Green Rust

Definition - What does Green Rust mean?

Green rust is a type of iron compound with unstable properties that is formed by metal corrosion. Its formation also corresponds to the depassivation of steel and usually takes place when the chloride concentration or ratio to ions of hydrogen is greater than 1.

Green rust is also known as fougerite.

Corrosionpedia explains Green Rust

Rust is the term given to a large group of iron oxides. Rust can be found when there is unprotected steel or iron that is exposed to the elements. It can form in various colors like yellow, brown, orange and green, which is known as the green rust.

Green rust is an unstable product commonly produced in environments with low oxygen concentration like rebar in chlorine-rich environments, where there can be a release of iron hydroxides. In such a case, the steel’s depassivation leads to the formation of green rust when the ratio between hydroxide ions and chloride ions is higher than 1. Thus, rebar in materials such as concrete is protected from green rust if the concrete’s alkalinity is adequately high.

Since depassivation of steel results in green rust formation, it is vital that industries put into place methods to keep it from forming. In green rust protection, the following must be considered:

  • Depassivation does not take place if the environment’s alkalinity is adequately high. In practice, corrosion does not occur if the alkalinity is adequately high to prevent the action of aggressive agents.
  • Corrosion occurs in concretes that are carbonated given the absence of elements such as chloride. Thus, in environments with a pH below 9.5, critical contents of chloride may induce depassivation.

By taking these points into consideration, the occurrence of corrosion in such environments can be monitored and prevented.

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