A macrocyclic compound, namely 2,3,9,10-tetraphenyl-6,13-dithia-1,4,5,7,8,11,12,14-octaaza-cyclotetrade ca-1,3,6,8,10,13-hexaene (PTAT) was synthesized to study the corrosion inhibitive effect on pickling of mild steel in 20% H2SO4 at 95 degreesC. The synergistic effect of this compound was studied by weight-loss, potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen permeation studies. The results of this investigation have shown enhancement in inhibition efficiency (IE) for 500 ppm PTAT in the presence of 0.25% KI as a synergent. Formulated inhibitor has shown better performance than commercial corrosion inhibitor (Metasave). Potentiodynamic polarization studies showed that PTAT is a predominantly cathodic inhibitor whereas Metasave is mixed in nature.