Galvanized and mild steel compatibility

[AndreiR]

Hello everybody,
Alright, this is not really a welding question but rather about metal compatibility.
I'm planning to build an outdoor sculpture combining a structural mild steel frame (or COR-TEN if necessary) with bolted (to the frame) galvanized steel sheets. I am aware of the galvanic effect between two different metals (in my case - steel and galvanized steel). But at the same time I'm a bit confused: how come nothing bad happens between the zinc coating and the base steel itself? And if there is no problem within galvanized steel, would there be a problem with the combination mentioned above?
Any input about this would be appreciated.

[Rojodiablo]

Just paint the steel before you attach the galvanized to it, you'll be fine. Most issues with galvanic action have electricity and or water in the mix. If the steel frame will be completely shielded from water, and not visible, you don't even need to paint it.

[Sandy]

how come nothing bad happens between the zinc coating and the base steel itself? And if there is no problem within galvanized steel, would there be a problem with the combination mentioned above?

We'll just stick with the hot dip galvy for this question. With a good hot dip process there are multiple distinct 'micro' layers created, with the inner most layer being an alloy of zinc and the parent metal. The outer most layer being zinc. What you have then is a staged bond and/or smooth transition from the parent metal to the zinc. Electrically they are one and the same.

When butting Mild steel up against some nonferrous materials, like zinc or a galvanized mild steel, the differences in electrical properties as well as metallic composition will nearly always result in either galvonics or electrolysis, depending on current flow.

BTW, bad things can happen between zinc coatings and the base metal itself if not done properly.

[AndreiR]

Thank you Rojodiablo and Sandy for your advise.
Unfortunately painting is not really an option due to the size of the pieces - about 10' in length - it would be far beyond the budget. Plus the client wants the look of raw metal.
The idea was to combine white metal (galvanized) with rusty metal. Do you think galvanized steel would also destroy COR-TEN? How about aluminum with steel? They are in the same galvanic group.

[Sandy]

The idea was to combine white metal (galvanized) with rusty metal. Do you think galvanized steel would also destroy COR-TEN? How about aluminum with steel? They are in the same galvanic group.

I'm clueless on the cor-ten. A third element that generally complicates things further is the fastensers, if you're looking for a totally corrosion free joint.

Aluminum and steel is worse yet, if you are the aluminum any way. I doubt either will present a real visual problem tho. Some light white dusting over time. A lot depends on the environment of course.

I'd have to wonder about some product vendors definition of galvanized tho. The real heavy galvy is about a thing of the past.

[AndreiR]

Originally Posted by Sandy

The real heavy galvy is about a thing of the past.

I didn't know that. But what do I know...?
I'm coming to the conclusion that the best way would be to galvanize the whole thing - make separate pieces, galvanize them all, and then bolt them together with (galvanized) bolts.
What I read about COR-TEN is it's low alloy steel that develops a thin layer of rust that actually protects and doesn't rust through (well, not as fast) as the mild steel would. Bridges and high voltage line towers are good examples of using COR-TEN. Many sculptors use it for outdoor sculptures if they want a look of rusty brown metal.