Galvanic Erosion

[Raz]

8 minutes ago, mark99 said:

Ok thanks. Marina anywhere a DC railway?

Our permanent mooring was an end of garden farm mooring located between the two Swanley Locks on the Llangollen Canal near Nantwich, NOT in a marina, although it was a local marina that we used exclusively for all our preventative maintenance work during our own ownership over 5 years, including the last occasion. Shore power was provided by the mooring land owner who had a solar panel system installed on a barn to back up his own mains supply from where our boat supply was taken. As far as I am aware - no nearby electrified railway.

[Raz]

Something that always stuck in the back of my mind after the event. Look at the images of the prop. It is in perfect condition and is the original 2004 fitment. After my own research on this subject I would have thought the prop would have been eaten away by corrosion, but it was the steel hull that was. Although the anodes were discovered to be more worn that I would have preferred when I first saw the underwater hull ashore, and should have been replaced as I asked before this work, they too are original and not that worn compared to the steel hull.

[jddevel]

Is there any value in having both a Galvanic Isolator and a Isolating Transformer please (belt and braces) ?

[Robbo]

2 minutes ago, jddevel said:

Is there any value in having both a Galvanic Isolator and a Isolating Transformer please (belt and braces) ?

Nope, just get a IT if your always on shore power, a GI if your on it occasionally.

[jddevel]

Thank you Robbo

[Robbo]

7 minutes ago, jddevel said:

Thank you Robbo

If an IT fails you get no electric so it's obvious if it goes wrong, there is no physical connection between the shore and onboard systems so you will always get protection.  With a GI there is a physical connection, so if it fails you don't get the protection, their is however a simple test to confirm if working tho which should be performed regularly, but I'm guessing most people don't.

Airlink do a decent low cost IT at around £300.

edit to add link; https://airlinktransformers.com/category/boating-transformers

Edited April 14, 2017 by Robbo

[WotEver]

Specifically this one:

https://airlinktransformers.com/product/boating-transformer-bt3231

[mark99]

2 hours ago, Raz said:

Something that always stuck in the back of my mind after the event. Look at the images of the prop. It is in perfect condition and is the original 2004 fitment. After my own research on this subject I would have thought the prop would have been eaten away by corrosion, but it was the steel hull that was. Although the anodes were discovered to be more worn that I would have preferred when I first saw the underwater hull ashore, and should have been replaced as I asked before this work, they too are original and not that worn compared to the steel hull.

 

If you had a large emv forcing its way through the steel hull; the normal galvanic cell created between prop and other dissimilar would pale into insigificance.

 

One line of thought re solar.

http://ieeexplore.ieee.org/document/7066917/?reload=true

 

 

Edited April 14, 2017 by mark99

[Mikexx]

1 hour ago, Raz said:

Something that always stuck in the back of my mind after the event. Look at the images of the prop. It is in perfect condition and is the original 2004 fitment. After my own research on this subject I would have thought the prop would have been eaten away by corrosion, but it was the steel hull that was. Although the anodes were discovered to be more worn that I would have preferred when I first saw the underwater hull ashore, and should have been replaced as I asked before this work, they too are original and not that worn compared to the steel hull.

You've got me thinking. The bronze propellor will itself cause a galvanic current to flow. The reason why the prop is in such good condition is the current flow, and more importantly the direction, prevents any corrosion. Phosphor bronze is more noble than copper, so worse than a copper rod used for a local earth than many rave about.

It's not uncommon for the stern to suffer more corrosion and pitting than the bow end, and perhaps this is the cause. That is certainly my experience, and my the stern anodes have deteriorated faster then others.

Perhaps it suggests that anodes should be fitted as close as possible to the propellor, in addition to using an isolating transformer when using a shore line.

Or use a 'steel' prop? Or plastic one?

[Phil.]

I am no expert but my understanding of stray current corrosion is, the point of corrosion is generally where the current leave to hull. Looking at the two holes being where they are, would suggest that current was leaking in the bilge area, if indeed it was a wet bilge, and a bilge pump was present, then a faulty bilge pump or connection would be a probable cause. If that was the case then a GI or IT would not have helped.

[Mikexx]

At the currents involved the hull can be considered at the same potential. I therefore don't see have a wet bilge should make any difference unless the corrosion was bilge side.

The current would be dependent on the local 'water' potential and the insulating properties of water and any surface paint.

[Flyboy]

12 minutes ago, Phil. said:

I am no expert but my understanding of stray current corrosion is, the point of corrosion is generally where the current leave to hull. Looking at the two holes being where they are, would suggest that current was leaking in the bilge area, if indeed it was a wet bilge, and a bilge pump was present, then a faulty bilge pump or connection would be a probable cause. If that was the case then a GI or IT would not have helped.

Highly unlikely.

