The Characterastion Of Self-Assembled Monolayers On Iron And Stainless Steel Surface And The Study By Electrochmical Method

Self-assembled monolayers (SAMs) offered by organic substances have advantages for studying the inhibition of metal corrosion because uniform and closely packed films form on the metal surface and provide an insulating layer. The SAMs are organic assemblies formed by the adsorption of molecular constituents from solution or the gas phase onto the surface of metal electrodes. SAMs on the iron surface are studied so few because iron is one active metal which is easily corroded in the air. Although there have been some reports about protection to metal caused by self-assembled films formed on the surfaces of iron and steel, for there usually exist a film of iron at the surface of engineering iron. The main research systems are limited to the alkyl thiols. Some self-assembled films are unstable and have poison. Therefore, it is essential to select proper systems and prepare strong and stable self-assembled films that are durable in various environments, especially in corrosion environments. We found that imidazoline (IM) and triazole (AAMT) are innocuous and high efficiency corrosion inhibitors. It is quite effective for the imidazoline and triazole to inhibit the corrosion of iron in acid solution. In this thesis, we wide the applied range of the IM and AAMT which can form can form self-assembled films on the surface and inhibit the iron corrosion. It is important for the practical application of IM and AAMT.In practical point of view, it is very important to study those environmentally friendly inhibitors. The most widely used inhibitors in the petroleum industry are nitrogen containing compounds such as specially imidazolines and their derivatives. So it is important to search for new, nontoxic, effective organic inhibitors to the iron in order to prevent corrosion, because most corrosion inhibitors impose a toxic burden on the environment. In this regard, inhibitors have a promising alternative for the design of "green" corrosion inhibitors which will satisfy the environmental requirements. Surface analysis and molecular modeling techniques were used to study adsorption and film formation of inhibitor on the iron surface.The software of Cerius2 was employed which was produced by Accelrys accompany. Molecular simulation studies that the molecular structure and behavior are simulated with the molecular model in atomic level by using computer, and the physical and chemical properties of the system were calculated, Molecular simulation method was used to, research some critical subjects such as the ways of chemical reaction, transition state, reaction mechanics and so on. Furthermore, it was applied to design new materials substituting some experiments of chemical synthesis, analysis of structure and physical detection .At the same time, the period of research and the cost of development have been cut down greatly. In this paper, we used molecule mechanics method to study imidazoline and triazole self-assembled film on Fe(110).Scanning electrochemical microscopy (SECM) is a new electrochemical technology with high space resolving power. Using the SECM to study the corrosion electrochemical system, we can get some very important information which cannot be obtained using the common general electrochemical methods. SECM was used to characterize the morphologies of iron. It is widely used for the characterization of the chemical nature of interfaces. SECM was carried out with the SECM270 (Uniscan Instruments Ltd.. UK) using a standard four-electrode cell. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a Pt wire. The SECM tip was made of Pt wire (10μm diameter) sealed in a glass capillary. The tip was polished with 0.05 μm alumina. The distance between the tip and iron substrate is 40 μm.The purpose of this work was to search for a kind of new, low toxicity inhibitor with the high inhibition efficiency for iron. We synthesized IM and AAMT compounds and investigated the inhibition efficiency of iron with SAMs in acid solution using electrochemical methods (EIS, Tafel, SECM). It was confirmed that the self-assembled film of AAMT on the iron surface had the higher charge transfer resistance and the lower corrosion current densities. Furthermore, we interpreted the inhibition mechanism of IM and AAMT inhibitor using XPS, SEM, EDS and molecular simulation. 