Galvanic Erosion

[Chewbacka]

2 hours ago, WotEver said:

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

It is rust, but instead of slowly expanding in a circle it 'grows' under the paint film and can disrupt it over several inches eventually breaking the paint film allowing lots more rust to form under the paint, but not always that visible until big areas start to flake.  So even undamaged (externally) paint can harbour rust underneath.  That said in the car industry, normal corrosion - like you get on chassis parts etc - is considered to be about 0.1mm per year if untreated.  Which on car panels - which are often only 0.7mm, that gives a life of about 7 years from corrosion start to perforation, hence the use of phosphates and ED coating to stop it starting.  Pitting corrosion on vehicles is not a big problem as tiny pits don't greatly reduce the structural integrity, but on a boat, it is of course, the other way around, where pitting is very serious, and general rust on thick panels can be managed.  Sorry, but I am rambling again.

[WotEver]

44 minutes ago, Chewbacka said:

It is rust, but instead of slowly expanding in a circle it 'grows' under the paint film and can disrupt it over several inches eventually breaking the paint film allowing lots more rust to form under the paint, but not always that visible until big areas start to flake.  So even undamaged (externally) paint can harbour rust underneath. 

Like we used to find when peeling off underseal - loads of rust which has travelled underneath it. 

1 hour ago, mark99 said:

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.

Yeah, what we've referred to in the past as the Steel Bug. 

[jddevel]

Comments have been made about bare steel and corrosion plus I`ve also read that it`s not really necessary to "black" the bottom of the hull/ I`m confused. Comments would be appreciated please.

[WotEver]

7 minutes ago, jddevel said:

Comments have been made about bare steel and corrosion plus I`ve also read that it`s not really necessary to "black" the bottom of the hull/ I`m confused. Comments would be appreciated please.

You'll hear arguments for and against. In many waters the base plate gets so covered in mussels that there are no bare bits left. Two other argument are that it's much thicker than the rest of the hull and that there is very little oxygen a couple of feet down so little opportunity for oxidation. 

[peterboat]

On 14/04/2017 at 20:49, WotEver said:

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. 

Like you Tony after looking at the pictures of the sunk boat I reckon it was crap steel. A friend of mine had a boat built by somebody other than a boat builder, he shot blasted it and 2 packed it to no avail, it has corroded very badly. So is it mooring next to metal piling? or is it crap steel? Me I would go for crap steel

[WotEver]

I'm no chemist nor metallurgist but it seems logical to me that if a piece of steel has thousands of microscopic inclusions then each of these areas can set up localised Galvanic Corrosion spots all by themselves. I suspect this is what was meant by 'poor quality steel' but I have no idea if this theory has any merit. 

[Sea Dog]

11 hours ago, peterboat said:

Like you Tony after looking at the pictures of the sunk boat I reckon it was crap steel. A friend of mine had a boat built by somebody other than a boat builder, he shot blasted it and 2 packed it to no avail, it has corroded very badly. So is it mooring next to metal piling? or is it crap steel? Me I would go for crap steel

I so hope you're right! I can go back to sleeping at night then! 

[Mikexx]

1 hour ago, WotEver said:

I'm no chemist nor metallurgist but it seems logical to me that if a piece of steel has thousands of microscopic inclusions then each of these areas can set up localised Galvanic Corrosion spots all by themselves. I suspect this is what was meant by 'poor quality steel' but I have no idea if this theory has any merit. 

 

This article:

 http://www.corrosionclinic.com/types_of_corrosion/pitting_corrosion.htm

suggests that inclusions of Manganese is a major culprit of pitting. I would expect the inclusion of a more noble material would cause pitting.

  https://en.wikipedia.org/wiki/Pitting_corrosion

highlights the anodic effect of an area that has lost its surface rust. I presume any layer of rust is more cathodic in nature than the steel below.

 

[WotEver]

So perhaps the theory of poor steel has merit in some cases.

[Chewbacka]

1 hour ago, WotEver said:

So perhaps the theory of poor steel has merit in some cases.

If you ever look at rebar removed from concrete you will often find bright shiny patches and close by corroded areas.  This is due to local corrosion cells forming because of steel composition is a bit variable.  I can't see why that can not happen on a boat as well.

[peterboat]

I have seen a blowhole in a piece of steel that was to be a baseplate, it had half a mil of steel each side, how long before it corroded through? It wasnt made in britain, germany, china, or sweden and the whole lot went back so no boats sank because of it.

[jddevel]

I note that not only on my sailaway but all craft ashore have their sacrificial anodes -specifically those on the hull-  fore and aft. However corrosion appears all over the hull so is there any merit in having more attached along the hull sides? At the pace most travel is it going to affect maneuverability.

[WotEver]

20 minutes ago, jddevel said:

I note that not only on my sailaway but all craft ashore have their sacrificial anodes -specifically those on the hull-  fore and aft. However corrosion appears all over the hull so is there any merit in having more attached along the hull sides? At the pace most travel is it going to affect maneuverability.

