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Baseplate corrosion concern


Oldngrumpy

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The order of the first two photos is back to front. The photo above is what the patch of MIC you can see in the photo below looked like after it had been scraped with a blade. There is no traditional "rust" behind the orange - just bright metal  - the corrosion is thought to consume the sulphur in the steel and so some higher sulpher content steels may be worse affected than others. 

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it was bubbles like this that drew my attention straight away, this hull was only blacked in December 

 

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The last photo is of the baseplate. It isn't anything like as scary as it looks as the pits are less than .5ml It is a 12ml baseplate so, as we have managed to secure an early docking to get it sorted next month we have nothing to worry about - apart from an unexpected bill! Sadly whoever blacked the hull in December had not heard of MIC and so now we have a boat where the contamination is potentially trapped behind the fresh blacking It will be treated with bleach and grit blasted next month 

Edited by cheshire~rose
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I have to say it looks to my untrained eye like the same thing, just not in such an advanced state. The entire centre section of the baseplate I looked at yesterday was covered in very similar 'growths'. My state of depression is deepening. Will hit the phone now to sound out local boatyards.

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54 minutes ago, Oldngrumpy said:

I have to say it looks to my untrained eye like the same thing, just not in such an advanced state. The entire centre section of the baseplate I looked at yesterday was covered in very similar 'growths'. My state of depression is deepening. Will hit the phone now to sound out local boatyards.

 

Good luck - at least if it is MIC you have caught it 'early'. Another few months and it could have been a different story.

If it turns out not to be MIC - at least you'll know.

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Don't be depressed! Corrosion is corrosion after all. All boats get it, it's just a matter of ensuring the correct treatment so it is halted in its tracks. 

Did you scrape any off and if so was it shiny behind?

 

The treatment is just bleach, applied and left on for 24 hours before washing off. Not all boatyards will do it because if their hardstanding drains directly into the cut they won't want neat bleach going in there. 

We are going down the route of blasting for two reasons. Firstly because we want to be certain no MIC is trapped behind the recently applied blacking. Secondly because we want to apply 2 pack. 

It's possible a thorough industrial jet wash followed by bleach treatment will sort it, if it is MIC but you would do well to get the cause properly identified first

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Hasn't 'Keelblack' had a lot of bad reviews. I had a long chat with them at last years Crick & came away  with nightmares, envisioning this steel eating microbe munching it's way around the canal system.

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7 minutes ago, cheshire~rose said:

It's possible a thorough industrial jet wash followed by bleach treatment will sort it, if it is MIC but you would do well to get the cause properly identified first

 

 

The bacteria are often found inside oxidised welds or in areas which contain physical defects such as porosity, overlap or poor weld penetration. They can cause corrosion from beneath existing blackings or seek out pinpricks in the coating and cause the reaction to occur from the outside.

Sulphite films are highly corrosive. They are usually found under muddy and slimy surfaces, often behind paint coatings and a careful inspection is necessary to locate them.   MIC cannot be discovered by non-destructive testing such as ultrasonic thickness measurement or other methods familiar to most yards and marine surveyors. The only effective method is visual.

It is also worth noting that MIC cannot be treated with rust-preventatives neither is iteradicated by simple power washing. Whilst the outward physical signs may disappear under a high pressure lance, the microbes can continue to thrive beneath previous blackings unless correctly treated.

Out of sight is most definitely not out of mind.

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.

Final finishing

If the pressure washing has exposed areas of bare steel, it is recommended that a good quality zinc-phosphate rust prevention system be applied. This should be allowed to work over a 24-hour period and must be thoroughly washed off with water and a brush to ensure that only the bare steel retains the treatment before any blacking top coat is applied. This is essential to ensure that any subsequent coating properly attaches to the hull.

Nevertheless, the microbes can still live underneath remaining adjacent blackings so the only certain way to arrest the process and reduce the risk of future attacks is by blasting back to bare steel – an expense many owners may not wish to contemplate.

Edited by Alan de Enfield
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No - not bought it yet - sitting on the fence until I know for sure what the risks are and what the bill is likely to be. Any suggestions for a suitable place to get the work done? Ideally south of Birmingham, closer to London or even Gloucester would be best.

 

Thanks again - all of you.

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9 minutes ago, Alan de Enfield said:

Good point - it would appear not.

 

So its 'walk-away time' or get £1000's knocked off the price.

I agree.

 

While I do find the general narrowboaters’ view that steel baseplates will rust through and sink their boat somewhat akin to the Gaulish villagers’ fear of the sky falling on their heads this one is different.

 

JP

Edited by Captain Pegg
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3 minutes ago, Oldngrumpy said:

No - not bought it yet - sitting on the fence until I know for sure what the risks are and what the bill is likely to be. Any suggestions for a suitable place to get the work done? Ideally south of Birmingham, closer to London or even Gloucester would be best.

 

Thanks again - all of you.

