CORROSION RESISTANCE
Corrosion will affect the surface finish of the pin as well as the running clearance. As a pin corrodes, there are several consequences to the operation of the joint. The three main problems introduced into the joint are increased pin OD, rougher surface finish, and oxide material contamination. The increased pin diameter could result in an interference fit and increased torque needed to turn the pin. The increased roughness of the pin surface will serve to additionally increase the torque required to operate the joint as well as increase the rate of wear. The addition of contaminates, in the form of the metal oxide, will further degrade the pin and bearing. Protecting the pin and bearing from contamination will lead to long-term predictable wear regardless of the frequency of operation of the equipment.
Polygon recommends a corrosion resistant surface treatment for pins used in dry running applications. Customers have used Hard Chrome, Electroless Nickel, and Nitrocarburization successfully for many years. Pins with these surface treatments will have hardness exceeding Polygon recommendations for moderate to highly contaminated environments. Thickness or depth of the treatment should be determined based on the chosen materials. Typical values are 0.0005 inches for both thickness and white layer depths. These values are not intended as recommendations, only as examples to allow designers a value to begin a conversation with pin suppliers.
Softer surface treatments are also used with Polygon bearings. Yellow Zinc Dichromate (YZD) is a cost-effective alternative to the previous treatments given. YZD can be thought of as metal paint. The coating will be removed in the contact area by the bearing very quickly. The break-in period between the bearing and the base material will begin after the YZD has been worn away. There will be some added wear resulting from worn away YZD acting as contamination. Seasonal equipment will also have a possibility of pin corrosion at the contact area. The transferred PTFE will act as a corrosion inhibitor if a sufficient amount has been transferred during the first season of operation. Most designers say the main purpose for YZD is corrosion protection for the surface not in contact with the bearing. The result is a decrease in the migration of metal oxide particles into the joint compared to an untreated pin.