Anode bolts on corvett 32

[jamie langstone]

A friend of mine lifted the boat today and one bolt that holds the anode has dissolved/sheared off and the other bolt has nearly dissolved away .
Any ideas why the bolts have dissolved

 

[BruceK]

Zinc formed a white crust. Visually looked like there was meat on the bone but actually inert. Is the water brackish perchance? (notwithstanding I doubt SS bolts were used, probably mild steel)

 

[jamie langstone]

Its in sea water but on a drying mud pontoon berth if that makes much differance. My boat is next to his but i have not suffered the same.

 

[Portofino]

Is the hull steel ? Looks it .
A whole new minefield re corrosion prevention , the bolts have acted as secondary anodes to the hull .
They have gone albeit at a reduced rate to the zincs .So theres not enough protection.

 

[jamie langstone]

its Grp

 

[Plum]

A friend of mine lifted the boat today and one bolt that holds the anode has dissolved/sheared off and the other bolt has nearly dissolved away .
Any ideas why the bolts have dissolved
View attachment 97242

I had the same problem this year during lockdown. Nowhere near as bad as your friend's but my bolts had started to waist and they were only one year old. I usually get 5 to 8 years out of the zinc plated steel bolts. The difference this year was that I was unable to renew the anode and virtually all the zinc of the anode had gone just leaving the steel bar joining the two bolts and so there was no zinc to protect the bolts so they corroded fast. To maintain protection of my prop and shaft I rigged up a hanging anode during lockdown.

www.solocoastalsailing.co.uk

 

[superheat6k]

I would agree this is due to the anode having wasted then the mild steel bolt rapidly corrodes (it does not corrode all the time the anode is effective). My bolts were 10mm, but this Winter past I increased to 12mm as I am using heavy (4kg) MG Duff anodes and felt the 10mm were simply not up to the job.

Might also be worth checking there are no stray + DC (check flappy type bilge pump switches as they are really bad for causing this) or worse Mains AC voltages hopping on to the bonding circuit. Any positive DC voltage or especially an AC voltage wil see a mild steel bolt disappear in short order once the zinc is wasted.