---
title: "Does Stainless Steel Rust? When and Why It Can"
source: "https://www.laxconsteels.com/does-stainless-steel-rust/"
description: "Stainless steel rusts when its chromium oxide film is broken and cannot heal: chlorides, embedded iron, low oxygen, the wrong grade. Causes, grades, repair."
---
# Does Stainless Steel Rust? When It Can, Why It Does, and How the Surface Is Restored

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Published 11 September 2026 8 min read

Stainless steel can rust. It resists rusting because its chromium, at least 10.5 percent of the alloy, forms a thin chromium oxide film over the surface that stops oxygen and water from reaching the iron beneath, and because that film repairs itself within seconds wherever it is scratched. Rust appears when something prevents the film from re-forming: chloride ions from salt, seawater, pool chemicals or cleaning agents; particles of ordinary iron embedded in the surface by carbon steel tools; a crevice or deposit that starves the surface of oxygen; heat tint from welding; or a grade with too little chromium and molybdenum for the environment it was put in. In each case the rust is local, it begins at the point of damage, and it can usually be removed and the film restored. The word "stainless" was a description of the steel's behaviour in ordinary service when it was introduced, not a guarantee against every environment.

## Why stainless steel normally does not rust

Rust is hydrated iron oxide, the product of iron reacting with oxygen in the presence of water. On carbon steel the oxide is porous and flakes away, exposing fresh metal, so the attack continues until the metal is consumed. On stainless steel the chromium in the alloy oxidises first, and its oxide is different: dense, continuous, tightly bonded to the metal, and only a few nanometres thick. This passive film separates the iron from the environment. When it is scratched, the exposed chromium reacts with the oxygen in air or in aerated water and the film closes again. As long as the surface has oxygen and is free of the ions that dissolve the film, the steel stays bright indefinitely. Nickel, molybdenum and nitrogen in the higher grades make the film more stable and quicker to heal, which is why [316](https://www.laxconsteels.com/grades/aisi-316/) outlasts [304](https://www.laxconsteels.com/grades/aisi-304/) in salt and why the duplex grades outlast both.

## The causes of rust on stainless steel

Nearly every case of rust on stainless steel in service traces to one of the following. They are listed in the order a metallurgist would check them.

**Embedded iron.** The commonest cause on new fabrications. A grinding disc, wire brush, drill, clamp, chain or workbench that has touched carbon steel leaves iron particles pressed into the stainless surface. The particles rust in the first damp weather, producing brown spots and streaks that look like the stainless steel itself rusting. The film beneath is intact; the rust is the iron contamination, and it can start a pit if it is left. Fabricators keep separate tools for stainless steel and test finished surfaces with a ferroxyl solution that turns blue where free iron is present.

**Chlorides.** Salt spray on a coastal building, road salt on a vehicle part, seawater, brine, hypochlorite pool water, and bleach or hydrochloric acid cleaners all carry chloride ions, which dissolve the passive film at a point and start a pit. The attack is worse where the chloride concentrates by evaporation on a warm surface, and worse again in a crevice. The mechanisms, and the grade selection that answers them, are set out in the article on [corrosion of stainless steel](https://www.laxconsteels.com/corrosion-of-stainless-steel/).

**Lack of oxygen.** The film needs oxygen to heal. Under a gasket, a label, a deposit of dirt or scale, a stacked sheet or a bar resting on a rack in the rain, the trapped water soon loses its oxygen, the film cannot re-form, and crevice corrosion begins. Stagnant water inside a tank or a dead leg of pipe does the same.

**Heat tint and welding.** The blue and brown oxide bands beside a weld are chromium oxide that has grown thick at high temperature, drawing chromium from the metal beneath it. That chromium depleted layer rusts and pits where the bright metal does not. Welds and heat affected zones are pickled or mechanically cleaned to bright metal and then passivated.

**Sensitisation.** An austenitic steel held between about 425 and 815 degrees Celsius forms chromium carbides at its grain boundaries and loses the chromium beside them. The grain boundaries then corrode. Low carbon grades such as [304L](https://www.laxconsteels.com/grades/aisi-304l/) and stabilised grades such as [321](https://www.laxconsteels.com/grades/aisi-321/) avoid it.

**The wrong grade.** A ferritic grade such as [430](https://www.laxconsteels.com/grades/aisi-430/), with 16 to 18 percent chromium and no nickel or molybdenum, is liable to stain in a kitchen and to rust in coastal air. Grade 304 pits in seawater, and grade 316 pits as chloride concentration and temperature rise. Every grade has a limit, and rust beyond that limit is a selection error rather than a defect in the steel.

**Tea staining.** A light brown discolouration on stainless steel in marine and coastal atmospheres, most visible on rough or unwashed surfaces, caused by chloride deposits and pitting on a microscopic scale. It is cosmetic rather than structural, and a smoother finish, a higher grade and regular washing reduce it.

## Grade by grade

Where the grade itself is the cause, the composition places it in an order. The table gives the service each grade normally carries without rusting and the conditions under which attack starts on it. The entries are conditional rather than absolute: the same grade and the same chloride can leave one part clean and pit another, and the variables that decide it are listed below the table.

