Why Are High-Strength Bolts Usually Black?

The black appearance of high-strength bolts is due to important functional considerations: it is not that black signifies high strength, but rather that the requirements for manufacturing high-strength bolts make “black” (blackening) the most suitable and safe surface treatment option, primarily due to surface treatment processes and corrosion resistance.

Main Reasons

1. Blackening (Oxidation Treatment)

High-strength bolts typically undergo a blackening process (also known as oxidation treatment), which is a chemical surface treatment method: the bolts are immersed in a hot solution containing sodium hydroxide, sodium nitrite, and other substances, forming a layer of iron(III) oxide (Fe₃O₄) on the steel surface. This layer appears black or bluish-black and is approximately 0.5 to 1.5 micrometers thick.

2. Corrosion Protection

Although the oxide film is thin, it effectively blocks air and moisture, slowing down corrosion. Compared to bare steel, it offers significantly improved corrosion resistance at a low cost and is suitable for mass production.

3. Friction Reduction and Wear Resistance

The surface of the oxide film has a certain degree of oil absorption, which reduces friction between threads, facilitates tightening, and acts as a lubricant during assembly.

Why Not Use Other Colors?

High-strength bolts (especially grades 10.9S and 12.9S) are extremely sensitive to hydrogen embrittlement. Electroplating processes (such as zinc plating) may introduce hydrogen atoms, leading to sudden bolt failure during use and posing serious safety hazards. Therefore, high-strength bolts used in steel structures prioritize processes that carry no risk of hydrogen embrittlement, such as blackening or phosphating.

Not all high-strength bolts are black, and bolt strength grades cannot be determined solely by color.

Hot-dip galvanized bolts: These have a silvery-white surface and are commonly used in highly corrosive outdoor environments such as bridges and towers.

Dacromet-treated bolts: Available in various colors such as silver-gray, silver-white, and black, their corrosion resistance far exceeds that of traditional blackening, withstanding salt spray tests for over 200 hours.

Nickel-plated or Teflon-coated bolts: These can appear silver, colored, or even multicolored and are commonly used in specialized fields such as food processing machinery and aviation.

The true criterion for determining whether a bolt is high-strength is the performance grade marked on its head, such as Grade 8.8, Grade 10.9, and Grade 12.9. Among these, only Grade 8.8 and above are classified as high-strength bolts. They are typically made of low-carbon alloy steel or medium-carbon steel and undergo quenching and tempering treatment.