The dip coating method (also known as immersion plating or hot-dip coating) for finned tubes is a manufacturing process where the finned tube assembly—after the fins are mechanically threaded onto the base tube—is immersed in a molten metal bath to achieve structural bonding and comprehensive surface corrosion protection. The primary coating metals utilized are zinc (hot-dip galvanizing) and tin (hot-dip tinning), with cadmium applied in specialized environments.

The primary objective of dip coating is to fill the micro-clearance gap between the fin root and the base tube with molten metal, thereby completely eliminating contact thermal resistance while forming a dense protective coating across the entire assembly surface.

Achieving effective wetting and bonding between the molten metal and the substrate requires the complete removal of surface oxides and organic grease. Differentiated chemical pickling regimens are applied based on metallurgy:

• Chromium-Molybdenum Alloy Steel (Cr-Mo Alloy Steel): Cleaned using a combination of hydrochloric acid, sulfuric acid, and oxidizing agents (such as nitric acid) to strip stubborn high-temperature scale.
• Carbon Steel: Cleaned via cold hydrochloric acid or warm dilute sulfuric acid solutions to remove rust and surface mill scale.
• Copper and Copper Alloys: Cleaned strictly using dilute sulfuric acid to prevent excessive etching of the copper base.

Following degreasing and pickling, the assembly undergoes fluxing followed by molten bath immersion in strict sequence:

• Fluxing Treatment: Except for copper fins, the cleaned finned tubes are treated with an ammonium chloride solution containing stannous chloride to suppress re-oxidation and enhance liquid metal wetting.
• Molten Bath Immersion: The finned tube assembly is submerged into a molten metal bath. The molten zinc bath temperature is maintained at 400°C, while the molten tin bath is kept at 230°C.
• Capillary Filling and Solidification: Driven by capillary action, the liquid zinc or tin completely penetrates and fills the clearance gap between the fin foot and the outer tube wall.
• Coating Formation: Upon cooling and solidification, a rigid bond is established, forming a uniform protective metallic coating with a thickness of 20–50 μm across the fin and tube surfaces.

The dip coating process significantly reduces contact thermal resistance, enhances the overall heat transfer coefficient, and offers superior atmospheric and fluid corrosion resistance. However, metal consumption and manufacturing costs are relatively high:

• Thermal & Anti-Corrosion Performance: The 20–50 μm metallic protective layer isolates the substrate from corrosive elements, while the capillary metal filling eliminates air-gap thermal resistance.
• Metal Consumption: Taking hot-dip galvanizing as an example, zinc consumption ranges from 1.25 to 1.30 kg per square meter (m²) of heat transfer area, requiring economic evaluation during engineering selection.