Localised corrosion on steel boat

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alan006

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I have got my steel boat in dry dock at the moment and was very surprised to find that there is one small area about 3ft square where there has been severe corrosion and I am getting a new steel plate welded in. The rest of the hull was very good.
I asked the welder what might have caused this and he thinks it was caused by a pair of pliers that had fallen into the bilges.
Does this sound likely? The pliers were very pitted.
Could some other problem have caused this? I want to be sure that when the new plate is welded in the problem does not reoccur.
 
No, it is not near the heads or shower. The corrosion is on the inside, the outside appears fine.
I'm putting extra anodes on so hopefully that will help in the future. It just amazes me that a pair of pliers could do so much damage.
 
What was the condition of the anticorrosive coating in the area before the rusting? Was it different from the surrounding area? If it has broken down there through age / different system to surroundings etc the problem may have been there for some time but only just come to light.
Is there any wiring in the affected area? If so has it frayed and shorted onto the steel?
What were the pliers made of?
Your bio talks of old boats. If this is old and the anodes have been sufficient before you may create problems by adding more.
I suggest you find the cause of the problem before attempting corrective action
 
The pliers explanation is not very credible. Assuming the suggestion to be galvanic corrosion, the pliers would need to be made from a metal more noble than the steel hull, such as copper, gold or something similar, and be completely immersed in water together with the affected area of the steel. Since you say this was inside the hull it doesn't seem likely.

Far more probable is that adhesion of the protective paint film was poor and the paint has had air beneath it. Once this gets damp, especially with salt water, it will never dry and will corrode rapidly. This is how cars used to be before modern protection systems became common. Large areas of paint would flake off, leaving heavy corroded areas beneath.
 
Could it have been chocked up in that area? or possibly the people that painted it ran short of paint and thinned it.
 
It is just possible that this problem has occurred over time and only just been discovered.
The boat is a coded workboat and had a full ultasonic survey done about 2 years ago where it got very good readings.
If the boat was blocked on the bad area it is just possible that the surveyor could not get to it to take a reading....if that was the case in a way I'm happy because once repaired there is no reason for the problem to reoccur.
I'm not a steel boat type person all my boats have been GRP or timber, so I am curious as to if the pliers theory seems reasonable. It just seems very surprising to me that they could cause such a big problem.
The pliers are just standard steel pliers.
There seems to be no difference between that part of the bilges and the surrounding areas.
I can't see any signs of damaged wiring in that area.
I will check to see if there is any evidence of battery drain, but I would have expected that if present to attack the hull evenly not in such a localised way.
I'm surprised to read that excess anodes could cause a problem, I assumed that extra could only help.
 
I think you are probably right.
The stern gland was weeping so there was always a couple of inches of salt water in this part. If it was badly protected by a poor paint job then I'm sure this would do what you suggest.
I think it must have been going for some time but been missed on the last survey because it was on a block.
I'm relieved ( in an odd way) because once replaced and properly painted it should not reoccur.( I hope).
Thanks for putting my mind at rest.
 
Having had steel boats for many years, you must keep the bilges free of water at all times .You say the stern gland was leaking,this is bad news especially salt water. Could you possibly fit a Volvo seal? as you will not get any leakage then.The plier idea is not likely,The anodes will only help corrosion on the outside of the boat.
 
Thanks for your suggestions. In this case I don't think a Volvo seal could be fitted. I'll have to repack the sterngland properly while she is in the dock.
What paint do you use for your steel bilges?
 
One thing definitely did allow the corrosion - poor paintwork on the steel. Had there been a continuous film of solid paint there could have been no corrosion.

With poor paint there are all sorts of possible initial triggers that can accelerate corrosion but a pair of steel pliers isnt one that comes to mind.
 
[ QUOTE ]
With poor paint there are all sorts of possible initial triggers that can accelerate corrosion but a pair of steel pliers isnt one that comes to mind.


[/ QUOTE ] But could the pliers have been dropped from sufficient height to have damaged the paint coating locally.


Anodes? external anodes have no relevance to corrosion that has occurred internally, are we therefore talking about internal anodes in the bilges?

As far as having too many anodes goes, the answer is yes I believe you can on a steel hull. "Over-protection" can lead to blistering and failure of the paint coating.
 
Thanks for all the suggestions everyone.
I'll make sure the repaired area is well protected with a good paint job. I think the pliers are not likely to be a major factor in this situation, as people have said if its painted properly then job done.
 
Another possibilty. The pliers might have been dropped in a place where they moved about in a limited area with the movement of the boat at sea, and scraped through the paint in that area as they slid about.
 
I'm putting extra anodes on so hopefully that will help in the future
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Putting extra anodes on will not help internal corrosion unless you plan to flood the area and fit the anodes internally. This I would advise against .. /forums/images/graemlins/laugh.gif

Anodes only work when they are imersed in water and only protect steel or other metals which are also imersed in the same water. Anodes on the outside of the boat do not protect the steel on the inside of the boat!!!!!!

It would seem you need a good paint protection and also not have inches of sea water swilling around.

You should be able to get the glands reasonably watertight by repacking and ensuring the greasers are working.
 
[ QUOTE ]

As far as having too many anodes goes, the answer is yes I believe you can on a steel hull. "Over-protection" can lead to blistering and failure of the paint coating.

[/ QUOTE ]

Thats interesting Vic . How does that work?
 
When I had my first steel boat it had magnesium anodes(fresh water), I used to notice that the paint in certain areas on lift out had gone, to be replaced by magnesium oxide.When I bought my first Dutch Steel boat I noticed the anodes were aluminium, I asked why and they said magnesium were too reactive and tended to effect the paintwork, so as Vic S said it is possible that if you have to many anodes you could effect the paintwork.
 
nope

pliers wouldn't
I'd go for the stress put into the steel sheet in that area when it was being built either by poor lay out and to much welding or by the plate being bent and or pushed about causing the steel to be stressed
I'd use cold tar epoxy to re paint it, you will need to do it when it's not so cold
re anodes
what were your old ones like
were they situated in the right places
Use the correct type and what's recommended re anodes, me I prefer weld on because bolt on jobs are just a pain

As for not allowing water in through the stern gland !!!!!!!!!!!
if you over tighten the packing and not let a slight amount through you will damage the shaft, the importance of this I cant stress enough
cheers
Mick
 
[ QUOTE ]
Thats interesting Vic . How does that work

[/ QUOTE ] Not at all sure about this but I think because ordinary paint films are not entirely impervious to water and (therefore) do not present a total barrier to a current flow, with sufficient "driving force" the pH close to the surface rises, that itself being detrimental to the paint, and possibly bubbles of gas (hydrogen) can be formed under it.

I don't know if this can really happen with just sacrificial anodes but I believe it can with impressed current systems if the voltage applied to the anodes is set too high.

May be complete rubbish but nedmin's experiences with Mg anodes suggests there is potential for some problems although the deposit he observed was probably calcium (and magnesium) carbonate. I'd expect the problem to be confined to the area close to the anodes or to any areas where the paint film is thin or damaged.
 
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