Well, some corrosion is inevitable. Salt water is basically the enemy of everything. And, in particular, it likes to eat metal. So even if you're not soaking the tent in the ocean, the moist marine air will attack anything metal—or anything else, for that matter.
What you do NOT want to do is try to "treat" the zippers in any way. Lubricating them, putting wax on them, anything like that will only make things worse. Those sorts of things will attract sand and grit like, well, magnets. And then you'll have real problems.
The best solution is simply to perform a little extra maintenance. After each trip, rinse the tent in tap water, taking special care to get the zippers well-rinsed. Then make sure the tent thoroughly dries before storing it in a loose, breathable sack. Do not put the tent in a clothes dryer—that can cause the waterproof coating to de-laminate.
Zippers are replaceable, and I suppose at some point that might be an option if one gets really stiff and hard to use. But the salt water (and the sun) also are attacking the tent fabric. It may look fine, but at some point is going to degrade to well below its original strength. Best just to find a new tent rather than go to the expense of replacing zippers, then seeing tent disintegrate in a storm.
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