*Service each common stainless steel grade normally carries without rusting, and the conditions under which attack starts*

| Grade | Chromium, molybdenum | Normally carries without rusting | Attack starts as |
| --- | --- | --- | --- |
| AISI 430, ferritic | 16 to 18 percent Cr, no Mo | Dry indoor air, dry food contact, appliance trim | Chloride reaches the surface at all, or condensation is allowed to stand |
| AISI 304, austenitic | 18 to 20 percent Cr, no Mo | Inland atmospheres, fresh water, food and dairy, most kitchens | Chloride approaches a few hundred parts per million, or a crevice or deposit concentrates it |
| AISI 316, austenitic | 16 to 18 percent Cr, 2 to 3 percent Mo | Coastal atmospheres, mild brines, chemical plant, marine fittings above the waterline | Chloride and temperature rise together, and in crevices under immersion |
| [Duplex 2205](https://www.laxconsteels.com/grades/duplex-2205/) | 22 to 23 percent Cr, 3 to 3.5 percent Mo, nitrogen | Seawater, hot chloride process streams, desalination | Hot concentrated chloride meets a crevice, or the medium is a strong reducing acid |
| [Super duplex 2507](https://www.laxconsteels.com/grades/super-duplex-2507/) | 24 to 26 percent Cr, 3 to 5 percent Mo, nitrogen | Seawater immersion, offshore, hot brine | The medium is a strong reducing acid, or the chloride is hot and concentrated in a crevice |

Five variables move a grade across those columns. Chloride concentration, since a trace in air behaves differently from immersion in brine. Temperature, because the concentration at which a grade pits falls as the metal gets hotter. Crevice geometry, since a gasket face, a lap joint or a deposit concentrates chloride and excludes oxygen at the same time. Surface finish, because a ground or polished surface holds fewer deposits and fewer initiation sites than a rolled or coarsely abraded one. And cleaning, since a surface washed with fresh water before salt dries on it stays inside the limit that the same grade would fall outside unwashed. Molybdenum and nitrogen are what raise the limit itself, and the [PREN reference](https://www.laxconsteels.com/pren-stainless-steel-what-your-grade-guarantees/) gives the ranking, while the [304 vs 316](https://www.laxconsteels.com/304-vs-316-stainless-steel/) comparison sets out the choice between the two commonest grades.

## Removing rust and restoring the film

Surface rust on stainless steel is removed, not painted over, and the surface beneath is then passivated so that the film re-forms evenly. Light staining and embedded iron come off with a non-abrasive cleaner or a paste of mild abrasive applied along the grain of the finish, never across it, and never with steel wool or a carbon steel brush, which add the iron that caused the problem. Heavier rust, heat tint and scale are removed by pickling in a nitric and hydrofluoric acid paste or bath to ASTM A380, which dissolves the oxide and a few micrometres of the chromium depleted metal beneath it, or by glass bead or stainless shot blasting. Passivation follows: immersion in or wiping with nitric acid or citric acid to ASTM A967 dissolves any remaining free iron and leaves the surface to build a fresh, uniform film in air. A cleaned and passivated surface resists again as the grade allows, so where the cause was contamination or a deposit the repair holds. Where the service itself is beyond the grade, pitting returns, and the answer is a higher grade or a change to the conditions rather than a repeat of the cleaning.

Prevention costs less than removal. Surfaces are washed with fresh water where salt can settle; chloride cleaners are kept away from stainless steel; crevices are designed out and standing water is drained; carbon steel tools and fixtures are kept apart; welds are cleaned and passivated before the part is put into service. A smooth surface finish, ground or polished rather than hot rolled, holds fewer deposits and fewer initiation points.

## Stainless bar in the annealed and pickled condition

Laxcon Steels supplies its hot rolled stainless products in the annealed and pickled condition, in which the rolling scale and the chromium depleted layer beneath it have been removed in the pickling line and the surface has re-formed its film: [HRAP flat bars](https://www.laxconsteels.com/products/hrap-flat-bars/), angles and [hot rolled round bars](https://www.laxconsteels.com/products/hot-rolled-round-bars/) in the austenitic and duplex grades, and [duplex steel](https://www.laxconsteels.com/products/duplex-steel/) for chloride service. Bright bar is drawn, peeled or ground after pickling, and its smoother surface is the finish described above.

## Frequently asked questions

### Why is stainless steel rusting?

The chromium oxide film that protects it has been broken and cannot heal. The usual reasons are iron particles left by carbon steel tools, chloride from salt or cleaning agents, a crevice or deposit that keeps oxygen away, heat tint beside a weld, or a grade too low for the environment. The rust is local to the damage and can normally be removed.

### Does 304 stainless steel rust?

Not normally in inland air, fresh water or ordinary kitchen and food service. It is liable to pit in seawater, pool water, road salt and hot brine, and sooner where a crevice or a deposit concentrates the chloride, which is why 316 or a duplex grade is specified for those conditions.

### Does 316 stainless steel rust?

It can, though less readily than 304. Its molybdenum keeps the film stable in salt that pits 304, and in coastal air and mild brines it normally stays clean. Attack becomes likely as chloride concentration and temperature rise together, in crevices under immersion, and where iron has been embedded in the surface.

### How is rust removed from stainless steel?

Light rust and embedded iron with a mild abrasive along the grain or a citric acid cleaner; heavy rust, scale and heat tint by pickling to ASTM A380 or by blasting with stainless or glass media. The cleaned surface is then passivated with nitric or citric acid to ASTM A967, never with steel wool or a carbon steel brush.

## Sources

- ASTM A380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.
- ASTM A967, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts.
- ASTM A276 and EN 10088-1: composition windows of grades 430, 304, 316, 2205 and 2507.
- International Stainless Steel Forum and Nickel Institute publications on the passive film, tea staining and post-fabrication cleaning of stainless steel.
- ASM Handbook, Volume 13B, Corrosion: Materials.

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