Corrosion Inhibitor - Chemical Classification, Carbon Steel Corrosion
A corrosion inhibitor or anti-corrosive is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness of a corrosion inhibitor depends on fluid composition, the quantity of water, and flow regime. Corrosion inhibitors are common in the industry, and also found in over-the-counter products, typically in spray form in combination with a lubricant and sometimes a penetrating oil. They may be added to water to prevent the leaching of lead or copper from pipes.
A common mechanism for inhibiting corrosion involves the formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal. Permanent treatments such as chrome plating are not generally considered inhibitors, however: corrosion inhibitors are additives to the fluids that surround the metal or related object. Corrosion inhibitors have a widespread application in suppressing or at least mitigating the corrosion process of metals in different fields, spanning from industrial sectors to construction materials to surface treatments for cultural heritage. According to a standard definition, a corrosion inhibitor is a “chemical substance that when present in the corrosion system at a suitable concentration decreases the corrosion rate, without significantly changing the concentration of any corrosive agent.” It is generally effective in small concentrations. This excludes any chemical that reduces the corrosion rate by substantial pH variation, or oxygen and hydrogen sulfide scavengers, causing the removal of aggressive species from the solution.
Classification of Corrosion Inhibitors
Corrosion inhibitors may be classified as cathodic, anodic, or mixed, depending on whether their influence is mainly in retarding the cathodic or anodic reaction of the corrosion process or both of them. As a result, they cause a shift of the corrosion potential of the inhibited metal toward respectively either the cathodic or the anodic directions or they substantially leave the metal corrosion potential more or less unchanged. 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 of 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 to reduce the inhibitor concentration.
Inhibitors forming adsorption protective films are mainly organic substances. They often have the molecular structure of a surfactant, with a hydrophilic group capable of bond 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, reducing the corrosion rate.
Vapour Phase Inhibitors/Vapour Phase Corrosion Inhibitors
Vapor phase corrosion inhibitors are volatile compounds introduced in a closed system for corrosion protection. These inhibitors possess a high vapor pressure at normal temperature 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. Inhibiting anions such as nitrite (NO2−) and amine (NH2−) substituted into organic structures increase vapor pressure, increasing their protective ability against aggressive ions in atmospheric corrosion. The inhibition mechanism of VCI is not clear. It is assumed that VCI forms a weak bond as an adsorbed monolayer that protects the surface from water inclusion and aggressive ions such as sulfates and chlorides. The adsorbed monolayer changes anodic (dissolution of the metal) and cathodic reaction kinetics.
Carbon Steel Corrosion Inhibition
Corrosion inhibitors are one of the additional prevention methods used to delay or stop chloride-induced corrosion in concrete structures. Nowadays, both inorganic compounds (mainly based on nitrites) and organic substances (based on amines) are proposed worldwide. However, there are conflicting opinions about the effectiveness of these products: there is a lot of interest in studying new mixtures and in understanding the mechanism of inhibition. There are results on the effectiveness of some organic substances in preventing chlorides induced corrosion. Tests were performed on carbon steel rebar samples in a simulated concrete pore solution. Potential and corrosion rate measurements, as well as EIS spectra, were performed. Results have been discussed in terms of the influence of organic compounds on time-to-corrosion and in understanding the mechanism of inhibition of the analyzed substances.
Corrosion Inhibitor Manufacturer, Supplier, Exporter in India
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