Method for forming non-chromium conversion coatings on aluminium alloys

Abstract: The present invention relates to a compound, non-chromium conversion coating for a part formed from an aluminum alloy. The coating is formed by providing a first solution containing an anodic inhibitor species, providing a second solution containing a cathodic corrosion inhibitor species, and immersing the part to be coated in a first one of the first and second solutions and thereafter in a second one of the first and second solutions. Suitable anodic inhibitor species include tungstates, permanganates, vanadates, molybdates, and mixtures thereof. Suitable cathodic corrosion inhibitors include cobalt, cerium, other lanthanide elements, and mixtures thereof. In one embodiment, the conversion coating is formed using a cerium containing solution and a tungstate containing solution.

Description: The present invention relates to a method for forming a compound, non-chromium conversion coating on a part formed from an aluminum alloy. Chromate conversion coatings are used to protect parts manufactured from aluminum alloys from corrosion. These coatings are formed by treating the aluminum surface of the part with solutions containing hexavalent chromium. Hexavalent chromium is an International Agency for Research on Cancer (IARC) Group 1 or proven human carcinogen. Thus, such coatings are to be avoided where possible. US-A-5873953 discloses a process for conversion coating Al comprising first dipping in a solution containing cobalt followed by treating with a solution containing 6.5-10 g/l Na 2 WO 4 at pH 6.8-7.2. EP-A-0492713 describes a process for rinsing conversion coatings on metal surfaces, for example phosphate coated Al, with a solution containing 0.01-1 g/l Ce at pH 3-6. US-A-5419790 discloses a method for forming a non-toxic corrosion resistant conversion coating for aluminium comprising degreasing and abrading the Al, followed by dipping in a first solution containing 20-100 g/l CeCl 3 at pH 2-5 and rinsing in an alkaline molybdate solution containing 10 g/l sodium molybdate. US-A-5194138 describes a process for conversion coating Al comprising first dipping in a cerium nitrate solution, then in cerium chloride and finally in an alkaline molybdenum solution. EP-A-0902103 discloses a method for treating metal materials (e.g. Al) with corrosion resistant films composed mainly of oxyacid compounds or rare earth elements or mixtures thereof wherein the films may be of a thickness of 1-10 micron. The method includes treating with a solution containing a rare earth element (e.g. Ce or La) and an oxyanion (e.g. phosphate, tungstate, molybdate and vanadate). EP-A-1191073 discloses a metal substrate (for example Al) having a corrosion inhibiting coating including an anodic corrosion inhibitor and a cathodic corrosion inhibitor. The anodic corrosion inhibitor (e.g. tungstate) may be present in an amount of between 269-2690 mg/m 2 and the cathodic corrosion inhibitor (e.g. cerium) may be present in an amount of between 269-2690 mg/m 2 . Accordingly, it is an object of the present invention to provide a method for depositing a non-chromium conversion coating on a part formed from an aluminum alloy. In accordance with the present invention, there is provided a method for forming a non-chromium conversion coating on an aluminum alloy part comprising the steps of: providing a first solution containing an anodic corrosion inhibitor species; providing a second solution containing a cathodic corrosion inhibitor species; and immersing said aluminum alloy part in a first one of said first and second solutions and thereafter in a second one of said first and second solutions; characterised in that said first solution providing step comprises providing a solution having a pH in the range of from 11 to 12 and containing from 10 g/L to 20 g/L tungstic acid in ammonium hydroxide and wherein said aluminum alloy part is immersed in said first solution for a time period in the range of from 3 minutes to 15 minutes, and in that said second solution providing step comprises providing a solution having a pH in the range of from 3.5 to 3.6 and containing from 10 g/L to 50 g/L cerium (III) nitrate in deionized water and said aluminum alloy part is immersed in said second solution for a time period in the range of from 3 minutes to 15 minutes. In one embodiment of the present invention, the cathodic corrosion inhibitor comprises from about 10 g/L to about 30 g/L cerium (III) nitrate in deionized water and the anodic inhibitor solution is a solution comprising 10 g/L tungstic acid in ammonium hydroxide. A compound non-chromium conversion coating produced in accordance with the method of the present invention may comprise Ce 2 (WO 4 ) 3 having a thickness in the range of about 0.96 µm to about 1.51 µm. Certain preferred embodiments of the present invention will now be described by way of example only. The present invention relates to a method for forming conversion coatings based on sequential deposition of anodic and cathodic corrosion inhibiting compounds on a part formed from an aluminum alloy, such as aluminum alloy 6061 which consists essentially of 1.0 wt.% magnesium, 0.25 wt.% copper, 0.6 wt.% silicon, 0.25 wt.% chromium and the balance aluminum and inevitable impurities, through an immersion process as defined in claim 1. It has been found that the coating weights achieved by the process of the present invention are comparable to those achieved by a chromate conversion coating process. The coating weights are in the range of from about 4350-8600 mg/m 2 (400-800 mg/sq. ft). Prior to having a coating in accordance with the present invention applied to it, the surface or the surfaces of the aluminum alloy part to be coated are sanded using a 200-400 grit paper. After sanding, the surface(s) to be coated are washed in a mild detergent and rinsed sequentially with tap water, deionized water and ethanol. After the part has been abrasively cleaned, washed and rinsed, it is first immersed into a solution at room temperature without any agitation, the solution containing from 10 g/L to 20 g/L tungstic acid in ammonium hydroxide and which has a pH in the range of from 11 to 12. For example, a useful solution is one which contains 10 g/L tungstic acid in ammonium hydroxide and a pH of 11.82. The aluminum alloy part is immersed in the solution for a time in the range of from 3 minutes to 15 minutes. Following the first immersion the aluminum alloy part is immersed in a solution at room temperature without any agitation, the solution containing from 10 g/L to 50 g/L, preferably from 10 g/L to 30 g/L, cerium (III) nitrate in deionized water having a pH in the range of from 3.5 to 3.6. The aluminum alloy part is immersed in this solution for a time period in the range of from 3 minutes to 15 minutes. It has been found that aluminum alloy 6061 parts treated in accordance with the present invention show a 10x improvement in barrier properties and spontaneous corrosion rates over untreated aluminum alloy 6061. To demonstrate the method of the present invention, the following example was performed. EXAMPLE Conversion coatings were applied to 6061 aluminum test coupons using three solutions. The solutions were: Solution #1: 10 g/L Cerium (III) Nitrate in Deionized Water, pH = 3.60; Solution #2: 30 g/L Cerium (III) Nitrate in Deionized Water, pH = 3.5; and Solution #3: 10 g/L Tungstic Acid in Ammonium Hydroxide, pH = 11.82 The test coupons were sanded using 220 and 400 grit paper, washed with a mild detergent, and rinsed with tap water, deionized water, and ethanol. The samples were all dipped at room temperature with no agitation using three different methods. The methods are described in the following table. Method #1 1 st Dip: Solution #3 (3 min.) 2 nd Dip: Solution #1 (3 min.) #2 1 st Dip: Solution #3 (15 min.) 2 nd Dip: Solution #1 (15 min.) #3 1 st Dip: Solution #2 (3 min.) 2 nd Dip: Solution #3 (3 min.) Peak Height of Ce 103 counts 82 counts 137 counts Coverage of Ce 990 mg/m 2 (92 mg/ft 2 ) 786 mg/m 2 (73 mg/ft 2 ) 1313 mg/m 2 (122 mg/ft 2 ) Peak Height of W 192 counts 174 counts 262 counts Coverage of W 2497 mg/m 2 (232 mg/ft 2 ) 2270 mg/m 2 (211 mg/ft 2 ) 3412 mg/m 2 (317 mg/ft 2 ) Thickness of Ce 2 (WO 4 ) 3 1.12 µm 0.96 µm 1.51 µm An x-ray fluorescence spectrometer was used to confirm aluminum alloy part and to estimate the coating weight. Typical coating compositions determined by this method are listed above. The quality of the conversion coatings was evaluated using electrochemical impedance spectroscopy. The impedance spectra for the coatings shown above confirms that the coatings provide corrosion protection and that best results are obtained by treating first with the tungstate solution and then with the cerium (III) nitrate solution. If desired however, the aluminum alloy part could first be immersed in the solution containing the cerium (III) nitrate solution and then into the solution containing the tungstate solution. Coatings formed in accordance with one embodiment of the present invention comprise Ce 2 (WO 4 ) 3 having a thickness in the range of from about 0.96 µm to about 1.51 µm.

