Inhibitive impacts extract of Citrus aurantium leaves of carbon steel in corrosive media

ABSTRACT Relatively inexpensive, stable plant extract, namely Citrus aurantium leaves, was employed as highly efficient inhibitor of carbon steel corrosion by corrosive acid. The inhibition efficiency was estimated based on the weight loss method. Inhibition impacts of researched inhibitor increase with the concentration of the plant extract increase. The inhibition efficiency depends on three factors: molecular structure, concentration, and molecular weight of the inhibitor. Inhibition efficiency of 81.2% was achieved with 20% (v/v) of the extract in 1 M hydrochloric acid during 3 h at 25°C. The effect of temperature was also investigated and activation parameters were evaluated. Inhibition adsorption characteristics were approximated by Langmuir adsorption isotherm. Chemical adsorption mechanism was proposed for the studied inhibitor from the trend of inhibition performance and temperature degree in addition to activation energy values and heat of adsorption. GRAPHICAL ABSTRACT


Introduction
Corrosion is an event that was considered either chemical or physical in nature, denatures the mineral characteristic, and of alloys make them inefficient for specified function (1)(2)(3)(4)(5). The main reason of the corrosion of alloys is their tendency to revert to the steady state. Most of the alloys are inherently unsteady and their natural trend to seek self-demolition through interacting with the environment to reach to lower energy state via producing metal compounds. These were the states that where most of the minerals were found (6). As there are many techniques for controlling the corrosion of alloys, the employ of inhibitor is the superior way for protecting alloys from corrosive solutions (7). Inhibitors were compounds that added in little quantities to corrosive solutions in order to prevent the reaction of surface of metal with the corrosive environment (8). They were added to considerable systems, such as cooling, refinery and gas production units (9). Anticorrosion coatings are generally employed to inhibit the average of corrosion and increase the longevity of the mild steel. A broad range of organic adsorption inhibitors presently applied in the corrosion domain is expensive (10)(11)(12)(13)(14)(15)(16). Inhibitor adsorption on mild steel is affected by the nature of the mild steel, type of electrolyte, and molecular structure of the inhibitor (17)(18)(19)(20)(21). Inhibitor molecules are adsorbed on the surface of mild steel forming a barrier and consequently preventing reactions (cathodic or anodic) from processing at the surface of mild steel. These inhibitors could react with the iron atom at the mild steel surface to form inorganic complexes, blocking the surface of mild steel (22)(23)(24)(25)(26). Due to the high cost, toxicity, and hazardous of synthesized corrosion inhibitor (27)(28)(29), the examination of plant extract as corrosion inhibitors is presently interesting (30)(31)(32). Herein we focus on simple, practical, and economical corrosion inhibitors, so in this study, extract of Citrus aurantium leaves was used as green corrosion inhibitor for CS "Carbon Steel" in corrosive media.

Specimen preparation
CS "Carbon Steel" of thickness 3 × 10 −1 cm was obtained locally and was mechanically cut into coupons with dimensions of 65 × 10 −1 cm, 25 × 10 −1 cm, 3 × 10 −1 cm having a hole with the diameter of 3.0 mm. First, coupons were polished, washed with distilled water, acetone and dried.

Collection of plant materials
Fresh C. aurantium leaves were obtained from Citrus orchards in Iraq and identified in College of Agriculture of the University of Basra, Basra City. The leaves were air-dried, grinded, and made into a fine powder using laboratory mortar and kept in a sterile air-tight container to avoid contamination.

Preparation of extract
Two hundred grams of dried leaf powder was dissolved in 1000 ml of ethanol for 48 h and centrifuged at 3000 rpm to enable paper diffusion of the active ingredients into the extraction medium. Filtration was later carried out using Whatman's filter paper and the filtrate was stored at 5 o C in a refrigerator.

Impact of temperature
The influence of degrees of temperature on the corrosion parameters namely, WL "Weight Loss", CR, and    Figure 3 and Figure 4) indicated that CR of CS without and with the extract increase with increasing in temperature, although corrosion rate is lowered in the presence of the extracts comparing with a blank solution, but the IE increases with increasing temperature. These results showed that the adsorption of the extract on the CS in corrosive solution is chemisorption (36, 38).

Activation parameters of the inhibition process
As shown in Figure 5, the CR of CS getting from WL measurements versus the temperatures 25°C, 45°C, and 65°C, with a slope of −E a /2.303R. In addition, Figure 6 has a plot of log CR/T versus 1/T with a slope of −DH a /2.303R that the DH a values were calculated. E a and DH a values are listed in Table 3.

Adsorption isotherm
Increasing of extract molecules that adsorb on the surface of CS and the interaction could have qualified though adsorption isotherm. The data have been tested graphically though appropriate to the Langmuir model. This model is given by (41, 42) By plotting values of C inh /u versus C inh (Figure 7), a linear plot was obtained indicating that the adsorption of the studied plant extract as corrosion inhibitor was harmonious with the assumption of Langmuir adsorption isotherm and the slope obtained is close to unity.

Conclusion
Citrus aurantium leaves extract acts as efficient corrosion picking inhibitor on CS in 1 M corrosive media.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The authors thank UKM-YSD Chair on Sustainable Development for the grant 020-2017 "Malaysia" for supporting this work.