| Magnesium alloys have high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, which make it valuable in a number of structural applications including aerospace, electronics, computer parts and automobile fields. The standard potential is -2.37V, which is lower than iron (-0.44V) and aluminum (-1.66V). Therefore, the application of magnesium alloys has been limited due to the undesirable properties, including poor corrosion and wear resistance. To improve the practical usage of magnesium alloys, many researchers have attempted to develop anticorrosive and high wear-resistance strategies.Electroless deposition process experienced numerous modifications to meet the challenging needs of a variety of industrial applications since A. Brenner and G. Riddell invented the process in 1946. Among the various types of electroless plating, electroless nickel has gained immense popularity due to their ability to provide a hard, wear and corrosion resistant surface. The electroless Ni-P based alloy deposits with good quality and uniformity can be obtained without special requirements for substrate geometries and with capability of depositing on either conductive or nonconductive parts, which has been widely applied in many industries. Therefore, electroless Ni-P based alloys are considered as the most effective method to modify the physical and chemical properties of the substrates.However, the electroless plating on magnesium alloys, has many challenges in the processing of plating and there is limited reports on magnesium alloys. The magnesium alloy is extremely susceptible to galvanic corrosion that pit severely on the metal resulting in an unattractive appearance as well as decreased mechanical properties. It is noted that in many previous reports on the electroless plating on magnesium alloys, there are currently two general solutions to treating magnesium prior to plating: zinc immersion and conversion treatment in a fluoride-containing bath. Magnesium alloy was etched in a solution of chromium oxide and nitric acid and soaked in HF solution to form a MgF2 film before electroless plating. Furthermore, the nickel ions were provided by basic nickel carbonate or acetic nickel in most Ni-P plating bath for magnesium alloy. Therefore, in this study, an electroless Ni-P coating on AZ91D magnesium alloy was proposed from a plating bath containing sulfate nickel. The nucleation mechanism of the electroless Ni-P plating on the AZ91D magnesium alloy was studied by using XRD and SEM. The deposition motivation at the initial deposition stage was attributed to the electrochemically heterogeneous surface of the AZ91D magnesium alloy. From the XRD results and the determinations of the phosphorus contents in the Ni-P deposits at different plating intervals, it can be seen that the electroless Ni-P deposits had preferentially nucleated on theβ-Mg17Al12 phase and then extended to other phases of the AZ91D magnesium alloy. The effect of the acid pickle treatment time of the magnesium alloy substrate on the electroless Ni-P plating was also investigated. The acid pickle pretreatment provides surface pits to act as sites for mechanical interlocking to improve adhesion.Nevertheless, metal finishing industries have to look for alternative materials or specifically deposition methods to replace the hexavalent chromium compounds, which are progressively restricted due to their high toxicity on environment and HF also exhibits strong corrosive that can't be easily controlled. Thus, the environmental and health friendly technology have been extensively studied to effectively inhibit the magnesium corrosion recently. Therefore, the phosphate-manganese conversion film had been studied. The pretreatment layer reduces the corrosion of magnesium and decreases the potential difference between matrix and the second phase. Both the precipitation of magnesium phosphate and manganese and increase of the volume fraction ofβphase may improve the corrosion resistance of the substrate. The reactions during the pretreatment have been deduced from the experiment observation and analysis. The pretreatment process improves the deposition rate and replaces the hexavalent chromium compounds.As a result, the subsequent Ni-P plating became more compact and defect free. The XRD pattern indicates that the structure of the as-deposited Ni-P coating was a mixture of amorphous and nanocrystalline nickel, which consisted with the phosphorus contents 5.57wt.