had an amp 12 hour pack for a couple of years, use it on a lot of weekends
noticed the zips are getting hard to work on the belt pockets, had a closer look and noticed corrosion on the zips
the main compartment zip is a different steel, its brown alloy and had no problems.
given you're exercising with the pack the zips need to be resistant to salt from sweat, not to mention rain, theres no point using steel that isnt resistant to corrosion
i've got black diamond packs that use the brown steel on their pockets.
i've written to macpac and they said they will take it in for repair assessment...