Surface Treatment Processes for Cable Trays: Pre-Galvanizing, Hot-Dip Galvanizing, Electro-Galvanizing, and Powder Coating
Surface treatments for cable trays can be categorized based on the timing of application. Pre‑galvanizing is applied to steel sheet before fabrication—the steel is galvanized at the mill, then formed, cut, and punched into cable trays. Hot‑dip galvanizing, electro‑galvanizing, and powder coating, by contrast, are applied after the cable tray has been fully fabricated from raw steel.
This article examines the corrosion protection mechanisms of each process and provides selection recommendations for different application environments.
Pre‑galvanizing, also known as mill galvanizing, is a continuous process performed at the steel mill. Steel coils are passed continuously through a bath of molten zinc, coating both sides of the sheet. A thin iron‑zinc alloy layer forms on the steel surface with a pure zinc layer on top. The pre‑galvanized coils are then slit to size and fabricated into cable trays through roll forming, shearing, punching, and welding.
Pre‑galvanized zinc coating protects steel through two mechanisms:
- Barrier protection: The zinc layer physically isolates the steel substrate from air and moisture.
- Zinc, being more electrochemically active than steel, corrodes preferentially when the coating is damaged, protecting exposed steel areas at cut edges and scratches. This “self‑healing” effect protects cut edges up to a material thickness of 2 mm.
However, because the galvanizing is performed before fabrication, cut edges and welded areas are left uncoated. The coating thickness is relatively light, typically ranging from 10 to 20 μm.
Pre‑galvanizing is suitable for indoor, dry, heated environments such as office buildings and commercial spaces, as well as low‑corrosion environments where the corrosion rate of zinc is minimal. It is also a budget‑friendly choice. It is not recommended for outdoor installations exposed to weather, aggressive or humid environments, or industrial settings with chemical exposure.
Hot‑dip galvanizing after fabrication is a batch process in which fully fabricated cable trays—after all cutting, welding, and drilling operations are complete—are thoroughly cleaned (degreased, pickled, and fluxed) and then completely immersed in a bath of molten zinc at approximately 440–460 °C. A metallurgical reaction occurs between the molten zinc and the steel substrate. The resulting coating consists of a zinc‑iron alloy layer (inner layer) metallurgically bonded to the steel, with a pure zinc layer on top. Coating thickness typically ranges from 45 to 85 μm and can exceed 85 μm in severe environments.
Hot‑dip galvanizing provides dual protection. The thick, dense zinc layer forms an impervious barrier that prevents corrosive agents from reaching the steel substrate. In the atmosphere, a thin, dense zinc oxide layer forms on the surface, which is difficult to dissolve in water and provides additional protection. Additionally, when the coating is scratched or damaged, zinc corrodes preferentially to protect the exposed steel. This cathodic protection extends to cut edges, drilled holes, and mechanical damage sites.
The zinc‑iron alloy layer provides exceptional adhesion and unique corrosion resistance, particularly in marine salt spray and industrial atmospheres. The thick coating also offers significant mechanical durability and wear resistance. Furthermore, the hot‑dip galvanizing process effectively anneals the steel, improving its mechanical properties and relieving forming and welding stresses.
Hot‑dip galvanizing is the preferred choice for outdoor installations exposed to weather, aggressive environments including chemical plants, oil and gas facilities, marine/offshore applications, and industrial atmospheres with corrosive media. It is also ideal for applications requiring maximum corrosion protection and long service life—often exceeding 40 years in appropriate environments. The initial cost is higher than pre‑galvanizing or electro‑galvanizing, typically ranging from $6 to $10 per metre, but the extended service life usually justifies the investment.
Electro‑galvanizing, is a process in which a zinc coating is deposited on steel by electrolysis from a bath of zinc salts. The fabricated cable tray is immersed in an electrolyte containing dissolved zinc compounds, and an electric current is passed through the bath to deposit zinc ions onto the steel surface. The resulting coating is thin and uniform, typically ranging from 5 to 15 μm (with some specifications requiring a minimum of 12 μm). This finish is standard for cable tray hardware and some accessories for pre‑galvanized systems.
