A PROCESS FOR THE PREPARATION OF CORROSION RESISTANCE SEALED ANODIZED COATINGS ON ALUMINUM ALLOY

Abstract

Development of an alternative process to conventional toxic chromic acid anodization (CAA) with equivalent corrosion resistance is a challenging task. The present invention provides a chromate free process for the manufacture of corrosion resistant sealed anodized coating for long term corrosion resistance of aerospace grade aluminum alloy. This method includes the steps of cleaning, chemical etching, anodizing in Tartaric-Sulphuric acid electrolyte followed by dipping the specimen in the sealing bath containing at least two water soluble either Mn and Mo or Mn and V oxyanions as corrosion inhibitors and a sufficient amount of alkali metal ion based nitrates at a temperature range between 60 and 80° C. for about 20 to 40 minutes at a pH range of 7 to 9. The sealed anodic coatings developed from this invention showed improved corrosion resistance in neutral 5% NaCl fog environment for greater than 2000 h of exposure. The sealed anodic coatings developed by this invention also showed self-healing protection in NaCl environment.

Claims

1. An improved process for the manufacture of corrosion resistant sealed anodized coating on metal and metal alloy substrate comprises the steps of: (a) cleaning of substrate by wiping with acetone or ultrasonication; (b) rinsing of cleaned substrate with distilled water; (c) etching of cleaned substrate in sodium hydroxide solution having concentration between 0.3 to 1 molar; (d) rinsing of etched substrate with distilled water; (e) deoxidizing the treated substrate as obtained in step (d) in 1:1 aqueous solution of nitric acid followed by rinsing; (f) developing an anodic oxide layer on substrate by anodization; (g) sealing of anodized substrate followed by rinsing with distilled water; (h) air drying of sealed anodized substrate; wherein the improvement lies in anodization of substrate in tartaric sulphuric acid at 28-30° C. in tartaric-sulphuric acid electrolyte with current densities between 10 to 30 mA/cm.sup.2 for a period of 30 to 120 minutes and sealing of said anodized substrate by dipping in based sealing bath containing solution of transition element salts and additives, for about 20 to 40 minutes at pH range of 7 to 9 at a temperature range between 60 to 80° C.

2. (canceled)

3. An improved process as claimed in claim 1, wherein the tartaric sulphuric acid anodization is carried out in either sweep or constant current density mode.

4. An improved process as claimed in claim 1, wherein sealing bath contains water soluble transition elements and nitrates.

5. An improved process as claimed in claim 1, wherein sealing bath contains at least two water soluble transition elements based corrosion inhibitors preferably Manganese, Molybdenum, Vanadium oxyanions or combinations thereof.

6. (canceled)

7. An improved process as claimed in claim 1, wherein the first water soluble transition element is Mn, Mo, V, Ti, Zr, W salts preferably a Mn based oxyanion.

8. An improved process as claimed in claim 1, wherein the second water soluble transition element is Ce, Mo, V, W salts, preferably Mo or V based oxyanion.

9. An improved process as claimed in claim 1, wherein nitrates in sealing bath are selected from alkali metals preferably lithium, sodium and potassium.

10. An improved process as claimed in claim 1, wherein sealing bath contains Mn oxyanion in the range of 2 to 17 g/l, Mo/V oxyanion in the range of 1 to 10 g/l and alkali metal nitrates in the range of 3 to 8 g/l.

11. Sealed anodized substrate obtained by using the process as claimed in claim 1, has coating composition (wt. %) containing: a. Manganese in the range of 0 to 7; b. Aluminum in the range of 20 to 50; c. Oxygen in the range of 40 to 70; d. Iron in the range of 0 to 1; e. Copper in the range of 0 to 1; f. Magnesium in the range of 0 to 1; g. Vanadium in the range of 0 to 20; and h. Zinc in the range of 0 to 1;

12. Sealed anodized substrate as claimed in claim 9 has coating thickness in the range of 3 to 12 μm, salt spray resistance of >336 h, adhesion with epoxy primer of >4B and electrical breakdown voltage of >50 V.

