Review on anti-corrosion properties of expired antihypertensive drugs as benign corrosion inhibitors for metallic materials in various environments

Discovering new classes of organic corrosion inhibitors, such as expired drugs, can provide cheaper, low-toxic, and highly efficient alternatives to the toxic organic counterparts that serve as an efficient recycling or disposal strategy. Antihypertensive drugs constitute one of the cheapest classes of drugs, globally, and have been reported as highly effective corrosion inhibitors for a variety of metallic materials in various corrosion environments. The corrosion inhibition mechanism of these drugs, generally, involves the adsorption on the metallic surfaces and the formation of a protective organic film that retards the attack of substrates by corrosive entities in the media. The drug molecular structure usually contains π-bonds, phenyl rings, and polar functional groups (such as –NH2, –OH, –CN, and −Cl) which influence the electron density within the inhibitor framework and also facilitate adsorption of the drug on the metal surface. Generally, most antihypertensive drugs act as mixed-type inhibitors and usually obey Langmuir adsorption isotherm revealing physisorption adsorption mechanisms except telmisartan, losartan potassium, and atenolol when used in nitric acid. The reported inhibition efficiency of the drugs is generally above 90% with spironolactone having the highest efficiency of 98.1% at 30 °C.