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SPOTTY/INCOMPLETE ANODIZE SULFURIC BLACK

Hello my name is Joe and I work for an aluminum extrusion company that does value add to our customer's product.

We manufacture extrusions from aluminum 6063/T5, MIL ŻA8625E sulfuric Type II Class II black. The problem we are seeing is that the coverage is incomplete, spots in places, which appear to look like a gas out or an oxidation. My question is has anyone experienced this condition and have an answer of why this is happening and how to correct it.

Joe DaSilva
Buyer/Customer - Randolph, MA, USA
February 20, 2009

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Cleaning? Silicates in the cleaner? Weathered oxide before etching? Unrealizeble electrical contact in the anodizing tank? Non-uniform air agitation in the anodizing tank? Galvanic current in the dye tank between the titanium and the stainless steel (especially in the presence of chloride)? Many many more - send us more information .

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

February 26, 2009

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March 6, 2009

Cleaning:
Soap- sodium borate
Etch- sodium hydroxide
Rinse
Anodize

Silicates in the cleaner:
None

Weathered oxide before etching:
Extrusions normally don't sit around for more than 4-5 weeks. If an order is cancelled it could be longer.

Electrical contact in the anodizing tank:
Aluminum rack with 4 contacts.

Non-uniform air agitation in anodizing tank:
Not with air, agitation is done through the chilling process.

Galvanic current:
No, tank is made of PVC.

We are using 3 different anodize vendors and we are seeing the same results form all of them.

Joe DaSilva
- Randolph, Ma, USA

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First of two simultaneous responses -- March 6, 2009

My feeling before any of the answers was that the extrusions are probably the main problem. Now, since you use three outside vendors (plus your in house?) it is about as close to 100% that the extrusions are the cause.
I hate castings and I do not like many extrusions as bad ones have extrusion lubricant on the surface and in pits /voids in many cases. The anodizer can not fix problems with the parent material. Bad extrusions sometimes have flaws in not being uniform alloy and sometimes have wildly different surface conditions because of the cooling not being uniform.
Check a large number of posts in the archive for other problems with extrusions.

James Watts
- Navarre, Florida

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Second of two simultaneous responses -- March 8, 2009

If possible, please post a photograph of the defect...
Thanks,
S. Basu

SRIMAY BASU
- Dubai, U. A. E.

In my opinion, this caused by the impure raw material. When disgas is not enough in casting process or some oxide was extruded in the product in extrusion process, It will be like this. Even polishing could not get rid of this, but you could decrease time dip in sodium hydroxide and it will looks better.

Richard Ku
- Taicang City, China
March 10, 2009

Assuming your cleaning is perfect. Then an introduction of a nitric acid dip is advisable before passing through an alkaline etch. The nitric acid can soften organic compounds if present and also remove oxides e.g. magnesium oxide so that it will not present problems in the etching process.

Lee Leong Tee
plating company - Malacca, Malaysia
March 11, 2009