INCI name
Ascorbic Acid
Also listed as: Vitamin C, L-ascorbic acid
Ascorbic acid is vitamin C. In skincare it is used in serums, usually at a low pH, because the pure acid is unstable in water and oxidises to a yellow or brown colour when it degrades. Many products use derivatives instead, such as sodium ascorbyl phosphate, ascorbyl glucoside or tetrahexyldecyl ascorbate, each with its own INCI name and its own stability profile.
A brown, rusty-smelling vitamin C serum has probably oxidised. The colour change is a practical clue, not a precise assay.