Dacromet Bolt Process

In the field of bolt surface treatments, alongside common cold galvanizing and hot-dip galvanizing, there is a highly corrosion-resistant coating technology known as Dacromet treatment. Originating from abroad, Dacromet is a relatively new surface coating process for bolts. Its salt spray test duration far exceeds that of electrogalvanizing-specially treated Dacromet bolts can withstand over 1,000 hours of salt spray testing, a performance unattainable by electrogalvanizing.

In early domestic applications, many installation environments required bolts with excellent corrosion resistance. Electrogalvanized bolts often failed to meet design requirements, while hot-dip galvanized bolts, though corrosion-resistant, suffered from insufficient thread precision due to process limitations. Dacromet treatment perfectly filled this gap. Dacromet bolts offer 10 times the corrosion resistance of electrogalvanized bolts. Unlike hot-dip galvanizing, they do not damage threads (allowing normal nuts to fit seamlessly), eliminate hydrogen embrittlement risks, and withstand high temperatures. As a result, most bolts used in automobiles and motorcycles today employ Dacromet treatment.

The Dacromet process traces back to the 1930s in Europe. At the time, cold climates and airborne chlorides (e.g., salt spray) rapidly eroded metal substrates, causing severe damage to transportation equipment. Scientists developed a water-based coating material using zinc flakes, aluminum flakes, and chromic acid. After being applied to metal surfaces and sintered at 300–320°C, this material forms a dense inorganic passivation film on the metal surface. Later adopted by the U.S. military as a defense-grade anti-corrosion technology, it has been used ever since. After decades of development, it has fully civilianized and is widely applied in industries such as electricity and construction.

The Dacromet treatment process is straightforward but fundamentally different from electrogalvanizing (a non-electrolytic dip-coating and sintering process): Bolts are immersed in or sprayed with a slurry containing zinc and aluminum flakes, then sintered at high temperatures to form a uniform, compact inorganic coating. Unlike traditional zinc coatings, Dacromet coatings exhibit unique properties, including exceptional oxidation and salt spray resistance. After treatment, no polishing, brightening, passivation, or hydrogen removal is needed (eliminating hydrogen embrittlement risks due to the absence of electrolysis), and it meets environmental standards, making it a mainstream choice for export-oriented bolts.

Dacromet bolts offer significant advantages:

Compared to electrogalvanizing (with a thin 5–15μm zinc layer prone to damage from friction), Dacromet's 8–12μm coating resists minor scratches during transportation and installation, preventing exposed base metal and subsequent rust.

Compared to hot-dip galvanizing (with a thick 50–80μm layer that can clog nut threads), Dacromet's moderate coating thickness fully covers internal threads without filling them, ensuring smooth assembly and precise threading.