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Corrosion inhibitors

Dr. Dmitri Kopeliovich

A corrosion inhibitor is a chemical compound, which when added to an environment (aqueous solution, oil, fuels, atmosphere) reduces the corrosion rate.

Cathodic corrosion inhibitors

Cathodic inhibitors reduce the corrosion rate due to retarding cathodic reactions.
A cathodic inhibitor causes formation of insoluble compounds precipitating on the cathodic sites in form of a barrier film. The effective cathode area is one of the Factors of galvanic corrosion therfore its reduction results in decrease of corrosion rate.

The following compounds are used as cathodic inhibitors:

  • Zinc salts (zinc hydroxide, zinc phosphate);
  • Calcium salts (calcium carbonate, calcium phosphate);
  • Magnesium salts;
  • Polyphosphates.

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Anodic corrosion inhibitors

Anodic inhibitors reduce the corrosion rate due to retarding anodic reactions.
An anodic inhibitor shifts the equilibrium of the corrosion process to the passivation zone (Pourbaix diagrams) causing formation of a thin invisible passivation oxide film on the anodic sites, which increases the anode potential and depresses the oxidation process.
Reduction of the effective anode area results in decrease of corrosion rate.

Anodic inhibitors has a serious disadvantage: at low concentrations they cause increase of corrosion rate therefore it is important to avoid decrease of the inhibitor content below the optimum level.

The following compounds are used as anodic inhibitors:

  • Chromate (CrO42-);
  • Nitrite (NO22-);
  • Molybdate (MoO42-);
  • Orthophosphate (PO43-).

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Adsorption corrosion inhibitors

Adsorption inhibitors reduce the corrosion rate due to Polarization of the metal by extremely thin layer of their molecules adsorbed on the surface.
Decrease of the effective surface area results in reduction of the corrosion rate.
Adsorption inhibitors are substances (mainly organic) capable to form chemisorbed bonds with surface metal atoms.

The following compounds are used as adsorption inhibitors:

  • Amines (R-NH2);
  • Carboxyls (R-COOH);
  • Thiourea (NH2CSNH2);
  • Phosphonates (R-PO3H2);
  • Benzonate (C6H5COO-);
  • Antimony trichloride (SbCl3).

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Volatile corrosion inhibitors (VCI)

Volatile corrosion inhibitors (VCI) reduce corrosion in closed spaces (package bags).
VCI compound is emitted (vaporized) by the material enclosing the space.
The vapors condense on the metal surface in form of microscopic crystals, which dissolve in the moisture film present on the surface.
The ions of the dissolved VCI displace water molecules from the metal surface and form monomolecular invisible protection film reducing the corrosion rate.
Volatile corrosion inhibitors may be added to various package materials: polymer film (eg. low density polyethylene), paper, foam, powder, oils\, etc.).

The following compounds are used as Volatile corrosion inhibitors (VCI):

  • Cyclohexylamine;
  • Dicyclohexylamine;
  • Guanidine;
  • Aminoalcohols;
  • Nitrites.

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corrosion_inhibitors.txt · Last modified: 2012/05/31 by dmitri_kopeliovich
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