Corrosion Inhibition of Metal Using Lawsonia Inermis Extract

Introduction

Metal and alloy are widely used in various fields of industries. It’s commonly used in big industries such as manufacturing, processing and others. But metal and alloy used tend to face the corroded state due to the exposure to different chemical and substances that can lead to the corrosion. This phenomenon has become important especially in acidic media because of the increased industrial applications of acid solutions. For examples in industrial refining crude oil, acid pickling, industrial cleaning, acid descaling, oil-well acid in oil recovery and the petrochemical processes. Corrosion happens when the equipment surface was attacked by a strong acid. So in order to prevent the corrosion happen, inhibitors have been used to reduce the rate of metal corrosion.

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Corrosion inhibitor is a chemical that can slow down the acid attacked which can lead to the corrosion on the drill pipe, tubing or any other metal that contacted with acid during treatment. This chemical when added even in small concentration on corrosive media, it will prevent the reaction between metal and media. Many studies have been carried out to find the suitable compound that can be used as inhibitors. These studies reported that, both synthetic compound and plant extract can be used as corrosion inhibitors. But, usage synthetic compound as inhibitor has been limited. Most of them are highly toxic to both human and environmental.

Therefore, non-toxic and natural inhibitors such as plant extract have been used because of their environmentally acceptable, readily available and renewable source. One of the plant extract that have been used as inhibitor is Lawsonia inermis (henna leaves) extract. Henna is an herb which has dyeing properties used in Asia and North Africa. Inhibitive action of henna extract as a cheap, eco-friendly and naturally occurring inhibition on some metal such as aluminium, iron, zinc and nickel in acidic, neutral and alkaline solution has been investigated.

Literature Review

In order to avoid from use synthetic compound, there are other alternative compound which can be used as an inhibitor. Previous research has been carried out and proved that plant extract can be use in corrosion inhibition. Plant extract consists of chemical compositions lead to prevent metal from corroded. But Buchweishaija (2008) state that among of the plant extract, Lawsonia inermis (henna leaves) was the good one due to the high inhibition efficiency of henna leaves extract. Furthermore, the large amount of henna leaves exist which makes it easy to find it in the Asia and North Africa. Also the most important thing is the unique chemical composition present in Lawsonia inermis by functioning as an inhibitor.

 – Chemical Composition of Henna Leaves

Research was carried out to determine the chemical composition contain in henna leaves that can act as corrosion inhibitor. It has been discovered that henna leaves contain soluble matter, Lawsone (2- Hydroxyl-1,4-naphthoquinone, C10H6O3), gallic acid (3,4,5-trihydroxybenzoic acid, C7H6O5) and dextrose (α-D-Glucose, C6H12O6) (Ostovari et al., 2009). Henna leaves also contain other chemical such as resin, coumarins, sterols and tannin (Ostovari et al., 2009). The main components of henna extract are hydroxy aromatic compound like tannin and Lawsone. Tannin or also known as tannic acid present with a core glucose and methyl gallate groups (El-Etre et al., 2005). Tannin which acts as an inhibitor attributes the formation of a passivating layer of tannates on the metal surface (Boot & Mercer, 1964).

Besides, Lawsone also a main part of henna extract that had been mentioned earlier. The structure of Lawsone is shown in Figure 2.1 below.

Figure 2.1: Lawsone structure (C10H6O3) (Hamdy &Nour, 2012)

The Figure 2.1 above shows that, Lawsone structure consists of benzene, p-benzoquinone and phenolic group (Abdolahi & Shadizadeh, 2012). By looking at the structure in Lawsone, we can know that henna extract is an organic compound and the IUPAC name for the Lawsone structure is 2-hydroxy-1, 4-naphthoquinone (Rajedran et al., 2009). Lawsone molecule is a ligand that can chelate with various metal cations and then forming a complex compound. So it can perform corrosion inhibition on metal through absorption method (Ostovari et al., 2009).

