That is a tough one, it comes with being on the salt. As we sailed all over the oceans zippers were always a problem no matter what type of make. Lots of warm fresh water hopefully with pressure behind it will clean out the white corrosion powder. Make sure zipper is laying flat against a cement surface before pressure cleaning zipper or else more than likely you will tear away the thread holding the zipper to material. We have found no chemical or grafite that works in keeping zipper free of corrosion. Most stuff that you can put on zippers attract more salt than they keep out, WD 40 is really bad at bringing salt to your zippers. We just use fresh water and exercise the zippers as often as we can, works best.