Q: Why does a zipper blacken in humid storage?
Blackening is a different corrosion product from white oxidation and often points to sulphur, chlorine or contact with certain packaging materials. Comparing a stored part with a dry control is the fastest way to see whether the environment is responsible.
- Identify which parts blacken: slider, teeth, or the tape edge near metal.
- Check packaging for rubber, some papers and adhesive films that release sulphur.
- Review the storage area for rubber goods, cleaning chemicals or damp cardboard.
- Compare blackened parts with a control sample kept dry and separate.
- Wipe a sample with a soft cloth to see whether the black film sits on the surface.
Blackening across a whole carton stored with rubber or certain papers is a packaging interaction rather than a zipper fault. Repeat blackening in clean, dry packaging should be raised with the maker with the lot and storage history. Changing the packaging is often faster and cheaper than changing the finish.