As the fundamental component for mechanical connections, fasteners often operate in complex environments characterized by humidity, corrosion, and friction. Surface treatment processes not only directly affect the appearance and texture but, more critically, enhance corrosion resistance, improve wear resistance, ensure connection reliability, and extend service life. In industrial production, galvanizing, chrome plating, phosphating, and blackening are four widely used surface treatment processes, each with its own technical characteristics and suitable application scenarios. The rational selection of these processes requires careful consideration of various factors, including operational conditions and cost budgets.
 

1. Galvanizing Process: A Cost-Effective Choice for Corrosion Protection

Galvanizing is one of the most widely used surface treatment processes for fasteners. Its core principle involves depositing a zinc layer onto the fastener’s surface through either electrochemical methods or hot-dip immersion. By leveraging zinc’s sacrificial anode protection mechanism, this process isolates the substrate from contact with air and moisture, thereby providing corrosion resistance. Depending on the specific process used, the two main types are electro-galvanizing and hot-dip galvanizing.

Electro-galvanizing boasts features such as uniform coating, controllable thickness (typically ranging from 5 to 20 μm), and a bright, lustrous appearance. It offers high production efficiency and moderate costs, making it suitable for applications with stringent aesthetic requirements and relatively mild corrosive environments—such as fasteners used in household appliances and automotive interior components. Hot-dip galvanizing, on the other hand, involves immersing fasteners into molten zinc, resulting in a thicker zinc coating (20–50 μm) with strong adhesion and superior corrosion resistance. This process is ideal for outdoor, humid, or industrially corrosive environments—for example, high-strength bolts used in structural steelwork, bridges, and power equipment. However, hot-dip galvanized surfaces tend to be relatively rough, and the high-temperature treatment may affect the dimensional accuracy of certain precision fasteners.

II. Chrome Plating Process: Dual Advantages of Wear Resistance and Decoration

The chrome plating process involves depositing a chromium layer onto the surface of fasteners through electrolysis. It is divided into two types: decorative chrome plating and functional chrome plating. Its advantages include high coating hardness, excellent wear resistance, as well as good luster and chemical stability.

The chromium layer used for decorative chrome plating is relatively thin (0.1–0.3 μm) and is often applied as a top coat over a base layer (such as copper or nickel). It combines both aesthetic appeal and basic corrosion resistance, making it suitable for applications where appearance and texture are critical—for example, furniture hardware and exterior fasteners in automobiles. By contrast, the chromium layer used for functional chrome plating is thicker (2–50 μm), with a primary focus on enhancing wear resistance and corrosion resistance. Such coatings can withstand moderate frictional loads and are ideal for fasteners in mechanical transmission systems that frequently experience contact and sliding, such as engine connecting rod bolts and internal transmission fasteners. However, the chrome plating process is relatively costly, and the wastewater generated during electroplating is difficult to treat, posing stringent environmental requirements.

III. Phosphating Process: Common Pre-treatment Methods for Enhancing Adhesion

The phosphating process involves immersing fasteners in a phosphating solution, where a chemical reaction forms an insoluble phosphate film on the surface. This film is porous and has limited intrinsic corrosion resistance, but it exhibits strong adsorption capabilities. It is commonly used as a pre-treatment step prior to subsequent coating or oiling processes.

The primary function of a phosphating film is to enhance the adhesion between subsequent coatings and the substrate, preventing coating delamination while also providing temporary rust protection during short-term storage. Depending on the composition of the phosphating solution, common types include zinc-based, manganese-based, and iron-based phosphating treatments. Among these, zinc-based phosphating is widely used and suitable for most steel fasteners. Manganese-based phosphating films exhibit higher hardness and better wear resistance, making them ideal for applications requiring subsequent oil lubrication, such as engine piston pins and fasteners inside gearboxes. Phosphating processes have low production costs, a simple manufacturing process, and are more environmentally friendly than chrome plating. As a result, they are extensively employed in mass production across industries such as automotive and mechanical engineering.

4. Blackening Process: A Low-Cost, Short-Term Rust-Prevention Solution

The blackening process (also known as bluing) involves immersing steel fasteners in a high-temperature alkaline solution, where an oxidation reaction forms a black film of magnetite on the surface. This process is characterized by low cost, high production efficiency, and a simple operational procedure.

The blackening process forms an extremely thin oxide film (0.5–1.5 μm thick), which hardly affects the dimensional accuracy of fasteners. However, its corrosion resistance is relatively weak, making it suitable only for dry, non-corrosive indoor environments or as a temporary rust-preventive treatment for short-term storage and handling. In addition, the blackening film also provides a certain degree of lubrication, reducing frictional resistance during fastener assembly. Common application scenarios include fasteners used in non-critical internal parts of general machinery and tool-related fasteners. It’s important to note that if blackened fasteners are exposed to humid environments over a prolonged period, they still require supplementary anti-rust measures such as oiling to ensure adequate protection.

Under extreme operating conditions such as high temperature, high pressure, and strong corrosion, ordinary materials often fail to meet the requirements, necessitating the use of special-function alloys. Although these materials are costly, they can ensure the reliability of fasteners even in harsh environments.

High-temperature alloys, exemplified by Inconel and Hastelloy, can maintain stable strength and corrosion resistance in high-temperature environments above 600°C. They are particularly suitable for fasteners used in high-temperature equipment such as aircraft engines, boilers, and gas turbines. Titanium alloys, on the other hand, combine high strength, lightweight properties, and excellent corrosion resistance. Their strength is comparable to that of steel, yet their weight is only about half as much. Moreover, they exhibit superior resistance to acid, alkali, and seawater corrosion compared to stainless steel. Titanium alloys are primarily used in high-end applications such as aerospace, medical devices (including implantable fasteners), and deep-sea exploration equipment. However, their relatively high cost currently limits their widespread use in civilian applications.

In summary, the selection of fastener materials must precisely match the operational requirements: For general applications, carbon steel or alloy steel is preferred; in corrosive environments, stainless steel is the better choice; if electrical conductivity and lightweight design are required, non-ferrous metals can be considered; and in extreme operating conditions, fasteners made from special functional alloys are necessary. At the same time, it is essential to comprehensively weigh factors such as cost budget, processing difficulty, and compatibility with surface treatments in order to fully harness the fastener’s connection capabilities and ensure the overall operational stability of the equipment.