Black Anodising Problem

I too had similar problem on 6061 aluminum. I was forced to change to black painting after CrO2 anodizing. You may, however, carryout trials using Sandoz dye stuff.

There can be several reason for your problem:

1-Most such problems come out from surface pretreatment. Your degreasing bath may be out of use. Try to change it.

2-If you are using silicon based mold realese agent again such problem can come out. Try not to use silicon based mold realease agent.

3-Your metal may contain too much silicate so after degreasing those silicates can come out. You can see if such thing happen after degreasing, try to dip them in to %1 max. nitric acid solution and wash them well before anodising.

4-wach out is there any oil drops on the surface of the passivation or anodising if there is try to clean them with a papper or change it.

I have not seen the problem so what I can say are those. Hope they help you to solve the problem.

Best Regards

Hasan

LM25 won't take a good anodic coating because it has 6.5-7% silicon content to help it flow during the casting process.