How Does Hot-Dip Galvanizing Protect Offshore Anchors from Corrosion?

An offshore Anchor that holds a mooring line in 80 meters of water is exposed to four different corrosion zones at the same time. The section buried in the seabed is in the mud zone. The section in the water column is in the full immersion zone. The section at the waterline is in the tidal zone. The section above the waterline is in the splash zone. Each zone corrodes at a different rate, and the splash zone is the most aggressive. Hot-dip galvanizing is the most common protection method for these anchors because the zinc coating adapts to all four zones. This guide explains how the coating works and what to verify when specifying it.

1. Why Does an Offshore Anchor Face a Corrosion Environment That Land-Based Steel Never Sees?

Seawater is a highly conductive electrolyte with a chloride content of about 19 grams per liter. The chloride ion breaks down passive films on steel and accelerates the corrosion reaction. The corrosion rate of unprotected carbon steel in seawater is 0.1 to 0.5 millimeters per year, depending on the zone. In the splash zone, the rate can reach 0.5 to 1.0 millimeters per year because the steel is alternately wet and dry, and the oxygen supply is abundant. An offshore Anchor is designed for a service life of 20 to 30 years. Without protection, a 50 millimeter thick section would lose 10 to 30 millimeters of thickness in that time, which compromises the structural capacity. The table below shows the corrosion rate and the dominant mechanism in each zone.

In our factory, we manufacture offshore Anchor units for mooring systems in Southeast Asia and the Middle East. The anchors are hot-dip galvanized after fabrication to ensure that the coating covers the welds and the edges. LIG MARINE GROUP LTD has been supplying marine mooring equipment for over 15 years.

2. What Does the Zinc-Iron Alloy Layer Actually Do at the Steel Surface?

Hot-dip galvanizing is not a paint or a spray. The steel is immersed in molten zinc at about 450 degrees Celsius, and a metallurgical reaction forms a series of zinc-iron alloy layers on the surface. The typical coating has three zones. The innermost zone is a thin layer of gamma phase, which is zinc-rich and iron-rich. The middle zone is the delta phase, which is a columnar alloy layer. The outer zone is the pure zinc eta phase. The alloy layers are harder than the pure zinc and provide a strong bond to the steel. The pure zinc layer provides the sacrificial protection. When the coating is scratched, the zinc around the scratch corrodes first and protects the exposed steel. The table below shows the relationship between coating thickness and expected service life in seawater.

In our factory, we specify a minimum coating weight of 610 grams per square meter for the splash zone and 915 grams per square meter for the sections that are difficult to access for maintenance. We verify the coating weight with a magnetic thickness gauge at multiple points on each anchor. LIG MARINE GROUP LTD provides the coating measurement report with each shipment.

3. How Does Coating Thickness and Adhesion Determine Performance in the Splash Zone?

The splash zone is the most demanding zone because the coating is exposed to the highest oxygen level, the highest salt concentration, and the most frequent wet and dry cycling. A thicker coating provides a longer diffusion path for the chloride ions and a larger reservoir of zinc for the sacrificial protection. The adhesion of the coating to the steel must be high enough to resist the mechanical damage from waves, debris, and the mooring chain. The adhesion is determined by the surface preparation before galvanizing and by the cooling rate after galvanizing. A rough surface profile of 40 to 80 micrometers provides the best mechanical key for the zinc-iron alloy layer. In our factory, we prepare the surface by abrasive blasting to Sa 2.5 and a profile of 50 to 70 micrometers. We also control the cooling rate to avoid the formation of a brittle alloy layer. LIG MARINE GROUP LTD performs a bend test and a hammer test on a sample from each batch to verify the adhesion.

We also recommend that the designer specify a post-galvanizing treatment for the splash zone sections. A chromate conversion coating or a zinc-rich paint topcoat can extend the service life by 30 to 50 percent. The topcoat must be compatible with the zinc surface and must be applied after the galvanizing is complete.

4. Why Do Welding, Bending, and Handling Damage Threaten the Galvanized Coating on an Anchor?

Fabrication after galvanizing is not permitted because it destroys the coating. The correct sequence is to complete all welding, bending, and machining before galvanizing. The welds must be inspected and cleaned before galvanizing, because welding defects can trap air or moisture and cause coating defects. The bending radius must be large enough to avoid cracking the coating on the outside of the bend. The handling equipment must be padded to avoid scratching the coating during transport. In our factory, we fabricate the offshore Anchor completely, including the welds and the bends, before sending it to the galvanizing line. We also protect the coated surfaces with wooden blocks and soft slings during loading. LIG MARINE GROUP LTD provides a handling and installation guide with each anchor.

If the coating is damaged during transport or installation, the damaged area must be repaired with a zinc-rich cold galvanizing compound. The repair procedure must include surface cleaning, application of the compound to the specified thickness, and a cure time before the anchor is immersed. We recommend that the repair kit be included in the offshore installation package so that the crew can repair minor damage on the deck.

5. How Should Engineers Specify and Verify Hot-Dip Galvanizing for an Offshore Anchor?

The specification should include six items. The first is the galvanizing standard, such as ASTM A123 or ISO 1461. The second is the minimum coating weight for each zone. The third is the surface preparation requirement. The fourth is the adhesion test method. The fifth is the repair procedure for damaged areas. The sixth is the inspection report format. The verification should include three checks. The first is the coating thickness measurement at the specified number of points. The second is the adhesion test on a sample. The third is the visual inspection for bare spots, runs, and cracks. In our factory, we perform these checks on every offshore Anchor and provide the inspection report with the shipment. LIG MARINE GROUP LTD also offers a third-party inspection service for critical projects.

We recommend that the engineer specify the coating weight for the splash zone separately from the submerged zone, because the splash zone requires a thicker coating. A single specification for the whole anchor may result in a coating that is either too thin for the splash zone or unnecessarily thick for the submerged zone.

Summary for Marine Corrosion Engineers

Hot-dip galvanizing protects an offshore Anchor from corrosion by forming a zinc-iron alloy layer that provides both a barrier and a sacrificial protection. The coating thickness determines the service life in each corrosion zone, and the splash zone requires the thickest coating. The adhesion of the coating depends on the surface preparation and the cooling rate. Fabrication must be completed before galvanizing, and any damage during transport or installation must be repaired with a zinc-rich compound. The specification should include the galvanizing standard, the coating weight for each zone, the adhesion test, and the repair procedure. LIG MARINE GROUP LTD has been supplying hot-dip galvanized offshore Anchor units for over 15 years and provides full coating specification and verification support for marine projects.