Reusing expired streptomycin and neomycin drugs as potential corrosion inhibitors for high brass alloys in NaCl solution: quantum, chemical, surface and electrochemical investigations.

Although drugs are essential for humans, they are perilous for the ecosystem when disposed of improperly. There are challenges in reusing them for other useful purposes, most notably as corrosion inhibitors; corrosion inhibition is an important industrial and economic aspect of protecting metals and the environment. Copper alloys are widely used in outdoor applications, especially in casting decorative items, but they have shortcomings in corrosion resistance. Herein, the ability of two expired drugs, streptomycin (I) and neomycin (II), to resist the corrosion of a high Cu-Zn alloy in a 3.5% NaCl environment was studied in detail via electrochemical, gravimetric and microscopic techniques as well as theoretical calculations. The electrochemical results showed that there was an increase in the corrosion rate of the examined alloy with increasing [NaCl]. The addition of 500 mg L-1 of the tested drugs strongly inhibited alloy corrosion, and the inhibition efficiencies (% IEs) reached 87.4% and 91.6% with drugs I and II, respectively. The obtained gravimetric outcomes indicated that the values of % IEs were augmented with increasing drug dosages, while they decreased as [NaCl] and solution temperature increased. The higher % IEs were attributed to the strong adsorption of the drugs on the brass surface. This adsorption was suggested to be physical and obeyed the Langmuir adsorption isotherm. Polarization assessments indicated that these drugs were mixed-type inhibitors with an anodic priority. All thermodynamic/kinetic parameters were determined through full adsorption assays, and the possible inhibition mechanism was proposed. Computational methods provided further support and clarification of the inhibition mechanism of these drugs. All results of the applied techniques were in excellent conformity with each other and with the employed computational methods.