Corrosion Inhibition and Tribological Behavior of S-N Functional Molecules as Multifunctional Additives

Through electrochemical experiments, friction tests and quantum chemical simulation, the corrosion inhibition abilities as inhibitors in hydrochloric acid corrosive solution and tribological performance as additives inbase oil of mercaptomethylthiadiazole (MMT) and dinonylnaphthalenesulfonic acid (DNS) were investigated in this work. Results of electrochemical experiments showed that DNS molecule exhibited poor corrosion inhibition effect on the attack of corrosive solution against the mild steel surface. Then, the calculated maximum corrosion inhibition efficiency was 58.6%. MMT molecule formed a more complete adsorption film on the mild steel surface that the surface coverage was about 93.3%, effectively obstructingthe assault of aggressive solution tomild steel. The adsorption behavior of MMT molecule obeys Langmuir adsorption isotherm and Gibbs free energy \mathrm\bigtriangleupG_\textads^\text0 was about −32 kJ/mol, indicating that the adsorption mode of MMT molecule involved a mixed adsorption manner with physisorption and chemisorption. Corrosion inhibition efficiency enhanced with the increase of the amountof inhibitor and the largest valuecould reach 93.3%, demonstrating that MMT molecule could suppress the corrosion process well. Results of friction tests exhibited that the friction coefficient under lubrication of DNS molecule was not changedobviously compared with that of the base oil, but the wear volumebecame huge. Surpisingly, the friction coefficientand wear volume of base oil wereremarkably reduced after the addition of MMT molecule. Analysis of SEM-EDS results indicated that the worn morphology was flat and smooth with some dark areas likely adsorbed molecule or tribo-film covered on the surface in the case of MMT molecule lubrication. It was further comfirmed by XPS results that there were tribo-chemical products(Fe₂(SO₄)₃, FeS₂), oxide and nitrogen-containing degradation products with friction-reducing and anti-wear effect enshrouding on worn scar surface. It can be observed from SEM micrographs that the worn morphology under DNS lubrication was similiar with that of base oil. Combined with XPS results of DNS molecule lubrication, it could be inferred that oxide, FeSO₄ existed on worn scar surface that played a role in friction-reducing. Moreover, extreme pressure experiments were also carried out under the load from 100~700 N. The outcomes suggested that the base oil sample exhibited poorextreme pressure property. As the addition of DNS molecule, extreme pressure performance was barely enhanced. It could bededuced that the molecule layer adsorbed on the metal surface was not enough compact so that the friction coefficient sharply rised causing lubrication failure. To our surprise, the addition of MMT moleculeimmensely improved the extreme pressure performance of base oil. The load capacity of MMT molecule as additive in base oil was up to 700 N and the friction coefficient can keep stable during the whole process. Therefore, MMT molecule can be treated as an effective inhibitor and extreme pressure anti-wear reagent.