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Corrosion Inhibitor

By: Michelle Otutu | Reviewed by Raghvendra GopalCheckmark | Last updated: May 24, 2023

What Does Corrosion Inhibitor Mean?

A corrosion inhibitor is a substance that, when added to a liquid or gas (typically a metal or alloy), reduces or prevents the corrosion of metal surfaces exposed to the environment. Corrosion inhibitors function by creating a barrier between the metal surface and the corrosive environment, either by adsorption or by forming a protective film.

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Corrosionpedia Explains Corrosion Inhibitor

Corrosion inhibitors work by reducing or preventing the electrochemical reactions that lead to corrosion. There are two main types of corrosion inhibitors:

  1. Anodic inhibitors.
  2. Cathodic inhibitors.

Anodic inhibitors function by blocking the anodic reaction, which is the oxidation of the metal surface. They do this by forming a thin film on the metal surface that prevents the metal ions from entering the solution and being oxidized. Anodic inhibitors are typically used for metals that corrode in acidic environments, such as iron and steel.

Cathodic inhibitors function by blocking the cathodic reaction, which is the reduction of oxygen at the metal surface. They do this by consuming the oxygen in the environment or by providing a more easily reducible species for the cathodic reaction. Cathodic inhibitors are typically used for metals that corrode in alkaline environments, such as aluminum and magnesium.

Other common types of corrosion inhibitors include mixed inhibitors and volatile corrosion inhibitors (VCIs). Mixed inhibitors are film-forming compounds which reduce both cathodic and anodic reactions -- for example, silicates and phosphates used in domestic water softeners to prevent the formation of rust water. VCIs are compounds being transported in a closed environment to the site of corrosion due to the process of volatilization from a source -- for example morpholine or hydrazine transported with steam to prevent corrosion in condenser tubes in the case of boilers.

A corrosion inhibitor's effectiveness can be quantified using various parameters, including the corrosion rate, inhibition efficiency and polarization resistance.

The corrosion rate is a measure of the metal loss due to corrosion and is typically expressed in millimeters per year (mm/y). The inhibition efficiency is a measure of the reduction in the corrosion rate due to the inhibitor and is expressed as a percentage. The inhibition efficiency percentage can be calculated as:

Inhibition efficiency = [(CR_blank - CR_inhibited) / CR_blank] x 100%

Where:

  • "CR_blank" is the corrosion rate of the metal in the absence of the inhibitor.
  • "CR_inhibited" is the corrosion rate of the metal in the presence of the inhibitor.

Polarization resistance is a measure of the metal's resitance to the flow of electrons, and is related to the corrosion rate by the Tafel equation:

icorr = B exp[(E-Ecorr) / (aC)]

Where:

  • "icorr" is the corrosion current density.
  • "B" is a constant.
  • "E" is the applied potential.
  • "Ecorr" is the corrosion potential.
  • "a" is the Tafel slope.
  • "C" is the concentration of the electrolyte.

The effectiveness of a corrosion inhibitor also depends on the quantity of water, fluid composition and flow regime. A common mechanism for corrosion inhibition involves the formation of a coating, mostly a passivation layer, which helps prevent the corrosive substance from accessing the metal. The corrosion inhibitor acts as an additive to the fluids surrounding the metal or related object.

Corrosion inhibitors can be organic or inorganic compounds and can be added to a variety of environments, including aqueous solutions, gases and oils. Common organic inhibitors include amines, imidazolines and organic phosphates. Common inorganic inhibitors include chromates, nitrites and molybdates.

Overall, the choice of corrosion inhibitor depends on the type of metal, the corrosive environment and the application requirements.

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