GlossaryKind of term: Manufacturing termPassivation
What is passivation?
Passivation is a chemical treatment that removes free iron from stainless steel so that its protective passive layer can form properly.
Stainless steel protects itself with a thin, chromium-rich passive layer that forms spontaneously in air. Machining, grinding and welding can leave free iron from tools, workholding or chips in the surface, and rust can start there, even on 1.4404. Passivation is a treatment in a nitric or citric acid bath that dissolves this free iron so the passive layer can re-form evenly. The part has to be degreased first. No layer is added and the dimensions change by a negligible amount.
What does passivation depend on?
The right bath, temperature and time depend on the grade. Free-machining grades with sulphur, such as 1.4305, and martensitic grades are more sensitive: a badly chosen bath can attack the surface instead of protecting it. Welded work is pickled or mechanically cleaned first, because passivation does not remove heat tint and scale. After passivation the part is rinsed and dried; residual acid in narrow crevices or blind holes is a risk.
What does the standard say about passivation?
Drawings or purchase terms often refer to ASTM A967 or ASTM A380. These specifications describe baths and treatments, and also tests to demonstrate that no free iron remains on the surface, such as a humidity or copper sulphate test. The requirement is only clear once the specification, the method and the test are named; without them, ask before you quote.
How does passivation work in practice?
Passivation is usually subcontracted; the passivator charges per batch or per basket, with a minimum charge and a lead time of its own. For food machinery, medical parts and semiconductor equipment it is often a fixed requirement, sometimes with a certificate per batch. No allowance is needed, but the part must arrive clean and free of oil. Welded parts need pickling as well. In the route, passivation belongs after the last operation that can leave free iron behind.
What is passivation often confused with?
Passivation is not pickling: pickling removes scale and heat tint and lightly etches the surface, passivation only dissolves free iron. Nor is it a coating: no layer of another material is added, the passive layer comes from the steel itself. And it is not technical cleanliness, which concerns particles and film residues, not corrosion resistance.
How do you see passivation in Blake?
In Blake the finishing sits with the part as a proposal from the drawing; you confirm or correct it, and who did so and when stays visible. You add the subcontracted passivation as its own line from your subcontract library; without a price, that line stays open rather than showing zero. More on estimating.
Frequently asked questions
Does passivation change the size of a part?
Not in practice. It dissolves free iron at the surface and lets the passive layer recover; nothing is added and what comes off cannot be measured with ordinary gauges. No allowance is needed, but clean, oil-free delivery is.
When is passivation required on the drawing?
Often for food machinery, medical parts and semiconductor equipment, or when stainless steel will sit in a damp or aggressive environment. The requirement is on the drawing or in the purchase terms, sometimes with a specification and a certificate per batch.
Is passivation needed after pickling?
Not necessarily. Pickling also leaves the surface passive, but some specifications still ask for a separate passivation step or a test. What applies follows from the drawing or the purchase terms.