1. GV Extract—A New Corrosion Eco-Inhibitor

A notable aspect of this work is the focus on Galium verum, a plant species previously underexplored from this perspective, or corrosion inhibitor. This study bridges gaps in knowledge, opening avenues for further research on its multifaceted applications.

Comprehensive chromatographic analyses (HPLC, GC-MS) identified critical metabolites responsible for bioactivity ^[1].

Substances such as kaempferol, umbelliferone, quercetin, gallic acid, vanillic acid, and caffeic acid were found in abundance, contributing to the extract’s efficacy.

This study differs from previous works by using a plant that has not been previously investigated as a corrosion inhibitor. In addition, a detailed characterization of the active substances responsible for the inhibition, comparing their efficiency with that of other natural and synthetic inhibitors. The results obtained are comparable to those reported in the literature, but our contribution consists in highlighting the potential of a completely new and sustainable source.

In Table 1 there is a comparison that highlights that plant-based inhibitors, and so is GV extract, are a promising alternative for sustainable applications, although classical inhibitors remain preferred in stringent industrial environments.

The GV extract has all the advantages of green inhibitors, extracted from plants, but it also has some specific advantages: GC is a spontaneous plant, widespread in Europe, Asia, and North Africa, with no environmental impact. Plant culture could be used both for phytotherapeutic and industrial purposes.

As a corrosion inhibitor in acid media, the efficiency is very good, with a maximum value of 91.82%, close to classical ones.

Results of analytical investigations on ethanolic GV extract proved it to be a polyphenol-based green inhibitor ^[1]. In comparison with other chemical categories of substances discovered in plant extracts, polyphenols from GV extract act as a mixed inhibitor by forming protective films on metal surfaces. The main known categories of phytochemicals responsible for corrosion inhibition are alkaloids ^[2], phenolic compounds ^[3], tannins ^[4], and triterpenes ^[2]. The complex chemical compositions of these plant extracts make it very difficult to attribute the inhibitory action to a particular component or group of components. Thus, phenolic compounds generated more interest because of their high redox potential, which allows them to act as reducing agents, hydrogen donors, and singlet oxygen inhibitors, and also because of their metal chelating potential ^[5]. In Table 2, a comparison between different chemical categories of plant inhibitors is presented.

Table 1. Comparison of GV extract vs. other plant inhibitors and classical inhibitors.

Table 2. Comparison of corrosion behavior of different types of chemical compounds from plant extracts.

Detailed explanation of polyphenol action as corrosion inhibitors is given below. Derived from renewable natural resources, the GV extract could be a part of a circular economy and reduce the utilization of classical, non-renewable, synthetic inhibitors. Because the plant is readily available, it will involve low production costs. Being already part of pharmacopeia, not only in Eastern Europe but also in other regions, GV provides fewer risks to human health or the ecosystem.

The research also explored potential sustainable applications for this extract, leveraging the eco-friendly and biodegradable nature of plant-based products.

The findings pave the way for future studies on enhancing extraction and/or investigation techniques and broadening applications to include not only therapeutic but also industrial fields. These advantages make GV extract, a plant-based inhibitor, a promising solution for industries looking to adopt greener and more sustainable practices.

2. Consideration on Inhibitor Corrosion Mechanism of GV Extract

From the electrochemical experimental results, Galium verum extract was shown to be a mixed type of inhibitor that acts on both the anodic metal dissolution reaction and the cathodic hydrogen evolution reaction.

At carbon steel corrosion in an acidic medium, a sequence of steps could be identified in the corrosion mechanism [61]. Thus, the next electrode processes could be mentioned: the anodic carbon steel dissolution reactions and the cathodic hydrogen evolution reactions: On the steel surface a film is formed by the adsorption of GV extract, by the interaction of iron ions and plant extract compounds. Absorption inhibitors are substances that can selectively adsorb either the anodic zones or the entire metal surface, forming passivation films that slow down the dissolution of metal or stop the diffusion of the depolarizer in the cathodic areas.

Adsorption inhibitors are polar molecules that adsorb on the metal surface, inhibiting both electrode processes but especially the cathodic process of hydrogen discharge. These inhibitors are particularly effective in very small quantities (10⁻⁵ ÷ 10⁻³ mol/L) because they act in almost monomolecular layers. The greatest effectiveness of adsorption inhibitors is in acidic media.

Adsorption of inhibitors is a particular case, electrosorption occurring through a substitution reaction of solvent molecules oriented towards the metal surface, and it is also influenced by the electrical variable of the interface, i.e., the electrode potential. Adsorption of organic molecules influences the surface tension and the differential capacitance of the electrochemical double layer, their maximum adsorption occurring near zero charge. In practice, adsorption inhibitors are surface-active substances or surfactants (Figure 2).

