Devaraj, G. (1982) Mechanism of Corrosion Inhibition-Aldehydes as inhibitors from mild steel in acid solution. PhD thesis, Madurai-Kamaraj University.

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Corrosion rate of mild steel in different acids with and without inhibitors has been studied by weightloss potentiodynamic polarization and small amplitude cyclic voltametry. The hydrogen uptake during the dissolution of mild steel in acids with and without inhibitors has been studied by electrochemical permeation method. The adsorption behavior of aldehydes on mercury has been studied by electro capillary method. The corrosion products formed in presence of aldehydes have been analysed using IR and NMR. Corrosion rate of mild steel in acids depends on the anions and the present investigation clearly indicates the chloride ions are more corrosive nature follows the order HCL > H2SO4 also agrees with the order of absorbability of anions on steel surface namely Cl - > SO4 -- > CIO4-. But in dilute solution of acids i.e. at 0.1N the corrosive nature of the anions follows the order H3PO4 > HCL > H2SO4 > HCLO4. The present investigation indicates that the weight loss increases with time. It has also been found that oxygen has little effect on the corrosion rate of mild steel in HCL whereas in other two acids the effect of oxygen is more pronounced and increases the corrosion rate. All the aldehydes studied act as good inhibitors of corrosion of mild steel in the three acids. Among them cinnamaldehyde shows the highest efficiency. The inhibitive action of aldehydes is more in HCL than in the other two acids. The higher inhibition efficiency of aldehydes in hydrochloric acid has been explained as due to the specific adsorption of chloride ion. The inhibition efficiencies at a comparable concentration of 10 mM indicate that the substituent in benzaldehyde increase the inhibition efficiency. The low inhibition efficiencies of salicylaldehyde and p-hydroxybenzaldehyde are attributed to the presence of hydrogen bonding which decrease the electron density at the carbonyl oxygen. In sulphuric acid solutions the inhibitors perform less effectively than in the other two acids. The presence of extended conjugation in cinnamaldehyde has been considered to be responsible for its high inhibition efficiency. Cinnamaldehyde which exhibits maximum corrosion inhibition efficiency also reduces the hydrogen permeation to the greatest extent. The electrochemical methods include measurement of corrosion potential. Corrosion current by extrapolation of Tafel slopes and by linear polarization resistance using SACV and also measurement of double layer capacity from the hysteresis currents obtained in SACV. In HCl and HClO4 the values of anodic Tafel slopes are noted. However the values of cathodic Tafel slopes do not change in presence of aldehydes. The corrosion currents calculated by extrapolation of Tafel slopes indicate that the corrosion rate follows the order H2SO4 > HCl > HClO4. In the presence of aldehydes the corrosion currents are reduced considerably in all the three acids. The inhibition efficiencies calculated from corrosion currents are higher in HCl and HClo4 than in H2SO4 as in weight loss measurements. The results obtained by weight loss methods are compared with those of electrochemical methods. The corrosion rates in pure acids as well as in presence of aldehydes obtained by electrochemical methods are lower than those obtained by weight loss methods. The corrosion rate in HCl is greater than in H2SO4 in weight loss method but the electrochemical methods indicate higher rate for H2SO4. But at the same time both the corrosion current and weightloss increases rapidly after a specific period. The inhibition efficiencies calculated by weight loss methods are higher than those calculated by electrochemical methods in HCl and H2SO4 whereas in HClO4 solution the latter shows higher inhibition efficiency than the former. The extent of adsorption does not vary linearly with hammett constants. But the difference in the nature of the substituent and their position are reflected in adsorption of aldehydes. The lower extent of adsorption than expected by Hammett constant values for salicylaldehyde can be considered to be due to hydrogen bonding present in this compound. Comparison of the decrease in interfacial tension values at e.c.m. with inhibition efficiencies indicate that there is no direct relation between the two, except for the fact that cinnamaldehyde which shows the maximum inhibition efficiency is also adsorbed to the maximum extent in all acids. The absence of any relation between the extent of adsorption and inhibition efficiency can be due to the fact that adsorption over mercury and that on corroding iron cannot be considered as same. Moreover, the adsorbed molecules may interact differently on iron and mercury.

Item Type: Thesis (PhD)
Uncontrolled Keywords: corrosion inhibitor; aldehydes
Subjects: Corrosion Science and Engineering
Depositing User: Dr. N Meyyappan
Date Deposited: 11 Jun 2012 09:26
Last Modified: 11 Jun 2012 09:41

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