Description

DETAIL DESCRIPTION OF THE INVENTION

[0043] Present invention relates to a novel chromium-free sealing composition and the process for preparing corrosion resistant coating for anodized aluminum and aluminum alloys. The metal panels used in the tests were aluminium alloy panels of unclad 2024-T3 aerospace quality sheet. The nominal composition of the alloy (in weight per cent) was 5.0% copper, 1.5% magnesium, 0.7% manganese, 0.4% iron, 0.1% silicon and the remainder being aluminium. The present invention provides a process and composition for sealing anodic oxide developed on aluminum and high strength aluminum alloys wherein the composition of the invention is an aqueous solution containing either the mixture of Mn and Mo or Mn and V oxyanions. This process includes the following steps: [0044] (1) Developing an anodic oxide layer by anodization process on aluminum or aluminum alloy substrate [0045] (2) Providing a sealing solution comprising of a mild alkaline solution containing at least two water soluble non chromium metal oxyanions and alkali metal ion based nitrates [0046] (3) Contacting the substrate with sealing solution for sufficient amount of time to seal the developed anodic oxide layer

[0047] In producing a corrosion-resistant coating on an aluminum surface, generally the surface should be free of soil and oxides contamination which interfere the further coating process. The surface can be cleaned by any convenient method available in the market. Subsequent to the cleaning and rinsing, the cleaned specimen has to be anodized in suitable electrolyte. Preferably, Tartaric-Sulphuric acid anodization experiment for at least 30 minutes to 45 minutes. Most preferably, 50 to 60 minutes. Then the anodized aluminum surface is treated with the Mn and Mo/V oxyanion based sealing solution of this invention.

[0048] Various methods of contacting the anodized specimen with sealing solution such as by spraying, dipping, brushing art is acceptable most preferably dipping process. In this process the anodized surface is contacted with an aqueous solution containing soluble Mn and Mo/V oxyanions for at least 15 minutes, preferably 20 to 40 minutes. In most cases, excellent results can be achieved for about 30 to 40 minutes.

[0049] The Mn and Mo/V oxyanions are selected from alkali metal source (potassium, sodium or lithium). The preferred alkali metal source is sodium or potassium. Most preferably, the solution consists essentially of potassium permanganate and sodium metavanadate. The composition of this bath includes alkali metal Mn oxyanion from 2 to 17 g/l and alkali metal Mo/V oxyanion from 1 to 10 g/l. Preferably Mn oxyanion of about 8 to 12 g/l and Mo/V oxyanion of about 4 to 7 g/l.

[0050] Further the process of this sealing solution also contains alkali metal nitrates in the range of 3 to 8 g/l. The preferred alkali metal source is lithium (or) sodium (or) potassium. The alkali metal nitrates plays dual role in sealing process. The first reason for the addition of alkali/alkaline earth metal salts into the sealing is it improves the sealing quality and then the presence of nitrates acts as an activator for the sealing process and results in reduced processing time. In particular, the addition of highly soluble lithium ions favors the formation of insoluble alkali metal aluminum oxide complexes. The mixture of above mentioned alkali metal nitrates can also be used to achieve better sealing quality.

[0051] The sealing bath has a pH range of between 7 and 10, most preferably between 8 and 9. The temperature of the sealing bath is at least 60° C., more preferably from 70 to 75° C. The anodized specimens may be contacted with the sealing solution by immersion of about 15 to 40 minutes.

[0052] The surface treated with Mn and Mo/V oxyanion containing solution as per the above mentioned procedure has been exposed to 5% neutral salt spray test according to ASTM B-117 Standard. The sealed specimen has exhibited improved corrosion resistance performance during salt spray exposure. In order to understand the self-healing behaviour of this coating, a scribe mark has also been made on each panel and then exposed in salt spray chamber for minimum 500 h as per the above mentioned procedure.

[0053] On the Mn and Mo/V oxyanion sealed anodic oxide coated specimen, it has been identified that the Mn oxyanion forms an insoluble barrier oxide layer over the anodized aluminum surface. In the presence of Mo/V oxyanion, Mn oxyanions are incorporated along with the molybdate/vanadate oligomers during the sealing process on the alumina of the anodized aluminum surface. Molybdate/vanadate oligomers provide a compact polymeric network and which impede the penetration of corrosive species. Both these oxide layers formed by Mn and Mo/V oxyanions are found to be passive barrier during the corrosion process. It has also been identified that the presence of higher oxidation state soluble species of both Mn and Mo/V oxyanions in the sealed oxide layer. When there is a mechanical damage in the coating both these higher oxidation state soluble species migrates towards the corrosion area and simultaneously forms a passive layer. This has been identified as active corrosion inhibition (self-healing) performance of this developed coating.

[0054] This coating provides excellent corrosion resistance and paint adhesion for anodized high strength and high copper content and high corrosion prone aluminum alloy. This process can provide corrosion protection of greater than 2000 h of neutral salt spray exposure.

[0055] The following examples are given by way of illustrations and therefore, should not be construed to limit the scope of the present invention.

