DE68924012T2 - Chromium cleaning / deoxidation system. - Google Patents

Abstract

The invention is a metal treating process which uses a chrome-free deoxidizing bath. The process is useful for cleaning and deoxidizing aluminum substrates.

Description

This invention relates to a new process for cleaning and deoxidizing aluminum prior to a conversion treatment. The process is based on an acidic or basic cleaning system and a chromate-free deoxidizing system.

Traditionally, chromated deoxidizers have been used to enable aluminum samples to pass signature corrosion tests such as MIL-C-5541C. The aluminum is first cleaned in a relatively non-corrosive alkaline soak cleaner, deoxidized in an acidic chromate solution, and then chromated. Alternatively, if etching was desired, an alkaline etch or alkaline chemical stripping solution was used. This step was followed by a deoxidation step, which usually used a chromated deoxidizer to remove "grit" produced during etching. The aluminum was then chromated. In the past, attempts to replace chromate in the deoxidation step generally involved the use of iron salts such as ferric sulfate. However, iron-based deoxidizers have never produced results comparable to the chromate-containing deoxidizer systems.

U.S. Patent 4,451,304 to Batiuk provides a treatment for aluminum that includes an alkaline cleaning step and a deoxidation step. The deoxidation step uses a non-chromate cleaning agent that is an aqueous solution of sodium or potassium nitrite. This patent includes a detailed discussion of the prior art in this area.

US-A-4 459 216 discloses an aqueous acidic dissolving solution for metals, e.g. aluminum and aluminum alloys, comprising hydrogen peroxide, an inorganic acid and at least one aromatic compound, which solution can be used at a temperature of 10 ºC to 80 ºC and for a treatment time of 5 seconds to 15 minutes.

GB-A-2 186 292 discloses a method for cleaning aluminium surfaces using:

- a pre-wash with an aqueous solution, preferably an acidic pre-wash solution,

- a primary alkaline cleaning step to remove residual aluminium from the surface of the treated vessel without undesirable etching,

- a step of rinsing with slightly acidified water with an aqueous rinsing solution with a pH of not more than 6, which rinsing solution may contain fluorine and/or phosphate ions as an accelerator for the removal of the oxide coating, and

- an elective conversion treatment.

GB-A-2100 757 relates to a fluorine-free cryogenic solution for cleaning aluminium, which comprises sulphuric acid, phosphoric acid and surfactants.

The present invention is defined by the content of the main claims. Further embodiments can be found in the dependent claims.

The present invention is advantageous in that it eliminates the use of hexavalent chromium for the cleaning and deoxidation step and further provides improved Operation over previously disclosed methods for cleaning and deoxidizing aluminum prior to chemical conversion treatments.

In accordance with the present invention, aluminum is purified in an aqueous acidic or alkaline solution. The preferred acidifying agents are acids such as sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, citric acid, oxalic acid, acetic acid, gluconic acid, hydroxyacetic acid and the like, or the mixtures of two or more of these acids. Preferably, the acidic solution is a dilute solution which gives a low etch.

The alkaline cleaning solutions useful in the present invention generally contain alkali metal hydroxides or other water-soluble alkaline materials such as trisodium phosphate, alkali silicates, tetrasodium pyrophosphate, and the like.

The etchants may also include acidic solutions of fluoride compositions such as hydrogen hydrogen and fluorine salts, fluorine complexes and the like such as ammonium hydrogen fluoride, hexafluorosilicic acid, their salts and the like.

Following the cleaning step, the aluminum is then immersed in an aqueous acidic deoxidizer solution containing at least one of the following deoxidizer compositions, hydrogen peroxide or heteropolyvanadic acid or salts thereof. Optionally, the aluminum may be rinsed prior to contact with the deoxidizer solution. The use of the rinse is preferred.

In another embodiment of the invention, the aluminum article can be cleaned and deoxidized in a single step by using a deoxidizer compound how hydrogen peroxide is incorporated into the acidic cleaning solution.

In describing the invention, the term "aluminium" used in connection with the substrate metal to be treated includes aluminium metal as such and, in addition, those aluminium alloys which are generally subjected to a cleaning and deoxidation treatment prior to chromating or other chemical or electrochemical conversion treatment.

Aluminium metal as such invariably contains trace amounts of other metals. Examples of such contaminating metals are copper, manganese, nickel, zinc, titanium, vanadium, sodium and gallium.

