Research Article Investigation of Corrosion Protection of Austenitic Stainless Steel in 5.5M Polluted Phosphoric Acid Using 5-Azidomethyl-7- morpholinomethyl-8-hydroxyquinoline as an Ecofriendly Inhibitor Aimad Mazkour, 1 Souad El Hajjaji , 1 Najoua Labjar , 2 El Mostapha Lotfi , 2 and Mohammed El Mahi 2 1 Laboratory of Spectroscopy, Molecular Modeling, Materials, Nanomaterials, Water and Environment, CERN2D, Faculty of Sciences, Mohammed V University in Rabat, Rabat, Morocco 2 Laboratory of Spectroscopy, Molecular Modeling, Materials, Nanomaterials, Water and Environment, CERN2D, ENSAM, Mohammed V University in Rabat, Rabat, Morocco Correspondence should be addressed to Souad El Hajjaji; selhajjaji@hotmail.com Received 7 November 2020; Revised 30 March 2021; Accepted 19 April 2021; Published 15 May 2021 Academic Editor: Michael J. Schütze Copyright © 2021 Aimad Mazkour et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The use of 5-azidomethyl-7-morpholinomethyl-8-hydroxyquinoline (AMH) as a corrosion inhibitor for AISI 321 stainless steel in 5.5 M polluted phosphoric acid was investigated using the hydrogen evolution technique, linear polarization curves, and impedance spectroscopy. Impedance measurements revealed that the dissolution of AISI 321 in 5.5 M polluted phosphoric acid was controlled by an activation mechanism, unchanged even with the addition of AMH at different concentrations. Polarization results showed that the inhibition ability was enhanced with increasing inhibitor concentration. AMH acted as a mixed-type inhibitor by random adsorption on the alloy surface, whatever the nature of the reaction that is taking place. The adsorption of AMH on the AISI 321 surface was also discussed via the Langmuir adsorption isotherm. The influence of elevating the solution temperature on the corrosion inhibition performance was studied. A quantum chemistry study with the DFT method was also conducted, which supplied a logical and exploitable theoretical explanation of the adsorption and the inhibition action of AMH on AISI 321. 1. Introduction The different properties of stainless steels, namely, the mechanical strength and the corrosion resistance even in very aggressive environments, make them very commonly used in the phosphoric acid manufacturing plants. The oxide layer appearing on the surface of stainless steels in contact with air and/or water represents a barrier that protects these materials against undesirable and destructive reactions in surrounding environments [1]. Generally, the corrosion resistance of stainless steel depends mainly on the stability of the oxide film grown on its surface. However, the oxide film or passive film can be altered by the severe working conditions (high acidity, high temperature…) and the pres- ence of impurities in the industrial acids. In fact, most of the world’s phosphoric acid production is manufactured by wet process, which consists in reacting natural phosphate with a solution of sulfuric acid followed by a filtration. This technique produces a phosphoric acid containing several impurities (halogen, sulphates, gypsum particles…) that can break the passive film protection and seriously damage these materials [2]. To overcome this problem, the use of inhibitors is an effective and original method against corrosion phe- nomenon. In addition, the use of inhibitors for commonly used stainless steels spares the industry the economic burden of using materials with high levels of alloying elements that are too expensive. Inhibitors for corrosion protection are substances that slow down or stop the corrosion process Hindawi International Journal of Corrosion Volume 2021, Article ID 6666811, 15 pages https://doi.org/10.1155/2021/6666811