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Oldngrumpy

Baseplate corrosion concern

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Hi all - I'm hoping you can share some wisdom with me. Situation is that we are in process of buying our first boat and have found 'the one' we both like! 2008 Piper hull, 57 feet and standard 10/6/5/4 build. The boat has not had a lot of use in recent years and was out of water today for a hull inspection.

 

I was quite disturbed to find, underneath the accumulated crust, many large 'depressions' in the base plate steel. While the surrounding plate measured at 9.5 to 10mm, the depressions were up to 2.7mm deep and anything from 10 to 25mm dia. They are smooth depressions rather than sharp edged pits and are all over the central (bow to stern) area of the baseplate, everywhere I looked, while the bow and stern areas are not so bad. There are anodes fitted bow and stern but nowhere else. No evidence that the base has ever been blacked. The only other observation is that the water film betwenn the crust and the plate was black - very black, almost-like-ink black.

 

So, my questions are:-

  1. what do you think may have caused the corrosion/erosion?
  2. do you consider blacking or 2-pack epoxy will stop it in its tracks?
  3. should we walk away and look some more or are we likely to find this is common?
  4.  

Thanks for reading this far! I hope you can help me.

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Have you had a survey? Did the surveyor donthe thickness measurements? What was said about the depressions?

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7 hours ago, Oldngrumpy said:

I was quite disturbed to find, underneath the accumulated crust, many large 'depressions' in the base plate steel. While the surrounding plate measured at 9.5 to 10mm, the depressions were up to 2.7mm deep and anything from 10 to 25mm dia.

It sounds a little unusual.

However, if you take the 9.5mm and deduct the 2.7mm (worse case) you still have over 6mm which is the original base thickness of many, many boats built in the 80's that are still afloat today.

 

It will not start to leak until it gets down to 0.0mm thickness.

 

What did your surveyor suggest the reason for the depressions was ? (corrosion, impact damage ????)

 

NB - I'm in the "its a waste of time painting the base plate" gang.

 

If its the boat for you - use the 'corrosion' concerns to beat the price down a bit more.

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If there are 2.7mm "depressions" in the 10mm baseplate, what is the situation with the lower hull sides that were built in 6mm steel?  If there is anything similar there the remaining thickness would be less that the 4mm that many people seem to think is necessary to get comprehensive insurance.

Edited by alan_fincher

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7 hours ago, Oldngrumpy said:

Hi all - I'm hoping you can share some wisdom with me. Situation is that we are in process of buying our first boat and have found 'the one' we both like! 2008 Piper hull, 57 feet and standard 10/6/5/4 build. The boat has not had a lot of use in recent years and was out of water today for a hull inspection.

 

I was quite disturbed to find, underneath the accumulated crust, many large 'depressions' in the base plate steel. While the surrounding plate measured at 9.5 to 10mm, the depressions were up to 2.7mm deep and anything from 10 to 25mm dia. They are smooth depressions rather than sharp edged pits and are all over the central (bow to stern) area of the baseplate, everywhere I looked, while the bow and stern areas are not so bad. There are anodes fitted bow and stern but nowhere else. No evidence that the base has ever been blacked. The only other observation is that the water film betwenn the crust and the plate was black - very black, almost-like-ink black.

 

So, my questions are:-

  1. what do you think may have caused the corrosion/erosion?
  2. do you consider blacking or 2-pack epoxy will stop it in its tracks?
  3. should we walk away and look some more or are we likely to find this is common?
  4.  

Thanks for reading this far! I hope you can help me.

It's pitting.  I don't know why but folk seems to assume that pits manifest themselves as jagged sharp edged affairs, but what you describe is quite typical of localised corrosion especially on baseplates.  This is why it is such nonsense to suggest that the baseplate doesn't need protecting.  Ok at 2.7mm every ten years suggests it's going to be a long time before the hull is perforated but IMHO you would need to do something about it which would involve grit blasting and coating with something like epoxy, zinc, etc.

 

For certain it's a bargaining point.  

 

You could walk away and find a 10 year old boat that doesn't have pitting to the same extent, depends on how much you like this boat. 

 

Who knows why some boats are susceptible to this type of corrosion and others are not.  Anyone that can find a definitive answer is probably going to make a lot of money..

 

 

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2.7mm pits are evidence of quite aggressive corrosion. A 10 year old baseplate really shouldn’t have anything of note to report. The logic of having a 10mm baseplate versus 6mm can only be for additional protection in lieu of a protective coating system. So having used up the additional protection the logic now is to protect the hull with a good coating system. This may not be as simple as it sounds though. There is good reason why baseplates are often left unprotected.  

