Quick Galvanic Corrosion Question

[Trident]

Bronze is close to stainless and thus considered safe from galvanic corrosion or as most sources I read state its "unlikely"

My rudder stock is held by phosphor bronze bushes in the rudder shoe. The 316L shoe I made 6 years ago is still perfect . However, the last time I lifted the boat, the stock just above it (might not be 316 though non magnetic) having lived happily for 40 years had an area eaten out of it by corrosion that I clean and welded back in . I have checked and checked for stray currents and can trace none so all I am left with is galvanic. Could the bush (which I reused what I made new shoes as it was perfect ) be an issue? It seems like something changed - the stocks were fine for years, and on one rudder still are and the zinc on that rudder corrodes slowly but surely. The zinc on the damaged rudder was perfect after a year on, and after my repair I changed it and the new one still is completely untouched . (Another thread on rudders it was suggested a stainless rudder and stock don't even need an anode )

There is no damage after a year since the repair but some surface corrosion (probably from the stainless welding rod being different quality to the main metal) but I still can't figure out what cause the initial problem... except that after covid the boat sat in Chi Harbour mud for almost 3 years with only 4 - 6 hours a day afloat (and shore power connected without isolation but again no stray current from my boat and few hours in the day when others boats might have a circuit with me) . Would the mud perhaps destroy the passive layer and then inhibit further oxygen reaching the surface? I am now afloat continually and not even connected to shore power - I just want to put to bed what might have caused this so I can stop worrying that it may happen again.

 

[Hacker]

Could it be crevice corrosion rather than galvanic? If it was in the mud then no oxygen would get through.

 

[srm]

Stainless steel will set up internal galvanic cells when part is enclosed and part in an oxygenated water flow. Have seen this on oceanographic instruments where crevice corrosion formed around barnacles on a ss hull that had been in the sea for about nine months. Also on ss rudder heel fittings on a Prout catamaran; polished ss under the antifouling on the outside but a nasty uneven porous crystalline looking surface of crevices on the inside against the fibreglass and sealant.

 

[vyv_cox]

There are many tales of stainless chain corroding badly under mud. This might be sulphide stress corrosion, seems unlikely as the stress is not high, or crevice corrosion, again the mechanism does not seem right. (The reduced aeration mechanism needs a cell of full aeration in combination with partial aeration, which is not satisfied by a buried chain) So I am not sure but it does sound as if the mud was responsible. Wait and see seems to be the advice.

 

[Trident]

Stainless steel will set up internal galvanic cells when part is enclosed and part in an oxygenated water flow. Have seen this on oceanographic instruments where crevice corrosion formed around barnacles on a ss hull that had been in the sea for about nine months. Also on ss rudder heel fittings on a Prout catamaran; polished ss under the antifouling on the outside but a nasty uneven porous crystalline looking surface of crevices on the inside against the fibreglass and sealant.

The shoe itself, which I made is perfect and shiny on both sides and passivated before use and that is fine. It’s the stock directly above it on one side only that was the issue - it was the pontoon side which has deeper mud which seems to be the favourite cause of the issue

 

[Trident]

There are many tales of stainless chain corroding badly under mud. This might be sulphide stress corrosion, seems unlikely as the stress is not high, or crevice corrosion, again the mechanism does not seem right. (The reduced aeration mechanism needs a cell of full aeration in combination with partial aeration, which is not satisfied by a buried chain) So I am not sure but it does sound as if the mud was responsible. Wait and see seems to be the advice.

So far the wait and see is positive but next year I’m building a new rudder (separate thread here) and had planned GRP stocks solely on the passivity of the material and many suggested using what I have now and building the new foil around it (it’s just a flat plate stainless rudder someone made after the original grp was somehow damaged )

Before I can decide whether to start from scratch I want to be as sure as I can that I won’t be seeing the same issue again as long as I don’t burry her in the mud for years again

 

[vyv_cox]

Changing the bearing to acetal would remove any galvanic possibilities.

 

[rogerthebodger]

Changing the bearing to acetal would remove any galvanic possibilities.

I agree

My rudder bush's ae made from Visconte in a stainless carrier

 

[Trident]

I’ll change the bushes when she comes out in spring unless I change the whole thing to GRP