Green Rust - What It Is and How It Works

Rust is the name given to a collection of iron oxides. You'll find rust in all situations where unprotected iron or steel is exposed to the elements. Did you know rust comes in colors besides red? There is brown, orange, yellow and even green rust!

Green rust is an unstable corrosion product typically produced in a low-oxygen environment, such as on rebar in the chlorine-rich environment of seawater. The reaction between seawater and steel may result in [Fe^II₃ Fe^III (OH)₈]⁺ [Cl·H₂ O]^-, a series of iron hydroxides. Depassivation of steel to form green rust occurs when the ratio of concentration of chloride ions to hydroxide ions is greater than 1. Therefore, rebar in concrete, for example, may be protected from green rust if the alkalinity of the concrete is high enough.

Green Rust and Fougerite

There is a natural mineral which is the equivalent to green rust called fougerite. Fougerite is a blue-green to blue-gray clay mineral found in certain wooded regions of France. The iron hydroxide is believed to give rise to other related minerals.

Green Rust in Biological Systems

The carbonate and sulfate forms of green rust have been identified as by-products of ferric oxyhydoxide reduction in iron-reducing bacteria. For example, Shewanella putrefaciens produces hexagonal green rust crystals. Scientists speculate green rust formation by bacteria occurs naturally in aquifers and wet soil.

How to Make Green Rust

Several chemical processes produce green rust:

• Electrochemically oxidizing iron plates can form a green carbonate rust.
• Green rust may be prepared by bubbling carbon dioxide through a suspension of iron(III) hydroxide Fe(OH)₃ in iron(II) chloride FeCl₂.
• Green sulfate rust may result from mixing FeCl₂·4H₂O and NaOH solution to precipitate Fe(OH)₂. Sodium sulfate Na₂SO₄ is added and the mixture is oxidized in air.