– Mechanism of inhibition

The mechanism of inhibition involves blocking metal surface by inhibiting through adsorption. Adsorption process is affected by nature of metal and chemical structure of the inhibitor (Ostovari et al., 2009). The obtain results from previous studies indicate that Lawsonia inermis extract achieve good inhibition for the corrosion of any metal such as C-steel, nickel, and zinc in various mediums of solution (El-Etre et al.,2005). To perform the inhibitory action, there are some components in henna extract which will act as inhibitors. As had been stated earlier, the main components of Lawsonia inermis extract are hydroxy aromatic compound which consists tannin and Lawsone (El-Etre et al., 2005). Both tannin and Lawsone have its own function and mechanism to perform the inhibition action between the metal and henna extract successfully.

• Tannin

The first component is tannin. The inhibitive action of tannin was ascribed to the establishment of a passivating layer of tannates on the metal surface (Rajagopalan et al., 1969; Booth & Mercer, 1964). The formation of complex compounds with different metal cations can be form by tannin component especially in the basic media. El-Etre et al. (2005) reported that the formation of tannin complexes may be working well for the inhibition in the alkaline media rather than acidic solution it because tannins are limited inhibition in acidic media. Thus, tannin component in Lawsonia inermis extract is suitable to use in the manufacture of anti-rusting paints and coating.

• Lawsone

The other main element in the Lawsonia inermis extract is Lawsone which has higher concentration than other elements. Lawsone molecule is a ligand that can chelate with metal cations which will form complex compounds. Ligands typically consist in organic compound. Insoluble complex was formed when metal cations combine with Lawsone molecule and it was absorbed on the metal surface. This is how Lawsone compound give inhibition action to the metal. There is such method that can prove the formation of metal complex which the method is conductometric titration.

                 Mechanism of Conductometric Titration Method

Often, conductometric titration was conducted to obtain substantial support for the formation of metal complex (Amin, 2002). Several procedures were conducted to confirm the formation of metal complexes. Conductance measurement was carried out with a Jenway 4510 conductivity meter. Lawsonia inermis extract was titrated in separated experiments with some metal in solutions that have different pH value. As the result of the titration, non-complexes of metal ion was reacted with reagent first which leads it to a reduction in the conductivity of the solution. However, the higher amount of ligand (henna extract) does not change the value of conductivity (Ostovari et al., 2009). These results signify that the formation of metal-complex through the chelation of Lawsone molecules and the metal cations with stoichiometric ratio of 1:1 and 2:1 as shown in Figure 2.2.

Figure 2.2: Forms of Metal-Lawsonia complexes (El-Etre et al., 2005)

So, it can be conclude that the inhibition action of Lawsone was happen and prove that henna leaves extract can act as inhibitor (Abdolahi & Shadizadeh, 2012).

           Mechanism of Lawsone Rearrangement

In the acidic solution or known as acidic media, aromatic compound whose structure include a cyclic delocalized π-electron system are susceptible to electron delocalization, especially a ring containing only carbon such as benzene (Lide, 2006). Delocalization of π-electron is to stabilize the molecule. As the bond angles of carbon in benzene are trigonal (sp²), then the ring become flat. So these angles just fit the 120^o angles of a regular hexagon and this flatness allow the overlap of the p-orbitals in both directions that lead the delocalization and stabilization (Marrison & Boyd, 1983). So that’s why it needs delocalization to stabilize the structure.

In the other situation of Lawsone molecule in acidic media, delocalization of the lone pair of electrons on hydroxyl group takes place resulting in the rearrangement as shown in Figure 2.3.

Figure 2.3: Process of electron delocalization on the Lawsone molecule (Ostovari et al., 2009)

The migration of the hydrogen atom with a pair of electrons from an adjacent carbon to the carbon bearing caused the rearrangement occur. Carbon that loses the migrating group obtained the positive charge. This is the most common kind of the rearrangement which known as 1,2- rearrangement. Then, with the existence of metal cations, it will enhance the complex formation of reaction (Ostovari et al., 2009). Then the complex formation reaction of rearrangement was enhanced by adding henna extract in solution. So, the metal complexes are stabilized. The formation of these stabilized complexes between Lawsone molecule and metal cations give the high inhibition efficiencies in acidic media.

