And even bigger trouble: Keelband Pitting Corrosion

[Llafurio]

I had thought it only happens to guys with coppercoated hulls: Found pitting corrosion on my own boat's keelbands and fastening screws. Now that is serious, and I do not know how I can fix myself such a major defect.
I found it after my centreboard uphaul rope had torn, and the board had swung violently down and forward until hitting the forward wall of the centreboard case opening between the keelbands.  BANG!! There must have been a mighty shock on the pivoting pin also, and on the keelbands holding it in place.

So I inspected the keelbands which hold the board pivoting pin, and right, both keelbands are bent and bulging out down in the area of the pivoting pin. And, to my dismay, there is now also pitting corrosion of the keelband in that area, which was not there when I launched the boat in April. The two parallel stainless steel keelbands are what -only- holds the centreboard in place, and from sinking out of the cboard case and boat towards Nirvana. Rotting keelbands, I find that shocking, my boat is just a few years old, and spent most of that time in its shed, and in Rossiters Boatyard in Christchurch.

And no, there is definitely no electric current nearby my boats mooring, this is a remote tidal creek on the South coast of Ireland.

More pitting corrosion on other boats also? Wrong stainless steel unsuitable for seawater?
A repair, and a longterm reliable solution must be found.

[Sea Simon]

V sorry to hear this, bad news indeed.

This is your modified BRe, Homer?
How does the boat sail number/production dates compare with the other corrosion victims known, as detailed on here?

"More pitting corrosion on other boats also? Wrong stainless steel unsuitable for seawater?."
Seems a reasonable theory, in the scenario you describe.

Possibly a "bad batch", "counterfeit" or likely a wrongly labelled/miss-sold steel stock supply (in my experience at work).
In one particulalry crtical, and problematic application, its been necessary to resort to getting our own steel alloyed and cast (in France, not China!), witnessed by our independent surveyor, then all billets stamped up and fully traceable/auditable right thru production....and delivery to site. Expensive and very time consuming!
 
Afaik, the only way to fully understand what has happened in terms of what EXACTLY was fitted new, will be to take steel samples, and get them analysed by a "test house" or metallurgist? This may not be as difficult or expensive as you might think, but it will be a significant cost wrt this project cost.

Please keep us posted.

[Llafurio]

Plenty of info on the internet about stainless steel pitting corrosion. Key agent is chloride, and main driver is electrochemical current between one anode and one kathode.
Plenty of chloride in seawater, and some antifoulings contain copper, like mine, and there is the agent for electrochemical reaction, the driver.

From what I read, pitting corrosion occurs when the area of anode or kathode is small, so that the electrochemical current attack is not spread over a large area. Maybe I should not have antifouled the keelband(s) so thoroughly as this year but should have left the keelbands blank from antifouling. C.

[mainecoast]

I too had the same issue with Corvus back three or four years ago.

I had contacted Swallow.  Unfortunately, at that point they had not seen it.

Background: I keep Corvus in a marina, but don't have shore power. 

My only thinking was that while I had primed both hull and rub strakes, perhaps I did not coat the strakes as thoroughly as I should have and  somehow the copper in the antifouling on the hull reacted with the stainless. 

So, I took the strakes down to base metal.  Reprimed them.  Healthy amount for stainless.  Then I used a separate antifouling rated for metal on the strakes.

Over time that has seemed to minimize the plague, but it requires diligence and makes each spring something I dread as I have to go through the priming and touchup with more attentiveness than I'd prefer.

I'm honestly not sure if the cause was Micron CSC reacting with the stainless, but when I took the strakes to base and applied the primer things have gotten better.

I feel you on this.

Pete

Corvus, BC23 Hull 37

[Jeremy Apted]

I spoke to a technical person at Akzonobel recently as I have not yet decided on whether to anti-foul my BC 23.
He offered two options; International Ultra 2 being a hard anti foul or Micron Extra 2 which, with its polishing technology,. provides a continuous renewal of the active layer of the paint. Preparation of the underside is: 1. Sand back to the primer (with 80 grit paper angle grind the s/s rub strakes. 2. Apply Sikaflex after sanding to fill the gap between the s/s and the hull. 3. Apply 5 coats of Interprotect Primer including over the s/s. 4. When the last coat of primer is thumb print dry, apply the first coat of anti foul.

He thought it was particularly important to seal the gap between the s/s and hull with Sikaflex before applying the epoxy primer.

The anti-fouling (soft or hard) are also compatible so you can switch if you change your mind.

