Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers



Last updated: November 30, 2018

What Does Chromium Mean?

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, lustrous, hard and brittle metal which takes a high polish, resists tarnishing, and has a high melting point.

Chromium is unstable in oxygen; it immediately produces a thin oxide layer that is impermeable to oxygen and protects the metal underneath.

Chromium is used on a large scale in the metallurgical and chemical industries. The metallurgical industry commonly uses chromium for the production of stainless steels as well as for plating steel. In the chemical industry, chromium is used primarily in:

  • Pigments (CrVI and CrIII)
  • Metal finishing
  • Electroplating
  • Wood preservatives (CrVI only)
  • Leather tanning (CrIII only)

Chromium is commercially extracted from the chromite ore, which is iron chromium oxide (FeCr2O4).

Chromium metal has proven of high value due to its high corrosion resistance and hardness. Chromium is used in metallurgy to impart corrosion resistance and a shiny finish.


Corrosionpedia Explains Chromium

Chromium is a silver-gray, highly polished and hard metal. It does not tarnish in air, and when heated it burns and forms the green chromic oxide. It is present in the environment in several forms, the most common being trivalent chromium (Cr III) and hexavalent chromium (Cr VI). Chromium is found naturally in:

  • Rocks
  • Animals
  • Soil
  • Volcanic dust and gases

Chromium is used to harden steel and to form many useful alloys. Steel can be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel.

It is used in plating to produce a hard, beautiful surface and to prevent corrosion. Chromium is used to give glass an emerald green color. It is also widely used as a catalyst. The dichromates are used as oxidizing agents in quantitative analysis, as well as in tanning leather.

The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being the most popular metal coating with unparalleled combined durability.

Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal parts.

Chromium doesn't oxidize nearly as easily as steel. Chromium is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a structure only a few atoms thick and is very dense, preventing diffusion of oxygen into the underlying material. In iron or plain carbon steels the oxygen migrates into the underlying material, causing rust.


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