Engine Anode - covered by deposits

[Marqus]

I attach a picture of an engine anode removed from a 3-cyl inboard diesel (Bukh) for maintenance. Anode is about 5/16inch dia.

The engine is fresh water cooled, the coolant containing Ethylene Glycol. Anode has been in for one year. Engine run very little (less than 50 hours) in that period, sometimes unused for 2 months at a time.

When I scrape off the crud (brittle stuff), the anode underneath appears to have normal geometry; with barely any consumption of zinc. The picture shows the anode before scraping. I plan to flush and replace the coolant.

My question: What is the "stuff" that deposited onto the anode?

[DeepFrz]

The white stuff is probably aluminum oxide. I don't know what the brown stuff is. Check your owners manual to be sure you should have a zinc anode in the freshwater side. For hull protection in fresh water you should use magnesium anodes. Maybe its the same for the coolant side of the cooling system.

Maybe you could find a lab that would check out the deposits. Let us know what you find out.

Edit: changed aluminum to magnesium

[DeepFrz]

Here is a quote from Volvo Penta on anodes.

- If an anode looks yellow or is covered with white crust the anode has most certainly been pacified. If this happened brush the anodes using sandpaper or an emery cloth. Note, never use a wire brush with steel bristles. You have to use an emery cloth without iron oxide otherwise the anode might be pacified. Be carefull reusing yellow zink anodes since they sometime are not possible to brush up and make work again.

I think that for that application an aluminum anode would be best. But check with your engine manufacturer.

[Marqus]

Anode is made for Bukh, with composition:
# Cadmium: 0.025%-0.07%
# Copper: 0.005%max
# Iron: 0.005%max
# Lead: 0.006%max
# Aluminium: 0.1%-0.5%
# Zinc**: Remainder

So mostly Zinc and not appropriate, as DeepFrz suggested (thanks).

According to Martyr (anode manufacturer):
ZINC - For use in salt water
ALUMINUM - For use in salt and brackish water
MAGNESIUM - For use in fresh water

The anode's apparently not needed for the fresh water cooled engine, since the Bukh manual only mentions the need for 25% ethylene glycol in the coolant to inhibit corrosion. The heat exchanger has its own anode. I'll remove the zinc anode from the block.

So the crud that deposited on it is most likely a chemical reaction between something in the coolant and the zinc anode. Don't know what chemical reaction, but it seems it should not be unexpected. May take a chemistry post-graduate to explain that reaction.