ASTM D7590-09(2014)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D7590 − 09 (Reapproved 2014)
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 1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This guide covers the voltammetric analysis for quali-
responsibility of the user of this standard to establish appro-
tative measurements of primary antioxidants in new or in-
priate safety and health practices and determine the applica-
service type industrial lubricants detectable in concentrations
bility of regulatory limitations prior to use.
as low as 0.0075 mass percent up to concentrations found in
newoilsbymeasuringtheamountofcurrentflowataspecified
2. Referenced Documents
voltage in the produced voltammogram.
2.1 ASTM Standards:
1.2 This guide can be used as a resource for a condition
D1193 Specification for Reagent Water
monitoring program to track the oxidative health of a range of
D4057 Practice for Manual Sampling of Petroleum and
industrial lubricants which contain primary antioxidants. In
Petroleum Products
order to avoid excessive degradation of the base-oil, these
D4378 Practice for In-Service Monitoring of Mineral Tur-
primary antioxidants play a major role to protect the lubricants
bine Oils for Steam, Gas, and Combined Cycle Turbines
against thermal-oxidative degradation. This guide can help
D6224 PracticeforIn-ServiceMonitoringofLubricatingOil
users with interpretation and troubleshooting results obtained
for Auxiliary Power Plant Equipment
using linear sweep voltammetry (LSV).
D6304 Test Method for Determination of Water in Petro-
1.3 When used as part of oil condition monitoring practices,
leum Products, Lubricating Oils, and Additives by Cou-
it is important to apply trend analysis to monitor the antioxi-
lometric Karl Fischer Titration
dant depletion rate relative to a baseline sample rather than use
D6810 Test Method for Measurement of Hindered Phenolic
voltammetry for an absolute measurement of the antioxidant
Antioxidant Content in Non-Zinc Turbine Oils by Linear
concentration.The trending pattern provides a proactive means
Sweep Voltammetry
to identify the level of oil degradation or abnormal changes in
D6971 Test Method for Measurement of Hindered Phenolic
the condition of the in-service lubricant.
and Aromatic Amine Antioxidant Content in Non-zinc
Turbine Oils by Linear Sweep Voltammetry
1.4 The values stated in SI units are to be regarded as
D7214 Test Method for Determination of the Oxidation of
standard. No other units of measurement are included in this
Used Lubricants by FT-IR Using Peak Area Increase
standard.
Calculation
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum
Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-
mittee D02.09.0C on Oxidation of Turbine Oils. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedMay1,2014.PublishedJuly2014.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ε1
in 2009. Last previous edition approved in 2009 as D7590 – 09 . DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
D7590-09R14. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7590 − 09 (2014)
2.2 ISO Standards: standard test method has been developed for the detection of
ISO 4406.2 Hydraulic fluid power—Fluids—Method for other type of antioxidants by linear voltammetry, although
coding the level of contamination by solid particles
LSV also has detection capabilities for these types of second-
4 5
2.3 Other Standards: ary antioxidants (such as zinc dialkyl dithiophosphates).
VGB Guideline VGB-M 416 M In-Service Monitoring of
4.3 A measured quantity of sample is dispensed into a vial
Turbine Oils
containing a measured quantity of a selected test solution and
containing a layer of sand. When the vial is shaken, the
3. Oil Condition Monitoring Programs
antioxidants and other solution soluble oil components present
3.1 Mostindustriallubricantsconsistofmineralorsynthetic
in the sample are extracted into the electrolytic test solution
oilscompoundedwithoxidationandrustinhibitors.Depending
and the remaining droplets suspended in the test solution are
upon their application and the performance level desired,
agglomerated by the sand. The sand/droplet suspension is
specific required amounts of other additives such as metal
allowed to settle out and the antioxidants dissolved in the test
deactivators, pour depressants, extreme pressure additives, and
solution are quantified by voltammetric analysis. The results
foam suppressants can also be present.
arecalculatedandreportedasmasspercentofantioxidantoras
3.2 With modern formulations of industrial lubricants, the
millimoles (mmol) of antioxidant per litre of sample for
antioxidants play a major role in protecting the base-oil against
prepared and fresh oils and as a percent remaining antioxidant
excessive degradation. To prevent this base-oil degradation,
for in-service oils.
resulting in the eventual build-up of deposits, varnish and
4.4 Voltammetric analysis is a technique that applies elec-
sludge, the monitoring of the antioxidants represents a proac-
troanalytic methods wherein a sample to be analyzed is mixed
tive information on the remaining oxidative health of the
with an electrolyte and a solvent (acetone or ethanol based),
in-service lubricant. Oxidation is a chemical reaction between
and placed within an electrolytic cell. Data is obtained by
oxygen atoms with the base oil hydrocarbon molecules, which
measuring the current passing through the cell as a function of
are converting the hydrocarbon molecules into oxidation prod-
the potential applied, and test results are based upon current,
ucts and subsequently weak organic acids. The rate of oxida-
tion depends on the presence of antioxidant additives, which voltage and time relationships at the cell electrodes. The cell
controls the speed of oxidation, but eventually the antioxidants consists of a fluid container into which is mounted a small,
are consumed. Consequently as part of modern proactive easily polarized working electrode, and a large non-polarizable
maintenance strategies, it is vital to know at any time during
reference electrode. The reference electrode should be massive
the operating cycle of the lubricants, its condition by assessing relative to the working electrode so that its behavior remains
the remaining activity of antioxidants, to prevent the oxidative
essentially constant with the passage of small current; that is, it
degradation of the base oil.
