###### Abstract  
\(N\),\(N\)-dilhydroxylethyl-(5-methyl-[1,3,4]) thiadiazol-2-sulfur)-carbonyl acetamide was synthesized and used as an inhibitor to protect Q235 carbon steel in a 1 M HCl solution. The results showed an increased inhibition efficiency with the increase in the concentration of this inhibitor, and an inhibition efficiency higher than 96% at 40 mg/L can be obtained from weight loss, electrochemical impedance spectroscopy, and potentiodynamic polarization results. The inhibition effect was determined by the adsorption film according to the surface morphology and elemental distribution of the carbon steel surface. The adsorption consists of physical adsorption and chemical adsorption in view of the free adsorption energy of \(-\)40.64 kJ/mol derived from the Langmuir adsorption isotherm line. The lone pair electrons of N, O and S and \(\pi\) electronics of double bonds in molecules form stable covalent coordination bonds with the empty d orbitals of iron atoms, which is beneficial to chemical adsorption of the inhibitor. The high inhibitor efficiency of this inhibitor is important for the potential application in pickling field. thiadiazole derivative; carbon steel; HCl corrosion; electrochemical techniques; adsorption +
Footnote †: journal: Applied Sciences  
**Citation:** [PERSON]; [PERSON]; [PERSON]; [PERSON]. the activation energy of the corrosion reaction and decreasing the charge or material transfer related to the corrosion reaction, the corrosion reaction process of the anode and cathode is inhibited by the adsorption film of inhibitor molecules. Thiadiazole is a five-membered ring compound composed of two carbon atoms, two nitrogen atoms, one sulfur atom and two double bonds. Thiadiazole derivatives can be obtained by substituting the hydrogen atoms linked to the thiadiazole five-membered ring with substituents [17]. Thiadiazole derivatives contain some electro-negative atoms (N, O, P and S) and conjugated systems (benzene and thiadiazole rings) that can provide active electrons for organic adsorption films, making them excellent potential corrosion inhibitors [18; 19; 20]. Therefore, they are widely studied and used to protect carbon steel, copper, stainless steel, aluminum alloy and other metals in corrosion media containing hydrochloric acid [21; 22; 23], sulfuric acid [24; 25], carbonic acid [26; 27; 28], sodium chloride [29; 30] and so on [31]. For example, [PERSON] et al., synthesized N\({}^{\prime}\)-(4-hydroxy-3-methoxybenzylidene)-2-(5-p-tolyl-1,3,4-thiadiazol-2-ylthio) acetohydrazide and used it to protect mild steel in 0.5 M HCl solution. They obtained a high inhibition efficiency of 95% when the inhibitor concentration was higher than 5 mg/L [32]. [PERSON] et al., synthesized 2,5-bis(4-pyridyl)-1,3,4-thiadiazole and tested its inhibition effect on mild steel in 1 M HCl solution [33]. They obtained a high inhibition efficiency of 92% when the inhibitor concentration was higher than 2 \(\times\) 10\({}^{-4}\) M. [PERSON] et al., synthesized six thiadiazole derivatives and utilized them to protect the mild steel in 1 M HCl solution [34]. They found that the inhibition efficiency was related to the electronic properties of thiadiazole derivatives. In summary, the inhibition effect of thiadiazole derivatives on mild steel is mainly determined by the substituents linked to the thiadiazole rings. However, the synthesis route of most thiadiazole derivatives with high anticorrosion efficiency is complex, and most thiadiazole derivatives with simple structures have low anticorrosion efficiency. Herein, we designed a new thiadiazole derivative, namely, N, N-dihydroxyethyl-(5-methyl-[1,3,4] thiadiazol-2-sulfur)-carbonyl acetamide (TSCA), by substituting the two H atoms with methyl and hydroxyl groups, and synthesized the corrosion inhibitor with a simple two-step synthesis method. The inhibition effect of TSCA on Q235 carbon steel in hydrochloric acid solution was determined by weight loss, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements.