Anodes on underwater stainless?

[ricardio]

Following from NealB’s post about a new rudder for his Trident 24(www.ybw.com/forums/showthread.php?427278-Ideas-on-corrosion-protection-please) I have just had a rudder fabricated for my Trident 24 with both the shaft and blade in stainless steel.

I will leave the blade unpainted as the boat will be on a mud berth and the local mud (the Humber) has the happy quality of being an effective anti fouling. Would it be sensible though to fit a zinc to the rudder blade? It seems pretty well universal practice to fit a zinc to a prop shaft but I have always assumed this was to protect the propeller as much as the shaft.

 

[VicS]

Complex situation.

Stainless steel relies on a oxide film to make it corrosion resistant. The conditions in the mud will probably be anaerobic and even reducing leading to the loss of the oxide film.


Zinc will waste quite quickly on passive stainless steel ......... but the effect of the zinc will also tend to lead to loss of the oxide film, at least locally but the then active stainless steel will be protected by the zinc in the same way that mild steel would be.


I think you may well find that you will need anodes on the blade but I would feel inclined to see what happens without, being prepared to fit a pair if necessary.
Perhaps drill the blade so that a pair of disc anodes can easily be fitted if required

 

[ricardio]

Thanks for that. The wait and see approach seems sensible. It would be another story if the boat was somewhere prone to fouling but even then the best approach seems to be to leave the rudder unpainted and scrub regularly.

 

[rogerthebodger]

Following from NealB’s post about a new rudder for his Trident 24(www.ybw.com/forums/showthread.php?427278-Ideas-on-corrosion-protection-please) I have just had a rudder fabricated for my Trident 24 with both the shaft and blade in stainless steel.

I will leave the blade unpainted as the boat will be on a mud berth and the local mud (the Humber) has the happy quality of being an effective anti fouling. Would it be sensible though to fit a zinc to the rudder blade? It seems pretty well universal practice to fit a zinc to a prop shaft but I have always assumed this was to protect the propeller as much as the shaft.

The anodes on my steel hull are attached with A4 stainless bolts welded to the hull and the same on my steel rudder. The anode is not eroding too bad and is protecting the steel and A4 attachment bolt.

I also have a stainless steel rudder I made for my self steering gear but not fitted yet but intend to fit anodes in the same way at my main rudder.

I will also be painting with epoxy tar and anti-foul due to the temp of my local water which would need cleaning monthly if I didn't.