one for the galvanic erosion specialists

[UseleSS Enterprise]

I have read a lot about this and searched this forum for information but none seems to answer a specific problem. Old riveted steel butty, so no engine, propshaft or prop. There is no shoreline other than the occasional extension lead for 240v power tools or lights when working on the boat. No 12v battery on board. Not moored in a marina. No wiring, live or otherwise is earthed to the boat anywhere. Steel of the footings below the waterline are generally good but in certain parts, mostly toward the bows, after pressure washing off small mounds or rust and the jet black scab found underneath, there is bright silvery pitting. I understand this indicates strong galvanic and stray electrical current resulting in rapid erosion of the metal, usually to the point where there are no oxides present. But what on earth is the cause?. The only other clue is that where there is most pitting, is where it is moored up against a land surface water outfall which splashed down between the boat and the concrete wharf, when it rains.

[Giant]

I can certainly imagine the surface water outfall being much worse for corrosion than the canal water. Firstly, the water coming from the outfall could be much more oxygenated than the canal water the rest of the boat is sitting in. Secondly, it could be contaminated with all sorts of things picked up from running over the land.

[RD1]

Do you have any anodes, if so, how many and where? also what material are they, zinc, magnesium or aluminium?

Is the water the boat is moored on brackish?

[Tony Brooks]

Was there any slime under the rust scabs. If so it may be microbiological corrosion. Others who have experienced it have posted about it in the past.

[WotEver]

There's an interesting if somewhat old article about microbial corrosion: https://www.google.co.uk/amp/www.independent.co.uk/news/science/steel-eating-microbes-threaten-to-devour-britains-ports-738736.html%3Famp?client=safari

 

Loads more info follows a Google search.

[Sbg]

Pictures please!

[Alan de Enfield]

.................The only other clue is that where there is most pitting, is where it is moored up against a land surface water outfall which splashed down between the boat and the concrete wharf, when it rains.

 

I only know a little from what I have read.

 

Reinforced concrete (steel bars in concrete) can cause all sorts of problems for near-by structures.

 

Apparently the lime in the concrete reacts with the steel (iron) in the reinforcing, electrical currents generated that can 'leach out' to surrounding structures.

 

I see you are moored up against a concrete wall. ?

 

Edit to add -

having just done a google search, there is a humungous amount of research done into this, with the construction industry taking it all very seriously - the chemical formulas quoted are way above my head but the summary basically is :

 

"The corrosion of steel reinforcement in concrete is complex, but basically it is an electrochemical reaction similar to that of a simple battery. The composition of mild steel varies along its length and potential anodic (more negatively charged) and cathodic (positively charged) sites can be set up at various points".

 

Anodes are now recommended to be used in 'concrete-construction'

Edited January 20, 2017 by Alan de Enfield

[RD1]

There's an interesting if somewhat old article about microbial corrosion: https://www.google.co.uk/amp/www.independent.co.uk/news/science/steel-eating-microbes-threaten-to-devour-britains-ports-738736.html%3Famp?client=safari

Loads more info follows a Google search.

I was taking this seriously till they talked about hydrogen sulphate gas.. I checked the date of the report, and it wasn't April edition, so I just think it's a typo, and it should have been hydrogen sulphide.

Who recalls the polyestermite scare back in the 80's, a creature that bored into the grp of hulls, only when you read the date, April 1st you realised it was an April fool joke, a very good one too.

[Chewbacka]

Make sure the steel of your butty is not touching the steel of the bank side, if it is then you could set up a corrosion cell where the butty is close to the bank side steel, but not touching it. Use of fenders would be a good idea.

[Alan de Enfield]

Make sure the steel of your butty is not touching the steel of the bank side, if it is then you could set up a corrosion cell where the butty is close to the bank side steel, but not touching it. Use of fenders would be a good idea.

 

He is moored against a concrete wharf.

[Chewbacka]

 

He is moored against a concrete wharf.

I saw that, but sometimes there is a steel structure around the outflow to stop kids climbing in, and sometimes steel bars to protect the concrete from boats etc.

However if there is no earthed steel to touch the butty then ignore what I said.

[Grebe]

Could a patch repair, using steel of a different composition be setting up a 'battery' effect?

[WotEver]

Could a patch repair, using steel of a different composition be setting up a 'battery' effect?

Yes, but I'd then expect to see the corrosion around the patch, not remote from it.

 

So it would in theory be really obvious.

