Aluminum construction, exterior environment. Galv steel or SS screws?

[NorthCivil]

I am a structural engineer that is routinely designing aluminum structures (think gates, fences, railings, aluminum screens) that are often in external environments. Sometimes desert conditions, sometimes wet climates, sometimes very near the sea.

Aluminum members typically anodized or powder coated 6000 series. These would be large (100mm) odd shaped die extrusions & 3mm sheets, that are fixed together to form structures with a mixture of self tapping screws & bolts.

Wondering if it is better to specify stainless fasteners, or galvanized fasteners? Regular steel is closer to aluminum on the galvanic series. But mild steel does corrode on its own when exposed, galvanizing only lasts so long, and galvanized fasteners usually experience damage to the galvanization during the install process (particularly self tapping screws). separating with bushings or other separators is possible to a degree, but the fasteners will still contact the substrade, this is not avoidable practically.

Forgive me for the novice level of knowledge, I have done my homework before coming here, brushing up on galvanic corrosion, the anode-cathode scale, but I am by no means an expert.

 

[Dhurjati Sen]

Dear NorthCivil,

SS fasteners are definitely a better option. You have done your homework correctly.

Regards.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

This can be tricky.
Which are you most concerned about, cosmetics or durability?

First, in a dry environment it likely doesn't matter.
In a generic local it might not matter much.
Near the coast this is a big deal.
You need to use non-metallic washers with this application.
If you don't you will get one of the following:
-While the galv fasteners will bleed rust the Al will provide protection after the Zn fails.
These may look ugly and eventually the fastener will rust away.
-With SS screws the screw will be fine and there will be no rust.
All of the attack will be on the Al and the hole will bet larger, until it is the size of the screw head.

In the long run using plastic or rubber (non-porous and non-metallic) washers with SS fasteners will likely result in the best looks and durability.

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P.E. Metallurgy, Plymouth Tube

 

[NorthCivil]

Thanks for the input Edstainless, Dhurjati

specifically, for installations in a coastal environment.

we can specify nylon washers to separate the screw heads from the aluminum base the screw fixes to. However, the screw shank will still be in contact with the aluminum screw-chase. Will this corrode, to the point of eating away all elements in contact? I had suspected that a surface film of corrosion would cover the Stainless, and the aluminium, and the two would remain in contact through their corroded films.

there would be much greater volume of aluminum, (thus we have large volume of anode, small volume of cathode)

EDIT: as the engineer, i am most concerned with durability - though the architect is concerned with cosmetics. (both are a project concern)

would 316 be a better option, or 304 SS?

 

[doomster]

NorthCivil,

I would go for the 304 SS since it is much closer to the aluminum in the galvanic series. The lesser the gap the better, for it to lessen the chance of galvanic corrosion.



Regards!

 

[TugboatEng]

It's important to know which alloy of aluminum you are using.

 

[NorthCivil]

@tugboat

mostly 6060 series aluminum

 

[EdStainless]

With good washers the effects of galvanic corrosion are only limited to the actual contact.
So the hole in the Al gets a little larger over time, no harm done.
It is still retained firmly under the washer.

It is possible to get some crevice corrosion attack on the SS fasteners, but this is less likely.
I have only seen this in locations where the fasteners never get rained on (under roof but exposed).

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

 

[TugboatEng]

I know 6061 is the first alloy people flock to due to low cost and high strength but consider using a 5000 series aluminum such as 5052, 5056, or 5086. These weather better and are easier to weld.

Couple that with isolating washers (I prefer fiberglass or phenolic because they resist creep better and won't loosen or extrude out from under the head of the bolt.

 

[GarryB1957]

There are pro's and con's for the use of either type of fastner. As these are structures, where the aluminium is vastly larger than the fastners both in mass and surface area it could be argued that the aluminium structure should be anodic to the fastners, then any galvanic corrosion occurs on the aluminium but at a vastly slower rate than if the situation was reversed and the fastner was anodic.

