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94 Seiten, Note: 3
Chapter -1. Introduction of corrosion
1.1 General Aspect of Corrosion
1.2 Factors Influencing Corrosion
1.3 Types of Corrosion
1.4 Aim of Research work
Chapter -2. Performance and Impact of plant extracts as Metallic Corrosion 14 Inhibitors
2.1 Plant extract as Corrosion Inhibitors
2.2 Effect of Functional Group on Inhibitors
2.3 Classification of Inhibitors
2.4 Phenomenon of Adsorption
2.5Azadirechtaindica utilized in the present work its properties And uses as corrosion inhibitor
2.6Murrayakoenigii utilized in the present work its properties And uses as corrosion inhibitor
Chapter -3.Spectral studies on plants use as green inhibitor
3.1 Infrared spectroscopy
3.2 X-ray fiuoresence study
Chapter -4. Experimental Methods and Discussion of the Results
4.1 Experimental Method
4.2 Preparation of plant extract
4.3 Discussion of the Results
4.4 Effect of Inhibitor Concentration
4.5 Effect of Exposure Period
4.6 Effect of Temperature
4.7 Scanning electron microscopic study
Chapter -5. Summary of the Dissertation
5.1 Effect of Inhibitor Concentration
5.2 Effect of Exposure Period
5.3 Effect of Temperature
5.4 Scanning electron microscopic study
Aluminium, being a highly reactive metal, corrodes rapidly in acidic (pH < 6) and alkaline (pH > 12.5) media. Hence it has to be protected when it is likely to come in contact with such solutions, e.g., during cleaning or acid pickling. One method of protection is the addition of inhibitor to the corroding medium. In the present work, ethanol extract of Azadirechta indica and Murraya koenigii leaves have been investigated as corrosion inhibitor for aluminium in aqueous hydrochloric acid. The effect of inhibitor and acid concentration, exposure period, and temperature on the inhibitive action of the compound has been studied. Weight loss method has been used. The mechanism of the action of inhibitor has also been suggested. It was observed that the weight loss increases as the concentration of acid increases, and the same effect is observed with increase in temperature and time duration.
In acid containing the inhibitor, it was observed that at low concentrations of both the plant accelerate the attack on aluminium in 0.5 M HCl. The acceleration decreases with increase in inhibitor concentration. Finally, it depends upon the inhibitor and its concentration, the process of inhibition sets in and then inhibition increases with further increase in inhibitor concentration.
The results show that the plant extract studied function as accelerators of corrosion at low concentrations but as inhibitors at high concentrations. Thus at low concentrations they may be useful for removal or recovery of aluminium from galvanized articles provided they do not attack the base metal like steel. But when inhibition of corrosion is desired, higher inhibitor concentrations are required.
The corrosion of Alluminium in plain hydrochloric acid, as well as inhibited, is found to increase with a rise in temperature. Thus in uninhibited 0.5 M HCl the loss in weight due to corrosion for an exposure period of 60 min increases from 736 mg/dm at 35oC to 852, 922, and 958 mg/dm at 45oC, 55oC and 65oC respectively.
In inhibited 0.5 M HCl containing 1.30% of Azadirechta indica, it was observed that at 35oC and for an exposure period of 60 min Azadirechta indica confer 100.0% protection. As the temperature is increased, the extent of corrosion in inhibited acid also increases but the weight losses are much less than that in Murraya koenigii and in plain acid. As far as the inhibitor efficiency is concerned, it may be generalized that at 1.30% inhibitor concentration, the efficiency decreases with a rise in temperature, the effect being less pronounced in the case of Azadirechta indica.
