1.What are the process principles and zinc layer characteristics of hot-dip galvanized steel pipes?
Process Principle: The steel pipe is immersed in molten zinc at approximately 500°C. A chemical reaction occurs on the steel pipe surface, forming a composite coating consisting of an iron-zinc alloy layer and a pure zinc layer. Zinc coating features: Thick zinc layer (typically 60-150μm), strong adhesion, no pinholes, and a long corrosion resistance life (up to 15-30 years in outdoor environments).
2.What is the process flow of hot-dip galvanized steel pipe?
Pretreatment: Removing impurities from the steel pipe surface is crucial for ensuring zinc coating adhesion.
Pickling: Soaking in hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) removes surface scale and rust.
Rinsing: Rinsing residual acid with clean water to prevent corrosion of the steel pipe.
Flux: Soaking in a flux solution of zinc chloride (ZnCl₂) + ammonium chloride (NH₄Cl) forms a protective film on the steel pipe surface, preventing secondary oxidation and improving zinc wettability. Hot-dip galvanizing: The pre-treated steel pipe is slowly immersed in molten zinc (zinc purity ≥ 99.5%, temperature 500±10°C). The iron on the steel pipe surface reacts with the zinc solution:
First, an iron-zinc alloy layer (such as FeZn₇, Fe₂Zn₁₀, etc.) is formed, accounting for approximately one-third of the total zinc layer and firmly bonding to the steel pipe substrate.
Then, a pure zinc layer forms over the alloy layer, accounting for approximately two-thirds of the total layer, providing the primary "sacrificial anode" corrosion protection.
Post-treatment:
Cooling: After removal from the zinc solution, cool the zinc layer with cold water or air to prevent oxidation.
Passivation: In some cases, a chromate passivation treatment is performed to form a passive film on the zinc surface, further enhancing corrosion resistance.
Inspection: The zinc layer is inspected for thickness, adhesion (cross-hatch test), and appearance (for missing plating or blistering).
3.What is the process flow of cold-dip galvanized steel pipe?
Pretreatment: Similar to hot-dip galvanizing (pickling and rinsing), but without the need for a flux.
Electrolytic galvanizing: The steel pipe acts as the cathode and the zinc plate acts as the anode. These are placed in a plating solution containing zinc ions (such as zinc chloride). Direct current is applied, causing the zinc ions to deposit on the steel pipe surface, forming a pure zinc layer.
Post-treatment: Cleaning and passivation (if necessary). However, because the zinc layer is thin, passivation is a critical step in improving corrosion resistance.
4.What are the core advantages of galvanized steel pipes?
Sacrificial Anode Protection: The electrode potential of zinc (-0.76V) is lower than that of iron (-0.44V). When defects such as scratches or missing plating appear on the steel pipe surface, the zinc corrodes preferentially (becoming a "sacrificial anode"), protecting the iron substrate from rust through current transfer.
Physical Isolation Protection: A complete zinc layer (especially a thick one in hot-dip galvanizing) acts as a barrier, isolating the steel pipe from air, water, and corrosive media, preventing iron oxidation at the source.
5.What are the limitations of galvanized steel pipes?
Corrosion resistance has an upper limit: the zinc layer will corrode rapidly in highly corrosive environments (such as strong acids, strong bases, and environments with high chloride ion concentrations, such as seawater).
Cold-dip galvanizing has significant drawbacks: the zinc layer is thin and has poor adhesion, making it susceptible to rust when exposed to outdoor or humid environments for extended periods. Hot-dip galvanizing has gradually replaced it (cold-dip galvanizing is also declining in some areas due to environmental regulations).
Poor high-temperature performance: Zinc has a melting point of only 419°C. When temperatures exceed 200°C, the zinc layer softens and flakes off, losing its corrosion protection. Therefore, it is not suitable for high-temperature fluid transportation (such as steam pipes).