phosphor bronze for keel bolts

[seaweed]

We are putting a new keel on a 52foot 1918 copper sheathed admiralty launch. The old bolts appear to be a copper alloy, probably one of the bronzes. I am considering using phosphor bronze to make the bolts. Would a nickel aluminium bronze (admiralty bronze) be much better? Anyone know how well the phosphor bronze would perform in seawater near the copper sheathing.

 

[starship]

Silicon bronze is usually used for this purpose- I assume the keel is lead?

 

[BrianR]

The bolt has a much smaller area than the copper sheath and so to reduce galvanic corrosion, it should have an identical or slightly more positive galvanic voltage than copper. The more negative (anodic) part is sacrificial to the positive (cathodic) part and so if the bolt was the sacrificial metal, it would soon fail because it is much smaller than the copper sheet and the corrosion would be concentrated on it. If the voltage difference is kept very small, say <1V, corrosion is minimised.

The galvanic series in sea wateris:
1. copper, ~-0.3 to -0.37V (sacrificial metal)
2. 410/416 stainless (passive), ~-0.23 to -0.35V
3. silicon bronze, ~-0.25 to -0.29V
4. nickel aluminium bronze. ~-0.14 to -0.22V

The best choice would seem to be 410/416 (passive) stainless or silicon bronze.

 

[starship]

If considering stainless, I would usually recommend an austenitic such as 316 for its somewhat better resistance to corrosion in seawater. However, although the galvanic voltage for active 316 is -0.18v, the galvanic voltage of passive 316 is -0.05v.This might corrode your copper somewhat. My references say Admiralty brass is -0.29v, pretty close to the copper, as is maganese bronze at -0.27v. Silicon bronze is -0.18v. You should keep the voltage difference to .2v or less.

 

[BrianR]

I agree with Starship and had noted in horror that I had put 1V as the difference, when I meant 0.1V

 

[anusca]

Hello, could you tell me where can I find info about composition of bronze with phosphor??

Thank you