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Stainless steel resists corrosion because of a thin passive film that forms on its surface in oxidizing environments, not because the metal is inert. The film depends on a minimum chromium content of roughly 10.5%. It can break down locally where chloride or other halide ions are present, which produces pitting or crevice corrosion, while surface discoloration is not always corrosion at all. This guide is general materials information for lab, clinic and supply staff, not advice for a specific instrument or procedure.
Definition and scope
Corrosion resistance in stainless steel is a property of the alloy and its surface film in a given environment. Outokumpu describes the resistance as coming from “the thin passive film that forms spontaneously on its surface in oxidizing environments”, and states that this happens “if the steel has a minimum chromium content of approximately 10.5%”. The Australian Stainless Steel Development Association (ASSDA) likewise calls this film a passive layer that “exhibits high resistance to corrosion”. Because the resistance is a surface film, it depends on the environment: the same grade can stay clean in one setting and corrode in another.
How the passive film works and where it fails
The passive film forms spontaneously in oxidizing environments, so trouble starts when the environment prevents the film from holding. Outokumpu notes that stainless steels are particularly susceptible to pitting and crevice corrosion “in media containing halide ions such as chlorides”. Chlorides are the halide ions most relevant to know about in a lab or clinic setting.
Pitting
Pitting is highly localized: Outokumpu describes “discrete pits on the free surface of stainless steels”, and ASSDA defines pits as attack much deeper than their diameter.
Crevice corrosion
Crevice corrosion occurs in crevices and confined spaces. ASSDA gives “under fasteners or at lap joints” as examples and notes that crevices lower the temperature at which attack begins: the critical crevice temperature is always lower than the critical pitting temperature (CPT).
Staining and discoloration
Atmospheric exposure mainly causes staining, sometimes called tea staining, according to Outokumpu, which also states that not all discoloration is necessarily the result of corrosion. Staining is therefore a different problem from a pit, which is attack much deeper than its diameter.
Reading a stainless steel corrosion resistance chart
Resistance charts rank grades against environments, and they are only as good as the conditions they were built for. Two ideas from the sources explain what a chart can and cannot tell you.
- PRE (pitting resistance equivalent). Outokumpu gives PRE = %Cr + 3.3 × %Mo + 16 × %N. A higher value indicates greater resistance to pitting. ASSDA states that stainless steels with a PRE value greater than 40 generally exhibit resistance to ambient temperature seawater.
- CPT (critical pitting temperature). ASSDA explains that CPT is measured in an oxygenated chloride test solution and “is not indicative of service temperature”. It ranks alloys against each other; it is not a safe operating limit.
So a chart or a PRE number is a comparison tool. It does not guarantee that a given grade will survive a particular cleaner, saline or crevice in your equipment.
Where it matters for labs, clinics and supply decisions
The following checklist turns the sources’ mechanisms into a way of sorting what you see. It describes the damage type only and does not diagnose a specific item.
| What you see | Mechanism described in the sources | Source |
|---|---|---|
| Surface staining, sometimes called tea staining | Staining; not all discoloration is corrosion | Outokumpu |
| Small discrete holes on an open surface | Pitting: highly localized, deeper than wide | Outokumpu, ASSDA |
| Attack under a fastener, lap joint or in a tight gap | Crevice corrosion; occurs at a lower temperature than pitting | Outokumpu, ASSDA |
For lab-specific chloride questions, such as chlorine-based cleaners, the mechanism is the halide attack above. Cleaning and compatibility details are covered in the chemical compatibility chart for wetted materials, the biosafety cabinet cleaning guide and the bleach concentration guide. For choosing between common grades, see 304 vs 316 stainless steel; for tubing dimensions, see the stainless steel sanitary tubing size chart.
Common misunderstandings
- “Stainless never rusts.” The passive film can fail locally, especially with halide ions such as chlorides (Outokumpu).
- “10.5% chromium makes any steel fully corrosion resistant.” It is the approximate minimum for the passive film to form, not a guarantee for every environment.
- “Any discoloration is corrosion.” Outokumpu states that it is not necessarily so.
- “A CPT or PRE value is a service limit.” ASSDA says CPT is not indicative of service temperature.
FAQ
What is stainless steel corrosion?
It is the breakdown of the passive film at a point or area of the surface, most often as pitting or crevice corrosion in the presence of halide ions such as chlorides.
Does chlorine or bleach damage stainless steel?
Chlorides and other halides are the known trigger for pitting and crevice corrosion in stainless steels (Outokumpu). For concentrations and rinsing practice on laboratory equipment, follow the CASRAI pages linked above and the equipment manufacturer’s instructions.
What is crevice corrosion in stainless steel?
Attack in confined spaces such as under fasteners or at lap joints, which begins at a lower temperature than open-surface pitting (ASSDA).
Is staining the same as corrosion?
No. Atmospheric exposure mainly causes staining, and not all discoloration is necessarily corrosion (Outokumpu).
What does a stainless steel corrosion resistance chart show?
Relative ranking of alloys in stated conditions. Indices such as PRE rank pitting resistance; they are not service limits.
Sources
- Outokumpu, Corrosion resistance of stainless steel, checked 2026-10-30.
- ASSDA, Corrosion Resistance, checked 2026-10-30.
