Corrosion inhibitor: An overview

Corrosion is a broad topic. This discussion will be limited to metal corrosion caused by the electrochemical breakdown of metals. Metals are naturally prone to oxidation and corrosion. Corrosion inhibitors are chemicals that prevent metal corrosion. The inhibition of anodic or cathodic corrosion reactions can be due to the reduction of the active surface area of metal and/or to a change in the activation energy of the oxidation or reduction process in corrosion.

Why do metals have a tendency to oxidize?

Metals are electropositive with little affinity for electrons. Metals want to lose electrons and form positively charged ions.

But why it is so?

There can be two ways we can explain it. [1] atoms [ transition metal like Fe] have a big number of protons and big size with their outer valance electrons far away from the nucleus with relatively fewer attractions between protons in the nucleus and the far away located valance shell electrons. So, metals try to lose electrons. The second reason [2] is that transition metals have many electrons similar in energy, meaning that it is sometimes easy for many of those electrons to be lost. [This is also the reason why metals have a high electrical conductivity (i.e., their electrons can easily move from atom to atom]. So, to summarize, with little opportunity metal tends to lose electrons. The loss of electrons is oxidation.

Corrosion inhibitor definition

A corrosion inhibitor is defined as a "chemical substance that, when present in the corrosion system at a suitable concentration, reduces the corrosion rate while not significantly changing the concentration of any corrosive agent." In general, it is effective at low concentrations. This includes any chemical that reduces corrosion rate through significant pH variation, as well as oxygen and hydrogen sulphide scavengers that cause aggressive species to be removed from the solution.

Classification of inhibitors

Corrosion inhibitors are classified as cathodic, anodic, or mixed, depending on whether their influence is primarily in slowing the cathodic or anodic reaction of the corrosion process, or both. As a result, they cause a shift in the corrosion potential of the inhibited metal toward either the cathodic or anodic directions, or they substantially leave the metal corrosion potential more or less unchanged. The inhibition of anodic or cathodic corrosion reactions can be caused by a decrease in a metal's active surface area and/or a change in the activation energy of the oxidation or reduction process in corrosion. The combination of cathodic and anodic corrosion inhibitors frequently results in improved protection and allows for lower inhibitor concentrations.

What does it mean? What is corrosion potential?

The electrode potential of a metal is an indication of the tendency of the metal to dissolve and corrode in a certain electrolyte. The magnitude of the electrode potential of a metal is a measure of its relative tendency to lose or gain electrons, i.e., it is a measure of the relative tendency to undergo oxidation (loss of electrons) or reduction (gain of electrons). Eventually, this provides energy for corrosion]

Reference is also made to the "nobility" of the metal. The nobler the metal, the higher the potential is, the less the tendency it has to dissolve in an electrolyte.

What is the nobility of metal?

By simple definition, “noble metal is one which resists oxidation and corrosion like gold, silver, and copper.

The electrode potentials of different metals can be specified in relation to one another in the galvanic series for different electrolytes. The galvanic series of various metals in seawater is shown below.

Considering the steel-copper example, it will be noted from the table above that copper has a higher potential (is nobler) than plain carbon steel. The steel will be the anode and corrode, whereas the copper will be the cathode and not corrode.

What is Cathodic vs Anodic protection?

?The underlying science is very interesting.

Cathodic protection converts all anodic areas on a metal surface to cathodes so that corrosion ceases. Anodic protection, on the contrary, makes the entire metal surface anodic. The surface becomes so anodic that the metal completely passivates.

Obviously, then, this technique is limited to metals that can form protective passive films.

How do corrosion inhibitors work?

The inhibition of anodic or cathodic corrosion reactions can be due to the reduction of the active surface area of metal and/or to a change in the activation energy of the oxidation or reduction process in corrosion. The combination of cathodic and anodic corrosion inhibitors often determines improved protection and allows for reducing the inhibitor concentration.

What is the activation energy for corrosion?

Activation is the changing of a passive surface of a metal to a chemically active state. Activation polarization is usually a controlling factor during corrosion in media containing high concentrations of active species or strong acids. The increased corrosion rate results from increased activation energy for chemical and electrochemical reactions, increased diffusion rates in the electrolyte and increased transport through the electrolyte or environment and across films that may be formed on the metal surface.

Types of corrosion inhibitors

Whichever the reaction is hindered by the corrosion inhibitor, it will interact at the metal/solution interface by forming a film, which may be of three different types: (1) passivating film; (2) precipitation film, and (3) adsorption film.

Passivating film

Passivating oxide films are 30–200 ? thick and afford excellent corrosion protection. Among them, classical oxidizing inhibitors can be found, such as chromates and nitrites, now largely abandoned because of toxicity concerns. This class includes also nonoxidizing inhibitors, such as tungstates and molybdates, affording their passivating effect only in aerated solutions

Precipitation film

Precipitation film-type inhibitors are chemicals forming insoluble protective films by reaction with soluble species in the environments (e.g., phosphonates and polyphosphates, forming protective films with calcium ions in solution) or with the protected metal ions (e.g., copper-benzotriazole (BTA) salt film).

Adsorption protective film

Adsorption protective films are mainly organic substances. They often have the molecular structure of a surfactant, with a hydrophilic group capable of bonding with the metal surface and a hydrophobic part of the molecule protruding toward the solution bulk. Adsorbed inhibitor molecules limit oxygen diffusion and water access to the metal surface, so reducing the corrosion rate.

Vapor phase inhibitors/volatile corrosion inhibitors

In short, Vapor phase corrosion inhibitors are volatile compounds introduced in a closed system for corrosion protection. These inhibitors possess a high vapor pressure at normal temperatures and protect the metal by forming a bond and barrier layer on the metal surface. In general, a weak volatile acid or base that easily hydrolyzes provides the most effective inhibition. Volatile alkaline compounds such as octadecylamine and morphine are introduced in boilers with steam to inhibit corrosion in condenser tubes by neutralizing the acidic carbon dioxide.

Credit: Google