Study on properties of zinc-nickel alloy coating on steel fasteners

The zinc-nickel alloy coating is basically free of hydrogen embrittlement and has excellent corrosion resistance, so it becomes a suitable protective layer for steel fasteners. Through electroplating zinc-nickel alloy process test, the appearance and thickness of zinc-nickel alloy coating, nickel content, bonding strength, corrosion resistance and paint matching are analyzed and researched. The results show that electroplated zinc-nickel alloy is a reliable and cost-effective protective layer suitable for steel fasteners.

At present, galvanizing is the most widely used anti-corrosion surface treatment of steel fasteners. Generally, indoor fasteners are mainly considered to be decorative, and most of them use electro-galvanized as the preferred surface treatment method; outdoor fasteners are mainly anti-corrosion. In addition to electro-galvanized, hot-dip galvanizing is also selected as the surface protection layer (mainly M10 Fasteners above). However, for particularly harsh outdoor environments, such as marine environments with high humidity and high salt spray, the corrosion resistance of galvanized fasteners is often difficult to meet the requirements of use. The electroplated cadmium layer shows excellent corrosion resistance in the marine environment, and people began to use the cadmium plating layer as a protective layer fastener in the marine environment, and the effect is better [1,2]. As my country attaches great importance to environmental protection, the cadmium plating process has been included in the catalog of 'eliminating backward production processes and products' promulgated by the former State Economic and Trade Commission. Therefore, it is very necessary to carry out research on cadmium plating alternative processes.

In the early 1980s, my country began research in the field of electroplating zinc alloys, and successively developed and applied a variety of electroplating zinc-nickel alloy processes. When the nickel content in the zinc-nickel alloy coating is 8% to 15% (the best value is 13%), its corrosion resistance is excellent, which is about 3 to 5 times that of the zinc coating under the same thickness; and the nickel content is 15 The zinc-nickel alloy coating below% can also be passivated, and the corrosion resistance is greatly enhanced after passivation; 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, through related process tests, the performance of zinc-nickel alloy coatings is analyzed and researched.

1. Experimental part

1.1 Sample

The sample is a hexagon socket head screw, the thread specification du003dM6, the nominal length Lu003d20mm, the performance level is 8.8, and the material is 45# steel.

1.2 Process flow

Degreasing→hot water washing→cold water washing→weak erosion→two-stage cold water washing→electroplating zinc-nickel alloy→two-stage cold water washing→passivation→cold water washing→hot water washing→drying.

2. Results and discussion

2.1 Appearance and thickness of coating

The zinc-nickel alloy coating is passivated to form a bright rainbow-colored passivation film.

The thicker the zinc-nickel alloy coating, the longer the protection life. Taking into account the thread gap of fasteners, the thickness of the coating is generally selected (5μm~10μm) [4]. In this experiment, the coating of the screw sample was tested by a portable thickness gauge, and the thickness was about 8μm.

2.2 Nickel content in the coating

Using X-ray fluorescence spectroscopy (XRF) to analyze the zinc-nickel alloy coating, it is found that the nickel content in the coating is about 8%.

2.3 Coating strength

After the screw sample is kept at 200°C for one hour and quenched to room temperature, the coating is well bonded to the substrate.

The sample plate (material: st12) electroplated with the screw sample in the groove after the steel ball friction polishing test (GB5270-85), the coating is intact, and there is no phenomenon such as peeling of the coating from the substrate.

The results show that the bonding strength of the zinc-nickel alloy coating and the substrate is good.

2.4 Corrosion resistance of coating

According to GB/T10125, the neutral salt spray test (NSS) was carried out, and compared with the zinc-plated (color passivation) screw, the corrosion resistance of the zinc-nickel alloy coating is better than that of the zinc coating. Studies have shown [5] that when the zinc-nickel alloy coating is corroded, the presence of nickel causes its corrosion product to be ZnCl24Zn(OH)2, which is uniformly and densely covered on the surface and is not easy to conduct electricity, so it has a protective effect on the coating; Moreover, nickel has an inhibitory effect on the cathode process, which is also conducive to the improvement of corrosion resistance. However, the corrosion products of zinc coatings are mainly loose, unprotected semiconductor ZnO. These reasons reasonably explain the test results in Table 2.

2.5 The matching of coating and paint

The sample plate (material: st12, zinc-nickel alloy coating thickness is about 8μm) is used to investigate the compatibility of the coating with different types of primers, and the adhesion strength of the primer is tested according to the method specified in SJ/T10674, according to GB/T1732 The method is used to test the impact resistance of the primer, and the zinc-nickel alloy coating has a good match with epoxy and polyurethane primers.

Zinc-nickel alloy coating is an easy-to-operate and cost-effective protection method for steel fasteners; if used in combination with organic coatings, it will become a reliable long-term protection method for steel fasteners. However, compared with electro-galvanizing (or hot-dip galvanizing), the maintenance of electro-galvanizing-nickel alloy solution is more troublesome. Generally, the electroplating solution needs to be analyzed and adjusted every week. In the future, the development of easy-to-maintain electroplating zinc-nickel alloy solutions will be a development direction.