ASTM D7590-09

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Designation:D7590–09
Standard Guide for
Measurement of Remaining Primary Antioxidant Content In
In-Service Industrial Lubricating Oils by Linear Sweep
Voltammetry
This standard is issued under the fixed designation D7590; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Under normal thermal and oxidative working conditions, which degrade the chemical composition
of the oil’s basestock and gradually deplete the oil’s additive package, good oil condition monitoring
procedures are necessary to determine and planning corrective actions before the oil properties
changes have passed their warning limits.Antioxidant monitoring practices are a vital part of modern
oil condition monitoring practices to achieve lubrication excellence. This guide addresses the correct
guidelines for voltammetric data interpretation.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 This guide covers the voltammetric analysis for quali-
bility of regulatory limitations prior to use.
tative measurements of primary antioxidants in new or in-
service type industrial lubricants detectable in concentrations
2. Referenced Documents
as low as 0.0075 mass percent up to concentrations found in
2.1 ASTM Standards:
newoilsbymeasuringtheamountofcurrentflowataspecified
D1193 Specification for Reagent Water
voltage in the produced voltammogram.
D4057 Practice for Manual Sampling of Petroleum and
1.2 This guide can be used as a resource for a condition
Petroleum Products
monitoring program to track the oxidative health of a range of
D4378 Practice for In-Service Monitoring of Mineral Tur-
industrial lubricants which contain primary antioxidants. In
bine Oils for Steam and Gas Turbines
order to avoid excessive degradation of the base-oil, these
D6224 Practice for In-Service Monitoring of Lubricating
primary antioxidants play a major role to protect the lubricants
Oil for Auxiliary Power Plant Equipment
against thermal-oxidative degradation. This guide can help
D6304 Test Method for Determination of Water in Petro-
users with interpretation and troubleshooting results obtained
leum Products, Lubricating Oils, and Additives by Coulo-
using linear sweep voltammetry (LSV).
metric Karl Fischer Titration
1.3 When used as part of oil condition monitoring practices,
D6810 Test Method for Measurement of Hindered Phenolic
it is important to apply trend analysis to monitor the antioxi-
Antioxidant Content in Non-Zinc Turbine Oils by Linear
dant depletion rate relative to a baseline sample rather than use
Sweep Voltammetry
voltammetry for an absolute measurement of the antioxidant
D6971 Test Method for Measurement of Hindered Phenolic
concentration.The trending pattern provides a proactive means
and Aromatic Amine Antioxidant Content in Non-zinc
to identify the level of oil degradation or abnormal changes in
Turbine Oils by Linear Sweep Voltammetry
the condition of the in-service lubricant.
D7214 Test Method for Determination of the Oxidation of
1.4 The values stated in SI units are to be regarded as
Used Lubricants by FT-IR Using Peak Area Increase
standard. No other units of measurement are included in this
Calculation
standard.
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1 2
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum For referenced ASTM standards, visit the ASTM website, www.astm.org, or
ProductsandLubricantsandisthedirectresponsibilityofSubcommitteeD02.09.0C contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
on Oxidation of Turbine Oils. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Dec. 1, 2009. Published February 2010. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D7590–09
2.2 ISO Standards: other type of antioxidants by linear voltammetry, although
ISO 4406.2 Hydraulic fluid power -- Fluids -- Method for LSV also has detection capabilities for these types of second-
coding the level of contamination by solid particles ary antioxidants (such as zinc dialkyl dithiophosphates).
2.3 Other Standards: 4.3 A measured quantity of sample is dispensed into a vial
VGB Guideline VGB-M 416 M In-Service Monitoring of containing a measured quantity of a selected test solution and
Turbine Oils containing a layer of sand. When the vial is shaken, the
antioxidants and other solution soluble oil components present
3. Oil Condition Monitoring Programs
in the sample are extracted into the electrolytic test solution
3.1 Mostindustriallubricantsconsistofmineralorsynthetic and the remaining droplets suspended in the test solution are
oilscompoundedwithoxidationandrustinhibitors.Depending agglomerated by the sand. The sand/droplet suspension is
upon their application and the performance level desired, allowed to settle out and the antioxidants dissolved in the test
specific required amounts of other additives such as metal solution are quantified by voltammetric analysis. The results
deactivators, pour depressants, extreme pressure additives, and arecalculatedandreportedasmasspercentofantioxidantoras
foam suppressants can also be present. millimoles (mmol) of antioxidant per litre of sample for
3.2 With modern formulations of industrial lubricants, the prepared and fresh oils and as a percent remaining antioxidant
for in-service oils.
