Malaviya National Institute Of Technology

Metallurgical and Materials Engineering department

A REPORT ON “CORROSION INHIBITORS”

Submitted to: Submitted by:

Dr Munan Mandira Amit Bhardwaj

Associate Professor 2012UMT1302

(Metallurgical and materials B.tech. final year

Eng. Department) (Batch- T1)

Corrosion Inhibitors – Principles, Mechanisms and ApplicationsIntroductionCorrosion processes are responsible for numerous losses mainly in the industrial scope. It is clear that the best way to combat it is prevention. Among the various methods to avoid or prevent destruction or degradation of metal surface, the corrosion inhibitor is one of the best know methods of corrosion protection and one of the most useful on the industry. This method is following stand up due to low cost and practice method. Important researches have being conducted with government investment mainly in large areas such as development construction of new pipelines for shale gas and growth in construction. The focus of these researches has being the inhibitors applications in water and concrete for the protection of metals.Historically, inhibitors had great acceptance in the industries due to excellent anti-corrosive proprieties. However, many showed up as a secondary effect, damage the environment. Thus the scientific community began searching for friendly environmentally inhibitors, like the organic inhibitors. This chapter presents a revision of the corrosion inhibitors applications mainly the novelcompositions environmentally friendly. Is describes the mechanisms of action of inhibitors, main characteristics, environmental impact, technical analysis and calculation of efficiency.1.1. Mechanisms of actions of inhibitorsInhibitors are substances or mixtures that in low concentration and In aggressive environment inhibit, prevent or minimize the corrosion.Generally the mechanism of the inhibitor is one or more of three that are cited below:• The inhibitor is chemically adsorbed (chemisorption) on the surface of the metal and forms a protective thin film with inhibitor effect or by combination between inhibitor ions and metallic surface;• The inhibitor leads a formation of a film by oxide protection of the base metal;• The inhibitor reacts with a potential corrosive component present in aqueous media and the product is a complex. 1.2. Historic reviewThere are many industrial systems and commercial applications that inhibitors are applicable, such as cooling systems, refinery units, pipelines, chemicals, oil

and gas production units, boilers and water processing, paints, pigments, lubricants, etc. There are evidences of the use of inhibitor since the early XIX century. On that time they were already used to protect metals in processes such as acid picking, protection against aggressive water, acidified oil wells and cooling systems. Since years 1950's and 1960's, there was significant advances in the development of technology for corrosion inhibitor as the application of electrochemistry to evaluate corrosion inhibitors. Recent studies estimate that the U.S. demand for corrosion inhibitors will rise 4.1% per year to USD$ 2.5 billion in 2017. In 2012 they estimated that the market demand of inhibitors was divided on 26.6% to refining petroleum, 16.9% utilities, 16.7% gas and oil production, 15.3% chemical, 9.5% metals, 7.1% pulp and paper and 8.0% other. Now a days, due to changes occurred on the market

of corrosion inhibitors, some industrial corrosion inhibitors are being unused. Due to high toxicity of chromate, phosphate and arsenic compounds, related to various environmental and health problems, strict international laws were imposed. Reducing the use of these and therefore increasing the need for the development of other inhibitor to supply the lack in this area. Should, however, present a similar anti corrosive properties similar than a chromate inhibitor. An important number of papers have been published with the intention of develop an environmentally friendly corrosion inhibitors and a lot of research has been doing to development of the called “green” corrosion inhibitors. Also, has been increasing research in natural products, such as plant extracts, essential oils and purified compounds to obtain environmentally friendly corrosion inhibitors. The first evidence of natural product use as corrosion inhibitors is 1930’s. When extracts of Chelidonium majus (Celadine) and other plants were used on the first time in H2SO4 picklingbaths. Successful developments of researches to obtain natural corrosion inhibitors are growing as quickly as the environmental consciousness is gaining ground. Chromates as active inhibitors are being replaced by other components such as Molybdate compounds and rare earth metal salt, like cerium chloride. Also, drugs have been studied as corrosion inhibitors. Classification Depending upon the mechanism and mode of protection inhibitors can be classified under the following groups (a)Chemical passivator(b)Adsorption inhibitors(c)Film forming inhibitors(d)Vapour phase inhibitors(e)Green inhibitors

(A)Chemical passivator:Passivation, in physical chemistry and engineering, refers to a material ecoming"passive," that is, being less affected by environmental factors such as air and later. Passivation involves a shielding outer-layer of base material, which can be applied as a microcoating, or oxidation which occurs spontaneously in nature. As a technique, passivation is the use of a light coat of a protective material,such as metal oxide, to create a shell against corrosion. Passivation can occur only in certain conditions, and is used in microelectronics to enhance silicon.[1] The technique of passivation is used to strengthen and preserve the appearance of metallics.

