Wind Fence Failure

[Dusty Dave]

Hi all,

THIS IS A REPOST OF ONE SUBMITTED TO THE MATERIALS ENGINEERING FORUM
First post here, so be gentle! We have a system that mounts a porous high tenacity polyester onto a galvanised steel offset bracket (we call it an Omega bracket) that is mounted on a fence post, and a secondary annodised aluminium bracket (called a lockbar) is then screwed to the offset bracket using galvanised tec screws that have rubber washers to isolate them from the annodised bracket. The polyester is sandwiched between the Omega bracket and the lockbar under tension, and then stretched to another post for the next clamp section. The material is tensioned to 500 lbf as it is stretched across the posts.

These assemblies work very well generally, and we have installed in several coastal areas without incident - in fact we guarantee the system for 10 years and have not had a failure...until now.

We have an installation in Western Australia where we are seeing corrosion to such an extent (after 2 years) that the fence itself is degrading and the screw heads for the galvanised screws are popping off. I think that in this case, because the fence is located on a peninsula, where it is commonly opposing wind directions during the day, we are getting a lot of salt laden air (sea spray) that is allowing salty water to collect in the small gaps between the lockbar and Omega bracket, and this is causing corrosion over time. I am attaching photos that show what has happened.

We have had to warranty the fence material, but I don't want to have to do this again in a couple of years, so I wanted to get some advice on the potential cause and remedy.

Attached are photos showing what we found when we dismantled the fence. Any comments welcome

Regards - Dave

 

[Compositepro]

You have to fix the galvanic corrosion issue created by contact between aluminum and steel. This corrosion also creates a highly caustic environment where the corrosion occurs. Polyester fiber is being degraded by this caustic environment.

 

[EdStainless]

So is all of your steel double weight galvanized?
And is it galvanized after any forming or drilling?
In addition to a polymer washer, you may need to look at using a sealant on both sides of it to minimize moisture intrusion.
Have you looked at SS (martensitic) screws?

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P.E. Metallurgy, consulting work welcomed

 

[stevenal]

Another vote for stainless screws and sealant. Most steel screws are simply plated, but even proper hot dip galvanized screws don't hold up well in the salt. Unless your washers are shoulder style, you are not galvanically isolated from the aluminum (It's not aluminium there?). The salt water blows into the gap where it dries and where fresh water rain cannot clean it. You have the mechanical action of salt crystals working on the fabric in addition to the chemical attack. How's the back side of that lockbar look?

 

[PersonalProfile]

The detail has optimisation options, including:
1. Mixture of materials causes galvanic corrosion, as stated by others,
2. Self taping screws cause corrosion - consider fasteners etc if practicable,
3. Consider alternative material e.g. 316 stainless bracket/fixings, as generally stated by others, and
4. Consider barrier materials, if appropriate materials are impracticable,

The equivalent issues were generally addressed by the "owner's engineer" for your apparent product for a coal mine near Glenden/Nebo in Queensland, if I have guessed considering: your name, and the product; - perhaps consider reviewing the associated engineering drawings.

 

[EdStainless]

Don't use 304 or 316 SS.
They will not hold up and you may have cracking failures.
Personally, I think that you would be ahead to swap the Al bar for either heavy (double weight) galvanized steel or 430SS.
And use 400 series SS screws along with a sealant.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed