Corrosion protection for wharves and port structures: zone by zone

A wharf is protected zone by zone: a high-durability paint system in the atmospheric zone, a thick, impact-resistant system (often sealed metallizing or a high-solids epoxy) in the splash zone, a moisture-tolerant epoxy applied at low tide in the tidal zone, and a coating that works with cathodic protection in the submerged zone. ISO 12944-9 is the reference for systems intended for the CX category and seawater immersion.

Sheet piles, piles, fenders, ladders, bollards, crane rails, gangways and steel caissons: a port structure brings together, over a few metres of height, the most corrosive environments found in Quebec and Ontario. Salt or brackish water, de-icing salts, ice and tidal range all combine there. This guide helps maintenance managers and project managers build a protective coatings programme suited to each zone of the structure.

Which corrosion zones does a wharf have?

There are four zones, because corrosion rate and application conditions change completely from one to the next. A single system chosen for the easiest zone always fails in the most severe one.

The splash zone is generally the most aggressive: steel there is constantly wet and well oxygenated, and cathodic protection does not reach it. That is where metal loss concentrates and where protection effort has to be highest. The boundaries between zones are not fixed by a drawing convention: they are set on site from the tidal range, wave exposure and the height reached by spray, and recorded in the specification so that each system stops and starts at the right level.

What do ISO 12944-9 and the CX and Im2 categories mean?

ISO 12944-2 classifies environments: CX is the extreme atmosphere, including marine and offshore, and Im2 is immersion in seawater or brackish water (Im1 for fresh water). ISO 12944-9 then sets performance requirements and laboratory tests for systems intended for offshore and related structures, in the CX category and in immersion with cathodic protection.

Our article on ISO 12944 details how categories and durability ranges work. Three practical consequences for a wharf:

• Choose the right immersion category. A fresh-water wharf upstream on the river does not have the same exposure as a seawater wharf on the Côte-Nord. The category selected is justified by actual salinity.
• Require proven systems. For CX or Im2 exposure, the specification should call for a system tested as ISO 12944-9 or ISO 12944-6 requires, as applicable, not a standard plant system.
• Account for de-icing salts. The atmospheric zone of a wharf also receives salts spread on the deck. See our article on de-icing salt corrosion.

For comparison with systems for less severe environments, see ISO 12944 C3, C4 and C5 paint systems.

When is metallizing the right choice on a port structure?

Metallizing is a good choice in the atmospheric and splash zones when the steel can be blasted to the required grade and kept dry long enough for spraying and sealing. Sprayed aluminum or zinc-aluminum alloy, sealed and then topcoated if the specification calls for it, holds up well in salt water and protects damaged spots better than paint alone.

It is less suitable in the tidal zone, where dry windows are too short to chain blasting, spraying and sealing. Our comparison of zinc vs aluminum thermal spray explains the choice of metal, and our thermal spray metallizing service covers on-site execution.

Which coatings work in the tidal zone?

In the tidal zone, moisture-tolerant epoxies are used, designed to be applied on damp surfaces, or even underwater for some products, with a cure compatible with the returning tide. The choice depends on achievable preparation and the time available between two tides.

• High-solids surface-tolerant epoxy. Applied at low tide on a prepared, wiped surface; the data sheet sets the time before immersion.
• Underwater-applied epoxy. A paste or product applied by hand or roller by divers; useful for localized repairs.
• Jackets and wraps. On piles, jackets or petrolatum tape systems protected by an outer wrap are sometimes preferable to a liquid coating.

In the lower tidal zone and below water, the coating works with cathodic protection. It has to resist cathodic disbondment according to the test set by the specification.

How is wharf steel prepared before coating?

Steel is prepared in three stages: removal of marine fouling and salts, blasting to the required grade, then a soluble salt check before application. On a marine structure, chloride contamination is the leading cause of early blistering.

• Cleaning and desalting. Hydro-blasting removes fouling, loose rust and much of the salt, and suits the tidal zone well.
• Blasting. The target grade, often SSPC-SP 10 / NACE 2 or ISO 8501-1 Sa 2½ in the atmospheric zone, is set by the specification. See our NACE 2 / SSPC-SP 10 guide.
• Soluble salts. Measured per ISO 8502-6 and ISO 8502-9; the acceptable limit comes from the specification or the manufacturer's data sheet.
• Dust and dew point. Cleanliness checked per ISO 8502-3, along with the difference between surface temperature and dew point.

How is work organized around tides and ice?

Work is organized from tide tables, the ice season and wharf operations. The work window is often what decides the system, more than the theoretical performance of the product.

• Schedule. Low areas are treated at the lowest tides; the ice-free period is reserved for the tidal and splash zones.
• Access. Suspended scaffolds, lifts, barges or cofferdams depending on the zone; containment of blast debris over water is mandatory.
• Ice. Ice strips thin coatings in the tidal zone. Thick, abrasion-resistant systems are preferred, and an inspection is planned after each ice breakup.
• Operations. Work is coordinated with vessel movements and cargo handling.

Our crews and qualified subcontractors work in particular on the Côte-Nord, in Sept-Îles and in Quebec City.

Which inspections and maintenance should be planned?

Checks are planned at every stage of the job, then periodic inspection by zone, with particular attention to the splash zone. A measurement file makes it possible to follow the structure and plan touch-ups before the steel loses thickness.

• During the work. Cleanliness, profile, salts, ambient conditions, dry film thickness, holiday testing depending on the system.
• After the work. Visual inspection by zone, ultrasonic measurement of remaining steel thickness, defect survey.
• Maintenance. Localized touch-ups in the splash zone before corrosion spreads; monitoring of cathodic protection.

Our coating inspection service handles these checks and the report.

How do you get a quote for a port structure?

Prepare a description of the structure by zone, its current condition, water salinity and operating and tidal constraints. Pricing is on consultation: it depends on access, preparation, tide windows, containment and the system selected. Our protective coatings cover all four zones of a wharf. Send photos, drawings and surveys through the request a quote form: reply within 24 business hours.

Frequently asked questions

Is cathodic protection enough for a wharf?

No. It effectively protects submerged steel and part of the tidal zone, but it does not act in the splash zone or the atmospheric zone, which must be protected by a coating.

Can sheet piling be coated underwater?

Yes, with epoxies formulated for underwater application, applied by divers. It is mainly a localized repair solution; for full rehabilitation, a cofferdam gives better preparation.

Why does the splash zone corrode faster?

Because steel there is wetted and dried constantly, with plenty of oxygen and salt, and cathodic protection does not reach it. Ice and debris impacts also damage the coating.

Which ISO 12944 category applies to a fresh-water wharf?

Fresh-water immersion falls under Im1, and the atmospheric zone under a category set from the actual environment, often made worse by de-icing salts. The specification justifies the choice from site conditions.