Significance and Use

The quantitative determination of hindered phenol and aromatic amine antioxidants in a new turbine oil measures the amount of these compounds that has been added to the oil as protection against oxidation. Beside phenols, turbine oils can be formulated with other antioxidants such as amines which can extend the oil life. In in-service oil, the determination measures the amount of original (hindered phenol and aromatic amine) antioxidants remaining after oxidation has reduced its initial concentration. This test method is not designed or intended to detect all of the antioxidant intermediates formed during the thermal and oxidative stressing of the oils, which are recognized as having some contribution to the remaining useful life of the in-service oil. Nor does it measure the overall stability of an oil, which is determined by the total contribution of all species present. Before making final judgment on the remaining useful life of the in-service oil, which might result in the replacement of the oil reservoir, it is advised to perform additional analytical techniques (as in accordance with Test Methods D 6224 and D 4378 ; see also Test Method D 2272 ), having the capability of measuring remaining oxidative life of the in-service oil.

This test method is applicable to non-zinc type of turbine oils as defined by ISO 6743 Part 4, Table 1. These are refined mineral oils containing rust and oxidation inhibitors, but not antiwear additives.

The test is also suitable for manufacturing control and specification acceptance.

When a voltammetric analysis is obtained for a turbine oil inhibited with a typical synergistic mixture of hindered phenol and aromatic amine antioxidants, there is an increase in the current of the produced voltammogram between 8 to 12 s (or 0.8 to 1.2 V applied voltage) (see Note 1) for the aromatic amines, and an increase in the current of the produced voltammogram between 13 and 16 s (or 1.3 to 1.6 V applied voltage) (see Note 1) for the hindered phenols in the neutral acetone test solution (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- tert -butylphenol). Aromatic amine antioxidants detected by voltammetric analysis include, but are not limited to, phenyl alpha naphthylamines, and alkylated diphenylamines.

Note 1Voltages listed with respect to reference electrode. The voltammograms shown in Figs. 1 and 2 were obtained with a platinum reference electrode and a voltage scan rate of 0.1 V/s.

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

For turbine oils containing only hindered phenolic antioxidants, it is preferable to use a basic alcohol test solution rather than the neutral acetone test solutions, as there is an increase in the current of the produced voltammogram between 3 to 6 seconds (or 0.3 to 0.6 V applied voltage) (see Note 1) in basic alcohol test solution (Fig. 2: x -axis 1 second = 0.1 V) in accordance with Test Method D 6810 .

Note x -axis = time (seconds) and y -axis is current (arbitrary units) with top line in Fig. 2 showing the fresh oil.

1. Scope

1.1 This test method covers the voltammetric determination of hindered phenol and aromatic amine antioxidants in new or in-service type non-zinc turbine oils in concentrations from 0.0075 mass % up to concentrations found in new oils by measuring the amount of current flow at a specified voltage in the produced voltammogram.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

ASTM Standards

D1193 Specification for Reagent Water

D2272 Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel

D4057 Practice for Manual Sampling of Petroleum and Petroleum Products

D4378 Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines

D6224 Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment

D6810 Test Method for Measurement of Hindered Phenolic Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep Voltammetry

ISO Standards

Keywords

2,6-di-tert-butyl-4methylphenol; 2,6-di-tert-butylphenol; alkylated diphenylamine; aromatic amine antioxidant; hindered phenol antioxidant; in-service oils; linear sweep voltammetry; non-zinc turbine oils; phenyl alpha naphthylamine; turbine oil; Alkylated diphenylamines; Aromatic amine antioxidants; Hindered phenol antioxidant; Linear sweep voltammetry; Turbines;

ICS Code

ICS Number Code 75.160.20 (Liquid fuels)

DOI: 10.1520/D6971-09

ASTM International is a member of CrossRef.

ASTM D6971