Letter 42063

MIL-P-23377 suitable for Inconel?  

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I am reviewing an older drawing (2001) that calls for a coated area per MIL-P-23377. I read through the MIL spec & could not find any references to the type of material that this epoxy primer can be applied to except aluminum. The problem is that my part is made of Inconel 718. Can this primer be used on Inconel 718? Is there another MIL spec that has a similar epoxy primer that is used on Inconel 718?

Thank You!

Denny Falls
Genesis 3 Engineering - Woodland Park, CO, USA

Denny,

MIL-P-23377 should be able to be applied successfully to Inconel. It has more to do with the preparation of the surface to produce a key for the paint rather than the suitability of the paint.

We currently apply MIL-P-23377 to various materials including stainless steel, titanium, nickel, aluminium (usually anodised or alodined), alloy steels of various types and even to various composite and plastic materials.

What you need to do is probably to either mechanically work the surface such as alumina blast, bead blast or similar, or to chemically etch the surface, one of the nickel etches will probably do the job for you.

As always, when developing a process, run some test pieces first and optimise your conditions, ensuring you get good adhesion before you start applying it to production parts.

Brian Terry
Aerospace - Yeovil, Somerset, UK

1) Yes, and 2) Maybe yes, depending on use, if authorized.

MIL-PRF-23377J-A1 PRIMER COATINGS: EPOXY, HIGH-SOLIDS, Paragraph 6.1:
"...These primer coatings are formulated for the unique performance requirements of military aircraft. These requirements include adhesion to a wide variety of metals and composites..."

Paragraph 6.1.1 Compatibility. "For some applications, and only when authorized by the procuring activity, MIL-PRF-85582, Primer Coatings: Epoxy, Waterborne, may be substituted for MIL-PRF-23377..."
But MIL-PRF-85582 is another primer used mostly for aluminum, with the limitation (Para. 6.1):
"However, primer coatings containing water, such as these coatings, should not be used on iron or bare carbon steel, nor for the wet installation of fasteners or faying surfaces."
Inconel 718 is mostly Ni-Cr, so no flash rusting problem but maybe limited adhesion. And, no substitution if MIL-PRF-23377 is required for wet installation of slip fits or press fits, etc; see MIL-STD-7179 for more information.

Note Paragraph 3.11f. "Apply over pretreated metal..." The only referenced pretreatment suitable for non-aluminum metals and FRP is a wash primer in accordance with MIL-C-8514.

Ken Vlach - Goleta, California

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