Release time：2020-01-16 18:44 Browse：
The choice between Trivalent Blue-Zinc (Blue-White) and Trivalent Zinc-Iron (often called Color Zinc or Yellow Zinc) plating for extra-long screws is critical and depends on the application's specific requirements for corrosion resistance, appearance, and dimensional tolerances. Both are excellent, RoHS-compliant, environmentally friendly alternatives to the old hexavalent chrome plating.
Here are the key differences:
| Feature | Trivalent Blue-Zinc (Blue-White) | Trivalent Zinc-Iron (Color Zinc / Yellow Zinc) |
|---|---|---|
| Appearance | Bright, silvery-white with a slight blue iridescence. Clean, modern, and uniform appearance, similar to stainless steel. | Iridescent yellowish-olive, bronze, or rainbow hue. A more traditional, industrial appearance. |
| Corrosion Resistance | Good. Typically provides 72 to 120 hours to white rust (neutral salt spray test, ASTM B117). Suitable for most indoor and benign environments. | Excellent. Typically provides 120 to 200+ hours to white rust. Offers significantly higher corrosion resistance and is preferred for outdoor, humid, or harsh environments. |
| Coating Layer | Very thin, transparent passivation layer (0.2 - 0.5 µm). Color comes from light interference. | Thicker, colored conversion coating (0.5 - 1.5 µm). The color is intrinsic to the coating itself. |
| Dimensional Impact | Negligible. The ultra-thin layer does not significantly change the screw's dimensions, which is crucial for long screws with precise threading. | Slight. The thicker coating can marginally affect the pitch and fit of threads. This must be considered for high-precision applications. |
| Abrasion/Wear Resistance | Fair. The thin layer can be susceptible to scratching during installation or handling, which may compromise corrosion protection if damaged. | Good. The thicker conversion coating is generally more durable and better at resisting abrasion during transport and installation. |
| Cost | Generally lower. A more common and slightly less complex process. | Generally higher. The process is more complex and uses more expensive chemistry. |
Plating Uniformity: This is the foremost challenge. The longer the screw, the harder it is to achieve a perfectly uniform coating thickness and color from end to end. A reputable plater with proper racking and process control is essential to avoid thin spots or color variation.
Hydrogen Embrittlement: The electroplating process can cause hydrogen atoms to infiltrate the steel, making high-strength screws brittle and prone to cracking under stress. This risk is heightened for long screws that will be under significant tensile load.
Critical: Both processes require baking for hydrogen embrittlement relief within a few hours after plating. This is non-negotiable for any high-strength fastener, especially long ones.
Dimensional Tolerance: For very long, precision-threaded screws, the minimal dimensional change from Blue-Zinc might be advantageous over the slightly thicker Zinc-Iron coating.
Choose Trivalent Blue-Zinc (Blue-White) if:
The application is primarily indoor or in a controlled environment.
A bright, silvery aesthetic is important.
Maximum dimensional accuracy is absolutely critical.
Cost is a significant factor.
Choose Trivalent Zinc-Iron (Color Zinc) if:
The application is for outdoor, automotive, or high-humidity/industrial environments where superior corrosion resistance is the top priority.
Appearance is less important than function.
The screws may experience more abrasion during handling or use.
