Citation: Simovi´ c, A.R.; Grgur, B.N.; Novakovi´ c, J.; Jana´ ckovi´ c, P.; Bajat, J. Black Pine (Pinus nigra) Essential Oil as a Green Corrosion Inhibitor for Carbon Steel. Metals 2023, 13, 508. https://doi.org/10.3390/ met13030508 Academic Editor: Yanxin Qiao Received: 6 February 2023 Revised: 27 February 2023 Accepted: 28 February 2023 Published: 2 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). metals Article Black Pine (Pinus nigra) Essential Oil as a Green Corrosion Inhibitor for Carbon Steel An ¯ dela R. Simovi´ c 1 , Branimir N. Grgur 2, * , Jelica Novakovi´ c 3 , Pe ¯ da Jana´ ckovi´ c 3 and Jelena Bajat 2 1 Innovation Center, Faculty of Technology and Metallurgy, 11000 Belgrade, Serbia 2 Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Serbia 3 Faculty of Biology, Institute of Botany and Botanical Garden “Jevremovac”, University of Belgrade, 11000 Belgrade, Serbia * Correspondence: bngrgur@tmf.bg.ac.rs; Tel.: +381-11-3303681 Abstract: In this work, the essential oil of black pine (Pinus nigra J. F. Arnold) was used for the first time as a natural, ecological, and sustainable corrosion inhibitor for the acid cleaning of car- bon steel. The essential oil was extracted by hydrodistillation using a Clevenger-type apparatus, and the oil was analyzed by gas chromatography–flame–ionization detection (GC–FID) and gas chromatography–mass spectrometry (GC–MS). The most abundant components in the essential oil were α-pinene, germacrene D, (E)-cariophyllene, and β-pinene. The inhibition efficiency was deter- mined by electrochemical methods (electrochemical impedance spectroscopy and potentiodynamic polarization measurements). The results showed that the inhibitory efficiency of the black pine essential oil increases with time, reaching the highest values after 4 h of immersion for all inhibitor concentrations. It was also shown that black pine essential oil is a mixed-type inhibitor. The contact angle measurements confirmed that the black pine essential oil, as a new natural, environmentally safe inhibitor, is able to protect carbon steel from corrosion in a 1 M HCl solution. Keywords: steel; corrosion; plants; essential oil 1. Introduction Due to its excellent mechanical and physical characteristics, carbon steel plays a very important role in engineering, construction, packaging, many types of machinery, and military applications [1,2]. Steel is very susceptible to corrosion, especially in acidic media [3–6]. Of all the acids, HCl is the most commonly used in industry from an economic standpoint in terms of wastewater recovery. However, the use of aggressive acids to remove surface contaminants and deposits on the metal leads to corrosion, which causes the greatest economic losses [7,8]. A major challenge for scientists working in this field of research is the study and development of materials as corrosion inhibitors. Corrosion inhibition is necessary from an economic point of view, but it is also dangerous for the environment, causing mutagenic damage to biological systems and cytotoxicity [9,10]. The use of chemicals that slow the corrosion of metals in the water phase endangers the ecosystem and living organisms, necessitating the search for environmentally friendly alternatives worldwide [11]. The use of most inorganic compounds, such as chromates, and some synthetic organic molecules that act as good corrosion inhibitors is banned or restricted in most applications due to their carcinogenic, toxic, and hazardous natures, as well as their costly and complex synthetic routes [12–15]. The evolution of green inhibitors of corrosion is significant and necessary for food quality and safety standards [9]. Considerable efforts are being made to find natural products that would not influence the environment and that could slow down the corrosion attack on metals as much as possible. According to previous investigations, natural corrosion inhibitors are readily available, cheap, biodegradable, and environmentally friendly [16–18]. Metals 2023, 13, 508. https://doi.org/10.3390/met13030508 https://www.mdpi.com/journal/metals