Reduction of chromium waste water in an aluminum conversion coat processing line

Abstract

The present disclosure relates generally to the field of conversion coating. More specifically, the present disclosure relates to improved methods for improving efficiency of chromium conversion coat processing lines.

Claims

1. A method for reducing the amount of rinse water produced in a chromium-containing conversion coating process comprising the steps of: preparing a substrate having a substrate surface for immersion into a deoxidizing solution; immersing the substrate into a chromium-containing deoxidizing solution to deoxidize the substrate surface; removing the substrate from the deoxidizing solution; immersing the substrate into a chromium-containing conversion coating solution; and eliminating production of chromium contaminated rinse water in the chromium-containing conversion coating process; wherein the substrate is withdrawn from the deoxidizing solution without a rinse step.

2. The method of claim 1, wherein the substrate comprises an aluminum-containing component.

3. The method of claim 2, wherein the aluminum-containing component comprises an aluminum alloy.

4. The method of claim 2, wherein the aluminum-containing component comprises aluminum 2024-T3.

5. The method of claim 1, wherein the deoxidizing solution comprises chromium ions and an inorganic acid.

6. The method of claim 1, wherein the conversion coating solution comprises chromium-containing ions in the conversion coating solution.

7. The method of claim 6, wherein the chromium-containing ions are supplied to the conversion coating solution by a soluble chromium-containing compound.

8. The method of claim 1, further comprising the step of maintaining the conversion coating solution at a pH ranging from about 1 to about 2.

9. The method of claim 8, wherein the pH is maintained by adding a base to the conversion coating solution.

10. The method of claim 9, wherein the base comprises sodium hydroxide.

11. The method of claim 1, wherein, in the step of immersing the substrate into a chromium-containing conversion coating solution, an amount of deoxidizing solution is transferred into the chromium-containing conversion coating.

12. The method of claim 1, wherein, in the step of immersing the substrate into a chromium-containing conversion coating solution, said chromium-containing conversion coating solution further comprises a buffer.

13. The method of claim 1, wherein, in the step of immersing the substrate into a chromium-containing conversion coating solution, said chromium-containing conversion coating solution further comprises a conjugate base of nitric acid.

14. The method of claim 1, wherein, in the step of immersing the substrate into a chromium-containing conversion coating solution, said chromium-containing conversion coating solution further comprises an amount of ammonium nitrate.

15. The method of claim 1, wherein, in the step of immersing the substrate into a chromium-containing conversion coating solution, said chromium-containing conversion coating solution further comprises an amount of potassium nitrate.

16. The method of claim 1, wherein the deoxidizing solution comprises chromium ions and nitric acid.

17. A method for reducing the amount of flowing rinse water produced in a chromium-containing conversion coating process comprising the steps of: preparing a substrate having a substrate surface for immersion into a deoxidizing solution; immersing the substrate into a chromium-containing deoxidizing solution to deoxidize the substrate surface; removing the substrate from the deoxidizing solution; immersing the substrate into a dead rinse; removing the substrate from the dead rinse; immersing the substrate into a chromium-containing conversion coating solution; and eliminating production of flowing chromium contaminated waste water in the chromium-containing conversion coating process wherein the substrate is withdrawn from the deoxidizing solution without a rinse step.

18. The method of claim 17, further comprising the step of maintaining the conversion coating solution at a pH ranging from about 1 to about 2.

19. The method of claim 18, wherein the pH is maintained by adding a base to the conversion coating solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Having thus described variations of the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

(2) FIG. 1 is a flow chart of a preferred conversion coating process featuring a significantly reduced amount of generated chromium waste; and

(3) FIG. 2 is a flow chart of a preferred conversion coating process of FIG. 1 with a dead rinse station located between the deoxidizing bath and the conversion coating bath.

DETAILED DESCRIPTION

(4) Traditional chemical processing of metals follows a common pattern: processing, water rinse, processing, water rinse, etc. In the field of conversion coating, water rinse steps are employed throughout the line, especially between the deoxidizing step and the conversion coat step. More specifically, chromium conversion coat processing lines employ water rinse stages that follow a chromium-based deoxidizer and precede the chromium conversion coating of metal substrates (e.g. aluminum, etc.). Such rinse stages collect hexavalent chromium compounds in the rinsing stage that becomes hazardous waste water requiring regulated treatment, storage and disposal. Such rinse steps are universally incorporated into conversion coat processing lines. It has been conventionally thought necessary to maintain chromium-containing conversion baths in as low of a contaminated state as possible to prolong the useful life of the bath, and ensure acceptable coating results relative to a conversion coated substrate.