% analyzed by EDS. The porosity test, acid immersion test and electrochemical measurement reveal that the coating exhibited more noble anticorrosion properties. When the thickness reached 12μm, no red color spots were found on the tested coatings. In the immersion test, there were no hydrogen gas bubbles arising from the 32-μm Ni-P coating on the substrate after immersed in the 10% HCl solution for 122 min. While the electroless Ni-P coating with the same thickness on the magnesium alloy with tradition pretreatment only endured 50 min without corrosion. It seems the Ni-P coating exhibited better anti-corrosion performance. In summary, the chromium-free layer on AZ91D alloy could be as the pretreatment layer for further electroless nickel based alloys deposition to improve the corrosion resistance of the substrate.Although the binary Ni-P alloy could apply certain anti-corrosion for AZ91D magnesium alloy, but still lack of coatings which can be used in critical condition. It is thus of interest to assess some ternary nickel based alloys, which have also been developed to further improve these properties of binary systems by adding the metal salts to nickel solution for meeting some special demands. Among the possible metals, tungsten appears to be a significant importance due to its high hardness and high melting point. Codeposition of tungsten in binary Ni-P deposit has been of considerable interest because of its unique properties such as excellent corrosion-resistant and wear-resistant. One important modification of the Ni-W-P electroless bath is the addition of sodium carbonate, which is used as complexing agents, accelerators, and buffers to adjust the pH of the bath. Compositional analysis by EDS shows that the tungsten and phosphorus content in Ni-P matrix is about 4.5wt.% and 4.9wt.%, respectively. Ternary Ni-W-P deposits are typical dense nodular structure. When the thickness reached 8μm, the coating is thick enough and pores free to protect the substrate from corrosion. There were no hydrogen gas bubbles arising from the 24-μm Ni-W-P coating with chromium-free pretreatment on the substrate after immersed in the 10% HCl solution for 187 min.While the Ni-Sn-P alloy appears to be significant importance due to its excellent corrosion-resistance, heat-resistance and weldability, which can be used aerospace, electronics, computer parts and automobile fields. Therefore, an electroless Ni-Sn-P coating was deposited on AZ91D magnesium alloy in an alkaline-citrate-based bath where nickel sulphate and sodium stannate were used as metal ion sources and sodium hypophosphite was used as a reducing agent. The tin and phosphorus content in Ni-Sn-P alloy are 2.48 and 8.51 wt%, respectively. The ternary amorphous alloy is dense and uniform. Due to the presence of tin in Ni-P coating, the porosity is comparative low and the coating is more compact. The 10% HCl immersion test shows that the magnesium substrate with 35-μm coating can withstand about 10 h without corrosion.Since magnesium is one of the most electrochemically active metals, nickel alloy coating is cathodic to the magnesium alloy substrate and can only provide a physical barrier against the corrosion of substrate. Once the substrate is exposed to a corrosion environment, galvanic corrosion that pit severely on the metal will result in an unattractive appearance as well as decreased mechanical properties. Therefore, the use of multilayer or hybrid techniques in surface engineering has often been cited as the way to improve the mechanical, tribological and electrochemical properties of coatings.Duplex Ni-P (20μm)/Ni-B (15μm) coatings with the thickness of about 35μm on AZ91D magnesium alloy were prepared by electroless deposition using dual baths. The microhardness of the duplex coatings increases with increase in heat-treatment temperature. After heat-treatment at 350°C for 2 h, the microhardness of Ni-P and the Ni-B coating is 972 and 1245 HV, respectively, which is far higher than that of the AZ91D magnesium alloy substrate (about 100 VHN). The microhardness of the substrates is significantly improved by deposition of the duplex coatings, which has Ni-B coating as the outer layer, both in as-plated and heat-treated conditions.SEM of the cross-section view of the nickel duplex coatings reveals that the coatings are uniform and the compatibility between the layers is good. The results of the electrochemical measurements and the acid immersion tests revealed that the duplex coatings could provide electrochemical protection for the magnesium alloy substrate. Since the corrosion potential of the outer Ni-B layer was lower that that of the inner Ni-P layer in the duplex coatings, when the pitting corrosion potential penetrated the out layer to the inner layer, the outer layer would be corroded preferentially. The corrosion would change to the extended transverse corrosion, rather than the original longitudinal pinhole-corrosion. Therefore, the high corrosion resistance of the duplex Ni-P/Ni-B coatings on AZ91D magnesium alloy is expected and the coating on the substrate is an effective method to improve the corrosion resistance of the magnesium alloy.
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