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The role that Al3normalFe intermetallic inclusions play during initiation of pitting corrosion on aluminum alloys was investigated in 0.6 M NaCl. In aerated solutions microscopic observations showed the growth of cavities in the host metal adjacent to inclusions. The rest potential of synthetic Al3normalFe was measured in aerated and deaerated NaCl solutions over a range of bulk pH values between 2 and 12 and was found to act as a cathode. In aerated solutions rotating disk electrode experiments on synthetic Al3normalFe electrodes verified that the cathodic reaction corresponded to the reduction of dissolved oxygen. With scanning pH microelectrodes, measurements were carried out near synthetic Al3normalFe electrodes which were coupled in a galvanic cell with Al-6061 in NaCl solution. In addition, the pH measured over Al-6061 at a distance of 25–30 μm from the surface was observed to fluctuate between pH 4 and 8.5 for the first 2 h of immersion in NaCl solutions. Atomic force microscopy images of Al-6061 immersed in buffered aerated 0.6 M NaCl solution (pH 5.5) did not show any evidence of dissolution of the Al around intermetallics. These measurements support the view that Al3normalFe particles in Al-6061 serve as local cathodes, that a high pH develops around the intermetallic particles and creates cavities in the host metal, and that interactions among a cluster of intermetallic particles causes a large number of slowly dissolving alkaline cavities to evolve into a small number of rapidly dissolving acidic pits. © 1999 The Electrochemical Society. All rights reserved.

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The dissolution of MnS inclusions in stainless steels (SS) 303 and 304 during initiation of pitting corrosion in an aqueous solution containing 10 mM KI and 0.1 M NaCl was investigated with use of a scanning electrochemical microscope (SECM). The dissolution products of MnS were detected amperometrically at a carbon fiber SECM tip using I Ϫ /I 3 Ϫ as the redox mediator. SECM images showed that the sulfur species are slowly generated above previously identified MnS inclusions. The SECM tip current was stable up to 100 min, depending on the potential applied to the SS 304. SECM was found to be useful in mapping local concentration distributions of sulfur and their evolution in time during dissolution.

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In situ atomic force microscopy was used in conjunction with microlithography and scanning Auger electron spectroscopy to monitor localized corrosion near iron-rich inclusions in AI-666 I-T6 immersed in 0.6M NaCI and also sulfur=rich inclusions in 304 stainless steel (SS-304) in 0.5M NaCI. The local rate of aluminum corrosion was found to depend on the shape of the nearby iron-rich inclusion. At the corrosion potential, trenches were observed to form in the aluminum host matrix adjacent to the inclusions, and the corrosion sites gradually evolved into circular shapes owing to dissolution. During the dissolution process, the width of the dissolution area was an order of magnitude greater than the depth. Application of a 406 mV cathodic overpotential prevented corrosion initiation, while application of a 500 mV cathodic overpotential greatly accelerated the dissolution rate in comparison with that at the rest potential. On SS-304, exposure to 0.5M NaCl was accompanied by formation of deposits, which decorated the inclusion surface as well as the surrounding area up to four times the radius of the original inclusion.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.210.126.199 Downloaded on 2015-06-16 to IP

Ion-selective pH microelectrodes were used to measure hydroxide ion concentration fields produced by electrochemical reduction of oxygen on 10 and 25 p.m Pt disk electrodes. The sensors were mounted on a scanning system, and the hydroxide concentration profiles over the disk electrodes were obtained. Data were compared with predictions based on the diffusion equation. The pH microelectrode was also used to verify the buildup of hydroxide ions generated by cathodic reaction on Al3Fe that was in electrical contact with Al 6061 at the rest potential in 0.6 M NaCI.Investigation of corroding surfaces requires techniques that can directly probe local electrochemical activity and chemical composition with high spatial resolution at the microscale. While there exist techniques that provide morphology information on localized corrosion, such as pit initiation around the AI3Fe intermetallics in aluminum alloy 60611 and MnS inclusions in type 304 stainless steel,2-3 there is a need for additional methods to characterize the local chemical environment during localized corrosion. Microelectrodes have been utilized in corrosion studies often because they provide local electrochemistry in the vicinity of corrosion sites without significant perturbation of local events.4 Scanning electrochemical microscopy (SECM) has been also used to obtain electrochemical measurements with high spatial resolution.5-6 Recently, ion-selective microelectrodes were employed as scanning sensors to image concentration profiles of several ions.7 In the present work, pH microelectrodes were calibrated in an electrochemical system to investigate their suitability for corrosion studies.8-9Experimental Capillary tubes were prepared following the procedures of Wen and Oakley.° A double-barreled capillary tube (theta configuration 1ST 150-6, 1.5 mm od, World Precision Instruments) was pulled (P-77, Sutter Instruments) to a tip diameter of --5 to 7 p.m and bevelled with a micropipette beveler (Suffer Instrument Co.). Figure 1 shows a scanning electron microscopic image of a typical doublebarreled microelectrode tip used for pH measurements. The ion selective barrel was silanized and then filled by capillary action with hydrogen sensitive liquid neutral charge carrier (Fluka 95293) and 0.6 M NaCI solution. The reference electrode barrel was filled with 0.6 M NaCL. Ag/AgCI wires (250 p.m diam) were inserted into both barrels. The sensor potential was measured with an Axoprobe-1A microelectrode amplifier. Sensors were calibrated in a set of solutions prepared from 0.6 M NaCI and adjusted for pH by addition of H2S04 or NaOH.Sensors were positioned 370 from the normal direction (z) and maneuvered along the surface (the x-y plane) by a pair of stepper motors with 1 p.m resolution (Oriel 20010) and in the z-direction by an inchworm controller (Burleigh PZ-550) with 2 p.m resolution. Calibration studies were carried out by making pH distribution

Early stages of pitting corrosion in the vicinity of sulfide inclusions on Ni200 were observed by several techniques including scanning electron microscopy, Auger electron spectroscopy, microelectrochemical cell, and scanning electrochemical microscopy ͑SECM͒. These data provide information on the evolution of shape during early stages of dissolution along with elemental distribution over the surface, potentiodynamic behavior associated with individual inclusion sites, and the spatial/temporal distribution of local solution composition. It was found that pits were most likely to form near sulfur-rich inclusions, that a sulfurcontaining layer was formed on the walls of pits grown around such inclusions, and that the spatial distribution of the sulfurcontaining layer was dependent on fluid flow. The pitting potential exhibited a dependence upon the concentration of chloride and thiosulfate, as well as upon the presence of fluid flow. SECM data demonstrated that dissolved sulfur-containing species accumulated locally near sulfide inclusions before pitting began as well as during pit growth. In the presence of fluid flow, sulfide inclusions exhibited sulfur-containing region ͑tail͒ on the downstream side. As flow rate increased, the pit tail became slimmer and longer. Pits with diameters less than about 5 m did not exhibit a pit tail.