When exposed to air, many metals naturally form a hard, relatively inert surface, as in the tarnishing of silver, in contrast to metals such as iron, where uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. The amount by which a corrosion coating reduces the rate of corrosion varies, depending on the kind of metal and its environment, and is notably slower in room-temperature air for aluminium, chromium, zinc, titanium, and silicon (a metalloid); the shell inhibits deeper corrosion, and so is the key factor of passivation. The inert surface layer, termed the ‘’native oxide layer‘’, is usually an oxide or a nitride, with a thickness of a monolayer (1-3 Å) for a noble metal such as platinum, about 15 Å for silicon, and nearer to 50 Å for aluminium after several years.

MechanismsPourbaix diagram of iron.

There has been much interest in determining the mechanisms which describe how the thickness of the oxide layer on a material increases with time. Some of the important issues include: the relative volume of the oxide compared to the parent metal, the mechanism by which oxygen diffuses through the metal oxide to the metal - oxide interface and the relative chemical potential for the oxide to form. Boundaries between micro grains, if the oxide layer is crystalline, form an important pathway for oxygen to reach the unoxidized metal below. For this reason, vitreous oxide coatings – which lack grain boundaries – can retard oxidation. The conditions necessary (but not sufficient) for passivation are

recorded in Pourbaix diagrams. Some corrosion inhibitors help the formation of a passivation layer on the surface of the metals to which they are applied. Some compounds, dissolving in solutions (chromates, molybdates) form non-reactive and low solubility films on metal surfaces.

DiscoveryIn the mid 1800s, Christian Friedrich Schönbein discovered that when a piece of iron is placed in dilute nitric acid, it will dissolve and produce hydrogen, but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place. Schönbein named the first state the active condition and the second the passive condition. If passive iron is touched by active iron, it becomes active again. In 1920, Ralph S. Lillie measured the effect of an active piece of iron touching a passive iron wire and found that "a wave of activation sweeps rapidly (at some hundred centimeters a second) over its whole length".

Specific materialsSilicon

In the area of microelectronics, the formation of a strongly adhering passivating oxide is important to the performance of silicon.

In the area of photovoltaics, a passivating surface layer such as silicon nitride, silicon dioxide or titanium dioxide can reduce surface recombination - a significant loss mechanism in solar cells.

Aluminium

Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in most environments. Aluminium alloys, however, offer little protection against corrosion. There are three main ways to passivate these alloys: alclading, chromate conversion coating and anodizing. Alclading is the process of metallurgically bonding a thin layer of pure aluminium to the aluminium alloy. Chromate conversion coating is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys. Anodizing forms a thick oxide coating. This finish is more robust than the other processes and also provides good electrical insulation, which the other two processes do not.

For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and

peroxide alternating with deionized water. The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.

Ferrous materials

Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acidand further protected by surface coating. As the uncoated surface is water-soluble, a preferred method is to form manganese or zinc compounds by a process commonly known as Parkerizing or phosphate conversion. Older, less-effective but chemically-similar electrochemical conversion coatings included black oxidizing, historically known as bluing orbrowning. Ordinary steel forms a passivating layer in alkali environments, as reinforcing bar does in concrete.

Stainless steel

Stainless steels are corrosion-resistant by nature, which might suggest that passivating them would be unnecessary. However, stainless steels are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter (such as grinding swarf) allow water molecules to oxidize some of the iron in those spots despite the alloying chromium. This is called rouging. Some grades of stainless steel are especially resistant to rouging; parts made from them may therefore forgo any passivation step, depending on engineering decisions.

Passivation processes are generally controlled by industry standards, the most prevalent among them today being ASTM A 967 and AMS 2700. These industry standards will generally list several typical "types" of passivation processes that can be used, with the specific method to be decided between the customer and vendor. The "Method" refers to either the use of a nitric acid-based passivating bath (Method 1), or a citric acid based bath (Method 2.) The various 'Types' found listed under each method refer to differences in acid bath temperature and concentration. Sodium dichromate is often required as an additive to promote oxidation in certain 'types' of nitric-based acid baths.

