Pitting on 15 year old boat.

[sailor0500]

In the process of blacking my hull. It's 2 years since last blacking. There was some pitting of side plateing last time but seems to be a bit more extensive now. The pits look bad in the photo but in fact they are .5mm deep when measured from the mill scale with the odd one .75mm. so still 5.5mm plate left. I will grit blast and epoxy next time out of the water but just wondered if this amount of pitting is unusual on a boat of this age. Odd thing is bow and stern plates as new with mill scale intact. Galvanic isolator fitted and checked and side anodes fitted.

Edited August 9, 2020 by sailor0500

[PhilR]

4 minutes ago, sailor0500 said:

I but just wondered if this amount of pitting is unusual on a boat of this age.

You forgot to state the age of the boat

[sailor0500]

Sorry I thought I had said it's 15 years old.

[PhilR]

Apologies. I must remember to read the topic header!

[blackrose]

If the boat was blacked 2 years ago and the pitting has got worse in that time then it does make one wonder how well it was blacked. I'd expect the blacking would have started to come off around the waterline after 2 years but you should only have a bit of surface rust on that area, not pitting all over. I guess your anodes are at the bow and stern which might explain why those areas are ok?

Edited August 9, 2020 by blackrose

[Bee]

I wouldn't say its that unusual, 15 years is maybe a third of the life of some steel structures but it certainly shows what owners are up against. I am not an expert but I think that pitting can be caused by tiny electrical differences within the steel, a tiny piece of scale or dissimilar metal rolled in the steel will start a reaction. There is a line of weld in the pic that seems to have suffered and some of the pits seem to be in lines. Others will have better knowledge than me. Thing is though, how do you stop it? The photo doesn't seem to show any paint still on the steel so pretty much unprotected, the pits will almost certainly be shiny and active. I have always assumed that paint on the pits will 'turn them off', Epoxy is probably the best paint and shotblasting will give a good surface to stick to but you will not want to shotblast it every three or 4 years so lots of research is a good idea. I put 3 or 4 coats of a 'surface tolerant' Jotun on my boat 3 years ago and so far as I can see its still OK, it was pressure washed and I sanded the old paint (epoxy) to roughen it up a bit (cheap sander abused and binned) The bottom needs doing too, It corrodes despite what some people say. If it didn't boats would not need re bottoming.  Good luck.

[blackrose]

I assumed the blacking had been removed before the photo was taken?

 

Bee, I'm not sure if this is what you meant but you should only need to grit blast a hull once before epoxying. Next time it comes out the water just abrade the existing Jotun and put a couple more coats on. Mine was last done 5 years ago and it's still looking good. If it lasts 3 more years before needing re-doing I'll be happy.

 

My 15 year old baseplate still looked fine when I last looked at it 5 years ago. All I could see was a bit of surface rust that brushed off by hand. 

Edited August 9, 2020 by blackrose

[dor]

As your location is 'Cheshire East' there is a good chance you are on the Shroppie.  the notorious "Shroppie Shelf" will take its toll and leads to blacking being scraped off and allowing the consequent corrosion.

 

I have seen a lot of boats taken out of the water on the Shroppie system and even those with good two-pack have scraped through to the underlying metal.  This will give rise to corrosion, but it does not in itself lead to deep pitting.

[sailor0500]

I gave the boat six coats of bitumen 2 years ago onto well cleaned steel and when power washed last week everything below the waterline came of. The photo was taken today after cleaning down with angle grinder and 4" wire cup brush. Will now apply six coats ( first two by brush to stipple paint well into pitts) relaunch and see what happens but I guess it must be blasting and epoxy next time. Bitumen does not seem to do the job and the side anodes do not seem to help..

Edited August 9, 2020 by sailor0500

[matty40s]

If everything came off underwater with a powerwasher, it means it didnt adhere itself to the steel correctly. What time of year was it done last time??....and what blacking did you use?

The corrosion looks like classic galvanic corrosion, this seems to be getting worse each year, (especially in some marinas), and I'm not sure Galvanic Isolators give an awful lot of protection.

[Paringa]

I was just going to say are you in a marina? Moored next to metal piling? Plugged into shorepower?

Edited August 9, 2020 by Paringa
I see you have a galvanic isolator

[blackrose]

33 minutes ago, matty40s said:

 

The corrosion looks like classic galvanic corrosion, this seems to be getting worse each year, (especially in some marinas), and I'm not sure Galvanic Isolators give an awful lot of protection.