(5) It has now been recognized that chromium conversion coat processing for aluminum can be significantly streamlined by obviating or eliminating the rinse stages occurring between the chromium-based deoxidizing stage and the chromium conversion coating stage without sacrificing superior coating results. It has now been recognized that in the chromium conversion coat processing line, many deoxidizing and conversion coat steps have three major similarities: both are acidic and both contain hexavalent (hex) chrome and fluorine. It was postulated that a useful conversion coat bath would result and useful conversion coat results could be obtained, even if rinse steps between deoxidizing and conversion coating were eliminated.

(6) To test this possibility, a new 25 liter tank of a chromium based conversion coat (CC) was made up. In an effort to speed testing, this new CC make-up was not aged with dissolved aluminum as specifications normally requires. Five 360.032 2024-T3 bare aluminum panels were used as test specimens in all rounds of testing. These five panels were solvent wiped, alkaline cleaned, rinsed, deoxidized in a proprietary chromium/nitric acid solution and immersed in the CC with no rinsing after deoxidizing. All tanks were aged except for the CC. A final tap water rinse completed processing. The test panels appeared golden in color and somewhat mottled. After 24 hours, the panels were put into a neutral salt fog for 168 hours per ASTM B 117. The panels passed exhibiting less than three pits per panel.

(7) One liter of the new CC was then decanted and disposed of and replaced by one liter of the aged D. One liter of new deoxidizer (D) was made up and put into the D tank. The deoxidizer tank was significantly larger than the CC test tank85 liters. Five more test panels were then processed as above. The panels passed a 168 hours salt spray exposure. Three more iterations of this procedure were performed. The test panels became increasingly mottled in appearance and finally it seemed as if areas of bare aluminum were apparent. The panels processed during iteration 4 (15% of the CC removed and replaced with aged (D) failed 168 hour neutral salt fog testing.

(8) Analysis of the CC tank was performed and the pH was found to be significantly low; a function of the significantly lower pH of the deoxidizer being added to the conversion coat. Adjustment of the pH was made by adding sodium hydroxide to the CC tank to return the pH of the conversion coat to operating conditions. A new set of test panels were processed as before. Appearance of the panels was excellent. These panels passed 168 hour neutral salt fog testing. Testing continued with the pH being adjusted before test panel processing. Increasing amounts of solution were decanted and replaced in order to speed testing.

(9) The panels processed during run 10 failed salt fog testing. At this point, 40 percent of the conversion coat had been removed and replaced by aged deoxidizer. It was assumed that the conversion coat concentration was too low. A charge of make-up conversion coat material representing 40 percent of the operating volume of the conversion coat was made and the test was repeated. The panels passed neutral salt fog testing.

(10) TABLE-US-00001 TABLE 1 Amt. Decanted % Deoxidizer and contained in Replaced by Conversion 168 Hour Oxidizer Coating (CC) Salt Fog Run (liters) Bath Results Notes Baseline 0 0 Pass 1 1 4 Pass 2 1 8 Pass 3 1 11.6 Pass 4 1 15.1 Fail 4 (Repeat) Pass pH Adjusted 5 1 18.5 Pass 6 1 21.8 Pass 7 1.5 26.5 Pass 8 1.5 30.9 Pass 9 1.5 35.1 Pass 10 2.0 40.3 Fail 10 (Repeat) Pass Make-up CC add 11 2.0 45.1 Pass 12 2.0 49.5 Pass Both 168 and 336 hours 13 100 Pass

(11) Wet scribe paint adhesion testing was performed on additional panels processed during runs 8 and 12 per ASTM D3359. The panels passed with no loss of adhesion. Test panels from run 12 were exposed to both 168 and 336 hours of salt spray exposure. The appearance of the panels was excellent and the panels passed at both durations.

(12) After run 12, where the appearance of the test panels was excellent and the panels passed 336 hour neutral salt spray and paint adhesion testing, it was strongly suspected that an even larger amount of deoxidizer could be present in the conversion coat bath without harm. In order of further speed testing, a new tank of deoxidizer (10 liters) was set up. To this new tank was added the initial charge for ten liters of the dry conversion coat material. An adjustment was made by adding sodium hydroxide to the CC tank to raise the operating pH of the conversion coat to 1.3. Bare aluminum panels (2024-T3) were processed both with rinsing between the deoxidizer and conversion coat tanks, and without rinsing between the deoxidizer and conversion coat tanks. After one day of aging, panels were placed in neutral salt spray. Although the panels produced without rinsing had a grey metallic appearance, they passed the 168 hour exposure requirement. This is referred to as Run 13 in the above Data Table.

