I have not personally heard of any issues with the few folks with X's in Puget Sound. The steel in the Cal vs the steel being used today in many of the European yachts with "steel" frames like X-Yacht does, seems to be a better quality stainless. I am not sure of what grade steel Cal used either.
There is a fellow that post regularly on AS forums that is into X yachts, I believe lives in Denmark or close by. He might be a better person to get an answer from frankly. Or some of the other European folks that have access to more boats to guage if the framework is a problem. For me personally, I would not worry a lot. If there was a real issue, I would feel you would find enough posts online or equal about the issue(s) of their, and others steel frame work. Which frankly, looks way different that the piece shown from the C40. That looks like a steel version of a wood frame I have on my boat.
She drives me boat,
I drives me dinghy!