| 其他摘要 | Surface treatments for improving service performance of magnesium alloys are of great importance for their practical. Among various surface treatment techniques, electroless nickel plating is of special significance due to its advantages such as uniform deposition, good corrosion and wear resistance, good electrical and thermal conductivity, and good solderability.
The direct EN procedure has the advantages of simplicity and suitability for magnesium alloys with high aluminium content alloys, and is more attractive. Traditionally, before EN plating, the alloys should be pretreated with solutions containing CrO3 and hydrofluoric acid, which are surely harmful to the operators as well as to the environment. A phosphate conversion film for magnesium alloys was proposed as a intermediate between electroless nickel (EN) plating (HF-free/EN) and magnesium alloy substrate, to replace the traditional pretreatment by using chromium oxide plus HF. Compared with traditional process, the new pretreatment process is consisted of one step of activating. The subsequent nickel plating deposited on the intermediate film was then characterized by morphology observation, as well as measurements on adhesion and corrosion resistance. The phosphate film can dissolved in an electrless plating bath, therefore the possible inclusions incorporated in the interface of magnesium substrate and nickel coating may be decreased. Adhesion strength of the HF-free EN plating on the substrate after heat treatment reaches above 23 MPa measured by adhesive tensile test. Potentiodynamic tests measured that the corrosion potential was -0.36 V(Vs.SCE)for the new coating system, the corrosion current density of the HF-free/EN plating on AZ91 decreased by almost one orders of magnitudes in comparison with the HF/EN plating, and a passivation range in the potentiodynamic polarization curve, who clearly appear, which also proved an effective protectiveness of the coating system for magnesium alloys.
However, in severe corrosive environments, single electroless plated nickel coating may not be sufficient to meet the requirements for protecting magnesium alloys. Furthermore magnesium alloys substrate will be suffered from galvanic corrosion when some pores penetrate the coatings. It seems that the magnesium alloys with multilayered coatings, can keep longer life time in severe corrosive mediums. Subsequently a two layered coating system consisted of Zn-Ni alloy and Ni-P alloy designed. For that a Zn-Ni alloy coating was plated top of an electroless nickel plating using pulse voltage electric power source. The surface and cross-sectional morphologies of the two layered coatings were observed with scanning electron microscopy (SEM), the chemical compositions were analyzed by EDAX. The corrosion behavior was evaluated by electrochemical tests. It seems that Zn-Ni coating can provided electrochemical protection for the underneath electroless nickel plating. The experimental results indicated that Zn-Ni alloy coating provided a good protection as a top coating, therefore the two layered coating showed corrosion resistance superior to the single Ni-P coating.
Traditionally an EN plating on an anodized film may become a self-sustaining process only if the so called Pd activation technique is adopted. However, the Pd activation technique was inconvenient in practice due to the strictly prolix operation process and a high material cost，besides the activation solution may attack the anodized film. In this paper, a novel palladium-free activation EN plating process, by which a TiB2 powders contained intermediate film might play the role of as catalyst, was introduced for anodized magnesium alloy. On which a smooth and compact EN plating was obtained. TiB2 powders showed an autocatalysis function for EN plating. The deposition rate, 10μm/h, of the new process was close to that of the traditional process but somewhat lower than that of Pd-activation process on PEO film. The corrosion current density was about 10-7 A•cm-2 and the corrosion potential was -0.323 V for the new coating systems. Together with the observed passivation range in the potentiodynamic polarization curves, the electrochemical parameters revealed an effective protectiveness of the coating system for Mg alloy. The relatively good bonding strength about 11 MPa between the substrate and the catalytic layer might ensure the coatings a prospective application in industries. |
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