Failure of Wind Fence due to corrosion

[Dusty Dave]

Hi all,

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

 

[Ng2020]

Some guesses as to the mechanisms at play:
Salt spray accumulates on the polyster mesh, then concentrated salt solution wicks into the joint when it rains.

Regarding the mesh failure: corrosion products/salt crystals between the mesh and lockbar might be abrading the polyester under wind movement. (In addition to uv, heat degradation)

Estimated corrosion rates in oz for various galv coating thicknesses:

https://gaa.com.au/performance-in-various-environments/

 

[mrfailure]

Dave, I can see how the scenario you paint can work. Galvanizing is a sacrificial layer. Once it is eaten away, you will get corrosion of the steel substrate. Is the environment this particular fence sees any harsher than the other fences you installed?

I suggest looking at why the screw heads are "popping off". You should have a lab examine one of them to confirm fracture mode. The presented scenario suggests that they corroded to the point that service loads broke them but I am not so sure about that. Look at fracture mode such as fatigue, brittle fracture, ductile overload, or evidence of torsional overload when installed. Also confirm screw material properties.

 

[jessieleo99]

I think you should consider switching to more corrosion resistant fasteners, such as duplex stainless steel or fasteners designed for coastal environments.

Or you could add a corrosion-resistant coating to the material. For example, use a powder coating on galvanized steel, or a marine grade anodizing process on aluminum brackets. This can provide enhanced protection against salt corrosion.

https://powder.samaterials.com/

 

[Tensile_test_my_spine]

I second Jessieleo99's comment, and to take it a step further, as mentioned by mrfailure, I would have the bracket/fastener setup tested in a salt fog/salt spray chamber to verify that you are getting the lifespan you want. This way you can look at the whole bracket setup to see if there are any other design changes that might help ensure the service lifetime you're looking for. For instance, you might decide that using a gasket washer is exactly what you need to keep the salt water out, or you might find that with your assembly, the gasket washers are actually keeping salt water in the joint, which would accelerate some of the corrosion you're seeing.