Part 3 of 4
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This information in this article is copyright Texaco Inc., Texaco Magazine
LUBRICATION. This article may not be reproduced, copied, or retransmitted
in any form without express written consent of Texaco Inc.
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ANTIFREEZE AND COOLANT
Courtesy of Texaco Magazine LUBRICATION
Copyright Texaco Inc.
Inhibitors
Some of the typical inhibitors traditionally used in AF&Cs are borates,
phosphates, benzoates, nitrates, silicates, mercaptobenzothiazole and
tolyltriazole. In addition to direct inhibition, alkali metal borates and
phosphates provide extended metal protection by means of buffering action and
alkaline reserve. This maintains the coolant in an alkaline condition by
neutralizing acidic products that may form by deterioration of the coolant or
by entry of combustion products past a leaking cylinder head gasket.
The inhibitors must be present at sufficient concentration to provide metal
protection. The amount needed depends on operating conditions, corrosivity of
the water used and length of time at operating temperatures. Corrosion
inhibition is one of the reasons for maintaining the AF&C at 40 to 50 percent
concentration year-round. Since inhibitors are depleted with time through
interaction with metals or other substances in the system, the AF&C must be
replaced periodically.
Early inhibited AF&Cs contained triethanolamine and phosphoric acid, which
combine to form triethanolamine phosphate (TEP). TEP provides corrosion
protection to aluminum and cast iron. Sodium mercaptobenzotriazole (MBT) was
also added to protect copper or brass components used in radiators and
connecting pipework.
While TEP AF&Cs were being developed, other researchers suggested using
combinations of sodium nitrite and sodium benzoate. Sodium nitrite is
particularly effective for the protection of ferrous materials, but has the
serious disadvantage of promoting the corrosion of solder alloys. Sodium
benzoate, which is less effective for the protection of cast iron, is a good
inhibitor of solder corrosion. The benzoate-nitrite combination provided an
effective means for protecting both the ferrous and the solder alloys.
Another additive, sodium tetraborate, was also identified as a general
purpose inhibitor for aluminum and cast iron engine cooling systems.
In the U.S, phosphates, nitrates and silicates, as well as MBT and borates,
have been used as corrosion inhibitors. Nitrates provide aluminum protection,
while silicates provide general corrosion protection and act as a deterrent
to cavitation of aluminum coolant pumps.
As experience grew, some of the adverse properties of of the inhibitors and
inhibitor combinations in use came to light, as cited in the following
statements: Sodium mercaptobenzothiazole is relatively insoluble at low
temperatures and can form insoluble calcium salts in hard waters. Benzoates
are relatively inefective for the protection of cast iron and aluminum.
Silicates can polymerize and form gels on standing. Borates are not reliable
with light metals at high temperatures. Phosphates can be precipitated by
hard water.
While these disadvantages were being discovered, encouraging progress was
being made in the identification of new, improved inhibitors: Benzotriazole
(BTZ) and tolyltriazole (TTZ) replaced sodium mercaptobenzothiazole in most
formulations. BTZ and TTZ are more soluble than MBT and were found to have
remarkable inhibitor properties for the corrosion protection of copper
alloys. Sodium molybdate was found to display good inhibitor properties
towards a number of metals. Silicates, despite their disadvantages, remained
the inhibitor of choice for aluminum protection; silicate stabilizers have
alleviated some of the problems with polymerization and precipitation of
these chemicals. Disodium sebacate, the sodium salt of a dibasic carboxylic
acid, was found to be an effective corrosion inhibitor for both ferrous and
aluminum alloys.
U.S. AF&Cs have been formulated to provide high alkaline pH values (9.0 to
11.0) to protect cast iron. In Europe, however, pH values of 7.0 to 8.5 have
been considered desirable (neutral to low alkalinity) to avoid corrosion of
aluminum, which becomes more probable as pH is raised. This difference in
approach may have reflected the relative use of aluminum and cast iron
engines in the two regions.
Hard water compatibility is required in Europe and increasingly in the U.S. In
general, this requires avoidance of phosphate formulations. Phosphate
formulations are difficult to stabilize when hard water is used, as insoluble
calcium phosphates are readily formed.
As stated earlier, the life of an engine coolant depends essentially on the
continued effectiveness of its corrosion inhibitor package. Some inhibitors,
such as nitrates and silicates, have been found to be consumed quite rapidly
in service. Other inhibitors, such as phosphates and triazoles, are consumed
at moderate rates. Still others, such as borates and organic acid salts, have
slow depletion rates.
