Corrosion Behavior Of Carbon Steel In Aqueous Solutions:

Mohamed Sobhi Motawhe Awad

This thesis comprises three main chapters; introduction, experimental work and result & discussion.Chapter (1): contains the introduction part which deals with the following fields of interest:- Corrosion and corrosion prevention by inhibitors- The effect of the structure of organic compounds on their inhibiting properties- Literature survey for corrosion of carbon steel in aqueous solutions and its inhibition.Chapter (2): deals with the experimental work. It includes the chemical composition of the carbon steel (L-52) used in this study; preparation of the sulfuric acid solutions as corrosive medium; metal salts used (CuSO4, NiSO4 and Fe2SO4); organic additives used (3-meth. pyrazolone, imidazole and 2-meth. imidazole) and the techniques used for the corrosion measurements e.g. (weight loss, galvanostatic polarization, cyclic voltammetry, potentiodynamic anodic polarization, conductometric titration and spectrophotometry) techniques.Chapter (3) involves seven sections:Section (A): contains the results of weight loss of carbon steel under investigation in 0.5M H2SO4 solutions containing different concentrations of the metal cations ( Cu+2, Ni+2 and Fe+3), organic additives (3-meth. pyrazolone, imidazole and 2-meth. imidazole), and mixtures from them with different ratios. These results revealed that:- In general, for all additives, the increasing of the concentration of the metal cations, organic additives and mixtures from them leads to the decrease of weight loss, indicating the inhibiting effect of these additives.- The values of corrosion rate (k) of C-steel in 0.5M sulfuric acid was found to be decreased with increasing the concentration of the additives and obeyed the given trend:Metal cation < organic additive < mixtures from them- The percentage inhibition efficiency of the additives is arranged in the following orders:metal cation < organic additive < (metal cation + organic additives)- The variation of the degree of surface coverage (θ), with (logC) for C-steel in the presence of different additives, metal cations, organic additives and mixtures of them gives a straight line relationship. This means that, the adsorption of these compounds on the steel surface obeys the Temkin’s adsorption isotherm.Section (B): This section contains the results of gavanostatic polarization measurements of carbon steel in 0.5M H2SO4 solution in absence and presence of metal cations, organic additives, and mixtures of them with different ratios. These results permitted to conclude that:- As the concentrations of additives increase the values of corrosion current density (Icorr.) decreases, indicating the inhibiting effect of these additives.- The anodic (ba) and cathodic (bc) Tafel slopes are changed slightly indicating that these compounds, act mainly as mixed inhibitors. The higher the cathodic Tafel slope (bc) as the concentration of additives increased, as compared with that of blank solution. This behavior should be due to the decrease of the cathodic transfer coefficient, which can be ascribed to the thickening of the electric double layer or the formation of multi-molecular layers adsorbed on the metal surface, also the anodic Tafel slope (ba) in the inhibited solution is higher than that obtained in the acid inhibitor free solutions. This increase in ba suggests a mode of inhibition involving an interposition of the additives into the charge transfer process for the anodic reaction, which may lead to a morphological change of the electrode surface brought about by the anodic dissolution- The percentage inhibition efficiency values in the presence of different concentrations of the different additives at 30 C are comparable with those calculated from weight loss measurements, meaning that the conclusions obtained from weight loss and electrochemical methods are in good agreement.- The inhibition efficiency of the additives is arranged in the following order: (Metal cation + organic additive mixture) > organic additive > metal cation.Section (C): contains the results obtained from the potentiodynamic anodic polarization measurements of carbon steel in 0.5M H2SO4 in absence and presence of different additives at scan rate of 100 mV/sec. These results permitted to conclude that:- There is one anodic peak due to the dissolution of Fe to Fe+2 ion.- The anodic cyclic voltammograms of C-steel electrode in 0.5M sulfuric acid at different scanning rates, shows an increase in the peak current of all anodic peak with increasing the scanning rate. The relationship between the peak potential (Ep) and the scanning rate (ע1/2(gave a straight line indicating that, the passivation film exists on the C-steel surface is formed through the dissolution-precipitation mechanism.- Increasing the concentration of the additives shifts Ep to more positive values and ip to lower values, thus increasing the inhibition efficiency. This indicates an increased resistance to active dissolution which depends on both the type of electrode and the inhibitor.The inhibition efficiencies of all additives increase in the following order:metal cation < organic additive < (metal cation + organic additives)Section (D): contains the results of potentiodynamic anodic polarization of carbon steel in 0.5M H2SO4 containing different concentrations of Cl- ion in absence and presence of different additives. These results show that:- Higher Cl- ion concentration results in a sudden and marked increase of current density at some definite potentials denoting the destruction of the passive film and initiation of visible pits. The higher concentrations of C1- ion, the higher shift of pitting potential to active direction.- It was found that, the pitting potential of the C-steel electrode is shifted to more positive (noble) values with increasing the concentration of additives. This indicates the increased resistance to pitting attack.- Inhibition afforded by using different concentrations of the additives used decreases in the following order :(metal cation + organic additive) > organic additive> metal cation.Section (E): represents the conductometric titration measurements.- The data obtained gave a strong indication for the formation of a complex compound between the two additives (organic compound and metal cation) by a molar ratio of 1.0 (M+2 cation): 2.0 (additive). This reflects the great tendency of such organic compound to form a complex compounds with metal ions, which in turn increase the efficiency of acting as corrosion inhibitors through the formation of the complex species.Section (F): represents the spectrophotometric measurements (UV-visible spectra) of M+2 cation, the organic additives and M+2 cation + organic additive mixture obtained before and after polarization measurements which conclude that:- The spectra of M+2 cation, the organic additives and M+2 cation + organic additive mixture after polarization measurements shows the same absorption band values as obtained from those before polarization measurements but at lower intensities. This means that some of the organic additive as well as M+2cation + organic additive complex that formed in 0.5 M H2SO4 solution are adsorbed on the carbon steel surface.Section G: represents the inhibition mechanism suggested for the investigated additives, which conclude that:- The inhibiting action of M+2 cation is ascribed to the plating of the corresponding metal on the carbon steel surface, which forms a protective layer with consequent more complete anodic passivation of the anodic sections of the substrate.- The inhibition of carbon steel corrosion by organic additives is attributed to, the adsorption of the inhibitor molecules on the metal surface. Imidazole and pyrazolone molecules shows three different anchoring sites suitable for surface adsorption, the nitrogen atom with its lonely sp2 electron pair, the active hydrogen atom attached to the nitrogen atom and the p-bond of the aromatic ring.- In the case of addition of M+2 cation + organic additive mixture, the results demonstrated the complex formation between the two components, which was much more effective than the inhibiting action of each additive separately. The conductometric studies revealed the participation of two organic molecules in the complex formation with M+2 cation. This finding would suggest that the molecular size of the complex formed and consequently the number of adsorption centers play an important role in the enhancement of the protection of carbon steel against corrosion.