How old is the mast? (older masts were of different alloys that were not so tolerant to corrosion, particularly in a marine environment, however it was so long ago that most of these mast have corroded themselves out of a job by now anyway.)

Is it painted? (sometimes painting an aluminium extrusion can cause severe pitting corrosion because if water gets below the paint and in contact with un-oxidised aluminium the oxide layer may never form because it can't get any oxygen to form the oxide. It can be thought of as aluminium needing to breathe so that after a bit of a knock (which destroys the oxide layer) the oxide layer can reform and protect the aluminium again (the oxide layer is very hard compared with the base aluminium alloy). If the oxide layer never formed on aluminium, it would be a much less useful material. Thank you aluminium oxide layer!<img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />)

Was a halyard (wire or rope) bearing against the mast?

Got photos?