-- The Home Page of the Finishing Industry
23+ years of serious education, promoting Aloha,
& the most fun you can have in metal finishing smiley
    no popups, no spam
on this site
current topics
topic 5649

Black Film After Etching 7075 Aluminum


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


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


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


Andy D'Haen
- Belgium

This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Comment to THIS thread START a NEW threadView CURRENT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2018, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.