In A. indica, it is observed that the compound at 1.30% concentration show an efficiency 100.0% at 35°C. As the temperature is increased the efficiency decreases. In the case of A.indica, the efficiency decreases slightly and is found to be 72.96% at 65°C. It appears that in the case of Azadirechta indica, the adsorption is of physisorption type which decreases with a rise in temperature. The surface morphology of the Al samples in the absence and presence of A.indica and M.koenigii leaves extract was investigated after weight loss using SEM technique. The badly damaged surface obtained when the metal was kept immersed in 0. 5 M HCl for 60 min without inhibitor indicates significant corrosion. However, in presence of inhibitor the surface has remarkably Improve with respect to its smoothness indicating considerable reduction of corrosion rate. This improvement in surface morphology is due to the formation of a good protective film of inhibitor on aluminium surface which is responsible for inhibition of corrosion. The order of efficiency at 1.30% v/v inhibitor concentration in 0.5 M hydrochloric acid was found to be : Azadirechta indica (100.0%) Murraya koenigii ( 94.79%)
In recent years corrosion by chemical and electrochemical reactions has assumed great economic importance throughout the world. The estimated annual loss due to corrosion is enormously large. Studies worldwide have shown that the overall cost of corrosion amount to at least 2-3% of the Gross National Product and that 20-25% of the cost could be avoided by using appropriate corrosion control technology. The corrosion of a metal or a material is a global scientific problem as it affects different walks of life especially in metallurgical, chemical, materials and oil industries. Corrosion is defined as the degradation of materials or its properties due to a reaction with the environment. Corrosion exists in virtually all materials, but is most often associated with metals. Metallic corrosion is a naturally occurring process whereby the surface of a metallic structure is oxidized or reduced to a corrosion product such as “rust” by chemical and electrochemical reaction with the environment. The surface of metallic structures is attacked through the migration of ions away from the surface, resulting in material loss over time. The process of corrosion requires the presence of an anode, a cathode, an electrolyte and electrical circuit.
The problem of preventing the metallic corrosion is extremely complex but it is of great technological and economical importance. Corrosion of metals can be controlled by taking suitable preventive measures such as painting, plating, use of expensive alloys, use of inhibitors, etc. It is justifiable that several corers of rupees are spent on research for controlling corrosion.
The nature and extent of corrosion depend on the metal and the environment. The important factors which may influence the corrosion process are :
(i) Nature of the metal (ii) Environment (iii) Concentration of electrolyte (iv) Temperature (v) Electrode potential and (vi) Hydrogen over voltage
Different types of corrosion, more or less visible to the naked eye, can occur on metal, such as uniform (generalized) corrosion, pitting corrosion, stress corrosion etc. The predominant type of corrosion will depend on a certain number of factors that are intrinsic to the metal, the medium and the conditions of use. There is no form of corrosion that is specific to metal and its alloys.
- Uniform Corrosion : This type of corrosion develops as pits of very small diameter, in the order of a micrometer, and results in a uniform and continuous decrease in thickness over the entire surface area of the metal. The rate of uniform corrosion can be easily determined by measuring the mass loss, or the quantity of released hydrogen .
- Pitting Corrosion : This localized form of corrosion is characterized by the formation of irregularly shaped cavities on the surface of the metal. Their diameter and depth depend on several parameters related to the metal, the medium and service conditions. Unlike uniform corrosion, the intensity and rate of pitting corrosion can be assessed neither by determining the mass loss nor by measuring released hydrogen. In fact, these measurements do not make sense because a very deep and isolated pit results only in a small mass loss, where as a very large number of superficial pits can lead to a larger mass loss. Pitting corrosion can be assessed using three criteria : the density, i.e. the number of pits per unit area, the rate of deepening and the probability of pitting Pitting corrosion
- Transgranular and Intergranular (Intercrystalline) Corrosion: Within the metal, at the level of the grain, corrosion may propagate in two different ways : (i) It spreads in all directions, corrosion indifferently affects all the metallurgical constituents; there is no selective corrosion. This is called transgranular or transcrystalline corrosion because it propagates within the grains. (ii) It follows preferential paths: corrosion propagates at grain boundaries. Unlike transgranular corrosion, these forms of intercrystalline corrosion consumes only a very small amount of metal, which is why mass loss is not a significant parameter for assessment of this type of corrosion. It is not detectable which naked eye but requires microscopic observation, typically at a magnification of 50. When penetrating into the bulk of the metal, intercrystalline corrosion may lead to a reduction of mechanical properties and even lead to the rupture of components.
- Exfoliation Corrosion : Exfoliation corrosion is a type of selective corrosion that propagates along a large number of planes running parallel to the direction of rolling or extrusion. Between these planes are very thin sheets of sound metal that are not attacked, but gradually pushed away by the swelling of corrosion products, peeling off like pages in a book; hence the term exfoliation corrosion. The metal will swell, which results in the spectacular aspect of this form of corrosion.
- Stress Corrosion : This type of corrosion results from the combine action of a mechanical stress (bending, tension) and a corrosive environment. Each of these parameters alone would not have such a significant effect on the resistance of the metal or would have no effect at all.
- Crevice Corrosion : Crevice corrosion is a localized corrosion in recesses :
overlapping zones for riveting, bolting or welding, zones under joints and under various deposits. These zones also called crevices, are very tiny and difficult to access for the aqueous liquid that is covering the rest of the readily accessible surfaces. This type of corrosion is also known as deposit attack.