antioxidants play a major role in protecting the base-oil against
excessive degradation. To prevent this base-oil degradation, 4.4 Voltammetric analysis is a technique that applies elec-
troanalytic methods wherein a sample to be analyzed is mixed
resulting in the eventual build-up of deposits, varnish and
sludge, the monitoring of the antioxidants represents a proac- with an electrolyte and a solvent (acetone or ethanol based),
tive information on the remaining oxidative health of the and placed within an electrolytic cell. Data is obtained by
in-service lubricant. Oxidation is a chemical reaction between measuring the current passing through the cell as a function of
oxygen atoms with the base oil hydrocarbon molecules, which the potential applied, and test results are based upon current,
are converting the hydrocarbon molecules into oxidation prod- voltage and time relationships at the cell electrodes. The cell
ucts and subsequently weak organic acids. The rate of oxida- consists of a fluid container into which is mounted a small,
tion depends on the presence of antioxidant additives, which easily polarized working electrode, and a large non-polarizable
controls the speed of oxidation, but eventually the antioxidants reference electrode.The reference electrode should be massive
are consumed. Consequently as part of modern proactive relative to the working electrode so that its behavior remains
maintenance strategies, it is vital to know at any time during essentially constant with the passage of small current; that is, it
the operating cycle of the lubricants, its condition by assessing remains unpolarized during the analysis period. Additional
the remaining activity of antioxidants, to prevent the oxidative electrodes, auxiliary electrodes, can be added to the electrode
degradation of the base oil. system to eliminate the effects of resistive drop for high
3.3 Antioxidant monitoring guidelines have been part of resistancesolutions.Inperformingavoltammetricanalysis,the
International Standards such as Practice D4378, Practice potential across the electrodes is varied linearly with time, and
D6224, and VGB Guideline VGB-M 416 M, as well Interna- the resulting current is recorded as a function of the potential.
As the increasing voltage is applied to the prepared sample
tional OEM Maintenance Specifications. This guide presents
guidelines for the lubricant professionals using voltammetric within the cell, the various additive species under investigation
withintheoilarecausedtoelectrochemicallyoxidize.Thedata
techniques as part of their regular maintenance strategies, such
as data interpretation, oil analysis frequency, combination with recorded during this oxidation reaction can then be used to
other condition monitoring tests, etc. determine the remaining useful life of the oil type. A typical
current-potential curve produced during the practice of the
4. Summary of Linear Sweep Voltammetric (LSV) Test
voltammetric test can be seen by reference to Fig. 1. Initially
Method
the applied potential produces an electrochemical reaction
4.1 Linear Sweep Voltammetric (LSV) test can be per- havingaratesoslowthatvirtuallynocurrentflowsthroughthe
formed on any type of industrial lubricant containing at least cell. As the voltage is increased, as shown in Fig. 1, the
onetypeofantioxidant.Thevoltammetrictestisacomparative electroactive species (for example, substituted phenols) begin
test method. By establishing a comparison between its refer- to oxidize at the working electrode surface, producing an
ence oil (fresh oil or standard) and its used oil, this guide can anodic rise in the current.As the potential is further increased,
be used without the specific knowledge on the category to the decrease in the electroactive species concentration at the
which the antioxidants belong. electrode surface and the exponential increase of the oxidation
4.2 ASTM International has two standards, Test Method rate lead to a maximum in the current-potential curve shown in
D6810 and D6971, that shall enable the measurement of the Fig. 1.
remaining phenolic and aminic type of antioxidants. No
5. Significance and Use
standard test method has been developed for the detection of
5.1 The quantitative determination of remaining antioxi-
dants for in-service industrial oils by measuring the amount of
Available from International Organization for Standardization (ISO), 1, ch. de
la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http://
www.iso.org. “Remaining Useful Life Measurements of Diesel Engine Oils, Automotive
Available from VGB PowerTech e.V., P. O. Box 10 39 32, D-45039 Essen, Engine Oils, Hydraulic Fluids, and Greases Using Cyclic Voltammetric Methods,”
Klinkestraße 27 - 31, D-45136 Essen, http://www.vgb.org. STLE, Lubrication Engineering, Vol 51, 3, pp. 223 –229.
