Your experiences: Galvanic corrosion, electrolysis and anodes.

[Jack Haines]

Hello all,

In the May issue of MBY we are doing a feature on galvanic corrosion, electrolysis and anodes.

Would anyone on here like to provide us with a case study who has had an interesting experience with galvanic or electrolytic corrosion? It can be positive, or negative, you can tell us about any special techniques you have for keeping corrosion at bay or how to look after your anodes.

Even if you don't have a story to tell but have some high resolution pictures of munched up anodes or stern gear then we'd love to use them.

Please send any pictures to jack_haines@ipcmedia.com and email me with any stories to the same address. Alternatively, PM me or, best of all, put your experience on here for all to see!

Thank you very much in advance for any help.

Cheers,

Jack

 

[Firefly625]

Jack

most people on here have already seen this story, but I bought my Hardy Mariner a couple of years ago and the survey picked up corrosion on the prop due to Galvanic issues, As I was not present for the survey I didn't see the prop until the boat was lifted at the end of the season, and this is what it looked like;

after removing and cleaning up;

Now I found the prop shaft brushes were not resting on the prop shaft, also the spring that squeezes the prop shaft brushes together onto the shaft was corroded and had no spring. So I replaced that, and checked the connection between the prop and the anode and all was well. Now the prop was really rubbish, but since I had a couple of months off season time I worked on the prop in my workshop and made it look half decent and 12 months later that is still what is on the boat...

I would add that the boat had a survey 8 months earlier by a very well respected IOW based surveyor and nothing was picked up therefore this corrosion all happened in 8 months or less....

 

[Jack Haines]

Thank you

Hi Firefly625,

Thanks for the reply, apologies if this has been covered on here before, it must have been a thread on here that slipped past me, I'll see if I can search for it.

Those photos are excellent, do you think you could email them to the address in my original post please?

Many thanks,

Jack

 

[Firefly625]

Hi Firefly625,

Thanks for the reply, apologies if this has been covered on here before, it must have been a thread on here that slipped past me, I'll see if I can search for it.

Those photos are excellent, do you think you could email them to the address in my original post please?

Many thanks,

Jack

Jack,

No worries, just didn't want to bore others! I am away a couple of days, but will send you these pictures on Sunday or Monday.

James

 

[David2452]

My berthing neighbour and I both had problems with anodes being eaten rather rapidly, very similar boats and usage, the same anode size and configuration, launched on the same day. We are conducting a sort of control experiment this year; I have fitted a galvanic isolator and he still has his in a locker. We both launched at the same time, have cruised in company and moor 10 feet apart, we started out with the same anodes fitted to the same components so when we lift for a couple of weeks quite soon it will be interesting to see what if any difference there is.

 

[Jack Haines]

That's great

Hi Firefly625,

Thanks for the reply, apologies if this has been covered on here before, it must have been a thread on here that slipped past me, I'll see if I can search for it.

Those photos are excellent, do you think you could email them to the address in my original post please?

Many thanks,

Jack

Hi James,

That sounds great, thank you very much.

Jack

 

[Jack Haines]

Interesting...

My berthing neighbour and I both had problems with anodes being eaten rather rapidly, very similar boats and usage, the same anode size and configuration, launched on the same day. We are conducting a sort of control experiment this year; I have fitted a galvanic isolator and he still has his in a locker. We both launched at the same time, have cruised in company and moor 10 feet apart, we started out with the same anodes fitted to the same components so when we lift for a couple of weeks quite soon it will be interesting to see what if any difference there is.

Hi David,

Thank you for the post, we'd be keen to hear how you get on and interested in what the lift out reveals.

All the best,

Jack

 

[David2452]

OK, if it's significant I'll post up some shots.

 

[coreng]

Problems seen above looks more electrolytic corrosion than galvanic. Have the electric potential been checked and at what values ?

Checking electric potentials of hull, shaft, sterndrive, etc. gives information on anodes activity (over- or under-protection) and also helps identifying stray currents from board or shore power.

It is actually the state of the art to do so. And I do add you can make any electrical improvments to your installation, if you don't measure potentials you cannot know if the cure was useful. Why guess ?

For your article : anodes utility is such misunderstood I saw some just bolted on GRP boats with no wiring at all !!!

 

[coreng]

Over or under protection:

Diagnose cathodic protection problems:

Here is over-protection (due to hydrogen emission at cathode -the hull) causing paint blistering:

Here is under-protection (electrolytic corrosion due to stray currents) causing loss of metal, steel or aluminium:

As you can see, these pictures are fairly characteristics.

