Which nut and bolt to use that will avoid cold welding and corrosion 1

[scottkm]

Im designing a solar tracking frame that needs to be corrosion resistant for installations in coastal areas and UV resistant.
I am using 6061-T6 aluminum for the frame members.
There is an EPDM gasket between members to avoid crevice corrosion
I am securing the frame members together with 18-8 Stainless steel bolts through plastic sleeves to insulate the ss from the aluminum.
there is neoprene backed ss washers under the bolt head and nut.
We were using nylon insert ss nuts on the bolts but because of cold-welding, this inn't working.
Im considering using the "ultra corrosion-resistant coated" bolts from McMaster Carr and keeping the ss nuts. I am hoping that this could solve the cold-welding issue and maybe i wont need to use plastic sleeves to insulate the bolts from the aluminum.
Ideas?, Suggestions?

 

[CoryPad]

My suggestion is to eliminate all polymeric and steel hardware and replace with aluminum hardware. Similar exposed aluminum structures (think street signs and light poles) use the following:

Bolts: Alloy 2024-T4 according to ASTM F 468
Nuts: Alloy 6061-T6 according to ASTM F 467
Washers: Alloy 2024-T4 according to ASTM B 209

Fastenal will be a better supplier of these parts than McMaster-Carr.

 

[EdStainless]

I like the all Al approach. It lets you drop all of the plastic and rubber parts. This is the best way.
The other option would be to use marine grade Cu alloy hardware. Bronze and brass.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[scottkm]

I like the using all aluminum idea, thanks CoryPad. DO I need to take precautions for the 2000 series stuff to protect against corrosion and what about crevice corrosion?

 

[unclesyd]

I have to diverge from the recommendation of using a 2000 series, especially 2024 in a corrosive environment. 2024 aluminum regardless of temper has very poor corrosion resistance especially around salt water.

There are other Aluminum alloys available for your application. I would give someone like Aluminum Fasteners a call about your requirements

http://www.aluminumfastener.com/index.html

 

[CoryPad]

While 2024-T4 fasteners may not be ideal for severe environments, they are used in exposed applications and are actually available in multiple product forms. There is a dearth of engineered aluminum alloy fasteners outside of the aerospace and automotive markets, so options can be limited.

If 6061-T6 bolts and washers are available to you, use them instead. Just be sure to specify the ASTM standards (same ones above) so that you get parts with guaranteed properties.

 

[tbuelna]

scottkm,

I'm assuming that the aluminum frame has some sort of corrosion protection such as anodize or prime/paint. If so, then using 18-8 fasteners on that surface would be galvanically isolated.

Galling of your 18-8 threads can be prevented by applying a dry film lube like MoS2 to one of the threaded parts. Also, if using 18-8 parts in a marine environment, make sure they're passivated.

Hope that helps.
Terry

 

[boo1]

We don’t use ever aluminum threaded fasteners in the marine environment.

Use course treaded 300 SST (non 303) fasteners. Wet install fasteners head and shank with proseal polysulfide sealant, P/N 823E508 PRC Desoto. Changing nuts to a different hardness of the bolt will prevent galling. Fasteners not frequently removed: Anti-seize should used or consider a thin fluorocarbon fastener coating system.

Fasteners through heavy aluminum foundations, the aluminum should be sleeved with a SS bushing.