Common among all of the different specifications and types are the following steps: Prior to passivation, the parts must be cleaned of any contaminants and generally must undergo a validating test to prove that the surface is 'clean.' The part is then placed in an acidic passivating bath that meets the temperature and chemistry requirements of the Method and Type specified between customer and vendor. (Temperatures can range from ambient to 140 degrees Fahrenheit,

while minimum passivation times are generally around 20 to 30 minutes). The parts are then neutralized using a bath of aqueous sodium hydroxide and then rinsed with clean water, dried, and the passive surface is validated using exposure to humidity, elevated temperature, a rusting agent (salt spray), or some combination of the three. However, proprietary passivation processes exist for martensitic stainless steel, which is difficult to passivate, as microscopic discontinuities can form in the surface of a machined part during passivation in a typical nitric acid bath. The passivation process removes exogenous iron, creates/restores a passive oxide layer that prevents further oxidation (rust), and cleans the parts of dirt, scale, or other welding-generated compounds (e.g. oxides)

It is not uncommon for some aerospace manufacturers to have additional guidelines and regulations when passivating their product that exceed the requirements in a national standard. Often, these requirements will be flowed down using NADCAP or some other accreditation system. Various testing methods are available to determine the passivation (or passive state) of stainless steel. The most common methods for validating the passivity of a part is some combination of high humidity and heat for a period of time, intended to induce rusting. Electro-chemical testers can also be utilized to commercially verify passivation.

Nickel

Nickel can be used for handling elemental fluorine, owing to the formation of a passivation layer of nickel fluoride. This fact is useful in water treatment and sewage treatment applications.

Adsorption inhibitor :Adsorption occurs as a result of electrostatic forces between the electric charge on the metal and the ionic charges or dipoles on the inhibitor molecules.The potential at which there is no charge on the metal is known as the zero-charge potential (ZCP) . The charge on a metal surface in a given medium can be

determined from the corrosion potential (ECORR) and zero-charge potential.

FIGURE 4. Polarization diagram of an active-passive metal showing the dependence of the current onconcentration of passivation-type inhibitor

corrosion potential (ECORR) and zero-charge potential. When the difference, £CORR - ZCP, is negative, the metal is negatively charged and adsorption of cations is favoured. When £CORR - ZCP is positive, the metal is positively charged and adsorption of anions is favored. The charge on inhibitors depends on the presence of loosely bound electrons, lone pairs of electrons, rc-electron clouds, aromatic (e.g., benzene) ring systems, and functional groups containingelements of group V or VI of the periodic table. Most organic inhibitors possess at least one functional group, regarded as the reaction centre or anchoring group. The strength of adsorption depends on the charge on this anchoring group [rather on the hetero atom (i.e., atoms other than carbon including nitrogen, sulphur) present in the anchoring group]. The structure of the rest of the molecule influences the charge density on the anchoring group .Water molecules adsorb on the metal surface immersed in an aqueous phase.Organic molecules adsorb by replacing the water molecules:[Inhibitor]soln + [«H20]adsorbed <=> [Inhibitor]adsorbed + [nH2O] soln (6)where n is the number of water molecules displaced by one inhibitor molecule.

The ability of the inhibitor to replace water molecules depends on the electrostatic interaction between the metal and the inhibitor. On the other hand, the number of water molecules displaced depends on the size and orientation of the inhibitor molecule. Thus, the first interaction betweeninhibitor and metal surface is nonspecific and involves low activation energy. This process, called "physical adsorption," is rapid and, in many cases, reversible.

FIGURE 5. Polarization curve in the presence of cathodic inhibitor

FIGURE 6. Homologous series of organic molecules (the molecules differ only in the hetero atom).

Under favourable conditions, the adsorbed molecules involved in chemical interaction (chemisorption) form bonds with the metal surface. Chemisorption is specific and is not reversible.

The bonding occurs with electron transfer or sharing between metal and inhibitor. Electron transfer is typical for transition metals having vacant, low-energy electron orbitals. Inhibitors having relatively loosely bound electrons transfer charge easily. The inhibition efficiency of the homologous series of organic substances differing only in the heteroatom is usually in the following sequence: P> S> N> O. An homologous series is given in Figure 6. On the other hand, the electronegativity, that is, the ability to attract electrons, increases in the reverse order.Adsorption strength can be deduced from the adsorption isotherm, which shows the equilibrium relationship between concentrations of inhibitors on the surface and in the bulk of the solution. Various adsorption isotherms to characterize inhibitor efficiencies are presented in Table 2 . To evaluate the nature and strength of adsorption, the experimental data (e.g., corrosion rate) are fitted to the isotherm, and from the best fit, the thermodynamic data for adsorption are evaluated. The principle of soft and hard acids and bases (SHAB) has also been applied to explain adsorption and inhibition . The SHAB principle states that hard acids prefer to coordinate with hard bases and that soft acids prefer to coordinate with soft bases. Metal atoms on oxide-free surfaces are considered to be soft acids, which in acid solutions form strong bonds with soft bases, such as sulphur-containing organic inhibitors. By comparison, nitrogen-or oxygen-containing organic compounds are considered to be hard bases and may establish weaker bonds with metal surfaces in acid solutions.Whatever may be the mechanism of adsorption, the electron density of the functional groups, polarizability and electronegativity are important parameters that determine inhibition efficiency .

