Anode bolt failure.

[onesea]

GRP Boat
Engine bonded to prop shaft and anode.

6 month old anode bolts failed.
Aluminium anode and bolts new 6 months previously (MG Duff anode and bolts I believe).

Boat is not normally in marina, on a mooring in river brackish water.

All electrics isolated on leaving boat, except solar charger (Victron).

Why?

Is there anyway I can measure for stay currents?

 

[VicS]

If you could post a good clear closeup photo of the failed surfaces one of the resident metallurgists may be able to suggest a reason for the failure. I'd doubt if it was due to corrosion.

However
Out on a mooring any "stray current" will come from your own electrical system.

Turn the battery isolators to OFF
Cover the solar panels and disconnect the battery connections from the regulator. Make sure the wiring does not short itself.
Now look for any small current flowing from the two battery banks ( I'd do in on the negative connections but no real reason not to do it on the positives.)

Start with you meter on its highest DC amps range, just in case there's an unexpectedly big problem, and work down to the lowest range..

If you are wealthy enough to have an auto-ranging multimeter pay someone else to check it out.

 

[Plum]

GRP Boat
Engine bonded to prop shaft and anode.

6 month old anode bolts failed.
Aluminium anode and bolts new 6 months previously (MG Duff anode and bolts I believe).

Boat is not normally in marina, on a mooring in river brackish water.

All electrics isolated on leaving boat, except solar charger (Victron).

Why?

Is there anyway I can measure for stay currents?

Coincidence? I had both anode bolts fail flush with the hull last year. They both failed at about the same time, had one failed earlier it would have shown more rust. See photo of failures taken on the day I pulled the studs out after drying out. Anode went to the botton. Only some mastic was holding the stubs in place stopping the boat from sinking. No solar, no mains. The studs were the type with a thick circular washer/plate not the square type, that is fixed in place on the stud with no visible weld (friction welded?). The failure was right next to that washer.

 

[onesea]

@VicS thanks for response.
Sadly my phone doesn't have macro mode, I think these are the best I will get (same stud both photos).
Both appeared to have same failure move. There was no surface rust immediately when they where first seen. That appeared within hours after.

Regards stray currents, everything can be switched off onboard, including solar input.

Otherwise it's difficult to know how much power something is drawing and watch battery for stray currents / monitor battery useage.

The Victron and VCR are fused and are direct connection to battery no switches.

I have NASA BM1 battery monitor, that switches on with domestics breaker.

The BM battery monitor with everything turned off reads 0.1 amps.

The 0.1 I take to be the VCR & BM battery monitor and Victron.

I will put my cheap meter between battery terminal and positive with everything turned off tomorrow.

I tried tonight but I need to disconnect the engine battery as VCR is still closed (it's been a sunny day).

Out of interest I checked current between anode bolt and earth connection 0.05 amps, by my cheap meter on a swinging mooring only other boats 20m away.

 

[onesea]

@Plum
Thank you, glad I am not the only one!

I had square plates on mine, the rust marks on hull indicate to me the anode has been in place whilst bolts where rusting.

What type of anode did you have aluminium or zinc?

I was not so lucky with the sealant holding the bolt in place. Let's just say I had a bit of a moment. Holes plugged & bildges empty, in 20 minutes.

Back sailing with new anode and bolts, 48 hours later. Not bad when 150' from home in waters I have never sailed before.

I got very lucky.

 

[Plum]

@Plum
Thank you, glad I am not the only one!

I had square plates on mine, the rust marks on hull indicate to me the anode has been in place whilst bolts where rusting.

What type of anode did you have aluminium or zinc?

I was not so lucky with the sealant holding the bolt in place. Let's just say I had a bit of a moment. Holes plugged & bildges empty, in 20 minutes.

Back sailing with new anode and bolts, 48 hours later. Not bad when 150' from home in waters I have never sailed before.

I got very lucky.

I had a Zinc pear anode. We were both lucky. I was checking engine in the marina and saw a tiny drip of water next to a stud, touched the stud and it moved and a bit more water started to leak in. Stayed on board to monitor and moved to scrubbing posts early in the morning to dry out. phew.