- Galvanic Corrosion : When two dissimilar metals are in direct contact in a conducting liquid, experience shows that one of the two may corrode. This is called galvanic corrosion. The other metal will not corrode; it may even be protected in this way. This corrosion is different in its kind and intensity from the one that would occur if they were placed separately in the same liquid. Unlike other types of structural corrosion, galvanic corrosion does not depend on the metal’s texture, temper, etc. Galvanic corrosion may occur with any metal, as soon as two are in contact in a conductive liquid. It works like a battery. The appearance of galvanic corrosion is very characteristic. It is not dispersed like pitting corrosion, but highly localized in contact zone with the other metal. The zone affected by galvanic corrosion often has a shinier aspect than the rest of the surface.
- Erosion : Corrosion by erosion occurs in moving media. This type of corrosion is related to the flow speed of the fluid. It leads to local thinning of the metal, which results in scratches, gullies, and undulations, which are always oriented in the same direction, namely the flow direction. Avoiding erosion corrosion on Zinc.
The main purpose of the research work that has been described in the thesis centers around the possibility of finding excellent inhibitors for aggressive corrosive media. It has been our experience that it is difficult to find or to come across excellent effective inhibitors for metals corroding extensively in aggressive corrosive medium, e.g. for systems such as zinc in concentrated hydrochloric acid and sulphuric acid or aluminium in concentrated hydrochloric acid or in concentrated sodium hydroxide solutions. Aldehydes and amines are fairly good inhibitors for metals and alloys in acids and other corrosive media. Many of the commercial inhibitor formulations for acid solutions include aldehydes and amines [1-20].
To replace the environmentally hazardous chromates, several non -chromates have been used as corrosion inhibitors. Extracts of plant materials top the list. The plant extracts are environmentally friendly, non- toxic and readily available. These extracts contain many ingredients [21-30]. They contain several organic compounds which have polar atoms such as O, N, P and S. They are adsorbed onto the metal surface through these polar atoms; protective films are formed. Adsorptions of these ingredients obey
various adsorption isotherms. The films have been analyzed by many surface analysis techniques such as AFM, FTIR, UV, Fluorescence spectra and SEM. From this point of view, the following inhibitor has been synthesized and the performance of this inhibitor in retarding the corrosion of alluminium in hydrochloric acid has been studied. The natural inhibitors studied are :
1. Azadirechta indica (Bitter neem)
2. Murraya koenigii (Sweet neem)
The effectiveness of natural plant extract has been evaluated as an inhibitor by considering the following parameters:
(a) The effect of inhibitor concentration on inhibitor efficiency.
(b) The effect of acid concentration on inhibitor efficiency.
(c) The effect of exposure period on inhibitor efficiency.
(d) Adsorption isotherm followed by the inhibitor.
(e) The effect of temperature on the performance of the inhibitor.
From all the data obtained as above, an attempt has been made to suggest a probable action mechanism of this inhibitor.
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The exploration of natural products origin as corrosion inhibitors is becoming the subject of extensive investigation due principally to the low cost and eco-friendliness of these products, and is fast replacing the synthetic and expensive hazardous organic inhibitors. Plant extracts constitute several organic compounds which have corrosion inhibiting abilities. The yield of these compounds as well as the corrosion inhibition abilities vary widely depending on the part of the plant [1-5] and its location. The extracts from the leaves, seeds, heartwood, bark, roots and fruits of plants have been reported to inhibit metallic corrosion in acidic media [4, 5, 7-17]. A summary of plants extracts used as corrosion inhibitors have recently been given in Okafor et al. and Raja and Sethuraman.
Inhibitors can bind to metal surfaces not only by electrostatic interaction but also by electron transfer to the metal to from a coordinate type of linkage. This type of interaction is favored by the presence in the metal of vacant electronic orbitals of low energy as they occur in transition metals.
Electron transfer from the adsorbed species is favored by the presence of relatively loosely bound electrons, such as may be found in anions and neutral organic molecules containing lone pair of electrons or π-electron systems associated with multiple bonds or aromatic rings. In organic compounds suitable lone pair of electrons for coordinate bonding occurs in functional groups containing elements of group V and VI of the periodic table. The tendency to stronger coordination bond formation and hence stronger adsorption by these elements increases with decreasing electro-negativities in the order O < N < S < Se [19,20] and also depends on the nature of the functional groups containing these elements.