There are however some other considerations that need to be investigated. On the assumption that that the aluminium used is a good grade with a natural potential somewhere in the region of -950mVDC WRT Cu/CuSo4 reference electrode, stainless steel fasteners ss 302/304 have a natural potential in the region of -100mVDC WRT Cu/CuSO4 reference electrode and zinc plated fasteners have a natural potential somewhere in the region of -1035mVDC WRT Cu/CuSO4 reference electrode. Therefore the zinc fastner is anodic to the aluminium structure and the Aluminium is anodic to the SS fastner. However, the driving voltage between zinc and aluminium is small at approximately 85mVDC but the mass and surface area of the zinc fastners is significantly smaller than that of the aluminium therefore the the zinc coating on the fastner could be consumed in a relitively short space of time, leaving the aluminium being anodic to the steel part of the fastner with a driving voltage in the region of 400mVDC. This would suuggest that both aluminium and fastner will be prone to corrosion at some point.

Conversely if SS fastners are used the driving voltage is in the region of 850mVDC, however this would be supplied from the much larger aluminium structure, so the rate of corrosion will be very slow and pobably localized purely around the fastner as the aluminium is protected by a coating. There is another consideration with the aluminium being so much bigger in mass and surface area than the ss fastners in that when there is the presence of an electrlyte (there always is, even if we can't see it), then the aluminium will drive the potential of the stailess steel more negative, if this value should become more negative than -850mVDC WRT Cu/CuSO4 then the ss becomes reactive, stops producing its protective oxide layer and corrodes.This would suuggest that both aluminium and fastner will be prone to corrosion at some point.

All is not doom and gloom though, there are engineering solutions to these probelms. Firstly you could consider the use of rivets coated in a sealant to maintain an electrolyte proof joint of the same grade material as the aluminium being used in the structure, this does have a down side in the assembly as it is more time consuming and a certain level of experience is required. Secondly use the stailess steel fastners, but on assembly dip the threaded portion of the fastner on an epoxy resin to seal the joint where the fastner is. This will result in a virtual corrosion free fastner becase even if there is a minute amount of electrlyte and oxygen trapped between fastner and aluminium structure, once the oxygen has been consumed by the tiny bit of corrosion that occurs the corrosion cell dies.

 

[NorthCivil]

GarryB

Great post, thanks for the detail, I learned a lot from your response here - both the science behind & the practical realities. Going to make a point to study this in further depth.

much appreciated

 

[gtaw]

Have you considered using aluminum fasteners? Granted, you won't be able to find self tapping aluminum fasteners.

Best regards - Al

 

[Ron]

Agree with GarryB on the area effects. If you want a good example of how this works in uncoated aluminum/galvanized steel fastener applications, take a look at a chain link fence gate. You'll see that the fasteners (usually galvanized steel)have corroded and the aluminum has a light oxidation layer, but is not significantly affected.

I have designed many aluminum structures over the past 35 years or so, most of which were built in high wind/coastally influenced areas of Florida and other similar locations. We have always used stainless steel fasteners with very little in the way of corrosion issues over the years. Most of these structures are still in service ranging from simple canopies to long span trusses.

Coated aluminum is often subject to filiform corrosion that propagates from cut edges, drilled holes or even screw penetrations. Using epoxy coated fasteners will help this, particularly for self drilling/tapping fasteners.


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[WKTaylor]

I suggest high strength CRES fasteners [sheet metal, self-drilling or machine-screws/bolts/nuts/washers], with a tough environmental resisting coating system... such as baked ceramic or SFL.

Example. High-strength deck screws... designed for long-term strength and weather exposure... often come with a baked/colored finish for durability. I suspect this hardware already exists for sheet metal constructions... otherwise spec-it/buy-it.

Regards, Wil Taylor

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[NorthCivil]

WKTaylor

I think the reality is, any economically feasible coating for a fastener will get worn off during installation from the friction of the metal-metal contact when the screw penetrates into its screwchase. Some high performance coatings may suffice, but they are probably out of reach economically when looking at cheesy aluminium balustrades built to slap onto your porch for your average low budget condominium

 

[Corroneer]

If you have the option to go to lean duplex SS fasteners - UNS S32101;S3202; or32304 (with non-metallic washers as mentioned by @EdStainless), I guess that would be better than austenitic stainless steel for crevice corrosion resistance and strength.