Corrosion inhibition, surface adsorption and computational studies of new sustainable and green inhibitor for mild steel in acidic medium

Highlights

• After soaking in 0.4 g/L at 303 K, the inhibition efficiency reached 94.1% and 91.6% for OEAq and OEEt, respectively.

• SEM and EDX studies support the formation of inhibitors film on steel surface.

• Theoretical and experimental results are in good agreement.

Abstract

The inhibitory effect investigation of aqueous and ethanolic extracts of Origanum elongatum Plant (will be referred to in this paper, respectively, as “OEAq and OEEt”) on mild steel corrosion in HCl solutions has been evaluated. The analysis and characterization of both products extracted (OEAq and OEEt) were performed using HPLC-UV-ELSD-MS techniques. Various electrochemical techniques was used such as weight loss measurements, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in order to advance in the inhibition process. An appreciable inhibitory effect for both tested inhbitory has been obtained from all the combined results. Experimental results revealed that OEAq and OEEt can be considered as good green inhibitors and that OEAq is more efficient than OEEt with a maximum inhibition efficiency of 94.1% at 0.4 g/L at 303 K. The inhibition efficiency of each inhibitor increases sharply with increasing concentration and a decreasing of efficiency was observed with increasing temperature for both inhibitors. Both OEAq and OEEt were seen to act as a mixed type inhibitors from PDP study and according to EIS technical, their corrosion process is charge transfer controlled. Thermodynamic and activation parameters were determined to discuss the inhibitory behaviour of tested extracts molecules. The two extracts OEAq and OEEt obey to Langmuir adsorption isotherm model as a result of their adsorption on mild steel surface. Morphology of the metal specimen surface without and with inhibitors was investigated using Scanning electron microscopy (SEM) and EDX spectrum. The electronic characteristics of major compounds of OEAq and OEEt (MC-OEAq and MC-OEEt) were studied using the DFT method (density functional theory), the Fukui indices and MD (molecular dynamics) modelling in order to predict the spontaneous adsorption.

Introduction

Several industrial processes use acid solutions in the pickling processes of metals, as Hydrochloric and sulphuric acids, with the aim of removing of undesirable scale and rust [1], [2]. Through use of inhibitors, especially in acid solutions to limit metal breakdown and reduce acid consumption, is one of the most practical strategies for corrosion prevention [3], [4], [5], [6], [7]. The use of synthetic organic compounds containing heterotaoms, heterocylce and aromatic rings to lower corrosion aggression on steel has been widely studied in previous work [8], [9], [10], [11], [12], [13], [14]. Although these synthetic organic compounds have a significant inhibitory effect, they might, nevertheless, be toxic and harmful to the environment. As such, a strict environmental legislation as well as increased ecological awareness among scientists have led to the development of eco-friendly alternatives to help protect metal surfaces from corrosion [15], [16]. Lately, the use of natural compounds extracted of whole plants or their branches (Flowers, Leaves and Stems) gained prominence on the corrosion inhibitory field. For this reason, many researchers have tested some extracts as eco-friendly inhibitors so that they could avoid the toxic effects of the synthetic organic compounds [17], [18], [19], [20], [21]. These naturally occurring substances contain molecule families such as, e.g., polyphenols, amino and organic acids, glucosinolates, alkaloids, tannins, and which shown a good inhibitory effect against metal corrosion [22]. In addition, the obtaining of these extract is based on simple process with low cost. The new challenge in the corrosion inhibition arena today is that these natural substances must take the place of these synthesized organic inhibitors. The extracts of Punica granatum peel, Ginkgoleaves, Tinospora crispa, Fig leaves, Uncaria gambir, Pongamia pinnataseed and Azadirachta Indica can be cited as examples of good green inhibitors for mild steel corrosion in acid solutions with inhibition efficiencies exceeding 90% [23], [24], [25], [26]. The aerial extract of Cnicus Benedictus has given an important inhibition property (92.5%) as green and sustainable inhibitor for mild steel in 0.5 M HCl and the analysis techniques used showed a stronger interaction between the inhibitor molecules and metal surface [27]. Additionally, other study such as Resveratrol (kind of natural non-flavonoid polyphenols existing in polygonum cuspidatum and grape), chemically modified, showed an improved inhibition efficiency (IE) of 92.89% at 400 mg/L for Q235 steel in 1 M HCl and classified as an anode-type inhibitor [28]. Ginkgo leaves extract inhibitor adsorption on steel has obeyed Langmuir isotherm and shown better inhibition action in 1 M HCl than 0.5 M H₂SO₄ [29]. Fig leaves extracts inhibitor for mild steel in 2 M HCl aggressive solution has given 87% as maximal inhibition efficiency and the corrosion process was charge transfer controlled [30]. These advantages led to the selection of the plant Origanum elongatum, whose oils, dried whole plants, and aqueous extracts produced from the leaves had previously been tested for antibacterial activity against moulds, yeasts, and bacteria. On all microbiological species, this plant produced a 100% suppression [31], [32]. The entire plants were also inhibitory to some strains of yeasts, molds and bacteria [33]. Also, Origanum elongatum is a plant extremely popular in traditional medicine field, widely found in the market with very low costs. It is very beneficial because of its anti-diabetic direct effects; this has been proven by anti-hyperglycaemic tests as well as inhibitory effect of its extracts on pancreatic alpha-amylase enzyme activity [34].

In this connection, we suggest testing the inhibitory action of both aqueous & ethanolic extracts of Origanum elongatum (OEAq and OEEt) against mild steel corrosion in HCl (1 M) solution using various electrochemical techniques. Electrochemical measurements and surface characterization methods were used to evaluate the inhibitory behaviour of both inhibitors (OEAq and OEEt) on mild steel corrosion. Weight loss data were used to calculate the adsorption isotherm and thermodynamic parameters. Quantum method DFT was used to reveal the correlation between the major compound’s structures and inhibitory action of both OEAq and OEEt. Moreover, MD simulations were carried out to further explore the interaction between the molecular system and mild steel surface.