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


-----

Trouble in black anodizing

Dear Reader,

I have been doing anodizing for 1 year, and now I had situation, there were a lot of small spot around the surface, What do you guys think?

Should I change all of the chemical?or is that possible it's because of the sandblasting?

quick response will be great.

Best Regards,

Agung Dwi Prabowo
customer - Salatiga, Central Java, Indonesia
March 17, 2009

---

March 18, 2009

Not enough information given. Also, a picture would be helpful.
Why are you sandblasting before anodize? Pieces of sand get stuck in the metal and do not anodize.
The spots could be water spots, or it could be chemicals seeping out of pores or it could be spots of extrusion lubricant that do not let the aluminum anodize.

James Watts
- Navarre, Florida

---

March 19, 2009

Agung,

We need lots more information before you could get a sensible answer.

Firstly, what is your solution chemistry, what are your pretreatments and post treatments?

Secondly, what are you anodising? Is it castings, forgings, sheet, tube, bar? What alloys?

Thirdly, what do the spots look like and when do they occur? Are they little white spots, especially evident after black anodising has been completed for a while, or is it discrete pits?

Lastly, what investigations have you done yourself? We can help, but there is no point going over old ground.

As a guess, without any real detail to go on, I would say that you have solution leaching from castings after the anodising process has been completed, bleaching the black dye to white around the pores. To overcome this try alternately rinsing in warm (60C) and cold water, do this several times, it should improve things.

Brian Terry
Aerospace - Yeovil, Somerset, UK