[hounddog]

I have read a lot about this and searched this forum for information but none seems to answer a specific problem. Old riveted steel butty, so no engine, propshaft or prop. There is no shoreline other than the occasional extension lead for 240v power tools or lights when working on the boat. No 12v battery on board. Not moored in a marina. No wiring, live or otherwise is earthed to the boat anywhere. Steel of the footings below the waterline are generally good but in certain parts, mostly toward the bows, after pressure washing off small mounds or rust and the jet black scab found underneath, there is bright silvery pitting. I understand this indicates strong galvanic and stray electrical current resulting in rapid erosion of the metal, usually to the point where there are no oxides present. But what on earth is the cause?. The only other clue is that where there is most pitting, is where it is moored up against a land surface water outfall which splashed down between the boat and the concrete wharf, when it rains.

It's not much help but the year before last I experienced much the same thing. In this case there is 12V electrics but no engine, no 240V and no hull to negative connection. This boat is a working butty, never moored in a marina.

 

Generally moored on the inside of a more modern steel motor, again with 12V, no 240V but this time the negative was bonded to the hull through the engine.

 

We experienced exactly the same, in the footings, small mounds of rust with bright silver pits that flash rusted when they were cleaned. No slime or black deposits and we remain baffled as to cause.

 

 

Edited to add; we did conclude that is was ordinary rust, that the thing that kept it soft was being under water so it wasn't difficult to remove completely hence the shiny surface underneath.

 

Where i would differ from your quote is that there is oxide present, if it was a white metal deposit from galvanic action there would be no rust on top.

Edited January 20, 2017 by hounddog

[WotEver]

Probably more than you ever wanted to know about corrosion in this excellent article:

http://www.amteccorrosion.co.uk/m_corrosionguide.html

[UseleSS Enterprise]

Thanks for your thoughts (having huge trouble with talktalk email so sorry did not respond).

There was no slime.

Moored against rubber tyres. Hull does not touch concrete or metal.

Ordinary canal water - don't know if its brackish?

Grebe may have something though - corrosion apparent along weld line of replacement footing most noticeable next to water outfall although there are other small areas seemingly unrelated to anything.

Any theories will be welcome.

[blackrose]

Since your boat isn't connected to the mains I doubt your corrosion problems are galvanic. Identifying the cause(s) of corrosion from the appearance of the pitting is not something that even experienced metallurgists can do easily. Some types like corrosion from SRBs might be easy to spot, but in my experience people who tell you they can distinguish galvanic corrosion by just looking at the rust patterns or pitting are usually trying to sell you something, or are just bullshitters.

Edited January 21, 2017 by blackrose

[UseleSS Enterprise]

Probably more than you ever wanted to know about corrosion in this excellent article:

http://www.amteccorrosion.co.uk/m_corrosionguide.html

Very good article and some recognisable descriptions of what we found. Main advice I have from others who have seen the problem is that on such a large hull, welding on a few anodes would have less effect than a good steam clean of all corroded areas and into any pitting and then, paint with a good quality paint and paint again. As I understand it, the corrosive action can not take place if the surface of the steel is not exposed to the water.

Edited January 21, 2017 by UseleSS Enterprise

[WotEver]

Very good article and some recognisable descriptions of what we found. Main advice I have from others who have seen the problem is that on such a large hull, welding on a few anodes would have less effect than a good steam clean of all corroded areas and into any pitting and then, paint with a good quality paint and paint again. As I understand it, the corrosive action can not take place if the surface of the steel is not exposed to the water.

Yup, I'd agree with that. Use a brush not a roller so that you can be sure that the paint gets well into the pits. If it's covered it can't corrode.

 

Obviously ensure that the hull is dry when painting to ensure that the coating sticks. Probably not the best time of year to achieve this unless in a heated poly tunnel.

[Tony Brooks]

I suspect may instances of "galvanic" corrosion are in fact the result poor preparation and not forcing the blacking into all the small pits in the steel and welds.

 

I would probably use two different compatible products. A this one (less viscous) and a really old short hired brush to really scrub it into all the pits. If time repeat after thorough drying. The a fuller bodied on to build up the coating. I think these coats probably could be rollered on but I would still want to use a brush to work on in several directions.

[WotEver]

I suspect may instances of "galvanic" corrosion are in fact the result poor preparation and not forcing the blacking into all the small pits in the steel and welds.

Absolutely. When we bought Wot Ever there was a little rust between the hull and the guard rails at their lower edge. All we did was to ensure that the blacking was forced well into those crevices and the corrosion never progressed.