Galvanic Erosion

[jddevel]

During my nocturnal brain meandering and reading the forum last night regarding hull erosion and one specific comment- I think from Bizzard- it was noticed that on docking pitting was specifically notice on the outer hull in the vicinity of the internal 240volt equipment. In my ignorance on this cause and the subject I am asking the question would a anode on the hull in these locations help?

[Flyboy]

Are you connected permanently to a shore line and have you got a galvanic isolator or isolating transformer?

[WotEver]

1 hour ago, jddevel said:

During my nocturnal brain meandering and reading the forum last night regarding hull erosion and one specific comment- I think from Bizzard- it was noticed that on docking pitting was specifically notice on the outer hull in the vicinity of the internal 240volt equipment. 

I like Bizzard and respect his knowledge and experience. However on this point I think he's away with the faries. 230V appliances do not create a 'field' which would increase galvanic corrosion. They might warm the hull up slightly I guess. 

[Neil2]

I think you need to elaborate on what you mean by "240v equipment" are you referring to specific appliances or an inverter, charger, generator etc.

[mrsmelly]

33 minutes ago, WotEver said:

I like Bizzard and respect his knowledge and experience. However on this point I think he's away with the faries. 230V appliances do not create a 'field' which would increase galvanic corrosion. They might warm the hull up slightly I guess. 

I think Bizz is away with the faries all of the time....

[jddevel]

In Bizzards` defence his remarks were based on what he visually note and referred to on the 20th Sept. 2016. I`m sure he will comment more when he reads this topic. I have a 3000 inverter, washing machine and will use maybe toaster and TV. As a sailaway under fit out I`m continually researching my requirements.

[WotEver]

1 minute ago, jddevel said:

In Bizzards` defence his remarks were based on what he visually note

I know. I recall reading it. 

[Raz]

We lost our 9 year old boat to galvanic corrosion. Bought by us having recently become a 4 year old with a clean bill of health in March 2009 and blacked by our local marina every 12 months or so while in our ownership I started to notice waterline rust bubbles within 3 months of the 2013 blacking, so on the advice of the yard that undertook our regular maintenance, the next blacking was to be a grit blast followed by a two pack coating. Unfortunately, it appeared they had failed to notify us of a deteriorating hull over the 5 years in their annual care, and when we returned to collect her from their marina in February 2014 we found her sunk - on the bottom, (not their staff)! Three specialist surveyors inspected her during the aftermath, our own, one sent by our insurer, and another. The whole hull was severely pitted, with at least two having gone straight through the side at the stern, apparently unnoticed or ignored by those who had blacked her. The 4 anodes were still in fairly good condition. The worst pitting, including the ones with full penetration were on the side adjacent to the canal bank of our online mooring.

The opinion of the insurance surveyor was that the galvanic corrosion was due to our boat being left connected to shore power on a more or less a continuous basis between cruising. Another went further. He had been conducting research into galvanic corrosion, and it was his opinion that the cause was due to being connected to shore power, moored against galvanised steel piling and in flowing water (Llangollen Canal). Our own surveyor reported to us that another factor could have been poor blacking during the time we owned her - but I was paying a marina to do this for us, so was not aware of this possibility, until I saw her for myself out of the water at the end. All our electrics were checked, including our galvanic isolator, and were found to be functioning without issue.

Our boat was a 57 foot semi-trad, first used in September 2004 by her first owner and built by Dragon narrowboats. She sank due to galvanic hull corrosion in February 2014 in our ownership - her 2nd owners. The detailed story of how we lost her rather than having her repaired has been well discussed already on here and is another story. I hope our experience helps answer your question. Galvanic corrosion remains a very mysterious phenomenon.

