Suitable screws for galvanized posts?

[mungobongo]

I have galvanized fence posts that I am now going to attach wooden beams to and install a fence. However, I'm a bit unsure about what kind of screw would be suitable. It's about metal, so I want to avoid having the screw or the post rust.

 

[byggare henke]

do not take stainless bulr of any kind. carriage bolt?

 

[mungobongo]

Unfortunately, I don't have the option for through-mounting in the pole, so it has to be some type of screw. I am considering whether it might be possible to use some type of plastic expander. Does anyone have experience with this?

 

[frav]

If you want to prevent electrolytic corrosion, the best option is to use some form of equivalent material, in this case, hot-dip galvanized. Can it work with French wood screws since these are usually hot-dip galvanized?

 

[John J]

If you want to prevent electrolytic corrosion, the best option is to use some form of equivalent material, in this case hot-dipped galvanized. Could it work with coach screws since they are usually hot-dipped galvanized?

Exactly, you must have galv to galv and stainless to stainless, otherwise there will be galvanic currents.

 

[ArneTW]

It can certainly be beneficial to have equivalent materials in the screw and the material to avoid galvanic corrosion, but it is more important to ensure that the screw is more noble than the substrate.

That is, a copper screw in a steel plate is okay, but not a steel screw in a copper plate.

If the material you want to screw into is hot-dip galvanized, it means you have a relatively thick layer of zinc on your surface. Zinc is a less noble material, so you can use virtually any steel screw.

/ATW

 

[John J]

Of course, it can be good to have equivalent materials in screws and material to avoid galvanic corrosion, but more importantly, make sure the screw is nobler than the substrate.

That is, a copper screw in a steel plate is okay but not a steel screw in a copper plate.

If the material you want to screw into is hot-dip galvanized, it means you have a relatively thick layer of zinc on your surface. Zinc is a base material, so you can use practically any steel screw.

/ATW

If you put a copper screw in a steel plate, the plate will not fare very well.

 

[ArneTW]

If you put a copper screw in a steel plate, the plate will not fare particularly well.

No, it will not fare well with the copper screw. But since the plate is large compared to the screw, the oxidation of the plate will go relatively slowly, i.e., the plate is a sacrificial anode.

Whereas if you do it the other way around, i.e., an un-noble screw in a nobler material, the screw will disappear before you can blink. In other words, the screw is the sacrificial anode.

So the advice is that the screw should be equivalent to or nobler than the substrate material.

You want the largest material amount to be the sacrificial anode and not the smallest load-bearing detail.

/ATW