Since steel has been widely used as a construction material, corrosion protection has always been a major challenge. Today, corrosion protection has expanded from applications such as courtyard fences to landmark structures such as Shanghai Tower and the Hangzhou Bay Sea-Crossing Bridge.
Practice shows that corrosion can severely weaken the strength and durability of steel structures. By selecting a suitable protective coating system, a dense barrier can be formed on the steel surface to effectively isolate corrosive media such as water, oxygen, chloride ions, and chemicals, thereby improving the overall stability of the steel structure.
Studies show that if both coating selection and application meet the specified requirements, maintenance costs can be reduced by up to approximately 50%, creating significant economic benefits for owners.
A scientific and reasonable coating system selection process is the core factor for effective corrosion protection of steel structures. A well-established selection process can avoid two problems: (1) over-specification—choosing a coating system beyond actual needs, causing unnecessary cost; and (2) under-specification—choosing a system that does not match the service environment, thereby endangering structural safety. Selection of protective coating systems requires strict control by engineers based on professional expertise.
ISO 12944 is an internationally standard for corrosion protection of steel structures using coating systems. It provides a systematic evaluation method by classifying environments according to corrosivity.
This environmental classification system can guide engineers to select the most suitable protective coating system for specific service conditions.
The following three steps can be used to determine a protective coating system that meets ISO 12944 requirements:
Step 1: Select the corrosivity category according to ISO 12944-2
Use the table below to select the most suitable environment category for your project:
| Atmospheric corrosivity categories | External environment | Internal environment |
|---|---|---|
| C1 Very low | - | Heated buildings with clean air, such as offices, shops, schools, and hotels |
| C2 Low | Low pollution level, dry climate; mainly rural atmospheres | mainly rural atmospheres Unheated buildings where condensation may occur (e.g., warehouses, sports halls) |
| C3 Median | Urban and industrial atmospheres, moderate sulfur dioxide pollution, and low-salinity coastal areas | Unheated buildings where condensation may occur (e.g., warehouses, sports halls) |
| C4 High | Industrial areas and coastal areas with moderate salinity | Chemical plants, swimming pools, coastal ships and shipyards |
| C5 Very high | Industrial areas with high humidity and aggressive atmospheres, and coastal areas with high salinity | Buildings and areas where condensation and high pollution occur frequently and persistently |
| CX Extreme | Offshore areas with high salinity, and tropical/subtropical industrial areas with extremely high humidity and aggressive atmospheres | Industrial areas with extremely high humidity and aggressive atmospheres |
| Immersion corrosivity Environment | Environment | Examples |
| Im 1 | Fresh water | Installations in rivers, hydroelectric power stations |
| Im 2 | Sea water or brackish water | Immersed structures without cathodic protection (e.g., harbor areas such as sluice gates or breakwaters) |
| Im 3 | Soil | Buried storage tanks, steel piles, and steel pipes |
| Im 4 | Sea water or brackish water | Immersed structures with cathodic protection (e.g., offshore structures) |
Durability categories in ISO 12944-1 help predict how long a coating system can provide corrosion protection. Durability ranges from low (short-term protection) to very high (long-term protection), as follows:
An example of coating system selection is as follows:
|
Substrate: low-alloy carbon steel Surface preparation: substrates with rust grades A, B, or C; surface preparation to Sa 2.5 (According to ISO 8501-1) |
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|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Corrosivity | Primer | Top Coat | Coating System | Expected Durability | ||||||||
| Coating Type | Primer Category | No. of Coating Layers | Dry Film Thickness (μm) | Coating Type | No. of Coating Layers | Dry Film Thickness (μm) | Low | Medium | High | Very High | ||
| C2 | Epoxy | Others | 1 | 75 | Polyurethane | 2 | 125 | ✓ | ✓ | ✓ | ||
| C3 | 底面合一聚氨酯 (Integral-skin polyurethane) |
Polyurethane | 2 | 150 | 2 | 150 | ✓ | ✓ | ||||
| C3 | Epoxy | Others | 1 | 120 | Polyurethane | 2 | 180 | ✓ | ✓ | ✓ | ||
| C4 | Epoxy | Zinc-rich | 1 | 70 | Epoxy, Polyurethane | 3 | 200 | ✓ | ✓ | ✓ | ||
| C5 | Epoxy | Zinc-rich | 1 | 70 | Epoxy, Polyurethane | 3 | 280 | ✓ | ✓ | ✓ | ||
| C5 | Epoxy | Zinc-rich | 1 | 70 | Epoxy, Polyurethane | 3 | 320 | ✓ | ✓ | ✓ | ✓ | |
| Im 1 | Epoxy | Others | 1 | 200 | Epoxy | 2 | 400 | ✓ | ✓ | ✓ | ||
Genecoat can provide more categories of compatible protective coating systems. For details, please contact your local Genecoat sales representative.
To evaluate the performance of protective coating systems more efficiently and accurately, ISO 12944-6 defines multiple test methods. Coating systems must pass the standard tests above, and in some cases additional field exposure tests such as outdoor weathering are also required.
Passing these tests is a prerequisite for determining suitability for the target corrosive environment. For product certification, the required tests must be carried out by an authorized and qualified institution. When results meet the requirements, an official test report can be used to support the corrosion protection performance of the product.
| Corrosivity | Durability | Mode 1 | Mode 2 | ||
|---|---|---|---|---|---|
| ISO 2812-2 Water Immersion Method |
ISO 6270-1 Condensation Test |
ISO 9227 Slat Spray Test |
Cycling Aging Test | ||
| h | h | h | h | ||
| C2 | Low (L) | - | 48 | - | - |
| Medium (M) | - | 48 | - | - | |
| High (H) | - | 120 | - | - | |
| Very High (VH) | - | 240 | 480 | - | |
| C3 | Low (L) | - | 48 | 120 | - |
| Medium (M) | - | 120 | 240 | - | |
| High (H) | - | 240 | 480 | - | |
| Very High (VH) | - | 480 | 720 | - | |
| C4 | Low (L) | - | 120 | 240 | - |
| Medium (M) | - | 240 | 480 | - | |
| High (H) | - | 480 | 720 | - | |
| Very High (VH) | - | 720 | 1440 | 1680 | |
| C5 | Low (L) | - | 240 | 480 | - |
| Medium (M) | - | 480 | 720 | - | |
| High (H) | - | 720 | 1440 | 1680 | |
| Very High (VH) | - | 1200 | 2160 | 2688 | |
| Im1 | High (H) | 3000 | 1440 | - | - |
| Very High (VH) | 4000 | 2160 | - | - | |
| Im2 | High (H) | 3000 | - | 1440 | - |
| Very High (VH) | 4000 | - | 2160 | - | |
| Im3 | High (H) | 3000 | - | 1440 | - |
| Very High (VH) | 4000 | - | 2160 | ||
