Chromium makes steel stainless by forming an invisible oxide film on the surface that seals the metal from water and air and re-forms where it is scratched. It works only when the steel holds at least 10.5% chromium in solution at the surface, so anything that locks chromium away, such as carbide formation or surface iron contamination, switches the protection off locally.
Part of Stainless Steel Production
The self-repairing chromium oxide film that gives stainless steel its corrosion resistance, and the conditions that defeat it.
A Chromium Oxide Film Seals The Surface
The carrier describes the film as chromium oxide (Cr₂O₃), transparent, hard, adherent and self-healing. Unlike iron oxide it stays put and is non-porous, so once it forms it stops the metal beneath from being attacked. This is what the carrier calls passivation, and why a scratch on good stainless does not rust the way a scratch on plain steel does.
The film is extremely thin. I recall figures of a few nanometres but have no source in the corpus, so treat that as unconfirmed; the practical consequence is only that it cannot be seen, and that it is easily damaged by contamination, not by wear. The carrier says a scratch heals “within hours in air”; I recall repair in oxygen being much faster, but cannot source it. Either way, repair depends on oxygen reaching the scratch, which is why stainless corrodes first in crevices and under deposits where air cannot.
The 10.5% Figure Is A Surface Condition
The carrier states the threshold as at least 10.5% chromium. Two consequences follow, each of which shows up later in the tree.
- Chromium tied up in carbides does not count. If carbon in the steel combines with chromium at grain boundaries, the adjoining metal can drop below the threshold even though the bulk analysis reads above it. This is the “sensitization” of Carbon Control and the reason Solution Annealing exists.
- Free iron on the surface does not protect. Iron picked up from tools, scale or grinding dust rusts and the rust stains the stainless around it. Passivation Treatment removes it with acid.
Higher chromium does not just add margin; the carrier’s grade table runs from 12-14% for martensitic through 16-18% for ferritic to 18-20% for austenitic, tracking the corrosion resistance the user expects.
What The Film Cannot Do
- It does not survive every environment. Strong chloride conditions and reducing acids attack passive films; the carrier says only that stainless is impervious to water, most acids and food chemicals, and the word “most” is doing real work. Hydrochloric acid in particular is a poor choice for a stainless vessel as far as I recall, but I have no corpus source for the grade-by-acid compatibility, so test a coupon before committing a tank.
- It does not repair itself without oxygen. A part kept wet and sealed, with no air, may not re-form the film.
Common Mistakes
- Adding chromium to reach 10.5% and stopping. The carbon, the fabrication heat and the surface condition decide whether the chromium counts.
- Cleaning stainless with steel wool or a carbon-steel brush. Embedded iron particles rust and look like the stainless failing.
What’s Next
- Learn next: Chromite Ore - where the chromium comes from
- Combine with: Passivation Treatment - restoring the film after fabrication