Brake Fluid White Paper on Copper
The Use of Dissolved Copper to
Indicate the Age of Brake Fluid
Dean R. Wheeler
Ph.D. in Chemical Engineering,
University of California, Berkeley
March 23, 2006
Introduction
This report, prepared for Phoenix FASCAR, is an
analysis of the chemical changes that take place as
brake fluid is used in service. The report addresses how
the amount of dissolved copper in the fluid can serve as
an indicator of the age and protective ability of the
fluid. The conclusions here come from my interpretation
of experimental data made available to me, and my
professional scientific and engineering analysis. The
references section at the end gives some of the
information sources I used in preparing this report.
Brake fluid basics
Brake fluid is a hydraulic fluid mixture that must
function under many months of service and under periodic
high-temperature conditions. The main governmental
standard imposed on brake fluid is that it have a high
boiling point so that pockets of vapor will not form in
the braking system under severe braking conditions. For
instance, moisture-free DOT 3 fluids must have a boiling
point above 400 °F. This can be compared to the boiling
point of 387 °F for pure ethylene glycol (automotive
antifreeze). In fact, many of the molecules that make up
DOT 3 and DOT 4 brake fluids can be considered “larger
chemical cousins” to ethylene glycol. DOT 3 and DOT 4
brake fluids are hygroscopic, meaning they will mix with
and absorb water, which lowers the boiling point just
like with antifreeze. This has led many people to
incorrectly believe that a low boiling point caused by
water absorption is the only thing that can go wrong
with brake fluid.
Current automotive brake systems contain steel
components, such as cylinders and valves, connected by
lengths of copper-alloy-lined steel tubing. Both the
steel and the copper components are unavoidably subject
to corrosion. One need hardly mention that corrosion and
wear of the metal surfaces can interfere with the proper
operation of these components, leading to a diminished
margin of safety. Fortunately, the addition of standard
corrosion inhibitors by brake-fluid manufacturers
significantly slows the corrosion of critical steel
components, leading to much improved service life.
Recently there has been increased attention to the
fact that the protection offered by the corrosion
inhibitors in brake fluid does not last indefinitely. As
brake fluid ages in service, its chemical constituents
undergo a number of changes. Ordinarily none of these
fluid chemical changes are immediately catastrophic, but
cumulatively and over time they lead to decreased
braking-system protection and performance. As already
mentioned, decreased boiling point (associated with
water absorption) is well recognized as a sign of
brake-fluid aging. However, this is not the complete
picture. As discussed below, an increased level of
dissolved copper in the solution is an important and
reproducible indicator that the brake fluid is no longer
effectively protecting metal surfaces from corrosion.
The chemistry of corrosion
In order to better understand the
changes taking place in the brake fluid, it is necessary
to have a little background in corrosion science. The
main principle is that rust is a more natural and stable
state of iron than is a shiny machined steel part. Rust
is composed of iron mixed with oxygen. Similarly, other
metals such as copper corrode spontaneously by reacting
with oxygen. In practical environments it is impossible
to fully prevent corrosion; instead it is a matter of
trying to slow it down as much as possible.
For most metals (gold is a notable exception) exposed
to dry air, a thin layer of the metal on the surface
reacts with oxygen in the air to form a dense oxide
film. This film "passivates" and protects the rest of
the metal by acting as a barrier to greatly reduce
further reaction with oxygen. Unfortunately, when water
or a similar solvent contacts the metal, it partially
dissolves the protective metal oxide skin, leading to
increased corrosion in the presence of oxygen. The
problem is typically made even worse in situations where
there are aggressive chemicals or high temperatures
present. Note that most of what we know about metals and
corrosion is for the case of water mixtures; however,
the same principles apply to brake fluid.
A simplified corrosion reaction for a metal in liquid
looks something like this:
metal + dissolved oxygen + acid =
dissolved metal.
There are a few ways to "starve" this reaction and
therefore slow down the corrosion: First, one can
attempt to reduce the amount of dissolved oxygen in the
solution. In the case of brake systems, it is nearly
impossible to prevent oxygen from absorbing into the
solution due to the fluid-air interface in the master
cylinder, and the slow leakage of oxygen into the system
through rubber parts. A second scheme is to reduce the
acid in the system by adding chemicals that are
alkaline. This scheme is used in brake fluids. A third
scheme is to add chemicals to the system that stick to
and coat the metal surface, providing a barrier in
addition to the metal oxide film to slow things down.
This scheme is also used in brake fluids.
Water is known to degrade the integrity of the oxide
film on metals; however, water is not the only solvent
that can do this. Corrosion can take place in other
liquids, such as those that make up brake fluid.
Furthermore, there is no practical way to keep brake
fluid completely moisture free, so there will always be
some water present near the metal surface. I am aware of
only two scientific studies of corrosion in brake
systems (both are listed in the references section).
Neither showed that the amount of absorbed water in a
brake fluid was a main controlling factor in how fast
the metals corroded.
