Impeller plate recent corrosion

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Vicarage

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The impeller plate on our Yanmar 3YM30 was fine in Jan when we did the usual service to the engine which included a fresh impeller. Now I come back to the boat following lockdown and find that the plate has gone completely black/green with white "mould" growing round the edge.

Does anyone have any bright ideas on what might be causing the corrosion and how I can stop it?

Engine is definitely earthed to the anode, and she wasn't plugged into shore power during the time we left her. It ran up fine with plenty of water and I sat it under load for an hour to get everything warmed up and there wasn't any problems.
 
The white mould is salt deposits from a little weeping. The plate itself does go black/green over time, especially if you have cleaned all the grease off it.

There is nothing to worry about but you could remove it and thoroughly grease the O-ring and metal facing surfaces with silicon grease which should stop the salt forming.

Richard
 
Vicarage said:
The impeller plate on our Yanmar 3YM30 was fine in Jan when we did the usual service to the engine which included a fresh impeller. Now I come back to the boat following lockdown and find that the plate has gone completely black/green with white "mould" growing round the edge.

Does anyone have any bright ideas on what might be causing the corrosion and how I can stop it?

Engine is definitely earthed to the anode, and she wasn't plugged into shore power during the time we left her. It ran up fine with plenty of water and I sat it under load for an hour to get everything warmed up and there wasn't any problems.
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Slight water leakage as Richrd suggests.

Did you fit a new O ring seal for the cover plate or have you reused the old one. Whichever, fit a new O ring with a covering of with silicone grease, Water resistant grease if you do not have the silicone variety
 
The cover is 70/30 brass, susceptible to corrosion by seawater. I am guessing that this will not be too severe in the elapsed time. If necessary dress the inner face by rubbing on fine wet and dry paper on a flat base. Glass is ideal but a small piece of formica on ply may be good enough.
 
The 3YM30 impeller cover plate seems particularly prone to corrosion. Every season when I remove mine I find corrosion pits in the surface. These seem to correspond with periods where the engine hasn't run for a week or so. There's never a problem with the rest of the pump housing. I had problems with the pump priming at the start of the season when I first fitted the engine. I now routinely polish the plate before replacing it and have not had any problems since. Never understood why Yanmar used such a poor pump on an otherwise excellent engine or maybe why the pump manufacturer used such a poor material for the cover plate.
 
Thanks all. Was worried it was a sign of something worse!
I'll get it off, give it a good clean and see how it goes
 
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