A question about galvanised screws (or not)

[6ply]

Looking for advice on whether galvanised fittings really are necessary for this project.

I am re-purposing an old galvanised trailer chassis that is in generally good condition. Modifications will include a new wooden bed and a painted mild steel framework bolted onto the chassis. The framework (front, sides and rear) will be to hold the loads and stiffen the chassis up. I'm aware of regulations regarding self-build trailers (which this is not) and I'm not altering the chassis or braking system. Wherever possible I will be using existing bolt holes in the chassis but will need to add a few holes in strategic places for the frame. I aim to add as few holes as possible, and as I am drilling through the galvanised finish, I may slap some etch primer on these areas. I also need to replace a few rusty M8 set screws for aesthetic reasons in non-critical places.

Can I get away with using BZP fittings for all the above or is this a definite no-no on a galvanised frame? I'm assuming that galvanised fittings are preferred when joining galvanised steel to painted mild steel. I've not had to work with galvanised before and the choice of set screws / washers / nyloc nuts in hot-dip galvanised finish is much more limited than everyday BZP. As an aside, DIN 982 nuts are what I'm after, but DIN 985 (lower profile) seem to be much more readily available. As they will be used in a low-load application, I wonder if there's any difference to me.

Thanks in advance.

 

[lexi]

You can get proper Galvanised boat screws. A bit more spendy for a box and they are slotted heads but really good.

 

[Shedendman]

Most nuts and bolts are(or can be got) zinc plated,go with them

 

[James1979]

As you only want a handful and the difference per fastener is pence, would stainless be an option?

 

[merryman]

BZP means bright zinc plated, so exactly the same, only difference is how the zinc is applied, though galvanising tends to be thicker, so will last longer unpainted.

 

[Munkul]

^^^ what he said

galanised fasteners hold up to corrosion a LOT better than BZP. Sometimes I wonder the guys in charge of the BZP process must be instructed to put the least amount of zinc plating as possible on them.

The flip side is that galvanised fasteners are coarse coated and the threads don't often spin on easily. We wouldn't spec anything below 16mm as galvanised, its not worth the effort.

 

[pedrobedro]

Bright zinc plating and passivating doesn't last and only seems to be for the appearance of the bolts and screws. Any thicker coating would have to be allowed for in the thread cutting and could affect the torque settings for the clamping force in critical applications. BZP should be fine for this trailer application.

 

[Miranda]

I never used galvanised…. BZP or stainless depending on usage and how long you want them to last… personally, I only use stainless in a marine environment. . Beware, Stainless can be brittle and it’s easy to break off stainless screws.

 

[Wildefalcon]

Use the normal screws, spray with zg 90 or your favourite spray zinc.
If exposing to a harsh environment, salt, chemical, etc, then also coat with Bilt Hambler, or your favourite anti corrosion wax.
Top up whenever you can

 

[Wildefalcon]

Oh, and use a thread locker to reduce corrosion between the threads.

 

[500e]

Stainless also has a few Got you traits, just 2 Galling & Corrosion.