Bolts through Aluminum

[ReverenceEng]

Sample situation:

Aluminum angle ledgers attached to wall, wall sign with aluminum angle stringer to hang off ledger. What type of bolts are best to fasten along the length (exclude aluminum bolts)?

Steel manual and other sources show that a mechanically deposited or hot dipped bolt would be best, with stainless steel being a second best choice. All assuming exterior exposure, but not in a terribly corrosive environment or receiving salt spray. Couple this with aluminum and zinc being super close on galvanic series.


I am curious about this because I feel like literature says one thing, but I get comments back from plan checkers and other PE's from third party reviewers saying another thing..."people" aare frequently championing stainless steel "just because" of the name of it (I feel).

Can we field some comments please?

 

[BrianE22]

Be careful with stainless. In contact with aluminum, the aluminum will corrode.

 

[ReverenceEng]

That's what I was thinking...

 

[SlideRuleEra]

Stainless steel fasteners used with aluminum members can be acceptable. Depends of the size of the aluminum members. If there is a "large" amount of aluminum, the aluminum will corrode, but at a very slow (usually acceptable) rate. For "smaller" amounts of aluminum corrosion will be faster. See this British Stainless Steel Association info.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[drawoh]

ReverenceEng,

Aluminium and stainless steel are at opposite ends of the galvanic chart. Having said that, I have used a lot of stainless steel fasteners on aluminium on scientific equipment, most of it airborne, and I have not seen corrosion.

How are you finishing your aluminium. If you don't need to be conductive, you should anodize it.

--
JHG

 

[ornerynorsk]

Or isolate the hardware from the mounts through the use of non-conductive grommets, bushings, washers, etc.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[EdStainless]

How wet will it get?
How much salt will it see?
The worse the conditions the more it favors double weight galvanized steel fasteners.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[ReverenceEng]

All makes sense and thanks for the affirmation basically.

Large amount of aluminum area compared to bolts.

No salt spray, minimal direct water exposure. Basically, I called out galvanized fasteners to save my client money - I think it a better option all things considered, but I got a comment back from a plan checker to switch to stainless. Client doesn't want t incur cost and I don't think its necessary, in fact, I personally think its a worse decision...hence why I posted the question here...

 

[dvd]

I like these presentations on the galvanic series - "small car" (cathode/anode ratio) is easy to remember.

galvanic series

Fastenal Galvanic Corrosion