1. Studies of protection of iron corrosion by imidazoline self-assembled monolayersSome of the most effective corrosion inhibitors for oil field pipeline applications are the imidazoline (IM) compounds which exhibit good inhibition in acid solution. So IM inhibitors are used extensively as inhibitors of iron corrosion in acidic media, particularly in oil field applications. IM were effective corrosion inhibitors due to low their toxicity and special structure. The molecular structure of imidazolines can be divided in three different substructures: a nitrogen-containing five-membered ring, a long hydrocarbon chain and a pendant side chain with an active functional group. Because the lone electron pair of nitrogen filled in the unoccupied molecular orbits of iron atoms, the IM can adsorb on the iron surface. IM molecules had p-Ï€ conjugated system and -NH2, which made the nitrogen be easy to from chemical bonds.Stearic acid or rosin acid and diethylenetriamine (DETA) as raw materials synthesized IM inhibitor according to the references. Using EIS and Tafel curves, we got the inhibition efficiency of SAMs for iron. The films formed on iron electrodes were studied by XPS, SEM, SECM. The SEM shows that the iron with inhibitor is covered by a more uniform and coherent protective surface film. This observation suggests that the inhibition was due to the formation of a stable film through the process of adsorption of organic molecules on the metal surface. The XPS results showed that the IM molecules were adsorbed on the iron surface. Molecular simulation method is performed to study formation mechanisms and packing structures of monolayers on iron surface. Molecular simulation results show that the N atoms of the IM bound to surface iron atoms so that the IM molecules adsorb to the iron surface. These results open a new perspective for the molecular-scale characterization of complex self-assembled monolayers on metal supports.The experiments were performed at selected heights of the microelectrode above the iron substrate. The substrate generation/ tip collection experiment (SG/TC) is then performed. The X and Y axes show an area 1.0mm by 1.0mm. We did the area scanning and the line scanning on the iron surface using SECM. Among the above experiment methods, the area scanning, the line scanning can be used to investigate the concentration profiles of the corrosion products on the iron surface. The 3D images of SECM appear concave peaks, which shows that the Pt tip easily oxidate iron ions due to the dissolution iron substrate. But the values of tip current of peak decreases when SAMs modified iron substrate. That explains that SAMs has the blocking effect for dissolution of the substrate. The values of tip current of peak decrease when SAMs modified iron substrate. Futhermore, There are lower tip current when rosin imidazoline forms SAMs on iron than steric imidzoline does, which explains that rosin imidazoline has higher corrosion inhibition for iron than steric imidzoline has. The results of SECM were identical with those of electrochemical methods.2. The study of self-assembled films of triazole on iron electrodesOrganic substances have been used extensively as corrosion inhibitors during the last four decades. Of these, heterocyclic compounds containing one or more N, O and S atoms can affect the inhibition of corrosion, in aqueous acid solutions, such as triazole derivatives. Its effects of concentrations and functional groups on the corrosion of iron and steel have been studied. The efficiency of an inhibitor does not only depend on its structure, but also on the characteristics of the environment in which it acts the nature of the metal and other experimental conditions. Under certain conditions, the electronic structure of the organic inhibitors has a key influence on the corrosion inhibition efficiency to the metal. Triazole has special affinity toward iron due to an abundance of Ï€-electrons and unshared electron pairs on the nitrogen and sulphur atom that can interact with d-orbitals of iron to provide a protective film.AAMT acts as a mixed type inhibitor for iron in 0.1M H₂SO₄. The duration of immersion from 15min to 4h was increased, the inhibition efficiency was observed to increase. Furthermore, the polarization curves and electrochemical impedance spectroscopy were in good agreement. The XPS and SEM analysis showed that AAMT was adsorbed on the iron surface. The modeling of the adsorption of triazole on the iron surface was supposed by Molecular simulation, which proved that the five-ring was parallel to the metal surface. This method is a perspective for the molecular-scale characterization of complex self-assembled film on metal supports. The above results suggested that adsorption of AAMT occurred to the iron surface.3. The study of SAMs of imidazoline on 304 stainless steel electrodesIt was difficult to form SAMs on stainless steel owning to special structure with many elements. It is necessary to form SAMs, because it was widely used in industry. SAMs on the steel surface are studied so little. Corrosion of stainless steel is a major concern in the petrochemical industry due to the destructive attack of iron substrates by chemical and electrochemical reactions. In this paper, using SEM and SECM methods, we studied the SAMs of IM on the stainless steel. The 3D images of SECM appear concave peaks, which shows that the Pt tip easily oxidate Fe²⁺ due to the dissolution of substrate. But the values of tip current of peak decreases when SAMs modified stainless steel substrate. That explains that SAMs has the blocking effect for dissolution of the substrate. On the other hand, SAMs-covered stainless steel substrate surface becomes more and more uniform than bare. The existing of SAMs on stainless steel changed the chemical and physical property of the surface, and then inhibits the corrosion for stainless steel.