"Stainless steel does not rust" is shorthand. The accurate statement is: it is corrosion-resistant because there is a chromium-rich passive film on its surface. This film is only a few nanometres thick and can repair itself after being damaged — provided the surface is clean and oxygen is present. After welding, hot working, or iron contamination, the film is damaged but cannot repair itself, and corrosion starts there.
One-sentence judgement
If it has been welded, hot-worked, has touched carbon steel tools, or is used in food/pharmaceutical/high-purity water/coastal environments, pickling and passivation are mandatory; un-welded cold-rolled 2B/BA sheet used in dry indoor environments can go without.
When the passive film is damaged
- Welding: oxidation colours (blue, purple, black) appear on the weld seam and heat-affected zone; that is oxide scale, not passive film;
- Hot working/heat treatment: high-temperature oxidation forms oxide scale, which must be removed after cooling;
- Iron contamination: carbon steel fixtures, ordinary steel wire brushes, and cutting tables shared with carbon steel embed iron particles into the surface, which rust first when exposed to moisture;
- Machining residue: cutting fluid, oil, grinding debris.
Pickling and passivation are two different things
- Pickling: acid dissolves the oxide scale and the chromium-depleted surface layer, exposing a clean metal surface;
- Passivation: an oxidising medium (commonly nitric acid or citric acid) causes the surface to re-form a chromium-rich oxide film.
Pickling without passivation is like washing your face without applying skincare — the surface is clean, but the film has not grown back, and it will still rust when exposed to a humid environment. Many cases of "it still rusts after pickling" are simply missing the passivation step.
Relevant standards include ASTM A380 (cleaning and descaling) and ASTM A967 (chemical passivation) — the specific method and acceptance criteria are determined by project requirements.
Four scenarios where it is required
- Welded structures: weld seams and heat-affected zones must be treated, either by local treatment with pickling paste or by full pickling bath;
- Food/pharmaceutical/high-purity water: not only must oxide scale be removed, but no residue and no dead corners must be ensured (often combined with electropolishing);
- Coastal/chemical environments: even if the appearance is clean, iron contamination will show within half a year;
- Subsequent coating/lining: the surface must be clean, oil-free and rust-free, otherwise adhesion cannot be guaranteed.
Scenarios where it can be skipped
- Un-welded, un-hot-worked cold-rolled sheet (2B/BA) used for decorative and structural parts in dry indoor environments;
- Finished parts that were passivated at the factory and have not touched carbon steel throughout transport and processing.
Common pitfalls
- Using hydrochloric acid for pickling: hydrochloric acid contains chlorine and causes pitting, especially on 304; use nitric acid or a dedicated pickling paste;
- Insufficient rinsing after pickling: residual acid remains in crevices and actually accelerates corrosion;
- Brushing weld seams with a carbon steel brush: contaminating while treating;
- Treating only the visible weld seam and missing the back side and root;
- Delivering directly after passivation without testing (blue dot test, copper sulphate test).
Tell us the structural form (whether welded), the service environment and the applicable standard, and we will advise whether pickling and passivation are needed and which method to use; for workpieces that need treatment, we can also provide treatment records according to your acceptance method.
