Stainless steel resists corrosion because of a thin, self-healing chromium-oxide layer on its surface. Machining and welding can disturb that layer and leave free iron behind, which is where rust can start. Passivation fixes this.

Passivation treats the surface with an acid — nitric or citric — that removes free iron from the surface and helps restore a clean, protective chromium-oxide layer. It does not add a coating; it returns the steel to its natural passive state.

On drinkware, passivation is a food-safety and corrosion-resistance step applied to the drink-contact surface. It helps ensure the inner wall stays clean and resists pitting over years of contact with water, coffee, tea, and acidic beverages.

Practical takeaway: Confirm the inner wall is passivated, and ideally electropolished, as part of your food-contact quality plan rather than treating it as an optional cosmetic step.