Salt Spray Failure on Type I and Boric-Sulfuric Anodize Coupons
What can be done to keep Type I and boric-sulfuric anodize from failing salt spray testing?
Q. Some of our recent test coupons for both Type I and boric-sulfuric anodize have failed the salt spray testing. The coupons have developed pitting and turned black over nearly all of each panel. Test panels are 2024 T3 bare. The processing procedure is:
- Alkaline clean
- Rinse, rinse
- Rinse, rinse
- Rinse, rinse
- Dilute chromate seal
Rinse and dry Salt spray testing is done in accordance with ASTM B 117 with specimens inclined 6 degrees from vertical. All coupons were processed under correct solution concentration, temperature and time. We processed another complete set of test coupons, made up a new seal and then processed more coupons. The test results were the same. What can be done to solve the test failures? —W.W.
A. This sounds like a classic salt spray test failure. However, it appears that there is nothing wrong with your processing procedures and conditions. Additional information that you provided was that the panels started to turn dark at the third day and after seven days were 80 percent dark gray. You tested the unsealed coating weight on both types of anodized panels. The control limits that you stated for your deionized water are acceptable except for the conductivity, which you said is < 20 µS/cm. The acceptable upper limit of conductance of DI water is 5 µS. This is the same for DI water used in the anodizing process as well as the water used to make up the salt spray solution (according to ASTM D1193, Ty. IV).
Most salt spray failures end up being caused by a poor seal. First, I would verify the coating weight, with and without seal. If the dilute chromate seal is good, it should add about 15 to 25 percent to the coating weight. Also, if the seal is good, the panels should exhibit an obvious green color. If the coating weight is good, but you cannot see an obvious green color, then the seal is not working. Bad water is most likely the issue. The water quality and the sealing temperature are the most critical things. The deionized water used to make up the seal bath should have a quality of 5 µS or better (lower). The temperature should be 88°C or above. Sulfates, chlorides and fluorides should all be less than 30 ppm.
Since both the Type I coupons and the BSA coupons failed the salt spray testing, there is a very good chance that the salt spray testing procedure itself was not done correctly. Be sure that all of the requirements of both the ASTM B 117 testing method and ASTM D1193, Ty. IV water quality requirements are followed to the letter.
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Benefits of anodizing include durability, color stability, ease of maintenance, aesthetics, cost of initial finish and the fact that it is a safe and healthy process. Maximizing these benefits to produce a high–performance aluminum finish can be accomplished by incorporating test procedures in the manufacturing process.