(c)Film forming inhibitorsIn the industry related to mechanical machining, cutting tools and cooling systems are often subjected to aggressive conditions (turbulent flow, high temperature, etc.), involving the early apparition of corrosion. The addition of inhibitors in lubricating or rinsing solutions presents nowadays good results to prevent corrosion, and then prolong system lifetime . There have been few systematic studies reporting the effect of flow on the metals corrosion which have carefully controlled the flow parameters have studied the behaviour of the interface carbon steel in 200mg l-1 NaCl solution with inhibitor (fatty amines associated with phos-phonocarboxylic acid salts) using a rotating disk electrode. They have pointed out that the properties of the protective layer were dependent on the electrode rotation rate where current densities decrease when the electrode rotation rate increases. Mora-Mendoza et al. [6] have studied the corrosion inhibition of mild steel under turbulent flow conditions; he has demonstrated that the localized corrosion process does not occur at a specific rotation speed and they have shown that the changes in Ecorr to more positive values are more important in producing the localized process and removing the electrostatically adsorbed inhibitor from the surface sample, than increases in shear stress that generated from the turbulent flow. The molecule 4-Amino-5-phenyl-4H-1, 2, 4-trizole-3-thiol (APTT) has been studied as corrosion inhibitor by authors [7]. This molecule contain one co-ordinate (=N−, −NH2) and one covalent (−S −H) groups which can form a protecting film on the surface of mild steel through chemisorption. The aim of this reported work was focused on the turbulent flow effect on formation and stability of inhibitor (APTT) film. The influence of turbulent flow was investigated by changes in open circuit potential (OCP) with immersion time, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) with a rotating cylinder electrode (RCE). To separate the effect of oxygen from the effect of rotation rate, experiments were conducted in oxygenated and de aerated solutions. The Molecular structure of APTT is shown as

follows(scheme 1):

Scheme 1. Molecular structure of APTT 2. EXPERIMENTAL WORK The working electrode employed in this work was made from mild steel, whose chemical composition is as follows: 0.08 wt.% C, 0.25 wt.% Si, 0.45 wt.% Mn, 0.03 wt.% P, 0.03 wt.% S, balanced with Fe. The rotating cylinder electrode (RCE) was designed for application with a Gamry Instruments Company, model AFMSRCEP rotator. The dimensions of the RCE were diameter 1.2 cm and length 0.8 cm. This produced a RCE active area of 3 cm2.

High-density polyethylene rod was used as an inert insulating sheath. The experiments were performed at different electrode rotation rates: 400 RPM (U1= 25.12 cm s-1), 800 RPM (U2=50.24 cm s-1), 1200 RPM (U3=75.36 cm s-1), 1600 RPM (U4=100.48 cm s-1) and 2000 RPM (U5=125.6 cm s-1) at 30○C. The selection of these ranges was based on the conditions commonly observed at industrial facilities. The Reynolds number for a rotating cylinder electrode with outer diameter, d (cm) was calculated according the relation 8.

RESULT AND DISCUSSION 3.1. Open circuit potential (OCP) with immersion time measurements A corrosion potential evolution as function of time shows the best qualitative method for monitoring interface modification between a metal and its environment . The OCP of mild steel was monitored over 120 min. from the moment of immersion in the test solution at 30 ○C. The effect of different rotation rates on the variation of the OCP of mild steel with inhibitor in oxygenated and de aerated 1.0 M HCl solutions at 30 ○C were conducted and the results is represented in Fig. 1. It was found that the OCP becomes more positive in oxygenated solution compared to de aerated solution. It means that the surface becomes nobler in presence of oxygen. Moreover, it’s become nobler also with increased rotation rate due to more oxides formation on metal surface in high rotation rates. In fact increased presence of dissolved oxygen and H+ ions on the surface of metal at higher rotation rates leads to formation of more oxides and more positive values of OCP. While in de aerated solution the surface become less noble with rotation rate which due to decrease in the rate of cathodic reaction that provided by the adsorption of APTT on cathodic

site.

Figure Open circuit potential vs. rotation rate for mild steel in oxygenated and de aerated 1.0 M HCl solutions with 4× 10-4 M of APTT.

