Corrosion Protection and Electrical Conductivity Between Steel Thread Insert and Aluminum Plate 1

[jimbod20]

Hello,

My typical design practice is to install 300 series stainless wire thread inserts in wrought aluminum plates and use high strength steel fasteners for bolted assemblies. The stainless wire thread inserts provide a durable/repairable internal thread in the expensive/complex machined wrought aluminum part. Our typical design practice is to apply a two part epoxy primer to the hole when the wire thread insert is installed to prevent corrosion. The primer inhibits electrical conductivity. We have very good field experience with this design practice.

I have a new/similar design where my customer has asked me to provide an electric bond path through the fastener/insert. The fastener is high strength steel and the stainless wire thread insert is installed in a machined aluminum plate. My customer has design experience with this type of design and applies a 'thick grey paste' to the wire thread insert when the insert is installed in the wrought aluminum plate which provides good electrical connections while assuring corrosion protection. The 'thick grey paste' my customer has identified is manufactured only in Europe. My customer is obviously in Europe.

Can someone on this forum direct me to a similar product produced/available in the US. I need aerospace (flight vehicle) grade product.

 

[WKTaylor]

jumbod20

BE VERY CAREFUL about Your customer's asserting "good electircal bond experience" RE the method You describe.

There is electrical bonding... and then there is electrical grounding. True electrical bonding is repeatable and consistent; and is typically related in terms of very low mili-ohms... as opposed to ohms, where resistance is practically skyhigh and useless for precision grounding purposes. Grounding may be inadequate for high aperage/voltage lightning transients... or precision operation of certain electrical equipment.

NOTE.
It sounds like Your Euro customer has a special electrical grade polysulfide or polythioether sealant that has NO conductivity and very little resistance [not packed with corrosion preventative metallic pigments]. Wet-sealant installation of fasteners into helical coils will generally ensure elimination of moisture; which is a huge benefit for electrical bond-path corrosion resistance [especially bare CRES against bare aluminum in an electrical conduction path].

Take a hard look at the following...

MIL-B-5087 BONDING, ELECTRICAL, AND LIGHTNING PROTECTION, FOR AEROSPACE SYSTEMS
MIL-STD-464 ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR SYSTEMS
SAE ARP1870 Aerospace Systems Electrical Bonding and Grounding for Electromagnetic Compatibility and Safety.
FAA AC-25899-1 Electrical bonding and protection against static electricity
FAA AC25.1353-1 Electrical equipment and installations




Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[tbuelna]

jimbod20-

It is common aerospace practice to "wet install" cres thread inserts into aluminum substrates. However, the aluminum substrate usually has some form of corrosion protection (ie. anodize or chem film) already applied. The sealants/primers used for this application are intended to prevent galvanic corrosion by keeping moisture from getting into the joint between the dissimilar metals, as opposed to preventing an electrical conduction path.

In fact, it is common to see cres electrical bonding studs installed into clean aluminum substrates in order to ensure low electrical resistance. After checking the resistance of the installed cres stud, the exposed joint between the stud and aluminum substrate is encapsulated with a sealant compound to keep moisture out and prevent corrosion.

 

[jimbod20]

tbuelna,

We typically use a catalyst (C-1178-66) and 'green' epoxy paint (AMS3277 type II) when we wet install cres thread inserts into aluminum substrates. We normally anodize the aluminum substrate insert hole thread. We don't typically bond through fasteners/inserts.

In this application/install location we will chem film the aluminum substrate insert thread hole and then install the cres thread insert. The cres insert is a locking insert type design (MA3329 type insert). I'm currently working with my customer to 'negotiate' (hopefully) the bond path resistance requirement. The current requirement is 2.5 milli-ohms (end to end), however, I have several faying surface locations through the bond path and may require an allowance greater than 2.5 milli-ohms. I might be able to meet the requirement.

If I apply the paint/catalyst as in typical design practice how much will this adversely affect bond path resistance (I will test to quantify)? I do have a locking insert (ie metal to metal contact). If no significant adverse impact to bond path resistance I could simply use paint/catalyst on production basis.

Will I realize a significant benefit to corrosion control if I just apply paint/catalyst to the top of the insert after dry install and maintain low bond path resistance via metal to metal along thread length?

We do use a sealant AMS3277 type II on one product, however, I don't have room in the installation for this type of sealant.

 

[MikeHalloran]

One 'thick grey paste' is Never-Seez, which I think comprises metallic particles in oil or grease. It's probably conductive after everything gets cinched down, and does provide corrosion resistance in typical industrial environments.

Loctite published a white paper asserting that their anaerobic threadlocker, applied to a steel bolt threaded into bare tapped aluminum, provided electrical conductivity, thanks to metal to metal contact within the torqued joint, and that the conductivity is reliable, because the threadlocker excludes contaminants. I think they also mentioned that it's necessary to thoroughly clean out the cured threadlocker and reapply new upon reassembly in order to maintain the conductivity.

But installing a wire thread insert beforehand, with epoxy or paint, and letting the epoxy or paint cure before any load is applied, seems guaranteed to prevent conductivity, except by accident.

Anodizing the aluminum thread before installing an insert should reliably prevent conductivity, unless the insert has drive-in locking keys, like a Keen-Sert.

Maybe you should take a look at Japanese consumer electronics, where steel screws connect steel assemblies (with bare internal threads), and a self-wicking threadlocker (usually green) is applied after assembly and torquing.





Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

jimbod20-

There are metal filled epoxy adhesive compounds that provide some electrical conductivity, but I don't know if the cured adhesive joint would meet your requirements for corrosion protection, galvanic compatibility and electrical resistance.

I would also say that I agree with the advice provided by wktaylor. Electrical bonding is an issue that can result in huge problems in aircraft if not properly addressed during design, manufacturing and service. If this repair/retrofit procedure is for use on a component that is part of a certified aircraft then it may likely require some qualification effort.

Lastly, I am an aircraft mechanical systems designer but my work does occasionally involve electrical bonding issues. As a general rule I was taught never to use structural fasteners as primary electrical bonding devices. Primary electrical bonding is far more reliable when provided by dedicated components/installations designed specifically for the function.

Hope that helps.
Terry

 

[MikeHalloran]

Further to tbuelna's advice, in the world of terrestrial electronics, UL does not allow use of structural fasteners for electrical grounding purposes. They generally require dedicated tapped holes, and dedicated jumpers and screws for that purpose. I thought European requirements were similar, but I'm a little, er, rusty.





Mike Halloran
Pembroke Pines, FL, USA