ASTM D 943
Oxidation is a form of deterioration to which all
oils in service are exposed. It is a chemical
reaction that occurs between portions of the oil
and whatever oxygen may be present, usually
the oxygen in the atmosphere. The oxidation of
lubricating oils is accelerated by high tempera-
tures, catalysts (such as copper), and the
presence of water, acids, or solid contaminants.
The rate of oxidation increases with time.
Oxidation tends to raise the viscosity of an oil.
The products of oxidation are acid materials that
lead to depositing of soft sludges or hard,
varnish-like coatings. Paraffinic oils
characteristically have greater oxidation resis-
tance than naphthenic oils, although naphthenic
oils are less likely to leave hard deposits.
Whatever the net effect of oxidation, it is
undesirable in any oil that lubricates on a long-
term basis. Much has been done to improve
oxidation resistance by the use of selected base
stocks, special refining methods, and oxidation
inhibitors. As might be expected, moreover, a
great deal of study has been devoted to the
means by which oxidation resistance of an oil
may be evaluated.
A number of oxidation tests are in use. Some
may be better related to a particular type of
lubrication service than others. All are intended
to simulate service conditions on an accelerated
basis. At an elevated temperature, an oil sample
is exposed to oxygen or air, and sometimes to
water or catalysts, usually iron and/or copper. All
of these factors make oxidation more rapid.
Results are expressed in terms of the time
required to produce a specified effect, the
amount of sludge produced or oxygen con-
sumed during a specified period.
One of the more common methods of examin-
ing steam turbine oils is the ASTM method D
943. This test is based on the time required for
the development of a certain degree of oxidation
under accelerated conditions, the greater the
time, the higher the oil's rating. Here, oxidation
is determined by an increase in the oil's acidity,
a property measured by its acid neutralization
It is sometimes preferred, however, to reverse
this procedure and to measure the Increase in
neutralization number incurred during a speci-
fied test run. In all cases, at least two samples of
the oil under investigation are tested simulta-
neously in the same laboratory unit to reveal
any concealed irregularity in procedure.
Procedure: A sample to be tested under the
ASTM method is placed in a special oxidation
cell, and a specified amount of distilled water is
added to it. The cell is a large test tube fitted
with a water-cooled condenser at the top to help
prevent the escape of water vapor.
Oxygen enters through a glass tube around
which a catalyst of copper wire and iron wire Is
coiled. The coil is submerged in the distilled
water. Flow of oxygen through the cell is mea-
sured by a flowmeter.
The cell is immersed in a 203° F bath and
supplied with oxygen introduced to the bottom of
the tube at 3 liters per hour. Unless other
provisions are made, the test is concluded when
the acid number reaches 2.0
Reporting the Results: The oxidation life of the
oil is generally reported as the number of hours
required to reach the limiting acid number of 2.0
It may also be reported as the acid number, or
rise in acid number, obtained after a test of
specified duration. In many laboratories,
moreover, increasing attention is being given to
certain qualitative observations. Color and odor
of the sample at prescribed stages of the test,
also the presence of any rust or sludge,
condition of the catalyst, etc.. provide additional
data sometimes considered in specifications and
in product evaluation.
(FIST 2-4 11/90)