Claims: A method for forming a non-chromium conversion coating on an aluminum alloy part comprising the steps of: providing a first solution containing an anodic corrosion inhibitor species; providing a second solution containing a cathodic corrosion inhibitor species; and immersing said aluminum alloy part in a first one of said first and second solutions and thereafter in a second one of said first and second solutions; characterised in that said first solution providing step comprises providing a solution having a pH in the range of from 11 to 12 and containing from 10 g/L to 20 g/L tungstic acid in ammonium hydroxide and wherein said aluminum alloy part is immersed in said first solution for a time period in the range of from 3 minutes to 15 minutes, and in that said second solution providing step comprises providing a solution having a pH in the range of from 3.5 to 3.6 and containing from 10 g/L to 50 g/L cerium (III) nitrate in deionized water and said aluminum alloy part is immersed in said second solution for a time period in the range of from 3 minutes to 15 minutes. A method as claimed in claim 1, wherein said immersing step comprises immersing said aluminum alloy part in said first solution and thereafter into said second solution. A method as claimed in claim 1, wherein said immersing step comprises immersing said aluminum alloy part in said second solution and thereafter into said first solution. A method as claimed in any preceding claim, wherein both said first and second solutions are maintained at room temperature and said aluminum alloy part is immersed into said solutions without agitation. A method as claimed in any preceding claim, further comprising abrasively treating at least one surface of said aluminum alloy part to be coated, washing said at least one surface with a mild detergent, and rinsing said at least one surface prior to immersing said aluminum alloy part in said first one of said first and second solutions. A method as claimed in claim 5, wherein said rinsing step comprises rinsing said at least one surface sequentially in tap water, deionized water and ethanol.

Inventors: JAWOROWSKI MARK R, KRYZMAN MICHAEL A, JAWOROWSKI, MARK R., KRYZMAN, MICHAEL A.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 2001
Main classification: C23C22
IPC codes: C23C22