ASTM D6971 Standard Test Method for Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content in Non-zinc Turbine Oils by Linear Sweep Voltammetry
3. Summary of Test Method
3.1 A measured quantity of sample is dispensed into a vial containing a measured quantity of acetone based electrolyte test solution and a layer of sand. When the vial is shaken, the hindered phenol and aromatic amine antioxidants and other test solution soluble oil components present in the sample are extracted into the test solution and the remaining droplets suspended in the test solution are agglomerated by the sand. The sand/droplet suspension is allowed to settle out and the hindered phenol and aromatic amine antioxidants dissolved in the test solution are quantified by voltammetric analysis. The results are calculated and reported as mass % of antioxidant or as millimoles (mmol) of antioxidant per litre of sample for prepared and fresh oils and as a percent remaining antioxidant for in-service oils.
3.2 Voltammetric analysis is a technique that applies electro-analytic methods wherein a sample to be analyzed is mixed with an electrolyte and a test solution, and placed within an electrolytic cell. Data is obtained by measuring the current passing through the cell as a function of the potential applied, and test results are based upon current, voltage, and time relationships at the cell electrodes. The cell consists of a fluid container into which is mounted a small, easily polarized, working electrode, and a large, non-polarizable, reference electrode. The reference electrode should be massive relative to the working electrode so that its behavior remains essentially constant with the passage of small current; that is, it remains unpolarized during the analysis period. Additional electrodes, such as auxiliary electrodes, can be added to the electrode system to eliminate the effects of resistive drop for high resistance test solutions. In performing a voltammetric analysis, the potential across the electrodes is varied linearly with time, and the resulting current is recorded as a function of the potential. As the increasing voltage is applied to the prepared sample within the cell, the various additive species under investigation within the oil are caused to electrochemically oxidize. The data recorded during this oxidation reaction can then be used to determine the remaining useful life of the oil type. A typical current-potential curve produced during the practice of the voltammetric test can be seen by reference to Fig. 1. Initially the applied potential produces an electrochemical reaction having a rate so slow that virtually no current flows through the cell. As the voltage is increased, as shown in Fig. 1, the electro-active species (for example, substituted phenols) begin to oxidize at the working electrode surface, producing an anodic rise in the current. As the potential is further increased, the decrease in the electro-active species concentration at the electrode surface and the exponential increase of the oxidation rate lead to a maximum in the current-potential curve shown in Fig. 1.
4. Significance and Use
4.1 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 oforiginal (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 ofthe 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 ofan oil, which is determined by the total contribution of all species present. Before making final judgment on the remaining useful life ofthe 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 D6224 and D4378; see also Test Method D2272), having the capability of measuring remaining oxidative life of the in-service oil.
4.1.1 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.
4.2 The test is also suitable for manufacturing control and specification acceptance.
4.3 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 1 - Voltages 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.
4.4 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).
4.5 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 D6810.