Besides, Mikhael et al (2004) reported that henna constituent has antioxidant characteristic particularly by the naphthoquinones. It also state that, gallic acid also can be used as an oxygen scavenger in boiler system (Soderquist, 1990). Therefore, the oxygen scavenger characteristic of henna extract was attributed to another mechanism for inhibition. However in the acidic media, the main cathodic reaction at the surface is a hydrogen evaluation (Ait Chikh et al., 2005). Hence, the oxygen scavenging characteristic of henna is not the main responsible mechanism for the observed inhibition of henna extract. In addition, it is confirmed that the inhibition mechanism is the chemisorption of inhibitor molecule on the metal surface and it had been verified through thermodynamic parameter and conductometric titration. So as a conclusion, the main inhibitor mechanism chemisorption is through the chemisorption of inhibitor molecules on metal surface. While, the inhibitive action slightly enhanced through oxygen scavenging. (Ostovori et al, 2009).

• Comparison Henna Extract with Other Inhibition

There is another organic compound that can be used as inhibitor besides henna extract. Previous studies has been conducted to test the inhibition efficiency of henna and five other organic compound inhibitor used in acidizing treatment. Table 2.1 show the result of inhibition efficiency of these six inhibitors.

Table 2.1: Inhibition efficiency of MEA, DEA, TEA, and henna API steel (Abdollahi & Shadizade, 2012)

Refer to the Table 2.1 above; the inhibition efficiency was compared between Monoethanolamine (MEA), Diethanolamines (DEA), Triethanolamines (TEA), Dibenzylidene acetone (DBA), Di-N-dimethylaminobenzylidene acetone (DDABA) and henna. The inhibition efficiency of MEA, DEA, TEA and henna at temperature 28^oC are 82%, 80%, 78% and 85.98% respectively. Based on the result, it shows that henna has the higher inhibition efficiency than MEA, DEA and TEA.

Morever as shown in Table 2.1, the inhibition efficiency of henna extract at 28^oC is higher than inhibition efficiency of DDABA with 73.8% IE. Furthermore result also shown that inhibition concentration of henna extract is less than all inhibitors with the same temperature eventhough it have higher inhibition efficiency. So we can conclude that, with the small concentration of henna leaves extract, it can act as inhibitor with higher inhibition efficiency rather than other listed inhibitors which have larger concentration but average of inhibition efficiency is low than henna extract.

In addition, research still focused to the amount of henna leaves extract in order to apply it in industries as now the amount of henna is quiet small to apply it. So, the experimental must be conducted by using larger amount of henna leaves and various metals

Problem Statement

Nowadays, metal and alloy are widely used in various industries especially in manufacturing and processing. But the problem is the metals used tend to faced corrosive when the surface of equipment attacked by strong acid media or any other media. But mostly, acidic media are the main causes for corrosive to happen due to the increasing of industrial application an acid solution. So in order to prevent from corrosion of metal, inhibitor has been used. However the use of synthetic compound as inhibitor is extremely dangerous either to the body and the environment due to the toxicity. Thus, studies are conducted to find the non-toxic and natural inhibitors. The study shows that plant extract can be used as corrosion inhibitor. One of them is henna leaves extract. Besides being environmentally acceptable and readily available, henna leaves extract also cheap as it’s easy to find in Asia and North Africa and renewable sources.

Objectives

There are few objectives that has been studies in order to solve corrosion phenomenon and carried out the research on corrosion inhibitor of metal by using Lawsonia inermis extract. The objectives are:

1. Study of corrosion inhibition of metal by using henna leaves extract.
2. Analysis mechanism of inhibition towards metal.
3. Investigate the differences between henna leaves extract with other inhibitor.