Aluminum alloys generally contain larger amounts of other metals. Such alloying metals include silicon, chromium, lead, iron, copper, magnesium, manganese and zinc.

Such metals are always present in the deoxidizer solutions in the form of ions, as are aluminum ions, due to the action of such solutions on the metal and the dirt. It is an aspect of the present invention that the function of the deoxidizer solutions containing stabilizers can continue in the presence of such other metal ions, particularly iron, copper and manganese. The polyvalent metal ions tend to catalyze the decomposition of the hydrogen peroxide component. This is especially true for copper and manganese.

With reference to the first step of the invention, the aluminium substrate is subjected to cleaning and/or etching, preferably with a dilute aqueous acid solution at a pH of less than 2.

Preferably the pH is less than 2 and most preferably less than 1.5. The cleaning solutions generally contain from 0.005% to 5% or 10% by weight of the acids, but may contain up to 50% by weight and preferably from 0.2% to 8% by weight and more preferably from 0.4% to 2.5% by weight. It is also preferred to use a mixture of acids such as sulfuric acid and hydrofluoric acid.

While optional, suitable surfactants may be included in the cleaning solutions. They may be present in amounts of from 0% to 5% by weight, and preferably up to 2% by weight, and most preferably up to 0.5% by weight. Any suitable surfactant that is compatible with the cleaning solution and does not leave an undesirable film on the surface of the aluminum may be used. Such surfactants include the cationic, anionic, amphoteric and nonionic surfactants.

The cleaning and/or etching step can be carried out in an alkaline cleaning-etching solution. Known alkaline cleaning solutions containing alkali metal hydroxide, alkali metal carbonate, trialkali metal phosphate (such as trisodium phosphate), tetraalkali metal pyrophosphate (tetrasodium pyrophosphate), alkali metal silicates, alone or in combination, can also be used for the cleaning step of the present invention. Alkaline cleaning compositions such as Ridolin 53, P3-Almeco 18 and Ridolin 322 can be used in the cleaning step.

In an alternative embodiment of the invention, an acidic cleaning-etching step may be combined with the deoxidation step by including hydrogen peroxide and a hydrogen peroxide stabilizer in an acidic cleaning solution. The combined cleaning and deoxidation steps may be carried out at a temperature of from room temperature to 76.6 ºC (170 ºF), and preferably from 32.2 ºC (90ºF) to 65.5ºC (150ºF). The pH of the solution is preferably less than 3, more preferably less than 1.5, and most preferably less than 0.5.

It will be appreciated that the cleaning etching step can be performed in a short period of time with substantially little etching or can be performed for a longer period of time if a deeply etched surface is desired.

It is also appreciated that the cleaning-etching step can be carried out at room temperatures, but it is preferred to carry out the cleaning step at elevated solution temperatures. Higher temperatures increase the cleaning rate and/or etching efficiency of the cleaning solution and a certain degree of care must be exercised to ensure that the desired level of cleaning or etching is obtained. The temperature of the cleaning/etching solution is preferably 32.2°C (90°F) to 79.4°C (175°F) and most preferably 37.7°C (100°F) to 65.5°C (150°F).

With respect to the deoxidation step, it is preferred that the deoxidizer bath is acidic and preferably below a pH of 4 and more preferably at or below 2. When heteropolyions are used in the deoxidizer bath, they may be added to the bath as such or may be generated in situ from their constituents.

When heteropolyvanadic acid or its salts are used, the deoxidizer comprises 0.1 g/l up to the solubility limit of the material and an acid, thereby forming a composition with a pH below 3. Generally, mineral acids are used and nitric acid is preferred.

When the deoxidizer is hydrogen peroxide, it is preferred that the solutions contain from 0.03 to 30% by weight of hydrogen peroxide, and it is further preferred that one or more stabilizers for hydrogen peroxide also be used in the solution. With respect to deoxidizer solutions of hydrogen peroxide, it is also preferred to adjust the pH to 3 or less, preferably with nitric acid. The pH is preferably no greater than 1.5, and most preferably 0.5 or less. The deoxidizer solution is preferably maintained at a temperature of from room temperature to 65.5°C (150°F), depending on the length of time the aluminum article is in contact with the solution.

The aluminum object is in contact with the deoxidizer solution for preferably 1 to 25 minutes. The contact time is generally shorter at higher temperatures.

