10. Neutralization index (acidity and/or basicity)

Hydraulic fluid must not attack metals normally used in the circuit, especially steel and copper. Metal corrosion is generally caused by the presence of acids in the oil. In the case of new oil, the acids may come from acid refining, even after neutralization. In the case of used oils, acidity results from in-service deterioration, the consequence of a complex series of reactions involving oxidation phenomena.

In general, the acid number increases as the fluid ages in hydraulic operation. This characteristic serves as an indication of when, when a certain acid number value is reached, the fluid in service should be replaced with new oil. However, with synthetic oils, a high acid number does not necessarily mean that the fluid is unsuitable for use, as in the case of a mineral-based fluid with which sludge formation is feared. However, we must take into account the...