Figure 2. Schematic representation of the adsorption of corrosion inhibitors competing corrosion products on the metal surface.

The adsorption mechanism is dependent on the polarizability of the molecules. The asymmetric chemical structure, formed by two parts, one polar – acting as an anchor and one non-polar, oriented towards the corrosive environment, formed by the hydrocarbon residue and which forms the actual adsorption layer, which isolates the metal and prevents the corrosion reaction. Among the most effective anchor functional groups are -CH2-OH, -COOH, -CHO, -COOR, molecules that are found in abundance in the GV extract. In a previous study ^[1], a chemical characterization of Galium Verum extracts using analytical techniques identified and measured the presence of polyphenols: gallic acid, catechin, vanillic acid, caffeic acid, kaempferol, quercetin, and umbelliferon and others compounds including alcohols, phenols, fatty acids, hydrocarbons, esters and other aromatic compounds.

Size, spatial orientation, shape and electrical charge play an important role in the inhibition mechanism. The larger the volume of a molecule (aromatic compounds for example), the greater the inhibition capacity. The nature of the metal is also important. Metals in the Fe group, having free “d” electronic levels, tend to desorb organic molecules with “π” electrons, i.e. polar molecules, containing O, N, S.

Also, the greater the number of polar groups, as in the case of polyphenols, the more intense is the corrosion inhibition.

Taking into consideration the above arguments, among the substances present in plant extract to be corrosion inhibitors, polyphenols could be considered the main actors in limiting the corrosion rate of metals.

3. Conclusions

Due to the variety of existing plants and their constituent substances, in order to replace the dangerous toxic substances that have proven their effectiveness in protecting metals against corrosion, studies are being carried out to test plant extracts against corrosion. Most extracts are prepared from leaves, but they have also been reported for other parts of plants. Acidic environments are the most tested corrosive environments, being encountered in various industrial processes, and carbon steel is one of the most widely used industrial materials, therefore the protection of carbon steel is the most researched. The use of natural corrosion inhibitors, obtained from plants, brings significant environmental benefits. These biodegradable compounds reduce pollution and risks associated with synthetic chemicals, which can be toxic and difficult to break down. Furthermore, extracting inhibitors from plant sources promotes a sustainable process, avoiding the contamination of soil and water with hazardous substances. This approach helps protect ecosystems and reduce the negative impact on human health and nature. In essence, choosing nature is an important step towards a greener future.

In this work, the inhibitory effect of the extract of Galium Verum (GV) was evaluated as a corrosion inhibitor for carbon steel in 1M HCl solutions. Studies were carried out in the temperature range 20 - 50°C.

The determinations of the potential in the open circuit made at all the studied temperatures showed the displacement of the potential towards more positive values in the solutions with the addition of GV extract compared to the results obtained in the control solution of 1M HCl.

The results of the Tafel-potentiodynamic polarization studies showed that the presence of the GV extract changed the corrosion potential towards more positive values and the variation of the cathodic slopes was more pronounced than the anodic slopes, suggesting that the GV extract is a mixed inhibitor, predominantly cathodic. Electrochemical impedance spectroscopy studies showed an increase in polarization resistance in the presence of GV extract compared to the results obtained in its absence.

Green inhibitors have been reported to have an inhibition efficiency around 60-95% which means that Galium Verum is a very good one. The corrosion inhibitory efficiency of the Galium Verum ethanolic extract had a maximum value of 91.82%, for a concentration of 400 ppm polyphenol content, demonstrating the inhibitory potential of this green product comparable with synthetic inhibitors for steel in acid media. Obtained values of the inhibition efficiencies for mild steel in 1 M HCl were between 19.92% at the addition of 50 ppm GAE at 30°C and 91.82% at 400 ppm GAE at the temperature of 40°C. Temperature over 40°C decrease the inhibition efficiency of GV extract, due to less stability of polyphenols, the chemical species responsible for inhibition effect. Polyphenols and others polar molecules contained in GV-extract are adsorption corrosion inhibitors, acting after a mixed mechanism.

The presence of the GV extract led to the reduction of the corrosion rate, the results being comparable to the values obtained for other plant extracts, over 90%, therefore promising for the use of the GV extract as a green inhibitor of steel corrosion in acid.

GV-extract proved to be a new corrosion inhibitor, with specific advantages : spontaneous flora, widespread on globe, sustainability, cost-effectiveness, high inhibitor efficiency, reduced health risks.

Electrochemical, analytical and statistical techniques from this study establish the mechanism of corrosion and the chemical species involved, but is not exhaustive, further investigations will be continued, more powerful methods should be used to provide additional information, to analyze in detail the interaction of the inhibitor with the metal surface.