I thought bronze wasn't supposed to corrode?

P

prv

Well-Known Member
Joined
29 Nov 2009
Messages
37,358
Location
Southampton
Visit site
Not seacocks, but rudder fittings. The gudgeon and pintle at the bottom of KS's rudder were originally made of A2 stainless, which duly corroded away. We had a new set made, complete with bolts, from bronze by Classic Marine who I assume know what they're doing.

We could see when we pulled the boat out that not all was well, but my dad and I have just had a proper look today. The gudgeon and pintle themselves have some fairly heavy green surface corrosion, but are structurally ok. The bolts, though, have corroded at the heads until there is very little left, and the nuts were quite bad too.

This is after one season on a drying berth in Southampton mud.

Any suggestions how we can stop the bolts rotting away? A passer-by when we were lifting out suggested coating the whole lot in epoxy, which doesn't really appeal, whereas my dad is convinced that bolting a small anode to both the gudgeon and the pintle will solve the problem. I'm not convinced by that because it (should) all be the same metal so no galvanic circuit.

He's going to post the bolts back to Classic Marine and see what they think. You'd have thought they'd be used to mud on the East Coast.

Pete
 
Bronze does corrodes more than stainless steel. It starts with de-zincfication due to electrolysis, especially when in contact with other metals like stainless steel or steel.

If your bronze is not in direct contact with other metals, you are likely to have a source of stray currents near your boat, and you should be using anodes to protect the immersed metals.
 
haydude said:
If your bronze is not in direct contact with other metals, you are likely to have a source of stray currents near your boat, and you should be using anodes to protect the immersed metals.
Click to expand...

My bronze is not in contact with anything else. The boats immediately adjacent on the pontoons shouldn't be causing any stray currents, as there is no shore power available, although this is a built-up semi industrial river so it's not impossible that something of that nature is going on. But there still can't be a circuit, as my bronze is not electrically connected to anything except the water, so there's no return path.

Pete
 
Bronze does corrodes more than stainless steel. It starts with de-zincfication due to electrolysis, especially when in contact with other metals like stainless steel or steel.
Click to expand...
Bronze does not dezincify ..... it contains no zinc to start with. Its a copper tin alloy.

Electrolysis implies that the corrosion is driven by an applied external EMF , eg from a battery.

Galvanic corrosion occurs due to contact between "dissimilar" metals or alloys.
It's the steel that would corrode if bronze were in contact with it.

The situation is complex in the case of contact with stainless steel.
While the stainless steel remains passivated the bronze would tend to corrode but buried in Soton mud the stainless steel would probably lose its passivity in the anaerobic conditions then it would probably tend to corrode rather than the bronze.
 
Last edited:
prv said:
Not seacocks, but rudder fittings. The gudgeon and pintle at the bottom of KS's rudder were originally made of A2 stainless, which duly corroded away. We had a new set made, complete with bolts, from bronze by Classic Marine who I assume know what they're doing.

We could see when we pulled the boat out that not all was well, but my dad and I have just had a proper look today. The gudgeon and pintle themselves have some fairly heavy green surface corrosion, but are structurally ok. The bolts, though, have corroded at the heads until there is very little left, and the nuts were quite bad too.

This is after one season on a drying berth in Southampton mud.

Any suggestions how we can stop the bolts rotting away? A passer-by when we were lifting out suggested coating the whole lot in epoxy, which doesn't really appeal, whereas my dad is convinced that bolting a small anode to both the gudgeon and the pintle will solve the problem. I'm not convinced by that because it (should) all be the same metal so no galvanic circuit.

He's going to post the bolts back to Classic Marine and see what they think. You'd have thought they'd be used to mud on the East Coast.

Pete
Click to expand...

Id suspect that the bolts (and nuts) might be a different alloy to the fittings with just sufficient potential difference between them to be causing galvanic action. It seems unlikely though!

Anaerobic conditions in the mud perhaps causing the loss of protective oxide films (both in the case of the stainless steel and the bronze) and attack by sulphides.
 
