Now i have a cast Aluminium Injection Elbow

Depends on the spec of alloy used..... and the post process treatments.

I would wager slightly less than the CI bit. Have one fabricated out of stainless, mine has lasted 5 years now, and no signs of failure yet.
 
My Beta alloy elbow is original at 10 years old. 100hrs a year. Gave it a good clean at 8 years when the sail drive gaiter was changed and there was no evidence of corrosion then.
 
Yes, well, depending on your pattern of usage the Beta stainless ones may last longer than my 7 years - but I wouldn't recommend finding out the hard way :(. Mrs H and I added the experience to 'The Curse of Bradwell" (storm-bound, elbow-bound) - not that Bradwell ain't a nice place to be stuck (St Peter's, Green Man etc.).
 
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