Overplate or not

[Slim]

5 minutes ago, Alan de Enfield said:

 

Had you considered Microbial Corrosion, it has been shown to develop pits that can actually penetrate a thick-hulled commercial vessel in 18 months

 

It is now becoming quite widely found on the canals and more and more cases of attack on steel NBs are being reported.

Whilst it may have existed it was unheard of 23 years ago.

[Alan de Enfield]

41 minutes ago, Slim said:

Whilst it may have existed it was unheard of 23 years ago.

 

It was certainly recognised, and known about, by the Marine industry in 1966, but it did take a few years before anyone in the UK Inland waterways accepted its existence.

 

 

 

Edited October 2, 2023 by Alan de Enfield

[manxmike]

MIC and being bracketed on both sides by permanent live-aboards plugged into the mains, ate the hull on my Liverpool boat, down to 2mm in places on an 8mm base plate. My solution was to sell it.

Having said which I was honest, told the buyer and accepted a ludicrously low offer working on the principal that he/she could have the pleasure and expense of over-plating.

At £10k I didn't bother.

[blackrose]

30 minutes ago, manxmike said:

MIC and being bracketed on both sides by permanent live-aboards plugged into the mains, ate the hull on my Liverpool boat, down to 2mm in places on an 8mm base plate. 

 

That doesn't make much sense to me. How can you possibly tell how the corrosion occurred? At best it's just guesswork.

 

There's a great myth on the canals perpetuated by people who've heard of galvanic corrosion but don't really understand the subject, that plugging a steel boat into the mains will by itself cause the hull to galvanically corrode. That just isn't the case if the boat is galvanically isolated. So, assuming your boat wasn't on shore power (or was but was suitably isolated from earth) then having liveaboards on both sides shouldn't affect it at all even if their boats weren't isolated. 

[ditchcrawler]

1 hour ago, blackrose said:

 

That doesn't make much sense to me. How can you possibly tell how the corrosion occurred? At best it's just guesswork.

 

There's a great myth on the canals perpetuated by people who've heard of galvanic corrosion but don't really understand the subject, that plugging a steel boat into the mains will by itself cause the hull to galvanically corrode. That just isn't the case if the boat is galvanically isolated. So, assuming your boat wasn't on shore power (or was but was suitably isolated from earth) then having liveaboards on both sides shouldn't affect it at all even if their boats weren't isolated. 

So where would the current flow here, would it go from the bows of A to the stern of B, then through B's hull to C and the mains earth bond.

[blackrose]

8 hours ago, ditchcrawler said:

So where would the current flow here, would it go from the bows of A to the stern of B, then through B's hull to C and the mains earth bond.

 

I'm not sure how you can tell in which direction the current would flow unless you know the nobility of the steel of boats A & C, but let's not worry too much about that!

 

No it wouldn't flow through boat B because the hull of boat B isn't part of any large scale galvanic circuit. Boats A & C are connected to shore power power and share a common earth so assuming they aren't galvanically isolated one may become cathodic and the other anodic depending upon any relative variability in the nobility of their respective hull steel. On the other hand if they are both correctly isolated then nothing will happen. Boat B is already isolated so it isn't part of that weak galvanic circuit so won't become anodic or cathodic. If boat B was connected to shore power but correctly isolated it also wouldn't form part of any galvanic circuit created between boats A & C.

Edited March 2 by blackrose

[MtB]

58 minutes ago, blackrose said:

 

I'm not sure how you can tell in which direction the current would flow unless you know the nobility of the steel of boats A & C, but let's not worry too much about that!

 

No it wouldn't flow through boat B because the hull of boat B isn't part of any large scale galvanic circuit. Boats A & C are connected to shore power power and share a common earth so assuming they aren't galvanically isolated one may become cathodic and the other anodic depending upon any relative variability in the nobility of their respective hull steel. On the other hand if they are both correctly isolated then nothing will happen. Boat B is already isolated so it isn't part of that weak galvanic circuit so won't become anodic or cathodic. If boat B was connected to shore power but correctly isolated it also wouldn't form part of any galvanic circuit created between boats A & C.

 

 

Don't forget we are discussing AC which flows one way then reverses, 50 times a second. 

 

It also strikes me as quite a big leap to expect the microscopic currents involved here to be causing any metal loss in the first place. 

 

 

[blackrose]

1 hour ago, MtB said:

 

 

Don't forget we are discussing AC which flows one way then reverses, 50 times a second. 

 

It also strikes me as quite a big leap to expect the microscopic currents involved here to be causing any metal loss in the first place. 

 

I thought galvanic currents flowed in a one-way direction from anode to cathode. Surely that can't alternate 50 times per second? I think we might be confusing two different things here. Galvanic corrosion is caused by DC currents created by different metals with different nobilities on the galvanic scale with a potential difference between them, just like a battery with electrodes made of zinc & copper for example. It has nothing to do with AC. The reason you're confused about that is because the boats are plugged into shore power but that's only relevant because of the common earth connection between the boats.

 

I think galvanic currents causing metal loss is a fairly well established concept backed up by lots of data and evidence. It's basic physics. The galvanic currents may be weak but once they've been created they're "on" all the time, infinitely until something else changes like one of the boats unplugging. 

Edited March 2 by blackrose

[john.k]

Since most advanced countries mandated car rust standards ,car makers have discovered 'by magic' what causes steel sheet to corrode .......and it seems a major factor is titanium .........consequently car body steel is specified at close to zero titanium.content........Australian steel sheet for car bodies has always been atrocious for rust ......and titanium is abundant in Oz ........A good example is my old Coles crane ..mostly made in the UK ,with some sheet metal fitted in Australia ...........the UK sheet steel is still good,the Australian sourced steel rusted away.

[casper ghost]

On 02/10/2023 at 08:07, Morris said:

 

 

To op - overplating 6mm sides because of 1.5mm deep pits seems like madness to me. 

 

 

To me too...

[manxmike]

In answer to the queries about the "eating away" of the hull - I am no expert, I have a CSE in Chemistry (56 years ago) so I asked people with far more knowledge than I. Their view was that a combination of a boat either side permanently plugged in to the mains together with MIC in the water would in all probability produce something along the lines I described.

Whatever the cause, over a period of six years the same surveyor found that 8mm had reduced to 2mm. To me that seemed a trifle excessive in a relatively short period of time.

Edited March 11 by manxmike
spelling error

[john.k]

There is a monthly magazine called "Rust" for those who wish to delve into the subject of corrosion.

[blackrose]

9 hours ago, manxmike said:

...over a period of six years the same surveyor found that 8mm had reduced to 2mm. To me that seemed a trifle excessive in a relatively short period of time.

 

A trifle excessive! 😱