Not quite sure that your results will give any useful statistics.
For example, there are many more metal aeroplanes flying, and so you'd have to dig through all of these and find out how many had corrosion problem / replacements and come up with an average.
I can tell you though that our rallye had to be totally rebuilt in 2005 due to corrosion in the wings. Required new wings / spars, and a total bare metal re-spray / anti corrosion which cost about £38,000 in total, and took 6 months or so.