Friction Materials: Best Practices for the Evaluation of Corrodibility and Corrosion Mechanism

Abstract

Investigation of the corrosion performance of friction materials (FMs) plays a central role in: a) evaluating the corrosion resistance of braking pads [1]; b) elucidating galvanic couplings among different braking system components [2]; c) designing FMs with a negligible sticking effect upon coupling with a cast iron brake disc [3]. Corrosion performance can be evaluated by measuring proper electrochemical figures of merit [4] such as the: 1) corrosion potential (Ecorr); 2) corrosion current (Icorr); and 3) galvanic current (Igc); in agreement with a suitable test specification. Nevertheless, particular attention should be paid when measuring these quantities since several experimental details can lead to inaccurate or misleading results. At this regard, the work clarifies the effect of several test parameters (e.g. scan rate, stabilization time, potential ranges, etc.) on the measure of the corrodibility of a reference friction material. Most common errors and their effect on the electrochemical figures of merit are discussed, with the final aim of providing a solid guideline for designing braking pads with a reduced corrodibility. References:[1] Bertasi, F., Mancini, A., Bandiera, M., Pin, S., Casini, A., Bonfanti, A., “Interplay between Composition and Electrochemical Performance at the Pad-Disc Interface”, EUROBRAKE, Manuscript no. EB2019-MDS-018 (Stansted, UK: FISITA, 2019), p. 1.[2] Bertasi, F., Bandiera, M., Bonfanti, A., “Toward a Corrosion Proof Braking System”, SAE Technical Paper, Manuscript no. 2020-01-1625, 2020.[3] Bandiera, M., Mancini, A., Bonfanti A., Pavesi, A., Pin S., Bertasi, F. “Sticking Phenomena at the Brake Pad – Disc Interface: An Open Call for Electrochemists”, CORROSION/21, Manuscript no. C2021-16429, 2021. (under review).[4] Bandiera, M., Pin, S., Bonfanti, A., Bertasi, F., Mancini, A., “Physico-Chemical Characterization of Corrosion Scales in Braking Systems”, CORROSION/20, Manuscript no. C2020-14687, 2020.

Corrosion Stiction in Automotive Braking Systems

“…Since the stiction is controlled by corrosion phenomena at the pad–disc interface, several works in the literature explored the use of electrochemical methods for the investigation of the susceptibility to stiction of different friction materials [ 3 , 61 , 63 , 64 , 65 ]. In addition, a testing method based on electrochemical methods was recently patented by Sin et al [ 66 ].…”

“…In contrast, the application of electrochemical methods to investigate friction materials is a relatively new approach. Bandiera et al [ 63 , 64 , 65 ] performed open-circuit potential and linear-sweep voltammetry measurements on friction materials employing a typical three-electrode configuration. The friction material without an underlayer was the working electrode in the cell, graphite was employed as the counter electrode, and the reference electrode was a saturated calomel electrode [ 63 ].…”

“…Bandiera et al [ 63 , 64 , 65 ] performed open-circuit potential and linear-sweep voltammetry measurements on friction materials employing a typical three-electrode configuration. The friction material without an underlayer was the working electrode in the cell, graphite was employed as the counter electrode, and the reference electrode was a saturated calomel electrode [ 63 ]. Unfortunately, it is not indicated how the electrical connection of the friction material was made or if specific actions were taken to avoid wetting of the electrical connection during the electrochemical tests.…”

“…Unfortunately, it is not indicated how the electrical connection of the friction material was made or if specific actions were taken to avoid wetting of the electrical connection during the electrochemical tests. The testing electrolyte was a chloride-containing aqueous solution [ 63 ]. It is specified that large areas of the working electrode (at least 20 cm 2 ) are necessary to avoid artifacts related to the heterogeneous composition of the friction material.…”

“…The presence of copper in the friction materials has often been regarded as a possible source of galvanic effects with gray cast-iron rotors due to the more noble open-circuit potential of copper relative to iron in different electrolytes [ 58 , 62 , 63 ]. Tigane et al [ 58 ] investigated the stiction behavior of NAO friction materials containing copper in contact with gray cast iron.…”