[Raz]

Just now, ditchcrawler said:

Although I was a sparky in my working days I never had any formal learning on this subject so I would like to put forward the following question. What if the boat was chained to the pontoon, wouldn't this effectively bypass the GI

The causes of the initial problem were well reported by surveyors at the time, due to the fact my insurance company were attempting to wriggle out of accepting liability. Our mooring was inspected, and it was reported that as it was on a slight curve, even though I had fenders, it was still possible for the steel hull to be in permanent contact with the steel piling in the middle, if tied too tightly. Chain wasn't used by me, just rope, but the fact there was possible contact was mentioned. I am no expert, and had to rely on those who were involved in reporting on the causes of what had happened. In the end - 12 months later, my own insurer paid out due to the negligence of the marina who allowed her to sink after routine maintenance, a reason which was included in my schedule, whereas the hull corrosion could have gone down as lack of maintenance, which wasn't in my insurance schedule. The marina owner refused to provide his own insurer's details until I had accepted "full and final settlement" from my own insurer, which was by then too late to claim from his to obtain a full and detailed repair to the sinking damage - the reason I sold for salvage rather than repair.

[Captain Pegg]

Just now, Machpoint005 said:

Mods - any chance of correcting the original title -- the term is galvanic corrosion: the hull doesn't get 'eroded' because that would describe a purely chemical (as opposed to electrochemical) process. The hull isn't washed away, it reacts, molecule by molecule, with the electrolyte. It's no wonder the subject is so mysterious if the terms are inappropriately used.

All of the responding posters have correctly used the term corrosion.  

Yes it is correctly corrosion but so is a purely chemical process such as the ambient oxidation of steel to form rust. Erosion is a mechanical process.

JP

This appears to have posted ahead of the post it is in response to. Have I just invented time travel?

Edited April 13, 2017 by Captain Pegg

[Machpoint005]

Mods - any chance of correcting the original title -- the term is galvanic corrosion: the hull doesn't get 'eroded' because that would describe a purely chemical (as opposed to electrochemical) process. The hull isn't washed away, it reacts, molecule by molecule, with the electrolyte. It's no wonder the subject is so mysterious if the terms are inappropriately used.

All of the responding posters have correctly used the term corrosion.  

[WotEver]

Just now, Raz said:

The opinion of the insurance surveyor was that the galvanic corrosion was due to our boat being left connected to shore power on a more or less a continuous basis between cruising.

 

Just now, Raz said:

it was his opinion that the cause was due to being connected to shore power...

 

Just now, Raz said:

All our electrics were checked, including our galvanic isolator...

Firstly, let me say how sorry I am that you lost your boat.

It's important to understand that the above describes 'Stray Current Corrosion' rather than Galvanic Corrosion. Secondly, assuming that the GI tested satisfactorily then either it was being swamped for some reason, or the first two conclusions above are wrong. You can't have a functioning GI and then suffer corrosion 'because the boat is connected to shore power' quite simply because the whole purpose of the GI is to effectively disconnect the boat's hull from the shore power earth. UNLESS the GI was being 'swamped' by an excessive amount of leakage. 

Perhaps another argument in favour of an Isolation Transformer?

 

Just now, Raz said:

moored against galvanised steel piling

With Zinc being less noble than steel then Galvanic Corrosion would work in the boat's favour here with the piling coating being deposited onto the hull. 

[WotEver]

6 hours ago, ditchcrawler said:

Although I was a sparky in my working days I never had any formal learning on this subject so I would like to put forward the following question. What if the boat was chained to the pontoon, wouldn't this effectively bypass the GI

If it was chained with conductive links (most likely) then there would be no PD between the boat and the pontoon and therefore no Galvanic Corrosion possible. It's unlikely that a chain would make a 100% reliable connection though so some Galvanic Corrosion would still be possible (but as I said above it would be in the boat's favour). 

Stray Current Corrosion Is caused when a DC voltage might be present on the incoming earth connection which then leaves your boat via the hull and water. As it leaves it takes some hull with it (in simplified terms). Chaining the hull to the pontoon would only serve to lessen any corrosion of this nature as there would be a parallel path, reducing the current leaving through the water. A Galvanic Isolator should really be called a Stray Current Isolator as it doesn't actually have anything to do with Galvanic currents. 

I believe that it is possible that an adjacent boat with no GI could have leaked stray current to earth via OP's hull. This would have thickened the hull on the side closest to the 'faulty' boat and thinned it on the side closest to the pontoon. That is purely a guess though. 

