Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers



Last updated: April 21, 2015

What Does Chelate Mean?

Chelate is any of a class of coordination or complex compounds consisting of a central metal atom attached to a large molecule, called a ligand, in a cyclic or ring structure.

Many commercial dyes and a number of biological substances, including chlorophyll and hemoglobin, are chelate compounds.

Chelating agents are employed as extractants in industrial and laboratory separation of metals and as metal-ion buffers and indicators in analytical chemistry. Chelating agents are used in several applications, including:

  • Scale removal
  • Water treatment
  • Corrosion control

Corrosionpedia Explains Chelate

Chelate ligands, or chelates, are by definition ligands where the ligating atoms and the coordination center form at least one closed loop, when the connectivity of atoms is considered. The category of coordination compounds which is formed by chelating ligands is known as chelates. For example, iron has six coordination sites, as does EDTA (ethylenediaminetetraacetic acid). Iron ions entering a boiler (such as contamination from the condensate system) combine with EDTA. All coordination sites on the iron ion are used by the EDTA, and a stable metal chelate is formed.

Chelates are more stable than nonchelated compounds of comparable composition, and the more extensive the chelation — that is, the larger the number of ring closures to a metal atom — the more stable the compound. This phenomenon is called the chelate effect. The stability of a chelate is also related to the number of atoms in the chelate ring. In general, chelates containing five- or six-membered rings are more stable than chelates with four-, seven- or eight-membered rings. The stability of complexes is greatly increased by chelating ligand. The total number of ligand atoms that can bind simultaneously to one metal center is the density of the ligand.

The chelates play an important role in micro detection and determination of metal ions in analytical chemistry and water treatment.

Although chelating agents can be used for corrosion control, the formation of soluble chelates may provoke stimulation of a corrosion process. Uncontrolled agent application may lead to chelant corrosion. This can be prevented by applying the precise amount that does not leave residuals in the boiler.


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