CHAPTER 1: INTRODUCTION Corrosion operations are responsible for countless losses mainly in the manufacturing field and the best way to face this issue is prevention. From the different types and ways to prevent damages or deteriorates of the metal surface the corrosion inhibitor is considered as one of the best methods to fight the corrosion and protect the metal surface from rusting. Corrosion inhibitor following standards referring to low cost and practice methods. There are important researches that being done to invest in wide areas like development construction of new pipelines for shale gas and expansion in construction and the aim of this researches is to protect the metals from corrosion by using inhibitor application in water and concrete. In the past inhibitors were approved in the industries according to their great anti corrosive proprieties but, there was a side effect of some inhibitors that lead to effecting and damaging the environment. So the scientific society started on searching for friendly inhibitors such as organic inhibitors. This chapter shows a revision of corrosion inhibitor and describes a brief history review, classification of corrosion inhibitors, corrosion inhibitor mechanism techniques and consideration to employ inhibitors, toxicity, environmentally friendly inhibitors and industrial application of corrosion inhibitors. 1.1 Historic Review there are a lot of manufacturing process and applications that inhibitors can be applied for example pipelines, oil and gas production unit, cooling systems, chemicals, refinery units, paints, boilers and water processing, pigments, etc. The use of inhibitor was existed since the nineteen century and it was already applied to save metals in operations like protection against aggressive water, acid picking and acidified oil wells and cooling systems. A progress achieved in the improving of the technology for corrosion inhibitor since 1960s that’s lead to evaluate corrosion inhibitors by the application of electrochemistry. In 2017 the United States requests of corrosion inhibitors is going to increase by 4.1% per year to 2.5 billion according to new researches. The market requests of corrosion inhibitors in 2012 was categorized to 16.9% utilities, 15.3% chemical, 26.6% refining petroleum, 16.7% gas and oil production, 7.1% pulp and paper, 9.5% metals and 8% others. These days because of the replacement that is happening on the market of corrosion inhibitors some of manufactories stopped from working because of the toxic poisons of phosphate compounds and chromate compounds which is referred to many environmental and health issues and precise global regulations were imposed. Decreasing the use of these and subsequently growing the demand for improvement of other inhibitor to provide the shortages in these areas. A study has been published with the purpose of improve an environmentally friendly corrosion inhibitors and researching in natural products like essential oils and purified compounds to gain the called ( green) corrosion inhibitors. The first clue of using natural product as corrosion inhibitors is 1930s. When taking out of Chelidonium majus (Celadine) and other different plants were used on the first time in H2SO4 pickling baths. Successful improvement of studies to gain natural corrosion inhibitors are increasing as faster as the environmental consciousness is obtaining ground. 1.2 Corrosion Inhibitors Corrosion inhibitors are added to a corrosive environment to reduce or eliminate the rust process of the metal that wanted to be protected. According to the fact that the tools built with materials resistant to corrosion are too costly so it is obvious that the companies will use corrosion inhibitors as great economical substitutional. Corrosion inhibitor formulations are consist of one or more effective component and suitable vehicles that motivate compatibility with the environment and make it applicable to protect the metal surface and the properties that should be realized by corrosion inhibitor are:  Ability of decreasing corrosion process  Corrosion inhibitor must be in touch with the metal surface that wants to be protected  Must not have a secondary effect that damage the surrounding environment Occasionally, two compounds or effective component in formulation may have a bigger competence when they are blended together than that gained from the total of the efficiencies that are gained when they are alone at the same concentration and this impact is known as synergistic impact and is excessively used in formulation of corrosion inhibitor. 