Holder 20 keel corrosion

The centerboard keel on my holder 20 is cased in with fiberglass. Over the years, the keel has become increasingly difficult to lower (and raise) because the sides of the cased-in keel are binding on the sides of the shaft that it slides in.
I was puzzled why the problem got worse over time - but now theorize that the water-tight seal provided by the keel casing has failed, the keel itself is corroding (I assume it is cast iron), and expanding inside the case (see pictures).
My first question: does this theory make sense? That is, if iron corrodes inside a confined space does expansion occur, and could that expansion be forcing the fiberglass case to swell out?
Next, as I understand the design of this boat, the keel cannot be removed - so I probably cannot solve the problem completely, but might buy some time by treating the top of the keel, where the binding is occurring.
I'm thinking to remove (cut away) part of the casing so that I can grind the corrosion down to clean iron, and treat the iron with some kind of corrosion preventer. But I probably need to replace the casing that I remove to seal the keel from water, and I'm not sure how to approach that.
Any ideas are appreciated.

 

Welcome to the Forum Mike.

For other readers reference, here is some info on the Holder 20 -
https://sailboatdata.com/sailboat/holder-20
I presume that your keel is a daggerboard type, as shown in the drawing below?

Re your photos above - that split looks very ominous.
It should be possible to remove the keel from the casing surely?
Re the keel rusting, and the rust expanding, that is most probably what is happening.
I think that you will probably have to take off all the fibreglass over the cast iron, otherwise the problem will only get worse.
I don't think that you have to re-fibreglass it - just clean it up as best you can, and treat it / paint it.
Many yachts have bolted on cast iron keels that are simply painted.

 

bajansailor, thanks for the fast response.

Do you mean, remove the keel from the boat?
When lowered, the keel bottoms out on shoulders which seem to be an integral part of the hull - so the keel cannot be removed from below.
Since the entire keel and bulb are encased in fiberglass, it's impossible to see if the bulb is attached to the rest of the keel via bolts - or is welded. And the keel cannot be removed from above with the bulb attached.
I'm encouraged that the bare metal can be treated and painted. If I only treat the top, it will fix the binding issue, and perhaps provide a few more years of fun (I suspect the boat will outlive me

 

You do ideally want to remove the keel from the boat.
Re the top section of the keel, shown in your photos - is it possible that this is bolted to the cast iron keel? If you could get rid of it, then it might be possible to drop the keel completely?
Alternatively, you could see if the bulb is mechanically fastened to the keel underneath the layers of fibreglass.
If you can unfasten the bulb from the keel, is the companionway hatch large enough for it to come out vertically with a small crane?

 

That's an interesting thought. I'll watch out for that as I start removing the upper casing.