Last edited:
haydude said:
Bronze does corrodes more than stainless steel. It starts with de-zincfication due to electrolysis, especially when in contact with other metals like stainless steel or steel.

If your bronze is not in direct contact with other metals, you are likely to have a source of stray currents near your boat, and you should be using anodes to protect the immersed metals.
Click to expand...
I thought proper bronze was made from copper and tin?
Stu
 
There are lots of types of Bronzes - for example, my rudder post is made from magnesium bronze.
My money's with VicS's earlier comment - the bolts may be stock 'off the shelf' items and were a different type of bronze than the rudder fittings, with a slightly different electro-galvanic potential.
 
Last edited:
electrosys said:
There are lots of types of Bronzes - for example, my rudder post is made from magnesium bronze.
My money's with VicS's earlier comment - the bolts may be stock 'off the shelf' items and were a different type of bronze than the rudder fittings, with a slightly different electro-galvanic potential.
Click to expand...

And manganese bronze is not really a bronze at all. It's a copper zinc alloy so really a brass although it does contain a little tin as well as iron, aluminium and of course manganese. It is highly resistant to saltwater corrosion, hence its use for props.
In the wrong conditions it can dezincify.

ITYWF that some suppliers refer to it as manganese brass
 
haydude said:
Bronze does corrodes more than stainless steel. It starts with de-zincfication due to electrolysis, especially when in contact with other metals like stainless steel or steel.

If your bronze is not in direct contact with other metals, you are likely to have a source of stray currents near your boat, and you should be using anodes to protect the immersed metals.
Click to expand...

Real bronze does not suffer de-zincification, it is an alloy of copper and tin. Brass is an alloy of copper and zinc. Aluminium bronze contains a maximum of 0.5% zinc.

Sadly a lot of stuff sold and described as bronze is actually brass, not helped by the fact that so called "Admiralty Bronze" is actually a brass. "Commercial Bronze" is 90% copper and 10% zinc and is therefore really a brass.

I suspect that in the OP's case the nuts and bolts are brass rather than bronze, or if bronze they are an alloy containing some zinc, and are lower down the galvanic scale than the cast parts.

Real bronze normally forms a green oxide coating which inhibits further corrosion, but can suffer a different fate whereby the copper content is turned to copper chloride which ultimately corrodes the metal away.
 
If the river is a mix of salt and fresh water that mix creates an electrical current. Also, if the anodes aren't clean (i.e.covered in calcium) then they won't protect any metal fittings.
 
A couple of clarifications - firstly there's no stainless steel involved. That was the old fitting, the new one is all bronze (supposedly, anyway), together with its fastenings. Secondly, none of the parts are cast - instead they're fabricated from plate and bar, welded. Thirdly, there is currently no anode connected to these fittings, so KellysEye's suggestion that they're dirty is irrelevant.

Thanks for the suggestions; sounds like the consensus is that the bolts are a slightly different alloy to the fittings and are galvanically corroding. I suppose ideally Classic Marine would be able to supply (possibly make) new bolts of the same material as the fittings, but if not would a small anode on each fitting do the job? (I say one on each because they're insulated by a plastic bush in the pivot.)

Pete
 
but if not would a small anode on each fitting do the job?
Click to expand...
Worth try maybe. I'd not like to speculate as to whether or not they will be effective but I'll predict that they won't last long.
 
My dad's been on the phone to Classic Marine to order some new bolts. He and Mr MacPhail seem to have decided that the answer is to make sure the bolts are well bedded with Sikaflex, and then paint the whole lot externally with epoxy to keep the water/mud away from it.

Doesn't seem an unreasonable approach, as long as we can keep the epoxy well adhered all over.

Pete
 
As has been suggested, it would seem that interpretations of 'bronze' differ. If you use BS 1400 LG4 or LG2 with phosphor bronze bolts I suspect the problem should not occur. LG 4 has more tin and should be better but less widely available.

More info under Metallurgy>brassandbronze on my website.
 
You must log in or register to reply here.

Share this page

Top