Corrosion Inhibition and Tribological Behavior of Two Organic Functional Molecules at High Temperature and Heavy Load

In this paper, the corrosion inhibition properties of two organic functional molecules of B-N based additive (triethanolamine borate, TAB) and P based additive (trimethylphenol phosphate, TCP) in hydrochloric acid corrosion solution and their tribological behavior in polyethylene glycol (PEG) base oil under high temperature and heavy load were studied by electrochemical and tribological tests. Then, the micro morphologies of worn and corroded surfaces were studied by means of scanning electron microscopy and X-ray photoelectron spectroscopy. And the mechanisms of lubrication behavior, load-capacity and corrosion inhibition properties of two organic functional molecules were further analyzed and discussed. The load capacity of the two organic functional molecules as PEG additives at 150 ℃ exceeded 400 N, showing excellent extreme pressure properties at high temperature. In the high temperature and heavy load (150 ℃, 400 N) friction and wear experiment, TAB as an additive of PEG base oil could also significantly reduce the friction coefficient and wear amount, exhibiting good friction-reducing and anti-wear performance. The wear volume of the oil sample decreased to 2.39×10⁶ μm³ after the addition TAB in base oil at the concentration of 1%. The anti-wear effect of the base oil was obviously improved by 22.1%, compared with the wear volume of base oil (3.07×10⁶ μm³). For TCP as an additive of PEG base oil, the friction coefficient of oil sample could be reduced, but their wear volume would be increased. The electrochemical test results showed that both organic functional molecules could inhibit the corrosive attack of acid to mild steel, and the inhibition efficiency of TAB was better than that of TCP. Results of the scanning electron microscopy characterization on the steel surface after electrochemical tests showed that the hydrochloric acid aqueous solution system could cause serious pitting corrosion. After adding TAB and TCP, the number of pitting holes on the corroded surface was significantly reduced, and the pitting holes was less for TAB additive than those of TCP. The X-ray photoelectron spectroscopy test results confirmed the formation of BN and other compounds for TAB additive, and the generation of iron oxides, iron phosphide and phosphate for TCP additive, during tribological process. Combined with the results of tribological experiments, electrochemical tests and surface analysis, it could be found that TAB as an additive would form a strong adsorption film and a non-sacrificial friction film, dominated by borates, boron oxides and nitrides on the metal surface, which showed good corrosion inhibition and excellent extreme pressure and wear resistance properties at high temperature. As for TCP additive, a violent tribochemical reaction occurred on the steel surface, forming a dense friction film dominated by iron phosphate and iron oxide. Therefore, the extreme pressure and friction-reduction performance of TCP was fairly good, but its corrosion inhibition performance was poor, even aggravating the wear volume of frictional pairs. The synergistic effect between elements in additive was found in this study, which could help designing new types of multifunctional additives. The results could also provide some support and reference for the structural design and application of multifunctional additives with load capacity at high temperature and anti-corrosion performance.