As you know, I lovingly pack my paddles in their bag wet with salt water.
I don't let it go from getting off the water until in the bag, for fear of appearing Western Australian.
They then stay in the Kombi until next use - usually daily.
And are never removed.
No corrosion issues with Naish bags - these have been going 2 - 3 years now.
You can see salt staining around the zip to the left, but no corrosion on the zip.
Whole zip appears to be plastic but is unbranded.
I think the Flying Objects board bags have plastic YKK zips, also seem unaffected by corrosion.
New SIC paddle bag looks to be metal (upper right), so will watch and advise.
Lack of TLC may be the key.