Magnesium metal and its alloys are known to have excellent physical and mechanical properties suitable for a number of industrial and technological applications. Yet due to their highly reactive nature, magnesium alloys have continued to see little industrial use. This paper demonstrates an extension of the recently introduced, novel method established to deposit electroless Cu films on Mg alloys to the next step of direct electroless deposition of nickel phosphorous [Ni-P] and nickel-zinc-phosphorous [Ni-Zn-P] alloy films on polished AZ91D magnesium alloys. These reasonably adhered, quick depositing, continuous, low phosphorous, electroless coatings contain phosphorous, atomically, at, or below, 10% due to the use of an alkaline deposition bath.Magnesium [Mg] is an abundant, light, easily machined, and recyclable metal that possesses relative properties (property/density) equal to or better than most competitive materials. Mg alloys possess a specific strength (strength to weight ratio) between 2/3 and 3/4 that of aluminum, and a relative yield strength twice that of steel, ideal for many industrial applications. Despite many potential applications for Mg alloys in the automotive and aerospace industries, and well known beneficial properties including high thermal conductivity, high dimensional stability, and good electromagnetic shielding characteristics, 1 its industrial use is relatively scarce due to high reactivity, poor wear properties, and susceptibility to galvanic corrosion. 2,3 Galvanic coupling, and corrosion, is the primary concern regarding the industrial implementation of Mg and its alloys. Mg alloys readily form galvanic cells when brought into contact with a dissimilar metal in the presence of an electrolyte. It has been found that the ideal guard against the formation of galvanic cells is the isolation of the Mg alloy from the electrolyte and dissimilar metal by a metallic cladding; preferably one that forms an inter-metallic bond with the Mg-based substrate and retains the bulk conductivity of the Mg alloy.In addition to the formation of galvanic cells, the high activity of Mg alloys allows for the rapid formation of an oxide/hydroxide surface layer when exposed to air or water, often necessitating treatments prior to deposition, 4 commonly known as pre-treatment regiments, to ensure coating adhesion and/or uniformity. Common treatments include surface conversions 5,6 and organic coatings, 7,8 most of which result in the formation of an insulating layer between the Mg alloy substrate and the cladding. Additionally, a large variety of pre-treatment procedures are complex 9-11 requiring many different techniques and must be conducted carefully for optimum results. Others, especially those for electroless nickel [Ni] deposition, require the use of dangerous hydrofluoric acid [HF] 12,13 and/or hexavalent chromium [Cr 6+ ], 14,15 while others, such as the application of an organosilicon heat-resisting varnish interlayer, 16,17 ignore the bulk conductivity of the substrate, electrically isolating it...
Since the discovery of electroless deposition within the electroplating process, the two techniques have grown as two separate, yet parallel, means of deposition. This paper demonstrates the reunification of the two processes in what is herein named hybrid electroelectroless deposition, or HEED. Specifically, the novel reunification as outlined within this study demonstrates that electroplating and electroless deposition can be achieved from a single solution wherein each process specifically targets a different metal ion within the electrolyte. The successful production of a compositionally modulated Au/Co/Au tri-layer from a single electrolyte using modulated electroless and electro-plating is described. Additionally, the practical application of producing compositionally modulated Ni-Zn-P alloy layers on AZ91D Mg alloys is demonstrated. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0331410jes] All rights reserved. Modern applications of electro-and electroless plating, in both industry and academia, have kept the two deposition techniques as largely two separate processes. The advantages and disadvantages of both processes are well known and documented 1 with each process having complimentary attributes with respect to the other. Examples of the complimentary nature of the processes include the ability to easily coat recessed areas with electroless deposition as well as deposit pure metal, along with alloys, using electroplating. Combining electroless deposition with electroplating in a novel process we have termed hybrid electro-electroless deposition (HEED) a provides benefits from both processes and allows for the formation of many unique thin film coatings using aqueous electrolytes.Hybrid electro-electroless deposition (HEED), as defined in our work, is the targeted reduction of different metals, or alloys, by each of the plating techniques from a single electrolyte. As a matter of definition the process requires the reduction of the 'primary' metal/alloy by means of electroless deposition while a 'secondary' metal/alloy can also be electroplated at a later stage from the same electrolyte. The selection of the term 'primary' for the electrolessly deposited metal is due the electrolessly deposited metal/alloy having typically a higher nobility than the secondary electroplated metal, as well as the requirement that electroless deposition must remain uninhibited by the presence of the secondary metal. That is to say the secondary metal must not impede or prevent the occurrence of electroless deposition. The union of electroless deposition with electroplating using HEED allows for the deposition of multi-layered structures as well as the deposit of well controlled alloys and composites.Traditional methods of depositing both alloys as well as compositionally modulated multi-layers, including dual bath systems and electroplating alone, offer deposits of increased hardness 2 as well as enhanced corrosion 3-6 and wear resistance. 7,8 Additionally, multilayer deposits are known to possess beneficial and pra...
