Cerium conversion coatings, which have been used as protective coatings for aluminum alloys, are now being considered as an alternative to chromium conversion coatings for improving the corrosion resistance of magnesium alloys. This study investigated the evolution of conversion coatings on an AZ31 magnesium plate immersed in 0.05 M cerium nitrate solution. In addition to the expected growth of the conversion coating with immersion time, it was found that there may be an inherent adhesive weakness within the coating layers, which then led to partial detachment of the coatings from the magnesium plate while drying the samples at room temperature. Cross-sectional transmission electron microscopy characterization of conversion coatings revealed a three-layered structure comprising of porous, compact, and fibrous layers sequentially formed on top of the magnesium plate. Furthermore, the weakest bonding was identified as the interface between the compact and the fibrous layers. Based on the identified layer morphology and the respective composition, a possible formation mechanism for cerium conversion coating on magnesium alloy was proposed, which would serve as a basis for improving the adhesive strength of the coating on magnesium substrate.In light of their low density and high specific strength and stiffness, magnesium alloys are extensively used in electrical appliance and automobile industries. 1,2 However, most magnesium alloys are chemically reactive and tend to suffer severe corrosion during service. 2,3 Surface modification treatments are therefore indispensable for improving the corrosion resistance of magnesium alloys, which includes anodizing, 4-8 conversion coating, 4,9-17 electroless nickel plating, 18 and pure magnesium coating via physical vapor deposition ͑PVD͒. 19 Among the various surface modification techniques, conversion coating treatment is known for its low cost and simplicity in operation. For example, chromate conversion coatings have been extensively applied to magnesium alloys because of their simplicity in production and excellent corrosion resistance. 9,10 However, the toxicity of hexavalent chromium ions in the solution and exhaust fumes has imposed a strict restriction on the use of chromate conversion coatings. In seeking alternatives to chromate conversion coating, nonchromate solutions have recently been developed, such as phosphate, 4 phosphate/permanganate, 11,12 stannate, 13,14 and solutions containing salts of rare earth metals 15,16 or cobalt. 17 The rare earth conversion-coating process is recognized for its simple electrolytic constituents that usually contains nitrate, sulfate, and chloride of rare earth metals such as cerium, lanthanum, and neodymium. 15,16,[20][21][22][23] This simple electrolyte makes it easy to maintain and recycle, and more importantly, the solution is considered to be friendly to the environment. 21 Therefore, many efforts have been made to investigate how the solution composition affects the formation and properties of the conversion coatings, especia...

The properties of nickel-phosphorus ͑Ni-P͒ electrodeposits can be best related to their phosphorus content and microstructure. This study systematically investigated the microstructural evolution and mechanical properties of the deposits plated from nickel sulfamate baths containing 0-40 g dm −3 phosphorous acid ͑H 3 PO 3 ͒. Experimental results indicate that coarse nickel grains were substantially refined with the incorporation of phosphorus into the deposit. For example, as the deposit phosphorus content was increased from 0 to 14 wt %, the structure of the deposit changed in sequence from a coarse column to a mixture of column and lamella, followed by a well-defined lamella, and finally to a homogeneous amorphous matrix dispersed with nanosized grains. Accompanied with this structural evolution, the deposit exhibited a distinct change in deposit hardness and internal stress. These properties and microstructure relationships are discussed in terms of the lattice defects in the grains and proton discharge during electroplating.

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Real-time X-ray microscopy was used to study the influence of hydrogen-bubble formation on the morphology of ramified zinc electrodeposit. The experimental results show that when intense hydrogen bubbling occurs at high potential, the morphology of the ramified zinc deposit changes from dense-branching to fern-shaped dendrite. The fern-shaped dendrite results in part from the constricted growth due to hydrogen bubbles but also from the highly concentrated electric field. The fern-shaped dendrite morphology was observed during the early stages of electroplating for both the potentiostatic and galvanostatic modes; however, the deposit plated in the galvanostatic mode densified via lateral growth during the later plating stages. This indicates that potentiostatic plating for which the hydrogen-bubble formation steadily occurs throughout the electrodeposition process is better than galvanostatic plating for fabricating fern-shaped deposits, which are ideal electrodes for Zn–air batteries due to the relatively large specific area.

In this study, nickel-phosphorus ͑Ni-P͒ deposits were electroplated from the nickel sulfamate bath containing phosphorous acid using a pulse current, with emphasis on the effect of current density, duty cycle, and frequency of the pulse current. Experimental results show that both the deposit phosphorus content and current efficiency were substantially enhanced by employing the pulse current, preferentially at low duty cycles. The underlying difference for the dc and pulse currents on the effect of deposit phosphorus content and current efficiency can be explained by the detailed half-reactions relevant to incorporation of phosphorus into NiP alloys. Less variation in surface proton, Ni 2+ and H 3 PO 3 concentration due to the diffusion recovery during the time off of a pulse current is believed to play an important role in the improvement of the plating process.