PolarizationsPotentiodynamic measurements were obtained for mild steel in oxygenated and de aerated 1.0M HCl solutions with and without 4 ×10-4M of APTT at different rotation rates. In case of oxygenated solution, Fig.2, when the rotation rate increased the cathodic and anodic current densities are increased which due to an increase of the oxygen supply to the metal surface as well as it indicated an active dissolution of mild steel. While in de aerated solution, Fig.3, the cathodic current densities were decreased and the effect of the increased rotation rate on the anodic dissolution of metal is low, this is due to the inhibitor formed film as flow can increase mass transport of inhibitor molecules that causes more inhibitor presence at metal surface

The polarization data, including corrosion current density (icorr), Tafel slopes (βa and βc,), and Ecorr, for oxygenated and de aerated blank and inhibited solutions at 400, 800, 1200, 1600, and 2000 RPM are summarized in Table 1. The values of inhibition efficiencies (IE%) were also calculated using eq. [1] and depicted in Table 1 as follows:

It was observed that at higher rotation rate the higher corrosion current densities, while Ecorr followed the same trend of OCP measurement as mentioned in Section 3.1. Although the corrosion rate decreased with addition of APTT, the same trend decreases with increase in rotation rate in oxygenated solution in which indicating lower inhibitory action by APTT on the corrosion of the mild steel under given condition as it can be seen from IE% trend. IE% kept almost within narrow range of variation with rotation rate in de aerated solution. This trend may be attributed to the absence of oxygen effect.

(d) Vapour phase inhibitorThe process of vapor-phase inhibition involves two steps: transport of inhibitor

to the metal surface and interaction of inhibitor on the surface. AVPI may vaporize either in the molecular form or it may first dissociate and then vaporize. Amines vaporize in the undissociated molecular form. Subsequentreaction with water, present as moisture at the surface, dissociates the inhibitor. On the other hand, dicyclohexylamine nitrite dissociates liberating amine and nitrous acid, which deposit on the metal surface . Both in molecular and in dissociated forms VPIs adsorb either physically or chemicallyon the metal surface to inhibit corrosion.

(E)Green inhibitors Green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds. Some research groups have reported the successful use of naturally occurring substances to inhibit the corrosion of metals in acidic and alkaline environment. Delonix regia extracts inhibited the corrosion of aluminum in hydrochloric acid solutions , rosemary leaves were studied as corrosion inhibitor for the Al + 2.5Mg alloy in a 3% NaCl solution at 25°C , and El-Etre investigated natural honey as a corrosion inhibitor for copper [7] and investigated opuntia extract on aluminum . The inhibitive effect of the extract of khillah (Ammi visnaga) seeds on the corrosion of SX 316 steel in HCl solution was determined using weight loss measurements as well as potentiostatic technique. The mechanism of action is attributed to the formation of insoluble complexes as a result of interaction between iron cations, and khellin [9] and Ebenso et al. showed the inhibition of corrosion with ethanolic extract of African bush pepper (Piper guinensis) on mild steel ;Carica papaya leaves extract [11]; neem leaves extract (Azadirachta indica) on mild steel in H2SO4 . Zucchi and Omar investigated plant extracts of Papaia, Poinciana pulcherrima, Cassia occidentalis, and Datura stramonium seeds and Papaya, Calotropis procera B, Azadirachta indica, and Auforpio turkiale sap for their corrosion inhibition potential and found that all extracts except those of Auforpio turkiale and Azadirachta indica reduced the corrosion of steel with an efficiency of 88%–96% in 1 N HCl and with a slightly lower efficiency in 2 N HCl. They attributed the effect to the products of the hydrolysis of the protein content of these plants Umoren et al studied the corrosion inhibition of mild steel in H2SO4 in the presence of gum arabic (GA) (naturally occurring polymer) and polyethylene glycol (PEG) (synthetic polymer). It was found that PEG was more effective than gum arabic.Yee determined the inhibitive effects of organic compounds, namely, honey and Rosmarinus officinalis L on four different metals—aluminium, copper, iron, and zinc, each polarized in two different solutions, that is, sodium chloride and sodium sulphate. The experimental approach employed potentiodynamic polarization method. The best inhibitive effect was obtained when zinc was polarised in both honey-added sodium chloride and sodium sulphate solutions. Rosemary extracts showed some cathodic inhibition when the metal was polarized in sodium chloride solution. This organic compound, however, displayed less anodic inhibition when compared with honey. The main chemical components of rosemary include borneol, bornyl acetate, camphor, cineole, camphene, and alpha-pinene. Chalchat et al. reported that oils of rosemary were found to be rich in 1,8-cineole, camphor, bornyl acetate, and high amount of hydrocarbons. Recently, work has been emphasized on the use of Rosmarinus officinalis L as corrosion inhibitor for Al-Mg corrosion in chloride

