Types of Metal and Galvanic Corrosion.

[NightShadow1991]

Hi All,

Newbies here. Hope everyone's keeping well.

I wanted to ask a question that's been asked a thousand times, but I've never really worked out the best answer.

First off, what is the best metal on metal variation that's will cause the least reaction in an external environment.

I am welding mild steel frames, which will be hot dip galvanised. So I though using galvanised nylock nuts, washers and M10 bolts would be the best scenario. I can get all ironmongery in a galvanised finish, so one would assume galvanised bolts and nuts and washers, fastened up against galvanised metal frames (garden benches) would be the best option? Or can galv on galv still react without a gasket material/washer or suitable paint? I guess it's essentially zinc on zinc.

Also, if I wanted to galv the metal frames, which will then be bolted together. I'd be inclined to use A2 stainless, but I would have thought I would definitely need a gasket or rubber washer between the washers and the galvanised frame.

All options would work, I just wanted to find out the best option for least corrosion. I would have though keeping everything galv and not using a gasket or rubber washer to separate the metals when tightened would work?

Thanks all.
Best,
Jim

[kellys_eye]

Galvanic corrosion happens to DISimilar metals, never with two the same.

Your idea will work except for the fact that the threads may lose the galvanising to expose the original metal so the threads will, eventually, rust although this will happen so slowly I doubt it will ever be an issue.

[NightShadow1991]

Evening, thanks so much for the reply. I though that would be the case. Didn't think about the threads! At least that solves the problem of galv on galv, I shan't use a gasket between the washers and the galvanised frame. I was planning on using galv nylock nuts, but there would be little to no thread exposed beyond the locking section of the nut. As in the thread end of the bolt will maybe protrude 2mm beyond the nut. I could always coat the tips once they're fully fastened, or use copper greased prior to fastening.

My only other option would be either 8.8 grade zinc coated bolts, nuts and washers or A2 stainless bolts, nuts and washers. Though I'd use rubber washers or gasket washers between all washers and metal framework. I guess it comes down to cost effectiveness in the end. Though galv is my preferred choice.

When it comes to coating galvanised steel, is there a certain product/paint that would fair better than others, I seem to recall degreasing and priming on galv before applying a final weather proof coat. Though I guess for a few M10 and M8 bolts, this isn't essential. I will be fabricating 20 or more of these benches, so wanted to get the design covered on all bases.

Thanks again for the advice, much appreciated!

Best,
Jim

[kellys_eye]

Just use stainless throughout if you want to be totally safe. Unless you are in a particularly corrosive environment (seaside, salt air etc) then glavanised will last 'forever'.

[ayjay]

When it comes to coating galvanised steel, is there a certain product/paint that would fair better than others, I seem to recall degreasing and priming on galv before applying a final weather proof coat.

Mordant Solution is recommended for the initial treatment.

[aeromech3]

As I knew that zinc is used as a sacrificial metal in coating steel, I found this is interesting; I think using all galvanised is a better bet.
http://nordicgalvanizers.com/corrosion- ... -coatings/

[Nos]

I would go for stainless, I have just bought a load of different types of setscrews etc, from this firm "Kayfast.co.uk" huge choice and reasonable price's, until I now have to pay custom's duty

[NightShadow1991]

Thanks all, much appreciated.

I'm inclined to go galvanised, as the bolts will be touching the galvanised frames. I was planning on using recycled plastic joists to replace the 6x2 timber, it would be pressure treated timber, C24 green tanalith stained. I guess galvanised carriage bolts going through the timber will be fine. Much like any gate hanging bolts or galv nails for example. Or wood stainless fair better in wood, damp wood?

I may use galv M10 bolts and washers and nuts for securing the metal frame together, then A2 stainless carriage bolts for securing the bolts.

However, if I opt for the plastic joists over the timber, I will stick with galv.

Thanks again.
Best,
Jim

[Peloquin-UK]

At work we use galv on galv, zinc on galv and stainless on galv. These are on international power switches and pylons. They have a working life of up to 80 years, but that's only limited to that because the cost conductors have to be replaced and the ceramic/composite insulators have to be checked.