Local Evolution of pH with Time Determined by Shear Force‐based Scanning Electrochemical Microscopy: Surface Reactivity of Anodized Aluminium

Abstract

1IntroductionIn order to enhance the corrosion resistance of aluminium alloys [1] used in the aircraft industry,amulti-layer coatings ystem is formed on the metallic surface.T he first step generally consists in ana nodizing process performed in an acidic electrolyte such as sulphuric acid [2,3].T he resulting anodizedl ayer is constituted by an etwork of nanosized pores. Then this porous anodized layer is sealed in as olution containingc ompounds acting as corrosion inhibitors.C hromate is the most efficient one thanks to as pecific property of corrosion self-healing [4][5][6].H owever,d ue to its carcinogenic toxicity [7],c hromate should be banned from 2017 in the European Union.R ecently,n ew sealing solutions containing chromium(III) and zirconium(IV)s pecies have been developed and seem to be an interesting alternativet ou sual chromates [8][9][10].Nevertheless,t he efficiencyo ft he sealing processes mainly depends on the surface reactivity of the nanoporous oxide structureo nt he different anodized aluminium alloys.I nt his framework, the purpose of this work is to in-situ determine the time course of the pH change in the sealing solution, near the surface of the nanoporousa nodized structure formed on 7175 aluminium alloy.Scanninge lectrochemicalm icroscopy (SECM) was chosen here to monitor the pH at this interface using ap Hm icroelectrode.W hile SECM is historically based on monitoring the amperometric response of microelectrode [11],p otentiometric probes have rapidly been used for monitoring pH or metalc ation concentration at inter-faces [12][13][14] and applied to the analysis of metal surface treatment [15] and corrosion [14,[16][17][18][19][20]. In contrast to amperometric probes,m ost potentiometric probesc annot be positioned accurately in solution at ac ontrolled distance from the interface to be analyzed [ 21,22].I nt he present work, an additional difficulty is the positioning of the pH sensitive electrode with asufficient accuracy in air before the solutioni si ntroduced in the electrochemical cell. In this specific case, neither potentiometric nor amperometric response of microelectrode can be usedt op osition it near to the aluminium alloy surface.Shear force controlf or SECM (SF-SECM) is one of the technologies that has been proposedf or positioning am icroelectrode at ad efined distance from as ample independently from the electrochemical measurement [23][24][25].D etermination of local pH changes in shear forcebased SECM have been performede ither with iridium oxide based pH-sensor [17,20] or membrane-based ionselective electrodes [22].I nc orrosion studies,s hear force Abstract:S hear force-based scanning electrochemical microscopy is applied for pH monitoring during asealing reaction at the interface between anodized 7175 aluminium alloy and as olution containing Cr(III) and Zr(IV) species.p Hm easurement is carriedo ut with the electrode positioned near enough to the surface to be in the diffusion zone of the liquid/solid interface,i nt he reach of the surface-generated pH modulati...

The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

“…However, a recent publication by Etienne et al [271] reports some information on the local pH evolution with time 10 to 20 µm above anodized aluminum surfaces in selected sealing solutions in which local pH measurements in the first 2 to 5 minutes manifested increase in pH by initial bulk pHs of 3.4 and 4.6 the measured pH maxima were ≈5.5 and >5.5 respectively but decreased to values tending towards the initial bulk values during 20 min. Runge [272] had highlighted the existence of a pH profile across the depth of anodic oxide layer with pH increasing with depth during anodization.…”

Efecto del tratamiento de sellado en el comportamiento frente a corrosión de la aleación anodizada de aluminio-litio AA2099

“…In order to reduce the Al 2 O 3 layer porosity, two sealing solutions (H 2 O and 6 wt.% Na 2 Cr 2 O 7 ) were used to reduce the diameter of the pores. The efficiency of the sealing process depends mainly of the surface reactivity and structure of the passive layer (Etienne et al, 2016;Yang et al, 2019;Yu et al, 2020).…”

“…The anodizing process is used mainly for the treatment of Al alloy surfaces for industrial applications. However, in the aeronautic industry, such electrochemical processes are implemented because Al alloys such as 2xxx and 7xxx have a wide range in aerospace applications due to high strength and low density, allowing for an increase in payload and fuel efficiency (Keller et al, 1953;Etienne et al, 2016;Ma et al, 2016;Jaimes-Ramírez et al, 2018;Zhang et al, 2020).…”

“…The sealing treatment can be carried out through different solutions such as water, sodium dichromate (Na 2 Cr 2 O 7 ), nickel acetate, and nickel fluoride. The efficiency of a sealing process depends on the surface reactivity of the porous oxide structure in the different anodized Al alloys (Etienne et al, 2016;Runge, 2018). Sodium dichromate is considered to be one of the most effective sealing methods for corrosion prevention because it not only blocks pores, it also acts as a corrosion inhibitor, a specific property of self-healing corrosion (Zuo et al, 2003;Etienne et al, 2016;Yu et al, 2020).…”

Direct manipulation of metallic nanosheets by shear force microscopy

“…The detection of the changes is generally realised by attaching the probe to a mechanical oscillator, such as a tuning fork (Karrai & Grober, 1995;Schmidt et al, 2000), piezoelectric tube (Hsu et al, 1995;Lee et al, 1996) and bimorph (Shang et al, 2004;Shang et al, 2005;Lei et al, 2006), forming a shear force sensor. By contrast with the optical detection of cantilever deflection in SFM, remarkable advantage of such shear force sensors is their self-sensing principle of operation, which dispenses with the need for laser deflection or external excitation, and readily enables the integration of ShFM system into SEMs (Bercu et al, 2016) and scanning electrochemical microscopy (Etienne et al, 2016) . These integrated techniques have been a prized means of realtime and in situ observation for nanocharacterisation.…”