Galvanic corrosion?

[Cheshire cat]

I'd be interested in thoughts as to what is going on.

 

We moor in a marina and the boat is permanently hooked up to the electricity when not cruising.

I fitted a galvanic isolator and I last tested it 12 months ago. It showed almost identical voltage readings that lead me to think it is still working.

 

It's been two and a half years since we were last blacked and I could see some waterline damage so we pulled the boat out on Wednesday.  

On the port side which is usually the side moored against the pontoon the steel is nice and smooth with very little damage to the blacking. 

On the other side the situation is very different. At the time of purchase almost five years ago the surveyor noted "pitting occasionally up to 2.5 mm deep suggesting an external influence"

 

This is what caused me to fit the galvanic isolator.

 

Currently the starboard side has a rough texture to the bitumen as expected from the surveyors report. There are also quite a few small orange blisters that I have removed using the jetwash to reveal bright steel underneath. The guy who pulled the boat out tells me that this is a tell tale sign of galvanic corrosion but he has seen far worse. The anodes are in good condition. Ones added at the last blacking are barely touched and the ones from 5 years ago are only 50% consumed.

 

To add further to the mystery the side that is corroded has faced two empty berths on the next pontoon for the majority of the last two years.

I'm unsure if there's anything further I can do other than install a small solar panel and disconnect from shore power when we are away from the boat. 

 

Thought please.

[David Mack]

It's a thorny subject with no definite answers.

42 minutes ago, Cheshire cat said:

 

I'm unsure if there's anything further I can do other than install a small solar panel and disconnect from shore power when we are away from the boat. 

 

This. If you are not a live aboard, why do you need to be permanently plugged in? Solar is so cheap that you can easily keep the batteries topped up when the boat is unattended. And it works just as well when you are away from the home mooring.

[Bee]

My thoughts are that active corrosion / rust / call it what you will, is often silvery underneath, might not be anything to do with shore power, Anodes only have a limited 'range', most of the side will be out of range of any protection and if the shorepower is only used for battery 'trickle' charging I personally would disconnect the shorepower. In fact I take batteries home for winter, (I've only got three though) Others will have other views

[Alan de Enfield]

58 minutes ago, Cheshire cat said:

Currently the starboard side has a rough texture to the bitumen as expected from the surveyors report. There are also quite a few small orange blisters that I have removed using the jetwash to reveal bright steel underneath. The guy who pulled the boat out tells me that this is a tell tale sign of galvanic corrosion but he has seen far worse. The anodes are in good condition. Ones added at the last blacking are barely touched and the ones from 5 years ago are only 50% consumed.

 

There are other forms of corrosion.

 

Does it look like this ?

 

 

 

 

I have a number of documents on Microbial corrosion explaining causes and treatments but they are Pdf's so I cannot post them on the forum.

 

 

But this link may give you an overview.

 

http://handsonmarine.com/microbially-induced-corrosion/

[WotEver]

My first thoughts on reading...

1 hour ago, Cheshire cat said:

There are also quite a few small orange blisters that I have removed using the jetwash to reveal bright steel underneath.

... is that it’s MIC. See the link in the post above.
 

Rust is rusty all the way down and it’s unlikely to be galvanic corrosion if it’s only on one side (and that facing open water). 

 

However, it’s a complex subject with rarely any definitive answers. 

[peterboat]

To be on the safe side I would do the solar panel idea, when I used to be hooked up with my old  boat corrosion next to the metal pilings was an issue, however new boat no shore power no problems and that was before the shot blast and Zingering of the boat

[Alan de Enfield]

9 minutes ago, WotEver said:

.. is that it’s MIC. See the link in the post above.

One of the 'issues' with MIC is that it is not only happening where you can 'see it', but because it likes non-oxygenated areas, it lives behind the blacking and can be eating its way all along a hull without you knowing

 

A good summary but no doubt some will cast doubt on it due to the author's "interests"

 

........... If a hull is found with evidence of microbial attack, it is necessary to deal with it to try to prevent it recurring. A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left for twenty four hours. Afterwards a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only real solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process. The main problem is that the microbes can continue to live beneath the existing paint coatings and once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion from the inside out. No coatings are entirely proof against a microbial attack from the exterior. Minute pinpricks, mechanical damage below the waterline are all opportunities for the microbes to penetrate the steel and commence the process from the outside in..

