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.

AZ31 magnesium alloys were galvanostatically anodized in an alkaline solution without and with the addition of phosphate and fluoride ions. The microstructure and composition of the anodic film were investigated using cross-sectional transmission electron microscopy. Results show that during anodizing at 10 mA cm −2 in the KOH solution, the potential of the cell was relatively low and sparks were absent throughout the anodization. Adding fluoride ions in the KOH solution resulted in a sharp increase in cell voltage during the early stages of anodizing and subsequent sparking commenced at ϳ75 V. In contrast, phosphate ions in the solution hardly affected the potential response of the cell. The anodic film formed in the presence of fluoride ions consisted of a compact inner layer containing significant fluorine species, while that formed in the solution without fluoride ions comprised a highly porous inner layer. Sparking, which resulted from the dielectric breakdown of the compact inner layer, led to the formation of magnesium oxide layer. In contrast, the anodic film formed in the absence of sparks was primarily the magnesium hydroxide.Anodization treatment has been used extensively to modify the surface of magnesium alloys with better corrosion resistance, higher hardness, and improved decorative and wear-resistance properties, as well as enhanced paint adhesion. 1-5 Anodization of magnesium can be carried out in acid or alkaline media using the direct or alternating currents. DOW17 and HAE are, respectively, the typical processes performed in acid and alkaline media, in which fluoride and phosphate ions are the common ions for both solutions. 6,7 Although the DOW17 process produces anodic films displaying excellent corrosion protective properties, environmental concerns about the use of hexavalent chromium have stimulated the development of chromate-free anodization solutions. Recently, several industrial processes performed in nonchromate solutions have been realized, such as Tagnite, Anomag, Magoxid-Coat and Keronite processes, which are conducted in neutral or alkaline solutions with the addition of fluoride, borate, sulfate, phosphate, aluminate, and silicate. 3,5,[8][9][10] The properties of anodic films on magnesium can be closely related to its microstructure and composition, which depend largely on the composition of the electrolyte and the anodizing parameters of the process. For example, Mg-Al alloys can be passivated in a 1 M sodium hydroxide solution during constant voltage anodizing in both the low-voltage ͑less than 3 V͒ and high-voltage ranges ͑10 to 80 V͒. 2,11,12 Passivation of the alloys anodized at 3 and 80 V is, respectively, due to the coverage of magnesium hydroxide and oxide layers, in which the hydroxide layer is thicker than the oxide layer. 12 Consequently, the anodic film formed at 3 V exhibits better corrosion resistance than that formed at 80 V. Anodic films formed in the solution composed of hydroxide, silicate, and fluoride of potassium contain smaller and less-interconnected pore...

This study investigated the microstructure evolution and defects of the titanium oxide layer containing calcium (Ca) and phosphorus (P) formed by anodic oxidation in a solution containing Ca and P compounds. Results show that the anodic film exhibited a two-layer structure: a pore-containing amorphous titanium oxide layer dispersed with nano-sized crystallites formed prior to sparking, and a porous overlay dotted with craters formed after sparking. Ca and P were predominantly incorporated in the porous overlay, in which the amorphous region contained more Ca and P than the crystalline region regardless of the anodizing voltages. Moreover, the ratio of amorphous to crystalline regions in the porous overlay changed insignificantly with anodizing voltage. Increasing anodizing voltage enhanced the incorporation of Ca and P in the anodic film, but deteriorated the adhesion of the anodic film to the substrate. This deterioration was related to two inherent adhesive weaknesses: the aligned pores in the titanium oxide layer and the craters in the major overlay, signifying that a new anodic oxidation process that can produce high Ca- and P-containing oxide film at relatively-low anodizing voltages, i.e. approximately 200 V, is a necessity.

Anodization behaviors of AZ91 magnesium alloys in strongly alkaline solutions with and without the addition of fluorine ions have been studied with the emphasis on the effect of the Mg 17 Al 12 ͑␤͒ phase. In the absence of F − ions, the potential of the cell was relatively low and sparks were absent throughout galvanostatic anodizing at a current density of 1 A/dm 2 . After 10 min of anodization, a significant preferential anodization within an ␣ matrix resulted in a nonuniform anodic film in regions of large hemispherical pits. F − ions in strongly alkaline solutions resulted in a marked increase in the cell potential and induced sparks on the alloy. These effects associated with F − ions are likely due to the formation of Mg and Al fluorides, which effectively retards the preferential surface attacks in a strongly alkaline solution. Finally, the anodic film formed without sparking was mainly Mg hydroxide, whereas that resulting from sparking was primarily Mg oxide.In light of their superior specific strength and stiffness, magnesium ͑Mg͒ alloys have been used extensively in the applications where the weight of the structure is a primary concern. To extend their service life, many researches have been performed for better understanding the corrosion behavior of Mg alloys. 1-6 The AZ91 alloy ͑Mg-Al alloy͒, which consists of the ␣ phase ͑Mg solid solution͒ as the matrix and ␣ + ␤͑Al 17 Mg 12 ͒ eutectic phases, has received the most attention because of the potential galvanic corrosion occurring at the ␣/␤ interfaces. The Al content in the ␣ phase and the volume fraction, size, and distribution of the eutectic ␤ particles are of great interest in characterizing the corrosion behavior of Mg-Al alloys in different environments. Increasing the Al concentration of the ␣ phase improves its corrosion resistance due to the formation of a surface layer enriched with Al. 1 The ␤ phase, when present with a large volume fraction, is beneficial to the corrosion resistance of AZ91 by acting as an effective corrosion barrier; conversely, the presence of a small-fraction ␤ phase can be detrimental to the corrosion resistance due to severe galvanic corrosion. 3-5 In phosphoric acid buffered solutions, the ␣ + ␤ eutectic phase in a Mg-Al alloy is an effective anodic barrier at pH 7, but is preferentially attacked at pH 11. 6 Although the understanding of the corrosion behavior of AZ91 has been greatly advanced during the past decade, this alloy still relies on surface treatments for further improving its corrosion resistance properties or for forming a surface layer to enhance the adhesion of a painting layer. 7,8 Among the various surface treatments, the anodization treatment has received ever-increasing attention because anodic films impart Mg alloy with a better corrosion resistance, higher hardness, improved decorative and wear-resistance properties, as well as enhanced paint adhesion. [7][8][9][10][11][12] The factors governing the corrosion properties of Mg-Al alloys can potentially influence the anodization behaviors of the ...