They can get in the way in locks if they're in the middle. Hence they're usually fitted on narrower parts of the hull. 

[Raz]

Re the crap steel theory. 1) The boat was built by Dragon Narrowboats of Oswestry. Although I would class it at the budget end of the market there are hundreds out there with no issues, and they build narrowboat hulls as part of their main business - Premier Motor Bodies, building skips and refuse vehicle bodies. To me, that would negate the crap steel theory alone.

2) Also study the images I took again. The surveyor who was the specialist in this type of corrosion suggested that the side moored against the steel canal banking would be affected the most in the event of this type of corrosion taking hold. I always moored facing up the Llangollen Canal, starboard side against steel piling. The images show the most severe pitting on this side, with the holes that sunk her also on this side. The image of the port (canal side) - the one that includes the image of the anode, is nowhere near as badly pitted. This was the same right along the rest of the boat too. So if the crap steel theory is to hold up, then the port side must have been built with better steel than the starboard side?

3) When we acquired the boat in March 2009, having been first used in September 2004, there were no visible issues at all with the hull - 4 trouble free years from new. The previous owner had blacked her in 2005 and again in July 2008 - I have the receipts. When we took her over on the same mooring, she was severely damaged by hull corrosion within 5 years, even though we had (supposedly) kept up with regular blacking in less than two year intervals, three further times. The only difference in use between us and the previous owner is that we kept her connected to shore power permanently to keep her batteries charged between visits, and he did his own blacking.

The bottom line for me after the event is that I placed too much trust in those I paid to do the hull inspections and blacking. Had I seen it myself, I would have ordered much earlier investigations as to why the hull was pitting so badly between blacking and also ensured a good blacking job took place, because in my opinion the last blacking job was nowhere near a good job, with bare metal left everywhere apart from just painting over an obviously severely pitted hull that they had painted every time since September 2009.

[WotEver]

6 hours ago, Raz said:

Re the crap steel theory...

I don't believe that anyone was suggesting that this was the case with your boat, simply that it's another potential issue to be aware of - like microbial Corrosion. 

I believe it's obvious that with your boat the GI failed to do its job which, coupled with the poor job from the boatyard, resulted in the holed hull. 

[jddevel]

Am I right in assuming that galvanic corrosion only really takes place when the boat is moored and not really important when in motion? This may be a stupid question and you may wonder why I ask but I may have a "cunning plan" as one of my heroes would say> 

[WotEver]

3 minutes ago, jddevel said:

Am I right in assuming that galvanic corrosion only really takes place when the boat is moored and not really important when in motion?

Nope. Not really. 

[Robbo]

9 minutes ago, WotEver said:

Nope. Not really. 

Unless you have really long shore cable.

[bizzard]

I think it depends where you are. My cousin used to winter in Stoke-on-Trent. He said that most folk that moored there didn't bother with anodes because of the orange waters thereabouts, iron oxide from the old mines which seemed to prevent the attacks.  Most hire boat companies didn't used to bother either a number of years ago, they reckoned because their boats were on the move most of the time.

[jddevel]

No  not when in motion or no I`m not right? Un fortunately I asked two questions and you gave one answer or am I wrong on both counts?

[bizzard]

15 minutes ago, jddevel said:

No  not when in motion or no I`m not right? Un fortunately I asked two questions and you gave one answer or am I wrong on both counts?

To simulate the boat moving when it isn't you could moor in a fast flowing mill stream below a weir. At night time in black dark you should observe down stream of you in the slower waters, electricity all aglow, with sparks crackling, like St Elmo's fire that have just missed you.

Edited April 17, 2017 by bizzard

[jddevel]

So obviously the waters` movement has to be taken into account. Although a stupid idea I just wondered whether "temporary" anodes affixed magnetically when the boat is not in motion would avoid the lock problem and corrosion associated with other craft. Perhaps as a layman to this science I`m just making too much of a problem!! 

[WotEver]

26 minutes ago, jddevel said:

No  not when in motion or no I`m not right? Un fortunately I asked two questions and you gave one answer or am I wrong on both counts?

No you're not right. Galvanic Corrosion occurs between dissimilar metals in the presence of an electrolyte. The only thing that changes if the boat is on the move is the composition of the electrolyte. 

[bizzard]

I reckon fizzing anodes are due to folk spilling Peps iCola in the water. The eruption can be like Alka Seltzer. Or your anodes are made of Alka Seltzer.

Edited April 17, 2017 by bizzard

[Sea Dog]

Right, I've just hauled my boat out onto the towpath where it will remain until this gets sorted out!  Raz's sorry tale unsettled me last time it was discussed on here (I think he might have had a different name then), and now I'm scared to go back in the water!

If anyone tries to move me, I'll say I'm protesting peacefully against sustrans taking over the towpaths - I know my rights. Hurry up and diagnose Raz's hull piercing Galvanic Erosion though, cos I'm not much for rights that inconvenience everyone else and I was very disappointed with the beer in Ma Pardoes (yes, really) so I need to get to the Bull and Bladder for some Bathams pdq!