No registration or passwords; no pop-up ads -- just aloha, fun, & answers.
(as an eBay Partner & Amazon Affiliate earns from qualifying purchases).
Home /
T.O.C.
Fun
FAQs
Good
Books
Ref.
Libr.
Advertise
 
Help
Wanted
Current
Q&A's
Site 🔍
Search
pub  Where the
world gathers for metal finishing
Q&As since 1989



-----

Chemical Conversion Coating Appearance Problem





I have a small commercial chemical conversion coating line processing machined aluminum parts for local machine shops. I typically process 6061, 2024, and 7075 alloys. I often get a blotchy appearance on parts made of 7075 alloy. I have tried increasing cleaning and de-oxidizing times and Iridite bath times, but have made no progress to date. Solution strengths are good. Any suggestions?

Heinz Lauf
Shop owner - Porterfield, Wisconsin
2007



First of two simultaneous responses --

Be sure you use a deoxidizer that is mixed acids. Then consider that the zinc in 7075 "walks" slowly to the grain boundaries and the surface with time, and very fast with heat treatment, and in the case of work hardening there will be more zinc on the surface in the work hardened areas.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner
2007



Second of two simultaneous responses -- 2007

Mr. Lauf.
Please check your rinse water following deoxidizing. I've been experienced that overly effective rinsing after deoxidizing aluminum surfaces can adversely affect the surface appearance after conversion coat application. The rinses to get a little ¡°dirtier¡± and ultimately, improve the appearance of the coating.
Don't forget parts are to be processed from one step to the next without allowing the parts to dry.

D.Y.YOO
- BUSAN, KOREA



2007

Mr. Lauf,
I agree with D.Y.Yoo in that extending cleaning and deoxidizing times is not always the best solution to your problem. Sometimes cutting back your times a bit might be the trick (the less chemistry you have touching these pieces the less failure modes you'll have). Also, you should get good color in your Iridite bath within a couple minutes - and start getting problems after more than a few, try cutting those times back a bit too.

The idea is to remove the thin layer of natural oxide on the aluminum without going too deep beyond the surface. Try combinations of shorter cleaner times / longer deox times. Then longer cleaner times / shorter deox times. Don't look for a reason, you'll go crazy - just look for what works.

Depending on the volume you have going through these tanks, your "solution strengths" look good is half the battle, but how are your contaminant levels in your cleaners, deoxidizer, and Iridite?

Good luck to you!

Christopher D. Arfman
- Seattle, Washington, USA




(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully 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 might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

 
Jobshops
Capital
Equipment
Chemicals &
Consumables
Consult'g, Train'g
& Software


About/Contact  -  Privacy Policy  -  ©1995-2024 finishing.com, Pine Beach, New Jersey, USA  -  about "affil links"