The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Shelf life of chromic acid anodize coating? Need to seal?

Q. Dear Sir,
I want to be informed about the behavior of chromic acid anodized 7075 alloy aluminum without sealing process. Operators forgot the sealing operation after chromic acid anodizing process ... what kind of corrective actions can be taken for this situation? If these parts are used as is, what kind of problems will we face?

Betul Bicer
- Ankara Turkey
November 21, 2022

---

---

⇩ Related postings, oldest first ⇩

---

---

Q. We have an application that calls for unsealed chromic acid anodize as the surface preparation for 6061T6 aluminum. Normally a corrosion inhibiting primer would be applied immediately after the anodize, but for this application the customer has requested that no primer be used. My question is; what is the shelf life of unsealed chromic acid anodize, when it is used as a surface preparation for a structural adhesive bond? I have checked the sources available to me, and have been unable to find a quotable reference. Thank you in advance for whatever assistance you can offer.
Ken W [last name deleted for privacy by Editor]
1995

---

---

Re-anodizing non-stripped chromic film

Q. Hello, I have a long, hollow core 6061 aluminum extrusion that is chromic acid anodized and left unsealed. The interior is then coated with oil for storage. Once into the production line, the exterior only is then machined away to create a finished part. The part is then re-anodized for structural bonding on the exterior.

Question: Does the interior need to be stripped prior to the second anodizing? The interior will be coated with zinc chromate primer, so as long as the second anodizing cleaned away the oil and provided for primer adhesion I see no reason to strip it.

This is a fatigue critical part so I'm looking to minimize the etching on the interior surface. I will test to ensure the solution is adequate, but is this worth a shot?

Alex Sked
Aircraft Manufacturer - Central Point, Oregon, USA
2007

Q. Thank you for your response Terry-
We have to follow this order of operations due to the logistics of machining these extrusions. The duration of the machining cycle for these parts doesn't provide the ability to machine them all before they would be subjected to corrosion.
For example, an extrusion production run may be 25 parts and it would take over a year to machine them all.

Alex Sked
- Central Point, Oregon, USA

---

---

Shelf life of chromic acid anodized surfaces prior to primer application?

Q. Hello everybody!

I am an industrial anodizer and I have a new customer who wants to use Chromic Acid Anodizing for improvement of aluminum-rubber adhesive joints. The problem here is that the anodize process is carried out in my workshop and the adhesion application is done in the customer's workshop. My understanding is that in this case I should eliminate the sealing step which is normally used at the end of anodizing process. But I wonder how much time do I have before application of adhesive to the parts considering the hydration of aluminum oxide. I am sure if the hydration on anodized layer takes place before having the adhesive on the surface there will be no improvement in the adhesive joints. So I wonder what is the shelf life of chromic acid anodized surfaces prior to primer application?

Thanks in advance

Masih Ghorban
plating shop employee - Tehran, Iran
October 17, 2011