The effects of humidity and soluble water content on the lubricity testing of a n-hexadecane and palmitic acid test fluid

Abstract

Liquid fuel lubricity testing is known to be sensitive to the humidity of the surrounding atmosphere. The effect reported does however not seem to be universal for all diesel types according to available literature. This study was limited to the use of n-hexadecane as base fluid with addition of between 100 - 2000 ppm of palmitic acid (PA) as lubricity enhancer. The fuel spectrum was kept simple to allow a more fundamental approach by limiting the extent of possible chemical reactions. Both wear and seizure load testing were performed. The effect of water on both tests was found to be:  Wear and friction increased with higher soluble water content. Water therefore interferes with the action of the lubricating additive leading to decreased performance.  It is known that hydroxyl groups, from dissociative chemisorption of water, act as active sites for adsorption. It is however postulated that the associative chemisorption and/or physisorption of water interferes by either reducing the active site availability or reducing activity to ligand exchange.  No iron oxyhydroxides (FeOOH) were detected on the wear scars or tracks using Raman spectroscopy. The adsorbed water therefore likely only interact with the surface due to their low concentrations (10 ~ 50 ppm).Liquid fuel lubricity testing is known to be sensitive to the humidity of the surrounding atmosphere. The effect reported does however not seem to be universal for all diesel types according to available literature. This study was limited to the use of n-hexadecane as base fluid with addition of between 100 - 2000 ppm of palmitic acid (PA) as lubricity enhancer. The fuel spectrum was kept simple to allow a more fundamental approach by limiting the extent of possible chemical reactions. Both wear and seizure load testing were performed. The effect of water on both tests was found to be:  Wear and friction increased with higher soluble water content. Water therefore interferes with the action of the lubricating additive leading to decreased performance.  It is known that hydroxyl groups, from dissociative chemisorption of water, act as active sites for adsorption. It is however postulated that the associative chemisorption and/or physisorption of water interferes by either reducing the active site availability or reducing activity to ligand exchange.  No iron oxyhydroxides (FeOOH) were detected on the wear scars or tracks using Raman spectroscopy. The adsorbed water therefore likely only interact with the surface due to their low concentrations (10 ~ 50 ppm). For wear testing the following additional observations were made:  The electric contact resistance (ECR) took longer to increase at higher soluble water content, for all PA concentrations. The equilibrium values attained were also lower, when values were within the measurement range. This again highlights the reduced lubricity performance at higher soluble water content.  Wear scar appearance and colour were uniquely affected at each concentration of PA  Similar trends were observed at 60 °C and 80 °C. At the higher temperature:  Wear scar diameters increased.  Friction coefficients decreased and had less variance.  ECR values were similar, but took longer to increase to a steady value. For seizure load testing the following additional observations were made:  At higher water content, the oscillation-stroke on the test machine used tended to deviate more, before the actual seizure point was reached.  The effects of soluble water were more noticeable at lower PA concentrations.