Anodization: Recent Advancements on Corrosion Protection of Brake Calipers (SAE Paper 2020-01-1626)

Abstract

Brake calipers for high-end cars are typically realized using Aluminum alloys, with Silicon as the most common alloying element. Despite the excellent castability and machinability of Aluminum-Silicon alloys (AlSix), anodization is often required in order to increase its corrosion resistance. This is particularly true in Chlorides-rich environments where Aluminum can easily corrode. Even if anodization process is known for almost 100 years, anodization of AlSix -based materials is particularly challenging due to the presence of eutectic Silicon precipitates. These show a poor electric conductivity and a slow oxidation kinetics, leading to inhomogeneous anodic layers. Continuous research and process optimization are required in order to develop anodic layers with enhanced morphological and electrochemical properties, targeting a prolonged resistance of brake calipers under endurance corrosive tests (e.g. >1000 hours Neutral Salt Spray (NSS) tests). In this manuscript a lab-scale anodization setup is used to investigate the interplay between process parameters, oxide layer morphology and corrosion protection capability. The influence of high anodization steps (AS) and low rest steps (RS) in pulsed anodization waveforms is investigated with respect to the homogeneity and compactness of the obtained oxide layers. In comparison with a conventional set of anodization parameters, which is taken as a standard, the following level of performance are achieved: 1) increase of the corrosion potential (Ecorr) of +98mV; 2) increase of the anodic breakdown potential (Ebp) of +362mV; 3) reduction of the corrosion rate of a factor six; and 4) a polarization resistance 1.5 times higher. This work identifies key parameters in the anodization of Aluminum-Silicon alloys and propose new electrochemical figures of merit in order to: a) extend the corrosion resistance of future braking systems; and b) evaluate ex-situ the anodic layer electrochemical performance.