Galvanized brackets on pressure-treated wood, rusting.

[rehn]

I often see that galvanized fittings on pressure-treated wood rust quite quickly. I assume it's the copper in the wood that's the cause. What I've been thinking about, is it possible to protect the fittings with construction plastic (or similar) between the fitting and the wood. Has anyone tried that, or have any opinions on the method? Stainless steel screws are used.

 

[Totte_S]

By using stainless screws, corrosion is accelerated because the metals in the stainless material are nobler than the zinc in the galvanized fittings. Depending on how the installation is carried out, it can certainly work both better and worse. If the galvanized fitting is mounted with a galvanic separating material and is positioned so that it is not drenched by water that has previously run over the impregnated wood, it should theoretically be better. However, in this context, I would choose galvanized fasteners, which will then be the weak link, but often cheaper to replace than the fitting. In practice, I think it will be difficult to achieve a significant difference, but it's relatively cheap and fairly risk-free to try, so why not. Flat fittings can be separated with tar paper, for example.

 

[rehn]

OK
If I interpret you correctly, you believe it would be better with galvanized screws instead of stainless steel??
You may be right, your reasoning sounds reasonable.
Anyone else with opinions on the matter??

 

[ArneTW]

I think the difference in using galvanized screws instead of stainless steel is marginal, it would have had a greater impact if it were the other way around with stainless steel fittings and galvanized screws. Copper and various salts in the impregnation probably affect it somewhat, but I believe the biggest factor is that the fittings are positioned in an exposed location and the wood causes it to remain wet for a long time after rain, and that the rust follows because the zinc layer in the galvanization is too thin... there are probably not very many µm of zinc on a fitting.

​/ATW

 

[mycke_nu]

Use hot-dip galvanized fittings and screws and there should be no problems.

 

[ion]

You write that it rusts quickly, so I don't believe in galvanic currents. If it were the zinc going to the copper in the wood, the copper in the wood would go to the nickel in the stainless steel screw. No, it's probably due to poor quality in the fittings. Over a longer period, it might be as Totte S writes, but not quickly.