By John Smith
Magnesium chloride looked like a miracle cure for highway crews when it emerged as a de-icing option in the 1990s. It could be sprayed onto the surface of a road before freezing rain began to fall, ensuring that ice never had the opportunity to form. By reducing the amount of conventional road salt, transportation departments were even helping to protect nearby fields and greenery in the process.
Brakes were hardly that lucky.
This “miracle” de-icing compound also will seep into the smallest crevices and coat every piece of exposed metal such as the brake shoe tables found under a lining block. Unlike road salt, it will continue to suck moisture out of the air, even after a truck leaves the coated roads of the Northeast for the supposed shelter of a dry Texas highway.
Then the corrosion begins.
“Rust jacking does not instantaneously appear. It comes in stages,” Bendix Commercial Vehicle Systems engineer John Hawker explained this year during a presentation to the Technology & Maintenance Council (TMC).
Initially the problem will emerge as hairline cracks in the friction material – usually small enough to escape the attention of a roadside inspector. These eventually evolve into cracks that would require a lining replacement. As the oxidation builds up, the lining will begin to separate at the rivets before it completely comes off the brake shoe table, ultimately buckling around the rivets and causing cracks that run diagonally between the rivet holes.
