Bronze corrosion in seawater?

[HareBrain]

Hi, can anyone help with this? I need to know if bronze can survive a couple of thousand years exposed to shallow saltwater. What I've read so far suggests that the surface would corrode and protect the underlying metal, but I can't confirm this.

I've found references to Ancient Greek helmets etc recovered from shipwrecks, but they seem either to have been buried in sand, or the sources don't say whether they were exposed or not.

 

[Sapheron]

To sum up my answer, no. It can't. If you want to see my full and rambling answer, read on.

Iron, as we know, reacts with oxygen over time to make iron oxide. Contrary to popular belief, this actually takes a hell of a lot of time if left alone; iron in the middle of the sahara would take hundreds of years to noticably rust. However, the process is sped up by water, and even more by halite (salt) and many acids.

Bronze, as you most likely have realised already, is an alloy of tin and copper. Tin can react with oxygen over time, creating a white powdery type stuff. Copper reacts very slowly indeed with oxygen, barely at all. Instead, it reacts more quickly with carbon dioxide, to create copper carbonate, which is the green colour, or it can react with salt to create copper chlorite (green again) or with sulphur to create copper sulphate (black tarnish. very weird looking). Now, the chlorine and sulphur will react much quicker than the carbon dioxide with this.

In a shallow marine environment, you'll find very little sulphur, so this is effectively null. On the other hand, there's plenty of salt, and the shallow depth means the water will be rich in oxygen and carbon dioxide compared to a deeper marine environment. Assuming temperate temperatures (western europe, for example) and a normal level of salt in the sea (atlantic ocean) we're probably looking at pretty quick reactions here. Unless the mass of bronze is very large it won't last more than a few hundred years, and within a few decades it will be practically unrecognisable. A sword, for example, would probably become unusable within ten years.

Another factor to consider would be that shallow sea water is usually a high energy environment; part of why it would react so quickly is that it would be continually being swept about by currents, waves and living creatures, which would scrape off the reacted surface and allow easier access to the pure metal beneath.

You could slow these processes by lowering the temperature or salt content. Very still water would also help by lowing movement, oxygen and carbon dioxide. Really though, you want to bury it in fine mud or silt which will cut it off completely from the various reactants (as the helmets you mention were). To be honest, if it's in a shallow marine environment for two thousand years, burial is inevitable anyway; these areas have a large level of deposition.

 

[HareBrain]

Thanks for that thorough (if disappointing) answer. One point about burial though, is this inevitable? I've been to parts of the Turkish coast where the seabed is pretty much rock, and you can find bits of amphorae lying about that I assume date from Roman times. I'm guessing this is because if the lack of tides, the nature of the local rock (not sandstone) and lack of silt-bearing rivers outflowing into the sea. Would this be right?

 

[Sapheron]

Yes, I did generalise a little. It is possible for burial to not occur; it is, however, very unlikely. Pedantically, I must also point out that a lack of sandstone doesn't mean there won't be sand. You need sand to make sandstone, but the majority of rocks can be used to make sand. Other than that, yeah, you're right. Without rivers or a longshore drift system (movement of stuff along the coastline) there won't be any real build up. Add to that low levels of erosion in the place in question and sediment won't be made locally. In that case, yeah, stuff could just lie around.

If you want an object to last, put silver or gold down there. Or titanium (less of a traditional fantasy metal). All of these will last much better. It's concievable that all of them could last a few thousand years if submerged in sea water, especially gold.

 

[HareBrain]

Thanks again Sapheron. Unfortunately the thing I want to last is quite substantial, and having it made of solid gold would be ridiculous, unless gold were as plentiful as, say, copper. But it might be logical for it to be gold-plated, as it has to do with the sun. If one were to take something of bronze and coat it with a thin layer of gold somehow (maybe just by hammering thin strips of gold around it; I don't suppose any other techniques were around in the bronze age), would that stop the bronze corroding? And would it still be clearly gold-plated after centuries underwater, or does gold develop a surface corrosion/tarnish like silver does, which might disguise its value?

 

[Sapheron]

No, gold doesn't tarnish, so it wouldn't look any different however long you left it. The problem would be how soft gold is; it would easily be scraped off by pretty much anything. It would have to remain very still and be left completely alone for the duration of however long you want it to last.

Although I can't say it for certain, other goldsmith techniques may have been around back then. Due to the low melting point and high malleability of gold it can be manipulated quite easily. It would be quite easy to submerge an object in molten gold, assuming you gathered enough gold to do so, and gain a covering that way.

 

[HareBrain]

OK, cheers for all that. If I have any more geometallogicallurgical queries I'll know where to come!

 

[Sapheron]

Now I feel like a chemistry nerd... Bringing out the worst in me! Damn you Harebrain!