1.3 Classification of Corrosion Inhibitors Corrosion inhibitors are categorized into chemical either natural or artificial and could be labeled by the chemical nature as inorganic or organic, as oxidants or not oxidants and the mechanism of action as cathodic, anodic and by adsorption action. Generally the organic inhibitors have cathodic actions and anodic action and protective by a film adsorption. The inorganic inhibitors have either anodic action or cathodic. Figure 1.1: Classification of inhibitors 1.3.1 Organic Inhibitors Organic components applied as inhibitors sometimes they perform as anodic or cathodic or simultaneously as anodic and cathodic inhibitors. Though as a based rule applied through an operation of surface adsorption specified as a film forming the situation of molecules present a strong affinity for metal surfaces components presenting good inhibition quality and minimum environmental hazard. This inhibition made up a protective hydrophobic film absorbed molecules on the metal surface which making a barrier to the dissolution of the metal in the electrolyte. In the Figure 1.2, that shows a theoric potentiostatic polarization curve, it can be seen that the effect of the solution containing organic inhibitor on the metal presents an anodic and cathodic behavior. After the addition of the inhibitor, the corrosion potential remains the same, but the current decreases from Icor to I'cor- Figure 1.2 Is showed in Figure 1.3 the mechanism of actuation of organic inhibitors, when it is adsorbed to the metal surface and forms a protector film on it. .Figure 1.3 The efficiency and quality of an Organic inhibitor are reliable of:  The size of the organic molecules (chemical structure)  Aromaticity bonding such as carbon chain length  Kinds and counts of bonding atoms  Capability for a layer to be secure  The ability to build a complex with the atom such as solid in the metal lattice The quality of these organic corrosion inhibitors is referred to the existence of polar practical groups with S, O or N atoms in the molecule, heterocyclic components and pi electrons and the polar function considered as the reaction center for the starting of the adsorption process. Organic acid inhibitor which consists of oxygen, nitrogen or sulfur is adsorbed on the iron surface closing the active corrosion spots. Moreover the best efficient organic inhibitors are compounds that include bonds which show biological toxicity and environmental hazards. The focus of the inhibitor in the medium is crucial because of the iron surface wrapped in proportional to the inhibitor concentrates. Organic inhibitor examples are amines, urea, Mercaptobenzothiazole (MBT), benzotriazole e toliotriazol, aldehydes, heterocyclic nitrogen compounds, sulfur-containing compounds and acetylene compounds and also ascorbic acid, succinic acid, tryptamine, caffeine and extracts of natural substances. There are still some inhibitors that work in vapor phase (volatile corrosion inhibitor). Some examples are: dicicloexilamônio benzoate, diisopropylammonium nitrite or benzoate, ethanolamine benzoate or carbonate and also the combination of urea and sodium nitrite. 1.3.2 Inorganic Inhibitor 1.3.2.1 Anodic Inhibitors Work by decreasing anodic reaction which close the anode reaction and supplies the natural reaction of anodic metal surface and because of the building a film adsorbed on the metal. The inhibitors respond with corrosion product that immediately formed which resulting in cohesive and insoluble on the media surface. Figure 1.4 Figure 1.4 shows a potentiostatic polarization diagram of a solution with behavior inhibitor anodic. The anodic reaction is affected by the corrosion inhibitors and the corrosion potential of the metal is shifted to more positive values. As well, the value of the current in the curve decreases with the presence of the corrosion inhibitor. The anodic inhibitors reacts with metallic ions Men+ produced on the anode, forming generally, insoluble hydroxides which are deposited on the metal surface as insoluble film and impermeable to metallic ion. From the hydrolysis of inhibitors results in OH- ions. Figure 1.5 shows how the mechanism of the anodic inhibitory effect is. Figure 1.5 The cathodic current density at the main anodic potential becomes higher than the crucial anodic current density when the concentration of inhibitor rise enough that’s lead to shifting the potential for a noble sense and then the metal is passivated. The anodic inhibitor impact is very significant that the inhibitor focus must be high enough in the solution and unsuitable quantity of the inhibitors impact the formation of film protection due to not covering the metal completely which remain exposed areas that causes a localized corrosion. Generally concentration under the impact value are worse than without inhibitors at all which can cause pitting because of decreasing at the anodic sites referred to cathodic or can increase corrosion such as generalized corrosion according to full breakdown the passivity. Examples for anodic inorganic inhibitors are phosphates, molybdates, silicates, nitrates, hydroxides and sodium chromates. 