 

However it’s also worth considering the environment the boat has been kept in as this is almost certainly the key to why it has pitted. So simply changing the mooring, the means of electrical power supply and protection, or simply the cruising pattern may be enough to arrest the corrosion but the prospective buyer didn’t create the problem so why should they take the risk?

 

I’d be negotiating to get the hull protected but that could be a showstopper. The boat will probably eventually sell to someone less diligent if this sale doesn’t go through. That’s not a reason to take a risk yourself though.

 

JP

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1 hour ago, Neil2 said:

It's pitting.  I don't know why but folk seems to assume that pits manifest themselves as jagged sharp edged affairs, but what you describe is quite typical of localised corrosion especially on baseplates.

That's interesting as I have never seen any 'corrosion pits' up to 20mm diameter - not saying its impossible, just never seen any larger than a couple of mm, smooth and shiny. 

 

It may be worth the OP investigating Microbially Induced Corrosion which could POSSIBLY fit his description.

 

Out of the water– after pressure washing, MIC bacteria shows itself as 5-10 pence sized pits in the steel surface, often with vertical sides rather than sloping edges and frequently displaying a silver or white base to the pits resembling an earlier zinc treatment. The pits appear not to rust but in due course will do so

 

Boat owners and yards may know all about rust. But there is an increasingly prevalent threat to the boat owner – hull damage caused by Microbially Induced Corrosion.  Dismissed as normal galvanic or electrolytic corrosion and ineffectively treated as such by most yards and DIY-ers, MIC particularly affects boats lying in canals or rivers containing high levels of chemicals – usually from fertiliser run-off or decaying vegetable matter.

Microbially Induced Corrosion (MIC) is the result of the respiratory habits of a nasty organism that is supported by the nitrates and sulphites found in oil and gas – and therefore present in fertiliser by-products. MIC most frequently occurs in arable regions of the canal network, particularly on the east side of the UK and where there is intensive crop farming using non-organic chemical fertilisers.

Marinas fed by rivers are another risk area and, in salt and brackish water environments, it is well known that harbour muds are highly contaminated by the sulphites supporting these creatures, leading to catastrophic steel pile failure.  MIC can occur on any ferrous and non-ferrous surface including aluminium.

 

Dealing with a microbial attack

If a hull is found to be suffering from microbial attack, it must be dealt with to try to prevent it recurring.   A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left on the surface for 24 hours after which a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only total solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process.

The main problem is that the microbes are anaerobic and can continue to live beneath the existing paint coatings and, once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion out of sight.

This means that conventional solvent based blackings which usually leave a coating of around 300-400 microns in thickness can effectively conceal the problem until corrosion has reached dangerous levels. Thinner blackings allow the problem to be visible earlier which, whilst cosmetically unattractive, means that the problem can be discovered and dealt with before catastrophic steel failure occurs.

 

 

Edit to add :

 

I have never (knowingly) seen MIC, but an internet search shows :

 

Web-081-keelblack-pit.jpg

 

 

Edited by Alan de Enfield

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Has the boat been in a marina with a landline?  Does it have a working GI or IT?   Galvanic corrosion is a possibility, but why it should be limited to the middle of the boat is not clear.  Also if the sides are not pitted in the same way, why is it limited to the base plate?

Sorry I can't answer these questions, but i would be interested in any answers.

  • Greenie 1

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21 minutes ago, dor said:

Has the boat been in a marina with a landline?  Does it have a working GI or IT?   Galvanic corrosion is a possibility, but why it should be limited to the middle of the boat is not clear.  Also if the sides are not pitted in the same way, why is it limited to the base plate?

Sorry I can't answer these questions, but i would be interested in any answers.

 

One answer could be that the middle is often way out of the protection range of the anodes but I certainly don't know.

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1 hour ago, alan_fincher said:

If there are 2.7mm "depressions" in the 10mm baseplate, what is the situation with the lower hull sides that were built in 6mm steel?  If there is anything similar there the remaining thickness would be less that the 4mm that many people seem to think is necessary to get comprehensive insurance.

Not necessarily,  we had a boat out for survey earlier in the year with virtually nil pitting on the sides, baseplate had extensive pitting to depths of 5 and 6 mm.

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31 minutes ago, matty40s said:

Not necessarily,  we had a boat out for survey earlier in the year with virtually nil pitting on the sides, baseplate had extensive pitting to depths of 5 and 6 mm.

And it is logical. Stray current will find the path of least resistance to a point of lower/lowest potential from the source of the stray current. That source may very likely be closer to the baseplate than the sides and being uncoated the baseplate will conduct better than the sides and be more able to form a galvanic cell. It just depends on the particular circumstances.

