Corrosion Inhibition By Essential Oil Extracts: A Sustainable Method Of Controlling Carbon Steel Degradation

Iron has played a dominant role in the development of modern civilization since its discovery four millennia ago. Human society is heavily dependent on ferrous alloys, without which industrialization would not be as it is today.

The most common alloy of iron in existence, accounting for 80% of its production, are carbon steels. The steels possess desirable mechanical and physical properties coupled with its relatively low cost, availability, and recyclability. The ever-expanding industrial complex worldwide necessitates the increasing use of carbon steel in infrastructure, transportation, manufacturing, etc. Carbon steel is consistently exposed to aqueous environments composed of corrosive anions, especially in acid manufacturing, acid pickling of steel, chemical cleaning and processing, ore production, and oil well acidizing. These environments cause extensive corrosion damage and deterioration of the steels.

Corrosion is a huge concern in most industrial establishments and manufacturing plants due to the high cost associated with maintenance, damages, replacement of metallic parts and structures, and corrosion control. Effective corrosion control methods such as protective coatings and linings, cathodic protection, anodic protection, materials selection, etc. are available, but none of them are as economical as the use of corrosion inhibitors to extend the service life of industrial parts, structures, and components manufactured from carbon steel for application in aqueous environments.

Inhibitors are chemical compounds of organic and inorganic origin. In some cases, it combines both origins. Organic inhibitor compounds mostly function by forming a protective film of one or multiple molecular layers on carbon steels that suppress the transfer, diffusion, and electrolytic transport of corrosive anions responsible for redox electrochemical reactions and surface degradation of the steel. Most available corrosion inhibitors are unsustainable due to their toxicity, cost, and strict regulations governing their use.

Research on green organic chemical compounds has given very promising results to satisfy the unending thirst for cost-effective sustainable alternatives. This is particularly evident in the oil and gas industry, where a huge proportion of their fortune goes on inhibitors to mitigate the corrosion problem and justify their extensive use of carbon steels.

Essential oil extracts have proven to be effective inhibitors of carbon steel corrosion. These are condensed hydrophobic fluids containing volatile aroma components from plants. They have been determined experimentally to consist of components whose organo-metallic interactions lead to improved protection of metallic surfaces. They also have the advantage of ready availability, non-toxicity, and are economical. However, their performance from a comprehensive study of literature tends to be concentration-dependent, which is a huge setback in corrosion control. In many cases, lower inhibitor concentration accelerates corrosion faster than the control corrosive solution. Applications that require consistent monitoring of inhibitor concentrations, such as process environments, are particularly at risk, as the inhibiting compound only delays the onset of metal degradation.

The need for further research to develop green compounds with effective performance even at very low concentrations necessitates the study of the synergistic properties of two different admixed essential oils that are significantly different in major compositions. The choice of a rosmarinus officinalis and trypsin complex for performance evaluation of their combined corrosion inhibition properties has previously focused on their individual performance, especially at high concentrations. Their inhibition performance on medium carbon steel in dilute sulfuric and hydrochloric acid media using conventional electrochemical techniques was great.

Optical microscopy assessed the morphological changes of the steel which slightly varies from the untested specimen and significantly differs from the corroded steel in the aqueous environment with the inhibitor. ATF-FTIR spectroscopy exposed the major functional groups: nitrile, carboxylic acids, primary amines, aromatics, nitro compounds, aromatic amines, alcohols, aliphatic amines, and secondary amines functional groups responsible for corrosion inhibition. Results showed molecular coverage and chemisorption adsorption of the functional groups within the extracts stifled the corrosion reaction processes responsible for metal degradation. Studies also showed the extracts are significantly more effective in hydrochloric acid compared to sulfuric acid.