Passivation

Passivation is the formation or treatment of a material’s surface that creates a thin protective layer and sharply reduces further chemical reaction, especially corrosion.

Oxide layer: A surface film of oxygen compounds formed when a material reacts with oxygen. Many metals passivate when oxidation creates a compact film that slows further attack.
Stainless steel: An iron-based alloy containing enough chromium to resist corrosion through a protective surface film. Its corrosion resistance depends on chromium-rich passivation at the surface.
Corrosion: The deterioration of a material through chemical or electrochemical reaction with its environment. Passivation is a way corrosion slows when a protective surface film develops.
Coating: A material layer applied to a surface to alter its appearance, properties, or resistance to its environment. A coating is added from outside, whereas a passive film often forms through reaction of the substrate itself.
Pilling–Bedworth ratio: The ratio of oxide volume formed to the volume of metal consumed during oxidation. It helps explain whether a metal oxide film can cover and protect the underlying surface.
Nitric acid passivation: A surface treatment that uses nitric acid to remove free iron and promote a chromium-rich passive film on stainless steel. It is a widely specified chemical route for enhancing stainless-steel corrosion resistance.
Electrochemical potential: The energy per unit charge that governs the tendency of a species to undergo electrochemical reactions. A metal’s potential helps determine whether its surface dissolves actively or remains passive.
Cathodic protection: A corrosion-control method that suppresses metal oxidation by making the protected structure the cathode of an electrochemical cell. It protects by controlling electrode reactions rather than relying solely on a surface film.
Diffusion: The net movement of particles from regions of higher concentration to regions of lower concentration. A dense passive film limits the transport of ions and molecules needed for continued corrosion.
Citric acid passivation: A stainless-steel treatment using citric acid to remove surface contaminants and support formation of a passive film. It offers an alternative to nitric-acid treatment for cleaning and passivating stainless steel.

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