Supplemental Coolant Additives
Supplemental coolant additive (SCA) packages are sometimes used to boost the
useful life of an engine coolant by replacing the corrosion inhibitors that
deplete. Most of these products are intended for the heavy-duty market where
diesel tractors can average more than 100,000 miles per year, as opposed to
the 15,000 miles per year that are normal for light duty cars and trucks. The
aim is to extend the life of the coolant to one year under the severe
conditions that heavy-duty cooling systems often experience.
These products are added by gradual dissolution from a screw-on filter
canister or as a liquid additive package at certain mileage intervals. A
simple test kit is often used in conjunction with liquid additive additions
to indicate when the most rapidly depleting inhibitor, nitrite, is below a
level considered necessary for cylinder liner cavitation protection. The
packages contain all of the inhibitors that deplete and, when used correctly,
can maintain the correct overall inhibitor balance to provide excellent
protection for all of the cooling system metals.
However, the potential for over-inhibition is great. None of the methods used
to control the use of these packages is fool proof. Many people live by the
guiding principle that "more is better," but this philosophy will often lead
to problems in cooling system corrosion prevention. Too much nitrite can
promote corrosion of the solder used to hold the radiator and heater core
together. Too much silicate will result in gel formation and cause plugging
of the radiator and filter, thereby reducing the rate of heat exchange. This,
in turn, can contribute to shortened engine life through increased wear
caused by higher engine operating temperatures that result from poor heat
transfer. Finally, there may also be a lack of compatibility between SCAs and
some AF&Cs.
SCAs do not create life-time fill engine coolants. Scheduled replacement of
the coolant in these systems is still recommended. Even with the most
conscientious monitoring of SCA additions, chemical inhibitor imbalances will
eventually result. Following recommended practices, SCAs will help maintain
the engine at the peak of its efficiency throughout its life and minimize
overhauls and repairs.
Antifoamants
Excessive foaming, either in service or during vehicle manufacturing in the
filling lines, must be avoided. Foam that accumulates in a cooling system due
to exhaust gas or air entrainment is detrimental to heat transfer. Certain
higher alcohols or silicone polymers are added to AF&C to supress foam. these
substances lower the surface tension of the liquid to allow the bubbles to
break.
Dye
A dye is added to AF&C to differentiate it from other functional fluids, such
as brake fluid or automatic transmission fluid, and to signal its presence in
the cooling system. The dye should be stable during the period of use and
should not affect paint finishes in case of accidental spillage of coolant.
AF&C FORMULATIONS
United States
In the U.S., AF&Cs may contain a relatively large amount of phosphate (up to
about 1%) and minor amounts of nitrate, silicate and triazole. other
formulations, with less phosphate, contain borate in combination with
nitrate, silicate and triazole.
U.S. AF&Cs are usually nitrite-free, because of concern that carcinogenic
nitrosoamines can form in the presence of amines. However, nitrites are
common in coolant additive packages for heavy-duty diesel vehicles
(especially with wet sleeve cylinder liners) for field maintenance of cast
iron corrosion protection.
In the mid 1980's, the heady duty diesel industry experienced gel problems
with high silicate AF&Cs, caused by overtreatment with SCAs. The industry
responded by developing "Universal AF&Cs". By definition, these AF&Cs were
low silicate products, meeting ASTM standards D4985, D3306 and D4340. these
AF&Cs could be used in any engine and were compatible with both the high
silicate automotive AF&Cs and the specialized heavy duty AF&Cs. This allowed
a company with a mixed fleet of vehicles to buy only one AF&C. SCA suppliers
also responded by lowering the silicate levels of their products. As a result
of these two events, the problem of silicate gel in heavy duty diesel engines
has been significantly reduced.
Europe
With the exception of the triethanolamine phosphate (TEP) coolants used in
France, phosphate is not used in European AF&Cs because of the hard water
situation in Eruope. Most coolants have been based on a borate-benzoate
inhibitor package containing minor amounts of nitrite, nitrate, silicate, and
triazole.
In newer formulations, salts of organic aliphatic acids are uses as
substitutes for nitrite. A combination of disodium sebacate, the sodium salt
of a C[10] dibasic carboxylic acid, and benzotriazole is prescribed in the
British Ministry of Defence [sic] (AL39) and NATO S757 specifications and has
been used with success for over a decade. Aliphatic acid based coolants are
gaining credibility. Long life coolants based on this technology have been
adopted by a major manufacturer. The ultimate goal is lifetime-fill AF&Cs.
Joe Dille
Telford PA USA
(joe@dille.montgomery.pa.us)
Joe Dille
Telford PA USA
(joe@dille.montgomery.pa.us)
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