Stabilizers for peroxides are well known and are illustrated in U.S. Patent 4,059,678. Other suitable stabilizers for hydrogen peroxide are well known in the art and any of these can be used in a suitable manner provided they are compatible with acidic solutions. A preferred group of stabilizers is disclosed in U.S. Patent Application Serial No. 07/221,063, filed July 19, 1988 in the name of McMillen and entitled "Stabilization of Peroxide Solutions," corresponding to EP 0361 772 A1.

After the deoxidizer treatment, the treated aluminum substrate is then chromated by chromating treatments for aluminum, which are well known in the art. Chromating with chromating compositions such as Alodine 1200S or Alodine 600, a product of Parker + Amchem, a division of Henkel Corporation, are useful. The aluminum substrates are preferably rinsed before being treated with the chromating compositions. Rinsing is preferred to reduce contamination of the chromating composition by the components of the deoxidizer treatment composition.

The following examples are intended to illustrate this invention and are not to be considered as limiting.

EXAMPLE 1

A cleaning solution was prepared as follows:

A. 70 ml of a solution prepared by diluting 323 ml of 96% sulphuric acid with water to one litre.

B. 70 ml of a solution of hydrofluoric acid concentrate, prepared by diluting 214 g of 70% hydrofluoric acid to one liter. 70 ml of A and 70 ml of B were mixed and diluted to 7 liters with a mixture of 25% Ambler Borough (PA.) tap water and 75% deionized water. The mixture of tap water and deionized water had a conductivity of 250 uS.

A deoxidizer solution was prepared as follows:

First, a concentrate was prepared by mixing 3800 ml of 35% hydrogen peroxide and 200 ml of Dowfax 241 (sodium dodecyl diphenyl oxide disulfonate).

1610 ml of the hydrogen peroxide concentrate prepared as above and 475 ml of 70% nitric acid were diluted to 18.9 liters.

Plates made of aluminum alloy 2024-T3 were heated at a temperature of 48.8 ºC (120 ºF) for a period of ten minutes in the cleaning solution. The plates were then removed, rinsed with deionized water and immersed in the deoxidizer solution for 15 minutes at room temperature. The plates were rinsed and then chromated. The chromated plates successfully passed the MIL-C-5541C Neutral Salt Spray Specification test. The MIL-C-5541C specification and test procedures are expressly incorporated herein by reference.

EXAMPLE 2

A cleaning solution concentrate was prepared by mixing 100 ml of 96% sulfuric acid and 416 ml of 75% phosphoric acid and diluting to one liter.

A cleaning solution was prepared by mixing 140 ml of the cleaning solution and 70 ml of hydrofluoric acid solution concentrate as prepared in Example 1 and diluting to 7 litres. The deoxidizer solution used in this example was identical to that used in Example 1.

Aluminum alloy 2024-T3 panels were immersed in the above cleaning solution at a temperature of 48.8ºC (120ºF) for a period of 10 minutes. The panels were then removed, rinsed with water, and immersed in the deoxidizer solution at room temperature for a period of 15 minutes. The panels were then rinsed and chromated. These panels successfully passed the MIL-C-5541C neutral salt spray specification test.

EXAMPLE 3

A cleaning solution concentrate was prepared by mixing 256 ml of 96% sulfuric acid, 90 g of Mirawet B (amphoteric surfactant), 90 g of Surfonic LF-17 (ethoxylated alcohol) and diluting with water to one liter.

Four litre cleaning baths containing 50 ml of the cleaning concentrate were prepared containing varying amounts of hydrofluoric acid as shown in Tables I and II.

The amount of hydrofluoric acid added to the baths is given in Tables I and II and is based on the hydrofluoric acid concentrate as prepared in Example 1. With respect to the 0.17% solutions, 40 milliliters of the hydrogen fluoride concentrate were added. With respect to the 0.09% solutions, 21 milliliters of the hydrogen fluoride concentrate were added and with respect to the 0.009% solutions, 2 milliliters of the hydrofluoric acid concentrate were added.

A 4 liter deoxidizer bath was prepared in a manner comparable to that of Example 1 above. The ingredients used for the deoxidizer baths are given in Tables I and II below.

With respect to the baths illustrated in Table 1, it should be noted that 70% reagent grade nitric acid was used and that the stabilizer in Table I was Dowfax 2A1 (sodium dodecyl diphenyl oxide disulfonate). Furthermore, with respect to the baths in Table I, 10.2 ml of the stabilizer was added to obtain the stabilizer concentration of 0.255% and 0.4 milliliters of the stabilizer was added to obtain the stabilizer concentration of 0.01%.

