Corrosion or something else on Keels?

[CentaurSailorMan]

Hi all,
Just had the boat out for the winter and noticed some fairly large areas of
corrosion at the top of one keel. They were full of what looks like black iron
oxide and are about the size of a 2 pence piece and about 5-10 mm deep. They are quite smooth and it has been suggested they could be due to galvanic corrosion. pic attached (i think)

Does this seem like a possible explanation? Has anyone else had something
similar. I have cleaned them out and painted with fertan ( the pics are after
this) and plan to fill with epoxy filler.

My concern is whether ther may be other damage (as yet unseen) damage caused.
The keel bolts do show a little rust but dont seem to bad.
This has happened in the last 6 months.

Any views would be very much appreciated.

 

[vyv_cox]

It's a common problem with Westerlies. They are a casting defect, gas bubbles that float to the surface when the keels are cast (on their sides). See some similar photos on my website, under Metallurgy > Corrosion. May I nick your photo for the site, please?

 

[CentaurSailorMan]

The pics on your site do seem to be very similar but I am wondering why they have only just appeared now after about 40 years and in a number of places at the same time. The boat was pressure washed when it came out but from where I was standing there seemed to be a large amount of red rust running down the keels from this area before this was done so I dont think this disturbed the area.

You are welcome to use the pics. Below is a pic of the only other area with this problem.

It's a common problem with Westerlies. They are a casting defect, gas bubbles that float to the surface when the keels are cast (on their sides). See some similar photos on my website, under Metallurgy > Corrosion. May I nick your photo for the site, please?

 

[Robin]

Could this be where someone in the past had ground out a bad bit, then filled the holes and the filler has now become detached? The area seems to have some kind of paint on it?

 

[Seajet]

CentaurSailorMan,

whereabouts is your boat based ?

I had corrosion on my 1st lift keel, then discovered there is a known problem with 'accelerated corrosion' in Chichester Harbour, reaching to Portsmouth; I am not alone in suspecting leakage from nearby power cables.

Then again the 'expert' lecturer who came to look solemnly pronounced the pitting in the steel was caused by " the speed of the boat through the water" !!! - I was quite flattered, but made a mental note to avoid anyone this character had taught...

 

[CentaurSailorMan]

We are based in chichester harbour, up at emsworth. I ahve been told that another boat there may have a similar problem but haven't had a chance to look yet.

I always thought that if the keels are not connected to any thing then galvanic corrosion cannot occur?

CentaurSailorMan,

whereabouts is your boat based ?

I had corrosion on my 1st lift keel, then discovered there is a known problem with 'accelerated corrosion' in Chichester Harbour, reaching to Portsmouth; I am not alone in suspecting leakage from nearby power cables.

Then again the 'expert' lecturer who came to look solemnly pronounced the pitting in the steel was caused by " the speed of the boat through the water" !!! - I was quite flattered, but made a mental note to avoid anyone this character had taught...

 

[Seajet]

There may be something electrically nasty in the mud / water, which certainly connects your keels !

Your boat is not a million miles away from mine...

 

[CentaurSailorMan]

The holes were full of a soft black substance which looked like it had small slivers of metal in, it didn't look like filler. I just scrapped it out with a screwdriver and then coated it with fertan to stop further rusting. If it was previously filled I did wonder why they appeared in multiple places at the same time

Could this be where someone in the past had ground out a bad bit, then filled the holes and the filler has now become detached? The area seems to have some kind of paint on it?

 

[VicS]

I dont think it is galvanic or electrolytic corrosion. It would not be so localised and without another dissimilar metal involved it cannot be galvanic.

The smooth hollowed appearance is characteristic of impingement attack, aka erosion corrosion, assuming that is how the pits appeared without any cleaning out. I guess that prompted Seajets friend to suggest the speed of the boat through the water.

It does affect cast iron but my own experience of it is limited to copper pipes and cupronickel condenser tubes. Turbulence caused by a partial obstruction or even a bend can cause it if the overall flowrate exceeds, IIRC, something like 4m/sec. ( 4 m/sec is a bit less than 8 knots)

Unless you are moored in very fast moving water and there is something that causes turbulence local to these sites I reckon impingement attack can be ruled out.

A scientific explanation of "Chichester accelerated corrosion" is needed before accepting that as the cause.

That leaves Vyv's explanation.

 

[Seajet]

VicS,

hello, I thought you'd be along ! Casting bubbles doesn't explain the corrosion on my first mild steel keel, or the reported corrosion on everything from structures to buoys & ships in Porstsmouth harbour.

I suspect there's a certain amount of discretion, as places like Chichester, Langstone ( to a lesser degree ) & Portsmouth have money to make...

When Portsmouth Uni' Marine metallurgy Unit came to look at my first keel they were disappointed, as they had a pet theory that 'steel eating microbes' were responsible, but they were absent.

These microbes apparently excrete something which rapidly goes through steel, rather than actually chewing it !

The same wee beasties are supposed to be finishing off what's left of the Titanic.

I would point out that there is nothing electrical attached to or even near my keel inside the boat; but I have found an anode has halted the corrosion, how & why is another matter !

 

[[10753]]

If it was something to do with the Chi/Langstone/Portsmouth Harbours wouldn't we all have the problem ?

I have two anodes on each keel (mine are fabricated steel) and the anodes last 2 seasons if I'm lucky. After fitting the anodes I always check for an electrical current to make sure the anode is electrically joined to the keel.

Saying all the above I have a friend with the same type of boat moored in the same area. His AF is copperbot. However, his anodes don't seem to disappear any where near as quick as mine. Both boats keels are similar condition which is very good seeing as they are 40+ years old as well.