Dr. Federico Bertasi, Brembo S.p.A., ITALY

Dr. Marco Bandiera, Brembo S.p.A., ITALY

Dr. Alessandro Mancini, Brembo S.p.A., ITALY

Dr. Arianna Pavesi, Brembo S.p.A., ITALY

Dr. Andrea Bonfanti, Brembo S.p.A., ITALY

Prof. Massimiliano Bestetti, Politecnico di Milano, ITALY

Anodization plays a pivotal role in improving the corrosion resistance of Aluminum-Silicon alloys (AlSix) used in the production of brake calipers.[1] However, the presence of eutectic Silicon particles within the Al matrix can reduce the oxide layer growing rate, leading to inhomogeneous and porous coatings. Following this, tailored current/potential anodization waveforms have been developed, in order to overcome the presence of Silicon, thus obtaining anodic layers with enhanced morphological and corrosion-resistance features.[2][3]

In this scenario, a fervent lab-scale R&D activity has been carried out regarding the optimization of pulsed anodization in terms of current density and frequency of the used square wave, obtaining: 1) coated AlSix specimens (30cm2) showing a superior corrosion resistance; and 2) a set of refined anodization parameters to be used to treat AlSix –based materials.[4] Unfortunately, anodization of a prototype caliper, using the obtained optimized waveforms, is not straightforward and appears particularly more challenging with respect to the lab-scale treatment of small specimens. Indeed, the presence of: a) non-uniform Silicon distribution (machined vs. non-machined regions); and b) shielded areas and/or sharp edges; can strongly influence the oxide growth, leading to inhomogeneous coatings and a morphology-dependent corrosion resistance.

As a further step toward the implementation of the optimized parameters in an anodization pilot plant, an electrochemical bath is designed, aiming at: 1) anodize a single brake caliper; and 2) scale up the anodization parameters from specimens to caliper treatment. The manuscript will discuss the so-obtained anodized caliper in terms of oxide layer: a) morphology; b) wettability; and c) corrosion resistance. The effect of optimized vs. non-optimized parameters will be discussed as well. Results allow to outline the path for an advanced anodization process, that will briefly lead to obtain AlSix brake calipers with an extended corrosion resistance.

References:

[1] Bandiera, M., Bonfanti, A., Mauri, A., Mancini, A., Bestetti, M., Bertasi, F., “Corrosion Phenomena in Braking Systems”, CORROSION/20, Manuscript no. C2020-14550, 2020.

[2] Bandiera, M., Bonfanti, A., Bestetti, M., Bertasi, F., “Anodization: Recent Advancements on Corrosion Protection of Brake Calipers”, SAE Technical Paper, Manuscript no. 2020-01-1626, 2020.

[3] Fratila-Apachitei, L. E., J. Duszczyk, and L. Katgerman. "AlSi (Cu) anodic oxide layers formed in H2SO4 at low temperature using different current waveforms", Surface and Coatings Technology, 165.3, pp. 232-240, 2003.

[4] Bandiera, M., Mancini, A., Pavesi, A., Bonfanti, A., Bestetti, M., Bertasi, F., “Optimized Pulsed Anodization for Corrosion Protection of Aluminum Silicon Alloys”, CORROSION/21, Manuscript no. C2021-16431, 2021. (under review).

Dr. Federico Bertasi, Brembo S.p.A., ITALY

Dr. Marco Bandiera, Brembo S.p.A., ITALY

Dr. Arianna Pavesi, Brembo S.p.A., ITALY

Dr. Andrea Bonfanti, Brembo S.p.A., ITALY

Dr. Alessandro Mancini, Brembo S.p.A., ITALY

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.