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topic 3362

Paint adhesion failures over Mil-C-5541 chromate conversion coating

A discussion started in 1999 & continuing through 2017


Q. A customer of ours seems to be experiencing paint adhesion problems from time to time.They are applying a water based primer per MIL-P-85582 to aluminum parts. The strange thing is, there doesn't seem to be any problems related to anodized parts. Virtually all failures seem to be in conjunction with parts that have been chem filmed per Mil-C-5541 [link by ed. to spec at TechStreet] prior to painting. Also, there does not seem to be any adhesion problems when MIL-P-23377 primer is used over either anodize or chem film. Anybody had any similar experiences?

Kent Backus
Fort Worth, Texas, USA


A. Kent,

What is the time frame between chem film and coating? Are the parts getting dirty? Are they recleaned?

Alodine vs. anodize - If the pretreatment cycle and the coating cycle is longer for chem film than for anodize, contamination is probably your problem. Both surfaces will get contaminated, the longer they sit, the worse the contamination.

Kelly Draper
- West Plains, Missouri


Q. Hello,

For two years immersion of Mild steel components in the water, what are the best types of paints that can be used. Presently we are using epoxy 2 pack yellow zinc chromate. It fails (due to poor adhesion) within 2 months time. The surface is well cleaned with metal treatment solutions and it undergoes a good pretreatment prior to the paint application. Kindly advise me.

Santozh [last name deleted for privacy by Editor]
- Coimbatore, Tamil Nadu, India

September 12, 2017

Q. Does anyone know if there is a time limit (maximum time) before having to apply a primer after having treated 2024-T3 aluminium with Alodine/Alochrom 1200. If it is not painted within a set time, will it have an effect on paint adhesion? The Technical data sheets does not state such a limit, but are there aerospace industry rules-of-thumb?

Hanri Potgieter
Aerospace - Pretoria, South Africa

December 3, 2017

A. Hi Hanri,
Normally primer shall be applied no longer than 16-24 hours (depending on spec) after Alodine; note that it also has to be painted not less than 30 minutes after treatment

Wojciech Pach
aero industry - Mielec, Poland

December 5, 2017

A. Be reminded that the chromate coating should be very thin to be painted. Heavy yellow chromate will peel under paint.
MIL-DTL-5541 makes this clear. Also, never put "acid-wash-primer " over chromate as is deteriorates the coating.

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

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

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