Freshwater - calcareous buildup on propshaft

[mcswain29]

Any tips/tricks out there to mitigate buildup on running gear surfaces?


-Austenitic (non magnetic, but actual alloy unknown) SS prop shaft
-Mg shaft collar anode as well as Beneteau style propshaft nut Mg anode
-1.1 to -1.2v on an ag/agcl half cell in all possible combination of conditions (at and away from marina slip, AC power on and off, DC powered on and off, etc)
-located in Eastern Lake Ontario, freshwater
-boat sits at the slip for max 5-7 days at a time



After a season, I end up with a white calcareous buildup on the propshaft and prop. It's ugly but is removable with off the shelf descaling chemicals (CLR, or Zep, etc) but my main issue is the abrasive wear it causes to the cutless bushing, as it does form on the bearing surface of the propshaft


Everybody else that I see using (incorrect for freshwater) zinc anodes stuff looks clean as a whistle on haulout


Perhaps a switch to Al anodes?

[fstbttms]

Originally Posted by mcswain29

Perhaps a switch to Al anodes?

Umm... no.

[fstbttms]

My understanding (and I may be 100% incorrect about this) is that the calcareous buildup is actually the result of the actively depleting anode depositing ions onto the surrounding metals. That it is evidence of the anode doing its job.

How you mitigate it is beyond me however.

[rslifkin]

I've been using aluminum anodes on Lake Ontario with good results for a couple of years now. The anodes do degrade, my props, shafts, etc. aren't showing signs of corrosion and when I measured, I was showing around -800mv (if I'm remembering correctly), which is enough to protect bronze and stainless (my only underwater metals). I should probably dig up my tools and re-measure at some point, as I'm not 100% sure of what my measurements were.

So I wonder if the magnesium anodes may just be a bit too much in your case, particularly considering the voltage you measured. Lake Ontario isn't the world's purest fresh water, so unless protecting aluminum or other more fragile metals, it's possible that magnesium anodes are overkill and aluminum will provide enough protection. Only way to know for sure is to try it, however.

[jakkum]

MG is the reason.
Too big anode or not enough exposed metal that has to be protected. Buy an aluminum anode.
Or just don't mind. As long as there are no hydrogen bubbles on that shaft surface
When atomic H is formed because of the electro chemical reaction. with mild steel it can get in to the metal and can cause hydrogen brittle ess.
I do not know about your shaft.
No bubbles, no problem.