By: Arnold Anderson
What Happens to Copper Wear Particulate from Brake Linings?
Copper may be in brake linings as a powder, wire, or particle. Copper compounds may be used, such as copper sulfide and copper oxide. Brass chips and bronze powders also may be used. They all end up in essentially the same form after brake usage. Both the oxide and the sulfide will be reduced, producing metallic copper at the surface. This smears at the rubbing interface. The exposed surface, after brake usage, develops a visible oxide layer. At the end of a hard brake stop, the smeared copper develops wrinkles and tears, with some particles emitted as thin platelets in the wear debris.
Copper wear debris may go through the wear process several times. Using a laboratory visualization apparatus, copper has been seen to smear until it departs the trailing edge of the brake lining. Then, weakly attached to the counter surface, it reappears at the leading edge to begin another cycle. When wear rates are low, copper particles may repeatedly recycle themselves. Copper particles that leave the brake lining contact from the side edges are unlikely to get recycled.
I have observed copper smears on brake lining surfaces up to 60 millimeters long and 15 millimeters wide. Emitted copper wear platelets are much smaller, typically 5 to 30 microns in length,and a micron or less in thickness.Because they are thin, such platelets behave aerodynamically as very small particles, but lay flat on any surface they contact. Copper and ferrous metallic wear platelets foliate on automotive wheels and tire sidewalls, much like wet leaves. Platelet foliation is the reason brake wear deposits are so difficult to remove during car washing.
About half of larger brake lining wear particulate is retained around the brake. The rest is emitted to the air and falls quickly to the ground. No data was found specifically addressing the fraction of copper platelets that stay on the brake and wheel assembly. It is obvious that most will eventually 'get down to earth', whether along the road or around the car wash site. The critical items that should be known are:
what percentage of copper platelets get relocated by stormwater flow?
what percentage of relocated platelets reach the Bay?
what percentage of those that reach the Bay will be dissolved?
what percentage of those that do not reach the Bay get dissolved in place?
While none of these four issues have been documented in available literature, the chemistry and morphology of the copper wear platelets allows engineering judgments to be made with reasonable confidence. These follow the above numbering.
Once allowed to settle on a solid surface, flat platelets are hydrodynamically stable, and unlikely to be lifted and suspended by stormwater. Should stormwater disturb the surface substrate and suspend the particle, it should resettle quickly as soon as flow velocities drop. Perhaps as much as a percent or two of the platelets might get relocated.
The greater the distance from where the copper platelets first settle, the less likely they will ever reach the Bay, unless the stormwater provides a raging torrent of turbulent water flow all of the way to the Bay. The expectation of brake copper platelets reaching the Bay would be essentially zero.
The copper platelets have a moderately high surface-to-volume ratio, but true copper solubility is unlikely unless strong acids are present.For water pollution measurements, solubility is based on passage through an 0.5-micron filter. That is, the definition of solubility includes fine particulate as well as copper in solution. Brake wear copper platelets would have to have their length and width reduced substantially to meet this criterion. Considering the unlikely prospect of a copper platelet from brake wear reaching the Bay, solubility should be of trivial consequence.
Corrosion literature contains data about chemical attack of copper. Copper roofing and flashing, along with other exterior surfaces of copper and copper alloys are known to be attacked by the elements, bird droppings, and soon, but at a low rate. The copper from brake wear debris is known to be insoluble in fresh and saltwater. Solubility in the presence of various environmental agents, especially acids, would require experimentation to quantify. If copper from brake wear platelets reaches the Bay, acid attack on the ground would appear to be the most likely mechanism. However, this presumes a corrosive environment that could be hostile to almost everything, not just copper brake wear debris.
Brake Reports funded by Santa Clara Valley Non-Point Source Pollution Control Program
The WCC 1994 wear study showed that US made brake linings had less than 0.25% copper, while Asian and European brake linings contained up to 20% copper. This study assumed only original equipment linings in its wear estimate (about 85% of brake relines use aftermarket replacement brake linings), that 100% of brake linings wear (about 40% typically remains on the shoes), and that the high copper brake linings wear at the same rate as the low copper brake linings. This overestimates copper wear by a factor of two or more. Of greater consequence, this report assumed that copper brake wear debris behaves like 'generic' atmospheric dust or dirt. Consequently, a totally unsubstantiated 55% of brake copper was presumed to enter the Bay. The report concluded that "Automotive disc brake pads are a significant source of copper to storm water in Santa Clara Valley" and "non-domestic manufacturer's automobiles appear to contribute a larger load of copper to storm water than domestic manufacturer's automobiles." [Actually, non-domestic brake linings provided 99.8% of the estimated copper disc brake pad wear-only 0.2% came from US made brake linings.]
The WCC 1996 parking lot study showed "metals are predominantly in the dissolved phase (80-90 percent)" and concluded "the pollutants in stormwater runoff from parking lots are present mainly in dissolved phase rather than particulate phase." Interestingly the study also concluded that "the concentration of metals in runoff from mall parking lots is similar to the concentration of metals in runoff from most other urban land uses." The reported implications for pollution control were "The total concentration of metals in runoff from mall parking lots is less than the concentration of metals in runoff from other urban land uses."
One could conclude from the WCC data that:
brake lining copper wear debris is quite soluble in rainwater, OR
soluble parking lot copper comes from another source-not brake linings.
A reasonable and prudent researcher would have determined solubility of brake wear copper and examined the dominant(non-brake) portion of parking lot dust before reporting that brake linings provide 80% of the non-point copper loading to the Bay. WCC did not.
What are the Expected Consequences of Aircraft Brake Copper on the Bay?
If we assume that copper wear debris from brake linings is soluble, as did WCC, we would expect to see evidence of excess copper in waters around major airports near the Bay. San Francisco and Oakland International airports have heavy traffic and are right on the Bay. San Jose International Airport is adjacent to, and drains into, the Guadalupe River-which empties into the South Bay a few miles downstream. San Jose Airport has a large amount of its air traffic in the smaller and older aircraft that commonly use high copper content disc brake linings.
Researchers surely should have detected such a stormwater pollution source if it existed in the Estuary. If airports are not a copper pollution source, then Woodward-Clyde Consultant and Santa Clara Valley Nonpoint Source Pollution Control Program personnel should rethink the assumption that most brake wear copper is picked up by storm water, and their presumption that brake copper is a major source of copper in the South Bay.
Aircraft brake copper wear debris may have a greater oxide film thickness, due to generally higher bulk operating temperatures, than copper from automotive brakes. Greater oxidation should make the aircraft wear particulate slightly greater in solubility, although still nearly zero-and nowhere near 55%. If we consider the size and shape of copper brake wear particulate, aircraft brakes probably contribute an inconsequential amount of dissolved copper, either to the Estuary or to the South Bay.
The continuing trend in aircraft brakes is toward carbon-carbon friction materials. Therefore, the amount of copper wear debris from aircraft brakes will decrease with time, whether such debris has undesirable environmental consequences or not.