Ni-Mo nanocrystalline layers were electrodeposited using direct current from citrate-ammonia solutions. The quartz crystal microbalance investigation confirms that the discharge process starts with hydrogen evolution before the onset of the alloy deposition. The grain size was estimated from X-ray line broadening. It decreases when the molybdenum content is increased. It is smaller for layers deposited at pH 9.5 than 8.5. The microhardness exhibits a maximum close to 800 Vickers for s Mo around 17 wt%. For higher s Mo a softening is observed showing a deviation from Hall-Petch behaviour due to small grain size. In deaerated hydrochloric solutions, the layers show a large passivation domain without any pitting. The corrosion currents as well as the passivation currents, higher than for the bulk Hastelloy B alloy, decrease when s Mo is increased.

Electrochemical impedance spectroscopy experiments were performed on a microdisk electrode in a thin-layer cell using a scanning electrochemical microscope for controlling the cell geometry. Experimental data showed that when the thin-layer thickness diminished, an additional low-frequency response appeared. It was ascribed to the radial diffusion of the electroactive species and was strongly dependent on the thin-layer dimensions (both thickness and diameter). Moreover, the numerical simulation of the impedance diagrams by finite element method calculations confirmed this behavior. An equivalent circuit based on a Randles-type circuit was proposed. Thus, the diffusion was described by introducing two electrical elements: one for the spherical diffusion and the other for the radial contribution. A nonlinear Simplex algorithm was used, and this circuit was shown to fit the impedance diagrams with a good accuracy.

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The local breakdown of the iron passive layer and the resulting pitting corrosion were studied with a coupled scanning electrochemical microscopy/electrochemical quartz microbalance technique ͑SECM/EQCM͒. The initiation and the propagation of a single pit on iron deposited on the quartz gold electrode were controlled through the local production of chloride anion thanks to the silver chloride reduction at the tip of the scanning electrochemical microscope. The frequency response of the quartz was correlated to the pit evolution with respect to time. After the breakdown of the passive layer, the frequency changes were then directly linked to the generation of Fe͑II͒ during the pit growth. It was also shown that in 0.1 M NaOH solution, the local amount of Cl − necessary to initiate the pit was about 0.15 M. In addition to the presence of the oxidized Fe 3 O 4 passive layer and corrosion products like Fe 2 O 3 5H 2 O, ␤-FeOOH, and Green Rust, Raman analysis evidenced the formation of Fe͑OH͒ 2 inside the pit. Moreover, the influence of the pit size was also studied by varying the amount of chloride ions generated close to the iron substrate.

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OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

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Scanning electrochemical microscopy (SECM) is a powerful technique for performing quantitative measurements at a local scale. This paper covers the development of combinations of SECM with electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). Basic aspects are described and potential applications reported by several research groups are covered. The unique advantages of the coupled techniques--with additional information being obtained from each coupling--are also discussed.