Process for the preparation of corrosion resistance sealed anodized coatings on aluminum alloy
View Patent ↗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.
1. An improved process for the manufacture of a corrosion resistant, sealed, anodized, coated metal or metal alloy substrate, the process comprising the steps of:
(a) cleaning a metal or metal alloy substrate by wiping with acetone or ultrasonication;
(b) rinsing the cleaned substrate of (a) with distilled water;
(c) etching the rinsed substrate of (b) in sodium hydroxide solution having a concentration between 0.3 to 1 molar;
(d) rinsing the etched substrate of (c) with distilled water;
(e) deoxidizing the rinsed substrate of (d) in a 1:1 aqueous solution of nitric acid followed by rinsing with distilled water;
(f) forming an anodic oxide coating on the deoxidized substrate of (e) by anodization;
(g) sealing the anodic oxide coating on the substrate of (f) in a sealing bath containing a solution comprising as corrosion inhibitors, a combination of manganese oxyanions in the range of 2 to 17 g/l and vanadium oxyanions in the range of 1 to 10 g/l, and as an additive, one or more alkali metal nitrates in the range of 3 to 8 g/l, followed by rinsing with distilled water; and
(h) air drying the substrate having the sealed anodic oxide coating of (g) to form the corrosion resistant sealed, anodized, coated substrate,
wherein the anodization in (f) is performed in an 8 wt % tartaric acid-2.5 wt % sulphuric acid electrolyte at a temperature in the range of 28-30° C. and with current densities between 10 to 30 mA/cm 2 for a period of 30 to 120 minutes, and
wherein the sealing in (g) is performed by dipping the substrate with the anodic oxide coating from (f) in the sealing bath containing the solution comprising as corrosion inhibitors, a combination of manganese oxyanions in the range of 2 to 17 g/l and vanadium oxyanions in the range of 1 to 10 g/l, and as an additive, one or more alkali metal nitrates in the range of 3 to 8 g/l, for a period from above 20 up to 40 minutes at a pH in the range 7 to 9 and at a temperature between 60 and 80° C.
2. The improved process of claim 1 , wherein in (f) the anodization is performed in the tartaric sulphuric acid electrolyte in either a sweep or a constant current density mode.
3. The improved process of claim 1 , wherein the corrosion resistant, sealed, anodic oxide coating has a 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.
4. The improved process of claim 1 , wherein in (g) the alkali metal nitrates are selected from the group consisting of nitrates of lithium salts, sodium salts, and potassium salts.