Edited April 14, 2017 by WotEver
Typos

[blackrose]

1 hour ago, Raz said:

. Galvanic corrosion remains a very mysterious phenomenon.

It's actually a very well understood phenomenon, just not by most boaters.

[ditchcrawler]

Just now, WotEver said:

 

 

Firstly, let me say how sorry I am that you lost your boat.

It's important to understand that the above describes 'Stray Current Corrosion' rather than Galvanic Corrosion. Secondly, assuming that the GI tested satisfactorily then either it was being swamped for some reason, or the first two conclusions above are wrong. You can't have a functioning GI and then suffer corrosion 'because the boat is connected to shore power' quite simply because the whole purpose of the GI is to effectively disconnect the boat's hull from the shore power earth. UNLESS the GI was being 'swamped' by an excessive amount of leakage. 

Perhaps another argument in favour of an Isolation Transformer?

 

Although I was a sparky in my working days I never had any formal learning on this subject so I would like to put forward the following question. What if the boat was chained to the pontoon, wouldn't this effectively bypass the GI

[Raz]

Looking at the interesting posts on this thread I would now agree that our own particular issue was down to stray current corrosion rather than galvanic corrosion. As I said - I am no expert, but I suspect the specialists including those who inspected our boat on behalf of our insurance company reported our damage to the hull under the banner of galvanic corrosion as a general term. My own reading on the subject tells me that stray DC corrosion can be far worse than galvanic corrosion, and in my case our boat's hull was obviously desolving in the canal water at a very rapid rate over the 5 years we owned it. When it was brought ashore after the sinking, which was the day after being relaunched by the marina that did the "blacking" I could see for myself what state the hull was in - completely peppered with deep pitting holes. Perhaps the moral of this maintenance story might be don't rely on others that are doing work on your behalf to keep you informed of what is going on, or do the blacking yourself so any potential issues will be seen and attended to before you get to the stage ours did and became a write off due to the cost of putting things right and I was completely unaware of what was happening.

Just to finish off, our insurer Craftinsure passed our claim to their underwriters, Navigators and General. They declared the hull damage to be wear and tear and would not be covered by insurance, which I agreed with. After investigation and a report from their own surveyor they agreed to rectify the damage as a result of sinking by the marina. However, as my Craftinsure policy would not replace New for old, things like the submerged alternators and starter motor would only be funded to be rebuilt not replaced. The alternators actually worked once dry, so were not touched. This went against the contractor's advice who had been awarded the work as well as my own surveyor's advice. In fact they both suggested "proper" repairs to all damage as a result of sinking should come in around £20k, but Craftinsure would only agree to a repair estimate at £5k and wouldn't even go as far as replacing water stained oak interior panels for example because they remained serviceable.

As I said above, the marina owner was asked for his insurance details but we never got a reply until my claim with my own insurer was finalised, even with solicitors asking. So our choice was get a boat back with what in our opinion would be inferior internal and mechanical repairs, and still have to pay for hull repairs, or get shut. I chose to get shut. The marina then chose to charge us storage fees for the time the boat was with them as well as the cost of what in our opinion was a rubbish job, before we could remove the boat from their site - £2,500! In the end Craftinsure reclaimed their £5k outlay from the marina owner's insurance, so they refunded my £300 excess!

So, stray current corrosion or galvanic corrosion, whatever it may be called is something that is best not ignored.

[jddevel]

Firstly I must apologize for the incorrect terminology and am thankful that what I meant was actually understood. So could someone summarize in layman's terms the perfect scenario to avoid or at least greatly reduce this problem other than keeping your boat out of the water- which sort of defeats the object of having one. Thank you in anticipation.

[Raz]

46 minutes ago, jddevel said:

Firstly I must apologize for the incorrect terminology and am thankful that what I meant was actually understood. So could someone summarize in layman's terms the perfect scenario to avoid or at least greatly reduce this problem other than keeping your boat out of the water- which sort of defeats the object of having one. Thank you in anticipation.

