In addition to the common surface treatment processes of electrogalvanizing and hot-dip galvanizing for bolts, there is another surface coating process with excellent corrosion resistance, namely Dacromet treatment. Originating from overseas, the Dacromet process is a relatively new surface coating technology for bolts. Its salt spray test duration is several times longer than that of electrogalvanizing. Specially treated Dacromet bolts can withstand salt spray tests for over 1,000 hours, a performance level that electrogalvanizing simply cannot achieve.
In the early days, many domestic installation environments required bolts with good corrosion resistance. Ordinary electrogalvanized bolts often failed to meet design requirements, while hot-dip galvanized bolts, due to process limitations, could provide corrosion resistance but failed to maintain thread precision. The Dacromet treatment process just filled this gap. Dacromet bolts have corrosion resistance over 10 times that of ordinary electrogalvanized bolts. Compared with hot-dip galvanizing, Dacromet treatment causes no damage to threads, allowing the bolts to be directly used with standard nuts. Moreover, it eliminates the risk of hydrogen embrittlement and offers good high-temperature resistance. Therefore, most bolts used in automobiles and motorcycles currently adopt the Dacromet treatment process.
The Dacromet technology was first developed in Europe in the 20th century. At that time, the cold climate in the region led to rapid corrosion of metal substrate structures by chlorides in the air, causing severe damage to the metal components of transportation vehicles. To solve this problem, scientists developed a polymer-soluble coating composed mainly of metallic zinc powder, aluminum powder and chromic acid. The coating is applied to the metal surface and then sealed, baked and cured, forming a dense anti-corrosion protective layer on the metal surface. In the following years, this technology was incorporated into the procurement system of the US military, becoming a military anti-corrosion technology for national defense and still in use today. After years of development, Dacromet technology has been fully civilianized and widely applied in various fields such as electric power and construction.
The Dacromet bolt treatment process is actually not complicated, but it should be noted that it is completely different from the electrolytic process of electrogalvanizing. Dacromet is a coating-type process. The specific procedure is as follows: immerse the bolts in a water-based coating containing zinc powder, aluminum powder, chromic acid and other components, take them out after fully adsorbing the coating, and then conduct high-temperature baking and curing to make the coating bond tightly with the metal substrate, finally forming a protective layer. The Dacromet coating has unique properties, with super strong oxidation resistance and corrosion resistance. After Dacromet treatment, bolts do not require subsequent processes such as polishing, brightening and passivation, nor do they need an additional hydrogen removal step. Meanwhile, the entire production process meets environmental protection requirements, making it one of the main surface treatment methods for export bolts.
Dacromet bolts have numerous outstanding advantages. Compared with electrogalvanized bolts, they not only have significantly improved corrosion resistance, but also address the drawback of electrogalvanized coatings which are relatively thin. During logistics transportation or installation, slight collisions and friction can easily damage the zinc coating on electrogalvanized bolts, exposing the substrate and forming corrosion points, which will cause rusting within a few days. Compared with hot-dip galvanizing, hot-dip galvanized nuts often suffer from thread filling by zinc liquid due to the thick coating, leading to failure in screw assembly during later use. This problem does not occur with Dacromet nuts. The thickness of the Dacromet coating is about 12 microns, which can fully cover the internal threads of nuts without filling the thread gaps, ensuring smooth assembly.