 

[vyv_cox]

As VicS has suggested, in neither case do these look like corrosion failures. Straight fractures normal to the axis of the bolt are most characteristic of fatigue, although it is difficult to understand how that could occur in this application. I would expect stress corrosion cracking to produce a far rougher fracture face due to the branched cracking that this mechanism produces.

 

[onesea]

@vyv_cox
Thank you, so vibration might be the culprit.

Engine is 2GM20 I believe they are known for vibration. Although it has probably only had less than 100 hours running in this period.

So how do you prevent this?

@VicS
I have now disconnected and checked current across the system.

I get zero discharge on the BM1 when everything is off AND the VCR is open.

Switching breakers and switches gives expected currents matched with multi meter and BM.

I sumise that the intermittent 0.2v discharge with everything off is current flowing into engine battery.
Which is not monitored by BM, hence current would show following from domestics to engine battery.
Please correct me if I am wrong.

Long and short I don't believe I have current leak.

 

[Lee Shaw]

Have you any idea what material the bolts are? If they are high tensile strength e.g. grade 10.9 or 12.9 carbon steel cap screws, they would be susceptible to delayed fracture due to hydrogen embrittlement cracking. There's a big lump of aluminium nearby producing hydrogen, so I don't think this failure mode can be ruled. It would be best to lower strength bolts such as grade 4.6, or 6.6.

 

[vyv_cox]

Have you any idea what material the bolts are? If they are high tensile strength e.g. grade 10.9 or 12.9 carbon steel cap screws, they would be susceptible to delayed fracture due to hydrogen embrittlement cracking. There's a big lump of aluminium nearby producing hydrogen, so I don't think this failure mode can be ruled. It would be best to lower strength bolts such as grade 4.6, or 6.6.

No, they are mild steel, lowest grade available.
I don't think your hydrogen embrittlement mechanism is correct. It's a high temperature problem.

 

[Lee Shaw]

From one metallurgist to another, please forgive my sloppiness Vyv, I should have been more precise. I should have said hydrogen induced stress corrosion cracking which can and does occur in high strength steels at ambient temperatures. Though I accept this does not include mild steel. I also accept that a high temperature hydrogen embrittlement process can occur in steels of all strength levels but this is hardly relevant here.

 

[onesea]

Have you any idea what material the bolts are? If they are high tensile strength e.g. grade 10.9 or 12.9 carbon steel cap screws, they would be susceptible to delayed fracture due to hydrogen embrittlement cracking. There's a big lump of aluminium nearby producing hydrogen, so I don't think this failure mode can be ruled. It would be best to lower strength bolts such as grade 4.6, or 6.6.

@Lee Shaw
I would imagine @vyv_cox is right regarding steel, they where as I recall bought from the local chandlery, in branded packets.

I guess I will be checking anode and bolts at frequent intervals for a whilst.

I guess next I should go for S/S.

 

[vyv_cox]

@Lee Shaw
I would imagine @vyv_cox is right regarding steel, they where as I recall bought from the local chandlery, in branded packets.

I guess I will be checking anode and bolts at frequent intervals for a whilst.

I guess next I should go for S/S.

I have always assumed that the only benefit of stainless steel bolts is that they do not corrode in the boatyard Underwater the anode has to protect its fasteners before anything else.

 

[Plum]

I have always assumed that the only benefit of stainless steel bolts is that they do not corrode in the boatyard Underwater the anode has to protect its fasteners before anything else.

Agree, there is no benefit from having SS studs, other than, during lockdown my anode wasted away to nothing so once there was no zinc left, the studs started to corrode significantly!

 

[RivalRedwing]

Two similar failures makes me wonder if there is a supplier batch problem - have you enquired with Mr Duff?

 

[onesea]

Two similar failures makes me wonder if there is a supplier batch problem - have you enquired with Mr Duff?

There answer was it's an onboard electrical problem speak to a Marine Electrician.
As @Plum had a similar problem without solar, I am not investigating that fully. that is about the only thing left on when the boat was unattended.