Standard Guide for Measurement of Remaining Primary Antioxidant Content In In-Service Industrial Lubricating Oils by Linear Sweep Voltammetry

Importancia y uso:

6.1 The quantitative determination of remaining antioxidants for in-service industrial oils by measuring the amount of these additives that have been added to the oil as protection against oxidation. Industrial lubricants, such as turbine oils, compressor oils, gear oils, hydraulic oils, bearing lubricants and greases can be formulated with a wide variety of antioxidants types such as phenols and amines (as primary antioxidants), which are working synergistically and therefore all important to be monitored individually. For in-service oils, the LSV determines and compares the amount of original primary antioxidants remaining after oxidation have reduced its initial concentration.

6.2 This guide covers procedures for primary antioxidants such as amines and phenols, as described by Test Method D6971 and D6810.

6.3 LSV 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 used or in-service oil. In order to measure the overall stability of an oil (including contribution of intermediates present), and before making final judgment on the remaining useful life of the used oil (which might result in the replacement of the oil reservoir), it is advised to perform additional analytical techniques (in accordance with Practice D4378 and Practice D6224).

6.4 This guide is applicable to a wide range of industrial oils, both mineral or synthetic based, which can contain rust and oxidation inhibitors, antiwear additives such as zinc dialkyl dithiophosphates on gear oils, circulating oils, transmission oils and other industrial lubricating oils.

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

6.6 When a voltammetric analysis is obtained for a industrial lubricant inhibited with at least one type of antioxidant, there is an increase in the current of the produced voltammogram between 5 s to 8 s (or 0.5 V to 0.8 V applied voltage) (see Note 1) for the zinc dialkyl dithiophosphate type of antioxidant (Fig. 1), 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) (Fig. 2) (see Note 1) for the aromatic amines, and increase in the current of the produced voltammogram between 13 s and 16 s (or 1.3 V to 1.6 V applied voltage) (see Note 1) for the hindered phenols or carbamates in the neutral acetone solution (Fig. 2: x-axis 1 s = 0.1 V), or both. 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.

FIG. 2 Aromatic Amine and Hindered Phenol Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed

Note 1: Voltages listed with respect to reference electrode. The voltammograms shown in Figs. 1-6 were obtained with a platinum reference electrode and a voltage scan rate of 0.1 V/s.

FIG. 3 Hindered Phenol Voltammetric Response in Basic Test Solution with Blank Response Zeroed

FIG. 4 Voltammetric Reading for an In-service Oil Sample Comparing Aromatic Amines (additive #1) and Hindered Phenols (additive #2) Peaks (in the Neutral Test Solution)—Standard (top line) and Sample In-Service Oil (lower line)

FIG. 5 a Filming Problems Due to Oil Solubility

FIG. 5 b Filming Due to Additive Concentration (continued)

FIG. 5 c Filming Problems Due to Oil Solubility (continued)

FIG. 6 Shifting of Antioxidant Peaks Due to Oil Solubility

6.7 For industrial lubricants containing zinc dialkyl dithiophosphate type of antioxidants, there is an increase in the current of the produced voltammogram between 5 s to 8 s (or 0.5 V to 0.8 V applied voltage) (see Note 1) by using the neutral acetone test solution (see Fig. 1). There is no corresponding ASTM International standard describing the test method procedures for measuring zinc dialkyl dithiophosphates type of antioxidants in industrial lubricants.

6.8 For industrial lubricants containing only aromatic amines as antioxidants, there is 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) (see Note 1) for the aromatic amines, by using the neutral acetone test solution (first peak in Fig. 2) as described in Test Method D6971.

6.9 For industrial lubricants containing only hindered phenolic antioxidants, it is preferable to use a basic alcohol solution rather than the neutral acetone solutions, to achieve an increase in the current of the produced voltammogram between 3 s to 6 s (or 0.3 V to 0.6 V applied voltage) (see Note 1) in basic alcohol solution (Fig. 3: x-axis 1 s = 0.1 V) as described in Test Method D6810.

Subcomité:

D02.09.0C

Volúmen:

05.04

Número ICS:

75.100 (Lubricants, industrial oils and related products)

Palabras clave:

2,6-di-tert-butylphenol; 2,6-di-tert-butyl-4methylphenol; phenyl alpha naphthylamine; alkylated diphenylamine; turbine oil; linear sweep voltammetry (LSV); hindered phenol antioxidant; aromatic amine antioxidant; zinc dialkyl dithiophosphates;