In recent years, the anti-corrosive properties of natural extracts as environmentally friendly inhibitors have gained considerable interest. This study evaluates the potential of Marjoram (Origanum majorana L.) essential oil (OML), collected from Salé, Morocco, as a corrosion inhibitor for mild steel in 1 M HCl medium. The protection performance of OML was assessed using various electrochemical techniques, including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), as well as the weight loss method. The influence of OML concentration and temperature on the inhibition performance were investigated. OML demonstrated pronounced inhibitory benefits via increasing the corrosion resistance of mild steel in the corrosive HCl solution, thus reducing the corrosion rate to 0.11 mg cm⁻² h⁻¹ and increasing the inhibition efficiency to 87.1% at an inhibitor concentration of 500 ppm. PDP confirmed that the inhibitor works as a mixed-type inhibitor with cathodic supremacy. EIS revealed that the charge transfer mechanism is the main controlling factor for the corrosion process. The thermodynamic parameters suggested a key role of OML physisorption in inhibition, following the Langmuir isotherm. Importantly, SEM and EDX analyses suggested the formation of a protective layer of the extract onto the steel surface, which shields the surface from corrosive species. This is owed to the functional group-rich phytochemicals of OML. Therefore, the development of bio-based corrosion inhibitors is not only a step towards more eco-friendly industrial practices, but also meets the growing demand for sustainable materials in a world with constrained resources.

Phytochemical-rich natural extracts have recently attracted intense attention as green corrosion inhibitors and costly benign coating components for the protection of metallic structures of immense commercial importance. Herein, various methods were applied to assess the corrosion protection efficiency of a methanolic extract of parsley (Petroselinum crispum) (PCE) on carbon steel C37 in 1 M HCl. Initially, the chemical profile of PCE was analyzed using gas chromatography/mass spectrometry (GC/MS), and myristicin and apiol were identified as the main components. The results from the weight loss, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) techniques revealed a substantial reduction in the corrosion rate upon the use of PCE, with a maximum inhibition efficiency of 92% at 1 g L⁻¹ PCE. To optimize the performance, the corrosion behavior was investigated over a temperature range of 303–333 K and for concentrations of 0.1–1 g L⁻¹. The inhibition effectiveness increased at higher concentrations of PCE, whilst it decreased when the temperature was elevated. The query suggests that the adsorption process involves both physical and chemical mechanisms. The adsorption of PCE onto C37 was well described by the Langmuir adsorption isotherm. The data were used to determine the activation energy and thermodynamic parameters. The PCE coating acted as a mixed-type inhibitor, hampering both cathodic and anodic corrosion reactions. SEM further confirmed the formation of a protective coating film on the steel surface when exposed to PCE. UV-Vis and XRD were implemented to understand the inhibition mechanism and formed products at the microscopic and spectroscopic levels. Hence, the green PCE inhibitor may potentially be applied in corrosion mitigation due to its high corrosion protection efficacy and its environmentally benign nature.

This work demonstrates the recycling potential of a hydroalcoholic extract of spent coffee waste (HECG) as an environmentally friendly additive to soybean cooking oil (SWCO) biodiesel, alleviating its corrosive impacts on a Cr-rich stainless-steel alloy. The extract is efficiently produced using the Soxhlet extraction method, and its chemical profile is analyzed using GC-MS and FTIR. Furthermore, the anticorrosive properties of HECG are thoroughly evaluated using gravimetry, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The HECG additive shows excellent inhibitory efficacy (94%) on 307 L stainless steel. The polarization results underline the mixed inhibitory character of HECG. The metal surface is primarily protected against biodiesel-induced corrosion through adsorption of additive molecules onto it. Additionally, SEM-EDX and XRD techniques are used to evaluate the microstructure, crystal structure, and composition of the surface and formed products after immersion in this biodiesel. The results showcase the additional protection of the 307 L sample via creating a passive oxide film (CrO₂₃, FeO₂₃, FeOOH) on the metal surface, thus boosting its corrosion resistance. Importantly, DFT and Fukui function simulations identify the electrophilic and nucleophilic sites at the β-turmerone molecule, the major component of the extract, and reveal intrinsic retro donations in both ways – additive to metal and/or metal to additive.