Molecular oxygen as default oxidant
Definition
Oxygen is an oxidant present by default in nearly every water system, never added deliberately yet constantly seeking electrons.
It looks inert because the O=O double covalent bond satisfies both atoms valence requirements, but molecular stability does not mean indifference to outside electrons. Oxygen has two advantages no applied oxidant has: it is dissolved in almost every water system on Earth and is constantly replenished by contact with air. When present it keeps looking for electrons, which in water systems often means metals.
Related concepts
Kinetic restraint of oxygen · Henry’s Law · Temperature paradox
Part of The Blueprint of Water · 🧪 chemistry