 

JP

Edited by Captain Pegg

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10 hours ago, Oldngrumpy said:

Hi all - I'm hoping you can share some wisdom with me. Situation is that we are in process of buying our first boat and have found 'the one' we both like! 2008 Piper hull, 57 feet and standard 10/6/5/4 build. The boat has not had a lot of use in recent years and was out of water today for a hull inspection.

 

I was quite disturbed to find, underneath the accumulated crust, many large 'depressions' in the base plate steel. While the surrounding plate measured at 9.5 to 10mm, the depressions were up to 2.7mm deep and anything from 10 to 25mm dia. They are smooth depressions rather than sharp edged pits and are all over the central (bow to stern) area of the baseplate, everywhere I looked, while the bow and stern areas are not so bad. There are anodes fitted bow and stern but nowhere else. No evidence that the base has ever been blacked. The only other observation is that the water film betwenn the crust and the plate was black - very black, almost-like-ink black.

 

So, my questions are:-

  1. what do you think may have caused the corrosion/erosion?
  2. do you consider blacking or 2-pack epoxy will stop it in its tracks?
  3. should we walk away and look some more or are we likely to find this is common?
  4.  

Thanks for reading this far! I hope you can help me.

You mention the depression. You haven't made it clear whether the depressions are just that rather than they are actual reductions in steel thickness.  Was there an actual measurement made of the steel thickness or just a measurement of the depression? If there is no reduction in steel thickness then this appears to be mechanical damage rather than pitting. If, however, it is an actual reduction of steel thickness in way of the depressions it could well point towards electrolytic corrosion.

 

Howard

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As the owner of an older (41 year old ) boat I find this thread most interesting.

Can't add anything to what's been said , but one boat on the mooring had what the owner thought was microbial corrosion.It took the form of clearly visible bright orange patches below the waterline.When he blacked his boat he said these orange patches could only be removed with a wire brush. I have just noticed some small orange patches around the counter of my boat,but I have just sold it,so it's not really my concern anymore.

The boat that was badly contaminated was moored for several years in Ripon, and the owner seems to think that the contamination was picked up there.

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These hollows are not just some kind of depressions caused perhaps by welding stresses ? have they been checked out with a depth meter. 

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1 hour ago, matty40s said:

Not necessarily,  we had a boat out for survey earlier in the year with virtually nil pitting on the sides, baseplate had extensive pitting to depths of 5 and 6 mm.

I was only asking the question, and saying "if".  I realise the sides may not have the same problem.

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When I bought Vital Spark's bare hull in 1996 it was delivered into the water 'blacked' by the builder. It was about 3 years if not a bit more before it came out of the water for blacking. There was significant localized pitting on one side. 1 -1.5 mm. During that time it was moored alongside a new (1995) Colecraft that was used residentially. My boat was not permanently connected to the mains with all power for tools etc coming via a cable that was disconnected when not used. There were no 12 volt electrics, indeed no engine. Fortunately over the next few years I was able to fill in all the pitting with weld. Soon afterwards the Colecraft moved mooring and there was no more pitting. Strangely the Colecraft didn't ever suffer pitting itself.

Draw your own conclusions but I always put it down to the other boat.

I still take the boat out of the water every 12 - 18 months to check. Due out in about 2 months so fingers crossed.

 

Frank

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4 minutes ago, Slim said:

When I bought Vital Spark's bare hull in 1996 it was delivered into the water 'blacked' by the builder. It was about 3 years if not a bit more before it came out of the water for blacking. There was significant localized pitting on one side. 1 -1.5 mm. During that time it was moored alongside a new (1995) Colecraft that was used residentially. My boat was not permanently connected to the mains with all power for tools etc coming via a cable that was disconnected when not used. There were no 12 volt electrics, indeed no engine. Fortunately over the next few years I was able to fill in all the pitting with weld. Soon afterwards the Colecraft moved mooring and there was no more pitting. Strangely the Colecraft didn't ever suffer pitting itself.

Draw your own conclusions but I always put it down to the other boat.

I still take the boat out of the water every 12 - 18 months to check. Due out in about 2 months so fingers crossed.

 

Frank

in my opinion its not good to keep docking too regularly as keep exposing the baseplate to the fresh air oxygen will speed up rusting.

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Wow - thank you all for such a quick and detailed response.

 

First - a confession - I did not employ the services of a surveyor, being an engineer myself, so did all the visual inspection and measurements. Probably not my best call to date but there we are. Subsequent discussion with a surveyor suggested this was quite a common occurence - plenty of thickness left!

 

A little more history - first, I understand the boat has been moored for its entire life at an end-of-garden morring near Kidderminster - this would make it River Severn I assume. It has cruised of course but the majority of the time it will have been 'at home'. I also assume it will have had a shoreline connected for most of that time - no solar panels so shore power only. I have yet to see a GI on the boat. My thoughts at this stage are a combination of the presence of the shoreline and the absence of anodes through the centre section of the hull - but this is supposition, hence my shout out.

