Electrogalvanizing and hot-dip galvanizing differ by only one character in Chinese but are vastly different in practice. This article will analyze the differences between these two surface treatment methods for bolts.
Electrogalvanizing is achieved through an electrolysis process using cathode and anode in an electrolyte solution: zinc (or zinc alloy) at the anode loses electrons to form zinc ions, which then gain electrons and deposit on the surface of the cathode (the bolt to be plated), eventually forming a white zinc protective coating on the bolt surface. Surface treatment processes such as nickel plating, nickel-tin alloy plating, and chrome plating operate on a similar core principle to electrogalvanizing, with differences mainly in surface color and corrosion resistance.
Most bolts we commonly use have a black or natural surface finish. When exposed to air, these bolts are prone to corrosion over time; even metal products like guardrails result in thousands of tons of material waste annually due to corrosion. To save materials and improve the aesthetics and durability of products, protective coatings are applied to bolt surfaces through surface treatment. The two most common methods are electrogalvanizing and hot-dip galvanizing.
After electrogalvanizing, if the bolt comes into contact with humid air, the zinc coating on its surface can isolate the air from the bolt substrate, preventing corrosion. The electrogalvanized layer is uniformly distributed on the bolt surface, rarely leaving unplated areas, and the zinc layer across the entire bolt surface has a fine-grained structure with good corrosion resistance. Electrogalvanized bolts have a clean, attractive appearance and can be safely used in cleanrooms without generating oil stains or other contaminants. Their color can also be adjusted as needed, with common options including white zinc, blue-white zinc, colored zinc, and military green-these not only provide corrosion protection but also serve a decorative purpose in special environments. Electrogalvanized bolts are widely used across almost all industries and play a crucial role in the development of the basic industrial economy.
Electrogalvanized Bolts
Hot-dip galvanizing for bolts is also a surface protective coating process, but with better corrosion resistance than electrogalvanizing. Its principle is relatively simple: after pretreatments such as degreasing and derusting, the bolts are immersed in a molten zinc bath (this only describes the core principle, not the full process). A zinc layer approximately 70 micrometers thick forms on the bolt surface, giving hot-dip galvanized bolts superior corrosion resistance compared to electrogalvanized ones. However, hot-dip galvanized bolts have a narrower application range-they are mainly used in fields with high corrosion resistance requirements, such as wind power facilities, photovoltaic bases, and national power grid structures.
Hot-dip galvanized bolts have distinct characteristics compared to electrogalvanized ones: due to the 70-micrometer-thick zinc layer, they have a significant impact on thread precision. Generally, only bolts of M8 size and above are suitable for hot-dip galvanizing (smaller bolts tend to have thread blockage after hot-dip galvanizing). In contrast, the electrogalvanized layer is thin (usually 5–20 micrometers) and barely restricted by bolt size-bolts of any size can undergo electrogalvanizing. Hot-dip galvanized bolts only have a silver-gray surface and cannot be produced in other colors; their appearance is far less attractive than that of electrogalvanized bolts, and their surface finish is also much poorer. Therefore, hot-dip galvanized bolts require matching special hot-dip galvanized nuts during installation (standard regular nuts cannot fit the thread size after hot-dip galvanizing), while electrogalvanized bolts have no such concern and can be directly used with standard nuts.
Hot-Dip Galvanized Bolts
There is also a difference in processing costs between the two methods: currently, the processing cost for electrogalvanizing is approximately 1,500 RMB per ton, while that for hot-dip galvanizing is around 3,000 RMB per ton. Thus, for bolts of the same specification, the cost of hot-dip galvanizing is higher than that of electrogalvanizing-a point to note during procurement.
In terms of application compatibility: whether bolts have a black, natural, or electrogalvanized finish, their thread dimensions remain largely unaffected, allowing direct installation with standard nuts. However, hot-dip galvanizing increases the outer diameter of the bolt threads. To fit the existing thread holes in the base material, it is necessary to either select bolts with slightly larger thread tolerances in advance (to reserve space for the galvanized layer) or enlarge the thread holes in the base material; otherwise, fitting difficulties will occur.