Regarding the peroxide, 400 milliliters of 35% hydrogen peroxide diluted to one liter with water were used for the 10% concentrations and 240 milliliters for the 6% concentrations.

With respect to the deoxidizer baths of Table II, commercial grade nitric acid was used at 67.5% and the stabilizer was Monofax 1214 (alkyl phosphate ester). With respect to the nitric acid, 400 milliliters of nitric acid were used for the 10% solution, 240 milliliters for the 6% solutions and 80 milliliters for the 2% solutions. Furthermore, with respect to the peroxide, 400 milliliters were used for the 10% solution, 240 milliliters for the 6% solution and 80 milliliters for the 2% solution. TABLE I Cleaner Deoxidizer Sulfuric acid Surfactant Peroxide Stabilizer pH setting Period TABLE Cleaner Deoxidizer Sulfuric acid Surfactant Peroxide Stabilizer pH setting Period

Alloy 2024-T3 aluminum panels were cleaned in a specified cleaning bath as shown in Table I and Table II above. The panels were then rinsed and immersed in the deoxidizer bath shown in Tables I and II, each for a specified time period listed. After the deoxidizing step, the panels were removed, rinsed and chromated. The chromated panels were then examined with a neutral salt spray for 336 hours (twice the time required by MIL-C-5541). All panels showed no pitting or corrosion after testing.

It has been found that preferred systems for a strong caustic cleaner are those in which the cleaning solution contains 0.3% by volume sulfuric acid (96%), 0.17% by volume hydrofluoric acid (70%) and up to 0.2% by volume sodium 2-ethylhexyl sulfate as a surfactant. This cleaner should be used for 5 to 10 minutes at temperatures of about 43.3ºC (110ºF) to about 54.4ºC (130ºF).

It is preferred to then subject the aluminum plates to a deoxidizing step by immersing them in a deoxidizing bath of preferably 8% by volume hydrogen peroxide (35%), 3% by volume (42ºBé) nitric acid and up to 0.5% of a stabilizer for the peroxide (preferably MIRATAINE CBS (coco-amidopropyl hydroxysultaine)) for about 10 to 15 minutes.

If it is desired to have a less caustic cleaner, it is preferred to use a cleaning solution of 0.5% sulfuric acid (96%), 1.7% phosphoric acid (75%) and up to 0.2% of a surfactant combination which is a mixture of Triton N-100 and Surfonic LF-17, at a temperature of about 24.1ºC (110ºF) to about 60ºC (140ºF) for 5 to 10 minutes. used, followed by the same deoxidation step as introduced above for the 5 to 15 minutes.

EXAMPLE 4

A cleaning-etching-deoxidizing bath was prepared containing 8% H2O2, 2.5% HNO3, 1.5% phosphoric acid, and 2 ml/liter of Triton surfactant X-102 (octylphenoxypolyethoxyethanol). Aluminum panels were immersed in the cleaning-etching-deoxidizing solution at 48.8ºC (120ºF) for 10 minutes. The panels were rinsed and then chromated in Alodine 1200S chromating solution in accordance with the manufacturer's recommendations. The chromated panels were then tested in neutral salt spray for 336 hours. No pitting or corrosion of the aluminum panels occurred.

EXAMPLE 5

A cleaning-etching-deoxidizing bath was prepared containing 8% H2O2, 2.5% HNO3, 1 ml/liter of 79% HBF4, and 2 ml/liter of Triton X-102 surfactant. Aluminum panels were immersed in the cleaning-etching-deoxidizing solution at 48.8ºC (120ºF) for 10 minutes. The panels were rinsed and then chromated in Alodine 1200S chromating solution in accordance with the manufacturer's recommendations. The chromated panels were then tested in neutral salt spray for 336 hours. No pitting or corrosion of the aluminum panels occurred.

Combining the acid cleaning-etching step with the deoxidation step in one treatment step is an advancement in the art. Combining the two steps into a single step reduces the equipment required to provide a commercial process, and additionally reduces the processing time for each aluminum piece.

