The work aims to investigate the significant disparity in surface damage observed between anode and cathode slip rings in hydroelectric generator excitation systems. Despite experiencing identical mechanical loads and environmental conditions, these components exhibit markedly different wear patterns due to fixed current directionality. By comprehensively comparing performance parameters, including friction coefficient, wear rate, contact resistance, interface temperature, and surface morphology under two opposing polarity configurations (steel(+)-carbon(-) and steel(-)-carbon(+)), the fundamental understanding of how current polarity governs the current-carrying wear characteristics of steel/carbon friction pairs is established, providing theoretical guidance for engineering applications. A custom-designed pin-on-disc current-carrying tribometer was employed to simulate actual operating conditions of hydrogenerator collector rings. The experimental setup enabled precise control of electrical and mechanical parameters while simultaneously monitoring tribological and electrical responses. The friction pair consisted of 45# steel disc and D172 electrographite pin. Comparative tests were conducted under carefully selected parameters that reflected practical operating conditions: current density of 10 A/cm2, normal load of 1.5 N, sliding speed of 0.419 m/s, and test duration of 40 minutes. This parameter set ensured sufficient wear progression to reach steady-state conditions while remaining within realistic operational boundaries. Real-time data acquisition captured dynamic variations in friction coefficient, contact voltage drop, and interface temperature throughout the testing period. Post-test analysis employed scanning electron microscopy (SEM) for surface morphology characterization, 3D profilometry for quantitative topographical assessment, and energy-dispersive X-ray spectroscopy (EDS) for elemental composition mapping of worn surfaces. The experimental findings revealed profound polarity-dependent effects. Under steel(+)-carbon(-) configuration, the average friction coefficient reached 0.434, contact resistance measured 1.03 Ω, and interface temperature stabilized at 59.342 ℃. In contrast, the steel(-)-carbon(+) configuration yielded significantly lower values: friction coefficient of 0.349 (24.2% reduction), contact resistance of 0.869 Ω (18.5% reduction), and interface temperature of 49.897 ℃ (17% reduction). More importantly, the positive steel configuration exhibited substantially higher signal fluctuations, indicating unstable contact conditions. The wear behavior showed dramatic asymmetry: the anode carbon brush (in steel(-)-carbon(+) configuration) experienced a wear rate 2.24 times greater than the cathode brush (in steel(+)-carbon(-) configuration). Microstructural investigations elucidated the underlying mechanisms: when steel served as anode, its surface developed a thick, uneven oxide layer with oxygen content reaching 9.8% (compared to 1.9% for cathodic steel). This oxide layer acted as an abrasive medium, increasing friction and generating particulate debris. Concurrently, iron ions migrated directionally from the anodic steel to the cathodic carbon brush, forming an iron-rich protective layer that reduced carbon wear. When steel functioned as cathode, it remained in a reduced metallic state with minimal oxidation, providing superior electrical conductivity and friction stability. Current polarity fundamentally alters interfacial electrochemical reactions, material transfer phenomena, and wear mechanisms in steel/carbon current-carrying contacts. The steel(-)-carbon(+) configuration delivers superior electrical contact stability and lower friction coefficients but accelerates carbon brush wear through electrochemical oxidation. Conversely, the steel(+)-carbon(-) configuration protects the carbon component but promotes severe steel oxidation, leading to performance degradation and potential failure. For applications prioritizing system reliability and metallic component longevity, such as hydroelectric generator excitation systems, connecting the steel collector ring to the negative terminal (cathode) is strongly recommended, despite the accelerated carbon brush consumption which can be managed through scheduled maintenance.
Key words
current polarity /
current-carrying friction /
steel/carbon friction pair /
contact characteristics /
electrochemical oxidation
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Funding
The National Natural Science Foundation of China (52475202)
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