solution . It is believed that the catechin fraction present in the rosemary extracts contributes to the inhibitive properties that act upon the alloy. Ouariachi et al. also reported the inhibitory action ofRosmarinus officinalis oil as green corrosion inhibitors on C38 steel in 0.5 M H2SO4.Odiongenyi reported that the ethanolic extract of Vernonia amygdalina appears to be a good inhibitor for the corrosion of mild steel in H2SO4 and action is by classical Langmuir adsorption isotherm.The effect of addition of halides (KCl, KBr, and KI) was also studied, and the results obtained indicated that the increase in efficiency was due to synergism .Umoren et al. also investigated the corrosion properties ofRaphia hookeri exudates gum—halide mixtures for aluminum corrosion in acidic medium . Raphia hookeri exudates gum obeys Freundlich, Langmuir, and Temkin adsorption isotherms. Phenomenon of physical adsorption is proposed. Abdallah also tested the effect of guar gum on carbon steel. It is proposed that it acts as a mixed type inhibitor [14]. The mechanism of action of C-steel by guar gum is due to the adsorption at the electrode/solution interface. Guar gum is a polysaccharide compound containing repeated heterocyclic pyrane moiety as shown in Scheme 1. The presence of heterooxygen atom in the structure makes possible its adsorption by coordinate type linkage through the transfer of lone pairs of electron of oxygen atoms to the steel surface, giving a stable chelate five-membered ring with ferrous ions. The chelation between O1 and O2 with Fe++ seems to be impossible due to proximity factor presented as in Scheme 1:

Scheme 1: Guar gum.

The simultaneous adsorption of oxygen atoms forces the guar gum molecule to be horizontally oriented at the metal surface, which led to increasing the surface coverage and consequently protection efficiency even in the case of low inhibitor concentrations.

looked into the extracts of onion (Allium sativum), Carica papaya extracts, Garcinia kola, and Phyllanthus amarus . El-Etre, Abdallah M used Natural honey as corrosion inhibitor for metals and alloys. II C-steel in high saline water . Jojoba oil has also been evaluated . Artemisia oil has been investigated for it is anticorrosion properties . Oguzie and coworkers evaluated Telfaria occidentalis ,Occinum viridis, Azadirachta indica, and Sanseviera trifasciata extracts . Benda-hou et al., studied using the extracts of rosemary in steel, and Sethuraman studied Datura . Recently, studies on the use of some drugs as corrosion inhibitors have been reported by some researchers . Most of these drugs are heterocyclic compounds and were found to be environmentally friendly, hence, they have great potentials of competing with plant extracts. According to Eddy et al. drugs are environmentally friendly because they do not contain heavy metals or other toxic compound. In view of this adsorption and inhibitive efficiencies of ACPDQC (5-amino-1-cyclopropyl-7-[(3R, 5S) 3, 5-dimethylpiperazin-1-YL]-6,8-difluoro-4-oxo-uinoline-3-carboxylic acid), on mild steel corrosion have been studied and found to be effective.Eddy studied inhibition of the corrosion of mild steel by ethanol extract of Musa species peel using hydrogen evolution and thermometric methods of monitoring corrosion. Inhibition efficiency of the extract was found to vary with concentration, temperature, period of immersion, pH, and electrode potentials. Adsorption of Musa species extract on mild steel surface was spontaneous and occurred according to Langmuir and Frumkin adsorption isotherms and also physical adsorption. Deepa Rani and Selvaraj report the inhibition efficacy of Punica granatum extract on the corrosion of Brass in 1 N HCl evaluated by mass loss measurements at various time and temperature. Langmuir and Frumkin adsorption isotherms appear to be the mechanism of adsorption based on the values of activation energy, free energy of adsorption. Few researchers have summarized the effect of plant extracts on corrosion .Efforts to find naturally organic substances or biodegradable organic materials to be used as corrosion inhibitors over the years have been intensified. Several reports are available on the various natural products used as green inhibitors as shown in Tables 1 and 2. Low-grade gram flour, natural honey, onion, potato, gelatin, plant roots, leaves, seeds, and flowers gums have been reported as good inhibitors. However, most of them have been tested on steel and nickel sheets. Although some studies have been performed on aluminum sheets, the corrosion effect is seen in very mild acidic or basic solutions (millimolar solution

Mechanism of Action of Green Inhibitors :

Many theories to substantiate the mode of action of these green inhibitors have been put forth by several workers. Mann has suggested that organic substances, which form onium ions in acidic solutions, are adsorbed on the cathodic sites of the metal surface and interfere with the cathodic reaction.