I can quote from the information my insurance provided surveyor passed on to me.

1) His own opinion was that steel narrowboats should not be connected to shore power for continuous periods of time.

2) If they were to be connected at all, then he recommended the use of an isolation transformer rather than the more widely used Galvanic isolator diode.

3) He attached great importance to maintaining a good quality, undamaged underwater blacking, which again in his opinion reduces the effect of natural corrosion of any type.

4) As well as maintaining good blacking, anodes should be regularly inspected, replaced when starting to dissolve, (together with an investigation as to the reason why if this happened too quickly), as well as ensuring anodes left in place after blacking have not been painted over, which would reduce or impede their efficiency at preventing corrosion. Magnesium anodes for fresh water use, zinc ones for salt water or brackish use. New hull anodes attached next to worn ones, not in place of.

5) Generally keep an eye on the waterline for fizzy rust bubbles. As I said earlier, I had noticed this and advised the marina who had carried out previous blacking. Unfortunately they neither told me of the revealed damage after grit blasting, nor asked if I would like it investigating. They just painted over perhaps with the intention of returning my boat as quickly as possible to collect their fee. Unfortunately it sank immediately, launched at close of work on a Friday afternoon, found like that by me on Saturday morning.

As can be seen from above my opinion is that it appears that galvanic corrosion or stray current corrosion can't be prevented as it is a natural occurrence. What can be done though is act when the symptoms are there, and do your best with maintenance to minimise or prevent it taking hold in a severe way possibly due to an electrical fault either on your own boat, on a neighbouring boat or in the shore power supply. As I keep saying - I am no expert, and the above has been taken from those who were involved in my own particular experience of this.

i have attached images here of the condition of my hull AFTER the so called blacking job by the marina when back on shore. Ignore the bent rudder Skegness - they also damaged that when dragging out again! The holes that sank her are chalked by my surveyor.

[WotEver]

Number 1 is just silly. Thousands of boats in marinas around the country are permanently connected to the mains without dissolving. 

I thoroughly agree with all of your other points though. 

[Raz]

3 minutes ago, WotEver said:

Number 1 is just silly. Thousands of boats in marinas around the country are permanently connected to the mains without dissolving. 

I thoroughly agree with all of your other points though. 

I completely agree. I too kept on insisting there are thousands of narrowboats connected to shore power. I did wonder if he was speaking in his "insurance surveyor" capacity. However he would not have it. He kept on insisting that steel narrowboats should not be generally connected to shore power, and alternative methods such as wind generator or solar should be used instead. For information, I do believe he is a highly respected narrowboats surveyor though - Mike Carter of Marine Surveys Ltd. Advice to me on who to appoint as my own surveyor after the event often pointed to Mike. Unfortunately Navigators and General appointed him as theirs first.

[mark99]

Did you have a GI or GT?

http://www.safeshoremarine.com/Galvanic isolators in a nutshell.htm

 

[WotEver]

5 minutes ago, mark99 said:

Did you have a GI or GT?

http://www.safeshoremarine.com/Galvanic isolators in a nutshell.htm

 

See his OP: 

 

[Raz]

Our boat was fitted with a Safeshore Marine galvanic isolator which I tested annually according to their instructions using an electronic multi-meter. It was also tested after the event by my surveyor and reported to be in working condition.

[mark99]

47 minutes ago, Raz said:

Our boat was fitted with a Safeshore Marine galvanic isolator which I tested annually according to their instructions using an electronic multi-meter. It was also tested after the event by my surveyor and reported to be in working condition.

Ok thanks. Marina anywhere a DC railway?

[cuthound]

If the voltage driving the stray currents is higher than the turn on voltage of the GI's diodes, typically around 1.4 volts, then the GI  will be switched on permanently, and effectively become a hard wired earth connection. This is where an isolating transformer is better, as it always provides a physical isolation between shore and boat. 

However, in the vast majority of cases the voltage driving the stray currents is less than this, so the GI works perfectly well. If they didn't,  the cut would be littered with sunken boats resulting from story of galvanic currents.