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The objective in the present paper is a better understanding of acidic corrosion inhibition by polypyrazolylborates on iron electrodes. Structural and electronic effects of these compounds are discussed in relation to the inhibiting efficiencies and absorption.

Two polypyrazolylborates, i.e. dihydrobis(1‐pyrazolyl) borate (Bp) and hydrotris(1‐pyrazolyl)borate (Tp) were investigated as corrosion inhibitors for iron in 1 mol/l HCl and in 0.5 mol/l H₂SO₄ using Tafel measurements, linear polarization resistance and electrochemical impedance spectroscopy. Theoretical explanation between molecular parameters of these compounds and inhibitor efficiency (IE) was made using the MNDO SCF‐MO method. Morphological changes of the reacted surfaces were monitored via SEM.

Results obtained show higher inhibition effect of Bp than Tp. Inhibitors are more efficient in HCl than in H₂SO₄. Corrosion current and corrosion rate (CR) decrease with inhibitor concentration, while polarization and charge transfer resistance markedly increase from 10⁻⁴ to 10⁻² mol/l inhibitor concentration range. All impedance data are modeled with an equivalent circuit model. The semicircular shape of Nyquist plots is indicative of a charge transfer process occurring via a parallel resistor and capacitor.

This study provides constructive information relative to corrosion inhibitor behavior of polypyrazolylborates in acids. Using the molecular modeling calculations, the compounds were characterized in terms of their structural properties and inhibition activity. It was noticed how exposure to the inhibitor solution changes the film morphology.

The effect of some organophosphorus compounds on iron corrosion in 6 M HCl has been examined by means of polarization and capacitance measurements. It appears that these compounds inhibit iron corrosion by affecting both cathodic and anodic reactions. A quantum chemical approach was used to correlate the inhibition efficiency to the molecular structure of the compounds. It was found that there is not a noticeable relationship between their molecular structure and their inhibition effectiveness. Also based on these calculations, it can be suggested that the inhibitor molecules behave as electron acceptor when they adsorb on the iron metal.

Pyridine and some of its derivatives have been evaluated as inhibitors for the corrosion of API‐45 steel in NACE sour solution. Amongst the various methyl and halogen substituted pyridine compounds, 2,4,6‐trimethyl pyridine is found to afford maximum protection followed by 2,6‐dimethyl pyridine, >2,4‐dimethyl pyridine, >3‐bromopyridine, >2‐methyl pyridine, 4‐methyl pyridine, 3,5‐dimethyl pyridine, >2 and 3‐chloropyridine>pyridine. These results have been discussed in the light of availability of the electron densities with the nitrogen atom of the pyridine ring. The inhibitive periormance of all the inhibitors is found to deteriorate with increase in temperature from 30 to 60°C and with passage of time. The values of heat of absorption and free energy of absorption indicates that the inhibitors have a chemical type of absorption at the metal/electrolyte interface. Polarization diagrams and shift in open circuit potential in the presence of inhibitors indicate that these compounds retard the cathodic as well as anodic reaction process.

The rust inhibiting characteristics of barium/calcium petroleum sulphonates, prepared from two indigenously available base stocks of different origin have been determined in concentration ranging from 0.5 to 3.0% W/W in mineral oil as per ASTM D‐1748 method. The products have been characterised as ‘outstanding’, ‘good’ and ‘satisfactory’ on the basis of their performance in the humidity cabinet. It has been observed that Ba/Ca petroleum sulphonates possessing outstanding rust inhibiting characteristics show low oil/water interfacial tension and high water solubilization characteristics as compared to other sulphonates. It has also been observed that the rust inhibiting characteristics are profoundly influenced by the structural parameter of the hydrocarbon portion of the petroleum sulphonates. The larger the alkyl side chain, the better is the rust inhibiting properties.