Corrosion with different metals
There is one more complication I
need to introduce into the corrosion picture, namely
that individual metals differ in their susceptibility to
corrosion and also can corrode one another. This can
work to advantage or disadvantage depending on the
system. Here I consider three metals: zinc, iron, and
copper. Zinc is the least "noble" of the three - meaning
most susceptible to corrosion - while copper is the most
noble. I give some examples of this metal-to-metal
corrosion behavior below.
Galvanized nails used in home construction are steel
nails that have been dipped in molten zinc to form a
zinc coating on the outside. Because zinc is less noble
than iron, it will corrode before iron will. If the zinc
coating is ever broken, and dissolved oxygen gets to the
exposed steel surface, the surrounding zinc will
"sacrifice" itself and react with the oxygen before the
iron does, and thus protect the iron.
Copper, being the most noble of the three metals I
listed, is the best protected against oxidation or
corrosion under normal exposure to dissolved oxygen.
This is the reason that plumbing pipes in homes are
generally made of copper, not steel. However, in a
situation where copper metal has already been corroded
and dissolved into a liquid, it will attack any iron
metal (steel) it comes in contact with. This is because,
like zinc does for iron, the iron will sacrifice itself
for the copper. The result is that dissolved copper will
come out of solution and plate onto the surrounding
steel, while a proportional amount of iron will dissolve
and go into solution. While the initial corrosion
reaction of copper requires oxygen and acid, the second
reaction where dissolved copper corrodes the iron does
not have this requirement. This chemistry is important
in explaining what can happen in brake systems with aged
and degraded brake fluid.
The Highway Traffic Safety Administration of the U.S.
government conducted a six-year engineering analysis
(EA94-0038), culminating in a report in year 2000, to
investigate decreased performance and possible failure
of anti-lock braking systems on light trucks and SUVs.
During the course of the investigation the agency
contracted the services of the National Institute for
Science and Technology (NIST). The scientific tests by
NIST indicated that it was possible for corrosion to
take place in the brake system so as to form deposits of
foreign copper particles around the sealing surfaces of
the steel PWM valve. The effects of a leaking PWM valve
on vehicle braking performance were studied in a
separate report (EA95-026). The important lesson, as I
discuss below, is that copper is not necessarily benign
and inert in the presence of iron and could lead to
degraded braking performance.
The role of corrosion inhibitors in brake
fluid
Corrosion inhibitors come in many varieties, but the
ones used in brake fluid are typically based on a
chemical group called "amine." The amine-based
inhibitors are well known as being able to protect iron
or steel from corrosion in aggressive high-temperature
liquid environments. For instance, amines are widely
used as corrosion inhibitors in steam boilers.
Individual amine inhibitors work in one of two different
ways: (1) by reducing the acid level (neutralizing or
buffering amines) and (2) by forming a water-repelling
barrier film on the metal surface (filming amines).
In brake fluid, the amount of amines present is
usually reported in terms of "reserve alkalinity," a
scientific term that indicates how much acid can be
added to the brake fluid before the neutralizing ability
of the amines is exhausted. However, neutralizing amines
alone will not adequately prevent corrosion in
the presence of dissolved oxygen. This is because even
in a buffered alkaline solution (high pH) there is still
a small amount of acid present to slowly feed the
corrosion reaction. To give full protection, the
inhibitor package requires the help of the filming
amines as well. However, reserve alkalinity does not
necessarily account for the presence or absence of the
filming amines, and so gives only a partial picture of
how much protection is left in a given sample of brake
fluid.
A fact that is rarely appreciated is that the amines
do not protect copper as well as they protect
iron. This is backed up by the observation that
dissolved-copper levels in brake fluid begin rising
almost immediately upon the fluid being put into
service, and the levels rise consistently throughout
service. On the other hand, dissolved-iron levels do not
begin to rise noticeably until the corrosion inhibitors
have already been significantly depleted.
A significant experimental study was conducted
jointly by researchers at Delphi, Union Carbide, and
General Motors and published by the Society of
Automotive Engineers in 1997 (see references section).
The researchers examined the durability of corrosion
protection in brake fluids. They found that the
corrosion protection declined sharply with time in
service. The following numbers are telling: Reserve
alkalinity was between 10 and 20% of its initial level
for the tested fluids after 30 months of service (about
23,000 miles). Furthermore, they found that by 40 months
of service (about 34,000 miles) most of the amine
inhibitors were deactivated by thermal reactions that
turned them into non-inhibiting chemicals.
Interestingly, they found that around 60% of the
amines-both active and inactive-were lost entirely from
the brake fluid by this time. They believed this to be
due to the amines being volatilized (evaporated) into
the air space of the master cylinder and by permeating
out through rubber components.
