Anticorrosion Coatings for Metal Structures: Types and System Selection

Corrosion Costs Ukraine Billions Annually — and Most Losses Could Be Prevented

According to global estimates by AMPP (formerly NACE International), corrosion annually consumes 3–4% of the GDP of industrialized countries. For Ukraine, this is approximately 150–200 billion UAH per year: premature replacement of bridge structures, repair of pipelines, production downtime due to worn-out metal structures in workshops. A bridge designed to last 50 years requires major repairs after 15. A reservoir at an oil depot leaks due to bottom corrosion. A metal truss in a warehouse loses its load-bearing capacity due to reduced cross-section of elements.

At the same time, 70–85% of premature coating failures are not related to paint quality, but to errors in surface preparation and system selection. A correct anticorrosion system is not “paint and forget.” It is an engineering solution where each layer performs its function, and the combination of layers provides protection for 15–25 years even in aggressive conditions.

Corrosivity Classes of Environments: ISO 12944 as the Basis for Selection

ISO 12944 is the main standard that determines which protection system to apply. It classifies environments by aggressiveness from C1 (very low) to CX (extreme).

A common mistake is to apply a C3 system to an object with actual C5 conditions. Metal structures of a bridge over a river in an industrial city are at least C4, and the splash zone is C5. A load-bearing structure in a chemical workshop with acid vapors is CX.

At one industrial facility in the Mariupol district (before 2022), we designed a protection system for external metal structures of a workshop with a C5 corrosivity class. The customer insisted on a C3 system “to save money.” After 18 months, the coating began to delaminate over 30% of the area. Reworking cost three times more than the initial difference between the C3 and C5 systems.

Rule: always choose a system one class higher than it “seems.” The cost of an additional layer of paint is 5–10% of the system cost. The cost of premature repair is multiples of the initial painting cost.

Surface Preparation: 80% of Coating Success

Any anticorrosion system begins with surface preparation. Without it, even the most expensive paint will delaminate. For details on degreasing methods, see the article “Degreasing and Metal Surface Preparation”.

Preparation Grades According to ISO 8501-1

For most industrial projects, the minimum standard is Sa 2½ (near-white metal). This is achieved by sandblasting or shot blasting. The roughness profile (anchor profile) for typical epoxy primers is 40–75 µm.

Key parameters after preparation:

• Salt cleanliness: < 20 mg/m² NaCl-equivalent (Bresle method)
• Surface humidity: steel temperature at least 3 °C above dew point
• Time to primer application: maximum 4 hours after abrasive cleaning (at humidity < 85%)

Layers of an Anticorrosion System: What Each Does

An industrial anticorrosion system always consists of several layers. Each performs a unique function. Removing one layer is like removing one wall from a house.

Primer: The First Line of Defense

Primer provides adhesion to the metal and active or barrier anticorrosion protection.

Zinc-rich primers:

• Zinc dust content: 75–92% in dry film
• Mechanism: cathodic protection – zinc sacrificially dissolves instead of steel
• Two types: inorganic (ethyl silicate, higher heat resistance up to 400 °C) and organic (epoxy, easier to apply)
• Dry film thickness (DFT): 60–80 µm
• Application: C4–CX, bridges, offshore, critical structures

Epoxy primers:

• Mechanism: barrier – dense film prevents water and oxygen penetration
• Excellent adhesion to metal and chemical resistance
• DFT: 50–100 µm
• Limitation: chalking under UV – requires a protective topcoat

Alkyd primers:

• Budget solution for C1–C3 environments
• Easier application, better painting properties
• DFT: 40–60 µm
• Limitation: lower chemical resistance and durability compared to epoxies

Intermediate Coat: Thickness and Barrier

The main function is to increase the overall thickness of the system and enhance barrier properties. The intermediate coat compensates for defects in the primer and prevents penetration of aggressive environments.

