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how it works

The brakes on this new car picked up rust in only 48 hours in a parking lot.Richard Russell/The Globe and Mail

It is discouraging to look through those fancy alloy wheels of your pride and joy to see rusted rotors, brakes that were nice and shiny when you parked the car.

Rust can start to accumulate on the surface of exposed, and unprotected iron parts within hours. Rust has all but been eliminated on new vehicles through the use of a protective layer such as galvanized steel or by using non-rusting metals.

But up to now brake rotors, commonly made from cast iron, have to be free of any normal coating, in order to perform their function.

General Motors engineers and scientists have developed and are starting to deploy FNC coating for brake rotors. FNC stands for Ferritic Nitro-Carburizing, a chemical case-hardening process whereby nitrogen and carbon are diffused into the surface of a ferrous metal such as steel or cast iron. This exclusive, patented process is being applied to the brake rotors of a number of GM vehicles. It not only reduces or eliminates corrosion, it results in a finish that is harder and stronger, thus increasing the life of the rotors. The FNC protection lasts for about 60,000 km in highly corrosive environments such as the east coast and up to double that in less severe areas such as the prairies. The initial testing of the new rotors was conducted in Atlantic Canada.

The process involves heating the rotors in special ovens the size of a tour bus at 560 degrees Celsius where they are exposed to a nitrogen-rich atmosphere for 24 hours. The nitrogen atoms bond to the surface of the rotor.

The development team at GM's Canadian Regional Engineering Centre in Oshawa, Ont., which led the project of getting the FNC rotor to market, faced a number of challenges. Among them was the distortion caused by the heat treatment and the reduced friction and resulting increased braking distances caused by the new surface. It made continual adjustments to the process until these issues were addressed.

Rusted rotors are one of the leading causes of customer complaints for all car companies. A recent consumer study by GM found that 40 per cent of new-vehicle owners listed corrosion among the top three bothersome things about their cars. Most of the complaints were of the nasty sound when the brakes are first applied after sitting for a period and becoming coated with rust.

That same situation can lead to the pads and rotors becoming bonded.

Both situations lead to uneven rotor wear and a pulsation felt through the brake pedal – and the steering wheel if it is a front brake or the seat of your pants if a rear brake unit.

Uneven wear is a leading cause of rotors having to be turned on a lathe to generate a fresh surface or replaced when the thickness reaches a certain point. FNC-treated rotors have a 400-micron (one-tenth the width of a human hair) layer of protection. Once this layer is worn through, the cast iron rotor can be treated like a normal one and machined if necessary.

GM's internal warranty data show that, when FNC-treated rotors are used instead of conventional ones, the replacement and incident rate drops by between 50 to 80 per cent.

The first vehicle to benefit from FNC-treated rotors was the 2009 Cadillac DTS. It was followed in 2011 by the Chevrolet Impala, Malibu and Volt and the Buick LaCrosse and Regal. The Cadillac ATS and XTS are the most recent recipients. More than 80 per cent of GM vehicles sold in Canada will have FNC rotors by 2016.

FNC-treated rotors not only look better – shiny metal instead of rust seen through those fancy wheels – they last longer, saving money.

Another benefit: FNC rotors generate less brake dust, helping keep those wheels looking good longer.

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