Saildrive anode & saildrive corrosion

[Daydream believer]

In the years that I have had my boat ( from new) I started with a standard volvo fixed prop on a 120s saildrive. I never had to change the saildrive anode. Then 14 years ago I changed to a Brunto Auto prop with a rope cutter. The anode on the prop needed changing twice a year ever since & the saildrive anode just lasted a season. The boat is lifted out every winter. I do not leave shore power on unless cruising & that can be 90 nights of the year. This year I managed 78 nights aboar, but the boat was in the water for a month less than usual. The prop anode just about lasted but only just. It was hanging on by one bolt so barely making contact I suspect.
I bought 3 saildrive anodes from Parts 4 Engines 2 years ago.( to reduce postage per unit.), instead of paying volvo prices from Volspec.
When the boat was lifted last month the saildrive anode was barely corroded & I noticed an area of deep pitting in the trailing edge of the saildrive about 25mm * 8mm. * 2-3mm deep
To make matters worse I forgot I had spares so had just bought 2 more leaving me with 4 spare anodes.
So my question is- Have any other users had problems with anodes from this supplier?. Have I inadvertently been sold a freshwater anode? How does one tell if all 4 anodes are rubbish or if the one was just an isolated incident.
How does one tell where one is with the quality of anodes? How do I know if I have the correct anode?

Then to follow that up. I need to repair this area of corrosion. Any tips on how to deal with small areas ( in the trailing edge) before they become big ones

 

[vyv_cox]

SThere are three anode metallurgies, zinc, aluminium and magnesium. Magnesium in seawater will disappear in around two weeks, so that is definitely not what you have. Zinc and aluminium anodes are pretty much interchangeable - they are alloyed in such a way that their potential in seawater is almost the same, both more anodic than your saildrive. You can identify them by weight, zinc being far heavier.

 

[yotter]

I had a similar issue two seasons ago and managed to repair the leg. Can you post a photograph showing the area of damage ?

 

[Daydream believer]

"Zinc and aluminium anodes are pretty much interchangeable - they are alloyed in such a way that their potential in seawater is almost the same, both more anodic than your saildrive. "

So any idea why my anode has not corroded please?
I will look to see if I have an old anode. If i can photo the difference I will post a picture

 

[Tranona]

Poor continuity between the leg and the anode. Unlikely with a 120 style anode but worth checking.

 

[Daydream believer]

I had a similar issue two seasons ago and managed to repair the leg. Can you post a photograph showing the area of damage ?

Sorry no pics but it is on the trailing edge about the size I indicated. I use a sponge infill in the void rather than a rubber outer seal because it always stays in place flush with the hull. I will remove it later to see if there is any corrosion in the area up in the space under the main seal. Unfortunately it was so cold today that just refitting the prop resulted in frozen hands so I gave up.

 

[Daydream believer]

Poor continuity between the leg and the anode. Unlikely with a 120 style anode but worth checking.

Yes, there was a brown stain on the saildrive where the surfaces met. I did not think to look at the back of the anode. Never gave it a second thought. But the retaining screws were so tight I had to get assistance from a passing engineer as I was just not strong enough to turn the screws. So there would have been continuity there at least. It was certainly clamped very tight.

 

[yotter]

It was almost certainly poor continuity which prevented my anode from performing and the damage to the leg. This leg was successfully repaired, and a new bearing carrier fitted. Still no water in the oil, The fixing holes in the (old) bearing carrier do not look great.