51314-4078-In Situ Crevice Corrosion of Nickel-Aluminium Bronze Using a Boron-Doped Diamond Microelectrode Array

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Product Number: 51314-4078-SG

ISBN: 4078 2014 CP

Author: Julian Wharton

Publication Date: 2014

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This investigation demonstrates the ability of a boron–doped diamond microelectrode (BDDME) to assess in situ metal–ion concentration from nickel–aluminium alloy (NAB) crevice corrosion. The BDD substrate was utilised for its outstanding chemical and physical properties such as a wide potential window low capacitance current and high stability. The BDDME (consisting of five electrodes with a diameter of 500 ?m) was located inside a crevice former which in turn was immersed in a 0.6 M NaCl solution. Differential pulse voltammetry (DPV) was carried out hourly on the BDDME sweeping the potential in either the anodic or cathodic directions. In the absence of cathodic protection extensive crevice corrosion has been experienced on NAB when immersed in chloride containing solutions [1]. As opposed to other copper–based alloys where generally the corrosion attack takes place at the exposed surfaces immediately adjacent the crevice [1] the crevice corrosion occurring at NAB has been reported to proceed through a selective phase attack and a microstructure dealloying as the crevice solution pH varies due to the different phase stabilities present within its complex microstructure [2]. In particular it has been reported that above pH 4 the NAB corrosion behaviour is controlled by the oxidation of the copper–rich alpha–phase whereas below pH 4 the NAB corrosion is driven by the dissolution of the aluminium– iron– and nickel–rich kappa(I) kappa(II) and kappa(IV)–phases [2]. The DPV within two weeks of NAB immersion showed only the presence of copper(I) within the crevice. The copper(I) concentration ranged from zero to ca. 0.3 mM after the first week and attained a concentration plateau (approx. 0.4 mM) over the second week of immersion. The concentration of other metal–ions such as copper(II) iron(III) or nickel(II) were determined to be negligible via either BDDME or ex situ capillary electrophoresis. The negligible concentration of copper(II) confirmed the low oxygen concentration which diffuses slowly within the crevice whereas the absence of metal-ions such as Al(III) Fe(II) Ni(II) indicated that during the monitoring time the local pH did not decreased below 4 where the NAB kappa–phases undergo corrosion [2]. Overall the in situ electrochemical testing at the BDDME allowed the NAB crevice corrosion mechanism to be closely defined via assessment of the metal ion concentration.[1] J.A. Wharton K.R. Stokes Electrochim. Acta 53 (2008) 2463-2473.[2] S. Neodo D. Carugo J.A. Wharton K.R. Stokes J. Electroanal. Chem. 695 (2013) 38-46.