In actual mechanical equipment and important structures, the prominent black bolts are typically high-strength bolts. Why is this? Is it for aesthetics?
The black appearance of high-strength bolts is not for aesthetics, but rather for crucial functional considerations: black does not represent high strength, but rather the requirement to manufacture high strength makes "black" (usually a blackening treatment) the most suitable and safest surface treatment choice, primarily related to surface treatment processes and corrosion resistance.
Main Reasons
1. Blackening Treatment (Oxidation Treatment)
High-strength bolts typically undergo a bluing/blackening process (also known as oxidation treatment), a chemical surface treatment method: the bolt is immersed in a hot solution containing sodium hydroxide, sodium nitrite, etc. (approximately 135°C~145°C), forming a layer of iron(II,III) oxide (Fe₃O₄) on the steel surface. This film appears black or bluish-black and is approximately 0.5~1.5 micrometers thick.
2. Corrosion Protection
Although the oxide film is thin, it effectively isolates air and moisture, slowing down rust. Compared to exposed steel, it offers significantly improved corrosion resistance and is cost-effective, making it suitable for mass production.
3. Friction Reduction and Wear Resistance
The oxide film surface has a certain degree of oil absorption, reducing friction between threads, facilitating tightening, and providing lubrication during assembly.
4. Anti-reflective Properties (Secondary)
In outdoor projects such as bridges and buildings, black does not reflect light, reducing light pollution. However, this is a secondary benefit, not the primary purpose.
Why Not Use Other Colors?
The following are various surface treatment methods for high-strength bolts.
The key reason for not using other colors is that high-strength bolts (especially 10.9S and 12.9S grades) are extremely sensitive to hydrogen embrittlement. Electroplating (such as zinc plating) may introduce hydrogen atoms, causing the bolt to suddenly break during use, posing a significant safety hazard. Therefore, high-strength bolts for steel structures preferentially use processes such as blackening or phosphating, which do not pose a risk of hydrogen embrittlement.
Therefore, the black color of high-strength bolts is actually a visual indicator of safe manufacturing processes-it conveys a clear message to engineers and users: the bolt has undergone a surface treatment that eliminates the risk of hydrogen embrittlement.
Not all high-strength bolts are black.
Nor can the strength grade of a bolt be judged solely by its color.
For example:
Hot-dip galvanized bolts: Silver-white surface, commonly used in highly corrosive environments such as outdoor bridges and towers;
Dacromet-treated bolts: Available in various colors including silver-gray, silver-white, and black, with corrosion resistance far exceeding traditional blackening, achieving over 200 hours of salt spray testing;
Nickel-plated or Teflon-coated bolts: Can be silver, multicolored, or even multi-colored, often used in specialized fields such as food machinery and aerospace.
The true standard for determining whether a bolt is high-strength is the performance grade marked on the head, such as 8.8, 10.9, and 12.9. Only bolts of grade 8.8 and above are considered high-strength bolts, typically made of low-carbon alloy steel or medium-carbon steel and subjected to heat treatment.