Hopefully this will allay the corrosion of the s/s.

[Sea Simon]

This thread may be worth a revisit?
Topic: BRe - Lifespan of Self-Bailers
https://www.swallowyachtsassociation.org/smf/index.php/topic,1954.0.html

[Llafurio]

Some photos here. The more I look the more pits I find. Plenty more .

[TimLM]

Thinking aloud:
a lot of the corrosion is around the screws and if they were not of the same alloy ratio would this affect the corrosion.
Does not stainless steel remain stainless due to the presence of oxygen as a gas so land based boats might not be so badly affected? Admittedly mine is mostly out of the water and sometimes in fresh water so is no comparison, but I would be inclined to pull it out regularly and jet wash the hull. Yes I know it helps having an electric winch but I am a grumpy old man, hurrumph.
 

[Sea Simon]

Thinking aloud:
a lot of the corrosion is around the screws and if they were not of the same alloy ratio would this affect the corrosion?

Does not stainless steel remain stainless due to the presence of oxygen as a gas, so land based boats might not be so badly affected?

1. Correct.
2. Also correct (aluminium alloys are broadly similar).
Yes, However there is normally enough dissolved oxygen in non-stagnant water to satisfy the needs of the SSteel.
But, even "the best" SSteels can suffer badly in stagnant water, as seen around stationary pump/prop shafts, valve glands etc.
Hence some commercial applications for Monel, Titanium etc...

Similar but different?
 We seem to have suffered an unusually bad fouling season here this year. There also seems to be more mud/silt in the river? The hull itself is very slimy, thick with soft green weed/algae around the waterline. I have just scrubbed off for the second time this year.
My (factory applied) Coppercoated bottom does not cover the SSteel keel bands etc.
The keel bands are all now heavily fouled with small barnacles. I spent some time this week scraping off what I could reach.
Relevance? I fear that the barnacles could prevent oxygen reaching the SSteels. We will see at haul out...

Llafurio...did you resolve your issues?

[Llafurio]

Llafurio...did you resolve your issues?

No. I took the boat out, and will remove all the antifouling from the s/s keelbands, I have a theory that covering the keelband completely with antifouling was the mistake, and that the removal of the antifouling will stop the pitting corrosion from digging in deeper.

BTW it is not the different alloys of screws and keelband which causes the pitting corrosion. The pitting corrosion is powered by the copper plus chlorine --> chloride electrochemical eaction on the s/s, and the obvious points of attack are where the s/s is strained / stressed most by screws screwed in hardest, or, for instance where the c/board pivoting pin exerts force and strain on the keelband. In metallurgy it is a common feature that mechanical stresspoints are attacked first.

[Sea Simon]

Will be interesting to see how your keel bands fair now the paint is stripped off?

Will update re mine, underneath the barnacles, once I'm hauled out.

Wrt your corrosion, imho, you are probably seeing forms of both galvanic or crevice (across adjacent different metals, fastener/keel band)  and also pitting driven/activated at the stress points you describe.
This is thought to be because of the flex or movement in the materials repeatedly breaking the protective oxide skin/crust on the metal surface as they move/flex.

...not that these thoughts really offer you any solution!

I sometimes think there is a large helping of "art" alongside the science of corrosion....

[Sea Simon]

Will be interesting to see how your keel bands fair now the paint is stripped off?

Will update re mine, underneath the barnacles, once I'm hauled out.

I sometimes think there is a large helping of "art" alongside the science of corrosion....

Hauled out, scraped off.
Fouling is the worst I've seen in 25 years here.
Proper full on weed and mussels growing on the bailers, and the outboard opening plastic frame, right up under the stern.
Stainless predominantly fouled with barnacles, some the size of my thumb nail.

But NO CORROSION!
Odd, but relieved!

Now to the centreboard lift pulley project.....

[Llafurio]

Just to remind all BR and BRE owners to be on the lookout for signs of pitting corrosion on the stainless steel (s/s) keel bands and bilge runners.
And, as a precaution against, NOT to paint antifouling with copper content such as International Offshore VC20 over same.
I am still not certain about the quality of the s/s bands, whether they are ordinary s/s or marine grade, but it appears that also marine grade s/s can develop destructive electrochemical pitting corrosion from copper chloride.
CR

[JSFolk]

I don't have a Swallow Yachts boat, so I'm not familiar with the size/shape of these pieces... are they complex pieces? Could they be replaced with bronze?