Lube-Tech
4.5.3 Top up with an ester based HF: does the mixture remain hydrolytic stable? In unfavourable operating conditions (high water content, high temperature), ester based hydraulic fluids tend to hydrolysis (more critical with partially unsaturated ester). The acidic organic decomposition products from the ester can chemically attack materials and components.
The test ASTM D2619: Hydrolytic Stability of Hydraulic Fluids (Beverage Bottle Method) covers the determination of the hydrolytic stability of petroleum or synthetic-based hydraulic fluids.
This test method differentiates the relative stability of hydraulic fluids in the presence of water under the conditions of the test. Hydrolytically unstable hydraulic fluids form acidic and insoluble contaminants which can cause hydraulic system malfunctions due to corrosion, valve sticking, or changes in the viscosity of the fluid. The degree of correlation between this test method and service performance has not been fully determined.
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change of the copper specimen is measured. The acid number change of the fluid and acidity of the water layer are determined.
In the literature we also found the following incompatibilities: - HF with rust inhibitor that contains succinic acid mixed with rust inhibitor calcium-based used in antiwear HF: HL and HM fluids may result in the formation of precipitates that can cause valve sticking and filter plugging.
- HM type hydraulic fluid with ZDTP mixed with another HM containing Zn-free or ashless AW additives may not be compatible with each other.
- Water-glycol fluids are highly alkaline due to the presence of amine-based corrosion inhibitors. As a result, this fluid can attack zinc, cadmium, magnesium, and on-anodized aluminium, forming sticky or gummy residues. Fluorocarbon-based elastomers, such as Viton, are also incompatible with amines.
Figure 6. Description of hydrolytic stability test ASTM D2619
In this test, a copper test specimen and 75 g of test fluid plus 25 g of water are sealed in a pressure-type beverage bottle (Cola bottle which is resistant to a high pressure). The bottle is rotated for 48 h in an oven at 93 °C. Layers are separated and the weight
Table 2.
LUBE MAGAZINE NO.166 DECEMBER 2021
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