D7590–09
FIG. 1 Zinc Dialkyl Dithiophosphate (ZDDP) Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
these additives that have been added to the oil as protection gram between 5 to 8 s (or 0.5 to 0.8 V applied voltage) (see
against oxidation. Industrial lubricants, such as turbine oils, Note1)forthezincdialkyldithiophosphatetypeofantioxidant
compressor oils, gear oils, hydraulic oils, bearing lubricants (Fig. 1), an increase in the current of the produced voltammo-
and greases can be formulated with a wide variety of antioxi- gram between 8 to 12 s (or 0.8 to 1.2 V applied voltage) (Fig.
dants types such as phenols and amines (as primary antioxi- 2) (see Note 1) for the aromatic amines, and increase in the
dants), which are working synergistically and therefore all current of the produced voltammogram between 13 and 16 s
important to be monitored individually. For in-service oils, the (or 1.3 to 1.6 V applied voltage) (see Note 1) for the hindered
LSV determines and compares the amount of original primary phenols or carbamates in the neutral acetone solution (Fig. 2 :
antioxidants remaining after oxidation have reduced its initial x-axis1s=0.1V),or both. Hindered phenol antioxidants
concentration. detected by voltammetric analysis include, but are not limited
5.2 This guide covers procedures for primary antioxidants to, 2,6-di-tert-butyl-4-methylphenol; 2,6-di-tert-butylphenol
such as amines and phenols, as described by Test Method and 4,4’-Methylenebis(2,6-di-tert-butylphenol). Aromatic
D6971 and D6810. amine antioxidants detected by voltammetric analysis include,
5.3 LSV is not designed or intended to detect all of the but are not limited to, phenyl alpha naphthylamines, and
antioxidant intermediates formed during the thermal and oxi- alkylated diphenylamines.
dative stressing of the oils, which are recognized as having
NOTE 1—Voltages listed with respect to reference electrode. The
some contribution to the remaining useful life of the used or
voltammograms shown in Figs. 1-6 were obtained with a platinum
in-service oil. In order to measure the overall stability of an oil
reference electrode and a voltage scan rate of 0.1 V/s.
(including contribution of intermediates present), and before
5.7 For industrial lubricants containing zinc dialkyl dithio-
making final judgment on the remaining useful life of the used
phosphate type of antioxidants, there is an increase in the
oil (which might result in the replacement of the oil reservoir),
currentoftheproducedvoltammogrambetween5to8s(or0.5
it is advised to perform additional analytical techniques (in
to 0.8 V applied voltage) (see Note 1) by using the neutral
accordance with Practice D4378 and Practice D6224).
acetone test solution ( see Fig. 1). There is no corresponding
5.4 This guide is applicable to a wide range of industrial
ASTM International standard describing the test method pro-
oils, both mineral or synthetic based, which can contain rust
cedures for measuring zinc dialkyl dithiophosphates type of
and oxidation inhibitors, antiwear additives such as zinc
antioxidants in industrial lubricants.
dialkyl dithiophosphates on gear oils, circulating oils, trans-
mission oils and other industrial lubricating oils. 5.8 For industrial lubricants containing only aromatic
5.5 The test is also suitable for manufacturing control and aminesasantioxidants,thereisanincreaseinthecurrentofthe
specification acceptance. produced voltammogram between 8 to 12 s (or 0.8 to 1.2 V
5.6 When a voltammetric analysis is obtained for a indus- applied voltage) (see Note 1) for the aromatic amines, by using
trial lubricant inhibited with at least one type of antioxidant, the neutral acetone test solution (first peak in Fig. 2)as
there is an increase in the current of the produced voltammo- described in Test Method D6971.
D7590–09
FIG. 2 Aromatic Amine and Hindered Phenol Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
FIG. 3 Hindered Phenol Voltammetric Response in Bas
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