Experimental Study of the Inhibitory Property of Expired Promethazine-theoclate for Mild Steel Corrosion in Hydrochloric Acid

Abstract

The inhibitory properties of expired promethazine hydrochloride on mild steel corrosion in a 1-molar hydrochloric acid (HCl) solution were investigated. Fourier Transform Infrared (FTIR) and gas chromatography-mass spectrometry (GC-MS) analyses were performed to characterize the expired drug regarding functional groups and chemical constituents. Electrochemical polarization and Scanning Electron Microscopy (SEM) were used to derive the inhibition efficiency of the drug and access the surface morphology of the mild steel specimens. The potentiodynamic polarization curves displayed the inhibition efficiency, suggesting a significant reduction of corrosion current density by the inhibitor, which appeared to function as a mixed-type inhibitor. Accessing the surface morphology of the specimens, the results demonstrated notable inhibition of the corrosion process with higher values of R² and lower values of RMSE and SEP, respectively. Analysis of the gas chromatography-mass spectrometry peaks aligns with the FTIR results. It also suggests the presence of corrosion-inhibiting compounds, including carboxylic acids, alkenes, alkyl, carbonyl groups, alcohols, phenols, and esters, in varying intensities. The negative value of the heat of adsorption indicates that heat flow occurred from the interface between mild steel and inhibitor. Langmuir isotherm provided the best fit and applied to the drug with the inhibitor molecules, as it is governed by physisorption. The optimum efficiency of the drug was 93.27% at 35°C. Therefore, adding an inhibitor at the steel-solution interface benefits the charge transfer resistance, as observed in the impedance analysis. Earlier works did not explore the inhibitory actions of expired promethazine theoclate, therefore, the present research addresses that gap by evaluating the inhibition capabilities of this drug.