Investigation of corrosion of cast aluminium alloy piston in brake fluid O.O Ajibola*1,2, D.T Oloruntoba3 and B.O Adewuyi3 1 Department of Materials and Metallurgical Engineering, Federal University Oye‐Ekiti, Nigeria 2 Department of Chemical Engineering Technology, University of Johannesburg, South Africa. 3 Department of Metallurgical and Materials Engineering, Federal University of Tech., Akure, Nigeria
*Correspondence:
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ABSTRACT
Keywords: Corrosion Cast aluminium Alloy substrate Brake fluid Moisture Brake Master cylinder
The work studied corrosion of cast aluminium alloy specimens in hydraulic brake fluid with and without moisture. Aluminium scrap was sourced from hydraulic brake master cylinder pistons. Cast piston (CP) specimens were produced by melting; poured at a temperature of 800±50 oC, sand cast and systematically cooled. Samples were immersed in brake oil with and without 1% water. The corrosion of cast pistons CP were compared with purchased pistons (PP) or (control sample) in brake fluid with and without moisture. The corrosion rates, electrode potential EP and pH are measured. In all cases, test piston samples corrode less in brake oil containing 1% water. Cast piston sample CP was more aggressive in both media than purchased pistons (PP) samples. Hence, the production of pistons form scrap by cottage aluminium industries for use in the hydraulic brake master cylinder should be discouraged; because PP and CP have 0.0292 & 0.0334 mg/mm2/yr wear rates in oil only while the wear rates are 0.0157 & 0.0159 mg/mm2/yr in oil plus 1% water. On the other hand, when considering the reliability of the mechanical properties (hardness and strength) of cast aluminium alloy, surface protection such as electroplating and electroless‐ nickel plating may be optional.
Received: 24 October, 2016 Accepted: 21 December, 2016
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Introduction
Literature reports are scarcely obtained on corrosion of aluminium alloys in hydraulic fluids. An ideal brake fluid should not compress or corrode metal parts in it (http//:www.sae.org/servlets/product). It is unfortunate that when brake systems are not perfectly sealed and air penetrates and with time, the fluid absorbs moisture in the air. This reduces the boiling point of the fluid and makes it more compressible at high temperatures. The absorption of moisture into the fluid increases the tendency for corrosion (Bosch, 2007). The brake oil has its pH ranging from 10.0 to 11.5 (Dot Vol.2 No.2 2016
Chemicals, 2009); making it have a strong potential to attack aluminium alloy components in the brake system. Protection against corrosion and compressibility are made by adding formulated quantity of inhibitors to the base fluid (wiki/brake fluid.) The choice of quality material plays a prominent role in the performance and life span of any machine component under service (Degarmo et al. 2003; Egunlae, 2008). To this effect, a very large quantity of materials of high economic value are lost to wear and corrosion (Ajibola and Jimoh, 2014; Ajibola and Oloruntoba, 2014). Among the types of corrosion of aluminium alloys that have
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Investigation of corrosion of cast aluminium alloy piston in brake fluid been reported in the literature are the crevice, fretting, erosion and tribocorrosion (Wood, 2007; Pokhmurskii et al, 2014), and uniform corrosion. The general dissolution is the most prominent in aluminium alloy corrosion (Adewuyi, 2002; Blucher et al. 2006, Fasuba et al, 2011). Aluminium alloys get extensively corroded mostly in strongly acidic, basic salt or strongly alkaline solutions though there are specific exceptions such as 4043, 5052, 5183, 6005A and so on (Adewuyi, 2002; Ajibola et al, 2012 ‘a’; Ezuber et al, 2008; Fasuba et al, 2011). Since early 2010, Ajibola and Jimoh, and Ajibola et al, had been involved in understudying the properties, application and problems of wear and corrosion of cast aluminium alloys used in brake master cylinders with hydraulic fluid. Three prevailing conditions have been identified to be causes of wear losses in the hydraulic brake master cylinders: the contacting surfaces of aluminium alloy pistons and the wall of cylinder without fluid (mechanical wear); and with fluid when the cylinder is engaged (tribocorrosion); and chemical influence of brake fluid on both parts when the automobile is left or parked for very long period (corrosion), (Ajibola et al, 2014 ‘a ‐ c’). Hence, the present work investigates the corrosion of cast aluminium alloy substrate in hydraulic brake fluid. The reports by Ajibola et al, 2014 ‘b ‐ d’ suggested the application of surface treatment and overlay of durable metallic electroless nickel film as remedy to reducing wear and corrosion of this functional machine part in brake oil. The reports of such findings have been well received within the metallurgical and materials engineering research community (Ajibola and Jimoh, 2011; Ajibola et al, 2012 ‘b’; Ajibola et al, 2014 ‘a – d’). Table 1: Chemical compositions of alloy samples. Samples purchased As‐received Cast Al
Al 98.6 98.97 98.44
Si 0.68 0.38 0.32
Mg 0.4 0.40 0.29
Fe 0.2 0.13 0.16
Mn 0.001
