Have I cursed myself with galvanic corrosion through welding? (saildrive)

[SvenH]

Last winter I had a repair done on my saildrive, by a friendly welder.

Next winter on the hard I found a lot of corrosion.

Can it be that using "standard aluminium filler" started a whole heap of corrosion that would not have started if I had left things alone?
The boat lives in fresh water and all anodes look fine, and were not changed before or after the welding.

Or can it be that my paint method was so poor that corrosion finds it origin there?
I applied 2 pac primer immediately after sanding.

 

[Tranona]

Suspect the corrosion is between the original aluminium and the welding. The anode on the drive is to protect against the stainless in the fastenings and shaft and the unpainted aluminium. Corrosion on the leg usually starts when the anode fails or wears out then spreads behind the coating adjacent to the anode mounting. If it elsewhere on the leg then it is probably coating failure.

 

[SvenH]

Ah... the corrosion is at the contact where the anode ring is, which is also where the welding was done.

Bummer. Probably time for a new saildrive leg.
Or can the repair be redone by removing the welded up material and redoing it using the proper aluminium filler rod?

 

[VicS]

You say the boat is kept in fresh water.
Are you using magnesium anodes ?
Is there a good electrical connection between the anode and the sail drive leg ?

 

[SvenH]

I'm using the anodes that had been there before the welding took place.
When the welding was done, the previous repair was removed, which was done with durafix, a solder that if I am well informed contains zinc.
That was being eaten away...
Except for that there was no corrosion to speak of.

There were three thing different when the corrosion appeared:
- the welding
- the new 2-c paint
- a stainless washer between the leg and the lid where the propshaft goes through.

Before assembly the contact surfaces had been cleaned and I did not paint them.

 

[SvenH]

Could the corrosion have started or worsened because of the anode like spline of the radice folding prop deteriorating?
When removing the prop, quite a bit of the material had gone, going from the rear towards the front of the boat.

I have now bought a used all bronze maxprop plus a new anode.

 

[vyv_cox]

I believe that most saildrives are cast in LM5, or 5xx alloy, containing about 5% magnesium. If the weld repair filler contained zinc it was probably a 7xx series alloy, strong but not exceptionally corrosion resistance. There will undoubtedly be a galvanic voltage between the two but I doubt that this would be large enough for serious corrosion.

Ensure that the protective paint film is in good condition throughout the saildrive but especially at the repair. A good anode should protect the repaired area despite the mixed materials. Don't try to save on this, an anode is a lot cheaper than a new saildrive!

 

[SvenH]

Thanks Vyv. Maybe my painting did not go well.
I did paint 2- pac immediately after sanding.
Anode on the saildrive was 1 season old or similar.

 

[VicS]

I'm using the anodes that had been there before the welding took place.
When the welding was done, the previous repair was removed, which was done with durafix, a solder that if I am well informed contains zinc.
That was being eaten away...
Except for that there was no corrosion to speak of.

There were three thing different when the corrosion appeared:
- the welding
- the new 2-c paint
- a stainless washer between the leg and the lid where the propshaft goes through.

Before assembly the contact surfaces had been cleaned and I did not paint them.

Could the corrosion have started or worsened because of the anode like spline of the radice folding prop deteriorating?
When removing the prop, quite a bit of the material had gone, going from the rear towards the front of the boat.

I have now bought a used all bronze maxprop plus a new anode.

I asked if you were using a magnesium anode because you said the boat lived in fresh water. Magnesium anodes are the only ones suitable for fresh water. If your existing anodes are not magnesium then you should consider changing to magnesium. Magnesium anodes are available for your Yanmar saildrive and for the Maxprop.

As Vyv says there may be a small galvanic potential difference between the sail drive leg and the repair alloy . A good paint coating and the appropriate anode should prevent any corrosion this may cause..

Any type of bronze propeller should be electrically insulated from the shaft when fitted to a sail drive. If the insulation in the hub of your Radice propeller had broken down, or was bridged, it could well have been the cause of the corrosion.

Check that your replacement Maxprop is in fact electrically insulated from the shaft and the saildrive leg.

 

[thinwater]

Durafix is a high zinc brazing alloy, and you are correct, it does not hold up in seawater. Bad in a few months.

The weld is more confusing. If the sail drive is 5xxx and most boat hulls are 5xxx, the filler should have been appropriate. The only TIG rods I have used around boat hulls are 5356, and I didn't see galvanic corrosion.

Pictures please.

 

[SvenH]

Thanks for the replies guys. It is a complicated puzzle, this really helps.

Forgot to reply to the anode type, it is magnesium.
And my boat is wood, so the issues should only be from the sail drive, shouldn't they?

When working on the saildrive last time, I removed the corrosion and it seemed way more shallow than I thought at first.

"Any type of bronze propeller should be electrically insulated from the shaft when fitted to a sail drive. If the insulation in the hub of your Radice propeller had broken down, or was bridged, it could well have been the cause of the corrosion."

Is that even possible with any prop? Except kiwiprop maybe?
The maxprop I bought has the anode close to where the spline touches the prop.

Maybe my painting was wrong somehow?
I kept the contact surfaces between the drive and the lid (where the spline shaft comes out) uncoated, maybe I should have coated those?

 

[SvenH]

Revisiting...

The boat is out again.
The area that showed the corrosion when going in had no new corrosion, totally clean. Phew!

So that does suggest that the change to the new prop with a fresh and much larger anode made a big difference.

 

[Neeves]

Revisiting...

The boat is out again.
The area that showed the corrosion when going in had no new corrosion, totally clean. Phew!

So that does suggest that the change to the new prop with a fresh and much larger anode made a big difference.

As mentioned what is the reluctance to taking a few pictures. The pictures would have shown the hull was wood, the state of the sail drive, detail of the corroded area - you could have had much more informative and focussed suggestions.

But do not worry you are not alone in forgetting 'a picture is worth a thousand words'

You may find, if needed, that you can source another leg minus the contents. It is common for legs to be retired because the mechanics are at fault, not the leg.

Jonathan

 

[VicS]

I'd suspect the old prop was not properly insulated from the shaft.
.