A corrosion inhibitor is a chemical substance which, when added in small concentrations to an environment, minimizes or prevent corrosion. Corrosion inhibitors are used to protect metals from corrosion, including temporary protection during storage or transport as well as localized protection, required, for example, to prevent corrosion that may result from accumulation of small amounts of an aggressive phase. One example is brine, in a non-aggressive phase, such as oil. An efficient inhibitor is compatible with the environment, is economical for application, and produces the desired effect when present in small concentrations.
Classification of corrosion inhibitors is somewhat a subjective exercise. Some of the more commonly encountered descriptive are anodic, cathodic, passivating, oxidizing, film-forming, organic, vapor phase, volatile and ‘safe’ or ‘dangerous’ inhibitors.
Inhibitor selection is based on the metal and the environment. A qualitative classification of inhibitors is presented in Figure 1. Inhibitors can be classified into environmental conditioners and interface inhibitors.
1. Environmental Conditioners (Scavengers)
Corrosion can be controlled by removing the corrosive species in the medium. Inhibitors that decrease corrosivity of the medium by scavenging the aggressive substances are called environmental conditioners or scavengers. In near-neutral and alkaline solutions, oxygen reduction is a common cathodic reaction. In such situations, corrosion can be controlled by decreasing the oxygen content using scavengers (e.g., hydrazine ).
Interface inhibitor control corrosion by forming a film at the metal/environment interface. Interface inhibitors can be classified into liquid and vapour phase inhibitors.
Temporary protection against atmospheric corrosion, particularly in closed environments can be achieved using vapour-phase inhibitors (VPI). Substances having low but significant pressure of vapour with inhibiting properties are effective. The VPIs are used by impregnating wrapping paper or by placing them loosely inside a closed container . The slow vaporization of the inhibitor protects against air and moisture. In general, VPIs are more effective for ferrous than non-ferrous metals.
Liquid phase inhibitors are classified as anodic, cathodic, or mixed inhibitors, depending on whether they inhibit the anodic, cathodic, or both electrochemical reactions.
Anodic inhibitors are usually used in near-neutral solutions where sparingly soluble corrosion products, such as oxides, hydroxides, or salts, are formed. They form, or facilitate the formation of, passivating films that inhibit the anodic metal dissolution reaction. Anodic inhibitors are often called passivating inhibitors.
When the concentration of an anodic inhibitor is not sufficient, corrosion may be accelerated, rather then inhibited. The critical concentration above which inhibitors are effective depends on the nature and concentration of the aggressive ions.
Cathodic inhibitors control corrosion by either decreasing the reduction rate (cathodic poisons) or by precipitating selectively on the cathodic areas (cathodic precipitators).
Cathodic poisons, such as sulfides and selenides, are adsorbed on the metal surface; whereas compounds of arsenic, bismuth, and antimony are reduced at the cathode and form a metallic layer. In near-neutral and alkaline solutions, inorganic anions, such as phosphates, silicates, and borates, form protective films that decrease the cathodic reaction rate by limiting the diffusion of oxygen to the metal surface. Cathodic poisons can cause hydrogen blisters and hydrogen embrittlement due to the absorption of hydrogen into steel. This problem may occur in acid solutions, where the reduction reaction is hydrogen evolution, and when the inhibitor poisons, or minimizes, the recombination of hydrogen atoms to gaseous hydrogen molecules. In this situation, the hydrogen, instead of leaving the surface as hydrogen gas, diffuses into steel causing hydrogen damage, such as hydrogen-induced cracking (HIC), hydrogen embrittlement or sulfide stress cracking.
Cathodic precipitators increase the alkalinity at cathodic sites and precipitate insoluble compounds on the metal surface. The most widely used cathodic precipitators are the carbonates of calcium and magnesium.
About 80% of inhibitors are organic compounds that cannot be designated specifically as anodic or cathodic and are known as mixed inhibitors. The effectiveness of organic inhibitors is related to the extent to which they absorb and cover the metal surface. Adsorption depends on the structure of the inhibitor, on the surface charge of the metal, and on the type of electrolyte.
Mixed inhibitors protect the metal in three possible ways: physical adsorption, chemisorption and film formation. Physical (or electrostatic) adsorption is a result of electrostatic between the inhibitor and the metal surface. When the metal surface is positively charged, adsorption of negatively charged (anionic) inhibitors is facilitated.
Positively charged molecules acting in combination with a negatively charged intermediate can inhibit a positively charged metal. Anions, such as halide ions, in solution adsorb on the positively charged metal surface, and organic cations subsequently adsorb on the dipole. Corrosion of iron in sulphuric acid containing chloride ions is inhibited by quaternary ammonium cations through this synergistic effect.