Standard Test Method for Measurement of Antioxidant Content in Lubricating Greases by Linear Sweep Voltammetry

Importancia y uso:

5.1 The quantitative determination of antioxidants in new greases measures the amount of the chemical compounds that were added to the base oil as protection against grease oxidation. For in-service oil greases, the voltammetric test method measures the amount of original (individual) antioxidants remaining after grease oxidation have reduced its concentration. Before making final judgment on the remaining useful life of the in-service grease, which might result in the replacement of the grease reservoir, it is advised to perform additional analytical techniques, such as Test Method D942 and D5483, which may be used to measure the remaining oxidative life of the used grease.

5.1.1 This test method is applicable to mineral oil-based and synthetic oil-based greases, based on all type of applied thickeners. This test method is applicable to greases containing at least one type of antioxidant. The presence of other types of additives like corrosion inhibitors or metal deactivators will not interfere with this test method.

5.2 When a voltammetric analysis is obtained using a neutral acetone test solution for a grease inhibited with a typical synergistic mixture of hindered phenol and aromatic amine antioxidants, there is an increase in the voltammogram current between 8 s to 12 s (or 0.8 V to 1.2 V applied voltage), see Note 1, for the aromatic amines, and an increase in the voltammogram current of the produced voltammogram between 13 s to 16 s (or 1.3 V to 1.6 V applied voltage), see Note 1, for the hindered phenols. In Fig. 1, x-axis = 1 s = 0.1 V. Hindered phenol antioxidants detected by voltammetric analysis include, but are not limited to, 2,6-di-tert-butyl-4-methylphenol; 2,6-di-tert-butylphenol and 4,4’-Methylenebis(2,6-di-tertbutylphenol). Aromatic amine antioxidants detected by voltammetric analysis include, but are not limited to, phenyl alpha naphthylamines, and alkylated diphenylamines.

5.3 For greases containing only aromatic amines as antioxidants, there will only be an increase in the current of the produced voltammogram between 8 s to 12 s (or 0.8 V to 1.2 V applied voltage) for the aromatic amines, by using the neutral acetone test solution (first peak in Fig. 1).

5.4 For greases containing ZDDP as antioxidants, there shall be an increase in the voltammogram current between 6 s to 10 s (or 0.6 V to 1.0 V applied voltage), see Note 1, for the ZDDP, when evaluated in the neutral acetone test solution.

5.5 For greases containing only hindered phenolic antioxidants, basic alcohol test solutions are recommended for use as described in Test Method D6810. In basic alcohol test solutions, the voltammogram current for phenols increases between 3 s to 6 s (or 0.3 V to 0.6 V applied voltage), see Note 1. In Fig. 2, x-axis = 1 s = 0.1 V are as described in Test Method D6810, where x-axis = time (seconds) and y-axis is current (arbitrary units). Top line in Fig. 2 is fresh grease.

Subcomité:

D02.09.0E

Referida por:

D7918-23

Volúmen:

05.04

Número ICS:

75.100 (Lubricants, industrial oils and related products)

Palabras clave:

2,6-di-tert-butyl-4methylphenol; 2,6-di-tert-butylphenol; alkylated diphenylamine; aromatic amine antioxidant; grease; hindered phenol antioxidant; linear sweep voltammetry; phenyl alpha naphthylamine;