The corrosion inhibiting effect of tramadol visualized by Scanning Electron Microscopy.Drugs that have passed their expiry date could be recruited into service as corrosion inhibitors in industry. It may sound unlikely, but the anti-corrosion properties of several drugs are being actively explored by research groups worldwide. In the journal Materials Discovery, researchers led by Mumtaz Quraishi at the Indian Institute of Technology, Banaras Hindu University in India describe the potential of expired tramadol—a common painkiller—for inhibiting the corrosion of steel.
Professor Quraishi and his colleagues initially began with an experiment on expired samples of the statin drug atorvastatin, comparing their corrosion inhibiting performance with fresh drug. "Excellent corrosion inhibition greater than 95% was obtained, which prompted us to explore this work using samples of other expired drugs that we ourselves were using for various ailments,” explains Quraishi. “Rather than throwing them in the garbage we discovered that some could work as effective corrosion inhibitors.”
After immersing some steel in a dilute (0.01%) solution of tramadol, the team subjected it to a corrosion-inducing solution of hydrochloric acid. At the level of just 100 milligrams per liter, the tramadol achieved a corrosion inhibition efficiency of 97.2%. The protective effect of the drug was monitored by several sophisticated methods of analysis, but it is also readily apparent when the metal surfaces are examined by scanning electron microscopy.
The researchers also investigated the precise chemical mechanism that allows the drug to form a thin film on the metal surface and inhibit corrosion. They identified specific interactions between the drug molecules and electrons available on the metal surface. These strengthened adsorption of the drug molecules to the metal and mediated its anti-corrosion effects.
The anti-corrosion activities do still depend on the molecular structure of a drug, but Quraishi says he was “astonished” to learn that some suitable drugs retain 90 percent of their activity, and therefore molecular integrity, for up to 10 years after their expiry date. “This strengthened our belief in the potential of expired drugs as corrosion inhibitors,” he explains.
Organic (carbon-based) inhibitors cause less environmental damage than inorganic alternatives, but making them can be complex and expensive. Although most synthetic drugs are also expensive to make, when they expire, they generally become worthless waste. At that point, they could instead become valuable and highly cost-effective corrosion inhibitors. The annual global cost of metal corrosion is around 2.7 billion US dollars.
“We are now investigating the key issues that need to be resolved to move this work from the lab into practical use,” says Quraishi. In addition to the chemical studies required, he is also reviewing the global availability of expired drugs to find the best matches between usefulness and potential supply.
“Connections need to be made between retailers, manufacturers, and scientists,” Quraishi adds. With good cooperation he is hopeful that progress in the field will eventually have sufficient economic and environmental benefits to make it a mainstream aspect of corrosion protection.
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Quraishi, M. A. et al.: "DFT and experimental studies on the inhibition potentials of expired Tramadol drug on mild steel corrosion in hydrochloric acid," Materials Discovery (2017)