Bright Anodized parts Don't Pass the Cass Test

Ask an Expert From: Products Finishing, from Anodizing Technologies

Posted on: 11/1/2006

Question: I have a roll-formed part made of 5052 alloy aluminum sheet.

Question:

I have a roll-formed part made of 5052 alloy aluminum sheet. This part is required to have a bright anodized finish, but the part being produced now will not pass the CASS requirements. I’m wondering if there is another anodized finish that will pass the testing requirements and still have the visual appearance that is required. I’m thinking in particular about hard anodize. Is there a cost difference between what we are doing now and hard
anodizing? B.C.

Answer:

There could be a couple of issues with the part not passing the CASS test. For the benefit of the readers let me first review the requirements of the CASS Test. CASS stands for “Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing.” It is performed in accordance with the procedures set forth in ASTM B 368. It essentially follows the procedures of ASTM B 117 Salt Spray Test, i.e., carried out in the same type of salt spray chamber, but to the standard 5% (wgt) salt solution (NaCl adjusted to pH 6.0 to 7.0) is added 0.25 grams of copper chloride. This solution is then acidified with acetic acid to a pH of 3.1 to 3.3. The length of testing is determined by the customer/end user. It is generally anywhere from six to 720 hours. The performance criteria is also up to the customer. For example, the customer should call out the number of corrosion sites allowed, their size, etc., in accordance with the end use requirements.

Two processing issues immediately come to mind. First, the sealing process simply may not be performing properly. Second, there may not be enough anodic coating thickness to give the protection required from exposure of the salt spray. Putting a hardcoat anodized finish on the part may not be the answer since it sounds like you are producing a specular (highly reflective) part. Hardcoat anodizing can be used on some bright parts to obtain a pleasing finish, but usually hardcoat anodizing is associated with industrial or high performance parts where appearance is not as critical as performance. Hardcoat anodizing done properly certainly will give a hard, dense anodic coating. My suggestion would be to try it and see if it makes a difference in the test performance and is still acceptable visually. You did not say what coating thickness is required. The coating thickness can affect both the performance and the brightness of the part.

Another possible solution to the problem is simply to do a better job of sealing. That’s relative because I have no way of judging the quality of seal you are getting now. We know that whatever it is, it is not good enough to pass the performance criteria of the test as determined by the customer.

You don’t mention what sealing bath/process you are using, so it’s difficult to give you specific trouble shooting hints. For a mid-temperature nickel acetate bath, for example, the bath temperature should be around 1,800°F ±5. Bath concentration should be up to the recommended strength in accordance with the manufacturer’s instructions. This is generally between 0.5 g/liter and 3.0 g/liter. The bath should be largely free of contaminants. Have you tried dumping the existing bath and making up a new one? Be sure to use only deionized (DI) water with a minimum electrical resistance of 200,000 ohms. This is the same as five micro-Siemens specific conductivity. If the bath is more than 60–90 days old, it may be too contaminated to permit a good quality seal. Of course, the degree of contamination depends on how well the bath is maintained. Ideally, there should be a good quality deionized rinse just prior to the seal and the sealing bath should be continuously filtered using a 5µ filter media, preferably pleated or wound polypropylene filter cartridges. I would be interested to find out more specifics about your seal. What are you using, what concentration and temperature, status of DI rinses, filter system, size of your tanks, etc. Any of that kind of information you could give me would be helpful. Please let me hear from you about this and we can start zeroing in on the cause of the problem.

Yes, there would most likely be a cost difference if you were to change the anodizing spec to hardcoat. I can’t say how much because I have no details about your current finish. Check with your anodizer. Hardcoat finishes rank right at the top as far as cost is concerned.

 


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