The surface treatment of rectangular nuts requires a precise balance between corrosion resistance and thread fit accuracy. This requires a coordinated design involving process selection, parameter optimization, tolerance control, and post-processing. While electroplating provides basic corrosion protection through metal coatings such as zinc and nickel, the skin effect often results in a coating thickness lower than the surface thickness inside the thread, potentially causing a shift in the tolerance band. In this case, processes with greater coating uniformity should be prioritized. By controlling current density and time, variations in coating thickness can be minimized to ensure that the thread pitch diameter tolerance remains within standard requirements.
Although hot-dip galvanizing forms a dense zinc-iron alloy layer with significantly superior corrosion protection compared to electroplating, the coating thickness is relatively thick, which can easily lead to thread fit issues. To address this conflict, a compensation allowance can be added during the tapping stage of rectangular nuts, increasing the major diameter of the thread by a certain amount, bringing the actual thread size back to the standard range after hot-dip galvanizing. For applications requiring high precision, some processes utilize Spiralock internal thread technology. Its specialized tooth profile allows for thicker coatings, ensuring screwability without compromising corrosion resistance.
Mechanical plating forms the coating through cold welding of metal powder, avoiding dimensional changes caused by thermal processing. The coating thickness is more controllable than hot-dip plating, but wear resistance is slightly inferior to zinc-iron alloy coatings. This process is suitable for applications with moderate corrosion protection requirements and frequent disassembly and assembly, such as machinery maintenance. Its key advantages include no risk of hydrogen embrittlement, high coating thickness uniformity, and minimal negative impact on thread accuracy, meeting the requirements of precision assembly for rectangular nuts.
Dacromet coating, based on a scaly zinc, aluminum, and chromate matrix, is formed through sintering to form an electroless coating. It offers excellent corrosion resistance and can withstand extended periods of neutral salt spray testing. However, the coating thickness can fluctuate widely, potentially affecting the thread depth of smaller nuts. In practice, multiple dip coating processes are required, with each layer thickness controlled within a specific range. Preheat treatment is also required to eliminate internal stress in the coating and avoid tolerance violations caused by dimensional shrinkage after sintering. Furthermore, this process requires strict control of the use of chromates to comply with environmental requirements.
Phosphating improves wear resistance by forming a phosphate conversion film, but its corrosion protection is weak when used alone and requires a complementary oiling process. This process has minimal impact on thread accuracy, and tolerance band deviations typically do not exceed a minimal range. It is suitable for indoor dry environments or for short-term corrosion protection needs. For extended protection periods, composite phosphating solutions containing nanoparticles can be used. Their film density is significantly improved compared to traditional processes, significantly extending the neutral salt spray test time.
Blackening, which produces a dense oxide film through an oxidation reaction, is low-cost and simple to process, but its corrosion protection is weak and requires subsequent oiling for protection. This process has little impact on thread accuracy and is suitable for precision instruments with stringent dimensional tolerances. To enhance protection, room-temperature blackening technology can be used. By optimizing the bath concentration and treatment time, the oxide film thickness can be highly uniform. Combined with high-grade anti-rust oil, this technology can provide extended salt spray protection.
The balance between corrosion resistance and thread precision requires process selection based on the application scenario. Hot-dip galvanizing or Dacromet coating is preferred for outdoor, highly corrosive environments, but tolerances must be controlled through dimensional compensation or multiple coatings. Phosphating or blackening treatments can be used for indoor precision assembly to minimize impact on thread fit. Regardless of the process used, strict adherence to relevant standards and graded control of coating thickness are essential during the manufacturing phase to ensure functional stability of the rectangular nut throughout its lifecycle.
