A visual study of acid-neutralizing ability of marine cylinder lubricants
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Description
The combustion of high level sulfur-containing (2--5%) fuels generates sulfuric acid in marine diesel engines. In the absence of any control, sulfuric acid attacks metal surfaces and is the prime cause of ring and cylinder bore wear. One of the major requirements of the marine cylinder lubricant (MCL) is therefore their ability to neutralize the sulfuric acid and any other acids formed during the combustion process. The MCL's additives for acid neutralization are basically oil-soluble Group-II basic metal (calcium or magnesium) overbased sulfonates, or phenates. Those overbased detergents are in the form of reverse micelles with a calcium carbonate core, which is stabilized by an outer alkyl/aryl sulfonic acid shell In this study, a capillary video-microscopy technique was used to develop an experimental protocol for ranking the ability of marine cylinder lubricants (MCL's) to neutralize acids. The technique allows the direct observation of an acid droplet from the instant of its formation inside an oil-filled capillary. It was applied to several commercial oils with known rates of acid-neutralization ability as well as model lubricants made in the lab to qualitatively, yet successfully, test this ability. The optical observations allow one to propose a plausible interfacial neutralization mechanism by which the overbased detergent additive system performs the task of acid neutralization. According to this mechanism, the overbased sulfonate reverse micelles are the site for the neutralization reaction, which occurs at the oil-acid interface. The results confirm that the detergent surfactants are chemically bound to the micellar core. In the case of overbased calcium-carbonate detergents reacting with sulfuric acid, the formation of solid crystals was observed which may form a protective antiwear film on engine metal surfaces. By measuring the shrinking rate of nitric-acid drops in MCL, the acid-neutralizing ability of commercial MCL's as well as model lubricants can also be determined quantitatively. The shrinking rate of a nitric-acid drop in MCL was found to be constant during the reaction period and independent of the initial drop size. Analysis of the experimental results, following an interfacial neutralization reaction mechanism, suggests that the 'adsorption' of overbased reverse micelles is the controlling step in the reaction process The effects of two key additional additives---nonionic surfactants and dispersants---to MCL were also investigated. The rate of the neutralization reaction is enhanced by the presence of the nonionic surfactants in the model MCL's and is consistent with the formation of the mixed micelle systems which makes the basic core of the overbased sulfonate more accessible to the aqueous acid due the increased size and flexibility of the overbased sulfonate reverse micelle. Presence of small amounts of dispersants (1 wt.%) decreases the reaction rate for both nitric and sulfuric acid, while further increasing the dispersants, concentration (up to ca. 4%) does not lead to additional reduction of the acid-neutralizing ability. In the case of sulfuric acid, the dispersants act so as to also effectively disperse the hydrated calcium sulfate crystals produced by the neutralization reaction