Aluminium zirconium tetrachlorohydrex gly is what causes pit stains. Sometimes called AZG for short, it is commonly found in antiperspirant for deodorant.
It has the ability to close skin pores and prevent sweat from producing. In its anhydrous form it can additionally absorb moisture. Simply put, something in anhydrous form doesn’t contain any water.
AZG contains hydroxide, chloride, and glycine. The anhydrous AZG diffuses in the sweat glands and creates a type of plug that limits the flow of sweat to the surface of the skin. Gradually the plug is broken down and normal sweating then occurs. A problem with AZG is that when mixed with sweat it can create a yellowish tint and stain clothing.
It can also make the clothing become stiff where it has dried, even after washing it out. The yellowish tint can be removed with vinegar or mild bleach. Deodorants and antiperspirants that do not contain aluminium zirconium tetrachlorohydrex gly do not have the problem of staining clothing yellow.