As core components in connection and fixing assemblies, the surface treatment of bolts not only affects their appearance but also plays a crucial role in their service life and overall performance. Adopting an appropriate surface treatment method can effectively improve bolts' corrosion resistance, wear resistance, and fatigue strength, thereby extending their service life and ensuring stable and reliable operation of equipment. Next, we will explore several common surface treatment methods for bolts, analyze their differences, and identify which method offers the best rust prevention effect.

1. Galvanizing

1.1 Introduction

Galvanizing is a commonly used surface treatment for bolts. Its principle is to form a zinc protective layer on the bolt surface to enhance corrosion resistance and appearance. Galvanizing is mainly divided into two categories: hot-dip galvanizing and cold galvanizing (electrogalvanizing).

Hot-dip galvanizing: Bolts are immersed in molten zinc, forming a relatively thick zinc layer (usually 10-60 microns) with a uniformly rough surface. It has strong protective performance and is suitable for outdoor, high-humidity environments and occasions requiring long service life.

Electrogalvanizing: A thin zinc layer (typically 5-15 microns) is deposited on the bolt surface through electrolysis. It features low cost and good appearance, making it suitable for indoor dry environments and light protection scenarios with low corrosion resistance requirements.

1.2 Usage Limitations

Hydrogen embrittlement is likely to occur if hydrogen relief treatment is incomplete during electrogalvanizing. Small-sized bolts (M2.5 and below) have a higher risk of fracture. Electrogalvanized bolts without effective hydrogen relief can hardly meet the strength standards of Grade 10.9 and above, so they should be used cautiously in high-strength connection scenarios.

1.3 Process

The specific process flow is: Degreasing - Cleaning - Weak acid activation - Electrogalvanizing - Cleaning - Passivation - Cleaning - Drying.

1.4 Corrosion Resistance

Electrogalvanized bolts are available in colors such as white zinc, blue-white zinc, colored zinc, and black zinc. The color difference results from different passivation processes after galvanizing; different passivation solutions form passivation films of varying colors. The corrosion resistance ranking is: Black zinc > Colored zinc > Blue-white zinc > White zinc.

2. Nickel Plating

2.1 Introduction

Nickel plating is a process that deposits a nickel layer on the metal surface through specific technologies to improve bolts' comprehensive performance, including corrosion resistance, hardness, and appearance. It is mainly divided into two categories: electrolytic nickel plating and electroless nickel plating.

Electrolytic nickel plating: Nickel ions are deposited on the bolt surface to form a nickel layer through electrochemical action.

Electroless nickel plating (also known as autocatalytic nickel plating): Under specific conditions, nickel ions in the aqueous solution are reduced by a reducing agent and deposited on the bolt surface to form a coating.

2.2 Coating Characteristics

Electroless nickel plating: It has excellent coating thickness uniformity, avoiding the thickness variation of electrolytic nickel plating caused by uneven current distribution. It is suitable for treating bolts with complex shapes, deep holes, or precision requirements.

Electrolytic nickel plating: It has a fine crystalline structure and excellent polishing performance. After polishing, it can present a mirror-like luster, making it often used in scenarios with high decorative appearance requirements.

2.3 Environmental and Cost Characteristics

Electroless nickel plating: It uses special chemical additives, producing no obvious harmful substances and offering better environmental friendliness. However, the maintenance cost of the plating solution is relatively high, making it more suitable for small-batch production or workpieces with complex structures.

Electrolytic nickel plating: It has simple equipment configuration, mature process flow, and high production efficiency, suitable for mass processing. However, the electroplating process generates wastewater containing heavy metals, requiring supporting environmental treatment equipment.

3. Black Oxide Treatment

Black oxide treatment (also known as bluing or oxidation treatment) is widely used due to its low cost and simple operation. It is a process that forms a black oxide film (mainly composed of iron oxide, Fe₃O₄) on the metal surface through chemical reactions. Common types include:

3.1 High-Temperature Alkaline Black Oxide (Traditional Process)

Principle: Iron reacts with oxidants in an alkaline solution (mainly composed of sodium hydroxide and sodium nitrite) at 135°C to 150°C to form a dense black oxide film.

Advantages: Simple equipment, low cost, and high processing efficiency.

Disadvantages: The treatment solution contains harmful components such as sodium nitrite, resulting in significant environmental pressure. After black oxide treatment, post-treatments such as saponification and oil immersion are required to significantly improve rust prevention ability.

3.2 Room-Temperature Chemical Black Oxide

Principle: A black film is formed on the bolt surface through redox reactions using a room-temperature blackening agent containing copper, selenium, sulfur, and other components.

Characteristics: Low treatment temperature and fast speed, suitable for scenarios requiring rapid processing. However, the film layer is thin (usually 0.5-1.5 microns), and its rust prevention effect is limited when used alone, requiring matching oil immersion sealing treatment.

4. Dacromet Treatment

Dacromet treatment involves coating the bolt surface with a coating mainly composed of zinc powder, aluminum powder, chromate (environmentally friendly formulas are chromium-free), and deionized water via dipping or spraying, followed by baking at 180-250°C to form a dense zinc-aluminum composite oxide film. This process does not require pickling, thus avoiding hydrogen embrittlement, and is particularly suitable for high-strength bolts. The appearance is mostly silver-gray or silvery-white.

Core Characteristics

Superb corrosion resistance: Although the coating thickness is only 4-8 microns, its rust prevention ability is 7-10 times that of traditional electrogalvanizing. The neutral salt spray test can reach 1200 hours without red rust.

High-temperature resistance: It can withstand high-temperature environments below 300°C, making it suitable for bolts used in high-temperature components such as engines and exhaust pipes.

Environmental friendliness: Environmentally friendly Dacromet is free of hexavalent chromium and does not discharge wastewater or waste gas, complying with environmental standards.

Mechanical properties: The coating has a high hardness (HV 300-500), better wear resistance than electroplated layers, and strong adhesion, which can penetrate into complex structures such as deep holes and gaps.

No hydrogen embrittlement risk: It does not require pickling or electrolysis processes, eliminating hydrogen embrittlement. It is especially suitable for stressed components such as springs and high-strength bolts.

Summary of Rust Prevention Ability, Advantages, and Disadvantages of Common Surface Treatment Processes

Rust Prevention Ability Ranking (Based on Neutral Salt Spray Test)

Dacromet > Electrophoretic black > Galvanizing (hot-dip galvanizing > electrogalvanizing) > Nickel plating > Black oxide/Phosphating

Note: The rust prevention ability of stainless steel stems from its material itself (containing elements such as chromium and nickel) rather than surface treatment processes, so it is not included in the ranking. If stainless steel is combined with surface treatment (such as passivation), its rust prevention ability will be further improved.

Core Comparison Points

Dacromet: Best rust prevention, no hydrogen embrittlement, high-temperature resistance, but relatively high cost and single appearance color.

Galvanizing: Moderate cost, diverse appearance options. Hot-dip galvanizing has better rust prevention than electrogalvanizing, but electrogalvanizing has the risk of hydrogen embrittlement.

Nickel plating: Good decorative effect and high hardness, suitable for precision or decorative bolts, with medium rust prevention ability.

Black oxide: Lowest cost and simple operation, but weak rust prevention ability, requiring post-treatment, suitable for low-cost and short-term protection scenarios.

Would you like me to sort out a comparative table of common bolt surface treatment processes, covering core indicators such as corrosion resistance, cost, applicable scenarios, and precautions, for quick reference and selection?