Metal Coating Technology of Fasteners

Metal coating technology is mainly used on the surface of metal materials to form a thin layer of metal, in order to give metal materials decorative or protective surface treatment technology. In urban rail vehicles, the metal plating technology of fasteners is mainly galvanized, and another special metal plating (chrome plating, nickel plating, cadmium plating, silver plating, etc.).

Galvanizing

Zinc is miscible with iron, and its standard electrode potential is -0.76V. For the steel matrix, zinc coating belongs to the anode coating, and can better protect the steel matrix, so galvanized technology is widely used in fasteners. There are three commonly used galvanizing methods: hot dip galvanizing, electric galvanizing and mechanical galvanizing.

Hot dip galvanizing

Hot dip galvanizing refers to the steel parts immersed in molten liquid zinc, so that a series of physical and chemical reactions occur on the surface of the workpiece, so as to form a metal galvanized layer. Hot dip galvanized coating is very thick (up to 30 ~ 60 μm), and its corrosion resistance is very good, so it is widely used in outdoor steel parts (such as TV towers, highway guardrails, etc.). For fasteners, hot-dip galvanizing – is generally suitable for M6 and above bolts, but it cannot be used for high-strength fasteners. This is mainly because the operating temperature of the hot dip galvanizing process is very high (400C~500C), which is easy to temper and soften high-strength fasteners.

Electro-galvanizing

Electro galvanizing refers to the formation of uniform, dense and well-combined galvanizing layer on the surface of iron and steel parts by electrolysis. The zinc layer of electro-galvanizing is thin (5~30μm), and its corrosion resistance is the worst in galvanizing anticorrosion treatment. But it is simple in process, low in cost, and has little influence on the problem of screw screwing, so it is widely used in fasteners. Electro galvanizing cannot be used for high-strength fasteners because of its high sensitivity to hydrogen emittance and its difficulty in complete dehydrogenation (the surface of the electro-galvanizing layer will peel or fall off above 100C).

Mechanical galvanizing

Mechanical galvanizing refers to the surface treatment process of a galvanized layer formed by the impact medium on the surface of iron and steel parts under the action of zinc powder, dispersant and accelerator and other chemical substances. The thickness of the mechanical galvanized layer is generally 5 ~ 50 μm, the surface of the coating is compact and uniform, the decorative effect is good, and the corrosion resistance is excellent. And the coating has no high-temperature tempering, no hydrogen embrittlement and other shortcomings of hot dip galvanizing and electric galvanizing, so mechanical galvanizing is a surface treatment process especially suitable for fasteners anticorrosion.

There are also other metal coatings.

Chrome plating

As a metal coating, chromium has strong adhesion, good wear resistance, excellent decorative effect, and high heat resistance (under 500C can be used normally), so the chromium coating as a metal coating of fasteners is very ideal.

Chrome plating has the following disadvantages:

1) The process is complex, and nickel or copper must be plated before chrome plating.

2) Expensive.

3) The chromium coating is hard, brittle and easy to fall off.

Nickel plating

Nickel as a metal coating has good electrical conductivity, high hardness, good decorative effect, and good heat resistance (below 600C can be used normally), so the fastener using nickel plating treatment is relatively ideal.

Nickel plating treatment mainly has the following disadvantages:

1) The process is complex, and copper must be plated before chrome plating.

2) The nickel coating is porous, and the thin coating will accelerate the corrosion of the matrix.

3) Expensive.

Cadmium plating

As a metal coating, cadmium is an anodic coating, which has strong hydrochloric acid corrosion resistance, low hydrogen brittleness and good decorative effect. It is especially suitable for fasteners used in Marine environments, such as Hainan Sea aircraft and oil drilling platforms.

Cadmium plating treatment mainly has the following disadvantages:

① High environmental pollution, cadmium melting gas and soluble cadmium salt are toxic.

② It is expensive.

Silver plating

As a metal coating, silver has excellent electrical conductivity, excellent reflective performance, good lubricity and excellent heat resistance (below 870C can be used normally), so silver plating treatment is widely used in electronic and electrical, high-frequency components and other fields (such as generator conductive bolt, vehicle battery lead terminal).

Silver plating has the following disadvantages:

① The process is complex, and copper must be plated before silver plating.

② The price is very expensive.

Galvanized nickel

Zinc-ni composite coating is a new type of alloy metal coating optimized on the surface treatment technology of zinc-Ni, which has many advantages.

1)NSS can reach 500-1500Hrs.

2) The electrode potential of the coating is between Fe and Zn, which is more suitable for aluminum parts assembly.

3) High coating hardness, decorative effect is very good.

4) Almost no hydrogen embrittlement, can be used for high-strength fasteners.

5) Good heat resistance (under 8009C can be used normally).

The main shortcoming of zinc nickel coating: is its higher price (about 6 times galvanized), but its excellent comprehensive performance has been more and more widely recognized.

As a one-stop fastener company, we can provide a wide range of fasteners including stainless steel nuts, carbon steel nuts, 4 prong t nuts, bolts, screws, washers, and so on. Non-standard custom fasteners are also available as long as you provide the corresponding requirements and drawings. If you want to wholesale fasteners or want to consult fastener-related knowledge, please contact us, and our experienced service team will reply to you as soon as possible.