Is anode corroding too fast?
QI’ve owned my Sigma 362 for a couple of seasons and keep her at an East Coast marina. The zinc shaft anode eroded away rapidly during the 2018 season, resulting in some corrosion of the bronze Flexofold propeller.
A more robust shaft anode was fitted and this has lost 340g from its original 660g over eight months. The 316 stainless steel shaft remains in good condition and the prop has corroded no further, but I do have some new corrosion on the PSS 316 stainless shaft seal rotor where it mates with the carbon stator.
Is this rate of zinc loss usual? If not could there be stray current problems?
There is no bonding on the boat and the shaft/prop is isolated from the engine via a plastic R&D coupling. The hull is GRP and the iron/lead keel is bonded only to the mast. There is a simple AC ring main with galvanic isolator and shore power is only connected when I’m on board. Plenty of other boats around mine have AC shore leads connected 24/7 and some are in less than ideal condition. Jeremy Muntus, Sheffield
COLIN BROWN REPLIES: The loss of just over half an anode in eight months is a bit faster than you might expect, but not exceptional.
You’ve done all the right things but if you’re in a marina with other less diligent owners there may be unusual and unpredictable potentials between the shore supply, marina infrastructure and other boats that could accelerate anode wasting. You may have to experiment with adding more zinc until you have enough to last a full season.
Corrosion on the shaft seal may be the result of a simple galvanic couple between the stainless steel and the carbon ring. Carbon in the form of graphite is at the very top of the galvanic table with passive 316 and 317 stainless steel somewhere around 0.2V more negative.
Another possibility may be sea water trapped around the edge between the shaft and the stainless steel collar allowing a crevice to form and potentially low oxygen levels in the static water.