Various mechanisms of action have been postulated for the corrosion inhibition property of the natural products.It is a contaminant of mustard seeds containan alkaloid berberine which has a long-chain of aromatic rings, an N atom in thering, and, at several places H atoms attached to C are replaced by groups, –CH,–OCH3, and –O. The free electrons on the O and N atoms form bonds with theelectrons on the metal surface. Berberine in water ionizes to release a proton,thus the now negatively charged O atom helps to free an electron on the Natom and forms a stronger bond with the metallic electrons. These propertiesconfer good inhibition properties to Argemone mexicana .

GarlicIt contains allyl propyl disulphide. Probably, this S-containing unsaturatedcompounds affects the potential cathodic process of steel.CarrotIt contains pyrrolidine in aqueous media, pyrrolidine ionizes, and the N atomacquires a negative charge, and the free electrons on N possess still highercharge, resulting in stronger bond formation at N Carrot does not ionize inacidic media and thus does not protect in acids.

INDUSTRIAL APPLICATIONS OF CORROSION INHIBITORS

1. Petroleum ProductionCorrosion in the hydrocarbon industries may be divided into two types, "wet corrosion" and "dry corrosion." At low temperature (i.e., below the boiling point or dew point of water), material corrodes due to the presence of an aqueous phase (wet corrosion). At higher temperature (above the boiling point of water), corrosion occurs in the absence of an aqueous phase (dry corrosion). Wet corrosion is influenced by pressure, temperature, and compositions of aqueous, gaseous, and oil phases. In refineries and petrochemical plants, the amount of

water is usually small, but the

corrosivity is high and is localized at regions where the aqueous phase contacts the metal. The water may contain dissolved hydrogen sulfide (H2S), carbon dioxide (CO2), and chloride ions (Cl ~).Corrosion may occur even when the produced water content is as low as 0.1%, or corrosion activity may not begin until after several years of production.Refineries and petrochemical industries employ a variety of film-forming inhibitors to control wet corrosion. Most of the inhibitors are long-chain nitrogenous organic materials, including amines and amides. Water-soluble and water-soluble-oil-dispersible type inhibitors are continuously injected, or oil-soluble and oil-soluble-water-dispersible type inhibitors (batch inhibitors) are intermittently applied to control corrosion.Film-forming inhibitors anchor to the metal through their polar group. The nonpolar tail protrudes out vertically. The physical adsorption of hydrocarbons (oils) on these nonpolar tails increases film thickness and the effectiveness of the hydrophobic barrier for corrosion inhibition . Because inhibitors are interfacial in nature, they are active at liquid-liquid and/or liquid-gas interfaces and can lead to emulsification. As a result, foaming is sometimes experienced in the presence of inhibitors.2. Internal Corrosion of Steel PipelinesGathering pipelines, operating between oil and gas wells and processing plants, have corrosion problems similar to those in refineries and petrochemical plants. The flow regimes of multiphase fluids in pipelines influence the corrosion rate. At high flow rates, flow-induced corrosion and erosion-corrosion may occur, whereas at low flow rates, pitting corrosion is more common.Corrosion is related to the amount and nature of sediments. High-velocity flow tends to sweep sediments out of the pipeline, whereas low velocity allows sediments to settle at the bottom, providing sites for pitting corrosion. Internal corrosion of pipelines is controlled by cleaning the pipeline (pigging) and by adding continuous and/or batch inhibitors.3. Water

Potable water is frequently saturated with dissolved oxygen and is corrosive unless a protective film, or deposit, is formed. Cathodic inhibitors, such as calcium carbonate, silicates, polyphosphates, and zinc salts, are used to control potable water corrosion. Water is used in cooling system in many industries. In a recirculating system, evaporation is the chief source of cooling. As evaporation proceeds, the dissolved mineral salt content increases.Cooling systems may consist of several dissimilar metals and non-metals. Metals picked up from one part of the system can be deposited elsewhere, producing galvanic corrosion. Corrosion is controlled by anodic (passivating) inhibitors including nitrate and chromate as well as by cathodic (e.g., zinc salt) inhibitors. Organic inhibitors (e.g., benzotriazole) are sometimes used as secondary inhibitors, especially when excessive corrosion of copper occurs.4. AcidsAcids are widely used in pickling, in cleaning of oil refinery equipment and heat exchangers, and in oil well acidizing. Mixed inhibitors are widely used to control acid corrosion.5. AutomobileInhibitors are used in an automobile for two reasons: (1) to reduce the corrosivity of fluid systems (internal corrosion), and (2) to protect the metal surfaces exposed to the atmosphere (external corrosion). Internal corrosion is influenced by the coolants, flow, aeration, temperature, pressure, water impurities and corrosion products, operating conditions, and maintenance of the system. Some common inhibitors dissolved in antifreeze are nitrites, nitrates, phosphates, silicates, arsenates, and chromates (anodic inhibitors); amines, benzoates, mercaptons, and organic phosphates (mixed inhibitors); and polar or emulsifiable oils (film formers) .Atmospheres to which automobiles are exposed contain moist air, wet SC>2 gas (forming sulfuric acid in the presence of moist air) and deicing salt (NaCl and CaCl2). To control external corrosion,the rust-proofing formulations that are used contain grease, wax resin, and resin emulsion, along with metalloorganic and asphaltic compounds. Typical inhibitors used in rust-proofing applications are fatty acids, phosphonates, sulfonates and carboxylates.6. Paints (Organic Coatings)Finely divided inhibiting pigments are frequently incorporated in primers. These polar compounds displace water and orient themselves in such a way that the hydrophobic ends face the environment, thereby augmenting the bonding of the coatings on the surface. Red lead (Pb3C>4) is commonly used in paints on iron. It deters formation of local cells and helps preserve the physical properties of the paints. Other inhibitors used in paints are lead azelate, calcium plumbate and lead suboxide .