Epoxy intermediates:

• Most common choice for industrial systems
• Often reinforced with MIO (micaceous iron oxide) flakes – they create a labyrinthine structure, making it harder for water and oxygen molecules to penetrate
• DFT: 80–150 µm per coat
• Excellent intercoat adhesion (to primer and to topcoat)

Polyurethane intermediates:

• Higher elasticity – better for structures subject to vibrations
• DFT: 60–100 µm
• More expensive than epoxies, but better mechanical resistance

Topcoat (Finish Coat): UV Protection and Aesthetics

The topcoat is the outermost layer of the system. It protects the underlying layers from UV, atmospheric influences, and abrasive wear. It determines the color and appearance of the structure.

Polyurethane (PUR):

• Standard for external structures C3–C5
• High UV resistance (no chalking or fading for 10–15 years)
• Glossy or matte finish
• DFT: 50–80 µm
• Excellent chemical resistance to weak acids, alkalis, solvents

Acrylic:

• More economical than PUR
• Good color retention, but lower chemical resistance
• DFT: 40–60 µm
• Application: C2–C3, where environmental aggression is moderate

Silicone and silicone-containing:

• Heat resistance: up to 200–600 °C (depending on formula)
• Application: chimneys, heat exchangers, exhaust manifolds, hot gas pipelines
• Limitation: lower resistance to chemical environments at room temperature

Typical Anticorrosion Systems by Corrosivity Class

Duplex Systems: Hot-Dip Galvanizing + Paint

A duplex system is a combination of hot-dip galvanizing (HDG) with a paint coating. The synergy of the two methods provides a protection period 1.5–2 times longer than the sum of each separately.

How it works:

1. The structure is immersed in molten zinc (450 °C) – a Zn layer 60–100 µm thick is formed

2. After surface preparation (sweep blasting to a profile of 25–40 µm), the paint system is applied over the zinc

3. Zinc provides cathodic protection, paint provides barrier protection + protects the zinc from premature dissolution

Typical duplex system (C5, 25+ years):

• HDG: 85 µm
• Epoxy primer for zinc: 60 µm
• Epoxy MIO intermediate: 100 µm
• PUR topcoat: 60 µm

Where applied: bridges, power line towers, highway barriers, gantry cranes, structures in coastal areas.

Key nuance: compatibility of the primer with the galvanized surface. Not all primers work on zinc – special adhesion formulas or wash-primers based on polyvinyl butyral are required.

Temporary Anticorrosion Protection

Multi-layer coatings are not always necessary. Temporary measures are used for protection during storage, transportation, and inter-operational pauses.

VCI – Volatile Corrosion Inhibitors

VCI molecules sublimate from a carrier (paper, film, tablets), are transported by air, and settle on the metal surface, forming a monomolecular protective layer.

• Forms: VCI paper, VCI film, VCI emitters (capsules for enclosed volumes)
• Protection period: 6–24 months in sealed packaging
• Application: protection of parts after degreasing and before painting, equipment preservation, transportation of spare parts
• Advantage: does not require application and removal – the VCI layer evaporates upon unpacking

Protective Oils and Preservation Greases

• Thin-film oils: DFT 2–5 µm, protection 3–6 months indoors
• Thick-film waxes: DFT 30–80 µm, protection 12–24 months, for outdoor storage
• Water-soluble preservatives: easily washed off before further processing, protection 1–3 months

More details on corrosion inhibitors for pipelines – in the article “Corrosion Inhibitors for Oil and Gas Pipelines”.

System Selection by Object Type

Bridges and Overpasses

Environment: C4–C5 (water splashes, reagents, vibrations from traffic).

System: duplex (HDG + paint) or zinc-rich primer + MIO intermediate + PUR topcoat. Total DFT: 320–500 µm. Special attention to the splash zone and joint connections. Bolted connections are painted separately after assembly.

Pipelines (Above-Ground and Underground)

Above-ground: epoxy systems C4–C5 with heat-resistant topcoat in sections near compressor stations.

Underground: three-layer polyethylene insulation (3LPE) or FBE (fusion-bonded epoxy) DFT 300–500 µm + cathodic protection.

Tanks and Vessels

External surface: standard atmospheric system according to corrosivity class.

