(589e) Illustrating the Significance of ??Electrons in the Adsorption of Corrosion Inhibitors

Carbon steel rebars are incorporated in concrete to increase it tensile strength, hence improving its structural integrity and durability. In this high alkaline environment (pH~12.6), carbon steel rebar is naturally protected by a passive film. However, under extended exposure to chloride-rich environments (on-shore/off-shore marine atmospheres and de-icing salts) passivity breakdown can occur. Cl^‒ ions ingress into the concrete matrix reaching the surface of the rebar, and once the concentration is above the critical Cl^‒ threshold, an autocatalytic acid iron hydrolysis reaction will initiate causing a local pH drop, promoting passivity breakdown, and initiating corrosion. Consequently, corrosion products will build up on the surface of the rebar causing an increase in the crystallization pressure ‒ compromising the integrity of the structure. Many corrosion mitigation strategies can be utilized to avoid this issue, however, according to literature, corrosion inhibitors are found to be the most effective due to it being cheap, efficient, and easy to apply.

Organic corrosion inhibitors have gathered a lot of attention due to its low cost, ease of preparation, and purification steps. The inhibiting capability of organic corrosion inhibitor arise from its ability to create an adsorption film on the surface of the electrode, hence protecting it from the corrosive electrolyte. Adsorption films are typically capable of bonding to the metal surface through a hydrophilic group, while a hydrophobic group faces the solution, repelling water molecules. Most popular organic corrosion inhibitor classes are amines, alkanolamines, and carboxylates due to them having polar groups, showing that molecular structure can play a vital role in the inhibition/adsorption phenomenon. One of the most common structural features that has been attributed to have great electron donating abilities are π–bond electrons, suggesting that structures possessing this fragmental feature in a molecule tend to have better inhibitive performance. However, no studies have been found where this structural feature was investigated, although a seeming connection between inhibition performance and π–electrons has been noticed. To this end, the impact of π–bonds electrons on the inhibition performance was studied electrochemically utilizing cyclic potentiodynamic polarization, and structurally using a quantitative structure property relationship with atomic Signatures as descriptors.

Seven corrosion inhibitors were tested to find the influence of π–bond electrons on the inhibition performance. Four compounds did not posses π–electrons (propylamine, cyclohexylamine, triethanolamine, and dimethylamine), while three did (oxalic acid, glutaric acid, and citric acid). Grade 75 carbon steel rebar was used as the working electrode in a 0.1 M Cl^‒ contaminated deaerated simulated concrete pore solution at 25 ºC. A 1:1 molar ratio of inhibitor to Cl^‒ was applied and a cyclic potentiodynamic polarization was used to find the pitting potential of the carbon steel reinforcement in the absence and presence of different corrosion inhibitors.

According to electrochemical testing, carboxylates were able to increase the pitting potential significantly, followed by amines and alkanolamines. The great inhibiting performance of carboxylates can be attributed to the high electron donating properties of π–electrons found in the carboxyl group. To further illustrate this, density-functional theory calculations were utilized, and high density of highest occupied molecular orbital energies were found around π–bonds. Hence, increasing the tendency of donating electrons to the appropriate vacant d–orbital of the carbon steel reinforcement, forming the protective adsorption film on the electrode/electrolyte interface. Amines and alkanolamines, on the other hand, had moderate increase in the pitting potential. The main mode of adsorption of amines/alkanolamines is through the nitrogen lone pair electrons, hence initiating adsorption. Different surface characterization including scanning electron microscopy and energy dispersive X‒ray spectrum were used to further elucidate this phenomenon.

Additionally, a novel approach to show the impact of molecular structure on the corrosion inhibition phenomenon was applied by developing a quantitative structure-property relationship using Signature molecular descriptors. The Signature molecular descriptor is an approach that help to encode the local neighborhood of a molecule. A Signature describes the connectivity of an atom within a 2D structure of a molecule, out to a predetermined number of bonds away called height. This method correlates the occurrences of atomic Signatures in a dataset to a property of interest (i.e. pitting potential) using a forward stepping multilinear regression. It was concluded that the most influential fragmental part of the corrosion inhibitors studied was [O](=[C]) which encompass the oxygen double bonded to a carbon present in the carboxyl group which is the main absorption site for carboxylates ‒ corroborating electrochemical tests. Furthermore, the second most important Signature found was the [N]([C][C][C]) representing a nitrogen atom with lone pair electrons ‒ the main cause of adsorption for amines and alkanolamines.