Corrosion and electrolysis

[coreng]

Hello canalworld community,

I am a new user and I am specialized in pleasure boats corrosion problems. I will be glad to help - free advices - if needed. As I do not live in UK, you won't see me there except if on holidays... And as I am not english, you may find some errors in my posts... So, please be kind...

[jake_crew]

How kind of you to offer and welcome to the forum. Where are you based.

 

 

What is your opinion on the use of galvanic isolators for shore to ship mains power supplies.

 

Should the mains neutral be connected to the boat earth ?

 

Will this affect the life of the anodes ?

[coreng]

The largest cause of electrolytic corrosion (called electrolysis in boating world) is the earth (ground) connection of the shore power. It always carries erratic currents which are harmful to metal hulls. To prevent them, you must have a galvanic isolator or best, an isolation transformer. This device physically cut the earth line without affecting the working of the GFCI breakers, which are mandatory for security of each installation.

 

Stray currents affects longevity of anodes as they are fighting it, but the fight is quite unfair and stray currents will win causing quick destruction of anodes and corrosion of the metal.

 

Remember corrosion is ALWAYS the consequence of an outgoing electric current at the point of corrosion.

 

I forgot : my boat is currently at CAEN (Normandy).

Edited January 25, 2011 by coreng

[coreng]

Furthermore, to answer fully your question :

 

Abstract of ISO 13297 - Small Craft Electrical Systems AC installations

 

4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator,

the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral

shall be grounded through the shore-power cable and shall not be grounded on board the craft.

 

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray

galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to

withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s.

symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After

three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be

unchanged.

 

Nota : Prefer an isolation transformer to a galvanic isolator and wire according to instructions of the maker.

[Sunset Rising]

Did he pass the test ?

[Tim Doran]

I think so, but then again I know diddly squat about galvanic isolation

 

Welcome to the forum

[Gibbo]

Did he pass the test ?

 

Absolutely yes.

[Doorman]

Absolutely yes.

 

Hi Gibbo,

 

following your sound advice on this subject back in 2008, I specified an isolator transformer to be installed on our new build narrow boat. After cruising through thick ice (reluctantly) last winter and in consequence removing some of the blacking, when we had the boat re-blacked some six months later, we discovered evidence of galvanic corrosion/erosion

on the hull.

 

Whilst I accept that even an isolator transformer cannot afford 100% protection against this phenomena, I was a little disappointed with the effects of such a short term exposure of the hull to the canal water.

 

What are your thoughts on this situation...?

 

Mike

[Gibbo]

Hi Gibbo,

 

following your sound advice on this subject back in 2008, I specified an isolator transformer to be installed on our new build narrow boat. After cruising through thick ice (reluctantly) last winter and in consequence removing some of the blacking, when we had the boat re-blacked some six months later, we discovered evidence of galvanic corrosion/erosion

on the hull.

 

Whilst I accept that even an isolator transformer cannot afford 100% protection against this phenomena, I was a little disappointed with the effects of such a short term exposure of the hull to the canal water.

 

What are your thoughts on this situation...?

 

Mike

 

Couple of points.

 

1. How do you know it's galvanic corrosion? What are you using as an indicator that it's that as opposed to plain old rust?

 

2. Have you checked the installation? I know lots of "boat electrical engineers" who don't know how to wire an isolation transformer up and still get the incoming shorepower earth connected to the hull.

[Tony Brooks]

Hi Gibbo,

 

following your sound advice on this subject back in 2008, I specified an isolator transformer to be installed on our new build narrow boat. After cruising through thick ice (reluctantly) last winter and in consequence removing some of the blacking, when we had the boat re-blacked some six months later, we discovered evidence of galvanic corrosion/erosion

on the hull.

 

Whilst I accept that even an isolator transformer cannot afford 100% protection against this phenomena, I was a little disappointed with the effects of such a short term exposure of the hull to the canal water.

 

What are your thoughts on this situation...?

 

Mike

 

 

My immediate thought is that did Julian or someone not discover one of the major makes of ITs manage to cock up the earthing on the unit so it did absolutely nothing to combat corrosion?

 

Perhaps we can have the makers/importers name.

 

Also are you sure it has been correctly fitted with the hull bond on the boat side of the transformer and not on the shoreline side. Is the case earthed to the shoreline earth and it has been screwed to the metalwork?

