Phenothiazines as Corrosion Inhibitors for Zinc in NH 4 Cl Solution

The effect of phenothiazine and its fluoro, methyl and methoxy ring substituted derivatives, N-formyl and N-acetylphenothiazines, on the electrochemical and corrosion behaviour of zinc in NH₄Cl solution was studied using galvanostatic and potentiostatic techniques. All the compounds investigated were found to be inhibitors of mixed type, inhibition occurring by simple blocking of the electrode surface through chemisorption. The higher efficiency of N-acetylphenothiazine as compared with the parent compound was attributed to the interaction of π-electrons of the benzene ring with the electrode surface. With the exception of N-acetylphenothiazine, adsorption of the inhibitors conformed with the Langmuir isotherm, that for the N-acetyl derivative with the Temkin isotherm. The mechanism of the hydrogen evolution reaction and of zinc dissolution was found to be the same in the uninhibited and inhibited states. The apparent activation energy for zinc corrosion wasfound to increase slightly with the concentration of N-formylphenothiazine, although it was slightly decreased at concentrations ≥ 10⁻⁵ for the N-acetyl derivative.