Passivation req'd after reaming a bushing? If so.... 3

[M.E.]

Hi all,
Got a frustrating problem that seems like it should be pretty straightforward but I can't find the "definite" answers I'm looking for.

Background:
Commercial aerospace application, exterior crown area of the aircraft, with indirect (i.e. covered but not sealed) exposure to surrounding environment. Likely to see moisture, possibly de-icing fluid, misc. crown of the aircraft stuff.

I have a supplier that provides us aluminum "clevis" fittings with press-fit 17-4PH CRES AS14237 and AS14238 ream-type bushings installed in each lug. The bushings are installed wet with P/S 870 sealant. The thru-holes in the bushings are currently reamed to size after the bushings are installed in the aluminum housing, through both bushings at the same time, to get good hole alignment. The bushings come passivated prior to install, but obviously the thru-hole passivation is removed when they are reamed to size.
The "pin" that is installed through these bushings is an A286 CRES bolt. There is freedom of movement between the bolt and the bushings, but only to accommodate aircraft deflection, thermal expansion/contraction, etc. Aside from that, this is a "fixed" structural joint.

Questions:
1) How critical is it that the newly exposed surface of the reamed thru-hole be passivated? (Or otherwise treated?) Is the reamed surface acceptable as-is?
2) Some fittings that have already been produced need to be reworked; the fittings themselves are fully primed and painted. I've seen one product (CitriSurf) that has promise for a brush-on citric acid passivation, but it isn't universally accepted as meeting any aerospace passivation spec when brushed on; seems it needs to be a tank-dip before everyone agrees it meets spec. Are there any other options for locally passivating just the reamed bushing hole that is widely accepted as meeting an aerospace passivation spec?
3) Any other suggestions?

Per usual...timeline is critical...

Thanks; looking forward to feedback!

Best Regards,
-ME

 

[tbuelna]

Interesting question. I've never experienced this particular situation, but I can imagine problems with applying the chemical solutions used for passivation of stainless steel to the ID surface of a bushing installed in an aluminum fitting with carefully coated/sealed surfaces in close proximity. You can prevent any ferrous material contamination of the cres bushing bore surface during finish machining by using a carbide tipped reamer. And since there will be some relative sliding between the A286 pin body and the 17-4PH bushing bore in service, you might consider applying some type of brush-on DFL coating (like moly disulfide) to the bore surface after reaming.

 

[israelkk]

A286 Bolts should be silver coated to avoid galling.

 

[M.E.]

Good tips....carbide tipped reamer will at least risk mitigate...
The bolts are currently passivated A286, no silver plating...they do get installed with lube, but not the dry-film type.
I know that A286 sometimes gets silver plated for high temp applications where you need some lubrication....any issues with 17-4PH vs. silver galvanically?

Best Regards,
-ME

 

[WKTaylor]

M.E....

1. Before I address Your issue…

CAUTION. You have not mentioned the finish on the CRES bushing OD [???]. A CRES-bush pressed-into an aluminum hole DEMANDS that the outer surface of the bushing [and generally the ends] be cadmium/Zn-Ni plated [plus chromate treat] or have an aluminum IVD coat or have an aluminum-pigmented coating system applied. The aluminum hole MUST be isolated from the highly dissimilar CRES-bush OD by MORE than just sealant!

NOTE. You did NOT mention what the finish on Your A286 bolt is: bare-passivated, cadmium + chromate or aluminum pigmented coating [???]. This is the surface that will match against the bare-CRES [reamed/machined] bushing ID.

NOTE. In my opinion, P/S 890 is NOT an optimum choice for sealant in this application. P/S 890 [QPL listed VS AMS-S-8802] is an integral fuel tank sealant than deliberately omits corrosion protective elements [‘pigments’], such as chromates/etc, which provides the most durable fuel/fluid seal at the price of reduced corrosion inhibition. In general P/S 870 [QPL listed VS MIL-PRF-81733] sealant with corrosion-inhibitive elements would do a better job in this instance. The reason for -8802/-890 use as an integral fuel tank sealant, VS -81733/-870, is that the corrosion inhibitive elements [finely dispersed inorganic particles] reduce sealant cohesion, adhesion and sealant toughness [critical for sealing chemically aggressive fuel ] and will ‘leach’ from the sealant over-time [further reducing fuel tightness] to fulfill their corrosion-protective function. In the case of a CRES bushing installed in an aluminum alloy hole the enhanced corrosion inhibition of -81733/870 sealant is a vital enhancement to isolate the Bush from the hole.

NOTE. The reamed/bored/deburred hole in the aluminum part should have a chromate conversion coating [CCC, MIL-DTL-5541 Class 1A or equivalent] applied for enhanced isolation. MY practice is to apply one of (2) finishes to a corrosion and strength/fatigue critical aluminum part…

A. CCC all surfaces [including reamed/bored-holes] + apply corrosion inhibited epoxy primer on all exterior surfaces [not holes] + install bushes and fasteners ‘wet’ with corrosion inhibited sealant or primer.

B. Thin-film anodize all surfaces EXCEPT precision reamed/bored holes [mask-off or pilot-only during anodize] + CCC in finished holes and on radiused hole-lips + apply corrosion inhibited epoxy primer on all exterior surfaces [not holes] + install bushes and fasteners ‘wet’ with corrosion inhibited sealant or primer.
WARNING. Anodize on the edges and bores of holes with interference-fit parts [bushes, bolts] is a fatigue-crack initiation nightmare.

