Automotive vehicles are suspended on springs to reduce the loads or vertical accelerations transmitted from the wheel to the body and its occupants. Springs store and release the energy imparted to the vehicle from the road surface. Cars and trucks also use suspension dampers (also known as shock absorbers and suspension struts) to convert the kinetic energy of the vehicle and springs into thermal energy and dissipate it to the atmosphere and the vehicle chassis. Such suspension dampers are necessary for the safety and comfort of the occupants of the vehicle.
Suspension dampers usually come in two forms on automotive vehicles, and a plurality of the dampers are used on each vehicle. A shock absorber is typically a hydraulic device that controls the sprung and unsprung masses of the vehicle by converting the kinetic energy into thermal energy. It is usually used in combination with a spring operating between the vehicle chassis and an axle to dampen the compression of the spring. A strut damper is more of a structural member of an automotive system that is designed to be capable of withstanding the forces and bending moments resulting from braking, acceleration and handling maneuvers. A strut damper, of course, also serves as a shock absorber. A typical shock absorber or strut damper comprises a cylindrical piston rod that slides in and out of a cylinder tube in sealing engagement with a closure portion of the tube. One end of the tube and one end of the piston rod are respectively connected to the chassis and axle. The other end of the rod is connected to a suitable piston structure that reciprocates within the tube. The piston, of course, displaces hydraulic fluid which is the shock absorbing medium of the device.
Piston rods for shock absorbers, strut dampers and the like are produced by the millions each year. Typically, the rods are formed of a suitable steel to provide the strength for its function. The exposed portion of the rod outside of the damper cylinder tube encounters water, salt, air and other aggressive materials that can cause corrosion of the rod. Since the rod must have a true round sealable surface for engagement with the seal portion of the device, corrosion must be minimized. For this reason, piston rods have traditionally been provided with an electroplated chromium coating. The chromium coating provides both corrosion resistance in the environment in which the damper operates and some abrasion resistance for the engagement of the rod with the sealing portion of the damper cylinder. Such electroplated chromium coatings have served well in damper piston rods for years. However, the hexavalent chromium plating bath and vapors are toxic to workers and present a disposal problem in the environment. There is a need for an alternative to the electroplated chromium coating practice for protecting automotive suspension damper piston rods.