Since cathodic protection regulation is essential for metal hulls, here is an anodes calculator for metal boats on any waters (hope not going over forum rules, otherwise I will delete link):

www.galvatest.com/quickcalc.html

 

[smee]

A few years ago I had a Moonen steel motorsailer, I had it for a number of years and had it lifted and changed the anodes every two years, which was what a friend with another steel boat in the same marina had done (indeed the longest he went was 4 years without problem). This worked fine until the last time when I stretched it out to 2 and half years and didn't think anything of the wreck that had moored next to me for the duration of that time.
What I found when it came out was the hull & keel full of holes about half a cm max, predominantly down the side where the wreck had been alongside and frankly I am baffled why it didn't sink. The surveyor said it was like a seive. It was a costly lesson, which proved that just because a boat managed to survive perfectly for a year or two for many years running, if someone else comes and berths next to you, it can change the whole situation.
More recently I have been working on a boat with leaking P bracket, my first fear naturally after the above episode was electrolysis, and since it's been lifted it's proven correct, 4 of the bolts were pitted and in a bad way, although everything else is fine.

 

[coreng]

Thats why...

... hull or equipments potential should be monitored every 3 months and on suspicion. It is the commonly used practice in naval industry and it is totally ignored in the pleasure one (???). Furthermore, it is the best practice recommended by NPL (UK National Physical Laboratory) and ABYC (American Boat and Yacht Council). It would save every boaters huge amounts of money going in hull repairs, shaft and props changing, etc.

Questions please to improve understanding :
a) were you connected to shore power without protecting device?
b) was the other boat connected?
c) were you beetween 2 metal boats (incl. the other one)?

Thanks,

 

[jollyroger47]

Electrically isolated sacrificial anodes

I have a question rather than a case study, but it might add to the debate.

I have a 2006 Merry Fisher 805 that is fitted with a steel “prop protector”, which would appear to have been treated with a light zinc plating. This has progressively broken down towards the forward end, where there was surface rust and some pitting. This I have now treated with a proprietary rust converter and will overcoat with an anti-corrosive paint ( Hammerite No.1 Rust Beater or similar) before anti-fouling.

The boat is currently fitted with a transom anode, prop. anode and shaft anode, which are electrically bonded. These were partly depleted, but there was no signs of corrosion around the stern gear. It would be relatively simple to fit zinc anodes to the steel brackets that secure the prop protector to the GRP hull, which is where the worst corrosion is, i.e. the area most distant from the existing anodes, but not practical to bond electrically to the existing system. Do you think fitting extra anodes will help alleviate the problem, or could I cause more problems by not bonding them to the existing system?

 

[DAKA]

posted this for interest but photo belongs to flowerpower from this thread

 

[crazy4557]

posted this for interest but photo belongs to flowerpower from this thread

Blimey, and I thought I had problems with a few slight corrosion marks on my props!!

 

[coreng]

JollyRoger...

As far as I know, there is no use bonding systems that are protected with
their own anodes, just because bonding is intended to create an equipotential between all bonded items and then, protect them with a single anode.

Mind underwater paints must resist current flowing in the limit of
protective potential difference values (-1050 to -850 mV in the case of steel), which is not the case of surface paints.

Yes you can (should) put a zinc anode on the bracket (but not too big !) and
without bonding it. This will ensure the anode protection is dedicated to
the bracket and not another equipment.

 

[Ferry bank]

The importance of anodes!!

Heres what happens to your props and shafts if you dont have anodes
we bought a boat recently knowing it had been neglected for 5 years
This is what we found when the props and shafts were removed at a cost of 12,000!!
View attachment 10147

View attachment 10148

View attachment 10149

 

[Firefly625]

posted this for interest but photo belongs to flowerpower from this thread

I wonder if the chap noticed any drop in performance....?

 

[Ferry bank]

impotance of anodes

I dont know as he hardly ever used the boat, but we are looking forward to her performance now though!!

 

[NoviceRod]

Not as bad as some of those above, but here are some shots of my Merry Fisher 705:

New (no antifoul present) June 2010, then (with antifoul) after 9 months in Torquay.

Note prop anode has completely corroded, but not too much evidence of prop corrosion, so I guess it did its job. I was assuming it might last a year, but perhaps I was optimistic.

Significant but not total corrosion of transom anode