[coreng]

May I suggest an isolation transformer is designed to deal with stray currents issued by the shore power and that is all. If coating has been removed, natural corrosion does not need stray currents to occur and the isolation transformer, in this case, is no use... Nothing abnormal thus...

[Speedwheel]

And as I am not english, you may find some errors in my posts... So, please be kind...

 

Well I wish I could speak a foreign language as well as you speak English (or even half as good!).

[tosher]

Couple of points.

 

1. How do you know it's galvanic corrosion? What are you using as an indicator that it's that as opposed to plain old rust?

 

2. Have you checked the installation? I know lots of "boat electrical engineers" who don't know how to wire an isolation transformer up and still get the incoming shorepower earth connected to the hull.

 

 

 

I had a similiar experience but with a GI fitted. What I took for galvanic corrosion were small areas about the size of a pea, rusty looking round the edges and almost like shiny solder in the centre, a silvery colour. Is that what galvanic corrotion looks like ?? Does anyone have any pictures of the real thing? tosher.

[RLWP]

I had a similiar experience but with a GI fitted. What I took for galvanic corrosion were small areas about the size of a pea, rusty looking round the edges and almost like shiny solder in the centre, a silvery colour. Is that what galvanic corrotion looks like ?? Does anyone have any pictures of the real thing? tosher.

 

Now, those ones are interesting. I was very sceptical when someone posted last year, but it seems they could well be microbial action

 

Richard

[coreng]

I had a similiar experience but with a GI fitted. What I took for galvanic corrosion were small areas about the size of a pea, rusty looking round the edges and almost like shiny solder in the centre, a silvery colour. Is that what galvanic corrotion looks like ?? Does anyone have any pictures of the real thing? tosher.

2 questions :

1) did you notice any regular bubbles of different sizes in the coating around (1 to 4 cm diameter)?

2) were those "peas" in little groups, mostly around anode's areas?

Edited January 26, 2011 by coreng

[david and julie]

I have a question on anodes, during the recent bad weather I noticed my anodes were covered in ice, which also contained lots of frozen bubbles.

 

I haven't been on a shoreline for very long so I'm uncertain whether this is normal or not.

 

I have an Airlink Isolation Transformer, which is fitted exactly as per the makers instructions and everything seems to be working OK.

[Gibbo]

I had a similiar experience but with a GI fitted. What I took for galvanic corrosion were small areas about the size of a pea, rusty looking round the edges and almost like shiny solder in the centre, a silvery colour. Is that what galvanic corrotion looks like ?? Does anyone have any pictures of the real thing? tosher.

 

They are not easy to identify, that's why I posed the question.

 

Rampant galvanic corrosion causes the steel to erode so fast that it actually doesn't have time to "rust over". This results in very shiny patches of steel.

 

If the anodes are doing their job, they will plate any bare spots of steel with zinc/magnesium. This too results in very shiny pathces of steel.

 

To the untrained eye they are very difficult to tell apart yet one is an indication that all is fine and the other is an indication that there is a serious problem.

[coreng]

No guess!

It's quite easy to know if your anodes works properly and if you suffer from stray currents AC or DC. Just measure regularly the electric potential of your hull. This is the method applied in the naval industry where ships are automatically monitored in "real time". It manually works perfectly well with pleasure boats. You just need a reference electrode Ag/AgCl and a simple voltmeter (and the referential of course).

[tosher]

2 questions :

1) did you notice any regular bubbles of different sizes in the coating around (1 to 4 cm diameter)?

 

2) were those "peas" in little groups, mostly around anode's areas?

 

 

 

 

1 There were just a few bubbles that seemed to be full of water when I scraped them off but the metal under these was not shiny.

 

2 No - they were spaced randomly along the hull and on the base plate, not around the anodes.

[coreng]

[/color]

 

 

 

 

1 There were just a few bubbles that seemed to be full of water when I scraped them off but the metal under these was not shiny.

 

2 No - they were spaced randomly along the hull and on the base plate, not around the anodes.

Great. This speaks to me. Love playing Dr House (but, like he, I might need some investigations...).

1) So full of liquid and not shiny... so, rusted? Bubbles like picture?

2) Anodes aluminium? magnesium (guess)? zinc? and in pretty good shape when you noticed?

Edited January 27, 2011 by coreng

[Doorman]

Couple of points.

 

1. How do you know it's galvanic corrosion? What are you using as an indicator that it's that as opposed to plain old rust?

