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Is a shiny finish possible after Chromic anodizing?

December 10, 2010

We received a part for Type I (Chromic Acid) anodizing that required a hot dichromate seal. In areas where the old anodizing was still present, the color was a really smooth and shiny yellow (picture attached). This is something new to us as we've never had any other color but grey on our finished parts. (We conform to AMS2470; sealing is by boiling water only.)

I used up to a 10% by weight dichromate solution in the lab to see if we can get the same color and luster but could not. We could get a greenish hue but it's matte (picture attached).

We also noted that the customer did not require or specify any dyeing step in their procedure and from what I've read, chromic anodizing is not good for dyeing anyway.

The coating met all other quality requirements, except the color was different from the original and was thus rejected. Since we've never processed anything like it before, I'm curious if there's something we missed or must do differently - for example, would an acid strip have made a difference? Or is this really what it should look like after chromic anodizing? I am not very familiar with Type II but does sulfuric anodizing produce a shinier finish? Could it be the part was previously sulfuric anodized instead of chromic?

Romina Garcia
Plating and Surface Treat Engineer - Winnipeg, MB, Canada

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December 14, 2010

Romina,

Based on your photos the top one definitely looks more like type II with dichromate than type I.

As you have surmised type I is not good for dyeing or for dichromate sealing. The colour does tend more toward a pale green with a little iridescence, it certainly won't take on the bright hue of the first picture.

Brian Terry
Aerospace - Yeovil, Somerset, UK