Claims (21)

1. A method of treating an aluminum article, comprising deoxidizing and conversion coating the aluminum article, characterized in that the aluminum article is deoxidized by contacting the aluminum article with an acidic stabilized hydrogen peroxide solution, an acidic heteropolyvanadic acid solution, or an acidic solution of a salt of heteropolyvanadic acid. 2. The method of claim 1, wherein the pH of the acidic stabilized hydrogen peroxide solution is less than 3. 3. A method according to claim 1, wherein the aluminum article is cleaned by contact with an alkaline cleaning solution. 4. The method of claim 3, wherein the cleaning solution has a temperature of 32ºC (90ºF) to 80ºC (175ºF). 5. The method of claim 3, wherein the deoxidizing solution is a stabilized hydrogen peroxide solution at a temperature of from ambient to 65ºC (150ºF) and a pH below 3. 6. The process of claim 5, wherein the pH is below 0.5. 7. A method according to claim 1, wherein the aluminum article is cleaned by contact with an acidic aqueous solution at a pH below 2. 8. The method of claim 7, wherein the aluminum article after contact with the acidic cleaning solution is contacted with an acidic, stabilized hydrogen peroxide solution at a pH below 1.5. 9. The method of claim 8, wherein the pH of the deoxidizing stabilized hydrogen peroxide solution is below 0.5. 10. The method of claim 7, comprising a. bringing an aluminum article into contact with an acidic solution at a temperature of 32 ºC (90 ºF) to 80 ºC (175 ºF) at a pH of less than 2 for 1 to 15 minutes to clean the aluminum article; and b. contacting the cleaned aluminum article with an acidic stabilized hydrogen peroxide solution at a temperature of from ambient temperature to 65ºC (150ºF) for 1 to 25 minutes to deoxidize the aluminum article. 11. The two-step process of claim 1, wherein the combined step of cleaning and deoxidizing comprises: contacting an aluminum article with an acid stabilized hydrogen peroxide cleaning-etching-deoxidizing solution at a temperature of 32°C (90°F) to 80°C (175°F) for 2 to 25 minutes, the pH of the solution being below 2. 12. The method of claim 11, wherein the pH of the solution is below 0.5. 13. The method of claim 11, wherein the cleaning-etching-deoxidizing solution comprises: a) hydrogen peroxide in an amount of 0.03 to 30% by weight, b) 0.1 to 5 g/l stabilizer for the hydrogen peroxide, c) nitric acid in an amount of 0.5 to 10% by weight 420 Baumé, d) a source of fluoride ions resulting in 0.009 to 0.17 wt% HF. 14. The method of claim 11, wherein the cleaning-etching-deoxidizing solution comprises: a) hydrogen peroxide in an amount of 0.003 to 30% by weight, b) 0.1 to 5 g/l stabilizer for the hydrogen peroxide, c) phosphoric acid, d) Nitric acid in an amount of 0.5 to 10% by weight 42º Baumé. 15. The method of claim 11, wherein the deoxidized article is conversion coated by contact with a chromate-containing conversion coating composition. 16. The method of claim 1 wherein the aluminum is cleaned by contacting a cleaning solution comprising 0.3% sulfuric acid (96%), 0.17% hydrofluoric acid (70%) and 0.2% sodium 2-ethylhexyl sulfate and wherein the aluminum substrate is cleaned for a period of 5 to 10 minutes at a temperature of 43°C (110°F) to 54°C (130°F). 17. A process according to claim 1, wherein the deoxidizing solution comprises 8% by volume hydrogen peroxide (35%), 3% nitric acid (42º Bé) and wherein the aluminum is immersed in the solution for a period of 10 immersed in the deoxidizing solution for 15 minutes at ambient temperature. 18. The process of claim 1 wherein the aluminum is cleaned by contacting it with a cleaning solution comprising about 0.5% sulfuric acid (96%), 1.7% phosphoric acid (75%) and up to 0.2% of a surfactant, and wherein the aluminum is treated in said solution for a period of 5 to 10 minutes at a temperature of 43°C (110°F) to 60°C (140°F). 19. The method of claim 1, wherein the aluminum article is cleaned by contact with an acidic cleaning solution containing up to 10% by weight of at least one acid and 0 to 5% by weight of a surfactant, wherein the pH of the acidic cleaning solution is less than 2. 20. The process of claim 1, wherein the stabilized hydrogen peroxide solution contains 0.03 to 30 wt.% hydrogen peroxide and has a pH not higher than 1.5. 21. The method of claim 1, wherein the step of conversion coating comprises contacting with a chromate containing conversion coating composition.