Internal surface: determined by the stored product. For petroleum products – epoxy coatings DFT 300–500 µm. For drinking water – coatings approved for contact with drinking water. Tank bottom – high-risk area: condensate + sediment = pitting corrosion.

Industrial Buildings and Warehouses

Internal load-bearing structures: C1–C2, alkyd or acrylic system is sufficient.

Structures with high humidity (pools, car washes, food processing plants): C4–C5, epoxy or vinylester systems with high chemical resistance.

Marine and Offshore Structures

CX – maximum aggressiveness. Inorganic zinc-rich primer + thick-film epoxy (glass-flake reinforced) + PUR topcoat. Submerged zone and tidal zone are protected by a separate system: high-build epoxy 500–1000 µm + cathodic protection. More details on phosphating and conversion coatings as an element of preparation.

Inspection and Quality Control of Coatings

Applying an anticorrosion system without control is like building a bridge without checking the welds. Inspection standards are defined by ISO 12944-7 and ISO 19840.

DFT – Dry Film Thickness

Measured with a magnetic or eddy current thickness gauge. For each layer, the following are established:

• Nominal DFT – target thickness
• Minimum DFT – 80% of nominal (no measurement can be lower)
• Maximum DFT – usually 3× nominal (excessive thickness = risk of cracks and delamination)

Measurement frequency: minimum 5 points per 10 m² of surface.

Adhesion

Cross-cut test (ISO 2409): a grid of cuts through the coating to the metal, tape is applied and removed. Rating from 0 (perfect) to 5 (complete delamination). Passing score for industrial systems: 0–2.

Pull-off test (ISO 4624): a dolly is glued to the coating and pulled off with a hydraulic device. The pull-off force in MPa is measured. Minimum for industrial systems: 3–5 MPa (depending on specification).

Holiday Detection (Defect Search)

For coatings operating in immersion (internal surface of tanks, underground pipelines), electrical methods are used:

• Low-voltage wet sponge (up to 500 µm DFT): a damp sponge with a voltage of 67.5–90 V is passed over the coating. A signal is triggered at pores and cracks.
• High-voltage spark test (> 500 µm DFT): voltage 5–30 kV depending on thickness. A spark penetrates the coating at the defect location.

FAQ

How many layers are needed for anticorrosion protection?

From two (primer + topcoat) for C1–C3 environments to four or more for C5–CX. A standard industrial system is three layers: primer, intermediate, topcoat. Total DFT depends on the corrosivity class and expected service life.

What is better: hot-dip galvanizing or painting?

It depends on the conditions. Hot-dip galvanizing is an excellent basic protection (20–40 years for C2–C3), but it does not offer color choice and has limited resistance in acidic environments (pH < 5). Painting offers flexibility of choice. The best option for critical structures is a duplex system (zinc + paint), which combines the advantages of both methods.

How often should anticorrosion coating be renewed?

A properly selected system: 15–25 years until the first major repair. Maintenance repair (local touch-up of damaged areas): every 5–7 years. The key factor is regular inspection: detecting a defect at the 2% damage stage is much cheaper than repainting the structure at 30%.

Can anticorrosion coating be applied in winter?

Most paint and varnish materials have a minimum application temperature of +5–10 °C. The metal surface temperature must be at least 3 °C above the dew point. In winter, this is achieved by temporary enclosures and heating the painting area. Some special formulas (moisture-tolerant) work from 0 °C, but their cost is higher, and the application window is narrower.

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SVK produces anticorrosion chemicals for industrial protection of metal structures: epoxy and alkyd-based primers, conversion coatings, protective oils, VCI agents, degreasers, and surface preparation agents. We will develop a system for your operating conditions and corrosivity class. Contract manufacturing of anticorrosion materials under Private Label – from formulation to finished products with your brand.

👉 Send an inquiry for anticorrosion chemicals

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Read also:

• Degreasing and Metal Surface Preparation
• Phosphating vs. Nanoceramics
• Corrosion Inhibitors for Oil and Gas Pipelines
• Glossary of Industrial Chemistry