2. Now to directly address Your issue.

In general, for press-fit bushes, reamed-to-size-in-place…

Bare CRES-Bush ID V A286/pass-Bolt
Bare CRES-Bush ID V HRA/pass-Bolt [Heat Resistant Alloy, such as I-718, MP35, etc]
Bare Bronze/CuBe-Bush ID V CRES or HRA/pass-bolt

Process: ream the bushing with a high quality reamer or boring-process; attain a good quality machined finish [125-RA]; deburr; then install the bolt [steel/Cd, CRES/pass or Cd, HRA/pass or Cd] ‘dry or with ‘wet’ epoxy primer. Further treatment of the bushing ID is generally NOT required: LEAVE IT BARE!

NOTE. Trying to passivate the bushing bore is inviting corrosion from the passivation acid which could migrate-into/penetrate/wick-into fay surfaces [even sealed fay-surfaces are not 100%] or onto adjacent organic finishes that may/may-not be affected by the acid].

IF I felt the need for enhanced corrosion protection for the bare CRES bush ID, then I have cleaned the surface with light abrasives and solvents [water-break-quality]; then applied wet epoxy primer to the surface; agitated [scrubbed/stirred] the wet epoxy primer across the surface/edges for 5-10 minutes; then GENTLY wiped off the majority of the primer with a fresh process wipe, leaving a bare trace/film of epoxy to ‘cure-in-place’ [0.0002—0.0004-DFT]. In general, this fine epoxy film does not significantly interfere with precision-fit mating parts, but does provide a ‘good-bump’ in corrosion enhancement [at least makes me ‘feel’ better].

NOTE. For passivated bolts or Cd-plated bolts, an OEM applied dry lube [sealant friendly], or a temporary dry lube applied by the mechanic, such as acetyl-alcohol, etc, will enhance assembly [bolt-installation, nut-torque, etc] without adding or subtracting any to corrosion situation. Generally.

NOTE. A Cadmium finish on the bolt will do (2) ‘good’ things for corrosion inhibition for CRES or HRA bolts: cadmium is a natural lubricant; and [with CCC coating] really enhances corrosion protection, overall.

CAUTION. An aluminum IVD or pigmented aluminum coating on a CRES or HRA bolt inserted into a bare CRES hole is generally ‘worse’ than NO [bare-pass] coating!

CAUTION. For the odd combination of an alloy steel-bolt installed in a CRES bush, Cd-plating + wet-assembly, is essential for protecting the low alloy steel bolt from being destroyed [dissimilar corrosion] by the CRES bush. CRES/HRA alloys contain a high nickel content which is very dissimilar to alloy steels.

CAUTION. CRES and HRA alloys are highly corrosion resistant materials: however they are generally VERY dissimilar to all other alloys [exception: titanium alloys]. Coating applied to CRES/HRA parts help isolate these alloys from affecting the less resistant materials: aluminum, steel, mag, etc.

The devil is in the details!


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

israelkk ...

I totally disagree with You RE installation of a silver coated bolt.

First off, NUTS [threads!] are generally silver-plated... bolts are 'never' silver-plated. Please advise if You are aware of a silver-plated bolt or screw spec.

Silver-plate is used in high temperature situations where no other coating survives. Silver does NOT 'play' well with other alloys [especially aluminum, steel or titanium] and should be avoided for low temp applications.

Galling problems are typically minimized/eliminated between CRES alloy or HRA alloy or CRES-HRA alloy mixes by:

(a) having dissimilar alloys/tempers, such as bushes MF 15-5PH-HT180 with A286-HT160 bolts.

(b) use of anti-seize compounds used very sparingly. These are especially important for 'similar alloy' mixes of bolt/SL-nut threads under high torques.

(c) by application of SFL coatings [and antiseize-compounds] to one or both surfaces.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[M.E.]

Wil...Thank you!

Sounds like we *may* be not too far off...with some checking to do on the bushing OD corrosion protection.

Key notes:
-The bushing is 17-4PH CRES, and comes fully passivated prior to install.
-The bolt is A286 CRES, passivated.
-The bolt is installed with lubricant, but not a dry-film type...don't believe it is cetyl alcohol either; I need to check.
-We are using P/S 870 to install the bushing, not 890; agree, absolutely need the corrosion inhibitors
-The fitting gets a MIL-DTL-5541 class 1A CCC after machining, prior to bushing install.




Best Regards,
-ME

 

[WKTaylor]

Dooohhhhh...

Sorry about brain-dead thing 'RE P/S 870... VS... P/S 890'.... I was working another problem recently where P/S 890 [class] was inappropriately used.

I do STRONGLY suggest You install Bushes with Cd or Zn-Ni or IVD aluminum coat on the OD. Every layer of Isolation between dissimilar materials is vastly important.

In Your case...

CRES [Passivated=Bare] + Sealant [chromated] + CCC - Aluminum alloy.

In my recommended case...

CRES [Passivated=Bare] + Cd/Zn-Ni+CCC -or- IVD aluminum + Sealant [chromated] + CCC - Aluminum alloy

NOTE. For Cd & Zn-Ni plated parts the type II or 2 specified application of a chromate conversion coating [CCC] treatment to the 'shiny' bare plating for enhanced isolation.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]