EXAMPLE 1

[0056] Samples (1.5 in.×5 in.) of AA2024 were coated as per the following procedure: [0057] 1. Each sample was cleaned with acetone and then ultrasonicated in the same solution for about 10 to 15 minutes. Each was then immersed in hot sodium hydroxide solution and finally treated with nitric acid in order to remove the residual organic and inorganic impurities from the surface. The specimens were washed with distilled water for about 2 minutes after every treatment. [0058] 2. Each cleaned specimen was then anodized in 2.5% sulphuric acid and 80 g/l tartaric acid for about 120 minutes with constant current density of about 20 mA/cm.sup.2. [0059] 3. The anodized specimen then exposed to salt fog corrosion testing according to ASTM B117. [0060] 4. In order to check the self-healing corrosion resistance property, the freshly anodized specimen was mechanically damaged (cross-hatched) and then subjected for salt fog corrosion testing for about 500 h. [0061] 5. In order to check the adhesion performance with paint, the anodized specimen was then coated with volatile organic compound (VOC) compliant epoxy-polyamide primer. Then adhesion test was carried out after complete curing. [0062] 6. The properties of this specimen are listed in Table I.

[0063] As seen from the Table 1, the coated specimen showed the formation of pit within 50 hrs of salt spray exposure and completely corroded in cross hatched area.

EXAMPLE 2

[0064] Samples (1.5 in.×5 in.) of AA2024 were coated as per the following procedure: [0065] 1. Each sample was cleaned with acetone and then ultrasonicated in the same solution for about 10 to 15 minutes. Each was then immersed in hot sodium hydroxide solution and finally treated with nitric acid in order to remove the residual organic and inorganic impurities from the surface. The specimens were washed with distilled water for about 2 minutes after every treatment. [0066] 2. Each cleaned specimen was then anodized in 2.5% sulphuric acid and 80 g/l tartaric acid for about 120 minutes with constant current density of about 20 mA/cm.sup.2. [0067] 3. Each anodized specimen was then sealed in boiling water (conventional sealing for comparison) by immersion for about 30 minutes. The solution temperature was maintained at 95-100° C. [0068] 4. The boiling water sealed anodized specimen was then exposed to salt fog corrosion testing according to ASTM B117. [0069] 5. In order to check the self-healing corrosion resistance property, the freshly prepared specimen was mechanically damaged (cross-hatched) and then exposed to salt fog corrosion testing for about 500 h. [0070] 6. In order to check the adhesion performance with paint, the anodized specimen was then coated with volatile organic compound (VOC) compliant epoxy-polyamide primer. Then adhesion test was carried out after complete curing. [0071] 7. The properties of this specimen are listed in Table I.

[0072] It is observed from the Table 1 that anodized alloy sealed with boiling water exhibited the formation of pit within 400 hrs of salt spray exposure and found completely corroded in cross hatched area.

EXAMPLE 3

[0073] Samples (1.5 in.×5 in.) of AA2024 were coated as per the following procedure: [0074] 1. Each sample was cleaned with acetone and then ultrasonicated in the same solution for about 10 to 15 minutes. Each was then immersed in hot sodium hydroxide solution and finally treated with nitric acid in order to remove the residual organic and inorganic impurities from the surface. The specimens were washed with distilled water for about 2 minutes after every treatment. [0075] 2. Each cleaned specimen was then anodized in 2.5% sulphuric acid and 80 g/l tartaric acid for about 120 minutes with constant current density of about 20 mA/cm.sup.2. [0076] 3. Each anodized specimen was then sealed in a sealing solution containing 5 g/l of potassium permanganate, 2.5 g/l of sodium vanadate, 5 g/l of sodium nitrate and 4 g/l of lithium nitrate for a period of 30 minutes. The solution temperature was maintained at 70° C. [0077] 4. Then coated specimens were removed from the sealing solution followed by washed with distilled water for about 2 minutes and then air-dried. [0078] 5. This sealed anodized specimen was then exposed to salt fog corrosion testing according to ASTM B117. [0079] 6. In order to check the self-healing corrosion resistance property, the freshly prepared specimen was mechanically damaged (cross-hatched) and then exposed to salt fog corrosion testing for about 500 h. [0080] 7. In order to check the adhesion performance with paint, the anodized specimen was then coated with volatile organic compound (VOC) compliant epoxy-polyamide primer. Then adhesion test was carried out after complete curing. [0081] 8. The properties of this specimen are listed in Table I.

[0082] Anodized alloy sealed with Mn-V oxyanion showed no formation of pits even after 2000 hrs of salt spray exposure and also displayed no corrosion in cross hatched area.