7. MiscellaneousInhibitors are used to control corrosion in boiler waters, fuel oil tanks, hot chloride dye baths, refrigeration brines, and reinforcing steel in concrete, and they are also used to protect artifacts.

OTHER FACTORS IN APPLYING INHIBITORSSome factors to be considered in applying inhibitors are discussed in the following paragraphs.

1. Application TechniquesA reliable method should be applied for inhibitor application. A frequent cause of ineffective inhibition is loss of the inhibitor before it either contacts the metal surface or changes the environment to the extent required. Even the best inhibitor will fail if not applied properly.If the inhibitor is continuously applied in a multiphase system, it should partition into the corrosive phase, usually the aqueous phase. This partitioning is especially important when using water-soluble, oil-dispersible inhibitors.

In batch treatment, the frequency of treatment depends on the film persistency. It is important that the corrosion rates are measured frequently to ensure that a safe level of inhibition is maintained. It is also important that the inhibitor contacts the entire metal surface and forms a continuous persistentfilm. When using volatile inhibitors, care must be taken in packaging to prevent the loss of inhibitor to the outside atmosphere. Inhibitors are added to the primers used in paint coatings. When moisture contacts the paint, someiinhibitor is leached from the primer to protect the metal. The inhibitor should be incorporated in such a way that it protects the areas where potential corrosion can take place, and not leach completely from the primer during the service life.2. Temperature EffectsOrganic molecules decompose at elevated temperatures. In general, film-forming inhibitors that depend on physical adsorption become less effective at elevated temperatures, so that larger treatment dosages may be required to maintain protective films. Chemisorption, on the other hand, increases with temperature due to the strengthening of chemical bonds. As a result, inhibitor efficiency increases with temperature up to the temperature at which decomposition of the inhibitor occurs.3. PoisoningInhibitors for hydrogen damage should reduce not only the corrosion rate, but also the rate of absorption and permeation of hydrogen into the steel. For example, corrosion rate of steel in sulfuric acid is decreased , while hydrogen flux through a steel membrane is increased by adding thiourea . Although

thiourea inhibits corrosion, it poisons the hydrogen recombination reaction, so that much of the hydrogen produced at the steel surface enters the steel, causing hydrogen damage, rather than recombining with other hydrogen atoms to form bubbles of hydrogen that escape from the system.4. Secondary InhibitionThe nature of the inhibitor initially present in acid solutions may change with time as a consequence of chemical or electrochemical reactions. Inhibition due to the reaction products is called secondary inhibition. Depending on the effectiveness of the reaction products, secondary inhibition may behigher or lower than primary inhibition. For example, diphenyl sulfoxide undergoes electrochemical reaction at the metal surface to produce diphenyl sulfide, which is more effective than the primarycompound . On the contrary, the reduction of thiourea and its alkyl (e.g., methyl, ethyl) derivatives gives rise to HS ~, which accelerates corrosion.5. Synergism and AntagonismIn the presence of two or more adsorbed species, lateral interaction between inhibitor molecules can significantly affect inhibitor performance. If the interaction is attractive, a synergistic effect arises, that is, the degree of inhibition in the presence of both inhibitors is higher than the sum of theindividual effects. For example, because of this synergistic effect, the inhibition efficiency of a mixture of formaldehyde and furfuralimine is higher compared to the inhibition efficiency when these inhibitors are used separately (Fig. 11). On the other hand, when narcotine and thiourea are

used as mixed inhibitors, there is an antagonistic effect and a decrease ininhibitor efficiency compared to that which exists when these inhibitors are used separately (Fig. 12) .