The hot-dip galvanizing and dactron coating of the bolts, the differences in surface treatments
As core fasteners in the industrial field, the surface treatment of bolts directly determines their corrosion resistance, service life and application scenarios. Hot-Dip Galvanizing and Dacromet are two widely used surface treatment processes, with significant differences in principles, performance and application scenarios. The detailed analysis is as follows.
1.Hot-Dip Galvanizing involves immersing derusted bolts into molten zinc solution (temperature around 440-460℃), forming a uniform zinc alloy coating on the bolt surface. Its core is to achieve metallurgical bonding between zinc and base iron through high-temperature infiltration, forming a dense protective layer. The process flow mainly includes: pickling derusting, rinsing, fluxing, hot-dip galvanizing, cooling and passivation.
2.Dacromet, also known as zinc-chromium coating, is a water-based coating liquid mainly composed of zinc powder and chromic anhydride. It covers the bolt surface by dipping, spraying or brushing, and forms a protective layer after curing by baking (temperature around 200-300℃). Its protection mechanism is the sacrificial anode protection of zinc powder and the passivation and sealing effect of chromate. The coating has a porous structure and needs to be matched with a sealant to improve protection performance.
| Performance Index | Hot-Dip Galvanizing | Dacromet |
| Coating Thickness | Thicker usually 85-120μm,adjustable according to needs,with longer protection cycle. | Thinner, generally 5-15μm,good coating uniformity, without affecting bolt assembly accuracy. |
| Corrosion Resistance | Excellent performance in atmospheric and soil environments, salt spray test can reach hundreds of hours; but easy to fail in strong acid and alkali environments. | Stronger resistance to salt spray, damp heat and chemicals, salt spray test can reach thousands of hours, suitable for harsh corrosive environments. |
| Adhesion and Wear Resistance | Strong metallurgical bonding force, excellent adhesion and wear resistance, able to withstand a certain degree of impact and friction. | Weak adhesion and wear resistance, easy to fall off when impacted, not suitable for high-frequency friction scenarios. |
| High Temperature Resistance | Good high temperature resistance, can be used for a long time below 200℃, and the coating is easy to oxidize and fail above 300℃. | Poor high temperature resistance, long-term service temperature not exceeding 150℃, easy to decompose at high temperature, resulting in decreased protection performance. |
Application Scenario Differences
Hot-Dip Galvanizing
Suitable for outdoor atmospheric environments, building steel structures, bridges, power towers, highway guardrails and other scenarios. Its thick coating can effectively resist rain and wind erosion, and has strong wear resistance, which can adapt to complex outdoor physical environments. At the same time, the hot-dip galvanizing process has relatively low cost, suitable for surface treatment of mass-produced and standardized bolts. However, it should be noted that the size of the bolt will increase slightly after hot-dip galvanizing, and assembly clearance should be reserved in advance.
Dacromet
More suitable for high-corrosion environments, such as marine engineering, chemical equipment, automobile chassis, ship accessories and other scenarios. Its excellent salt spray resistance can resist the erosion of seawater and chemical media, and the thin coating does not affect the precision assembly of bolts. In addition, Dacromet coating has no hydrogen embrittlement risk, and can be used for surface treatment of high-strength bolts (hot-dip galvanizing is prone to hydrogen embrittlement and requires additional dehydrogenation treatment). However, the Dacromet process has high cost and poor wear resistance, not suitable for heavy-load and high-frequency friction scenarios.
Summary and Selection Suggestions
Both Hot-Dip Galvanizing and Dacromet have their own advantages and disadvantages, and the selection should be comprehensively judged based on the service environment, performance requirements and cost budget. For ordinary outdoor corrosive environments, pursuing high cost performance and wear resistance, Hot-Dip Galvanizing is preferred; for high-corrosion environments such as marine and chemical industries, or for requirements on assembly accuracy and no hydrogen embrittlement, Dacromet can be selected. In addition, it should be noted that Dacromet coating contains chromium, and chromium-free Dacromet alternatives should be selected in areas with strict environmental standards.