1.What is the coating structure of galvanized pipe?
The electroplated zinc layer is mainly pure zinc (purity ≥ 99.5%), which is deposited on the surface of the steel pipe through electrolytic reaction. Depending on the process differences, it may include the following structures: Basic pure zinc layer: The thickness is usually 5-20μm (commonly 8-15μm), which is the core layer of protection.
Passivation film (optional): Traditional process: Hexavalent chromium passivation (such as Cr₂O₃・CrO₃) is used to form a rainbow or blue-white film layer with a thickness of about 0.5-1μm, which can increase the corrosion resistance by 3-5 times (salt spray test is extended from 24 hours to 72-100 hours).
Environmentally friendly process: gradually switch to trivalent chromium passivation or chromium-free passivation (such as silane, titanate), the corrosion resistance is close to the hexavalent chromium process (salt spray test 50-80 hours), and it complies with environmental protection standards such as RoHS.
2.What are the characteristics of hot-dip galvanized coating structure?
Zinc-free iron alloy layer: Unlike hot-dip galvanizing, the electroplated zinc layer is only bonded to the steel pipe substrate by physical adsorption (no metallurgical reaction), and there is a clear boundary line at the interface.
Porosity: There are microscopic pores (pore size of about 0.1-1μm) on the surface of the pure zinc layer, which need to be filled with a passivation film or sealant to improve corrosion resistance.
3.What are the adhesion characteristics of hot-dip galvanized pipes?
Adhesion test: The adhesion level of the electroplated zinc layer is usually 2-3 (hot-dip galvanizing can reach level 0) using the cross-hatch method (ISO 2409), that is, the edge peels off slightly after cross-hatch.
Failure mechanism: Mechanical processing damage: Operations such as bending, flaring, and welding can easily cause cracking of the coating, exposing the steel substrate and causing corrosion.
Hydrogen embrittlement risk: Hydrogen atoms penetrate into the substrate during the electroplating process, which may cause brittle fracture of high-strength steel (such as Q345) (dehydrogenation treatment is required, baking at 180℃ for more than 2 hours).
Surface stress: There is residual tensile stress in the electroplated zinc layer. Long-term use may cause cracking of the coating due to stress release, especially in an environment with large temperature differences.
4.What is the coating thickness distribution of galvanized pipes?
Axial uniformity: The coating thickness error at different positions of the same steel pipe is ≤±1μm, which is suitable for scenarios with high precision requirements (such as precision instrument catheters).
Radial difference: Inner wall coating: Affected by the current distribution, the coating thickness of the inner wall of the steel pipe is usually 60%-80% of the outer wall (such as 12μm on the outer wall and 8-10μm on the inner wall).
Edge effect: Due to the high current density at protruding parts such as pipe mouths and threads, the coating thickness can reach 1.5-2 times the average thickness ("zinc nodules" may be formed, which require subsequent grinding).
5.What are the surface characteristics of galvanized pipes?
Glossiness: The surface gloss can be adjusted through the electroplating process, which is divided into matte (roughness Ra 1.6-3.2μm) and bright (Ra 0.4-0.8μm) to meet decorative needs (such as furniture pipes, bathroom accessories).
Defect type: Missing plating: Due to incomplete pre-treatment, there is no coating locally (the incidence rate is about 0.5%-1%), and manual re-plating is required.
Water marks/passivation spots: If the post-treatment is not cleaned thoroughly, water stains may remain, affecting the appearance (the incidence rate can be reduced by ultrasonic cleaning).