The role of dissolved copper in brake fluid
Experiments by both Phoenix Systems and the industry
researchers mentioned above have found that dissolved
copper levels in brake fluid increase nearly constantly
with time of service. The SAE paper reports copper
levels at 150 to 300 ppm (parts per million) after 30
months of service. In contrast, the respective levels of
dissolved iron and zinc are significantly smaller and do
not follow as clear of a trend with time. It is true
that dissolved iron could be used as an indicator of a
problem, because elevated levels of dissolved iron
clearly show that corrosion has occurred. However, this
may not be the best practice in a routine maintenance
program that is intended to keep corrosion low at all
times, rather than respond to a problem after it
develops. In summary, copper concentration level in the
fluid is one of the clearest available indicators of
time-in-service for brake fluid. It can serve like wear
indicators on brake pads do, warning when a problem is
imminent rather than just warning when a problem has
already developed.
Moreover, copper is much more than a benign indicator
of brake-fluid service time. Copper plays a key role in
the chemistry of corrosion for the brake system. The
problem as discussed above is that relatively
unprotected and large copper surfaces can corrode almost
from the outset of fluid service. The corrosion of the
copper-lined tubing is less worrisome than it is for the
moving steel parts in the brake system, because close
tolerances are not as essential for the tubing. The
problem, however, is that the dissolved copper then goes
on to attack and deposit itself on the steel surfaces
once the corrosion inhibitors are sufficiently depleted.
The presence of high levels of dissolved copper in the
brake fluid indicates that the steel surfaces in the
brake system are already or will soon be under attack.
The SAE study included an attempt to create
artificially aged braking fluid for testing purposes.
The researchers found that two things were required to
create fluid that behaved similarly to fluid that had
seen many months of vehicle service: (1) significant
amounts of added copper and (2) elevated temperatures in
order to thermally degrade the corrosion inhibitors.
Simple thermal degradation without adding copper did
not lead to fluid that correctly mimicked the
corrosive action of truly old brake fluid. In fact, the
researchers speculated that the copper metal added to
the system acts as a catalyst to promote the degradation
of the amine-based inhibitors.
My analysis suggests that the presence in the brake
system of copper as well as amine-based corrosion
inhibitors is an unfortunate combination that in the end
works to promote iron corrosion. It is known that amines
associate strongly with dissolved copper. Any filming
amines that associate with copper in solution cannot at
the same time do their job of protecting iron.
Therefore, elevated levels of dissolved copper may
interfere with the effectiveness of the filming amines
in preventing corrosion of the steel surfaces.
Additional factors in fluid age
There are additional factors that can aggravate the
corrosion problems mentioned above. For instance,
anti-lock braking systems create greater circulation of
brake fluid in the system. This circulation causes
dissolved oxygen and dissolved copper to transport more
freely throughout the system, likely leading to greater
corrosion exposure that inhibitors must then counteract.
This could lead to more rapid depletion of inhibitors
than in a non-ABS system.
Similarly, city driving with its more extensive use
of braking will lead to elevated temperatures in the
system. Spontaneous chemical reactions always speed up
at higher temperatures. Therefore, higher temperatures
accelerate all of the undesirable corrosion-both of
copper and iron-as well as the processes that degrade
the inhibitor package. Therefore, an automobile that has
seen "hard driving" with frequent use of brakes is
likely to show greater depletion of the inhibitors and
loss of corrosion protection, as well as greater copper
concentration, for a given time or mileage in service.
So the use of copper concentration as an indicator will
naturally account for some degree of variation in user
abuse of the braking system. On the other hand, elevated
temperatures will tend to reduce the amount of
water that would otherwise be in the brake fluid. This
is because water, with its lower boiling point, will
volatilize more strongly than other components as
temperature is increased.
Other measurements of fluid age
In contrast to dissolved-copper measurements,
boiling-point and reserve-alkalinity measurements are
less effective as indicators of fluid-service time. This
is because these two quantities can vary so widely from
one manufacturer's brake-fluid formulation to the next.
Unlike in a laboratory experiment, a technician in the
field has no foolproof way of knowing the baseline level
of either quantity. For relatively new cars it is
reasonable to assume that they contain the
OEM-formulated brake fluid, but for a car that has been
in service for a few years the brake fluid is a big
question mark and could by that point even be a mixture
of different manufacturers' brake fluids. For instance,
the SAE paper notes the wide variations, with reserve
alkalinity levels for fresh commercial brake fluids
ranging from a low of 3 to a high of 120. These
researchers also warn that reserve alkalinity only
measures general acid-buffering ability and not the
concentration of particular inhibitors. Therefore, it
would be unlikely that one could reliably predict either
(1) months of service of the brake fluid or (2)
remaining strength of the full corrosion-inhibitor
package using boiling point or reserve alkalinity.
References
G.L. Jackson, P. Levesque, and F.T. Wagner, "Improved
Methods for Testing the Durability of Corrosion
Protection in Brake Fluids," Paper 971007, SAE Technical
Paper Series (1997).
R.E. Ricker, J.L. Fink, A.J. Shapiro, L.C. Smith, and
R.J. Schaefer, "Preliminary Investigations Into
Corrosion in Anti-Lock Braking Systems," Internal Report
6233, National Institute of Standards and Technology,
U.S. Dept. of Commerce (1998). |
