Besides using a bio diesel additive, one other suggestion:

When you replace the strainer, you might try giving the flange area where you are seeing pitting a couple of coats of POR-15. POR-15 is used for coating fuel tanks, so it will stand up to diesel or gasoline. It should protect the aluminum parts you can coat from whatever is causing the corrosion. (But not the screen!)

By the way, a search brings up lots of stuff on bacterial corrosion of aluminum. Aircraft tanks have apparently had this problem for years.

Too bad they didn't switch to a polymer strainer.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5