1. The reason for the zipper made of stainless steel is:
When used on stainless steel products or wool products, the stainless steel zipper will blacken some of the teeth due to metal oxidation. This is caused by the gas of the product which causes the discoloration of the stainless steel zipper when the leather is used to make the leather and the bleaching agent on the woven fabric, etc., and remains on the product during the treatment.
Examples of chemical reactions: oxidative bleach (H2O2) → black (CuO) or red (CuO2);
2. The reason for the discoloration of the metal zipper bundled by the rubber band:
The rubber band itself contains sulfides, which are vulcanized (blackened) when the metal zippers are bundled with rubber bands.
An example of a chemical reaction: a condition in which sulfide or HS2 gas is excessive → black [CuS].
3. Causes of discoloration caused by contact with an acidic compound or a chromium compound.
Examples of chemical reactions: acidic compounds and chromium compounds [Cr2O3] → black [CuO, red [CuO2] or blue [CuSO4].