9. Resistance to hydrolysis

Some ester-functional constituents of lubricating oils can be hydrolyzed in the presence of water (reverse esterification reaction), according to the reversible reaction : $\mathrm{acide}\mathrm{\hspace{0.17em}}\mathrm{organique}+\mathrm{alcool}\mathrm{\hspace{0.17em}}\underset{\mathrm{hydrolyse}}{\overset{\mathrm{estérification}}{\rightleftarrows }}\mathrm{\hspace{0.17em}}\mathrm{ester}+\mathrm{eau}$

Acids formed in this way lead to corrosion of reactive metals, particularly copper metals.

Hydrolysable products are mainly esters (diesters, neopolyol esters, phosphoric esters, etc.) and certain additives such as zinc dithiophosphates, widely used to improve the anti-wear properties of hydraulic fluids...