 

Can you explain the difference in appearance between classis galvanic corrosion and normal pitting because I've no idea and I assume most others don't either.

 

As long as it's working correctly a galvanic isolator should give 100% protection against galvanic corrosion. If the mains earth lead is isolated how could it be otherwise? It's either in circuit or it's not. If it is then the GI isn't working properly.

 

A GI is doing exactly the same job as an isolation transformer. It's argued that an IT will do the job more reliably, but as long as the GI is working then then there's no difference. Both either isolate the mains earth or they don't, i.e. they either work to block 100% of galvanic currents passing through the mains cable or they don't work and block 0%.

Edited August 9, 2020 by blackrose

[Tony Brooks]

Except given a high enough voltage a GI will be forced into conduction where as there is no connection between the supply side earth and the boat hull bond with an IT.

[Tracy D'arth]

33 minutes ago, blackrose said:

 

Can you explain the difference in appearance between classis galvanic corrosion and normal pitting because I've no idea and I assume most others don't either.

 

As long as it's working correctly a galvanic isolator should give 100% protection against galvanic corrosion. If the mains earth lead is isolated how could it be otherwise? It's either in circuit or it's not. If it is then the GI isn't working properly.

 

A GI is doing exactly the same job as an isolation transformer. It's argued that an IT will do the job more reliably, but as long as the GI is working then then there's no difference. Both either isolate the mains earth or they don't, i.e. they either work to block 100% of galvanic currents passing through the mains cable or they don't work and block 0%.

GI gives protection up to about 1.2 volts, over that it conducts.  Is yours still working though?  May be worth finding out.

[sailor0500]

Yes I am moored in marina with shore power. Regularly check the diodes on galvanic isolator. Wooden jetty on steel pilings.

[sailor0500]

Anyone know how to check for stray currents in marina shore power. I know this is a very complicated subject but maybe someone knows of a simple method.

[BEngo]

There are no simple methods.  That is why it is a very complicated subject.

 

The OP pictures show a perfectly normal piece of steel that has been in water with inadequate protection  against corrosion generally.

There might be a PhD or an EngD in investigating and explaining the how, the why and the wherefore. Not that that will change much.

Shotblast it and epoxy it and the problem will go away. 

 

Until the coating  is damaged.

 

N

 

 

[Bee]

2 hours ago, blackrose said:

I assumed the blacking had been removed before the photo was taken?

 

Bee, I'm not sure if this is what you meant but you should only need to grit blast a hull once before epoxying. Next time it comes out the water just abrade the existing Jotun and put a couple more coats on. Mine was last done 5 years ago and it's still looking good. If it lasts 3 more years before needing re-doing I'll be happy.

 

My 15 year old baseplate still looked fine when I last looked at it 5 years ago. All I could see was a bit of surface rust that brushed off by hand. 

Yes, that's what I meant, wasn't very clear though. You are right. Once is enough.

[blackrose]

58 minutes ago, Tracy D'arth said:

GI gives protection up to about 1.2 volts, over that it conducts.  Is yours still working though?  May be worth finding out.

 

Mine gives protection up to 2.4v and yes it's been tested regularly over the past 14 years and it's working fine.

Edited August 9, 2020 by blackrose

[bizzard]

Six coats !!!!.  that would need about a fortnight in the dry dock, in the summer if adhering to the correct drying times. I reckon the first 5 coats were still soft and indeed the sixth coat was when dunked back in the water, that's why it blew off easily, the hull may have been damp too.

[beerbeerbeerbeerbeer]

11 minutes ago, bizzard said:

Six coats !!!!.  that would need about a fortnight in the dry dock, in the summer if adhering to the correct drying times. I reckon the first 5 coats were still soft and indeed the sixth coat was when dunked back in the water, that's why it blew off easily, the hull may have been damp too.

I thought 6 was a lot. 
 

Mine gets just 2 coats, every two or three years. 

 

[sailor0500]

I was out of the water for a month. Off to bed now. Very tireing day again. Thanks for your replies everyone.

Edited August 9, 2020 by sailor0500

[bizzard]

11 minutes ago, sailor0500 said:

I was out of the water for a month. Off to bed now. Very tireing day again. Thanks for your replies everyone.

Ok.  I beg your pardon.

[sailor0500]

10 hours ago, bizzard said:

Ok.  I beg your pardon.

No problem bizzard

[Keith M]

There is growing evidence that a GI can be switch on by stray current within the supply circuit.

The correct solution is either a Isolation or Polarization  Transformer.

 

Check out the Smartgauge web site.