1.Where does the extreme corrosiveness of the marine environment to the galvanized layer manifest itself?
High salinity: High concentration of chloride ions in seawater has strong penetrability, destroying the passivation film of zinc and accelerating electrochemical corrosion.
Alternating wet and dry: The splash zone (above the average high tide level + splash zone) is the most corroded area. In the repeated wetting-drying cycle, the surface salt of the galvanized layer continues to concentrate, and the consumption rate of zinc is much higher than that of the full immersion area.
The life of the galvanized layer in this area may be less than 10 years, which cannot meet the design life requirement of more than 25 years for wind power pile foundation.
High oxygen content: High dissolved oxygen accelerates the cathode reaction and promotes the consumption of zinc sacrificial anode.
Microbial corrosion: Marine organism attachment and microbial metabolites accelerate local corrosion.
2.What are the failure symptoms of galvanized coating in marine environment?
The sacrificial anode is consumed too quickly: The standard potential of zinc is more negative than that of steel, and it will continue to dissolve in the electrolyte to protect the steel matrix.
In the highly conductive environment of the ocean, the consumption rate of zinc can be more than 10 times that of the land environment. Even thick coatings (≥150μm) are difficult to support for a long time.
Pitting and uneven corrosion: Tiny defects (pores, scratches) in the coating become the origin of pitting in a high-chlorine environment, and local perforation causes the steel matrix to be exposed.
The alternating dry and wet conditions in the splash zone are prone to form a "large cathode (zinc layer)-small anode (exposed point)" galvanic couple, which accelerates local perforation.
Risk of coating shedding: The zinc-iron alloy layer is highly brittle and may crack and peel off under stress, deformation or impact of the pile foundation.
3.What are the structural size and process limitations?
The problem of galvanizing super-large components: The diameter of a wind power pile is often over 6 meters, the length is over 80 meters, and the weight is thousands of tons. There are very few galvanizing plants in the world that can handle such giant components, and the size of the hot-dip galvanizing tank cannot meet the requirements.
Welding damages the coating: On-site welding (such as grouting section connection and accessory installation) will burn the galvanized layer around the weld, forming a weak point for corrosion.
It is difficult to perform high-quality post-weld surface treatment and anti-corrosion coating in the offshore environment.
Lifting and transportation damage: When hoisting giant pile foundations, friction and collision of steel wire ropes can easily scratch the galvanized layer, and the offshore installation process further increases the risk of damage.
4.What are the risks associated with over-protection?
Over-protection risk: The underwater area of the pile foundation usually adopts the combined protection of "coating + sacrificial anode cathodic protection".
If the galvanized layer coexists with the aluminum-based sacrificial anode, the potential of zinc is between that of steel and aluminum, which may lead to:
The galvanized layer becomes the cathode and loses the sacrificial anode function;
The aluminum anode applies excessive negative charge to the galvanized layer, triggering the hydrogen evolution reaction → hydrogen embrittlement and peeling of the galvanized layer.
5.What are the life cycle cost and maintenance dilemmas?
High initial cost: The galvanizing processing fee for giant components is extremely high, and transportation is limited.
Inaccessible for repair: Underwater repair is required after corrosion failure in the splash zone, but the wave energy in this area is large and the diving operation window is extremely short. Traditional repairs (cold galvanizing paint + topcoat) have poor durability in the marine environment and require frequent maintenance.