Hot-dip galvanized pipes are produced by reacting molten metal with an iron matrix to form an alloy layer, which bonds the matrix and the coating together. The process involves first pickling the steel pipes to remove iron oxide from their surfaces. After pickling, the pipes are cleaned in tanks containing an aqueous solution of ammonium chloride, zinc chloride, or a mixture of both, before being sent into a hot-dip galvanizing bath.
Hot-dip galvanizing boasts advantages such as uniform coating, strong adhesion, and long service life. In northern regions, a direct zinc replenishment process using galvanized strips for coiling pipes is commonly adopted.
Hot-dip galvanized pipes achieve their coating through the reaction between molten metal and the iron matrix, creating an alloy layer that integrates the matrix and the coating. This involves complex physical and chemical reactions between the steel pipe matrix and the molten coating solution, resulting in a corrosion-resistant, tightly structured zinc-iron alloy layer. This alloy layer merges seamlessly with the pure zinc layer and the steel pipe matrix, contributing to its strong corrosion resistance.
Uniformity of Zinc Coating: Steel pipe samples must not turn red (copper-colored) after being continuously immersed in a copper sulfate solution five times.
Surface Quality: The surface of galvanized steel pipes should have a complete zinc coating, free from uncoated black spots and bubbles. Small rough areas and localized zinc protuberances are permissible.
Weight of Zinc Coating: Depending on the buyer's requirements, the weight of the zinc coating on galvanized steel pipes can be measured, with an average value not less than 500g per square meter. No individual sample should weigh less than 480g per square meter.
