1.How to optimize the zinc coating?
Increase the thickness of the zinc layer
Use a hot-dip galvanized layer of ≥85μm
Upgraded coating type
Zinc-aluminum-magnesium alloy coating (ZAM): corrosion resistance is 3-5 times that of pure zinc, excellent resistance to cut-edge corrosion, suitable for highly corrosive soils.
Galfan (Zn-5%Al-RE): better soil corrosion resistance than pure zinc, good flexibility.
2.How to improve protective coating systems?
High-performance primer + thick film intermediate paint + wear-resistant topcoat
Primer: Epoxy zinc-rich primer: provides cathodic protection and is compatible with galvanized layers (low zinc type or special formula is required).
Solvent-free epoxy primer: high adhesion, shielding water vapor (surface sandblasting is required).
Intermediate paint: Epoxy micaceous iron intermediate paint (100-150μm): flaky micaceous iron oxide enhances shielding and resistance to penetration.
Topcoat: Polyurethane/modified epoxy coal tar (water-resistant and bacteria-resistant), glass flake epoxy (super wear-resistant).
Fusion bonded epoxy powder (FBE) coating
Single-layer FBE: Standard for pipeline corrosion protection, thickness 300-500μm, strong adhesion and chemical corrosion resistance.
Double-layer FBE: The bottom layer is anti-corrosion epoxy, and the outer layer is toughened and wear-resistant epoxy, which has better impact resistance. Polyethylene/polypropylene tape (3LPE/3LPP)
Structure: epoxy primer + polymer adhesive + polyethylene/polypropylene outer sheath.
Advantages: Strong mechanical protection, suitable for rocky areas or backfill containing gravel scenes.
3.Is cathodic protection necessary for long-term underground use?
Necessity and relationship with galvanizing: The galvanizing layer can provide initial sacrificial protection, but in long-term underground use (>20 years) or in highly corrosive environments, additional cathodic protection must be applied.
Sacrificial anode method
Material: magnesium alloy (Mg), zinc alloy (Zn) anode.
Applicable: small structures in low resistivity soil and no stray current area.
Layout: anode 1.5-3m away from the structure, filled with special chemical packing material to reduce resistance.
Impressed current method
Material: high silicon cast iron/MMO anode, constant potential instrument, reference electrode.
Applicable: large pipelines, pipe corridors, high resistivity soil or scenes with stray current.
Protective potential: maintain galvanized steel structure at -0.95～-1.10V.
4.What are the enhanced measures for special scenarios?
Stray current interference: Install drains or polarity drains
- Cathodic protection using zinc strips or mixed metal oxide (MMO) anodes
Microbial corrosion: Adding fungicides (such as epoxy coal tar) to the coating
- Cathodic protection potential ≤-0.95V inhibits SRB activity
High risk of mechanical damage: Add polyethylene protective sleeve or concrete weight layer to the outer layer (seabed/river crossing)
- Use 3LPE/3LPP coating.
Welding and damaged points: Welding area sandblasted to Sa2.5 level, coated with solvent-free epoxy + glass fiber cloth reinforcement
- Damaged areas repaired with cold galvanized paint + coating
5.What are the key points of construction and acceptance?
Surface treatment: The surface of the galvanized layer needs to be lightly sandblasted (Sa2 level) or chemically cleaned + passivated to ensure coating adhesion (pulling strength ≥5MPa).
Coating construction: Strictly control the ambient humidity (<85%) and dew point (≥3°C). The interval between multiple layers of coating shall be implemented according to technical specifications.
Cathodic protection design: Design anode quantity and position according to ISO 15589-1 or NACE SP0169, and fully cover the protection potential.
Inspection and acceptance
Coating: spark leak detection, adhesion test.
Cathode protection: measure the protection potential 48 hours after power on.