Generally speaking in aerospace, the effects of anodizing bare* hole edges/lips and hole-bores can be very disruptive and damaging. Major fatigue life reductions have been noted.
MIL-A-8625 spec, and all design manuals, generally recommends masking fastener holes and most small pass-thru holes. At a certain large diameter [++], open holes with smoothly radiused edges/lips can be anodized successfully.
'Hard' fasteners [Steel, CRES, Ti, I718, etc] installed in close-tolerance or [God-help-you] interference-fit... bearing against an anodized surface and sharp hole-lips, tend to have relative fast and irregular crack initiation. BAM... fatigue life goes down the toilet. I HAVE SEEN THIS ACTUALLY HAPPEN in structural parts.
* Bare = 'as machined' surfaces. Carefully radiused hole edges and shot-peening provide significant benefit for crack initiation retardation. However, shot-peening and anodizing have unpredictable effects on hole tolerances and quality... which is especially necessary for lose-tolerance/interference fastener installs.
I always mandate the following for a durable fastener installation.
Finish-machine the Part, EXCEPT for all bushed, fastener and small pass-thru holes, which can be pilot-drilled [at least 0.016 undersized]. Exception: only pilot drill holes that do NOT require mate drilling/fastening on installation... such as structural spares. Precision located holes [lugs, pivots, pass-thrus, etc] can be pilot drilled... omit all others that could have significant variation from airframe-to-airframe!!!
NDI the part.
Shot peen if mandated [preferred for durability critical parts]. Masking piloted holes is NOT required.
Anodize MIL-A-8625 Type I, IC or IIB. Masking piloted holes is NOT required.
Apply epoxy primer MIL-PRF-23377 Typ I class C1, C2 or N to all surfaces. Masking piloted holes is NOT required.
Finish ream/bore the piloted holes to drawing tolerance.
Radius hole edges/lips, R0.01-to-0.02 all holes up-to ~0.375; and ~R0.01-to-0.03 for larger holes.
Brush-apply chromate conversion coating [CCC] per MIL-DTL-5541 Class 1A to all bare aluminum holes and a lips.
NOTE.
And sometimes... I add the following... Swab-wipe holes/edges liberally with fresh epoxy primer; then wait ~5-to-10-minutes. Then gently wipe-off the primer from holes/edges using a fresh swab [preferred] or cheesecloth-pull-thru [etc], leaving a barely perceptible primer finish [translucent tint] in-place.
Match/mate step-drill/ream fastener holes when in-position on the structure. Disassemble and deburr all hole-edges/lips.
Brush-apply ‘no-rinse’ CCC to bare aluminum edges and edges/lips.
Re-assemble parts and Install male fasteners, bushes, inserts etc, ‘wet’ with sealant MIL-PRF-81733 or AMS3265 or AMS-S-8802 [etc]. CAUTION there are special techniques for placement of the sealant, just prior to fastener install.
OH, YEAH... never forget to specify that fasteners be installed using a typical cross-pattern sequence.
But, hey... Its still Your call...
Regards, Wil Taylor
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