Where did my anode go?

[Simondjuk]

We launched at the start of September with a new anode on a new anode stud. In mid October the anode was almost new looking with just a little fizzing to the surface. A couple of weeks ago I found it missing. More worryingly, not just the anode was missing, but the outer portion of the stud also. The remains of the stud were in the hole, secured only by the Sikaflex, which fortunately provides a strong bond.

I find it hard to imagine that the anode lasted a month in fine fettle, then disappeared entirely in the following five months, and the stud/anode backing plate also once unprotected by the AWOL anode.

The stud had a welded plate which forms the seal against the outside of the hull and I can't help but wonder if this weld had been badly made, rendering the stud weak at this point.

The anode/stud assembly cannot have been struck as it lies in an area protected by a skeg.

Had the bolt been seriously weak, I'd have thought it would have broken when I tightened things up.

The only physical trauma it can have suffered is prop wash from going hard astern, which I did do from reasonable speed by way of a test of the new engine/gearbox/shaft/stern gland installation, or, due to the anode coming loose on the stud and spinning, causing the backing plate to saw into the stud.

The remains of the stud are slightly waisted at the point at which it would have exited the thickness of the hull, where the plate would have been welded.

Has anyone else lost an anode complete with the outer portion of the stud, or is this a unique failrure? Or, do I have a huge stray current problem in my berth which could have eaten anode, stud and all?

All the other metal bits in the water are fine, seacocks, prop, shaft etc.

I want to put it down to a bad stud, but I can't quite convince myself. The next anode and stud will tell I suppose, but I'd appreciate any thoughts in the meantime.

 

[vyv_cox]

Do you have a close up photo of the fracture face? Post it and I will try to advise.

 

[Simondjuk]

Hi Vyv,

I'm not sure how good these will be as I've taken and posted them from my phone, hence not seen them on a big screen.

http://img.photobucket.com/albums/v249/syllogismcheck/IMAG0114.jpg

http://img.photobucket.com/albums/v249/syllogismcheck/IMAG0110.jpg

I seem to remember the stud having a more defined pit on the fracture face than appears to be there now. I'm either not remembering clearly or surface corrosion since I removed the stud has hidden it.

If it's a clue at all, the fracture face was blackened when first discovered. The discoloration transferring to ones finger easily when rubbed.

Thanks for your input.

 

[tinkicker0]

At first glance it looks like a tensile break to me.

 

[vyv_cox]

Yes, I think I agree. It doesn't look like a typical corrosion failure, too smooth/level a surface and the wasting of the final threads suggests a tensile fracture. It suggests you were a little too heavy with the spanner when tightening the nut. The bolts used for anode fixings are dead mild steel, having little strength.

 

[nathanlee]

We launched at the start of September with a new anode on a new anode stud. In mid October the anode was almost new looking with just a little fizzing to the surface. A couple of weeks ago I found it missing. More worryingly, not just the anode was missing, but the outer portion of the stud also.

http://www.bbc.co.uk/news/uk-england-shropshire-17706974

 

[Appledore]

Can't help you at all with your initial question, but the bolts themselves seem very expensive at about £20 each (with P & P).

I replaced my hull anode last spring, and used 2 x 10 mm zinc plated bolts, about 75 mm long. ( I can source these from where I work for pennies!!). I placed a nut the full length of the thread, (thinking this might stay in place if the head disintegrated) followed by a washer, threaded the bolt through the hull and pad, and double nutted the inside. As my boat is ashore every winter I thought that I could easily just replace the bolts annually if required.

When the boat was lifted last autumn, there was almost no rust or corrosion on the bolts externally, so I've left them alone. ( I did put a dollop of Hammerite on the heads when I was cleaning my brush after using the paint elsewhere! Not sure if it will stay on though!). And, yes, lots of Sikaflex used at the time.

 

[Simondjuk]

Thanks for the responses.

I'd be surprised if I over did the tightening as I'd considered how small the bearing surfaces of the plate outside the hull and the washer inside were prior to tightening, so went very gently in addition to my natural propensity to mechanical sympathy. When fitting the anode to the stud I just compressed the foam backing pad a little, writing the shakeproof washer off as ineffective due to the lack of bite imparted by the minimal tightening.

That said, I take the point about the studs being soft, so perhaps I did tweak a little to firmly, even with just finger and thumb on the spanner.

Perhaps a combination of slightly too firm a nip and a compromised stud one the weld line came together to cause a failure.

Nathan, It would be a pretty desperate metal thief who was prepared to wallow in knee deep mud on the off chance of finding a hidden kilo and half of zinc, but you never know these days.

Appledore, This was an M G Duff stud. About £8 for a pack if two I think. The stainless ones are even more disproportionately expensive in comparison with workaday nuts, bolts and studding.

 

[Appledore]

Thanks for the responses.


Appledore, This was an M G Duff stud. About £8 for a pack if two I think. The stainless ones are even more disproportionately expensive in comparison with workaday nuts, bolts and studding.

You were very fortunate I think, LOL! As I said, when I was looking for studs (they are, I think, about 100mm long), even on eBay they were £16 each, and postage on top. I could find nothing cheaper!

Geoff

 

[alahol2]

The shear is on the hull side of the welded plate so it is more likely you marginally overtightened the nut on the inside of the hull rather than the nut holding the anode on. I have re-used my stud for the last several years and it often takes quite a force to remove the old nuts holding the anode on. If mine were going to fail I'd expect them to go during removal. Next year I will remove the whole stud and check the bit inside the hull thickness.

 

[ghostlymoron]

Thanks for the responses.

I'd be surprised if I over did the tightening as I'd considered how small the bearing surfaces of the plate outside the hull and the washer inside were prior to tightening, so went very gently in addition to my natural propensity to mechanical sympathy. When fitting the anode to the stud I just compressed the foam backing pad a little, writing the shakeproof washer off as ineffective due to the lack of bite imparted by the minimal tightening.

That said, I take the point about the studs being soft, so perhaps I did tweak a little to firmly, even with just finger and thumb on the spanner.

Perhaps a combination of slightly too firm a nip and a compromised stud one the weld line came together to cause a failure.

Nathan, It would be a pretty desperate metal thief who was prepared to wallow in knee deep mud on the off chance of finding a hidden kilo and half of zinc, but you never know these days.

Appledore, This was an M G Duff stud. About £8 for a pack if two I think. The stainless ones are even more disproportionately expensive in comparison with workaday nuts, bolts and studding.

I'm amazed that it would be possible to overtighten a MS bolt to the point that it would fail using a normal spanner. There is obviously necking towards the end of the bolt though which seems to suggest its been stretched before breaking. As its holding on a zinc anode (or was) I wonder if there's some electro corrosion mechanism at play.