Potential-Controlled Boundary Lubrication of Stainless Steels in Non-aqueous Sodium Dodecyl Sulfate Solutions

In aqueous solutions, sodium dodecyl sulfate (SDS) surfactant can form a boundary film on metal surfaces to provide lubrication for sliding surfaces in contact. Previous studies have demonstrated that the boundary lubrication of SDS film can be inhibited or enhanced substantially by changing the surface potential of the rubbing metal surfaces. In this study, the SDS surfactant was added to a non-aqueous base fluid, propylene carbonate (PC), and the boundary lubrication behaviors of the solution for stainless steels were investigated under different potential conditions. Friction measurement, electrochemical impedance spectroscopy, cyclic voltammetry, and electrochemical quartz crystal microbalance techniques were employed to investigate the lubricating performance and adsorption film of the sodium dodecyl sulfate (SDS) film on two kinds of steels (AISI 316L, AISI 440C) in propylene carbonate (PC) solution. Similar to aqueous SDS solutions, the lubricating performance of the SDS/PC solution depends upon the electrode potential within the potential range from −1.5 to +1.5 V versus Ag/AgCl, which suggests the potential-dependent reversible change in the adsorbed film. When the potential is positive, both friction and wear of the tested stainless steels are relatively lower due to the presence of the adsorbed SDS film. As the potential is shifted to the negative regime, the DS chains in the adsorbed film are replaced by the PC molecules gradually, and friction coefficient increases by 100 % or more, depending on the load condition and the hardness of the stainless steels.