Hot-dip galvanized steel pipes are welded steel tubes with a hot-dip galvanized or electrogalvanized layer on their surface. Galvanization enhances the pipes' corrosion resistance and extends their service life. These pipes have a wide range of applications, including but not limited to pipelines for conveying low-pressure fluids such as water, gas, and oil. They are also used in the petroleum industry, especially as oil well pipes and oil transmission pipes in offshore oil fields. In the chemical and coking industries, they serve as oil heaters, condensers, and coal tar washing oil heat exchangers. Furthermore, they are utilized for pipe piles in trestles and as support frames in mine tunnels.
Hot-dip galvanization involves the reaction between molten metal and the iron matrix to form an alloy layer, thereby integrating the matrix with the coating. The process begins with pickling the steel pipe to remove iron oxide from its surface. Following pickling, the pipe is cleaned in a tank containing an aqueous solution of ammonium chloride, zinc chloride, or a mixture of both, before being immersed in a hot-dip galvanizing bath.
Hot-dip galvanization boasts advantages such as uniform coating, strong adhesion, and long service life. The steel pipe matrix undergoes complex physical and chemical reactions with the molten galvanizing solution, resulting in the formation of a corrosion-resistant, tightly structured zinc-iron alloy layer that integrates seamlessly with the pure zinc layer and the steel pipe matrix, thereby imparting exceptional corrosion resistance.
Uniformity of Galvanized Layer: The steel pipe sample must not turn red (copper-plated color) after being continuously immersed in a copper sulfate solution five times.
Surface Quality: The surface of the hot-dip galvanized steel pipe should have a complete galvanized layer, free from uncoated black spots and bubbles. Minor roughness and localized zinc nodules are permissible.
Weight of Galvanized Layer: Depending on the buyer's requirements, the weight of the galvanized layer on the steel pipe can be measured, with an average value not less than 500g/m² and no individual sample falling below 480g/m².