1.3.2.2 Cathodic Inhibitors The cathodic corrosion inhibitors prohibit the occurrence of the cathodic respond of the metal during corrosion operation. The inhibitors have metal ions that are capable to make a cathodic reaction according to their alkalinity, so making insoluble compounds that deposit selectively on cathodic areas. Precipitate over the metal built in and adherent film which minimum the diffusion of reducible species in these places. These inhibitors cause a big cathodic inhibition because of the increasing of the resistance of the surface and the diffusion restriction of the reducible areas which is the oxygen diffusion and electron conductive in these places. The Figure 1.6 shows an example of a polarization curve of the metal on the solution with a cathodic inhibitor. When the cathodic reaction is affected the corrosion potential is shifted to more negative values. Figure 1.6 The cathodic inhibitors form a barrier of insoluble precipitates over the metal, covering it. Thus, restricts the metal contact with the environment, even if it is completely immersed, preventing the occurrence of the corrosion reaction. Due to this, the cathodic inhibitor is independent of concentration, thus, they are considerably more secure than anodic inhibitor. The Figure 1.7 shows the illustration of mechanical effect of cathodic inhibitors to restrain the corrosion process. Figure 1.7 Some examples of inorganic cathodic inhibitors are the ions of the magnesium, zinc, and nickel that react with the hydroxyl (OH-) of the water forming the insoluble hydroxides as (Mg(OH)2, Zn(OH)2, Ni(OH)2) which are deposited on the cathodic site of the metal surface, protecting it. Also can be cited polyphosphates, phosphonates, tannins, lignin and calcium salts as examples that present the same reaction mechanism. It showed in hard water a kind of this mechanism of inhibiting according to the impact of the magnesium or calcium bicarbonate on it when through a short period hard water flows over the metal it can help the nucleation of carbonates which allows the reaction close to the equilibrium and forming deposit on the iron surface such as CaCO3 which covering the cathodic area to ensure the metal. For examples that might be mentioned the oxides and salts of arsenic, bismuth and antimony which are precipitated on the cathode area in acid medium cathodic inhibitors strict the release of hydrogen ions because of the phenomena that can strong the discharge of hydrogen called overvoltage. 1.4 Mechanims of Corrosion Inhibitors Mechanism of corrosion inhibitors are divided into the following  Adsorption by making a film that is adsorbed on the metal surface  Changing the media properties to produce deposit that remove or prevent an aggressive constituent  Inducting the formation of corrosion element for example iron sulfide which consider as passivizing species As we know the organic molecules inhibit corrosion by adsorption by building a barrier between the metal surface and the surrounding environment therefore the polar group of molecule is straightway connected to metal and the non-polar end is directly in a vertical way to the surface. An inhibitor may be effective in one system, while in another it is not therefore, it is convenient to consider the following factors: • Chemical structure of the inhibitor component. • Chemical composition of the corrosive medium. • Nature of the metal surface. • Operating conditions (temperature, pressure, pH, etc.). • Thermal stability of the inhibitor. - Corrosion inhibitors have temperature limits above which lose their effectiveness because they suffer degradation of the containing components. • Solubility of the inhibitor in the system. - The solubility of the inhibitor in the system is required to achieve optimum results in the metal surface protection; this depends on the length of the hydrocarbon chain. • The addition of surfactants to enhance the dispersibility or solubility of inhibitors. • Modification of the molecular structure of the inhibitor by ethoxylation to increase the polarity, and thus reach its solubility in the aqueous medium. 1.5 Techniques for Analysis of Corrosion Inhibitors Weight loss experiment and electrochemical measurements known as the best techniques to test the effectiveness of an inhibitor such as impedance measurement analyzing and polarization curve method. Moreover microscopy techniques are used to analysis the corrosion operation. 