Metal finishing Q&As since 1989

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Discoloration of copper and bronze brake pads

2007

I am a student at Oklahoma State and am currently working on my masters in Mechanical engineering. I have been assigned a case study on some discolored brake pads that is baffling me. Here are some pertinent facts:
These are brake pads, made by the millions. At the mid-section, their dimensions are 3" × 2.25". A problem of intermittent discoloration has arisen. While this is largely cosmetic, clients are perturbed. The issues are, thus, cause and prevention. The initial briefing, incomplete, secondhand and not guaranteed fully correct, is as follows. The base is copper-plated steel. The pads are believed to comprise bronze (~ 70 Cu, 5 Sn), graphite and an inert ceramic. The whole is a porous sintered product. The details of the attachment of the pad to the base are proprietary. The items pass on a conveyer belt through a sintering oven, whose maximum temperature is 1750°F. At the end, there is a cool-down chamber at 150°F. The furnace atmosphere is exothermic, cracked natural gas. Afterwards, the items are inspected and go into a bin or are stacked on end. When stacked, there are 40-50 together and they go into a box that way. They are not sealed in plastic. It is stated that the problem, when it arises, is over time; there are two stacks in a box. Occasionally, weeks later, when the box is opened at its destination (maybe overseas), one stack may all be "tarnished," as shown; the other may be as packed. Sometimes, tarnished items (damaged goods?) are found in the warehouse.
Initially I would have thought that overseas travel would be a factor, however, cases have been reported where there are two stacks of brake pads per box. Upon opening, one stack will be in pristine condition, while the other is discolored. If it were the atmosphere, wouldn't both stacks be discolored?

Alison Erzinger
Student - Stillwater, Oklahoma

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I think you already understand that you don't really have the necessary data, Alison. Normally, I feel that trying to look at discoloration or corrosion with high power instruments (like Auger analysis) isn't very useful until you at least have a hypothesis (because you can't see the forest for the trees), it may be different this time. If you have no idea what is done in the bonding step and they won't tell you, an elemental analysis of the discoloration might at least offer a hint.

Stains sometimes bleed out through copper plating but it sounds unlikely to me that they would bleed all the way through the sintered brake pad without the problem being obvious on the other side.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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First of two simultaneous responses --

Thanks so much! I'll try an elemental analysis on the discoloration. Lots of theories, few conclusions! *sigh*

Alison Erzinger
- Stillwater, Oklahoma
2007

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Second of two simultaneous responses --

When staining only happens in one layer, it could be because the boxes are not sealed and there is something in the plant atmosphere that is causing the top layer to stain and pulls all of the contaminant out before it can reach the bottom layer.
If it is the bottom layer, then I would suspect that there is a layer of plastic or of styrofoam on the bottom of the box. It may even be cardboard that is not acid free.

Step one is to find out where the stain is. The entire top or bottom layer or around the outside of the layers.

Step two is to get a complete list of what the box is made of, any paper or plastic or foam or VPI paper in ANY of the boxes. Specifically, is there any recycled paper used?

Good luck attacking the teacher.

James Watts
- Navarre, Florida
2007