I am only a mile from the ocean, but in a valley that does not get the salt fog. I do keep them in climate controlled container with dehumidifiers. I have knives stored in Leather that go back to 60s FWIW, only where BRASS is involved do I see leather reactions. I know leather is NOT good if moisture is involved.
I often , no longer than a couple months and certainly after our Oct-Feb rainy season (we get less than 40" of rain around here) wipe and lubricate the knives. Sheaths are Lexoled when they start to appear dry. I have used many lubes over the years; from veg oil, mineral oils, waxes, synthetic oils and various CLP products , Balistol , synthetic greases and finally arriving on Mad Dog Labs XF7 . XF7 is a non toxic waxy grease. I'll not espouse it's gun benefits but just say Kevin invented it as a way to protect his carbon steel blades from deep sea salt water immersion. My son buys small tubs of XF7 to use on deep submersibles and equipment FWIW and has found nothing close to it for that use. It is butt ugly when it dries looking like a thin soft paraffin coating .
Any way I don't want to o a sales pitch for XF7 and I am no way financialy involved with it , it is just a superb knife storage product. For me the sheath is part of a knife package and I store them that way, but I pay attention to what is happening.If I see a leather reaction starting with discoloration ect. then I ususually go to plan B , which would mean separating the two, and yes I have had it happen. For a using carbon knife most are in Kydex and I have NOT seen a reaction. I did recently get a reaction from my custom Skihn Dhu I wear in me sock when wearing me kilt. The blade is 5160 I think and the sheath is beech covered outside with black ostrich . After wearing it in me sweated stocking for a Celtic weekend I was horrified monday when I put her to bed to see the blade orange freckled which I Flitzed away and XF7ed. Kim Breeds Neker has been worn almost every day for a few months now and I keep it wiped with XF7 after use and every week or so. It is developing a patina, from cutting food I think which is VERY hard on non stainless knife color IMHO.