(13) The no-rinse scheme, according to methods of the present disclosure, produced acceptable panels having only a slightly different appearance than is commonly obtained with rinsing. Some panels were mottled or cloudy in appearance. Variations in color are acceptable as long as the coating is continuous (which is exhibited by passing salt spray testing). In addition, the pH of the conversion coat progressively decreased due to the more acidic deoxidizer being transferred from the deoxidizing tank into the conversion coat tank, along with an increased amount of metallic contaminants transferred into the conversion coat tank from the deoxidizing tank over time. Nevertheless, contrary to conventional thought, acceptable aluminum panels were surprisingly produced without employing any rinsing of the panels after the deoxidizing step and before the conversion coating step.

(14) FIG. 1 is a flow chart showing one preferred variation of the present disclosure. According to process 10, a substrate is preferably cleaned 12 and rinsed 14, and immersed in a deoxidizing bath 16. Alternatively a substrate is prepared for immersion in the deoxidizing bath 13. It is understood that the preparing step 13 may include the cleaning 12 and rinsing 14 steps, and also may include any additional preparatory steps for treating a substrate prior to deoxidizing the substrate. It is further understood that the preparatory step 13 may not specifically include cleaning and rinsing. The substrate is removed from the deoxidizing bath 18 and immersed into the conversion coating bath 20 with no intermediate rinse step. Optionally, to extend the life of the conversion coating bath 20, the pH of conversion coating bath 20 is periodically adjusted 22, preferably with a suitable base.

(15) While significant cost savings in a conversion coat line can be obtained by the methods of the present disclosure by obviating all rinsing of deoxidized aluminum prior to conversion coating, the present disclosure further contemplates a modified dead rinse protocol. According to such a dead rinse protocol, water rinsing after deoxidizing is still performed but flowing water is eliminated. This results in the virtual elimination of contaminated rinse water (but for the volume of rinse water contained in the dead rinse tank, which may be returned to the deoxidizer tank as evaporation allows) and its attendant treatment, further resulting in a significant cost savings as disposal costs are obviated. One significant advantage of employing the dead rinse step into conversion coat processing lines described herein is the improved appearance of the conversion coated aluminum panels.

(16) FIG. 2 is a flow chart showing another preferred variation of the present disclosure incorporating a dead rinse. According to process 24, a substrate is preferably cleaned 26 and rinsed 28, and immersed in a deoxidizing bath 30. Alternatively a substrate is prepared for immersion in the deoxidizing bath 27. It is understood that the preparing step 27 may include the cleaning 26 and rinsing 28 steps, and also may include any additional preparatory steps for treating a substrate prior to deoxidizing the substrate. It is further understood that the preparatory step 27 may not specifically include cleaning and rinsing. The substrate is removed from the deoxidizing bath 30 and immersed into and removed from a dead rinse tank 34. The substrate is then immersed into the conversion coating bath 36. Optionally, to extend the life of the conversion coating bath 36, the pH of conversion coating bath 36 is periodically adjusted 38, preferably with a suitable base.

(17) The vast majority of chemical conversion coat processing lines have at least one existing rinse tank following deoxidizing, and most have more than one rinse tank. This results in significant chromium-bearing waste water that requires expensive regulated handling and disposal. However, if the flow of water to such rinse tank(s) were turned off, a so-called dead rinse would be created. Use of dead rinses after deoxidizing would impact one aspect of the preferred methods of the present disclosure; namely, reduced processing time. However, incorporating a dead rinse into the methods of the present disclosure would significantly improve the appearance of processed parts and reduce the amount of deoxidizer that is transferred into the conversion coat tank, thereby improving part appearance, pH retention and reducing metallic contamination of the conversion coat tank. It is understood that water would only have to be added to the deoxidizing and dead rinse tanks due to evaporation (as with any processing tank). Therefore, the addition of fresh water to a dead rinse is thought to alleviate any issues of finished aluminum panel appearance, pH control and metallic contaminant drag-in. Compliant salt spray and paint adhesion results will still be obtained without any rinsing so that the use of a dead rinse only insures improved performance.

