Reviewed by Raghvendra GopalCheckmark | Last updated: July 27, 2021

What Does Anion Mean?

An anion is an ionic species with a negative charge. This is a type of atom that has gained electrons.

Anions are one of the two types of ions. The other type is called a cation. Anions are attracted to the anode, while cations are attracted to the cathode.

The concept of an anion is important in the corrosion industry because certain anions facilitate corrosion. When metals, such as mild steel, corrode in an aqueous solution, it is commonly accepted that certain electrolytes - notably chlorides and sulphates - are corrosive.


Corrosionpedia Explains Anion

Examples of anions include:

  • Hydroxide anion: OH-
  • Oxide anion: O2-
  • Sulfate anion: SO42-

Since they have more electrons than protons, anions have a negative charge. When the chloride anion is represented with Cl-, the "-" charge indicates that it has one less proton than the total number of electrons.

Electrons, due to their smaller mass and thus larger space-filling properties as matter waves, determine the size of atoms and molecules that possess any electrons at all. Thus, anions are larger than their parent molecule or atom. This is because excess electron(s) repel each other, and add to the physical size of the ion; an ion's size is determined by its electron cloud. As such, in general, cations are smaller than their corresponding parent atom or molecule due to the smaller size of their electron clouds.

Electrostatic attraction between positive and negative ions bring the particles together to create an ionic compound, such as sodium chloride. An anion is formed through the gain of electrons, called ionic bonding. Metals lose electrons, making them positive cations. Nonmetals gain electrons, making them negative anions. Nonmetals generally are more electronegative than metals, meaning that they have a stronger pull on their electrons. Thus, when they form ions, metals give up electrons and nonmetals take up electrons.

The effect of SO42−, Br−, Cl− , I−, ClO4−, NO3−, NO2−, WO42−, H2PO4−, HPO42−, and CrO42− on the corrosion of zinc when placed in an aqueous solutions, for example, is seen by measuring the potential of a pure zinc electrode in a neutral, strongly-aerated solution of anions as a function of both time and electrolyte concentration. The first six anions promote corrosion and the extent of corrosion-promotion is related to their catalytic effect on oxygen reduction in a solution.

With high dilutions, NO2−, H2PO4−, HPO42− and CrO42− develop potentials that are practically independent of the concentration. Corrosion is assumed to be occurring at an equal rate to that of film repair by oxygen. Ennobling of the potential takes place at a certain salt content and inhibition or passivation sets in at this point.

Zinc corrosion in acid solutions of NO3−, SO42−, Cl−, ClO4−, H2PO4− and CrO42−, when being examined via a thermometric method on the basis of Δ T-normality, a distinction can be made in curves between corrosive NO3−, SO42−, Cl− and ClO4− anions and inhibiting H2PO2− and passivating CrO42− anions, on the other.

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) have been modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to explain the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the fact that the rate of air-oxidation at this pH is very fast. As a result, dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe2+ and OH- ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH- ions results in strongly acidic solutions (pH 1.41), causing acid corrosion. It has been proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, thereby delaying the crevice- and acid-corrosion stages.


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