Experimental Investigation of Corrosion of Cast Iron Rotors in the Automotive industry
Electric and hybrid vehicles use regenerative braking, where application of the brake triggers the electric motor to drive as a generator to produce electricity, which in turn charges the battery. Although allowing use of up to ~60% of the kinetic energy of a vehicle, this ?electric braking? action is usually combined with the use of a friction brake, often called foundation brake, in the ?blended regime.? This not only saves energy, but results in much less frequent use of the friction brake and correspondingly reduces brake wear/pollution. The less frequent friction engagements, however, lead to changes of the corrosion and wear behavior, also changes in the performance of the friction brake [can you add a reference here?, if not just ignore, also further requests]. Numerous strategies have been adopted in order to eliminate the negative impact of corrosion, and this paper addresses the behavior of the grey cast iron rotors (ASTM A48, C30) tested in in 3.5wt% NaCl standardized solution after being subjected to four different conditions: i) virgin uncoated rotor, ii) virgin coated rotor (ceramic coat ), iii) friction tested uncoated, and iv) friction tested coated rotors, respectively. After a series of previous studies, when compared to automotive brake dynamometer standardized tests [2-4], the scaled-down SAE J2522 procedure and UMT Tribolab by Bruker) were adopted for friction tests with three types of pads: a) semi-metallic, b) low-metallic, and c) non-asbestos organic. The corrosion study has been performed using two different electrochemical methods: 1) cyclic voltammetry (CV), and 2) electrochemical impedance spectroscopy (EIS). Surfaces of corrosion tested samples were studied by scanning electron microscopy (SEM Quanta 450), equipped with X-ray electron microanalysis (EDX, Oxford Instruments, Inca software). CV experiments indicate a clear difference when obtained data are compared. Different materials exhibit different shifts in the corrosion potential and corrosion current density due to changes in the nature of the exposed surface. Both CV and EIS methods offered identical rankings when the measured corrosion resistance was compared. The least resistant is the virgin cast iron, followed by ceramic coated virgin material, and the friction tested samples, exhibiting very similar corrosion resistance to the virgin coated samples. This indicated that the friction process and related wear do not destroy the protective layer resulting from coating by ceramics. Nevertheless, the detected resistance of the friction tested samples to corrosion also differed. The coated rotors tested against the semi-metallic samples (SM) corroded more readily than the coated rotors friction tested against low-metallic (LM) pad samples, and the best and considerably lower corrosion resistance was detected in the systems employing the coated rotors tested against the non-asbestos organic (NAO) pads. SEM/EDX revealed that, the complex friction layer was formed on the surfaces of both tested pads and, importantly, rotors. These layers are not representing absolute barriers to the transfer processes (corrosion) but their chemistry differed. The character of the brake pad, and the resulting friction layer created by rubbing the brake pad against the rotor obviously plays a role in their corrosion resistance. The SEM/EDX revealed that the SM and LM pads, containing steel chips, also transferred ?corrosion active? ferrous constituents on the (coated) rotor surfaces, and were seen as more ?aggressive?