The figs depict the plot of OCP data against exposure time in 0.5 M H2SO4 and HCl solutions respectively, with and without the addition of predetermined concentrations of NNDT organic compound.
Due to desorption and lateral repulsion at 0.75% NNDT the potential values decrease suddenly to greater anodic potentials.
6 due to the differential mechanism of inhibition of NNDT in the solution.
The deviation in potentials to passivation potentials in H2SO4 and HCl depicts the improvement of the corrosion resistance of the steel with NNDT addition due to physiochemical interaction of the NNDT molecules unto the steel surface.
The electrochemical effect of NNDT on the corrosion polarization curves (anodic and cathodic) for mild steel in the acid media was investigated.
In HCl solution the %IE increased significantly after 0.25% NNDT concentration due to the presence of significant amount NNDT molecules to compete with and inhibit the electrochemical actions of the corrosive anions, block the active sites and form a protective film on the alloy surface in the acid solutions.
Table-3: Potentiodynamic polarization resistance results for mild steel in 0.5 M H2SO4 at predetermined concentrations of NNDT.
Table-4: Potentiodynamic polarization resistance results for mild steel in 0.5 M HCl at predetermined concentrations of NNDT
NNDT being an organic compound consists of oxygen, sulfur, nitrogen atoms, and bonds in its molecules which aids its adsorption on the metal surface  and are strongly polar in nature, thus providing an electron rich reaction center.
Due to the innate protonated hydrophilic amino and imino groups NNDT strongly adsorbs on the ionized steel surface, while their hydrophobic substituent (thiol) aligns the NNDT molecules during adsorption to form a protective film that shields the steel from the acid media.
Scanning electron micrographs of mild steel topography before and after weight loss test in acid solutions and after 432 h exposure time at predetermined concentrations of NNDT are shown in Fig.
The scanning electron microscopy image of the steel sample with NNDT as shown in Fig.