(18) Additionally, the addition of a buffer to the conversion coat tank will help maintain pH balance in the conversion coat. According to the present disclosure, with the rinsing steps obviated between the deoxidizing and conversion coating steps, residual amounts of both chromic acid and inorganic acid (e.g. nitric acid, etc.) are progressively transferred into the conversion coat from the deoxidizer. Adding a conjugate base of the nitric acid, such as a nitrate based chemical (in the case of nitric acid) to the conversion coat, will help to maintain the pH of the conversion coat tank at a desired level. Two chemicals were tried for pH adjustment of the conversion coating tank: ammonium nitrate and potassium nitrate. The results are shown below. Addition of either improved pH stability as demonstrated by the slope in Tables 2 and 3 below.

(19) TABLE-US-00002 TABLE 2 Control (No Ammonium 3.3 g/l 8.3 g/l Nitrate Ammonium Ammonium added) Nitrate Added Nitrate Added Mls of 10% HNO.sub.3 pH pH pH 0 1.65 1.27 1.24 5 1.58 1.24 1.2 10 1.52 1.2 1.17 15 1.48 1.18 1.14 20 1.43 1.15 1.11 25 1.4 1.14 1.09 30 1.37 1.11 1.06 35 1.34 1.1 1.03 40 1.31 1.07 1.01 45 1.28 1.05 0.99 50 1.23 1.01 0.96 slope 0.0388 0.0241 0.0272

(20) TABLE-US-00003 TABLE 3 Control (No Mls of Potassium Nitrate 3.3 g/l Ammonium 8.3 g/l Ammonium 10% added) Nitrate Added Nitrate Added HNO.sub.3 pH pH pH 0 1.71 1.46 1.25 5 1.64 1.36 1.20 10 1.59 1.31 1.16 15 1.54 1.26 1.13 20 1.50 1.21 1.11 25 1.47 1.17 1.08 30 1.44 1.14 1.06 35 1.41 1.11 1.05 40 1.38 1.08 1.02 45 1.35 1.05 1.00 50 1.32 1.02 0.99 slope 0.0369 0.0409 0.0248

(21) Again, it is important to note that, according to preferred variations, the conversion coat will continue to function adequately, even with a concentration of up to approximately 100% deoxidizer contained in the conversion coat bath. It may be desired to remove the metallic build-up in the conversion coating bath that may also occur due to the elimination of rinse steps. This metallic build-up can be remedied by removing the contaminants in a dead rinse tank(s) following the deoxidizer using an electrolytic technique common to purifying some plating baths known as dummying. Low voltage can be applied (while not processing any aluminum parts) causing the metallic ions to plate out thereby reducing metallic contamination that would be transferred into the conversion coat.

(22) According to the present disclosure, it has been shown that, due to the similar chemistries of a hexavalent chromium-based deoxidizer and a hexavalent chromium conversion coat, the rinse between these two processing steps for aluminum can be eliminated leading to significant savings and advantages. Such advantages include: 1) reduced processing time; 2) reduced water consumption and sewer disposal cost; 3) elimination of a hex chrome waste water stream and its treatment costs; 4) elimination of hex chrome hazardous waste, its disposal costs and long term liability; 5) reduced capital equipment cost (waste treatment); 6) reduced processing line footprint; 7) elimination of need to research alternative technology deoxidizers; 8) reduced conversion coat processing line footprint; and many others. Numbers 3 & 4 above would significantly, cut the hazardous hex chrome waste generated by a typical aluminum conversion coat line, in some cases by as much as 50% or more.

(23) The systems and methods set forth herein are contemplated for use with producing conversion coated components for use in manned or unmanned vehicles or objects of any type or in any field of operation, such as in a terrestrial and/or non-terrestrial and/or marine or submarine setting. A non-exhaustive list of contemplated objects include, manned and unmanned aircraft, spacecraft, satellites, terrestrial, non-terrestrial vehicles, and surface and sub-surface water-borne vehicles, etc., as well as stationary objects.

(24) While the preferred variations and alternatives of the present disclosure have been illustrated and described, it will be appreciated that various changes and substitutions can be made therein without departing from the spirit and scope of the disclosure. When introducing elements of the present invention or exemplary aspects or embodiment(s) thereof, the articles a, an, the and said are intended to mean that there are one or more of the elements. The terms comprising, including and having are intended to be inclusive and mean that there may be additional elements other than the listed elements. Although this invention has been described with respect to specific embodiments, the details of these embodiments are not to be construed as limitations.