Dissimilar metals

[gotribe]

We manufacture military equipment that is made from T6 aluminum. I need to attach lift rings to it and most off-the-shelf lift rings are alloy steel with a black oxide finish. Can I install an alloy steel, black oxide fastener into the aluminum without risking a corrosion related failure of the aluminum structure? The unit will be exposed to wet environments and possibly salt spray during transportation or storage. Would a sealant like ProSeal applied prior to assembly provide sufficient protection? Thanks in advance.

 

[rnd2]

Why not consider T6 or equivalent Al seamless pipe or tube and part off your own rings.

 

[israelkk]

As far as I know you can not use such fastenets unless you can guarantie constant maintenance with protecting oil or grease. All military components made of low alloy carbon steels used to be Cadmium plated. You should consult federal and military standards such as MIL-STD-889 DISSIMILAR METALS avaiable at

http://assist.daps.dla.mil/eAccess/index.cfm?ident_number=36032

and FF-S-86
Look for fasteners per NAS or MS standards such as NAS 1351, 1352 etc. SPS is one of the largest fasteners manufacturer you can contact them.

 

[CoryPad]

You should not use a black oxide fastener in contact with an aluminium part. Aluminium-friendly coatings for steel fasteners are available from:

http://www.themagnigroup.com

http://www.doerkenusa.com

http://www.geomet.net

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[WKTaylor]

gotribe...

There are LOTS of aluminum alloys capable of the -T6 temper. Each one has plus & minus capabilities... especially in a hostile corrosive environment. I PRESUME You mean 6061-T6 or some other 6XXX-T6 variant. Note: Certain 2xxx and 7xxx alloys do VERY poorly in an ocean environment and need to have extreme protective measures taken.

In general... You need to dramatically increase isolation between dissimilar materials for maximum corrosion protection.

In a aerospace "wheel-well" environment we might do the following...

Insure You anodize the aluminum part where feasible... and apply conversion coatings [lapping-over] to onto/into all other locations [such as small/blind/threaded] holes, etc.

Apply corrosion resistant primer [preferably epoxy] everywhere possible... including up-to/into lips of threaded holes.

Insure mating dissimilar parts are plated (with post plating chromate or phosphate treatment); or have an inert or aluminized coating applied.

Assemble/install dissimilar parts permanently "wet" with corrosion resistant sealant. Note: application of adhesion-promoting primer [for sealant] prior to assy/instl will greatly improve sealant adhesion and moisture-barrier potential.

Assemble/install dissimilar parts semi-permanently [IE: intended for many assy/disassy cycles] liberally "wet" with corrosion inhibiting compound [CPC]. Note: application of CPC prior to assy/instl will greatly improve moisture-barrier potential.

A decorative topcoat of paint, compatible with the primer and providing an "unbroken film" across all joints will enhance protection and will hold-up well for many years [before repaint].

If You get really eager... then over-coat [fully assembled] upper and critical surfaces/joints [more corrosion prone surfaces and "moisture-traps"] with a thin flexible rubberized coating... such as spray-on sealant like you see applied to truck-beds [before or after application of top-coat.. or colored to match the top-coat.

Also… fabrication details are critical... such as..

You can't paint [coat] sharp edges.

Insure NATURAL drainage away from joint: moisture [with salts and abrasive sand] retention is a killer.

Insure joints are tight to eliminate fretting/wear of critical coatings and seal-barriers.

If possible, apply light colors to minimize solar heating. Increase in temp of the coating in various wave-length [solar radiation absorption into the paint finish] accelerate paint/finishes break-down.


Regards, Wil Taylor

 

[WKTaylor]

gotribe...

I need to add to the following to supplement my previous posting...

DO NOT assemble/install dissimilar parts with grease or any other lubricant, unless You specifically intend the joint to be either: pressure-relubricated FREQUENTLY [IE: re-injected with grease or flowing oil]; or be disassembled and 100% hand-cleaned/relubricated FREQUENTLY... especially in a moisture-rich environment.

In my previous post I recommended use of corrosion protective compounds. Most CPC often looks and feels like grease [or wax] but it lacks the appropriate chemistry for good lubrication. What it does have is the appropriate chemistry for corrosion protection and moisture exclusion (IE: are Hydrophobic).

Lubricants [greases, oils, etc] on-the-other-hand have the chemistry for lubrication... but generally lack the ability to exclude moisture from being absorbed into the lubricant compounds. As a result, these materials tend to absorb and retain moisture… even to the detriment of the ability to properly lubricate (IE: are Hydrophilic). Temporary corrosion protection occurs with lubricants since the lube film” provides a physical barrier to moisture [or other electrolytes]. Frequent re-lubrication by pressure or hand methods, in-effect flushes out absorbed moisture [and other contaminates] by removing the contaminated lubricant from the joint. Failure to re-lubricate dissimilar material mechanical joints exposed to significant moisture [IE: aircraft landing gear pin-bearing joints] has resulted in incredible expense to aircraft operators due to massive wear and high potential for corrosion... not to mention actual seizures in operation that lead to mechanical failure and "accidents/incidents" [a triple whammy].

Also, try to use corrosion resistant steel [CRES] or titanium as Your dissimilar material to the aluminum... with appropriate coatings/sealants noted in previous posting. The reason is simple: steel, while VERY low-cost, reliable and tough is also very corrosion prone... and the platings/coatings applied to it must do "double duty", IE: protect the steel from rusting AND isolate the steel from the dissimilar aluminum alloy. CRES and titanium are, in general, immune to corrosion... so the coatings are "more capable" of performing the isolation function.


Regards, Wil Taylor