EXAMPLE 4

[0083] Samples (1.5 in.×5 in.) of AA2024 were coated as per the following procedure: [0084] 1. Each sample was cleaned with acetone and then ultrasonicated in the same solution for about 10 to 15 minutes. Each was then immersed in hot sodium hydroxide solution and finally treated with nitric acid in order to remove the residual organic and inorganic impurities from the surface. The specimens were washed with distilled water for about 2 minutes after every treatment. [0085] 2. Each cleaned specimen was then anodized in 2.5% sulphuric acid and 80 g/l tartaric acid for about 60 minutes with constant current density of about 20 mA/cm.sup.2. [0086] 3. Each anodized specimen was then sealed in a sealing solution containing immersed in 5 g/l of potassium permanganate, 2.5 g/l of sodium molybdate, 5 g/l of sodium nitrate and 4 g/l of lithium nitrate for a period of 30 minutes. The solution temperature was maintained at 75° C. [0087] 4. Then coated specimens were removed from the sealing solution followed by washed with distilled water for about 2 minutes and then air-dried. [0088] 5. This sealed anodized specimen was then exposed to salt fog corrosion testing according to ASTM B117. [0089] 6. In order to check the self-healing corrosion resistance property, the freshly prepared specimen was mechanically damaged (cross-hatched) and then exposed to salt fog corrosion testing for about 500 h. [0090] 7. In order to check the adhesion performance with paint, the anodized specimen was then coated with volatile organic compound (VOC) compliant epoxy-polyamide primer. Then adhesion test was carried out after complete curing. [0091] 8. The properties of this specimen are listed in Table I. Anodized alloy sealed with Mn-Mo oxyanion withstands the salt spray test up to 2000 hrs and also displayed no corrosion in cross hatched area.

EXAMPLE 5

[0092] To compare the performance of coatings of present invention with conventional method, Samples (1.5 in.×5 in.) of AA2024 were coated with the following procedure: [0093] 1. Each sample was cleaned with acetone and then ultrasonicated in the same solution for about 10 to 15 minutes. Each was then immersed in hot sodium hydroxide solution and finally treated with nitric acid in order to remove the residual organic and inorganic impurities from the surface. The specimens were washed with distilled water for about 2 minutes after every treatment. [0094] 2. Each cleaned specimen was then anodized in an aqueous acidic solution containing 65 g/l of chromic acid and 0.4 g/l of sodium sulphate. The temperature of the bath was maintained between 38 and 40° C. During anodization process the electrical voltage was increased from 0 V to 40 V at a rate of 5 V/min. Then the final voltage was maintained for 45 minutes. [0095] 3. Each anodized specimen was then sealed in boiling water by immersion for about 30 minutes. The solution temperature was maintained at 95-100° C. [0096] 4. This sealed anodized specimen was then exposed to salt fog corrosion testing according to ASTM B117. [0097] 5. In order to check the self-healing corrosion resistance property, the freshly prepared specimen was mechanically damaged (cross-hatched) and then exposed to salt fog corrosion testing for about 500 h. [0098] 6. In order to check the adhesion performance with paint, the anodized specimen was then coated with volatile organic compound (VOC) compliant epoxy-polyamide primer. Then adhesion test was carried out after complete curing. [0099] 7. The properties of this specimen are listed in Table I.

[0100] As seen from Table 1, coatings prepared using the above conventional method showed corrosion in cross hatched area.

[0101] From the above examples, it is observed that chromate free sealed oxide layers of present invention is capable of withstanding about 2000 h of continuous salt spray exposure and also exhibited self-healing behavior in corrosive environment. Coating also showed excellent paint adhesion rating which is at par with conventional method. This effect is due to anodization of treated substrate in Tartaric-Sulphuric acid electrolyte followed by sealing using transition element oxyanions as corrosion inhibitors and alkali metal nitrates as additives in sealing bath. Thus present invention qualifies the novelty and is an alternative for the coating obtained by conventional toxic chromic acid anodization process.

[0102] Main advantages of the present invention are as follows: [0103] 1. The present invention is a simple process and completely eliminates the toxicity of hexavalent chromium compositions generally used for this purpose and therefore it is more environmental friendly. [0104] 2. This process reduces the processing temperature than the conventionally used boiling water sealing or hexavalent chromium based sealing or other non-chromium based sealing processes. [0105] 3. This process is based on relatively abundant and low cost chemicals and hence it is highly economic. [0106] 4. Our example show this process provides highly durable corrosion resistant coating (greater than 2000 h of neutral salt spray test) on high strength aerospace grade aluminum alloy with improved adhesion and self-healing properties.