,000 hours according to ASTM B 117 using a 5% salt solution. The sample is removed and wiped dry. A tape (Permacel 99 or equivalent, 3/4 inch wide) is immediately applied over the scored area by pressing down firmly against the coating to eliminate voids and air pockets. The tape is then sharply pulled off at a right angle to the plane of the surface being tested.
To pass the salt spray resistance test, a minimum rating of 7 on the horizontal scribe or cut edges, and a minimum blister rating of 8 within the test specimen field is required, in accordance with the following Table 1 and Table 2 (Reference-Modification of ASTM D 1654).

Example I

Aluminum alloy 6061 coil stock, equivalent to 900 square feet of metal, was pretreated in an alkaline cleaning solution comprising about 6.5 g/l of sodium hydroxide and about 0.5 g/l of sodium gluconate. The solution had a temperature of about 140°F, and the coil stock was treated for about 15 seconds. The coil stock was removed from the cleaning solution and rinsed with tap water. The coil stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/l of hexavalent chromium, and about 2.4 g/l of amorphous fumed silica. The acidic coating solution had an initial pH of about 3.22. The coil stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution bath had increased to 3.52. The rise in pH was due to carry-over from the alkaline cleaning solution and the tap water rinse stage. This pH change in the acidic coating solution bath caused an instability problem with both the trivalent chromium and fumed silica therein, and bath separation was observed.
Aluminum alloy 6063 extrusions were processed in the acidic coating solution bath containing dissolved aluminum for about 10 seconds at a temperature of about 90°F. After treatment, the aluminum extrusions were static air dried at ambient temperature. After drying, the aluminum extrusions were painted with Pittsburgh Paint Glass Company Quaker brand High Solids Bronze paint code #UC 51044 . The film adhesion testing results were satisfactory, but salt spray corrosion resistance was borderline.
A fresh, comparable acidic coating solution was prepared for a pH traverse study. Increments of a fresh alkaline cleaning solution containing the same amounts of ingredients as above were added to the acidic coating solution until the pH was about 4.5 at which time the acidic coating solution bath separated. It was found that about 5.8% of the alkaline cleaning solution had been added to the acidic coating solution, indicating that about 5.8% contamination from the alkaline cleaning solution causes the acidic coating solution to separate and incur stability problems.
Samples were taken from the acidic coating solution bath used to treat the aluminum alloy 6063 extrusions for analysis of chromium, silicon and aluminum content. Samples were also taken from a fresh acidic coating solution bath; and the acidic coating solution bath wherein 400 square feet of the aluminum alloy metal had been processed. The analytical results confirmed silica and trivalent chromium separation from the acidic coating solution. From the start of treating the aluminum alloy to the end thereof, about a 20% loss of chromium and about a 90% loss in silica was observed due to bath separation. A total solids content determination was performed on the used bath for further examination as to bath separation. Aliquots thereof were taken from the top, middle and bottom regions. The total solids content gradient ranged from 0.295% at the top to 1.644% at the bottom of the bath. The fresh bath had a total solids content of 0.594%.
A further study was conducted using a fresh acidic coating solution bath which was contaminated with about 6%, and about 10%, respectively, of the alkaline cleaning solution bath. Aluminum extrusions were processed therein and then painted after drying. AAMA testing showed that 6% contamination with the alkaline cleaning solution of the acidic coating solution bath provided unsatisfactory 1,000 hour humidity and detergent testing results. Ten percent contamination produced unsatisfactory wet adhesion, detergent, and humidity testing results.
It was concluded that because of the volume of metal processed in conventional industrial aluminum processing lines, contamination of the acidic coating solution bath is going to occur to an unacceptable extent. Due to the alkaline sensitive nature of the acidic coating solution bath, an alternate cleaner system was pursued and developed as described in the following examples.

Example II

Aluminum alloy 6063 was pretreated in an acidic cleaning solution comprising about 4.5 g/l of sulfuric acid (66°Be) and about 1.0 g/l of a modified oxyethylated straight alcohol nonionic surface-active agent having an HLB value of about 10 to about 12. The cleaning solution had a temperature of about 140°F, and the aluminium alloy was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/l of hexavalent chromium, and about 2.4 g/l of amorphous fumed silica. The acidic coating solution had an initial pH of about 3.22. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.2.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the afore-mentioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Example III

Aluminum alloy 6063 was pretreated in an acidic cleaning solution comprising about 4.5 g/l of sulfuric acid (66°Be) and about 1.0 g/l of a modified oxyethylated straight alcohol nonionic surface-active agent having an HLB value of about 10 to about 12. The cleaning solution had a temperature of about 140°F, and the aluminum alloy was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/l of hexavalent chromium, and no amorphous fumed silica. The acidic coating solution had an initial pH of about 3.1. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.1.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the afore-mentioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Example IV

Aluminum alloy 6063 was pretreated in an acidic cleaning solution comprising about 4.5 g/l of sulfuric acid (66°Be) and about 1.0 g/l of a modified oxyethylated straight alcohol nonionic surface-active agent having an HLB value of about 10 to about 12. The cleaning solution had a temperature of about 140°F, and the aluminum alloy was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.09 g/l of trivalent chromium, about 0.22 g/l of hexavalent chromium acid, and no amorphous fumed silica. The acidic coating solution had an initial pH of about 3.45. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.45.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the aforementioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Example V

Aluminum alloy 6063 was pretreated in an acidic cleaning solution comprising about 5.5 g/l of sulfuric acid (66°Be), about 14.8 g/l of phosphoric acid and about 0.39 g/l of an alkyl polyoxyalkylene ether nonionic surface-active agent having an HLB value of about 10 to about 12 and about 0.61 g/l of a polyethylene glycol ether of rosin (nonionic wetting agent). The cleaning solution had a temperature of about 140°F, and the aluminum alloy was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/kl of hexavalent chromium, and no amorphous fumed silica. The acidic coating solution had an initial pH of about 3.1. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.1.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the aforementioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Example VI