 

Full document :

 

https://www.keelblack.co.uk/wp-content/uploads/2017/08/MICROBIOLOGICAL-CORROSION.pdf

 

Edited August 21, 2020 by Alan de Enfield

[WotEver]

4 minutes ago, Alan de Enfield said:

One of the 'issues' with MIC is that it is not only happening where you can 'see it', but because it likes non-oxygenated areas, it lives behind the blacking and can be eating its way all along a hull without you knowing

Yup. 
 

There are many scholarly articles about it but very few practical suggestions for mitigation. Basically the theory goes...

• Clean the area
• Use a biocide
• Protect the steel with a good coating

However, if there’s a high concentration of bacteria in the area then it’s going to come back, and only repeated treatment will keep it at bay. 
 

Another article here: https://www.corrosionclinic.com/types_of_corrosion/microbiologically_influenced_biological_microbial_corrosion.htm

 

Of course, it might not be MIC, and I’m unsure who can test it for you. 

[Alan de Enfield]

4 minutes ago, WotEver said:

Yup. 
 

There are many scholarly articles about it but very few practical suggestions for mitigation. Basically the theory goes...

• Clean the area
• Use a biocide
• Protect the steel with a good coating

However, if there’s a high concentration of bacteria in the area then it’s going to come back, and only repeated treatment will keep it at bay. 
 

Another article here: https://www.corrosionclinic.com/types_of_corrosion/microbiologically_influenced_biological_microbial_corrosion.htm

 

Of course, it might not be MIC, and I’m unsure who can test it for you. 

 

 

The last time I posted info about MIC  (2 or 3 years ago) I was accused of scaremongering and that there was no such thing 'in the canals' it only happens in muddy estuary's.

Although a member at the time did post to say his NB had had a severe attack of MIC.

 

Maybe some folks minds are now a little more open.

[WotEver]

I think I recall reading that an indicator is to sniff a bubble when you burst it. If it smells like vinegar then it’s likely MIC. 

[Alan de Enfield]

38 minutes ago, WotEver said:

I think I recall reading that an indicator is to sniff a bubble when you burst it. If it smells like vinegar then it’s likely MIC. 

That certainly true of Osmosis in GRP boats, could it be MIC as well ?

[J R ALSOP]

Nothing has made me change my mind about having a Isolating Transformer, boat is 41 years old, all hull 10mm thick with no signs of any trouble.

[WotEver]

1 hour ago, Alan de Enfield said:

That certainly true of Osmosis in GRP boats, could it be MIC as well ?

Ahh you’re right. It’s me age...

[Cheshire cat]

OK. Thanks for your input. I think I will go down the solar route asap. It is only hooked up to shore to keep the batteries in good condition. We can reconnect any time we decide to have a weekend in the marina and decide we temporarily need more power.

 

I don't think it looks like your photo Alan. I should have photographed it but it was more a case of a sprinkling of individual isolated blisters mostly less than a cm across and mostly oval in shape, and smooth on the surface. They really did look like blisters. Behind each one there is a small pit.

 

On the good side there were no blisters but there were some grey patches that seemed to be algae of some sort. They fell victim to the pressure washer leaving clean blacking behind them. 

[mark99]

Out of habit I always ensure my boat and the metal mooring berth in the marina never touch (big squashy balloon buoys).  Without a connection, the marina steel cannot act as a cathode creating an anode of my bote. Ie the circuit is not able to form.

 

I have been known to put a multimeter (on millivolts) between hull and steel to check there is no potential between the two whilst they have been touching.

Edited August 21, 2020 by mark99

[Alan de Enfield]

10 minutes ago, mark99 said:

Out of habit I always ensure my boat and the metal mooring berth in the marina never touch (big squashy balloon buoys).  Without a connection, the marina steel cannot act as a cathode creating an anode of my bote. Ie the circuit is not able to form.

 

I have been known to put a multimeter (on millivolts) between hull and steel to check there is no potential between the two whilst they have been touching.

 

If you are 'hooked up' - You only need water ;

 

 

[mark99]

Got solar. Also galvanic isolator anyhow. Electrician advice needed.... Does there need to be a fault in Alans example in neighbours boat to send current to earth to complete circuit?

 

Most marinas have a pole serving say 4 boats. If other three boats are not using shore power, then assume the common earth is not a hazard?

 

(The squashy buoy is to stop circuit between my bote and the marina steel).

Edited August 21, 2020 by mark99