 

2. Have you checked the installation? I know lots of "boat electrical engineers" who don't know how to wire an isolation transformer up and still get the incoming shorepower earth connected to the hull.

 

Hi,

 

In answer to your queries, the boat is only 12 months old and the hull sections where the rust was evident prior to grit blasting, had only been exposed to water/atmosphere for six months. If you look closely at the areas post grit blasting, the 'cratering' doesn't resemble

normal rust pitting.

 

Evidence of ice damage to the blacking on the hull.

 

 

Post grit blasting and evidence of some form of erosion/corrosion.

 

 

 

 

I too, considered whether the boatbuilder's electrician had wired the unit up correctly, specially as when I specified an isolator transformer during the build process, neither builder, or, electrician, had ever heard of such an item . Having checked it since I can confirm that it is wired per installation manual and the hull earth is connected to the outgoing neutral. I also wondered whether the 'boat out of water' link had been connected in error, but that was found to be ok! It's a Victron 3600W.

 

Mike

Edited January 27, 2011 by Doorman

[Doorman]

My immediate thought is that did Julian or someone not discover one of the major makes of ITs manage to cock up the earthing on the unit so it did absolutely nothing to combat corrosion?

 

Perhaps we can have the makers/importers name.

 

Also are you sure it has been correctly fitted with the hull bond on the boat side of the transformer and not on the shoreline side. Is the case earthed to the shoreline earth and it has been screwed to the metalwork?

 

Hi Tony,

 

That's a good point regarding whether the case has been inadvertently attached to the wooden bulkhead with say, over sized screws that extend through and possibly touch the metalwork of the hull! I'll check that out too.

 

Manufacturer: Victron 3600W

 

Mike

Edited January 27, 2011 by Doorman

[Tony Brooks]

Hi Tony,

 

That's a good point regarding whether the case has been inadvertently attached to the wooden bulkhead with say, over sized screws that extend through and possibly touch the metalwork of the hull! I'll check that out too.

 

Manufacturer: Victron 3600W

 

Mike

 

 

No names so no pack drill, but in view of that reply I would search the archive to see if you can get more on the wrong earthing of a certain make of IT. Although I think I read it on here it could have been on uk.rec.waterways.

 

I have just done a search and can not find a reference but I am sure I read a few years ago that one of the majors had cocked up.

[Gibbo]

No names so no pack drill, but in view of that reply I would search the archive to see if you can get more on the wrong earthing of a certain make of IT. Although I think I read it on here it could have been on uk.rec.waterways.

 

I have just done a search and can not find a reference but I am sure I read a few years ago that one of the majors had cocked up.

 

Right, here's the situation with the Victron IT

 

For galvanic corrosion protection it works. It does what it's supposed to do.

 

There is a link in there which is supposed to be fitted when the boat is using shorepower but has been craned/ramped/whatever out of the water and is on dry land. If that link is left in place when in water then the galvanic protection is no longer there.

 

The problem is that the case and interwinding screen should be connected to the shorepower earth. This is so that, if the transformer insulation fails, it fails to the shorepower earth and trips the circuit breaker or fuse. This is how it should be.

 

In the Victron the interwinding screen and casing is connected to the boat side earth so if the insulation fails it makes the boat hull live instead of blowing the fuse.

 

Admittedly, transformer failures of this type are incredibly rare (they are one of the most reliable components around) but the possibility still exists.

 

As far as I know Victron are the only people who do it this way and I have never seen any other diagram showing that as being the way to do it. Not a single one.

 

Absolutely everyone says it should be the other way.

 

Doorman:-

 

I have not got a clue what that is. I have never seen anything like it in my life but at only 12 months old (I'm not trying to scare you here) I think I'd be rather concerned. My boat now shows about the same depth of pitting as yours but it's 15 years old.

 

I'm not going to guess what you've got going on there.

 

I wonder if our new electrolytic member has any ideas?

[coreng]

If I can... Obviously on a new boat and after so little time this must be investigated.

 

1) Please check your inlet.

...For safety reasons the grounding terminal is usually internally connected to the metal housing of the inlet. When used on metal-hulled boats, this connection can disturb the galvanic insulation (or isolation?) between ships-ground and shore-ground when an isolation transformer is applied. This can cause corrosion of the ship’s hull!

 

If so, the internal connection should have been removed before commissionning.

 

2) Was the sand-blasting and paint system applied according to best practices?

Edited January 27, 2011 by coreng