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topic 5649

Black Film After Etching 7075 Aluminum




(2000)

I am etching 7075 aluminum parts,at ambient temperature, with a mixture of sodium dichromate,sulfuric acid & DI water in a polypro tank. The solution has been used for some time & has gradually darkened, picked up about 800 ppm iron but is otherwise unchanged(?)(i.e., by analysis of H2SO4 & Cr6+ approx. decrease of <10%).

The problem is that recent batches of parts are coated with a fine black film after final DI water rinse. A swab sample of this deposit, when examined by XRF (ED as attachment to SEM) shows only the presence of aluminum ( atomic # > oxygen). This film can be removed but I would like to know where it comes from. Anyone with an idea? PS The 7075 parts are racked on a 6061 frame.

G W Green
- Montreal, Canada


(2000)

Without regeneration,the material does not have an infinite lifespan. When enough aluminum and / or alloys are dissolved, they will immersion deposit. Buy purification equipment or replace the chemicals.

James Watts
- Navarre, Florida


(2000)

During the cleaning and etching operation, smut (a grey-to-black residual film) is deposited on the surface of the work. This deposit usually consists of iron, silicon, copper or other alloying constituents.For 7075 ; AlZn5.5 MgCu That's why after etching a desmutting operation is used! Desmutting by nitric acid solution or other products

sincerely.

Andy D'Haen
- Belgium



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