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Introduction:

An R & D program was initiated at Omega Research Inc. to determine the effects of a rubber vulcanizing curing cycle on epoxy prime adhesion. Primary test program objectives were to quantify the effects, if any, on adhesion of Mil-P-23377 primer to chem film and anodize surfaces previously exposed to a 2 hour temperature cycle at 370 deg. F.

Processing and Test Sequencing:

Step 1) Chem film treat panels - Set #1 Anodize panels - Set # 2 (Type I & 2 and Nickel acetate seal).
Step 2) Heat to 370 deg. F for 2 hours in air.
Step 3) Clean and prime with polyamide epoxy per Mil-C-23377
Step 4) Wet tape adhesion test per Mil-C-5541 and Mil-A-8625

Background:

Prior questions have been raised in the metal finishing industry on the adhesion of epoxy primer on chem film and/or anodized aluminum panels that had been exposed to an elevated temperature rubber vulcanizing cure cycle at 370 deg. F. prior to priming 2024-T3 aluminum panels processed according to the sequence noted above were tested.

Test Results:

A) The chem film treated aluminum panels showed no loss in primer adhesion.
B) The anodize treated aluminum panels, Type 1, 2 and nickel acetate sealed, all showed initial loss of primer bond strength. The adhesive failure of the primer was localized along the scribe lines, extending up to 0. 125" either side of the scribe lines. The nickel acetate sealed panels showed the greatest loss of primer adhesion as compared with Type I and 2.

Conclusions:

The mechanism of adhesion loss in the anodized coatings cannot be determined by the scope of this report. Potential causes could be dehydration of the sealed anodized, changed morphology of the anodize surface or other electromotive surface changes affecting the epoxy bond.



 
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NOTE:
The reader is cautioned that the subject nature of metal finishing and hydrogen embrittlement is technical and complex. Some simplification of metallurgical concepts may be presented in order for an easier understanding of the subject matter for the intended reader.

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