remains unpolarized during the analysis period. Additional
electrodes, auxiliary electrodes, can be added to the electrode
3.3 Antioxidant monitoring guidelines have been part of
system to eliminate the effects of resistive drop for high
International Standards such as Practice D4378, Practice
resistancesolutions.Inperformingavoltammetricanalysis,the
D6224, and VGB Guideline VGB-M 416 M, as well Interna-
potential across the electrodes is varied linearly with time, and
tional OEM Maintenance Specifications. This guide presents
the resulting current is recorded as a function of the potential.
guidelines for the lubricant professionals using voltammetric
As the increasing voltage is applied to the prepared sample
techniques as part of their regular maintenance strategies, such
within the cell, the various additive species under investigation
as data interpretation, oil analysis frequency, combination with
withintheoilarecausedtoelectrochemicallyoxidize.Thedata
other condition monitoring tests, etc.
recorded during this oxidation reaction can then be used to
4. Summary of Linear Sweep Voltammetric (LSV) Test
determine the remaining useful life of the oil type. A typical
Method
current-potential curve produced during the practice of the
voltammetric test can be seen by reference to Fig. 1. Initially
4.1 Linear Sweep Voltammetric (LSV) test can be per-
the applied potential produces an electrochemical reaction
formed on any type of industrial lubricant containing at least
havingaratesoslowthatvirtuallynocurrentflowsthroughthe
onetypeofantioxidant.Thevoltammetrictestisacomparative
cell. As the voltage is increased, as shown in Fig. 1, the
test method. By establishing a comparison between its refer-
ence oil (fresh oil or standard) and its used oil, this guide can electroactive species (for example, substituted phenols) begin
to oxidize at the working electrode surface, producing an
be used without the specific knowledge on the category to
which the antioxidants belong. anodic rise in the current.As the potential is further increased,
the decrease in the electroactive species concentration at the
4.2 ASTM International has two standards, Test Method
electrode surface and the exponential increase of the oxidation
D6810 and D6971, that shall enable the measurement of the
rate lead to a maximum in the current-potential curve shown in
remaining phenolic and aminic type of antioxidants. No
Fig. 1.
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 (2014)
FIG. 1 Zinc Dialkyl Dithiophosphate (ZDDP) Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
5. Significance and Use 5.6 When a voltammetric analysis is obtained for a indus-
trial lubricant inhibited with at least one type of antioxidant,
5.1 The quantitative determination of remaining antioxi-
there is an increase in the current of the produced voltammo-
dants for in-service industrial oils by measuring the amount of
gram between 5 to 8 s (or 0.5 to 0.8 V applied voltage) (see
these additives that have been added to the oil as protection
Note1)forthezincdialkyldithiophosphatetypeofantioxidant
against oxidation. Industrial lubricants, such as turbine oils,
(Fig. 1), an increase in the current of the produced voltammo-
compressor oils, gear oils, hydraulic oils, bearing lubricants
gram between 8 to 12 s (or 0.8 to 1.2 V applied voltage) (Fig.
and greases can be formulated with a wide variety of antioxi-
2) (see Note 1) for the aromatic amines, and increase in the
dants types such as phenols and amines (as primary
current of the produced voltammogram between 13 and 16 s
antioxidants), which are working synergistically and therefore
(or 1.3 to 1.6 V applied voltage) (see Note 1) for the hindered
all important to be monitored individually. For in-service oils,
phenols or carbamates in the neutral acetone solution (Fig. 2:
the LSV determines and compares the amount of original
x-axis1s=0.1V),or both. Hindered phenol antioxidants
primary antioxidants remaining after oxidation have reduced
detected by voltammetric analysis include, but are not limited
its initial concentration.
to, 2,6-di-tert -butyl-4-methylphenol; 2,6-di-tert-butylphenol
5.2 This guide covers procedures for primary antioxidants
and 4,4’-Methylenebis(2,6-di- tert-butylphenol). Aromatic
such as amines and phenols, as described by Test Method
amine antioxidants detected by voltammetric analysis include,
D6971 and D6810.
but are not limited to, phenyl alpha naphthylamines, and
5.3 LSV is not designed or intended to detect all of the
alkylated diphenylamines.
antioxidant intermediates formed during the thermal and oxi-
NOTE 1—Voltages listed with respect to reference electrode. The
dative stressing of the oils, which are recognized as having
voltammograms shown in Figs. 1-6 were obtained with a platinum
some contribution to the remaining useful life of the used or
reference electrode and a voltage scan rate of 0.1 V/s.
in-service oil. In order to measure the overall stability of an oil
(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
ASTM International standard describing the test method pro-
5.4 This guide is applicable to a wide range of industrial
cedures for measuring zinc dialkyl dithiophosphates type of
oils, both mineral or synthetic based, which can contain rust
antioxidants in industrial lubricants.
and oxidation inhibitors, antiwear additives such as zinc
dialkyl dithiophosphates on gear oils, circulating oils, trans-
5.8 For industrial lubricants containing only aromatic
mission oils and other industrial lubricating oils.
aminesasantioxidants,thereisanincreaseinthecurrentofthe
produced voltammogram between 8 to 12 s (or 0.8 to 1.2 V
5.5 The test is also suitable for manufacturing control and
specification acceptance. applied voltage) (see Note 1) for the aromatic amines, by using
D7590 − 09 (2014)
FIG. 2 Aromatic Amine and Hindered Phenol Voltammetric Response in the Neutral Test
...