Design of New Multifunctional Galling-Corrosion Testing Apparatus

Abstract

Galling of materials is a severe surface damage characterized by macroscopic material removal when two solid surfaces experience relative sliding under high load. This mode of wear is of particular interest in applications where design constraints require assembly components to slide against each other without adequate lubrication. Examples are high-pressure ball valve with metal-to-metal seal contact, sheet metal forming process and medical/food processing instruments where lubrication is sometimes non-existent due contamination concerns. In addition to sliding at high load, materials in these applications can be exposed to aggressive environments which lead to combined tribological and corrosive degradation phenomena. Progress in this field depends largely on the understanding of the interaction between corrosion and wear processes. In this paper, we describe a new testing instrument capable of testing galling and sliding wear behavior under dry and wet conditions. Design of this instrument is guided by ASTM standards G98 for galling, G99 for pin-on-disk, G133 for ball-on-flat and G199 for tribocorrosion. Galling configuration consists of two concentrically aligned hollow cylindrical specimen loaded along their longitudinal axis with constant load (max 5000 N). The upper specimen is held stationary while the lower specimen is rotated a number of cycles at a selected speed. During the test, the friction torque is measured using special torque/thrust load cell. For the combined galling/corrosion test, the specimen’s contact surface is submerged in an electrolytic solution and three-electrode type electrochemical cell is set-up using the specimens as working electrode. Galling/corrosion tests can be performed under open circuit potential or under controlled potentials. In the first type, the corrosion potential is monitored before, during and after rubbing, whereas in the second type, the corrosion potential and corrosion current are controlled before, during and after rubbing. In addition to galling, the instrument can also be used to conduct pin-on-disk and ball-on-flat tests under dry and wet conditions.