The further South you go the more expensive it becomes.

 

I bet the difference between  the North Midlands & 'London' is 2x.

 

Just phone up a few yards and ask them for a price to Sand blast back to bare metal, bleach soak & wash, and then twin pack epoxy coating.

 

Just a guess but I reckon £5,000 might cover it.

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I'm from the black the base plate camp and apply blacking every two years to my boat . In this case I suspect it is relevant  that the microbes  haven't attacked the sides because they were blacked .  I also believe that the reason most people do not black the base plate is because it is a pig of a job crawling underneath and many dry docks do not lift the boat high enough to be able to get underneath . Having said that a 10mm base plate will take a while to be 'eaten through'. 

In the pictures that Cheshire Rose has added it looks as if the microbes have crawled under the rubbing strake . That again highlights best practice of having the underside of the rubbing strake continuously welded. 

In the past those from the no need to black the base plate camp have claimed  lack of oxygen underwater  as the reason blacking is unnecessary ,I wonder what they will say knowing these microbes thrive without light and oxygen. 

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On 10/07/2019 at 17:48, Troyboy said:

In the past those from the no need to black the base plate camp have claimed  lack of oxygen underwater  as the reason blacking is unnecessary ,I wonder what they will say knowing these microbes thrive without light and oxygen...

... and love living beneath blacking.

 

Your argument doesn’t stack up, sadly. 

On 10/07/2019 at 17:48, Troyboy said:

In the pictures that Cheshire Rose has added it looks as if the microbes have crawled under the rubbing strake

Which was no doubt thoroughly blacked. 

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On 10/07/2019 at 16:39, Alan de Enfield said:

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.

 

That's a good argument for not blacking the baseplate in the first place. Another one is that on shallow canals (ie most of them) you polish the baseplate as you cruise along, so any surface treatment would get scraped off (yes, epoxy too).

 

I'm with you in the "don't black the bottom" club.

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On 10/07/2019 at 08:11, Alan de Enfield said:

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

 

 

Yes, because baseplates don't corrode, we all know that! ?

On 10/07/2019 at 00:26, Oldngrumpy said:

 

  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.

 

I agree with the others, but I can't help with question 4.

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On 10/07/2019 at 11:39, Mike the Boilerman said:

 

Ok, so 2.7mm lost in 11 years. 

 

That means the boat is scheduled to sink in 29.7 years from now.

 

How old are you? 

 

 

 

This assumes that the future corrosion occurs at the same linear rate and doesn't speed up (or slow down), and also implies that nothing needs to be done about hull corrosion until the boat is about to sink. 

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On 10/07/2019 at 12:01, Slim said:

 

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

 

 

I don't draw any conclusions from your post. There are too many other variables that you haven't described in your post to put the cause of your pitting down to the other boat.

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On 10/07/2019 at 17:25, Alan de Enfield said:

 

Just phone up a few yards and ask them for a price to Sand blast back to bare metal, bleach soak & wash, and then twin pack epoxy coating.

 

 

 

From what I read in your previous post and the other one earlier in the thread, if you're blasting back to bare steel you don't need to bleach it. 

 

If you're grit blasting then ideally you don't want to get the steel wet. Some 2 pack epoxies are surface tolerant and can go over flash rust, but it's a bad idea to initiate any level of corrosion prior to epoxying if you don't need to.

Edited by blackrose
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On 10/07/2019 at 17:48, Troyboy said:

In the pictures that Cheshire Rose has added it looks as if the microbes have crawled under the rubbing strake . That again highlights best practice of having the underside of the rubbing strake continuously welded. 

 

 

Both sides surely (if I understand you correctly) ? If the underside is fully welded and the top side isn't then any gaps behind the rubbing stake will just fill with water.

On 10/07/2019 at 17:48, Troyboy said:

 

In the past those from the no need to black the base plate camp have claimed  lack of oxygen underwater  as the reason blacking is unnecessary ,I wonder what they will say knowing these microbes thrive without light and oxygen. 

 

Indeed, but as others have suggested, what will those who've been proponents of blacking the baseplate say knowing that these microbes can thrive underneath ordinary blacking which simply hides the problem so that it goes undetected?

Edited by blackrose
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2 hours ago, blackrose said:

Both sides surely (if I understand you correctly) ? If the underside is fully welded and the top side isn't then any gaps behind the rubbing stake will just fill with water.

It looks like they can 'live in weld' irregularities as well.

Pretty aggressive little blighters !!!

 

The bacteria are often found inside oxidised welds or in areas which contain physical defects such as porosity, overlap or poor weld penetration. They can cause corrosion from beneath existing blackings or seek out pinpricks in the coating and cause the reaction to occur from the outside.

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This three letter acronym corrosion thing (I hate three letter acronyms, or TLA's as I call them):
Does it only live in fresh water, or is it found in salt waters? Does salt water kill it?

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