Aluminum alloy 6063 was pretreated in an acidic cleaning solution comprising about 4.5 g/l of sulfuric acid (66°Be), about 0.02 g/l of hydrofluoric acid, and about 1.0 g/l of a modified oxyethylated straight alcohol nonionic surface-active agent having an HLB value of about 10 to about 12. The cleaning solution had a temperature of about 130°F, and the aluminum alloy was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/l of hexavalent chromium, and about 2.4 g/l of amorphous fumed silica. The acidic coating solution had an initial pH of about 3.2. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.2.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the afore-mentioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Example VII

Heavily soiled aluminum alloy 6063 was pretreated in an alkaline cleaning solution comprising about 6.5 g/l of sodium hydroxide and about 0.6 g/l of sodium gluconate. The cleaning solution had a temperature of about 140°F, and was used to treat the aluminum alloy for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then treated in an acidic cleaning solution comprising about 4.5 g/l of sulfuric acid (66°Be) about 0.02 g/l of hydrofluoric acid, and about 1.0 g/l of a modified oxyethylated straight alcohol nonionic surface-active agent having an HLB value of about 10 to about 12. The cleaning solution had a temperature of about 130°F, and the aluminum stock was treated therewith for about 45 seconds. The aluminum stock was removed from the cleaning solution and rinsed with tap water. The aluminum stock was then sprayed with an acidic coating solution bath comprising about 0.47 g/l of trivalent chromium, about 1.09 g/l of hexavalent chromium, and about 2.4 g/l of amorphous fumed silica. The acidic coating solution had an initial pH of about 3.2. The aluminum stock was treated in the acidic coating solution for about 10 seconds at a temperature of about 90°F. At the end of the run, the pH in the acidic coating solution was about 3.2.
The aluminum alloy stock was dried as in Example I and then painted with PPG Quaker brand High Solids Bronze code UC 51044. After the painted aluminum alloy stock was dry, it was evaluated per the afore-mentioned AAMA test procedures for film adhesion, acid resistance, detergent resistance, and salt spray resistance. The painted panels satisfactorily passed the afore-mentioned tests.

Claims

1. The process of pretreating an aluminum or aluminum alloy surface prior to applying an organic coating thereto comprising contacting said surface with an acidic cleaning solution selected from sulfuric acid, phosphoric acid, hydrofluoric acid, or a mixture thereof, and a nonionic or anionic surfactant for a time sufficient to remove soil from said surface and etch said surface, rinsing said surface with water, contacting said surface with an acidic coating solution comprising trivalent chromium and hexavalent chromium until said surface is covered with said coating solution, and drying said surface.

2. The process in accordance with claim 1 wherein said nonionic surfactant is selected from a modified oxyethylated straight-chain alcohol having an HLB value of between about 10 and about 12, a polyethylene glycol rosin ester, and an alkyl polyoxyalkylene ether.

3. The process in accordance with claim 1 wherein said cleaning solution contains from about 1.5 to about 11.9 g./l. of sulfuric acid (66°Be) and about 0.3 to about 2.6 g./l. of said nonionic surfactant.

4. The process in accordance with claim 1 wherein said cleaning solution is at a temperature of from about 100°F to about 180°F.

5. The process in accordance with claim 1 wherein said surface is contacted with said cleaning solution for between about 15 seconds and about 3 minutes.

6. The process in accordance with claim 1 wherein said coating solution is at a temperature of from about 70°F to about 130°F.

7. The process in accordance with claim 1 wherein said coating solu tion contains from about 0.01 to about 0.7 g./l. of trivalent chromium and from about 0.02 to about 1.5 g./l. of hexavalent chromium.

8. The process in accordance with claim 1 wherein said surface is contacted with said coating solution for between about 5 seconds and about 2 minutes.

9. The process in accordance with claim 1 wherein prior to said steps, the step of contacting said surface with an alkaline cleaning solution comprising an alkali metal hydroxide and a complexing agent, and rinsing said surface with water.

10. The process in accordance with claim 9 wherein said complexing agent comprises an alkali metal salt of a hydroxy-carboxylic acid.

11. The process in accordance with claim 10 wherein said salt of said hydroxy-carboxylic acid is selected from sodium gluconate and sodium glucoheptonate.

12. The process in accordance with claim 9 wherein said alkaline cleaning solution is at a temperature of from about 100°F to about 150°F.

13. The process in accordance with claim 9 wherein said surface is contacted with said alkaline cleaning solution for between about 15 seconds and about 3 minutes.

14. The process in accordance with claim 9 wherein said alkaline cleaning solution contains from about 1.6 to about 16.3 g./l. of said alkali metal hydroxide and from about 0.12 to about 1.2 g./l. of sodium gluconate.

15. The process in accordance with claim 1 wherein said contacting steps are performed by immersion.

16. The process in accordance with claim 1 wherein said contacting steps are performed by spray application.

17. The process in accordance with claim 9 wherein said contacting steps are performed by immersion.

18. The process in accordance with claim 9 wherein said contacting steps are performed by spray application.

19. The process of preparing an environmentally stable painted surface of aluminum or aluminum alloy comprising the steps of preparing a surface of aluminum or aluminum alloy in accordance with the method of claim 1, and thereafter painting the prepared surface.

20. The process of preparing an environmentally stable painted surface of aluminum or aluminum alloy comprising the steps of preparing a surface of aluminum or aluminum alloy in accordance with the method of claim 9, and thereafter painting the prepared surface.