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A hydroxide oscillation model has been proposed to account for the observed "nanolaminated" structure in electrodeposited Zn and Zn-Co alloys produced under potentiostatic control. At a critical cathode potential the Zn deposition proceeds in a cyclically changing environment arising from the regular formation and decay of a zinc hydroxide layer near the cathode surface, which produces Zn-ZnO laminations in the deposits. Co was found to enrich at the Zn-ZnO interface. Such a model is supported by evidence obtained from electrochemical measurements and transmission electron microscopy observations, and this has given some insight into the anomalous behavior of Zn-Co deposition. lnfroduction In the electrodeposition of Zn-Co alloys from an acid sulfate solution, anomalous deposition' occurs in which the less noble metal Zn is deposited preferentially. A model proposed by Dahms et al.2 suggested that the deposition proceeded with the intermediate formation of a metal hydroxide located in the vicinity of the electrode and this inhibited the deposition of more noble metal.Higashi et al.4 have measured the pH in close approximation to the cathode while anomalous codeposition of ZnCo was taking place. It was shown that an abrupt rise in pH to values in the range 5.0 to 5.5 occurred, which would permit zinc hydroxide precipitation. These findings promoted the development of the "hydroxide suppression mechanism"24 for the anomalous deposition of Zn-iron group metal alloys which include Zn-Co.Observations by the authors6'7 using transmission electron microscopy (TEM) have shown that Zn and Zn-Co alloy deposits produced under galvanostatic conditions on a steel substrate contain a series of nanolaminations.These exist in a regular pattern with 50 nm thick Zn crystals surrounded by a ZnO layer which is 3 to 4 am thick. In the alloy the Co was found to be preferentially accommodated at the Zn-ZnO boundary. Here, the authors have carried out a polarization study on the Zn and Zn-Co plating solutions with polycrystalline steel and zinc singlecrystal cathode substrates. Zn and Zn-Co deposits have then been produced under potentiostatic control and examined by TEM and scanning electron microscope (SEM). Particular attention has been given to the onset of anomalous deposition. The information gained has been used to set up a new model to account for anomalous deposition of zinc and its alloys. ExperimentalPlating solutions.-Zn and Zn-Co coatings were electrodeposited from an acidic sulfate solution on to either a mild steel substrate or on to a (0001) faced single-crystal zinc substrate. The substrates were disk shaped, 1 cm2 surface area, which had been polished with 1 m diamond compound. Prior to plating they were given an ultrasonic clean. Table I lists the compositions of the solutions employed.Cathodic polarization study.-Polarization studies were carried out in a standard three-electrode cell. The cathode was immersed in 1 liter of aerated solution and Luggin capillary [in contact with an external saturated calomel electrode...

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