What is Hydrolytic Stability of Lubricant? What Lubricants Are Easier To Hydrolyze?

What is the hydrolytic stability of Lubricant?

Hydrolytic stability of lubricating oil refers to the ability of the components (base oil and additives) in lubricating oil to remain stable and not decompose in the presence of water. Lubricating oil with good hydrolytic stability is relatively stable.

 

The Test standard for lubricating oil hydrolytic stability:

ASTM D2619: Hydrolytic Stability of Hydraulic Fluids Beverage Bottle Method Tester Submerge Water Test Analyzer.
Chinese Standard NB/SH/T0301: 液压液水解安定性测定法(玻璃瓶法)

 

The composition of the hydraulic fluid is base oil and additives. The water in the hydraulic oil will damage the performance of base oil and also additives.

Hydraulic oil should have good anti-emulsification performance, but also has good hydrolysis stability, which means that the base oil & additives in the oil must not be hydrolyzed. If the additives are easily hydrolyzed, the additives' performance will disappear quickly.

During the use of hydraulic oil, too much moisture will cause the lubricant to deteriorate, breed bacteria (eventually lead to deterioration), performance decline, affect the strength of the oil film, weaken the load capacity, and cause machine corrosion.

 

What lubricants are easier to hydrolyze:

 

The lubricating oil itself is hygroscopic, so it will absorb moisture from the air. Some base oils are easy to absorb moisture from the air. e.g. Ester oils, especially polyol esters and phosphate esters, are easy to absorb water.

The hydrolysis stability is related to the formulation components of the hydraulic fluids. Some additives are easy to hydrolyze. e.g. some anti-wear oils contain ZDDP (zinc dialkyl dithiophosphate) anti-wear additive. This anti-wear additive is relatively easy to hydrolyze and produces acidic byproducts.

 

 

The hazards of lubricating oil hydrolysis:

 

The hydrolysis will change the physical and chemical properties of the hydraulic oil, resulting in poor lubrication and damage to the machine.

 

1. Hydrolysis of hydraulic oil will cause changes in the appearance (color, transparency) of the oil.
2. Formation of acidic substances: The most common consequence of lubricating oil hydrolysis is the formation of acidic substances, most of which are carboxylic acids which will damage the machinery.
3. Viscosity drops: the decrease in viscosity will damage the machine more directly and faster. When Viscosity drops, the bearing capacity of the oil film is not enough, friction between metals occurs and the parts wear out.