Like other zinc coatings, electro‑galvanizing provides both barrier and sacrificial protection. When exposed to air and moisture, the zinc forms a tough, adherent protective film consisting of zinc oxides, hydroxides, and carbonates that acts as a barrier against further corrosive attack. However, the thin coating thickness limits the duration of protection—corrosion resistance is directly proportional to coating thickness. The coating provides only limited protection, typically offering approximately 96 hours of protection in salt spray testing.
An important limitation is that electro‑galvanized surfaces can develop zinc whiskers—microscopic metallic filaments that grow from the electroplated surface over time. These whiskers can cause electrical failures, downtime in sensitive electronic environments such as data centers.
Electro‑galvanizing is suitable for indoor, dry environments with low corrosion exposure, and for data centers (where the zinc whisker risk must be carefully evaluated). It also works well for applications requiring a bright, aesthetically pleasing surface at low cost, such as offices, shops, and hotels (heated buildings with neutral atmospheres). It is not recommended for outdoor installations, humid or chemically aggressive environments, or applications requiring long‑term corrosion resistance. Cost‑wise, electro‑galvanizing is the most economical zinc coating option, making it a common first choice for internal environments.
Powder coating is a dry finishing process in which finely ground particles of pigment and resin (typically polyester, epoxy, or epoxy‑polyester hybrids) are electrostatically sprayed onto the cable tray surface. The coated tray is then heated in a curing oven, where the powder melts, flows, and chemically cross‑links to form a durable, continuous film. Standard powder coating thickness typically ranges from 60 to 80 μm. Powder coating may be applied directly to steel, but more commonly it is applied as a decorative and protective topcoat over a galvanized substrate—a configuration known as a duplex system.
Powder coating provides barrier protection—the organic coating physically isolates the steel from the corrosive environment. Unlike zinc coatings, any breach in the coating exposes bare steel to corrosion. However, powder coatings offer several distinct advantages: broad chemical resistance against acids, alkalis, and chlorinated compounds that would attack zinc; unlimited color options for colour coding and architectural coordination; a smooth, cleanable surface ideal for hygiene‑sensitive environments; and zero VOC emissions during application.
The optimal solution for many demanding applications is the duplex system—hot‑dip galvanizing followed by powder coating. This approach combines the sacrificial protection of zinc with the barrier protection of the organic coating. Research has shown that galvanized parts with an epoxy powder coating can achieve corrosion resistance up to 2.5 times greater than the sum of the two systems separately. The powder coating extends the life of the zinc layer, while the zinc provides backup protection at any coating defects.
Powder coating is well‑suited for indoor installations where appearance and colour coding are important (data centers, commercial buildings), environments requiring chemical resistance beyond what galvanizing provides, food processing facilities (smooth, cleanable surfaces), and applications where colour coordination with architectural finishes is desired. It is moderately priced, typically ranging from $4 to $8 per metre, making it cost‑effective for indoor applications.
When combined as a duplex system (hot‑dip galvanizing plus powder coating), the total coating thickness exceeds 100 μm, and the protective effect is significantly greater than either treatment alone. This system is the top choice for extreme environments such as coastal areas and chemical plants, albeit at the highest cost.
Shanghai Qiongkai Industrial Co., Ltd., recently shipped cable trays to Italy that feature this duplex coating system of hot‑dip galvanizing plus powder coating.
When selecting a surface treatment for cable trays, consider the following factors:
- Environmental exposure: Is the installation indoor or outdoor? Are there high humidity, chemical fumes, or salt spray?
- Service life requirements: How many years of maintenance‑free operation are needed?
- Aesthetic requirements: Is colour coding or architectural matching required?
- Budget constraints: Balance initial cost against lifecycle cost (maintenance, replacement).
For corrosive environments such as chemical plants and offshore installations, hot‑dip galvanizing (zinc coating ≥85 μm) or a duplex system (galvanizing + powder coating) is optimal. For indoor, dry conditions such as data centers and offices, electrostatic powder coating offers cost‑effective protection with aesthetic flexibility. Pre‑galvanized finishes are suitable for indoor, low‑corrosion environments where cost is a primary concern. Electro‑galvanizing provides an economical solution for indoor applications with minimal corrosion exposure.