Anodize Speckling 1

[swertel]

Continued from thread332-187869 because we still haven't been able to resolve the issue, even after changing the anodize process to the Sanford method.

To recap:
Base material is 7075-T6.
It is a forged billet and then machined to final dimensions.
Anodized per Type II, Class 2, Black now using Sanford Process. (I'm a little confused on that because all the references I see to the proprietary Sanford process consider it a Hard Anodize - Type III, but the vendor assures us it is Type II.)
All inspections certify that the anodize is done properly.
It gets to our customer who then heats it up a bit for a loading process.
They clean the parts after loading - they say with just water - speckling occurs on only the 7075-T6 part. There is a mating part made of 3003-O that does not exhibit speckling.

This time, I have a picture.

Like before, I'm looking for anything that may cause this. My team and I are running out of ideas.

--Scott

http://wertel.eng.pro

 

[coatingengineer]

Hi.

Your alloy, 7075, has a variety of unique structural properties over other alloys of aluminum. Which is great, except when it comes to hard anodize. Then it's a whole different animal.

Keep in mind those unique structural properties require unique metal chemistry. 7075 is far from homogeneous. Instead, you'll find a composition filled with magnesium, manganese, copper, and other elements. Each with its own free energy. Each capable of forming its own oxide.

Hard anodize does not like surfaces with pre-existing oxide inclusions. The result will be coating voids, adhesion loss.

So, often, it's not the coaters fault. Rather, it's in the machining.

Watch the rate and depth of your cuts. Slow down. While it may not be immediately evident, machining can affect grain boundaries, stimulate localized cell corrosion. Frictional heat can produce the same effects.

Machine a couple of samples for your coaters, using my suggestions, and you'll see what I mean.

If you'd like more help, just see:

<a href="

http://www.industrialcoatingsworld.com/Wear-Resistant-Coatings/Anodizing.html">Anodizing</a>

Good luck!

William Gunnar

http://www.IndustrialCoatingsWorld.com

 

[israelkk]

7075 should be oiled to protect from corrosion between and after all machining proceses. It is very sensitive and will corrode very rapidly if not protected. Make sure to clean and dry it after each machining process and use corrosion preventive oil to protect it until the anodizing. If you can sink the parts in water repelling oil it will even protect from imperfect drying before oiling.

 

[swertel]

Thanks for the information. We are all too well aware of the difficulties in anodizing 7075 and a requalification to use another aluminum is in the works. In the meantime, we still need to figure this one out.

William, the detailed information on machining is a new aspect not yet looked at. When you phrase it like that, it seems so self-evident, yet we have been overlooking this for quite a while. I'll get in contact with our machine shop and develop some test samples with varying speeds and feeds.

I have found out that bead-blasting (shot peening) has been found to be a way to make 7075 a little better for anodize. We haven't tried that yet because the material is very sensitive to the amount of blasting and we haven't found a house willing to attempt it considering the geometry of the part and what CANNOT be blasted. We also have to deal with residual stresses after blasting which will have an effect on the performance of this part. Too much variation in the process to consider blasting a long term solution. Changing the machining process is a much easier method to test that may have similar results.

I'll also verify the protection method used by the machine shop for temporary storage and transportation to the anodizer.

--Scott

http://wertel.eng.pro

 

[TVP]

Scott,

In addition to the machining parameters you will be investigating, I think you may want to re-visit the bead blasting aspect. Talk to a company that uses glass or ceramic media for blasting/peening aluminum parts, such as Metal Improvement. They will have expertise in handling aluminum aerospace parts, masking, delicate requirements, etc.