[Raz]

8 minutes ago, Phil. said:

I am no expert but my understanding of stray current corrosion is, the point of corrosion is generally where the current leave to hull. Looking at the two holes being where they are, would suggest that current was leaking in the bilge area, if indeed it was a wet bilge, and a bilge pump was present, then a faulty bilge pump or connection would be a probable cause. If that was the case then a GI or IT would not have helped.

Just for info. The bilge on our boat was notably dry. So dry in fact that the BSS surveyor usually commented on how pleasantly dry it was to work in. There was an electric bilge pump fitted and working, but the only time it got wet was when the bilge flooded as a result of the pitting holes. The bilge filled overnight to the top of the engine level where it then leaked into the cabin of the boat through pipes leading to the calorifier through the engine bay bulkhead. The water was around 4" above floor level right up to the kitchen when I found her on the Saturday morning.

I found an interesting article here on the different types of corrosion that will eat boats away.....

http://www.boatus.com/boattech/articles/marine-corrosion.asp

 

 

[WotEver]

For any Corrosion there must be a current flow, so as Mike points out a leaky wire to a bilge pump doesn't address what occurred on this hull.

If it's Galvanic Corrosion then two dissimilar metals have created a battery in the presence of an electrolyte (canal water). The less noble metal will migrate to the more noble metal. 

If it's stray current Corrosion then the circuit is from the shore to the boat via the earth lead, then from the hull to the pilings (and hence back to earth) via the canal. The latter part of this circuit sees the hull migrating to the pilings. 

I also had a chat with a surveyor who was in the process of pronouncing a 30 year old hull 'as good as new' who told me that the previous week he'd specified over £1000 worth of welding up pits in a 4 year old hull. He put it down to poor steel but I have no idea if he was right. 

[mark99]

And stray current corrosion is more (or can be) much more aggressive as the millivolts (emf) can be sig. higher.

Edited April 14, 2017 by mark99

[WotEver]

1 minute ago, mark99 said:

And stray current corrosion is more (or can be) much more aggressive as the millivolts (emf) can be sig. higher.

Absolutely. 

[cuthound]

Seems to me that having the boat epoxy blacked, including the bottom plate would help protect against galvanic and stray currents by effectively insulating the hull, particularly when used in combination with a GI or IT.

However, the protection would be compromised when the expoxy got damaged.

What would happen then?, would the small area of steel exposed then corrode faster or slower than the epoxy covered steel around it? 

[WotEver]

42 minutes ago, cuthound said:

What would happen then?, would the small area of steel exposed then corrode faster or slower than the epoxy covered steel around it?

The steel around it, still being insulated, wouldn't corrode. The exposed steel would. 

[Mikexx]

1 hour ago, WotEver said:

The steel around it, still being insulated, wouldn't corrode. The exposed steel would. 

Correct, except the whole point of anodes is that the exposed steel is at a negative potential and so shouldn't corrode through galvanic corrosion.

Of course, there are other forms of corrosion.

[WotEver]

59 minutes ago, Mikexx said:

Correct, except the whole point of anodes is that the exposed steel is at a negative potential and so shouldn't corrode through galvanic corrosion.

Galvanic yes, but you also mentioned Stray Current in your post. 

[Mikexx]

14 minutes ago, WotEver said:

Galvanic yes, but you also mentioned Stray Current in your post. 

 

I have mentioned local water potential but not stray currents directly.

I'd rather not use the term "stray" because corrosion is down to the magnitude and direction of current at any point on the hull. The hull is at one potential but the surrounding water at another depending on local conditions such as anodes and props, as well as a shore line with its own earthing arrangements.

[WotEver]

Just now, Mikexx said:

I have mentioned local water potential but not stray currents directly.

Sorry, no it wasn't you. I was replying to Cuthound who mentioned Stray Current Corrosion. 

[Chewbacka]

5 hours ago, WotEver said:

The steel around it, still being insulated, wouldn't corrode. The exposed steel would. 

You can get corrosion eating along the interface between paint and steel it grows in long trails.  From memory it's called filiform corrosion.  More a problem on thin steels.   -  I did some work on corrosion many years ago, but that was mainly steel in concrete with varying amounts of salt (NaCl) in the wet mix.

Edited April 15, 2017 by Chewbacka

[WotEver]

48 minutes ago, Chewbacka said:

You can get corrosion eating along the interface between paint and steel it grows in long trails.  From memory it's called filiform corrosion.  More a problem on thin steels.   -  I did some work on corrosion many years ago, but that was mainly steel in concrete with varying amounts of salt (NaCl) in the wet mix.

I'd suspect that's just ordinary common or garden rust though isn't it? 

[mark99]

There's MICROBIALLY INFLUENCED CORROSION (MIC) which is a becoming the focus of a bit more research including DNA sampling of the living things. Can be quite destructive.

General comment not related to OP.