Black anodising has red tinge or red splotches

A. You are getting a red tinge rather than a blue tinge because you are using a red-based dye rather than a blue-based dye :-)

Beyond that, it's hard to say. Is this something that started today after years of successful anodizing with no change in your process or is this a startup and you've never successfully blackened a part? Have you just dumped your dye and made a new tank today? More data is needed please.

A. Black dye may show up red when the thickness is not enough. This is common on 2024 when most people do not know that it requires higher voltage to get 12 ASF. So, put 21 volts on 2024.

Also, mechanically finished parts where the media has been used on steel, will embed iron in the soft "Kleenex-like" aluminum. Hey, two of the oxides of iron are red and you are "anodizing" iron to the red color. This is very common with sandblasting, wheel abrading, shot peening, time saving, etc.

Come back with more info and let's have another go at it.

A. At times when a tank gets too hot you will also get red and blue colors. When I say hot I mean around 80 °F. we currently run at 68.

Steve Tanner is right on the mark with the temperature of the anodizing solution. At 80 °F the pore is so large that it will not completely seal, then the dye bleeds, and some ingredients of the black recipe bleed worse than others - in your case the red bleeds when the pore is not completely sealed.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2005

A. As the question is vague, it leads me to believe that the individual behind the question is perhaps a hobbyist or beginner (we all start at the beginning - there is no shame in that). When I first started experimenting with type II anodizing I found myself working with common household dyes rather than commercial anodizing dye stuffs. For example, I found that RIT Dye (fabric dye) worked well in many applications except when it came to the black dye. In fact, if memory serves, the black fabric dye produced a reddish finish and I was never able to produce a true black with it. So I inclined to ask you this: What type of dye are you using?

Cheers!

A. Should Mr. Collins letter have been under a new number as it is a brand new case?

In either case the 7075 was not properly sealed and/or you have splashed something on it that removed the seal. Any alkali will remove the seal and allow color to bleed. Most cutting fluids will remove the seal and cause the dye to bleed.

----
Ed. note: Thanks Robert. There are a hundred readers for every poster, so we often group related topics so readers can see multiple situations and opinions to help them learn fast. Of course, sometimes we err and combine unrelated stuff and make a mess of things :-)
But we've now added some divider lines and structure to separate the two questions.