Acid-neutralizing of marine cylinder lubricants: Measurements and effects of dispersants

Considerable amounts of sulfuric acid are produced in marine diesel engines during the combustion of high-level sulfur fuels, thus causing corrosive w ear. The acid-neutralizing ability of marine cylinder lubricants (MCLs) is therefore a "measuring stick" of their quality. A new experimental protocol was previously published to qualitatively test the acid-neutralizing abili ty of marine cylinder lubricants. Using a capillary video-microscopy techni que and measuring the shrinking rate of the size of nitric-acid drops in MC L, the acid-neutralizing ability of commercial MCLs, as well as model lubri cants, can be determined quantitatively. The shrinking rate of the nitric-a cid drop size in MCL was constant during the reaction period and independen t of the initial drop size. Analysis of the experimental results, following an interfacial neutralization reaction mechanism, suggests that the adsorp tion of overbased reverse micelles on acid-oil interface is the controlling step in the reaction process. The presence of small amounts of dispersant (1 wt. %) in a model MCL on the neutralization reaction decreased the react ion rate for both nitric and sulfuric acid, while further increase in dispe rsant concentration (up to about 4%) did not reduce the acid-neutralizing a bility further. For sulfuric acid, dispersants act also to effectively disp erse the hydrated calcium sulfate crystals produced by the neutralization r eaction with sulfuric acid.