Element · Cr

Chromium

The element that makes stainless steel stainless, by growing an oxide layer thin enough to be invisible.

Chromium's contribution is a trick of self-repair. Above roughly eleven percent chromium, a steel forms a chromium oxide film a few nanometres thick that is transparent, adherent and — critically — reforms instantly wherever it is scratched. The metal underneath is protected by a layer too thin to see.

Without that threshold there is no stainless steel, and the surgical, food and chemical industries all look different.

Name origin

From Greek chroma, colour, after the vivid hues of its compounds. Chrome yellow, chrome green and the red of ruby all involve chromium.

Uses

Chromium's defining use is stainless steel. Above roughly a tenth chromium by weight, steel forms a self-repairing oxide film and stops rusting — the whole category of corrosion-resistant steel rests on that single behaviour.

Chrome plating gives the same protection plus a hard, bright surface. Chromium compounds are used as pigments, and chromium salts tan leather. The industrial forms differ sharply in hazard: metallic and trivalent chromium are benign, hexavalent chromium is carcinogenic.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is used as

Corrosion protection — application · the self-repairing oxide film that makes steel stainless
Alloying — application

is an alternative to

Steel — alloy · as the alloying addition that defines stainless grades
Nickel — element · as a plating: nickel goes on first and does the corrosion protection, chromium goes on top and is thin, hard and bright — which is why 'chrome' plating is mostly nickel

is a component of

Stainless steel — alloy · at least about a tenth by weight — below that the oxide film does not form
Nichrome — alloy · 15–25% · the whole point: it forms the tight oxide skin that stops the wire burning away at red heat, and re-forms if it is damaged
Nickel superalloy — alloy · 5–20% · for oxidation resistance, and reduced in the newest alloys because it competes with the elements that give strength

is an input to

Alloying and melting — process · the addition that makes steel stainless
Tanning — process · as basic chromium sulfate, which cross-links collagen in a day where vegetable tannins take weeks, and stays in the finished leather

is extracted from

Chromite — mineral · the only commercial source of chromium, which is what makes steel stainless Wikidata

is produced by

Smelting — process · as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal

is found in

Leather — material · at a few per cent in chrome-tanned leather, which is around eighty per cent of what is made — a significant destination for the metal, and the reason tannery effluent is a pollution problem
Tool steel — alloy · 0.5–13% · forms hard carbides and deepens hardening, so a thick section hardens through rather than only at the surface
Weathering steel — alloy · 0.4–1.25% · with the copper, densifying the rust layer so it seals rather than flakes
Cobalt-chromium alloy — alloy · 26–30% · the corrosion resistance, by the same passive film mechanism as stainless steel
Emerald — mineral variety · the colouring element, in quantities of a fraction of a per cent — and the reason emerald's geology is difficult, because chromium and beryllium occur in quite different rocks
Chromite — mineral Wikidata

Sources

Material World
Our own writing
Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
PubChem
US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
NIST — National Institute of Standards and Technology
United States Department of Commerce · US Government work — public information, credit requested

Where it comes from, and what it becomes

Follow Chromium back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
Chromium → is extracted from (the only commercial source of chromium, which is what makes steel stainless) → Chromite
Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (reduced with coke in a blast furnace) → Hematite
Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (the phosphate rock charged to the electric furnace, with coke to reduce it and silica to take up the calcium) → Apatite

These are the most distinct paths back. Chromium can be traced through others besides.

Downstream — what it becomes

Chromium → is an input to (the addition that makes steel stainless) → Alloying and melting → produces (tin into copper — the first alloy anybody made deliberately) → Bronze → is used to make (cast in a two-piece mould with a core, which is what bronze does better than the copper it replaced) → Bronze socketed axe → is associated with → Bronze Age complete chain
Chromium → is a component of (at least about a tenth by weight — below that the oxide film does not form) → Stainless steel → is produced at (discovered here in 1913 by Harry Brearley, investigating gun barrel erosion, and found on a scrap heap because it had not rusted) → Sheffield → is associated with (Bessemer built his first steelworks here in 1858, at the point iron gave way to steel) → Industrial Revolution complete chain
Chromium → is a component of (for oxidation resistance, and reduced in the newest alloys because it competes with the elements that give strength) → Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is associated with (the jet engine created the superalloy industry, because no existing material survived the turbine inlet — and turbine temperature has risen about 500 °C since, almost entirely on the blade) → The wartime materials programmes complete chain
Chromium → is a component of (the whole point: it forms the tight oxide skin that stops the wire burning away at red heat, and re-forms if it is damaged) → Nichrome → is used as (as a deliberately poor one: nichrome is chosen for its resistance, not despite it) → Electrical conduction
Chromium → is an input to (as basic chromium sulfate, which cross-links collagen in a day where vegetable tannins take weeks, and stays in the finished leather) → Tanning → produces (the process that makes a hide into a material — everything before it is preparation and everything after it is finishing) → Leather
Chromium → is used as (the self-repairing oxide film that makes steel stainless) → Corrosion protection

These are the most distinct paths onward. Chromium ends up in others besides.