Dichromate Sealing Problems

Question:

We have carried out salt spray testing on 2024-T4 bare chromic acid anodized parts. The test piece was sealed in sodium dichromate solution of concentration 45–100 g/liter and temperature 86°C (187°F) for 28 min. The test pieces failed the salt spray test. Could you advise us on what things we can try in order to solve this problem. We have done salt spray testing with parts sealed in DI water and have passed. Thanks for your help. Z.T.

Answer:

The corrosion inhibiting characteristics of chromium in the anodic coating have been recognized from the early days of commercial anodizing processes. The amount of chromium that is absorbed by the anodic coating is increased with increasing pH of the dichromate bath up to a value of around 7.0 –7.4. Temperature of the sealing bath is also important. A temperature of 96°C (205°F), ± 2°C, is recommended. Brace and Sheasby1 recommend the following bath conditions for best corrosion resistance: 1. A.W. Brace, P.G. Sheasby, The Technology of Anodizing Aluminum, Technicopy Limited, Gloucestershire, England.