Corrosion

[MMcK]

My Volvo MD2020 sea water pump has dezincified on the inlet pipe stub and the impeller plate, causing significant reduction in the wall thickness and pitting, the engine is only three years old with 300 hours, I believe the engine is electrically isolated from the sail drive so I doubt the stray current is coming from the shore power but can not think how this is occuring, has anyone had experience of this, if so what was the cause?

 

[boatmike]

Yes seen it before many times. If you leave the seacock open the pump inlet is directly connected to the water outside the hull and can suffer electrolysis just like any other vulnerable part. Happens worst if the inlet and seacock are very near to one another and connected through a pipe which is entirely below the waterline. Sometimes the seacock itself can act as a cathode and there is a small but significant cell built up between the two metals. Sometimes these pump bodies are brass and if there is any other material available to act as a cathode nearby they can fizz, especially if there are stray currents around like a shore supply...
One cure is to always turn off seacocks but a bucket filter above the waterline is a great cure sometimes as when the engine is off the water falls back from it and creates a break in the current flow.

 

[nigelhudson]

If the pump is electrically isolated from the saildrive then it is probably not protected by an anode. This is straighforward electrolysis and nothing directly to do with stray currents. You could bond it to the saildrive and accept the slightly accelerated depletion of the anode or arrange a separate anode for it.

 

[boatmike]

NO!!! On no account bond any component to the saildrive! They are electrically isolated for good reason. The saildrive leg is aluminium. Terrible advice.
And electrolytic corrosion can definitely be accelerated by possible stray currents from a shore supply, that's why people have galvanic isolators. In the case in question it can be further accelerated by crevice corrosion due to oxygen depletion. The advice I gave will assist in reducing if not eliminating both.

 

[MMcK]

Thanks for the quick and very informative advice, expensive lesson it is a pity the problem is not more widely known and thereby avoided

 

[boatmike]

In practice Mike every boat is different and predicting electrolytic corrosion is still more of an art than a science and opinions vary. Some people bond all their seacocks, others (like me) just fit high quality bronze and forget em. When you have a boat on a pontoon with a shore supply for any length of time a galvanic isolator is a good idea anyway, but when an engine is on a saildrive and therefore not "earthed" to the hull, I believe it is significant that the seawater inlet pipe is it's only connection to the water outside the boat. If your batteries are connected to the engine, then all your onboard systems are in fact "earthed" through this route aren't they? This at least creates an opportunity for stray currents to flow (at least that's my logic)