Degreasing and Pickling: Alkali Takes the Oil, Acid Takes the Oxide

Two soils, two chemistries, and they do not substitute for each other. **Alkali removes oil and grease**, by saponifying it or by emulsifying it. **Acid removes oxide and scale**, by dissolving it. An acid bath will not touch grease, and an alkaline bath will not touch oxide — so both are needed, in that order, and a rinse between them. This rung is hazard-gated because it is the only part of plating that involves hot concentrated alkali and strong acids, and because the two together produce the situations that cause real injuries: splashing, hydrogen, and mixing what should not be mixed.