Thanks, Tim. I think the metals have reacted. But I'll give these other methods a try.
Argh! That is a pain. I had that once with the threads of a TTL convertor and a sync cable. I could not separate them using "gentle" means. In the end I had to use a pair of pliers and force them apart - damaging one set of threads which I had to replace.
It may be "galvanic" action when two different metals form a unintended bond. A quick google search came up with this solution which sounds interesting....
Good luck with it. If you find a solution, could you let us know? I'm sure others would be interested.
There is some bright blue gunk you can buy which you can use to coat the threads which seems to stop galvanic action happening on typical u/w gear. I used it after my problem - it's called Duralac.
I think you can also fit a piece of zinc somewhere on your housing which can prevent the problem too - but others might correct me on that.
If you do a search on Wetpixel for Galvanic Action you'll find various posts about it.