Where the
world gathers for metal finishing
Q&As since 1989


-----

Anodizing to withstand high temperatures

I purchase aluminum parts with a gold anodizing. The parts must withstand a 275 degree C bake for 100 hours with little or no color change. I would like to include the anodizing process or chemical required in our drawing to ensure the parts meet this specification. At times, after receiving the parts from various Anodizers, the gold color varies from a bright gold to a deep, darker gold. What is the process or chemical that provides the deep dark gold appearance?  Your help in this matter is appreciated!

Sincerely,

Steve Post
Buyer - Manchester, New Hampshire, USA
2004

If you're not spec'ing it yet, and you're using a number of anodizing shops, it's hard to say. Color can be imparted a number of different ways, but the least expensive is probably sulfuric acid anodize and dye. Depending on the alloy, and what you mean by "gold", it's also possible that the aluminum itself and/or a sodium dichromate seal is imparting the gold color. Two-step anodizing where the anodized pores are impregnated with metal salts, or oxalic acid anodizing are other possibilities for a "gold" color. But my bet is they are dyeing the parts gold.

After dyeing, the parts are sealed--probably in either hot water or a hot solution of nickel acetate, but there are other options there as well. Baking can change the color because of its effect on the dye or on the remnants of the sealing solution.

I think you need to pick the gold color you like best and ask the anodizer generally what he is doing to get it. There may be some valid resistance to full disclosure of every detail, but he should be more than willing to tell you enough that his process doesn't get written out of the spec such that everyone loses.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004