 

The boat hasn't been lifted for 5 years, since it was shot blast and received 2-pack epoxy to the hull sides - this has stood up well but has a small number of breakthrough points from physical damage - nothing serious and certainly nothing like the attack to the baseplate. The edge of the chine is the first place to show any colour, with rusty looking 'bubbles' - picture attached (I hope).

 

Underneath is a different story as the attached pictures will show. Thickness measurements showed 9.5 to 10mm generally. The depressions don't look like impact damage or distrortion, simply hollowed out areas where the [late is thinned. As you can see, they are smooth sided and look just like finger tip and thumb prints in rolled out pastry. The crust was fairly thick in places, maybe 3 or 4mm and was black against the steel. You can see where I have scraped off varying size sections of it. The pictures are obviously upside down due to my position!

 

I note the comments above on MIC and hesitate to acknowledge the orange growths I saw.

 

Your comments are invaluable - thanks.

DSC_6583.JPG

DSC_6596.JPG

DSC_6598.JPG

DSC_6602.JPG

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15 minutes ago, bizzard said:

in my opinion its not good to keep docking too regularly as keep exposing the baseplate to the fresh air oxygen will speed up rusting.

I wouldn’t be concerned by simply lifting it out and exposing it to air while sat up on beams since the natural rate of corrosion of steel in air is extremely slow; as opposed to tending to negligible in water.

 

What’s not such a good idea is to keep exposing fresh steel by removing the oxidised layer off the surface of the steel. This layer always occurs because the surface of steel is chemically unstable. Once it is present it gives a degree of protection to the underlying steel. Even then a cycle of once per year isn’t a big risk, it would probably require a continuous process of oxidisation and exposure to be of any great concern.

 

JP

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A Kidderminster mooring would be on the Staffs and Worcs Canal.

Why do you assume it had a shoreline? If the electrics are turned off, batteries will stay charged from one trip to the next. A boat with self draining decks and/or suitable covers doen't even need an auto bilge pump left switched on.

 

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2 minutes ago, David Mack said:

A Kidderminster mooring would be on the Staffs and Worcs Canal.

Why do you assume it had a shoreline? If the electrics are turned off, batteries will stay charged from one trip to the next. A boat with self draining decks and/or suitable covers doen't even need an auto bilge pump left switched on.

 

Good point well made - I just assumed this was likely with it being a 'domestic' mooring.

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16 minutes ago, Oldngrumpy said:

I note the comments above on MIC and hesitate to acknowledge the orange growths I saw. 

As I previously said, I have no personal experience of MIC but your pictures show a very strong similarity to those found on line giving examples of MIC.

 

MIC doesn't "get better" without the correct treatment, if it was mine, for peace of mind I'd have it examined by someone who knows about MIC (and other types of corrosion)

 

Whilst it may be 'alarmist' I'd want to know  (extract from an article)

 

Untreated, within a year or two – it can be in a matter of months in very serious cases – all that may be between afloat and sunk could be your blacking. Not a particularly comforting thought.

MIC is a highly unpredictable process but the corrosion progress can be rapid, serious damage happening in as little as a matter of months. The oil, gas and mining industries spend millions protecting and repairing their pipelines and equipment but whilst the threat is the same, little is understood and even less done about it in the freshwater boating world.

MIC can destroy steel at four times the rate of electrolytic or galvanic corrosion and must not be ignored.

 

Web-081-keelblack-rusticles.jpg

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We have just bought a boat that has been diagnosed with MIC and two of the trip boats on our canal (unconnected section) have also had it. There is a school of thought that says that it is becoming more prevalent because the water in the canals is becoming cleaner (or perhaps rather than cleaner - subject to fewer toxins and instead full of run off from farmers fiels with many nutrients to encourage bacterial growth. 

 

MIC does not require oxygen t grow and so rather than the tell tale line of corrosion along the waterline of a boat that has not been blacked regularly it will attack the base plate which is often not protected. 

 

I have to admit (although I am no expert) the photo of your baseplate does not look like the MIC I have seen on 3 boats. Having seen it on the trip boats when our pre-purchse survey was done I walked straight over to a bright orange bubble of corrosion on the hull (that had been out and blacked as recently as last December) poked it with my finger and immediately asked our surveyor if it was MIC - he was astounded I even knew what it was!!! 

 

Usually MIC will manifest itself over a larger area rather than just a small pit. The corrosion will be a very bright orange and, if scraped off it leaves bright silver scab behind it. 

 

Thankfully the corrosion on our hull has hardly touched the thickness yet and we are booked in for it to be treated next month 

 

I will see if I can find the photos of our hull to share to give a comparison 

  • Greenie 1

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