Galvanic corrosion and seacocks

[awyatybw]

This seems to be one of those topics with many conflicting opinions and a certain amount of folklore. The wisdom of the panel would be appreciated.

GRP hull. None of our seacocks are bonded to the sacrificial anode. On the face of it – there being no electrical connection to any of them – this would seem right. Nonetheless, I have read texts that suggest one should bond everything near the back of the boat – somehow being 'near' to electrical stuff. How does that make any sense? What am I missing in the equation?

To add to the riddle, the galley seacock cone always comes up a little pink on the surface (when taken out for inspection). A little light grinding clears this up and both the valve body and cone give the right kind of 'ting' when knocked – not the hollow sound of wasted alloy. I have also shown the cone to someone much more experienced than me and they say it is nothing to worry about. So, I am not worrying, just curious. Why does it come out pink each year?

Alan.

 

[tcm]

the pink isn't good - the less noble alloys are being leeched.

I think you should bond anything pinkish (or been pink) to the same earth, and keep closer eye on the anodes.

Perhaps consider fitting another anode closer to anything with pink effect also.

 

[Blueboatman]

The best solution that I have ever seen (and copied myself,twice ) is a grp tube upstanding and glassed to the hull and coming up above the nominal waterline level at which point a valve is fitted and the water pipe carries on in the normal way to where ever it is to be plumbed.
Variants include a manifold arrangement for lots of pipes to engine ,galley,head etc and/ or 2 grp tubes one an inlet the other to deal with waste,cockpit drains etc..
The relief from knowing that there are now no below waterline through hulls is almost palpable,as is the slightly smug feeling that you aren't going to have to keep buying anodes and replacement bronze throughhulls over the years.

 

[tcm]

grp tube riser

um, but if anything happens to the joint twixt tube and hull, you're sunk. I'd feel less secure like this, not more secure. Risers of this nature are ok in metal, not grp, imho.

 

[Phoenix of Hamble]

Re: grp tube riser

I've seen a couple of these on boats and have always thought they look very vulnerable to a good clonk taking them out.....

 

[Talbot]

Re: grp tube riser

yes agree, even a bronze tube, extended that far will provide a lot of leverage so unless your through hull is heavily reinforced, you could be vulnerable.

 

[VicS]

[ QUOTE ]
the pink isn't good - the less noble alloys are being leeched

[/ QUOTE ] That's correct but provided it really is only the surface that is affected I would not worry unduly. Just inspect the galley seacock regularly and more critically than the others. It may be that that one is a different alloy. DZR brass rather than bronze perhaps. It maybe that what goes down the galley sink is a factor!

For anodes to be effective they have to be reasonably close. In this instance there is apparently just one anode. Where is it situated and what is it fitted to protect? Connecting this seacock to the existing anode if they are well separated probably won't do much good but it may be worth a try just to see if it does. Little to lose!

 

[tcm]

i heard anodes oughta be "line of sight" and altho that can't be utterly true i spose they ought be close by.

I massive reduced electrolytic activity with galnavic isolater, only £50ish, well worth it for boats cnected to shorepower a lot.

 

[Phoenix of Hamble]

I've got a galvanic isolater too... mainly as the boat is often left on shorepower on a float charger....

This year, the marina staff have commented on how much electrolytic activity there seems to have been in the marina, and suspicion lays at the door of new wiring regs requiring one common earth across all marina outlets.... several boats have come out with knackered props, including one boat with twin legs with duo props on them (ouch - £££££)

Even having an isolater, my anodes had really been chewed up... another few weeks and I might have had problems..... I am going to fit extra anodes this year.... but at least I know that my protection system is working....

Anyway, to the point..... my seacocks are not bonded, and despite the heavy conditions presented above, they are fine.....

 

[qetoo]

I had a prop that turned pink. Upon closer inspection it had corrosion cracks at the base of each blade. That was a $600 USD lesson in bonding. After that, I installed an electrical brush on the shaft and connected it to the anode bolted to the stern. That solved the problem, but I think the problem might have been caused by a neighboring boat in the marina, with bad wiring. All of my thru-hull fittings are electrically bonded to the stern anode.

 

[andy_wilson]

It begs the question about fouling and osmotic blisters in this un-inspectable place.

I think some Moodys had the decks put in place with cockpit drains connected directly to such an arrangement.

 

[andy_wilson]

Re: grp tube riser

My rudder tube looks tough enough.
I'll correct that, it looks tough enough now, it has had a repair in an earlier life and is nicely reinforced.

However, it should be easy enough to build a stack out of GRP that will be up to the job, after all, that's all a rudder tube is.

 

[tcm]

ah but

fair enough on the rudder tube with reinforcing on the hul and on the tube etc. But as talbot sugests, if the tube iself was indeed strong enuf to resist any knock...then the hull area to which it is fixed must/should also be heftily reinforced...

 

[Graham_Wright]

How does this prevent corrosion? There is still sea water passing through the tube is there not?

I accept that if one of the valves fails, it is not catastrophic.