1.6 Consideration to Employ Corrosion Inhibitors An environmental action factors must be considered for all the kinds of inhibitors due to some element like PH, metals, temperature, composition, geometry and impurities of the system which the concentration of inhibitor of one or more may change the anticorrosive system. For the employment of the inhibitors there are factors must be known as the real reason of the corrosion for example the value x benefit the reaction of inhibitor with the environment like the influence of catalyst, deposition or contamination. 1.7 Evaluating the Toxicity of Corrosion Inhibitors Prospective to be taken into minds in the improvement of corrosion inhibitors are their toxicity and effect on environmental pollution of both active and other components. 1.7.1 Toxicity Toxicity measured by using the LC50 which stands for 50 Lethal Concentration that is the concentration at which 50% of the test organisms are destroyed or measured by EC50 which can cause an adverse organism affection. For example the concentration reduces the emission of luminescent bacteria by the half or reducing the concentration growth of specific microorganism by the half. The toxicity degree may be classified according to the LC50 value, where these categories are described in Table 1.1. Category LC50 Super toxic 5 mg/kg of weight or less Extremely toxic 5-50 mg/kg Highly toxic 50-500 mg/kg Moderately toxic 0.5-5 g/kg Slightly toxic 5-15 g/kg Practically non-toxic More than 15 g/kg Table 1.1 1.7.2 Biodegradation Biodegradation is considered as testing measures that stability in the environment of the formulation components. The regulation test that should be used is the marine OECD and the allowable limit is more than the half after 28 days. 1.7.3 Bioaccumulation This test size the standard of product reinforcement in the body and it is measured by the partition coefficient. as this parameter can be correlated with the cell interface/water ratio. Po / w = concentration in octanol / concentration in water This means that the greater the partition coefficient, the more likely it is that the compound passes through the cell membrane, being bioaccumulated. EFCIs can be arbitrarily divided into two categories: natural products and low toxicity synthetic products. In this last category, special attention has been paid to a new class of low toxicity organic compounds known as ionic liquids. There are a few studies where CIs are evaluated according to the methodology described in this section and designed as low toxicity CIs or EFCIs 1.8 Environmentally friendly corrosion inhibitors The general prospective on corrosion inhibitor has change through these recent years because of increasing interest of the world to protect the environment from the chemical hazardous. An inhibitor should be adsorbed on the metal surface into eitherchemisorption or physisorption processes to be effective preventive against metal corrosion. Those adsorption processes relay essentially on the physicochemical characteristics of the inhibitor group for example electronic density, functional group and molecule structure. Organic molecules which have a big applicability that have been enormously researched and used as corrosion inhibitors which contain oxygen, nitrogen and sulfur atoms, as well as multiple bonds in their molecules. 1.9 Industrial Application 1.9.1 Acid Pickling Prevent the attack in the metal due to the acid solution in which metal gets cleaned of mill scale (bark lamination), and also prevented the subsequent hydrogen evolution inhibitors are added, typically organic, must be soluble or dispersed in the solution. Examples: thiourea and amino and its derivatives, propargyl alcohol. 1.9.2 Oil Industry sodium carbonates or organic amines complex are employed to reduce the corrosive effect of CO2, H2S and organic acids, enabling the use of more cheaper materials and less resistant to corrosion in wells extracting crude oil. Pipes for gasoline and kerosene are employed sulphonated oils, sodium nitrite. Oil well uses up fatty amines, fatty acids, imidazolines and quaternary ammonium salts. Internal pipe corrosion occurs in wet gas transportation due to condensation of water containing dissolved corrosive gases. Corrosion is caused by the dissolution of the corrosive gases, such as carbon dioxide and hydrogen sulfide as well as condensation of acid vapors. 1.9.3 Concrete To improve the durability of reinforced concrete structures, which are impaired due the high alkalinity, are used corrosion inhibitors, mixed with cement or concrete paste. An example is phosphate ions. 1.9.4 Boiler Thermoelectric use, in general, Ammonia, Cyclohexylamine, alkanol and Morpholine as inhibitors in boilers in various processes. The inhibitors, also, are added by the hydrochloric acid used for the solubilization of limescale to prevent the attack on pipes.
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