Practical Methods to Improve Adhesion of Fastener Surface Treatments

1. Enhancing Surface Pre-Treatment Quality

Thorough oil and rust removal is the foundation for improving adhesion. Using ultrasonic cleaning technology to process fasteners can completely remove oil stains, cutting fluid residues, and other contaminants from hidden areas such as thread gaps and blind holes through high-frequency vibration—this is over 40% more efficient than traditional immersion cleaning. For rusted parts, a combined process of pickling and electrolytic degreasing can be adopted: during pickling, control the hydrochloric acid concentration at 15%-20% and temperature at 50-60℃ to ensure complete removal of oxide scales, then immediately perform electrolytic degreasing to avoid secondary rusting.

Surface roughening treatment increases the contact area and enhances mechanical interlocking. For standard parts like bolts and nuts, sandblasting is applicable—use 80-120 mesh quartz sand and control the sandblasting pressure at 0.4-0.6MPa to achieve a surface roughness of Ra1.6-Ra3.2μm. For precision threaded parts, chemical etching is more suitable: soak the parts in a mixed solution of phosphoric acid and nitric acid for 3-5 minutes to form a uniform micro-rough structure. This method does not affect thread precision while improving the adhesion of the surface treatment layer.

2. Optimizing Surface Treatment Process Parameters

Different treatment processes require targeted parameter adjustments. In electroplating, the adhesion of zinc coatings can be controlled by adjusting current density: for acid zinc plating, set the current density at 1-3A/dm² and use a stepped current rise method to avoid coating stress concentration caused by excessive current. For phosphating treatment, the ratio of free acidity to total acidity must be strictly controlled (usually 1:8-1:15), and the temperature maintained at 50-70℃—this ensures the phosphating film has fine crystals and strong bonding with the substrate.

For coating-based treatments, primer selection is critical. Epoxy-based primers have excellent affinity with metal substrates: spraying a 5-10μm thick epoxy primer on phosphated fastener surfaces before topcoat application can improve adhesion by over 30%. During powder coating application, the curing temperature and time must be matched—for example, curing epoxy resin powder at 180℃ for 20 minutes forms a coating structure with higher crosslinking density, reducing adhesion loss caused by internal stress.

3. Introducing Transition Layer Technology

Adopting composite treatment processes to add a transition layer effectively mitigates performance differences between the substrate and the surface treatment layer. For high-hardness materials like ceramic coatings, first deposit a 5-10μm thick nickel-phosphorus alloy transition layer—its good bonding with metal substrates and compatibility with ceramics eliminate interface stress. For fasteners used in high-temperature environments, a zinc-nickel alloy coating can serve as the transition layer: its coefficient of linear expansion lies between that of steel and ceramics, reducing peeling force caused by temperature changes.

For irregularly shaped fasteners, local pre-plating technology is applicable. Pre-plate a 5-8μm thick copper layer on stress-concentrated areas (such as thread crests and bolt heads) before overall zinc plating. The copper layer acts as a buffer, preventing the treatment layer from cracking at weak points under stress. This method is widely used in the treatment of high-strength bolts for wind power equipment, enabling the adhesion retention rate after salt spray testing to reach over 85%.

4. Strictly Controlling Environment and Post-Treatment

Temperature and humidity control during treatment cannot be ignored. Electroplating workshops should maintain a temperature of 20-25℃ and relative humidity of 50%-60% to avoid coating pinholes caused by water vapor condensation. Passivation treatment should be performed immediately after phosphating: when using chromate passivation, control the pH at 1.5-2.0 and passivation time at 30-60 seconds. The resulting passivation film seals the pores of the phosphating film while enhancing adhesion with subsequent coatings.

Stress relief processes in the post-treatment stage reduce bonding risks between the treatment layer and the substrate. Baking electroplated fasteners at 180℃ for 2 hours releases internal coating stress, reduces hydrogen embrittlement risks, and promotes diffusion bonding between the coating and the substrate. For coated parts, low-temperature baking at 40-60℃ for 2 hours slowly releases internal stress generated during coating curing, avoiding adhesion loss caused by stress rebound.

These methods should be flexibly applied based on specific treatment processes and fastener materials. For example, high-strength bolts are suitable for the combined scheme of sandblasting + phosphating + epoxy primer, while precision nuts are better suited for the process route of chemical etching + zinc plating. Through refined control throughout the entire process, the adhesion of fastener surface treatment layers can stably meet high industry standards, providing solid support for the reliable operation of equipment.

Translation Notes

Term Consistency & Technical Accuracy
- Core terms such as "超声波清洗" (ultrasonic cleaning), "喷砂处理" (sandblasting), "化学蚀刻" (chemical etching), and "过渡层" (transition layer) adhere to materials science and manufacturing norms. Technical parameters (e.g., "15%-20% 盐酸浓度" (15%-20% hydrochloric acid concentration), "Ra1.6-Ra3.2μm 表面粗糙度" (Ra1.6-Ra3.2μm surface roughness), "1-3A/dm² 电流密度" (1-3A/dm² current density)) are retained exactly to maintain operational guidance value.
- Process-specific terms like "阶梯式升流" (stepped current rise), "游离酸度与总酸度" (free acidity and total acidity), and "氢脆风险" (hydrogen embrittlement risks) use precise industrial expressions to ensure clarity for engineers and technicians.
Contextual Adaptation of Functional Descriptions
- "机械咬合力" is translated as "mechanical interlocking" (a standard term for the physical bonding between a rough surface and a coating), rather than literal "mechanical biting force." "盐雾测试" is rendered as "salt spray testing"—the international standard method for evaluating coating corrosion resistance.
- "应力集中" (stress concentration) and "内应力" (internal stress) are accurately conveyed to reflect key factors affecting coating adhesion, ensuring the translation aligns with material mechanics principles.
Sentence Structure Optimization
- Long procedural clauses (e.g., the combined pickling and degreasing process) are split using em dashes and bullet points to improve readability. For example, "对于锈蚀件，可采用…… 避免二次锈蚀" is restructured into a clear step-by-step description in English.
- Passive voice (e.g., "can be adopted," "must be strictly controlled," "should be flexibly applied") is strategically used to focus on processes and requirements, aligning with the objective tone of technical guides.
Industry-Specific Detail Preservation
- Application scenarios (e.g., "风电设备高强度螺栓" (high-strength bolts for wind power equipment), "精密螺纹件" (precision threaded parts)) use industry-specific vocabulary to ensure relevance for professionals in fastener manufacturing and surface treatment.
- Material compatibility descriptions (e.g., "环氧底漆与金属基体的亲和性" (epoxy primer affinity with metal substrates), "锌镍合金线膨胀系数" (zinc-nickel alloy coefficient of linear expansion)) are translated precisely to reflect the scientific basis of the methods.

Practical Methods to Improve Adhesion of Fastener Surface Treatments