SONG INTRODUCTIONMagnesium (Mg) is the eighth most abundant element on earth, possessing several advantageous properties including a high strength to weight ratio and one of the lightest metals with a density that is only two-thirds that of aluminum and one-fourth that of iron at 1.74 g cm À3 . Additional properties that contribute to magnesium's versatility in the automotive, electronic, and aerospace industries include a high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability, and easy recyclability [1]. These properties make the utilization of magnesium of particular interest to the automotive and aerospace industries where the weight reduction provides a simple means to achieve higher fuel efficiencies without sacrificing structural strength.Despite its many valuable properties, magnesium remains a very reactive element, prone to a number of undesirable properties, including poor corrosion and wear resistance, poor creep resistance, and high chemical reactivity, which have limited its wider industrial use. As such, automobiles currently possess few magnesium cast parts, averaging only a few pounds per car. Pure magnesium corrodes rapidly in humid atmospheric and/or aqueous environments where anions such as Cl À , Br À , I À , and SO À x promote local and generalized corrosion [2][3][4][5][6][7][8][9][10][11][12]. Compounds like alcohols, ethers, and phenols also attack magnesium [13]. Although alloying magnesium with elements such as manganese, aluminum, zinc, zirconium, and rare earths [14,15] can improve its properties, magnesium and its alloys continue to be extremely susceptible to galvanic corrosion, which can cause severe pitting in the metal, resulting in decreased mechanical stability and an unattractive appearance of the surface. Although uniform attack [16][17][18] is generally the most prevalent form of electrochemical corrosion, occurring with equal intensity over the entire surface of the metal, in the case of magnesium, especially pure magnesium, the partially protective surface film makes localized galvanic [19][20][21][22][23][24] and pitting corrosion [25-35] more common and worrisome, largely due to the difficulty in their detection and suppression. Galvanic corrosion occurs when two metals/alloys having differing compositions, and thus standard electrode potentials, are electrically coupled in the presence of an electrolyte. This coupling results in the formation of a galvanic cell, which preferentially corrodes the more reactive, or electronegative, of the two metals. Since magnesium has a highly negative standard electrode potential (E 0 ¼À2.37 V) there are few metal couples with which serious and rapid corrosion does not occur. Pitting corrosion, defined by the formation of small pits on the surface of a metal/alloy, usually stem from galvanic corrosion of a surface defect and progresses similar to crevice corrosion, where stagnant liquid in a crevice begins oxidizing the metal and/or its pass...
Magnesium alloys are known to have excellent physical and mechanical properties suitable for a number of industrial and technological applications. The propensity of Mg to form galvanic couples limits further industrial acceptance. This paper examines the conditions for the electroless deposition of ternary nickel-phosphorous-zinc [Ni-P-Zn] alloy films on AZ91D Mg alloys comparing the deposition with similar deposits on more traditional surfaces. The stable, aqueous, alkaline solutions used in this study do not require any pretreatment specific to the deposition and are able to produce continuous coatings containing Zn up to around 24 wt% of the total cladding.The industrially advantageous qualities of magnesium [Mg] alloys include its abundance, light weight, equal, or better, relative properties (property/density) than most competitive materials. Additionally, high thermal conductivity, high dimensional stability, and good electromagnetic shielding characteristics of Mg alloys have long been established and provide further reasons for its use. 1 With respect to material properties, the chief barrier to industrial implementation of Mg alloys has been their propensity to succumb to corrosion, more specifically galvanic corrosion.Galvanic corrosion occurs when dissimilar metals are placed in contact in the presence of an electrolyte. The resulting corrosion weakens the more anodic of the dissimilar metals. Such contact between naturally active Mg and dissimilar metals results in corrosion that impairs the structural strength of the Mg alloy part and detracts from the many positive qualities of the alloys. 2,3 The ideal guard against the formation of a galvanic cell is the isolation of the Mg alloy from the electrolyte and dissimilar metal by means of a metallic cladding. Such a cladding, made of a dissimilar metal, is stable provided full sequestration of the Mg substrate is accomplished and that no electrolyte is present between the coating and substrate.We have previously established direct electroless plating techniques for the deposition of pure copper [Cu] as well as nickel phosphorus [Ni-P] alloys 4,5 on Mg alloys from alkaline deposition baths. These novel deposits, produced without any complex multi-stepped chemical pre-treatment, utilize the activity of Mg alloys as well as the more passive environment of an alkaline deposition bath to help form the deposit. In the case of Cu, removal of the native oxide layer was essential, while the Ni alloy deposition bath was shown to remove, and/or allow deposition upon, oxides obviating the need for any pre-deposit preparation.In our previous work, 5 we determined that, for best results, ammonia/ammonium [NH 3 /NH 4 + ] is necessary in the deposition bath, while the presence of chloride ions [Cl − ] is best avoided within the same. At the same time, it was shown that the deposition bath used for Ni-P lends itself well to the deposition of ternary alloys containing zinc [Zn] and that the deposition of Ni-P-Zn, while possible, had different needs than the Ni-P coatin...
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