The zinc-nickel alloy coating is basically free of hydrogen embrittlement and has excellent corrosion resistance, thus becoming a suitable protective layer for steel fasteners. Through the electroplating zinc-nickel alloy process test, the appearance and thickness, nickel content, bonding strength, corrosion resistance and paint matching of zinc-nickel alloy coating were analyzed. The results show that electro-galvanized nickel-nickel alloy is a reliable and comprehensive cost-effective protective layer suitable for steel fasteners. At present, the most common application of anti-corrosion surface treatment for steel fasteners is galvanizing. Generally, indoor fasteners are mainly decorative, and most of them are electroplated zinc as the preferred surface treatment method; outdoor fasteners are mainly anti-corrosion. In addition to electro-galvanizing, hot-dip galvanizing is also selected as surface protection layer (mainly M10). The above fasteners). However, for particularly harsh outdoor environments, such as high humidity, high salt spray marine environments, the corrosion resistance of galvanized fasteners is often difficult to meet the requirements of use. The cadmium plating layer shows excellent corrosion resistance in the marine environment. People began to use the cadmium-plated layer as a protective layer fastener in the marine environment, and the effect is better [1, 2]. With China's emphasis on environmental protection, the cadmium plating process has been included in the catalogue of “Removing outdated production processes and products†promulgated by the former State Economic and Trade Commission. Therefore, it is very necessary to carry out research on cadmium replacement process. In the early 1980s, China began research work in the field of electroplated zinc alloys, and successively developed a variety of electroplated zinc-nickel alloy processes and applied them. When the nickel-nickel alloy coating has a nickel content of 8% to 15% (optimum value of 13%), the corrosion resistance is excellent, about 3 to 5 times that of the galvanized layer at the same thickness; and the nickel content is 15 The zinc-nickel alloy coating below % can also be passivated, and the corrosion resistance after passivation is greatly enhanced; the zinc-nickel alloy coating is basically free of hydrogen embrittlement, and the weldability and ductility are also good [3]. Therefore, zinc-nickel alloy coating is a mature and reasonable protective layer suitable for fasteners. In this paper, the performance of zinc-nickel alloy coatings was analyzed by related process experiments. 1 experimental part 1.1 sample The sample is a hexagon socket head cap screw with thread size d=M6, nominal length L=20mm, performance class 8.8, material 45# steel. 1.2 Process Degreasing → hot water washing → cold water washing → weak etching → two-stage cold water washing → electroplating zinc-nickel alloy → two-stage cold water washing → passivation → cold water washing → hot water washing → drying. 1.3 plating solution composition and operating conditions In this experiment, the alkaline zincate electroplated zinc-nickel alloy process developed by Wuhan Institute of Materials Protection was used. The composition and operating conditions of the electroplating solution are shown in Table 1. 2 results and discussion 2.1 plating appearance and thickness The zinc-nickel alloy coating is passivated to form a brilliant rainbow-colored passivation film. The thicker the zinc-nickel alloy coating, the longer the protection life. Considering the thread gap of the fastener, the thickness of the coating is generally selected (5μm~10μm) [4]. In this experiment, the screw sample coating was tested by a portable thickness gauge and the thickness was about 8 μm. 2.2 Nickel content in the coating Analysis of the zinc-nickel alloy coating by X-ray fluorescence spectroscopy (XRF) revealed that the nickel content in the coating was about 8%. 2.3 plating bond strength After the screw sample was quenched at 200 ° C for one hour and quenched to room temperature, the coating was well bonded to the substrate. After the steel plate was rubbed and polished with the sample of the screw sample (material: st12) by the steel ball (GB5270-85), the plating layer was intact, and no peeling of the plating layer and the substrate was observed. The results show that the zinc-nickel alloy coating has good bonding strength with the substrate. 2.4 plating corrosion resistance Neutral salt spray test (NSS) according to GB/T10125, and compared with galvanized (color blunt) screws, the test results are shown in Table 2. As can be seen from Table 2, the zinc-nickel alloy plating layer is more excellent in corrosion resistance than the zinc plating layer. Studies have shown that [5], when the zinc-nickel alloy coating is corroded, the presence of nickel causes the corrosion product to be ZnCl2·4Zn(OH)2. The product is uniformly and densely covered on the surface and is not easily conductive, so the coating is protected. Moreover, nickel has an inhibitory effect on the cathode process, which also contributes to the improvement of corrosion resistance. However, the corrosion products of the zinc coating are mainly loose, unprotected semiconductor ZnO. These reasons reasonably explain the test results of Table 2. 2.5 plating and paint matching Using the sample (material: st12, zinc-nickel alloy coating thickness is about 8μm) to check the matching of the coating with different kinds of primer, the adhesion strength of the primer is tested according to the method specified in SJ/T10674, and tested according to the method specified in GB/T1732. The impact resistance of the primer is shown in Table 3. PU Custom Measuring Tape,Colorful PU Leather Tape Measure,Various Colors Tape Measure, Retractable Tape Measure, Vinyl Coated Fiberglass Tape Wintape Measuring Tape Company , https://www.wintapetapemeasure.com
As can be seen from Table 3, the zinc-nickel alloy plating layer has good matching with the epoxy-based and polyurethane-based primers.
Zinc-nickel alloy coating is an easy-to-operate and cost-effective way to protect steel fasteners; if used in combination with organic coatings, it will become a reliable long-term protection method for steel fasteners. However, compared to electrogalvanizing (or hot-dip galvanizing), the electroplated zinc-nickel alloy solution is more troublesome to maintain, and the plating solution needs to be analyzed and adjusted every week. In the future, the development of an easily maintained electroplated zinc-nickel alloy solution will be a development direction.