I've seen this phenomenon (internally tapered hard anodized aluminum seat sleeves splitting and corroding) on a number (a very small number like < 6) of high end 100% carbon masts from Itallica and Excel (I think they made the high end North masts a few years ago).
I suspect that the reason this is not a more common problem is that most of the hard anodized aluminum seat/collar sleeves get installed with a strong adhesive bond between the sleeve and the mast taper.
The adhesive is neither carbon nor aluminum, so it acts as a galvanic "buffer" between the 2 materials. Also, the hard anodizing puts a layer of aluminum oxide on all surfaces of the aluminum sleeve which pretty much kills any "reactivity" between the materials. Hard anodizing, with the correct post anodizing "seal" coat is used worldwide to make high strength aluminum parts withstand exposure to salt water.
If you get a mast where the adhesive bond breaks down (or wasn't fully bonded/buffered in manufacture) the sleeve gets pushed down the taper by the tremendous dowhaul forces we use in modern sails and as it slips down under pressure the hoop strength isn't quite up to the task so the sleeve begins to split at the lower (thinner due to the taper) end of the sleeve. Also, some masts have a little radiused "seat" at the bottom, but the sleeves do not have the same radius so a hard expansive "line contact" occurs when the sleeve slides down.
This splits/cracks the sleeve and if you sail in salt water, the corrosion starts as soon as the base metal aluminum (under the hard anodize coating) gets wet with salt water.
Then the crack corrodes, and as you continue to use the mast the sleeve slides further down the mast.
Fortunately, the taper of the ferrule often stops the top from sliding down further (when the internal and external tapers seat fully), but then the mast tends to be a little diffucult to get apart.
So, I suspect it's a bit of a manufacturing defect, but not something the factory QA guys can really check. How do you check the adhesive bond on an internal anodized aluminum to carbon interface?
I've had one or two masts (in 25 years of using windsurfing masts) with this problem.
One I turned in for a new bottom section and the replacement has not corroded/cracked in over 2 years) and the other one still has the crack and the corrosion, but it seems to have stabilized and it's not sliding or cracking any further.
Hope this helps,
Last edited by Roger; 7th February 2008 at 05:58 AM.